Biodiversity: it’s who you are

It is a massive understatement that ecologists and conservation biologists are profoundly interested in how disturbance affects biological diversity. Humans are disturbing ecosystems by degrading or destroying habitat, by fragmenting habitat into pieces that are too small to sustain populations, by directly overexploiting species for consumption or other purposes, and by introducing non-native species (and there’s more!). Some biologists argue that disturbance has gotten so severe that we need to modify our worldview of ecosystems. They argue, for example, that intact grasslands are so rare that we should stop talking about them as an ecosystem (or biome), but rather should more realistically explore the ecology of different types of croplands, which are, in actuality, primarily disturbed grasslands.

Some types of ecosystems, such as rainforests, have survived human impact more than others, but all have been highly disturbed. So it is fitting that conservation ecologists devote their attentions to understanding how disturbance influences biological diversity. Working in Cameroon in 1998, John Lawton and his colleagues assessed species richness (number of different species) in relation to level of disturbance experienced by eight different animal groups: canopy beetles, flying beetles, butterflies, canopy ants, leaf-litter ants, nematodes, termites, and birds. They discovered that more intense disturbances were associated with a significant reduction in species richness for many of the groups.

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Tropical forest in Cameroon. Credit: Earwig via Wikimedia Commons

Nigel Stork worked with Lawton on the original study, and recently reanalyzed the data in the context of changes that have occurred in how conservation biologists view biological diversity. For example, many biologists now argue that conserving biological diversity requires understanding which species are affected by disturbance, rather than the number of species. In addition, not all disturbances have similar impacts on biological diversity. For example, logging with heavy equipment removes trees and compacts soil, while logging with lighter equipment does not compact soil, so the two treatments may have very different impacts. Finally, it may be more informative to group species according to ecosystem function rather than by taxonomic group.

StorkFig1

Locations of sampling plots within the Mbalmayo Forest Reserve, Cameroon.  The three blown-up sites had multiple plots with different levels of disturbance, as indicated by the key.

Stork and his colleagues only had data for six of the original eight taxonomic groups. They categorized intensity of disturbance based on how much tree biomass was removed, level of soil compaction, time since disturbance, and tree cover and diversity at time of sampling. This allowed the researchers to assign a disturbance index to each plot, with 0 indicating least disturbed and 1.0 indicating most disturbed. This analysis showed no significant relationship between disturbance and species richness in five of the six taxonomic groups, with only termites declining in richness in response to disturbance.

StorkFig3

Species richness in relation to intensity of disturbance for six taxonomic groups considered in the study.

Stork and his colleagues used a slightly different approach to assess the response of species composition (the identity of species that are actually present in the community) to disturbance. They compared each pair of surveyed plots in relation to how different they were in disturbance. Plots with very different levels of disturbance had disturbance dissimilarities close to 1.0, while plots with similar levels of disturbance had disturbance dissimilarities near 0. They then looked at community dissimilarity to explore changes in species composition. Plots with a community dissimilarity near 1.0 had very different species, while plots with a community dissimilarity near 0 had very similar species.

Here’s what they found. For five of six groups, disturbance dissimilarity was associated with significant (solid line) or borderline significant (dashed line) increases in community dissimilarity. So even though the number of species was not affected very much by disturbance (excepting termites), species composition was affected in all groups, with the exception of canopy ants. They conclude that a disturbed forest has very different types of species in it, but not necessarily fewer species.

StorkFig2

Community dissimilarity in relation to disturbance dissimilarity. For five taxonomic groups, plots that had the greatest differences in disturbance also had the greatest differences in species composition.

Lastly, this study shows that response to disturbance is related to the functional group – the role that each species plays within the community. For example, beetles showed a strong response to disturbance, but in reality the strong response was only true for the herbivorous beetle functional group. Beetles that ate fungi or were predators or scavengers showed relatively little change in species composition in relation to disturbance.

So what should conservation ecologists do with this information? Given the diversity and intensity of disturbance globally, we need to develop a better understanding of how species and communities respond to global change. Species composition may be a more sensitive indicator of disturbance than is species richness. Functional groups may be more helpful than taxonomic groups in identifying how disturbance influences how ecosystems actually work. Perhaps monitoring particular functional groups can give us insight into how unrelated groups with similar ecology might respond to a world that promises to experience increasing levels of disturbance.

note: I discuss two papers in this blog.  The original is from the journal Nature. The reference is Lawton, J.H., Bignell, D.E., Bolton, B., Bloemers, G.F., Eggleton, P., Hammond, P. M., Hodda, M., Holt, R.D., Larsen, T.B., Mawdsley, N.A., Stork, N.E., Srivastava, D.S., and Watt, A.D. 1998. Biodiversity inventories, indicator taxa and effects of habitat modification in tropical forest. Nature, 391: 72-76. The second paper that reanalyzes the original data is from the journal Conservation Biology. The reference is Stork, N.E., Srivastava, D.S., Eggleton, P., Hodda, M., Lawson, G., Leakey, R.R.B. and Watt, A.D., 2017. Consistency of effects of tropical‐forest disturbance on species composition and richness relative to use of indicator taxa. Conservation Biology 31 (4): 924-933. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

Languishing Leatherbacks

Leatherback turtles, Dermochelys coriacea, are the largest of all sea turtles, tipping the scales at up to 900 kg. Unlike other sea turtles, the leatherback lacks a carapace covered with scutes; instead its carapace is covered by thick leathery skin that is embedded with small bones forming seven ridges running along its back. This turtle has a wonderful set of anatomical and physiological adaptations, such as huge flippers and an efficient circulatory system, that make it a powerful swimmer and deep ocean diver. Males spend their entire lives at sea, while females usually return to their birthplace along sandy beaches to dig nests and lay eggs.

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Leatherback female on the beach at Las Baulas National Park. Credit: Karla Hernández.

Unfortunately, from the perspective of conserving awesome animals in our world, some populations of leatherbacks are declining rapidly, and many are now listed as critically endangered by the IUCN Red List. Pilar Santidrian Tomillo wanted to know why leatherback populations in the Eastern Pacific Ocean have declined so much in recent years. Working at Las Baulas National Park in northwestern Costa Rica since 1993, Tomillo and her colleagues have tagged 1927 nesting females so they could measure survival and return rates to the nesting shoreline. They discovered an alarming trend of sharp decline as described by the graph below.

TomilloFig1Tomillo and her colleagues knew that many leatherbacks were killed every year as a consequence of bycatch – capture by fishing nets or lines cast by fishermen who are targeting other species. But leatherback bycatch is very difficult to monitor accurately, as few fishermen keep accurate records of dead turtles, and turtles may die after being entangled and subsequently freed. The researchers also suspected that climate variability could influence leatherback population size. El Niño Southern Oscillation (ENSO) is a large-scale atmospheric system that affects global climate. In leatherback foraging areas, El Niño years are associated with high atmospheric pressure and warm sea temperatures, while La Niña years are associated with low atmospheric pressure and cool sea temperatures. Importantly, cool sea temperatures stimulate upwelling of nutrient-rich water to the surface, increasing production of phytoplankton, thereby increasing the abundance of  jellyfish and other favored leatherback food items. So the researchers hypothesized that the leatherbacks might do better in La Niña years than in El Niño years.

But what do they mean by doing better? There are two important factors influencing population growth: survival and reproduction. Either one could be affected by climate. By recapturing marked individuals, Tomillo and her colleagues were able to measure both survival and one important aspect of reproduction, which is how often females return to lay eggs. Reproduction is a very energetically demanding process for leatherback females, as they must migrate long distances (often thousands of kilometers) from their feeding grounds, and their eggs are large and plentiful, so females require a huge investment in resources to reproduce. Consequently, at Tomillo’s field site, only 4.5% of females reproduced in consecutive years, while the average interval between reproductive events was 3.65 years.

Let’s consider leatherback survival. As you can see from the data below, annual survival probability is very variable from year to year, ranging from about 30% in 2012 to near 100% in several years. Disturbingly, the long-term trend is downward, and the overall mean adult survival rate of 0.78 is very low in comparison to viable populations of sea turtles. If survival rates do not increase, the future is very bleak for this population.

Tomillo Fig4

Annual survival probability of adult females tagged at Las Baulas National Park. Vertical bars indicate 95% confidence intervals.

How does climate variation influence survival and reproduction? The Multivariate ENSO Index (MEI) measures ENSO strength, with positive numbers (X-axis on graphs below) indicating El Niño years (with poor food availability), and negative numbers indicating La Niña years (with good food availability). The researchers found no climate effect on survival (top graph below), but a high reproductive rate associated with La Niña events (bottom graph below).

TomilloFig5

The question remains, why is survival so low? Climate does not appear to affect survival, so that brings us back to human impact. Tomillo and her colleagues recommend reducing bycatch levels and implementing beach conservation measures to eradicate egg poaching. They also warn us that increases in global temperatures reduce egg hatching success, and pose a severe stress to this and other critically endangered leatherback populations throughout the world.

note: the paper that describes this research is from the journal Ecology. The reference is Santidrian Tomillo, P., N. J. Robinson, A. SanzAguilar, J. R. Spotila, F. V. Paladino, and G. Tavecchia. 2017. High and variable mortality of leatherback turtles reveal possible anthropogenic impacts.  Ecology 98: 2170–2179. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Meta-analysis measures multiple mycorrhizal benefits to plants

Plants and fungi sometimes live together in peace and harmony. Arbuscular mycorrhizal associations are associations between plant roots and fungi, in which the fungal hyphae (usually branched tubular structures) grow between root cells, penetrating some cells with a network of branches or arbuscules.  Oftentimes these are mutualistic associations with both the plants and the fungi benefiting from living together. Though plants with arbuscular mycorrhizal fungi (AMF) tend to grow better than plants without AMF, it not always clear what causes them to do so.

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Kura clover, Trifolium ambiguum, grown with AMF (left) and without AMF (right). Credit: Liz Koziol.

Ecologists have traditionally viewed arbuscular mycorrhizal associations as a straightforward nutrient-carbon exchange. Fungal hyphae, with their vast surface area, pick up nutrients (such as nitrogen and phosphorus compounds) from the soil, which they deliver to the root cells in exchange for plant-produced carbon molecules.

But recently researchers have identified numerous other potential ways that the fungi help the plants, including the following: (1) promoting water uptake and transport, (2) helping to spread allelochemicals – toxic chemicals that some plants release to rid themselves of nearby competitors, (3) inducing chemical defenses against herbivores, (4) enhancing disease resistance, and (5) promoting soil aggregation or clumping, which stabilizes the soil near the roots, reduces erosion and promotes stable water flow.

Ecology Fig1

Camille Delavaux and her colleagues wondered whether these other plant benefits might actually be more important than we originally thought. Delavaux was planning to write a review paper for a 1 credit independent study, but she found so many papers on this topic that she decided to collaborate with fellow students Lauren Smith-Ramesh and Sara Kuebbing on a full-scale meta-analysis.

A meta-analysis is a systematic analysis of data collected by many other researchers. Delavaux and her colleagues used the Web of Science database to find 4410 studies on how AMF supplied plants with nutrients and 1239 studies on how AMF provided other plant benefits. That’s a lot of studies! But for the meta-analysis, the authors only used a small fraction of these studies because they set certain restrictions. For example, to be used in the meta-analysis the authors required each study to show some measure of variation for the data (such as standard deviation or standard error). In addition, the authors required each study to compare plants grown under two conditions: with AMF and without AMF.  In many studies the researchers collected soil, which they sterilized in a hot oven, and then set up a test group, which they inoculated with AMF spores or a plug of soil or root fragments that contained AMF. In addition, these studies also had a control group of plants that received only sterilized soil with no AMF added.

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A collection of eight different species of AMF spores. Credit: Liz Koziol.

Delavaux and her colleagues compared how plants performed with and without an AMF. Because each study was different, one might only have been looking at the effects of AMF on nitrogen uptake performance, while a second study might consider how AMF influenced soil aggregation. Effect size (Hedges d+) compares mean performance of the AMF plant to mean performance of non-AMF plants for a particular variable (such as nitrogen uptake or soil aggregation). A positive effect size means that the AMF plant did better. Of course we need to know how much better is biologically meaningful, so for each variable the researchers calculated the 95% confidence intervals of the mean effect size. If the 95% confidence intervals were positive, then Delavaux and her colleagues could be 95% confident that there was a biologically important effect of AMF on plants for that particular measure of performance.

As expected, the researchers found a positive effect of AMF on plant nitrogen uptake. The mean effect size was 0.674 with a 95% confidence interval of 0.451- 0.912. We can interpret this to mean that we are 95% confident that the true mean effect size on nitrogen uptake is between 0.451 and 0.912. But the greatest effect of AMF on plants was on soil aggregation (mean effect size = 1.645, 95% confidence interval = 1.032 – 2.248). AMF also had significant positive effects on phosphorus uptake, water flow and disease resistance.

EcologyFig2

Mean effect size (Hedges’ d+) of AMF on different factors considered in the meta-analysis.  The horizontal error bars are the 95% confidence intervals. n = number of observations.  If the error bars do not cross zero, inoculation with AMF had a significant positive effect relative to plants without AMF.

This meta-analysis shows that AMF help plants in many different ways. Researchers knew about the AMF impact on nitrogen and phosphorus uptake, but may be surprised to learn of equally strong effects on water flow, disease resistance and soil aggregation. Consequently, AMF may be very useful for forest management, agriculture, conservation and habitat restoration. As examples, conservation biologists and forest managers may need to consider adding AMF to soils that have suffered severe burns from fires, which may kill the existing soil fungi. Or agriculturalists intent on growing a particular crop may want to inoculate the soil with a specific group of AMF spores that enhance soil aggregation and water uptake, so their crop may thrive in a habitat that might otherwise not be suitable.

More than 3/4 of land plants form associations with AMF. Consequently, any attempts to restore habitats or to maintain high levels of species diversity in existing ecosystems require understanding what types of AMF inhabit the soils, and how these AMF influence ecosystem functioning.

note: the paper that describes this research is from the journal Ecology. The reference is Delavaux, C. S., Smith-Ramesh, L. M. and Kuebbing, S. E. (2017), Beyond nutrients: a meta-analysis of the diverse effects of arbuscular mycorrhizal fungi on plants and soils. Ecology, 98: 2111–2119. doi:10.1002/ecy.1892. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.