Tag Archives: New Zealand

Hot invaders thwart endemic New Zealanders

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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.

GiejsztowtPark

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.

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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.

GiejsztowtHeather

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.

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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).

GiejsztowtFig1A

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.

GiejsztowtFig1B

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.

Carbon dioxide’s complex personality

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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.

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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 21stcentury, 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).

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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.

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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.

ConnellFig1

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.