Invading hippos

August 9, 2020 by fredsingerecology

Jonathan Shurin was studying declining water quality in Lago de Tota, Colombia’s largest lake, when he learned about a local invasion of the common hippopotamus, Hippopotamus amphibius. Four hippos were imported to Colombia by the notorious drug lord Pablo Escobar to populate his private zoo. Following Escobar’s shooting death in 1993, the zoo fell into disrepair and the hippos wandered off free. The population now numbers between 65-80, and breeding individuals have been seen 150 km from the zoo.

Hippos wallow in a lake framed by cattle egrits. Credit J. Shurin

Common hippos are native to central and southern Africa; as their scientific name implies they divide their existence between land (mostly at night) and water (keeping cool during the day). These are huge animals, weighing up to 1500 kg and capable of running a surprising 30 kg/hr. Apparently it is very easy to annoy a hippo. From an ecosystem standpoint, hippos in their native Africa have been shown to have a strong impact on ecosystems by grazing on land at night and then releasing processed nutrients into lakes during the day. Their influence is greatest during the dry season when they’re concentrated at high densities. Jonathan Shurin and his colleagues wanted to know whether hippos were having a discernable effect on lakes and rivers in Colombia. Given an expectation that the hippo population will continue to grow, this question has important management implications.

A grazing hippo. Credit: J. Shurin

The researchers sampled 14 small lakes at Hacienda Napoles in Antioquia, Columbia during the wet season and the dry season. All lakes were sampled from shore because entering a lake containing hippos can be hazardous to a researcher’s health. peligrohippo

Two lakes were found to contain hippos, while the other 12 did not (though some had been observed with hippos on other occasions). The analysis compared the two lakes with hippos to the 12 lakes without hippos for nutrients, conductivity, pH, temperature and chlorohyll-a concentration (a measure of photosynthetic activity). The researchers sampled for phytoplankton, zooplankton and used dip nets to sample macroinvertebrates. They found few differences in most categories except for the composition of the phytoplankton community. As you can see below, lakes with hippos had considerably more cyanophytes (photosynthetic bacteria often associated with harmful algal blooms), and fewer chlorophytes and charophytes (types of green algae) than did lakes without hippos.

Mean relative density of different divisions of phytoplankton in the two lakes with hippos (left bar) and the 12 lakes without hippos (right bar).

Shurin and his colleagues also estimated net production of each lake by systematically measuring dissolved oxygen concentration throughout the day. Photosynthetic organisms in highly productive lakes should take up lots of carbon dioxide during the day, and release considerable oxygen into the water. Thus the difference in oxygen levels during the day (when photosynthesis occurs) vs. night (when there is no photosynthetic activity) would be greatest in highly productive lakes. The researchers discovered from multiple samples that the two lakes with hippos had an average range of 3.6 mg/L in dissolved oxygen levels which was significantly greater than the average range of 2.1 mg/L measured in three of the lakes without hippos (it was not feasible to measure all of the no hippo lakes). Presumably, this difference occurs from high photosynthetic rates during the day in the lakes with hippos.

Time series of dissolved oxygen in the sampled lakes. Notice how dissolved oxygen levels peak in the late afternoon (hour 12 = noon), but decline overnight without input from photosynthesis.

In addition to comparing the quantity of nutrients, Shurin and his colleagues wanted to know the source of the nutrients. Stable isotopes are forms of elements (in this case carbon and nitrogen) that differ in number of neutrons. They are called stable, because they don’t undergo radioactive decay. Stable isotope analysis measures the ratio of rare isotopes of a particular element in comparison to the more common isotope (for example ¹³C compared to ¹²C). Relevant to the hippo study, plants growing on land tend to have a higher (less negative for carbon, more positive for nitrogen) stable isotope ratio of carbon (delta¹³C) and nitrogen (delta¹⁵N) than do plants growing in water. So if hippos were bringing nutrients into the lakes, the researchers expected the two hippo lakes to have higher stable isotope ratios of carbon and nitrogen.

As you can see from the graph below, on average, the two hippo lakes had higher stable isotope ratios of carbon, but not of nitrogen. This indicates that hippos are importing carbon into the lake – presumably eating ¹³C rich plants during the evening, and then pooping out the remains when they return to the water. However there is no evidence that hippos are importing nitrogen into the lakes.

Stable C and N isotopic ratios for samples collected from lakes with (green) and without (orange) hippo populations. Solid circles are the mean values of multiple samples collected at different times from the same lake, and open circles are the individual observations from each sample.

Shurin and his colleagues acknowledge the difficulty of drawing conclusions on ecosystem impact based on only two lakes with hippos. On the other hand, finding significant differences with such a small sample is noteworthy, particularly when considering that the hippo invasion may be in its early stages. If we extrapolate, from four hippos in 1993 to the lower estimate of 65 hippos at the time of the study, and assume exponential growth, we should find 785 hippos by 2040 and over 7000 hippos by 2060. There are several assumptions with this extrapolation, but if unchecked the hippo population could expand dramatically, impacting ecosystem functioning in many different ways.

Observed (solid circles) and projected (open circles) growth of the hippo population in Antioquia, Columbia, assuming exponential growth.

But should we worry about this? After all, hippos are amazingly cool, and tourists have begun visiting Hacienda Napoles specifically to see the hippos. This is an example of a social-ecological mismatch, where the societal value placed on a species may oppose potential negative environmental impact. Conservation ecologists will need to work with the local community to devise a plan that serves the best interests of the ecosystem, and the humans who live there.

note: the paper that describes this research is from the journal Ecology. The reference is Shurin, J. B., Aranguren-Riaño, N., Duque Negro, D., Echeverri Lopez, D., Jones, N. T., Laverde‐R, O., Neu, A., and Pedroza Ramos, A. 2020. Ecosystem effects of the world’s largest invasive animal. Ecology 101(5):e02991. 10.1002/ecy.2991. 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.

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.

Stress frequency structures communities

May 21, 2020 by fredsingerecology

COVID-19 has amplified our experience of stress, but even in a COVID-free world, we share with most other organisms a continuously stressful existence, highlighted by situations affecting our survival (e.g. getting food and not becoming someone else’s food) and our reproductive success. Today we will discuss organisms that live in a very stressful environment – the subtidal zone off of the Galapagos islands – located just below the line demarcating the furthest extent of low tide. One serious stress for subtidal organisms is coping with dramatically fluctuating ocean currents. The speedy surgeonfish uses its powerful pectoral fins and slender, disc-shaped body to minimize drag, permitting feeding in high flow conditions brought about by powerful ocean waves. In contrast, the broad-bodied torpedo-shaped parrotfish is unable to do so; for it, fast water is too much of a drag.

Yellowtail surgeonfish (Prionurus laticlavius) stand out as voracious herbivores that can feed even in the most wave-swept coastlines of the Galapagos Islands. Credit: Dr. Alejandro Perez-Matus.

Waters near the Galapagos Islands are enriched by upwelling equatorial currents, which provide nutrients to a diverse community of plankton and benthic (attached to the ocean bottom) algae. These in turn support a high diversity of macroinvertebrates and herbivorous fish that feed on them, including the pencil urchin, Eucidaris galapagensis, a voracious feeder on algae, barnacles and coral. This species wedges itself among rocks and crevices during the day, and emerges to feed at night. It attaches itself (and moves very slowly) using its tube feet. Robert Lamb, Franz Smith and Jon Witman hypothesized that given the weak attachment strength of the pencil urchin’s tube feet, it might only be an effective feeder in locations where wave action was minimal.

Robert Lamb bolts experimental cages to the rock as Eucidaris urchins stand guard at the sheltered side of Caamaño. Credit: Salome Buglass.

To explore how wave action might affect the subtidal community, the researchers set up two research locations at Caamaño and Las Palmas – both off the Galapagos Island of Santa Cruz.

Effect of wave action (exposed – dark bar, sheltered – light bar) on abundance of some of the important members of the subtidal community off of the island of Santa Cruz.

At each location, they chose an exposed site with strong wave action and a sheltered site that had much reduced wave action. Mean flow speed was more than twice as fast at exposed sites than in sheltered sites. As you can see in the figure to your left, site differences in mean flow speed corresponded to differences in the subtidal community. Crustose coralline algae (red algae firmly attached to corals) were more common in sheltered sites (Figure A), while a variety of red and green macroalgae were more common at exposed sites (Figure B). Surgeonfish (Figure C) and parrotfish (Figure D) were much more abundant in exposed areas, while pencil urchins were much more abundant in sheltered sites (Figure E).

Lamb and his colleagues wanted to know why these differences exist. They set up a series of exclosures within each of these sites using wire mesh cages to either allow fish, but not urchins (+ fish treatment), allow urchins but not fish (+ urchins), or exclude both groups of herbivores (- all). They also had a control treatment that allowed all herbivores (+ all).

LambTreatments

In one experiment the researchers created sandwiches made up of the delectable green algae Ulva. For five days, they ran six replicates of each treatment at exposed and sheltered sites at Caamaño and Las Palmas. Lamb and his colleagues then harvested the sandwiches, weighed them, and calculated the percent remaining of each sandwich.

An Ulva sandwich

At exposed locations, urchins (without fish) consumed very little Ulva, while fish (without urchins) consumed about 2/3 of the Ulva (when compared to the –all controls). In contrast, at sheltered locations, urchins took some mighty significant bites from the Ulva sandwiches, while fish also ate substantial Ulva at Caamaño, but not at Las Palmas.

Percent of Ulva biomass remaining after five days of the Ulva sandwich experiment. Error bars are 1 SE.

In a related experiment, the researchers used the same cages to explore how macroalgal communities assemble themselves in the presence or absence of urchin and fish herbiores under different flow rates. If this was not enough to consider, they also ran these experiments both during the cool season, when nutrient-rich ocean currents lead to high production, and during the warm season when production is usually lower. Lamb and his colleagues bolted two 13 X 13 cm polycarbonate plates to the bottom of each cage, and after two months measured the abundance and type of algae that colonized each plate.

Several trends emerge. First, macroalgae colonized much more effectively during the cool season. Second, urchins profoundly reduced macroalgal colonization at sheltered sites, but had little effect at exposed sites. In contrast, fish herbivory reduced macroalgal colonization at exposed sites at Caamaño but not Las Palmas, during the warm and cool season.

Effect of herbivores on macroalgal community assembly, as measured by amount of algae colonizing the polycarbonate plates after two weeks.

In addition, the researchers set up video cameras and were able to document herbivory by 17 fish species, with drastically higher herbivory rates at exposed sites.

Lamb and his colleagues conclude that the dominant herbivores switched between urchins in low flow sites and fish in exposed sites. Fish can leave the resource patch when stress (flow rate) is unusually high, and return when flow rate drops, while the slow-moving pencil urchins do not have that option. The researchers argue that in many ecosystems, consumer mobility in relation to the frequency of environmental stress can predict how consumers influence community structure and assembly. They point out that the coupling of mobility effects with environmental stress is common throughout the natural world. As examples, many shorebirds feed on marine organisms that become available during low tides, or also between crashing waves. Large mammals in Africa can migrate long distances to escape drought-stricken areas, while smaller animals cannot undertake such long journeys. In locally acidic regions of the Mediterranean Sea, many fish species can enter, feed and leave before experiencing toxic effects from the acid water, while slow-moving urchins are excluded from feeding in those habitats. Thus, while extreme environmental stress often decreases consumer activity, there are also times when it doesn’t. In these cases, we need to understand how particular species will behave and perform in the stressful environment to predict how stress influences community structure and functioning.

note: the paper that describes this research is from the journal Ecology. The reference is Lamb, R. W., Smith, F., and Witman, J. D.. 2020. Consumer mobility predicts impacts of herbivory across an environmental stress gradient. Ecology 101( 1):e02910. 10.1002/ecy.2910. 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.

Turkey mullein trichomes gobble up protective pollen

February 21, 2020 by fredsingerecology

I’m always amazed at how brilliant plants can be. For example Billy Krimmel and Ian Pearse showed in 2013 that many plant species exude sticky substances that entrap small arthropods, thereby attracting predators, which then rid these plants of many herbivores that might otherwise consume their leaves or reproductive structures. Jennifer Van Wyk joined this research group (which included Laure Crova) in graduate school. They were hunting for predatory hemipterans (true bugs) for a different experiment, which involved looking for them on turkey mullein (Croton setiger). They found plenty of predators, but almost no prey. This was puzzling; what were these predators eating? Intrigued, the researchers swabbed the turkey mullein leaves for pollen and found relatively vast quantities of pollen trapped in the trichomes (hairlike protuberances) of the leaves. Much of the pollen was from other species, and the researchers suspected that the trichomes were removing pollen from pollinators (primarily bees) that came to visit. Presumably the predators, which included spiders, hemipterans and ants, were attracted to this highly nutritious pollen.

Turkey mullein with predaceous hemipteran on lower right leaf. Credit: Billy Krimmel – http://www.miridae.com/our-team

Van Wyk and her colleagues wondered whether pollen capture benefitted turkey mullein. If turkey mullein used pollen to attract predators, and predators ate herbivores, pollen extraction by trichomes would be an adaptation that formed part of turkey mullein’s defense strategy. If this is true, supplementing turkey mullein with additional pollen should increase visitation by predators, and decrease herbivore abundance. With fewer herbivores, the researchers predicted less leaf damage.

Turkey mullein with herbivore-damaged leaves

Supplementing turkey mullein with additional pollen presents its own set of problems – most importantly coming up with enough pollen – in particular pollen that predators want to eat (Van Wyk and her colleagues collected a pound of oak pollen, only to find that predacious bugs were not interested in it). The researchers grew sunflowers in greenhouses, secured squash pollen from friends’ gardens and used tuning forks to vibrate pollen from tarweed flowers.

The researchers then set up experiments using 60 turkey mullein plants from one population in 2013 and 80 plants from another population in 2014. Nearby plants were paired up, with one member of the pair receiving 150 mg of supplemental pollen each week from mid-August to mid-September. They surveyed all arthropods visible to the naked eye, and categorized each species as predator or herbivore based on its primary diet (many of the arthropods were actually omnivorous).

In accordance with expectations, predator abundance was substantially greater in the supplemented populations in both years of the study. Spiders showed the most consistent increase, while Orius (the minute pirate bug) increased significantly in the 2014 population. The 2014 population had fewer arthropods of all species, possibly because it was immediately adjacent to an agricultural field.

Mean predator abundance per plant in 2013 (top) and 2014 (bottom). Geocoris is a Genus of big-eyed bugs, while Orius is the minute pirate bug. ** p < 0.01, † p < 0.1. Error bars are 1 standard error.

The results are less clear-cut with herbivore abundance. Fleahoppers were 18% less abundant on supplemented plants in 2014, and slightly (not significantly) less abundant in supplemented plants in 2013. Plants with a greater number of spiders had fewer fleahoppers, suggesting that spiders were eating them (or scaring them away). The researchers were unable to measure the abundance of an important herbivore, the grey hairstreak caterpillar, which forages primarily at night, and retreats into the soil during the heat of the day.

Mean number of fleahoppers per plant in 2013 (left graph) and 2014 (right graph). Blue bars indicated plants with supplemented pollen. * p < 0.05.

Lastly, supplemented plants suffered much less leaf damage than did unsupplemented plants.

Mean number of damaged leaves per plant in 2013 (left graph) and 2014 (right graph). Blue bars indicated plants with supplemented pollen. ** p < 0.01.

Taken together, these experiments indicate that turkey mullein uses its trichomes to capture pollen and attract a diverse army of predators, which reduce herbivore abundance and reduce damage to the plant. It is possible that pollen supplementation could be used on a larger scale to reduce herbivore loads on agricultural crops. More generally, it will be interesting to see whether other plants with sticky trichomes, such as the marijuana plant Cannabis sativa, also use their trichomes to attract predators and reduce herbivore abundance.

note: the paper that describes this research is from the journal Ecology. The reference is Van Wyk, J. I., Krimmel, B. A., Crova, L., and Pearse, I. S.. 2019. Plants trap pollen to feed predatory arthropods as an indirect resistance against herbivory. Ecology 100( 11):e02867. 10.1002/ecy.2867. 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.

Birds and plants team up and trade off

February 7, 2020 by fredsingerecology

For many years, ecologists have been puzzling over the question of why the world is so green. Given the abundance of herbivores in the world, it seems, on the surface, that plants don’t stand a chance. The famous naturalist/ecologist Aldo Leopold was one of the first scientists to emphasize the role of predators, which provide service for plants by eating herbivores (his example was wolves eating deer, ultimately preserving the plant community growing on a hillside). As it turns out there are many different predator species providing these services. Colleen Nell began her PhD program with Kailen Mooney with a keen interest on how insectivorous birds locate their prey, and how this could affect the plants that are being attacked by herbivorous insects.

A Common Yellowthroat perches on Encelia californica. Credit: Sandrine Biziaux.

Plants are not as poorly defended as you might expect (having sat on a prickly pear cactus I can painfully attest to that). In addition to thorns and other discouraging structures, many plants are armed with a variety of toxins that protect them against herbivores. Thorns and toxins are examples of direct defenses. But many plants use indirect defenses that involve attracting a predator to the site of attack. Some plants emit volatile compounds that predators are attuned to; these compounds tell the predator that there is a yummy herbivore nearby. Nell and Mooney recognized that plant morphology (shape and form) could also act as an indirect defense, making herbivorous insects more accessible to bird predators. They also recognized that we might expect a tradeoff between how much a plant invests in different types of defense. For example, a plant that produces nasty thorns might not invest so much in a morphology attractive to predaceous birds.

California Coastal Cactus Wren eating an orthopteran insect on a prickly pear cactus. Credit: Sandrine Biziaux.

What is a plant morphology that attracts birds? The researchers hypothesized that birds might be attracted to a plant with simple branching patterns, so they could easily land on any branch that might be hosting a herbivorous insect (Encelia californica (first photo) has a simple or open branching pattern). In contrast, birds might have a more difficult time foraging on insects that feed on structurally complex plants that host herbivorous insects which might be difficult to reach.

Isocoma menziesii, a structurally complex plant. Credit: Colleen Nell.

The researchers chose nine common plant species from the coastal sage scrub ecosystem – a shrub-dominated ecosystem along the southern California coast. For each plant species they measured both its direct resistance and indirect resistance to herbivores. Plants of each species were raised until they were four years old. Then, for three months during bird breeding season, bird-protective mesh was placed over eight plants of each species, leaving five or six plants as unprotected controls.

Kailen Mooney and Daniel Sheng lower bird-protective mesh over a plant. Credit: Colleen Nell.

After three months, the researchers vacuumed all of the arthropods from the plants, measured each arthropod, and classified it to Order or Family to evaluate whether the arthropod was herbaceous.

Colleen Nell vacuums the arthropods from Artemisia californica. Credit: Colleen Nell.

Nell and Mooney evaluated the herbivore resistance of each plant species by measuring herbivore density in the bird-exclusion plants. Relatively few herbivorous arthropods in plants that were protected from birds would indicate that these plants had strong direct defenses against herbivores. The researchers also evaluated indirect defenses as the ratio of herbivore density on bird exclusion plants in comparison to controls (technically the ln[exclusion density/control density]). A density of herbivores on plants protected from birds that is much greater than the density of herbivores on plants that allowed birds would indicate that birds are eating many herbivores. Finally, Nell and Mooney estimated plant complexity by counting the number of times a branch intersected an axis placed through the center of the plant at three different angles. More intersecting branches indicated a more complex plant.

The researchers expected a tradeoff between direct and indirect defenses. As predicted, as herbivore resistance (direct defense) increased, indirect defenses from birds decreased among the nine plant species.

Tradeoff between direct herbivore resistance and indirect defense by predaceous birds, for nine common plant species in the coastal sage scrub ecosystem.

The researchers also expected that more structurally complex plants would be less accessible to birds because complex branching would interfere with bird perching and foraging. Thus Nell and Mooney predicted that structurally more complex plants would have weaker indirect defenses from birds, which is precisely what they discovered.

Indirect defenses (from birds) in relation to plant structural complexity .

Given that structurally complex plants received little benefit from birds, you might expect that they had greater direct defenses in the form of herbivore resistance. Once again the data support this prediction.

Direct defenses (herbivore resistance) in relation to plant structural complexity.

Initially, Nell was uncertain about whether increased plant complexity would deter insectivorous birds. She points out that the top predators in this ecosystem are birds of prey that circle overhead in search of vulnerable birds to eat. Structurally complex plants might provide refuge for insectivorous birds, which could result in them spending more time foraging in complex plants. But the research showed the opposite trend. Plant complexity reduced the foraging efficiency of these small insectivorous birds, who prefer foraging on plants with relatively simple structure, which are easier to access and tend to host more prey.

note: the paper that describes this research is from the journal Ecology. The reference is Nell, C. S., and Mooney, K. A.. 2019. Plant structural complexity mediates trade‐off in direct and indirect plant defense by birds. Ecology 100( 10):e02853. 10.1002/ecy.2853. 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.

Tadpoles shun trout across time

January 27, 2020 by fredsingerecology

At a young school child (so long ago I can’t recall exactly when) I was exposed to Ernst Haeckel’s dictum that “ontogeny recapitulates phylogeny.” More interested in language than biology at the time, I thought “cool – three words that I’m clueless about.” Though biological thinking about ontogeny – the processes of growth and development – has changed since Haeckel’s time, interest has, if anything, grown more intense across disciplines. Tiffany Garcia has explored her lifelong fascination with ontogeny by focusing her research on amphibians, which are famous for their distinct stages of development, each with unique habitats and ecological requirements. Working with eggs and tadpoles of the Pacific chorus frog (Pseudacris regilla), Garcia and her colleagues investigated whether stress associated with the presence of predators during one developmental stage (for example an egg) would carry over to influence behavior or development of subsequent stages.

The Pacific chorus frog (Pseudacris regilla). Credit Brett Hanshew.

A tadpole’s anti-predator strategy can be influenced by other factors besides carry-over from earlier developmental stages. For example, we might expect that tadpoles whose ancestors lived in association with predators for many generations might have evolved a different anti-predator strategy than did tadpoles whose ancestors lived in a less threatening environment (this would be an adaptive effect). Tadpoles may also show very short-term changes in behavior or development (this is termed plasticity) if exposed to a cue that indicated a possible predation threat.

Collecting newly laid eggs at Three Creeks Lake. Credit: Lindsey Thurman

These three processes operate over very different time scales (long term – adaptive; intermediate term – carry-over; short term – plastic). Garcia and her colleagues designed an experiment to explore how these processes might interact to influence a tadpole’s anti-predator strategy. To investigate long term adaptive effects, the researchers collected newly laid (fertilized) eggs from lakes with and without rainbow trout (Oncorhynchus mykiss). They investigated carry-over effects by conditioning these eggs with four different environments during development: (1) trout odor, (2) cues from injured tadpoles (alarm cues), (3) trout odor paired with alarm cues, and (4) a water control (no odors nor cues). The researchers created alarm cues by grinding up four juvenile tadpoles in 150 ml of water, and trout odor by housing 30 juvenile rainbow trout in a 200 L tank filled with well water. They then conducted behavioral and developmental assays on tadpoles to see how adaptive, carry-over and plastic effects influenced tadpole growth, development and behavior.

Overview of the experimental design.

Garcia and her colleagues discovered that early exposure to trout odor had very little effect on growth and development, with body size and stage of development equivalent to that of controls. In contrast exposure of eggs to tadpole alarm cues or to alarm cues + trout odor resulted in smaller, less developed fish (see table below). In addition there was no effect of evolutionary history – eggs from lakes with and without trout showed similar patterns of growth and development.

Tadpole size and development in response to the four conditioning treatments. Higher Gosner stage numbers indicated more developed tadpoles. A tadpole hatches at Gosner stage 21 and begins metamorphosis at Gosner stage 42.

The next question is how do tadpoles respond behaviorally from exposure to different environments over the long, intermediate and short time scale? To test tadpole anti-predator behavior, the researchers placed an individual tadpole into a tub that had a 6 X 8 cm piece of corrugated black plastic, which the tadpole could use as a refuge. The researchers added to each tub one of the following: water (as a control (C)), tadpole alarm cues (AC), trout odor (TO), or alarm cues + trout odor (AC+TO). After an acclimation period, a researcher noted the position of the tadpole (under the refuge or out in the open) every 20 minutes over a 3-hour time period.

There were no effects of evolutionary history on refuge use. Tadpoles from lakes with and without trout showed similar patterns of refuge use. However, embryonic conditioning to alarm cues and trout odor had a large effect on refuge use. The left graph below shows the response of tadpoles from all four conditioning groups (C, AC, TO and TO+AC) to the addition of water. As you can see, tadpoles that hatched from eggs that were conditioned with AC+TO were most likely to use refuges, while tadpoles from AC only or TO only eggs were somewhat more likely to use refuges. The pattern repeats itself when tadpole alarm cues are added to the water (second graph from left). However when trout odor is added to the water, the responses are much more extreme, but follow the same pattern (third graph). Lastly, when confronted with alarm cues and trout odor, tadpoles increase refuge use dramatically, but again show the same pattern, with tadpoles from control eggs using refuges the least, and tadpoles from eggs conditioned with alarm cues and trout odor using refuges the most (right graph).

Refuge use by tadpoles in response to embryonic conditioning and experimental exposure. C = water control, AC = tadpole alarm cue, TO = trout odor, and AC+TO = tadpole alarm cue and trout odor. Blue bars are means and gray bars are 95% confidence intervals.

There are two processes going on here. First, over the short term, tadpoles are more responsive to the strongest cues, increasing refuge use when exposed to both tadpole alarm cues and trout odor. Second, over the intermediate term, there is solid evidence for carry over effects. Tadpoles that hatched from eggs conditioned with alarm cues and/or trout odor showed markedly increased refuge use than did tadpoles that hatched from control eggs.

These predator-induced responses impose a cost to the tadpoles. Tadpoles exposed to alarm cues and trout odor while still in the egg were smaller and less developed, and probably metamorphosed into smaller frogs. Many studies have shown that smaller frogs have reduced reproductive success. The researchers recommend further studies to explore these trade-offs between survivorship, growth rate, development rate and size at metamorphosis. These studies are particularly essential, because rainbow trout are a non-native predator to these lakes. Studies such as these allow conservation ecologists to understand the evolution and development of predator-prey interactions when novel species are introduced into an ecosystem.

note: the paper that describes this research is from the journal Ecology. The reference is Garcia, T. S., Bredeweg, E. M., Urbina, J., and Ferrari, M. C. O.. 2019. Evaluating adaptive, carry‐over, and plastic antipredator responses across a temporal gradient in Pacific chorus frogs. Ecology 100( 11):e02825. 10.1002/ecy.2825. 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.

Forest canopy fixes nitrogen shortage

December 5, 2019 by fredsingerecology

The two billion hectares of forest canopy remaining on our planet are ideal habitat for nitrogen fixing microorganisms that can convert N₂ to ammonia.

View of the forest canopy at the research site. Credit: D. Stanton.

The forest canopy tends to be nutrient-poor because there is no access to nutrients that accumulate in the soils on the forest floor, and rainfall can leach away any nutrients that do accumulate in the canopy from atmospheric deposition. So if you are a microbe, and you want to enjoy the view from the canopy, it is to your advantage to be able to fix atmospheric nitrogen so you can build essential molecules such as proteins and ATP.

As I mentioned in a previous post (Nitrogen continues to confound convention) both phosphorus (P) and molybdenum (Mo) are essential nutrients for biological nitrogen fixation. Daniel Stanton and his colleagues hypothesized that nitrogen fixation in the canopy might be limited by the availability of P and Mo, so they designed a series of experiments to explore the role of these nutrients at the San Lorenzo Canopy Crane in San Lorenzo National Park in the Republic of Panama. The crane provides about 1 ha of canopy access to non-acrophobic ecologists.

The crane at the research site: Credit: D. Stanton.

In one experiment, Stanton and his colleagues filled thin nylon stockings with vermiculite to form 40 cm long cylinders of 4 cm diameter. Each cylinder was then soaked in either pure water (control), a molybdenum (Mo) compound, a phosphorus (P) compound, or a combination of Mo and P, thus establishing four treatments. They attached each of these stockings to five different trees and allowed them to reside in the canopy for six months, to be colonized by microorganisms.

Nylon stockings treated with nutrients (or untreated controls) and affixed to branches in the canopy. Credit: D. Stanton.

The researchers measured the rate of nitrogen fixation by cutting a 50 cm² rectangle from the area of densest growth on each stocking, and incubating it (along with the colonizing microorganisms) in a closed bottle that they had inoculated with heavy nitrogen (¹⁵N). They then measured how much ¹⁵N the colonizers took up during a 12 hr incubation period.

Samples incubating for 12 hr to measure the rate of nitrogen fixation. Credit: D. Stanton.

The most common colonizers were nitrogen fixing filamentous cyanobacteria. These cyanobacteria fixed nitrogen at a somewhat (but not statistically significant) higher rate with Mo addition and at a much higher rate with P addition, and even more so with Mo + P addition.

Nitrogen fixation rates for each experimental treatment. C = control. Note that the y-axis is logarithmic, so these differences in fixation rates are substantial. Non-overlapping lowercase letters above the bars indicate significant differences between the means.

Nitrogen fixation is complex and costly. Part of the complexity arises because nitrogenase, the enzyme that catalyzes the reaction, cannot tolerate oxygen. To deal with this problem, cyanobacteria have evolved heterocysts, which are specialized anaerobic cells where nitrogen fixation occurs. How does nutrient addition influence heterocyst abundance and function?

There are actually two aspects to this story. One finding is that Mo addition had no effect on heterocyst abundance, while P addition had a pronounced effect.

Heterocyst frequency for each experimental treatment.

A second aspect is that Mo addition had a pronounced effect on the efficiency of nitrogen fixation. For one analysis the researchers compared the nitrogen fixation rate per heterocyst for the phosphorus addition treatments either without or with Mo addition (in other words, they compared the P added treatment to the Mo + P treatment). Nitrogen fixation rates were much higher in the Mo + P treatments. So while Mo does not increase heterocyst abundance, it does dramatically increase heterocyst fixation efficiency.

Quantity of N fixed per heterocyst per day in relation to absence (left bar) or presence (right bar) of Mo. P was added for both treatments. Dark horizontal lines are the median values, quartile range is represented by top and bottom of each box, and the whiskers represent the range of values for each treatment.

Phosphorus acts by markedly increasing the overall cyanobacterial growth. It increases the amount of cyanobacteria that colonizes the canopy and also increases heterocyst density per filament. In contrast molybdenum’s effect is more nuanced as it increases the efficiency of the nitrogen fixation reaction without having any (obvious) effect on cyanobacterial structure.

How do these findings influence our understanding of tropical forests in the western hemisphere? It turns out that episodes of nutrient addition actually happen in nature, courtesy of vast plumes of nutrient-rich rock-derived dust that periodically blow over the Atlantic Ocean from the Sahara desert in western Africa. Preliminary estimates by Stanton and his colleagues indicate that nutrient enrichment from these dust plumes is sufficient to profoundly increase the rates of nitrogen fixation in tropical forests. This may require us to reconsider our understanding of how nitrogen cycles within and between ecosystems.

note: the paper that describes this research is from the journal Ecology. The reference is Stanton, D. E., S. A. Batterman, J. C. Von Fischer, and L. O. Hedin. 2019. Rapid nitrogen fixation by canopy microbiome in tropical forest determined by both phosphorus and molybdenum. Ecology 100(9):e02795. 10.1002/ecy.2795. 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.

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