Urchins in hot water

June 29, 2020 by fredsingerecology

The fabled Mediterranean Sea is under stress from overfishing, pollution, rapid warming, and the associated proliferation of invasive species that thrive in the warming waters. Two species of rabbitfish (Siganus luridus and Siganus rivulatus) crossed the Suez Canal into the Mediterranean Sea in the 20^th century, and now make up about 95% of the herbivorous fish in rocky habitats along the Levant Basin off the Israeli coast. These fish are voracious feeders on macroalgae that live in the Levant, and they have become much more abundant during the past 30 years in association with increased water temperatures of 2-3 degrees C.

The rabbitfish Siganus luridus. Credit: Roberto Pillon at Wikipedia.

While the Levant has been warming and rabbitfish have been proliferating, things have not gone very well for the purple sea urchin Paracentrotus lividus. Previously, it had been a very important consumer of macroalgae within the Levant, but its population has collapsed within the past decades. For his Masters program, Erez Yeruham decided to investigate why the sea urchin population collapsed. Initially, he and his colleagues thought it was likely that sea urchins were competitively excluded by the invasive rabbitfish. These fish overgrazed much of the algal meadows, forming barren grounds along much of the Israeli coastline. However, during the experiments they did to check that out, they noted that sea urchin mortality occurred in two consecutive summers, but not in other seasons. That led them to explore how sea urchin survival was affected by both the impact of warming water and by competition with rabbitfish.

Researchers construct cages to investigate to investigate the causes of sea urchin population collapse. See description below. Credit Erez Yeruham.

To investigate competitive exclusion of sea urchins by rabbitfish, the researchers bolted 25 metal cages (50 x 50 x 20 cm) to the rocks approximately 9 meters below the surface of the sea. They set up six different treatments: (1) fish only (F), (2) fish and sea urchins (FU), (3) sea urchins only (U), (4) no fish nor sea urchins (N), (5) cage control – a partial cage that allowed access to organisms (CC), and (6) no cage – an open control plot marked with bolts (NC). For the treatments with sea urchins (FU and U), the researchers introduced five sea urchins into each cage. For the treatments with fish (F and FU), the researchers cut oblong holes in the mesh large enough for rabbitfish to get through. There were five replicates of each experiment in the fall of 2011 and again in the spring of 2012.

Metal cage with five sea urchins (upper left corner of cage). Credit: Erez Yeruham.

Yeruham and his colleagues discovered that fish drastically reduced the abundance of soft algae, but that urchins had no discernable effect. The researchers suggest that sea urchin density in the cages was low enough that even though sea urchins were eating some soft algae, the effects were too small to be detected. Both fish and sea urchins had very little effect on the abundance of calcareous algae (algae with hard crusty surfaces).

Mean (+ standard error) dry weight (grams) of soft and calcareous algae for the six experimental treatments.

The researchers compared the amount of food in sea urchin guts when they were caged by themselves, or in cages with fish access. Sea urchins had 40% more food in their guts when fish were excluded (left graph below). In addition, they had a 30% greater gonado-somatic index (GSI) when fish were excluded (right graph below – the GSI measures the relative size of the gonads – a high GSI indicates good health and high reproductive potential). So when rabbitfish could visit the cages, sea urchins ate much less and suffered poorer health.

Mean dry organic gut content (left graph) and GSI (right graph) of sea urchins with and without fish.

The results of this experiment show that rabbitfish have strong competitive effects on sea urchin food intake and overall health. But do warmer waters also help to explain the collapse of sea urchin populations in the Levant? And might thermal stress interact with food limitation to influence sea urchin health? To answer these questions the researchers used seawater pumped in directly from the sea into tanks that housed eight sea urchins. Five tanks received ambient temperature seawater, while five other tanks received water that was chilled by 2 deg. C to mimic water temperatures before sea urchin populations collapsed. Each tank was divided in half by a partition so that four urchins could be fed (algae) three times a week, while the other four urchins were starved.

One important finding is that during the winter, feeding rates were similar when comparing sea urchins in ambient vs. chilled sea water (two left bars below – those differences are not statistically significant). However, feeding rates plummeted in the summer when water temperatures exceeded 29 deg. C in the ambient-temperature sea water.

Mean algal consumption by sea urchins in ambient vs. chilled water during the winter (two left bars) and summer (two right bars).

Respiration rates (measured as oxygen consumption) are a good measure of metabolic performance. Highest respiration rates were measured in the winter with fed sea urchins (ambient was slightly higher than cold) and in the summer with cold fed sea urchins. Most notably, when sea water temperatures increased above 29 deg. C in the summer, the respiration rates were very low, even in sea urchins that were well-fed.

Mean (+ standard error) respiration rate (measured as oxygen uptake) of starved and fed sea urchins in ambient vs. chilled water during the winter and summer.

What emerges from this series of experiments is that sea urchins feed much more poorly and have lower respiration rates at high temperatures, independent of the effects of competition with rabbitfish. The researchers also found that survival rates were lower at elevated temperatures. Yeruham and his colleagues conclude that the direct effects of high temperature and the indirect effects of competition with rabbitfish are important factors that together conspired to lead to the collapse of sea urchin populations in the Levant. They expect that as sea temperatures increase, rabbitfish will become more dominant in other regions that are now a bit cooler than the Levant. As warming continues and competition increases, Yeruham and his colleagues predict that sea urchin populations will collapse in those somewhat cooler ecosystems as well, changing the structure and functioning of coastal ecosystems across the Mediterranean.

note: the paper that describes this research is from the journal Ecology. The reference is Yeruham, E., Shpigel, M., Abelson, A., and Rilov, G.. 2020. Ocean warming and tropical invaders erode the performance of a key herbivore. Ecology 101( 2):e02925. 10.1002/ecy.2925. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Hot invaders thwart endemic New Zealanders

April 24, 2020 by fredsingerecology

Tongariro National Park in New Zealand’s North Island is changing in many ways. Over the past 50 years, the park, which has three large volcanoes, has increased in temperature at about three times the global average (about 1.5 deg. C) and is also receiving reduced annual rainfall. The park hosts a large number of endemic plants – species that are native to that region and found nowhere else.

Tongariro National Park in New Zealand. The plastic sheets in the foreground are open top enclosures used to experimentally raise air and soil temperatures. Credit: Justyna Giejsztowt.

Monoao (Dracophyllum subulatum), is an endemic shrub that thrives in low-lying areas between the volcanoes. Ecologically it is a facilitator, in that its growth form protects a variety of native species from heavy frosts, thereby promoting high species diversity within the plant communities.

The native monoao (Dracophyllum subulatum). Credit: Justyna Giejsztowt.

In addition to the threat of climate change, portions of Tongariro National Park are also being invaded by common heather (Calluna vulgaris), which has already caused a decline in many native and endemic plant species, and their associated insect communities. Justyna Giejsztowt had worked previously as a technician for a project that investigated how climate change affected plant communities. She noticed that the invasive heather had a stronger phenological response to warming than did the native community, flowering earlier and reaching peak floral density at an earlier date. Watching the countryside turn pink from the invasive flowers during that season, she wondered whether the pollinator community might be changing as well, which could affect the reproductive success of the surrounding native vegetation. So she and her colleagues decided to do some experiments.

The invasive heather, Calluna vulgaris. Credit: Justyna Giejsztowt.

Beginning in 2014, the researchers used hexagonal open-topped chambers to increase air and soil temperatures in experimental plots, while also maintaining unmanipulated control plots (you can see the plastic chambers in the top photo of Tongariro National Park). The researchers measured flowering dates for monoao and heather in each plot (and 11 other less abundant species as well), and estimated the number of flowers in each plot on a regular basis.

Daily mean temperatures (°C) over the 2015/2016 austral summer in experimentally warmed (red) and ambient temperature (blue) plots.

The researchers expected that experimental warming would cause more overlap between the time period when monoao and heather were both in flower. This is exactly what they found. Heather reached a high level of flowering much earlier in the year under experimental warming, increasing the percentage of flowering overlap from 2.79% (top graph below) to 11.27% (bottom graph).

Floral density of Calluna vulgaris (heather – dashed line) and Dracophyllum subulatum (monoao – solid line) under ambient (top graph) and experimentally warmed (bottom graph) temperature regimes. Shaded regions denote flowering overlap of monoao with high densities of heather.

This increase in overlap would increase the number of flowers open at a particular time, which might increase competition for pollinators leading to reduced reproductive success. On the other hand, increase in overlap could make a strong visual or olfactory impression on pollinators, drawing them into the area and thereby increasing plant reproductive success. Or both forces could be important and cancel each other out.

Giejsztowt and her colleagues set up a second experiment to explore how the ratio of native monoao to invasive heather in a patch, and also the total number of flowers of either type within the patch, influenced monoao’s reproductive success. They intentionally chose patches that had either (1) high monoao flower numbers and high heather flower numbers, (2) high monoao, low heather, (3) low monoao, high heather, or (4) low monoao, low heather. The researchers chose nine focal plants within each plot, and from these plants they set up four transects running north, east, south and west. Each transect was 25 meters long and 40 cm wide. The researchers estimated flower abundance in each transect. As their measure of monoao reproductive success, they collected seeds produced by each focal monoao plant, dried them and then weighed them.

Giejsztowt and her colleagues found that neither the ratio of native to invasive plants, nor total floral density had any direct effect on monoao reproductive success. However, the interaction of these two factors had a strong effect. Seed masses of focal monoao plants were heaviest in patches with a high ratio of native to invasive plants, but only if the patches had intermediate or high overall floral density. In contrast, monoao in patches composed of mostly invasive heather had consistently low seed masses, regardless of overall flower density in the patch.

Monoao seed mass (g) adjusted for the effect of plant height, relative to total floral density in the landscape. Colors denote native monoao (green) or invasive heather (black) dominance (making up more than 50% of the flowers).

The researchers were not surprised to find that heather responded more strongly to increased temperature than did monoao, as several studies have shown that invasive species tend to have flexible phenology in response to changing environmental conditions. By shifting its peak flowering earlier in response to warmer temperature, heather increased its flowering overlap with monoao, which could, and did, increase competitive effects on monoao reproductive success. When there were numerous flowers in a patch, but monoao was rarer than heather, monoao had relatively low reproductive success. In contrast, if monoao was more common than heather, it achieved much greater reproductive success.

Why does this happen? The researchers suggest that at high floral densities, heather may outcompete monoao for pollinators. The mechanism for this competitive effect is unknown; invasive species have been shown to influence pollinator behavior and the numbers and types of pollinator within the community. Because pollinators are declining globally, it is critical to understand how climate change and invasive species can interact to reduce pollination services to native plants within ecosystems.

note: the paper that describes this research is from the journal Ecology. The reference is Giejsztowt, J., Classen, A. T., and Deslippe, J. R.. 2020. Climate change and invasion may synergistically affect native plant reproduction. Ecology 101( 1):e02913. 10.1002/ecy.2913. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2020 by the Ecological Society of America. All rights reserved.

Warming Arctic forests diverge over nutrients

March 27, 2020 by fredsingerecology

Humans continue their unique uncontrolled experiment to see how increased atmospheric carbon dioxide and the resulting warmer temperatures influence biomes worldwide. One expected outcome of this global experiment is that trees in the extreme north will show improved growth resulting from a more benign physical environment. As it turns out, some regions of the north do show this trend, while others don’t – this lack of consistent response is known as divergence. For her graduate work, Sarah Ellison, working with Patrick Sullivan, Sean Cahoon and Rebecca Hewitt, wanted to document divergence in northern Alaska, and to figure out what might be causing it.

The Wind River, near Arctic Village in the Arctic National Wildlife Refuge, is the easternmost site in the study. Credit: Patrick Sullivan.

The researchers established four study sites in four watersheds across the Brooks Range in northern Alaska. They knew from previous studies that white spruce (Picea glauca) in the western Brooks Range have shown increased growth in response to climate warming, whereas those in the central and eastern Brooks Range have not responded. Some researchers hypothesized that warmer temperatures caused moisture limitation in the eastern Brooks Range, but previous plant physiological studies done by this research team show no evidence of water stress, even in the extreme eastern portion of the range.

The four study sites within the Brooks Range from west to east: Agashashok, Kugururok, Dietrich and Wind River.

So what’s causing divergence?

At each study site, the researchers set up climate stations which collected continuous data on air and soil temperature, wind speed and direction, solar radiation, snow depth and precipitation. (Check out the following (you may need to copy and paste into your browser) for an entertaining look at the challenges of doing this research: https://youtu.be/ty6vwio9LvU).

A weather station at the Wind River sight. Credit: Sarah Ellison.

They discovered that soil temperatures were consistently warmer in the western part of the range over the course of the season.

EllisonFig2

Soil

Temp.

(deg. C)

Colder soils are often associated with low levels of available nutrients, because bacteria are less active at colder temperatures, and thus less capable of breaking down nutrients into a form that can be taken up by roots. In 2014, Ellison and her colleagues measured soil nutrient levels at each site and found generally lower levels in the central (Dietrich) and eastern (Wind River) sites.

From top: ammonium, nitrate, phosphate and total free primary amine (TFPA – a proxy for amino acids) availability in the soils at the four sites during the 2014 growing season. Error bars are +/- 1 SE.

There were several other important physiological pieces to this puzzle. Plants in the west grew more quickly than did plants in the east. Rapid growth was associated with greater photosynthetic rates in the western watersheds. The researchers measured needle nutrient concentration and found that it decreased from west to east. Each year, there was a strong correlation between needle nutrient concentration and branch extension (the measure of tree growth), but the correlation with phosphorus was generally stronger than the correlation with nitrogen.

Branch extension in relation to needle phosphorus concentration (left graph) and needle nitrogen concentration (right graph) for three years of the study.

Armed with these findings, Ellison and her colleagues decided to experimentally test whether nutrient availability was limiting growth, particularly in the eastern regions of the Brooks Range. If so, this would support the hypothesis that cold temperatures and the resulting decrease in nutrient availability were primary factors causing divergence across this vast ecosystem. In June, 2015, the researchers fertilized five trees at each site with a mixture of nitrogen, phosphorus, and potassium fertilizer, and left five similar nearby trees as untreated controls. After one year, branch extension was greatly enhanced at the most eastern site, and only slightly (insignificantly) enhanced at the most western site.

EllisonFig7top

Mean annual growth (branch extension) before and after fertilization experiment for fertilized trees (gray circles) and control trees (black circles). Bars are +/- 1 SE. Fertilization occurred in 2015 (indicated by vertical dotted lines).

For many years, forest ecologists have believed that forests in young glacial soils are nitrogen limited. This study, and a few other recent studies, thrust phosphorus into prominence as a factor that can limit forest productivity. Over time, as the climate continues to warm, soils in the eastern Brooks Range will enjoy increased microbial activity, and may no longer suffer as much from nutrient limitation.

The Agashashok mesic treeline sits on a gentle slope above the Agashashok river. Credit Sarah Ellison.

One surprising finding was that mycorrhizal growth on fine roots was more extensive in central and eastern watersheds. Abundant mycorrhizae were associated with reduced branch extension, suggesting that these mycorrhizae may be parasitic, rather than mutualistic. The researchers are in the process of expanding their study to an even greater spatial extent of 20 sites distributed across the Brooks Range, which will allow them to further explore how general their findings are across this vast biome.

note: the paper that describes this research is from the journal Ecology. The reference is Ellison, S. B. Z., Sullivan, P. F., Cahoon, S. M. P., and Hewitt, R. E.. 2019. Poor nutrition as a potential cause of divergent tree growth near the Arctic treeline in northern Alaska. Ecology100( 12):e02878. 10.1002/ecy.2878. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Drought differentially diminishes ecosystem production

March 6, 2020 by fredsingerecology

Sometimes, even the most carefully conceived experiment is thrown for a loop by Mother Nature. Good scientists must embrace the unexpected. Ellen Esch, David Lipson and Elsa Cleland set out to explore how plant communities responded to high, normal and low rainfall conditions. The researchers set up rainfall manipulation plots that were covered with a clear plastic roof that would allow most light to pass through, but intercept all of the water. They then reapplied the intercepted water, with each plot receiving either 50%, 100% or 150% of the fallen rain. The plan was to simulate drought, normal and wet conditions. The natural world had other plans, however, as 2013-2016 were unusually dry years. Fortunately the researchers adjusted, by refocusing their question on how plant communities respond to severe drought (50% of intercepted rainfall), moderate drought (100%) and normal rainfall (150%).

Herbaceous plant community being irrigated (notice the rainbow). Credit: Ellen Esch.

Esch and her colleagues set up their experiment at the San Diego State University Santa Margarita Ecological Reserve, which has a Mediterranean-type climate with mild, somewhat moist winters and hot dry summers.

Exotic grasses (here showing recently senesced Bromus madritensis) dominated the herbaceous sites. Credit: Ellen Esch

They wanted to know how climatic variability brought about by climate change would influence plant phenology (the timing of periodic ecological events), specifically green-up date (when plants begin turning green) and senescence date (when they turn brown and curtail photosynthesis). They expected that the native species, primarily sage-type shrubs, would be more drought-resistant than the exotic herbaceous vegetation, which was dominated by brome grass. Climate change is predicted to increase climatic variability, which should increase the frequency and intensity of severe droughts (and also of unusually wet years).

An important measure of ecosystem functioning is its productivity – the amount of carbon taken up by an ecosystem, usually by photosynthesis. More productive ecosystems have more energy available to feed consumers and decomposers. More productive ecosystems also take up and store more carbon dioxide from the atmosphere, which can help reduce climate change. The researchers used a reflectance radiometer to calculate the Normalized Difference Vegetation Index (NDVI), which essentially calculates how green an area is, and is a good measure of productivity. Esch and her colleagues hypothesized that drought would reduce overall ecosystem NDVI, but that native vegetation would be more buffered against the negative effects of drought than would the invasive exotic vegetation.

A student from a plant physiology class at San Diego State University measures NDVI. Credit: David Lipson

Each year from 2013 – 2016, the researchers set up 30 3X3 meter plots; 15 plots were dominated by exotic herbaceous species such as brome, and 15 plots had mostly native shrub species such as sage. Plots were treated the same, except for receiving either 50%, 100% or 150% of the fallen rain, which corresponded to severe drought, moderate drought and normal rainfall, respectively. Periodically, the researchers used a radiometer to measure NDVI for each plot. They discovered that, as expected, drought reduced NDVI much more in the plots dominated by exotic herbaceous species (top graph below) than in the plots dominated by native shrubs (bottom graph).

NDVI on each measurement date for plots dominated by (top graph) exotic herbaceous species and (bottom graph) native shrub species. Red square = severe drought treatment, green circle = moderate drought, blue triangle = normal precipitation. Error bars = +/- 1 standard error.

What caused this difference in response to drought between exotic plant-dominated and native plant-dominated communities? Mechanistically, the native shrubs have deeper roots than the exotic grasses, which may allow them to take up more water. But how does this translate to differences in green-up date and senescence date?

A student measures stem elongation on a senescent native shrub, the black sage Salvia mellifera, near the very end of the growing season. Credit: Ellen Esch.

The researchers used two different NDVI measures to help answer this question. Maximum NDVI is the greatest daily NDVI measure over the course of the growing season. It is correlated with the maximum productivity of the plant community (at its greenest!). In contrast seasonally integrated NDVI is a measure of productivity summed over the entire growing season. Keeping those distinctions in mind, under extreme drought maximum NDVI was much lower in the exotic plots than the native plots. But exotic plot performance increased with rainfall, so that under the wettest conditions (normal rainfall), exotic plot maximum NDVI was similar to native plot maximum NDVI (graph a below). However, when considered over the entire growing season, native plots were consistently more productive than exotic plots (graph c below).

Effect of rainfall on (a) maximum NDVI (top left), (c) seasonally integrated NDVI (top right), (b) green-up date (bottom left) and (d) senescence date (bottom right). Colors indicate dominant plot community composition (yellow = herbaceous, green = shrub) and point shape indicates growing season year (circle = 2013, square = 2014, diamond = 2015, triangle = 2016).

Phenology played an important role accounting for these differences in seasonally integrated NDVI. At all rainfall levels, the native plant communities greened-up well before the exotic plant communities (graph b above). Exotic plants greened-up somewhat earlier as rainfall increased, while native plant green-up date was independent of rainfall. At all rainfall levels, native plots senesced about one month later than exotic plots, with increased rainfall delaying senescence in both native and exotic plant communities (graph d above).

Esch and her colleagues conclude that species composition (native shrub vs. exotic herbaceous plants) and drought both influence phenology and productivity in this important ecosystem. Climate change is predicted to increase the frequency of extreme droughts in this and other ecosystems. Consequently, drought coupled with invasion by herbaceous species threatens to sharply reduce ecosystem productivity, which will decrease the food available for consumers and decomposers, and simultaneously reduce the amount of carbon dioxide taken up and stored by the ecosystem, thereby contributing to further climate change.

note: the paper that describes this research is from the journal Ecology. The reference is Esch, E. H., Lipson, D. A., and Cleland, E. E. 2019. Invasion and drought alter phenological sensitivity and synergistically lower ecosystem production. Ecology 100(10):e02802. 10.1002/ecy.2802. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Nitrogen continues to confound convention

November 22, 2019 by fredsingerecology

Ah nitrogen… It is the most abundant molecule in the air that we breathe (close to 80%), yet plants always seem to be starving for it. Annually, nitrogen fertilizers are a $75 billion dollar industry. The problem is that the nitrogen gas that we breathe (N₂) is very nonreactive, because the two nitrogen atoms are held together by a massively powerful triple bond. So N₂ must be broken down to some other more usable form (such as ammonia) – a process we call nitrogen fixation. Most nitrogen fixers are microorganisms that live in soils or symbiotically within plants. Unfortunately, N-fixation is energetically very costly, so even organisms that can fix nitrogen will generally happily use nitrogen compounds from the soil or leaf litter (the layer of fallen leaves above the soil) if they are available, rather than expending enormous energy to fix it for themselves. The general formula for nitrogen fixation (ignoring protons, electrons and energy transfers) is…

DynarskiEquation

A few years ago Scott Morford, Benjamin Houlton and Randy Dahlgren (the first two are co-authors of the present study) stunned the ecological world by identifying a previously unsuspected source of nitrogen – weathering of bedrock such as the mica schist pictured below. This bedrock was formed from seabeds which were rich in organic matter and had a high concentration of nitrogen compounds When the rock breaks down, both carbon and nitrogen compounds leach into the soil. Katherine Dynarski became interested in nitrogen fixation as an undergrad at Villanova University, so it was natural for her to move to the University of California at Davis to begin her graduate work with Morford and Houlton on how nitrogen cycles through ecosystems.

Nitrogen-rich mica schist bedrock. Credit: Katherine Dynarski.

Dynarski got involved in this specific project essentially by accident. She was helping a fellow graduate student collect rocks at adjacent forests on contrasting bedrock (one high-N mica schist, and one low-N basalt), and figured that while she was out there, she might as well measure some N-fixation rates. In leaf litter and the soil below, most N-fixation is done by free-living soil bacteria. Dynarski expected higher N-fixation rates in the litter collected above the N-poor bedrock, reasoning that the microorganisms would need to fix nitrogen from the air, because there was little present in the litter. In contrast, she expected to find lower N-fixation rates in litter collected above the N-rich bedrock, reasoning that the micro-organisms could save considerable energy by using existing nitrogen that had leached into the soil and leaf litter layer. She was shocked when she ran the samples and found exactly the opposite of her expectation, which led her to develop a more substantial project looking at the relationship between bedrock and N fixing microbes.

Katherine Dynarski conducting gas incubations to measure N-fixation rates in the field. Credit: Scott Mitchell.

Working in northern California and Western Oregon, Dynarski and her colleagues identified sites whose bedrock was low in nitrogen (below 500 parts per million N) or high in nitrogen (above 500 ppm N). The researchers used soil and leaf litter samples from 14 paired sites – high N bedrock with nearby low N bedrock. They analyzed soil and leaf litter samples from each plot for concentration of nitrogen, carbon (C), phosphorus (P) and molybdenum (Mo) – the latter two elements have been shown in other systems to limit the rate of N-fixation. The researchers also collected samples of underlying bedrock and analyzed N and Mo content of these rocks.

Recall that the conventional paradigm is that microorganisms should have lower N-fixation rates in N-rich environments. There was negligible N-fixation occurring in the soil, but considerable N-fixation in the leaf litter above. Thus the conventional prediction was that N-fixation rates would be higher in leaf litter above low-N bedrock. As I mentioned previously, Dynarski found the exact opposite to be true in one site; would this unconventional finding be confirmed by the 14 sites explored in this study?

The answer is yes! Considerably more N-fixation was detected in leaf litter above high N bedrock than in leaf litter above low N bedrock.

Mean leaf litter N-fixation rates and low-N and High_N bedrock sites. Error bars are one standard deviation. P = 0.014.

You will notice the large error bars above the graph. As it turns out, N-fixation rates vary dramatically – even on a very small spatial scale, which is why the researchers took multiple samples from each site. Some sample sites (hotspots) have unusually high rates of N-fixation. These hotspots are also strongly correlated with high carbon concentration, with greater C in the leaf litter associated with much higher rates of N-fixation.

Litter N-fixation rates in relation to % soil carbon at N-fixation hotspots. Hotspots are defined as having fixation rates greater than 1 kg N per hectare per year.

Dynarski and her colleagues also discovered that, in general, leaf litter above high-N bedrock tended to have more C and P than did leaf litter above low-N bedrock. Given this finding (along with the hotspot finding) we are now ready to explore the question of why microbes are expending more energy to fix nitrogen in regions where more nitrogen is naturally available.

The researchers considered two hypotheses. First, it takes N to make N. N-fixation is catalyzed by N-rich enzymes. It may be that leaf litter above low-N bedrock is too N-poor to provide microbes with enough nitrogen make these enzymes. So the additional nitrogen from high-N bedrock is just enough to allow microbes to produce the N-fixation enzymes.

The second hypothesis is that the litter above low-N bedrock is also low in C, P and Mo, all of which are required for N-fixation. Thus the positive effect of these nutrients overwhelms the negative effect of additional nitrogen on the rate of nitrogen fixation. According to this hypothesis, the conventional paradigm of high nitrogen availability reducing the rate of N-fixation is correct, but other factors may be equally or more important in natural ecosystems.

Fortunately, this conundrum is easily resolved. Dynarski and her colleagues took some leaf litter samples and added a small amount of nitrogen to them. These N-additions significantly reduced N-fixation rates at both low and high bedrock N sites. Thus environmental N does reduce biological N-fixation, but other factors, such as the availability of other essential nutrients, can overwhelm the inhibitory effect of environmental nitrogen in natural ecosystems

A Douglas fir forest in the Oregon Coast Range, where some of this research was conducted. Credit: Katherine Dynarski.

The researchers conclude that nitrogen input from bedrock weathering leads to increased C storage and P retention, ultimately enhancing rates of N-fixation. About 75% of Earth’s surface is underlain by rocks with substantial N reservoirs, so we need to continue exploring the effects of weathering bedrock on ecosystem processes and functioning.

note: the paper that describes this research is from the journal Ecology. The reference is Dynarski, K. A., S. L. Morford, S. A. Mitchell, and B. Z. Houlton. 2019. Bedrock nitrogen weathering stimulates biological nitrogen fixation. Ecology 100(8):e02741. 10.1002/ ecy.2741. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Vacation’s changing tides

October 17, 2019 by fredsingerecology

Cindy and I and our dog (Cheyanne) recently returned from a two+ week vacation at North Carolina’s Outer Banks. We stayed in Avon, which is about eight miles north of the iconic Cape Hatteras lighthouse in a large house with a great ocean view. We got a large house, because we thought our kids might join us, but it turns out that one disadvantage of kids getting older is that their lives become more complex. Anyhow, several friends stayed with us for a few days, and a grand time was had by all.

Cheyanne and I ponder the ocean’s vastness. Credit: Cindy Miller

But the point of this post is the trip home. On Friday, we packed everything into our car, including Cheyanne, and began the eight-hour drive back to our home in Radford, VA. At Rodanthe (about 15 miles north), traffic just stopped. We sat in our car for a few minutes, disembarked, and spoke with many people walking by, who told us that the road (NC12) was flooded and covered with sand. We had heard rumors of flooding, but since the sun was out and the wind relatively calm, we assumed that was all in the past. Apparently the flooding was so bad that a motor home and the boat it was towing got totally caught up in the sand and water, and was wedged so efficiently that they could not even be towed out until serious excavation happened. That was not going to happen until Saturday.

Moving the dunes off of the road. Credit: Cindy Miller.

Saturday at 6 PM we got the call that the road was open and we could head home. We repacked the car, re-experienced Cheyanne’s baleful look, and set out, with an ETA of 3 AM at the earliest. Alas the high tide came in, water breached the dunes, and a very kind police officer knocked on our window, imploring us to return to Avon and wait for a better day. Cheyanne gave him a baleful look, but we obeyed.

Reconstituted dunes. Notice the tire tracks left by earth-moving machines. Credit: Cindy Miller.

The next morning we set out again; by now we could pack a car in just a few minutes. Our peanut butter on toast dinner of the previous night had left us a bit peckish, so we stopped off for some pastries and cappuccinos. We headed north once again and this time we were able to pass through the Rodanthe flood, and several others along the way. The water level was high, but our car had good ground clearance and our escape was relatively uneventful, but done at sub-breakneck speed.

Riding away through Rodanthe’s rising tides. Credit: Cindy Miller.

Why was this happening? The weather was beautiful – no rain, no wind and sunny skies. It just doesn’t get any nicer than this at the Outer Banks. As it turns out, there were two provocateurs. First, there was subtropical storm Melissa several hundred miles to our east, passing harmlessly out to sea, but increasing sea levels. Second, there was almost a full moon, which also tends to increase sea levels. But that’s it!

That shouldn’t be enough. In past years those two events might cause waves to crash to the dunes with increased vigor, but would not cause them to breach the dunes and spill onto the roads. But those were past years, and now is the present, and sea levels along the North Carolina coast have risen by about one foot in the past 50 years. Here are some data from Wilmington, NC – about 150 miles south of Avon.

Rising sea levels measured at Wilmington, North Carolina. Credit: National Oceanic and Atmospheric Administration and SeaLevelRise.org

You should note two things. First, there is substantial year-to-year variation in sea levels. Second, rates of sea level rise are accelerating. Scientists at the National Oceanic and Atmospheric Administration and the US Army Corps of Engineers expect this trend to continue. Here is the prognosticated change in sea levels between now and 2050 at Oregon Inlet (just a few miles north of Rodanthe).

Forecast sea level change between 2016 and 2050. Credit: National Oceanic and Atmospheric Administration, U. S. Army Corps of Engineers and SeaLevelRise.org

This is very bad. I’ve been vacationing at the Outer Banks for about 25 years; it has become a part of who I am. I don’t want to give up on this spectacular part of the world, but we must act. We cannot continue sticking our heads in the sand (which we can now oftentimes find on NC12), pretending that climate change is a construct of the liberal press or elite intelligentsia.

The first step in dealing with a problem is acknowledging that it exists. Climate change is here, and its impact is increasing. An estimated 50 million climate change refugees around the globe are being forced to abandon their homes. More will follow, including our neighbors from North Carolina’s Outer Banks. For their sake, and ours, let’s acknowledge the problem, and focus our resources, energies and talents to reducing the damage in the short term, and dealing with the causes of climate change over the next decades and centuries.

Decomposition: it’s who you are and where you are

February 11, 2019 by fredsingerecology

“Follow the carbon” is a growing pastime of ecologists and environmental researchers worldwide. In the process of cellular respiration, organisms use carbon compounds to fuel their metabolic pathways, so having carbon around makes life possible. Within ecosystems, following the carbon is equivalent to following how energy flows among the producers, consumers, detritivores and decomposers. In soils, decomposers play a central role in energy flow, but we might not appreciate their importance because many decomposers are tiny, and decomposition is very slow. We are thrilled by a hawk subduing a rodent, but are less appreciative of a bacterium breaking down a lignin molecule, even though at their molecular heart, both processes are the same, in that complex carbon enters the organism and fuels cellular respiration. However. from a global perspective, cellular respiration produces carbon dioxide as a waste product, which if allowed to escape the ecosystem, will increase the pool of atmospheric carbon dioxide thereby increasing the rate of global warming. So following the carbon is an ecological imperative.

As the world warms, trees and shrubs are colonizing regions that previously were inaccessible to them. In northern Sweden, mountain birch forests (Betula pubescens) and birch shrubs (Betula nana) are advancing into the tundra, replacing the heath that is dominated by the crowberry, Empetrum nigrum. As he began his PhD studies, Thomas Parker became interested in the general question of how decomposition changes as trees and shrubs expand further north in the Arctic. On his first trip to a field site in northern Sweden he noticed that the areas of forest and shrubs produced a lot of leaf litter in autumn yet there was no significant accumulation of this litter the following year. He wondered how the litter decomposed, and how this process might change as birch overtook the crowberry.

One of the study sides in autumn: mountain birch forest (yellow) in the background, dwarf birch (red) on the left and crowberry on the right. Credit: Tom Parker.

Several factors can affect leaf litter decomposition in northern climes. First, depending on what they are made of, different species of leaves will decompose at different rates. Second, different types of microorganisms present will target different types of leaves with varying degrees of efficiency. Lastly, the abiotic environment may play a role; for example, due to shade and creation of discrete microenvironments, forests have deeper snowpack, keeping soils warmer in winter and potentially elevating decomposer cellular respiration rates. Working with several other researchers, Parker tested the following three hypotheses: (1) litter from the more productive vegetation types will decompose more quickly, (2) all types of litter decompose more quickly in forest and shrub environments, and (3) deep winter snow (in forest and shrub environments) increase litter decomposition compared to heath environments.

To test these hypotheses, Parker and his colleagues established 12 transects that transitioned from forest to shrub to heath. Along each transect, they set up three 2 m² plots – one each in the forest, shrub, and heath – 36 plots in all. In September of 2012, the researchers collected fresh leaf littler from mountain birch, shrub birch and crowberry, which they sorted, dried and placed into 7X7 cm. polyester mesh bags. They placed six litter bags of each species at each of the 36 plots, and then harvested these bags periodically over the next three years. Bags were securely attached to the ground so that small decomposers could get in, but the researchers had to choose a relatively small mesh diameter to make sure they successfully enclosed the tiny crowberry leaves. This restricted access to some of the larger decomposers.

Some litter bags attached to the soil surface at the beginning of the experiment. Credit: Tom Parker.

To test for the effect of snow depth, the researchers also set up snow fences on nearby heath sites. These fences accumulated blowing and drifting snow, creating a snowpack comparable to that in nearby forest and shrub plots.

Parker and his colleagues found that B. pubescens leaves decomposed most rapidly and E. nigrum leases decomposed most slowly. In addition, leaf litter decomposed fastest in the forest and most slowly in the heath. Lastly, snow depth did not influence decomposition rate.

(Left graph) Decomposition rates of E. nigrum, B. nana and B. pubescens in heath, shrub and forest. (Right graph) Decomposition rates of E. nigrum, B. nana and B. pubescens in heath under three different snow depths simulating snow accumulation at different vegetation types: Heath (control), + Snow (Shrub) and ++ Snow (Forest) . Error bars are 1 SE.

B. pubescens in forest and shrub lost the greatest amount (almost 50%) of mass over the three years of the study, while E. nigrum in heath lost the least (less than 30%). However, B. pubescens decomposed much more rapidly in the forest than in the shrub between days 365 and 641. The bottom graphs below show that snow fences had no significant effect on decomposition.

Percentage of litter mass remaining (a, d) E. nigrum, (b, e) B. nana, (c, f) B. pubescens in heath, shrub, or forest. Top graphs (a, b, c) are natural transects, while the bottom graphs (d, e, f) represent heath tundra under three different snow depths simulating snow accumulation at different vegetation types: Heath (control), + Snow (Shrub) and ++ Snow (Forest) . Error bars represent are 1SE. Shaded areas on the x-axis indicate the snow covered season in the first two years of the study.

Why do mountain birch leaves decompose so much more than do crowberry leaves? The researchers chemically analyzed both species and discovered that birch leaves had 1.7 times more carbohydrate than did crowberry, while crowberry had 4.9 times more lipids than did birch. Their chemical analysis showed much of birch’s rapid early decomposition was a result of rapid carbohydrate breakdown. In contrast, crowberry’s slow decomposition resulted from its high lipid content being relatively resistant to the actions of decomposers.

Researchers (Parker right, Subke left) harvesting soils and litter in the tundra. Credit: Jens-Arne Subke.

Parker and his colleagues did discover that decomposition was fastest in the forest independent of litter type. Forest soils are rich in brown-rot fungi, which are known to target the carbohydrates (primarily cellulose) that are so abundant in mountain birch leaves. The researchers propose that a history of high cellulose litter content has selected for a biochemical environment that efficiently breaks down cellulose-rich leaves. Once the brown-rot fungi and their allies have done much of the initial breakdown, another class of fungi (ectomycorrhizal fungi) kicks into action and metabolizes (and decomposes) the more complex organic molecules.

The result of all this decomposition in the forest, but not the heath, is that tundra heath stores much more organic compounds than does the adjacent forest (which loses stored organic compounds to decomposers). As forests continue their relentless march northward replacing the heath, it is very likely that they will introduce their efficient army of decomposers to the former heathlands. These decomposers will feast on the vast supply of stored organic carbon compounds, release large quantities of carbon dioxide into the atmosphere, which will further exacerbate global warming. This is one of several positive feedbacks loops expected to destabilize global climate systems in the coming years.

note: the paper that describes this research is from the journal Ecology. The reference is Parker, T. C., Sanderman, J., Holden, R. D., Blume‐Werry, G., Sjögersten, S., Large, D., Castro‐Díaz, M., Street, L. E., Subke, J. and Wookey, P. A. (2018), Exploring drivers of litter decomposition in a greening Arctic: results from a transplant experiment across a treeline. Ecology, 99: 2284-2294. doi:10.1002/ecy.2442. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Recovering soils suffer carbon loss

January 24, 2019 by fredsingerecology

When dinosaurs roamed the Earth, and I was in high school, acid rain became big news. Even my dad, who as an industrial chemist believed that industry seldom sinned, acknowledged that he could see how coal plants could release sulfur (and other) compounds, which would be converted to strong acids, borne by prevailing winds to distant destinations, and deposited by rain and snow into soils. Forest ecosystems in North America and Europe are happily, albeit slowly, recovering from the adverse effects of acid deposition, but there are some causes for concern. At the Hubbard Brook Experimental Forest in New Hampshire, USA, researchers experimentally remediated some of the impacts of acid deposition by adding calcium silicate to a watershed (via helicopter!). A decade later, this treatment had caused a 35% decline in the total carbon stored in the soil. This result was very unexpected and alarming, as this could mean that acid-impacted temperate forests may become major sources of CO₂, with more carbon running off into streams, and some becoming atmospheric CO_2, as the effects of acid rain wane. Richard Marinos and Emily Bernhardt wanted to determine exactly what caused this carbon loss to better understand how forests will behave in the future as they recover from acidification.

The forest at Hubbard Brook in the Autumn. Credit: Hubbard Brook Ecosystem Study at hubbardbrook.org

The problem is that calcium and acidity (lower pH is more acid: higher pH is more alkaline) have different and complex effects on plants, soil microorganisms and the soils in which they live. Several previous studies demonstrated that higher soil pH (becoming more alkaline) caused an increase in carbon solubility, while higher calcium levels caused carbon to become less soluble. Soluble organic carbon forms a tiny fraction of total soil carbon, but is very important because it can be used by microorganisms for cellular respiration, and also can be leached from ecosystems as runoff. In general, soil microorganisms benefit as acidic soils recover because heavy metal toxicity is reduced, enzymes work better, and mycorrhizal associations are more robust. Complicating the picture even more, both elevated calcium and increased pH have been associated with increased plant growth, but increased calcium is also associated with reduced fine root growth.

To help unravel this complexity, Marinos and Bernhardt experimentally tested the effects of increasing pH and increased calcium on soil organic carbon (SOC) solubility, microbial activity and plant growth. They collected acidic soil from Hubbard Brook Experimental Forest, which formed three distinct layers: leaf litter on top, organic horizon below the leaf litter, and mineral soil below the organic horizon.

Soil excavation site at Hubbard Brook. Credit: Richard Marinos.

The researchers then filled 100 2.5-liter pots with these three soil layers (in correct sequence) and planted 50 pots with sugar maple saplings, leaving 50 pots unplanted. Pots were moved to a greenhouse, and that November given one of five treatments: calcium chloride addition (Ca treatment), potassium hydroxide addition (alkalinity treatment), Ca + alkalinity treatment combined, a deionized water control, and a potassium chloride control. The potassium chloride control had no effect, so we won’t discuss it further.

Potted sugar maple saplings used for the experiments. Credit Richard Marinos.

The following July, Marinos and Bernhardt harvested all of the pots, carefully separating plant roots from the soil, and analyzing the organic horizon and mineral soil levels separately (there wasn’t enough leaf litter remaining for analysis). The researchers measured SOC by mixing soil from each pot with deionized water, centrifuging at high speed to extract the water-soluble material, combusting the material at high temperature and measuring how much CO₂ was generated. The result is termed water extractable organic carbon (WEOC).

Remember that previous studies had shown that higher calcium levels decreased carbon solubility, while higher alkalinity increased carbon solubility. Surprisingly, Marinos and Bernhardt found that in unplanted pots, the Ca treatment reduced WEOC in both soil layers, while the alkalinity treatment decreased WEOC in the organic horizon, but not in mineral soil. In pots planted with maple saplings, the Ca treatment had no effect on WEOC, while the alkalinity treatment, and the Ca + alkalinity treatment, increased WEOC markedly.

Water-Extractable Organic Carbon in soil without plants (left column) and with plants (right column). Top graphs are organic horizon soils and bottom graphs are mineral horizon soils. Error bars are 1 standard error.

The next question was how might soil microorganisms fit into the plant-soil dynamics?

Soil respirations rates (top) over the short term (days 1-7 post-harvest) and (bottom) the long term (days 8-75 post-harvest). Error bars are 1 standard error.

Soil microorganisms use carbon products for cellular respiration, so the researchers expected that soils with more SOC would have higher respiration rates. They measured soil respiration 1, 2, 4, 8, 16, 35 and 72 days after the harvest, so they could evaluate both short-term and long-term effects. In unplanted pots, soil respiration rates were unaffected by treatment. But in planted pots, the alkalinity treatment increased soil respiration rates considerably in the short term (top graphs), but much less so in the long-term (bottom graphs). Putting the WEOC data from the figure above together with the respiration data from the two figures to your left, you can see that in pots with plants, increased alkalinity was associated with more SOC and higher respiration rates.

The researchers weighed the saplings after harvest and discovered that the sugar maples grew best in soils treated with calcium. Two previous studies had treated fields with calcium silicate and found better sugar maple growth in the treated fields. Marinos and Bernhardt argue that their study provides evidence that it is the Ca enrichment, and not the increased pH, that caused increased growth for both of those studies.

Perhaps the most surprising finding is that higher alkalinity increased soil microbial activity only in pots with plants, and had no effect on soil microbial activity in pots without plants. Somehow, the plants in an alkaline environment are increasing the rate of microbial respiration, perhaps by releasing carbohydrates produced by photosynthesis into the soil, which could then stimulate decomposition of SOC by the microorganisms. Finding that this effect largely disappeared a few days after harvest (bottom graph above), supports the idea that the plants are releasing a substance that helps microorganisms carry on cellular respiration. But this idea awaits further study. In the meantime, we have a better understanding of how forest recovery from acid rain affects one aspect of the carbon cycle, though many other human inputs may interact with this recovery process.

note: the paper that describes this research is from the journal Ecology. The reference is Marinos, R. E. and Bernhardt, E. S. (2018), Soil carbon losses due to higher pH offset vegetation gains due to calcium enrichment in an acid mitigation experiment. Ecology, 99: 2363-2373. doi:10.1002/ecy.2478. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Climate changes a bird’s life in shrinking grasslands

September 21, 2018 by fredsingerecology

Back in graduate school, a couple of my grad student buddies and I would get together to fish for brown trout in the Kinnickinnic River in western Wisconsin. We were students at the University of Minnesota (Twin Cities), but the Kinni was the closest trout stream. Tired of catching small brown trout, we consulted a trout fishing map and discovered that the headwaters of the Kinni were rich in brook trout. So early one morning, map in hand, we followed strange paths and found our sacred brook trout haven. Alas, the only thing it was rich in was corn, now about two feet high – though there was a modest depression where trout waters once had flowed. Our personal depression was perhaps more than modest – having been robbed of brook trout, and the opportunity to experience some pristine waters flowing through a beautiful grassland.

Grasslands, one of the biomes native to parts of Wisconsin and Minnesota, are globally one of the most endangered biomes, because they usually are relatively easy to convert into farmland and suburban developments. Native grasslands harbor a wide biological diversity; consequently conservation biologists are concerned about their continued loss.

Cool-season grassland in southwest Wisconsin. Credit: John Dadisman.

Ben Zuckerberg, Christine Ribic and Lisa McCauley wanted to know how environmental factors influenced the nesting success of grassland birds, in particular, because as obligate ground nesters, they might be susceptible to changing weather conditions that will be affecting the climate in coming decades. A nest built on the ground is much less insulated from the environment than one built in or on a tree or even a ledge.

Seven day old bobolink chicks in a ground nest. Credit: Carolyn Byers.

Zuckerberg and his colleagues used Google Scholar and the ISI Web of Science to comb the literature (1982-2015) for studies that explored the nest success of obligate grassland birds in the United States. They identified 12 bird species from 81 individual studies of 21,000 nests. Based on their experience and the literature, both precipitation and temperature were likely to influence nest success, which is the proportion of nests that fledge at least one young. They considered three precipitation time periods: (1) Bioyear – previous July through April of the breeding season, (2) May of the breeding season, (3) June – August of the breeding season. They considered breeding season temperatures during May, and during the period from June-August. The researchers were also interested in the size of the grassland (grassland patch size), reasoning that a larger grassland might provide more diverse microclimates, so, for example, a bird might be able to find a dry microhabitat for nesting in a large grassland, even in a wet breeding season.

Map of the identity and location of species considered for this study.

The researchers discovered that both temperature and precipitation were important. Nest success increased steadily with bioyear precipitation (Figure (a) below). Presumably, more rain led to more plant growth and more insect survival, which would help feed the young. Taller plants could also help shade or hide the nests. In contrast, nest success declined sharply with precipitation during spring and summer of the breeding season (Figure (b) and (c)). Heavy rains during the breeding season can flood nests, and also decrease the foraging efficiency of parents who might need to spend more time incubating nests during rainstorms. Lastly, extreme (low or high) May temperatures depressed nest success, which was highest at intermediate temperatures (Figure (d)). Egg viability depends on maintaining a constant temperature, and the parents may be more challenged to thermoregulate at extreme temperatures. Temperatures later in the breeding season did not affect nest success.

Effects of (a) bioyear precipitation (previous July – April of the breeding season), (b) May precipitation during the breeding season, (c) June – August precipitation during the breeding season, and (d) May temperature on nest success. Shaded area represents 95% confidence interval.

But all is not straightforward in the grassland nest success world. These main findings about precipitation and temperature interacted with grassland size in interesting ways. For example high bioyear precipitation, which overall increased nest success, only did so for smaller grassland patches (dashed line in top graph below), but not for larger patches (solid line). Extreme May temperatures had different effects on nest success in relation to grassland patch size. Low May temperatures were associated with high nest success in small patches (dashed line in bottom graph) and with low nest success in large patches (solid line). High May temperatures were associated with high nest success in large patches, and with low nest success in small patches.

Predicted nest success of grassland birds in relation to bioyear precipitation (top graph) and May temperature (bottom graph) in relation to grassland patch size. Solid lines represent large grasslands, while dashed lines represent small grasslands. Shaded area is 95% confidence interval.

The researchers were surprised to discover that patch size affected how weather influenced grassland bird nesting success. Some of the patterns seem intuitively logical; for example, in unusually hot breeding seasons birds had higher nest success in larger grasslands than in smaller grasslands. Presumably, birds were more likely to find a cooler microclimate for their nests in a large grassland. However it is puzzling why in unusually cold breeding seasons birds had higher nest success in smaller grasslands. The researchers are planning a follow-up study to better document and measure the existence of microclimates in grasslands of different sizes, and explore how different microclimates influence the nesting success of vulnerable grassland birds. Finding that warmer temperatures and drought generally reduce nest success to the greatest extent in small grassland patches is strong incentive for conservation mangers to establish large core grasslands as a tool to maintain bird populations in the wake of present and future changes to the climate.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Zuckerberg, B. , Ribic, C. A. and McCauley, L. A. (2018), Effects of temperature and precipitation on grassland bird nesting success as mediated by patch size. Conservation Biology, 32: 872-882. doi:10.1111/cobi.13089. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2018 by the Society for Conservation Biology. All rights reserved.

Carbon dioxide’s complex personality

September 12, 2018 by fredsingerecology

Carbon dioxide (CO₂) deservedly gets a lot of bad press because it is responsible for much of the global warming Earth is currently experiencing. Less publicized, but perhaps equally important, CO₂ is acidifying oceans, thereby threatening the continued existence of some critical biomes such as coral reefs and kelp forests (acid interferes with the ability of many marine organisms to build their shells). But carbon dioxide also has a kinder, gentler side, as it is an essential resource for plants, and in some cases higher CO₂ levels can increase a plant’s ability to carry on photosynthesis. Sean Connell and his colleagues explored this complex personality by studying a marine ecosystem that experiences naturally varying levels of CO₂. High CO₂ levels and acidity exist near CO₂-emitting vents at the study site – a volcanic island (Te Puia o Whakaari) off the coast of New Zealand.

White_Island_James Shook [CC BY 2.5 (https-::creativecommons.org:licenses:by:2.5)], from Wikimedia Commons

The volcanic Te Puia o Whakaari off the coast of New Zealand’s north island. Credit: James Shook [CC BY 2.5 (https-//creativecommons.org/licenses/by/2.5)], from Wikimedia Commons.

The major players in this ecosystem are the kelp, Ecklonia radiata, several species of turf-forming algae, and two grazers, the snail, Eatoniella mortoni, and the urchin, Evechinus chloroticus. The typical vegetation in the region is a mosaic of kelp forest, some scattered small patches of algal turf, and sea urchin barrens – hard rock without significant vegetation, a result of overgrazing by sea urchins. In contrast, extensive algal mats carpeted the rocks near these vents, and the researchers hypothesized that high CO₂ levels caused this shift in dominant vegetation.

Sean Connell collects data in a habitat dominated by algal turf (and numerous fish). Credit: anonymous backpacker.

Connell and his colleagues chose two vents and two nearby control sites at a depth of 6-8 meters. The CO₂ levels and acidification near the vents were approximately equal to the amount projected for the end of the 21^stcentury, but there were no differences between vents and controls in temperature, salinity or nutrient concentrations. The researchers estimated photosynthetic rates for kelp and turf algae by measuring the rate of oxygen production. They also estimated snail consumption rates by caging them for 3 days and measuring how much algal turf they removed. They used an analogous approach to measure sea urchin consumption rates.

Conditions at vents had a major impact on both producers and consumers. Kelp production decreased slightly, while turf production increased sharply at vents (Figures A and B below). Urchin density declined (almost to nonexistence) while gastropod density increased markedly at vents (Figures C and D). Lastly, consumption rates (on a per individual basis) by urchins plummeted, while consumption rates by snails increased sharply at vents (Figures E and F).

Comparison of production and consumption at control sites vs. carbon dioxide emitting vents.

These patterns converted the normal mosaic of kelp forest, small algal turf patches and urchin barren into turf-dominated habitats. Algal turf increased in size and frequency near the vents, while kelp forest shrank into near oblivion.

Frequency of patches of turf (light gray bars), urchin barren (medium gray) and kelp (black) in relation to patch size (diameter in meters) at control sites (top graph) and sites near vents (bottom graph).

These results can be pictured visually by the graph below. Under conditions of present-day pH and CO₂ levels, gross algal production is relatively low and urchin consumption is relatively high, which results in negligible net algal turf production (net production = gross production – urchin and gastropod consumption). High CO₂ levels sharply increase gross algal turf production while dramatically decreasing consumption by urchins. Even though gastropod consumption increases slightly at vents, the overall effect on vents is a dramatic increase of net algal turf production. Consequently, the ecosystem experiences regime shift from kelp to algal turf domination.

Summary of effects of CO₂ release by vents (bottom) vs Controls (top). Net algal production (red circle) = Gross algal production – urchin and gastropod consumption. Net algal production in dark green zone is predicted to be turf-dominated (as is found near vents), light green is a mosaic, while white zone represents urchin barrens (low production and high consumption). Error bars are 1 standard error.

Under current conditions, kelp is the dominant producer over turf algae in the near offshore ecosystem. High consumption by urchins keep the turf algae in check. But near CO₂ emitting vents, high levels of carbon dioxide have a dual effect on this ecosystem, disproportionately increasing turf algae production rate and decreasing urchin abundance and consumption rate. This allows the competitively subordinate turf algae to replace the competitively dominant kelp, resulting in a dramatically changed ecosystem. This occurs in the absence of an increase in ocean temperature. Given that ocean temperature will increase sharply by 2100 (along with CO₂ levels), many species interactions are expected to change in the next century, and ecosystem structure and functioning will be very different from what we observe today.

note: the paper that describes this research is from the journal Ecology. The reference is Connell, S. D., Doubleday, Z. A., Foster, N. R., Hamlyn, S. B., Harley, C. D., Helmuth, B. , Kelaher, B. P., Nagelkerken, I. , Rodgers, K. L., Sarà, G. and Russell, B. D. (2018), The duality of ocean acidification as a resource and a stressor. Ecology, 99: 1005-1010. doi:10.1002/ecy.2209 Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Fred's Ecology and Environmental Tales (AKA: The FEET)

How new research expands our understanding of the natural world

Category Archives: Global processes/Climate change