Metacommunity data and theory: the tortoise and the hare

Empirical approaches to metacommunities: a review and comparison with theory
Logue et al. 2011

The recognition that community composition is a function of both local and regional-scale processes, meaning that a community cannot be understood in isolation from the network of communities with which it interacts, is the fundamental idea behind metacommunity ecology. In a relatively short period of time, metacommunity ecology has integrated concepts from spatial ecology, metapopulation ecology, and community ecology with novel ideas, and developed a strong body of theory. However, metacommunity theory has advanced much more rapidly than empirical tests of that theory. In an interesting review in TREE, Logue et al. examine whether empirical data needs to catch up with the pace of theory development, or whether theory is moving too fast to incorporate the information available from empirical data.

The types of systems used in the 34 experimental and 74 observational studies that Logue et al. found were very limited – the most common experimental approach involved setting up aquatic microcosms of unicellular organisms.* Observational studies similarly tested microorganisms, usually in aquatic systems. The organisms so beloved in the rest of community ecology (plants? vertebrates?) barely feature. Most studies focus on aquatic systems composed of multiple patches (such as microcosms, ponds, pitcher plant communities) because systems with discrete boundaries are more amenable to testing current theory. However, natural systems are rarely configured into a clear “patch” versus “matrix” dichotomy. Instead they are complex and heterogeneous, and may lack clear boundaries.

Dynamics in metacommunities are generally described using four dominant paradigms: mass-effects, species sorting, neutral perspective, or patch-dynamics. These paradigms reflect the most important processes structuring communities, that is, either dispersal between communities, environmental differences between communities, dynamics driven by the tenets of neutral theory, or extinction and colonization, respectively. Strikingly, experimental studies mostly tested for mass-effects or patch dynamics, and observational studies mostly tested for species-sorting and mass effects paradigms. The neutral paradigm was rarely tested in any type of study. Logue et al. found that many studies had difficulty designing experiments that tested for evidence of specific paradigms, because natural communities are much more complex than the simple paradigms suggest. Most studies that did test for evidence of particular paradigms found evidence for multiple paradigms or had difficulty disentangling different mechanisms.

The metacommunity theory that has developed in the last five years is among the most exciting and interesting work in ecology. However, the slower pace of experimental work means that theory has developed with little feedback. For example, Logue et al. make a strong argument that the results from these studies suggest that it is time to integrate the four-paradigm system into a single, comprehensive framework (see figure). Theory is only valuable if it’s useful - this paper is an important reminder that there is an important feedback loop between theory and data, and successful science requires input from both.

*Important disclaimer: at this very moment I'm running aquatic microcosms of microscopic protists in the lab. We all have room for improvment. :)

Navjot Sodhi, conservation for all

I opened my e-mail to see the shocking and saddening news that Navjot Sodhi passed away yesterday (see here for more details). He was an absolute leader in tropical conservation biology from his base in Singapore. But more than this he made conservation biology accessible to the public and especially to those working on the front lines trying to protect biodiversity. His free edited book: 'Conservation Biology for All' set a new milestone in conservation biology and in the efforts of academics to step out of the ivory tower and reach out to broader communities.

Nature’s little blue pill

Something often happens to mature community ecology studies as they get older that we don’t like to talk about much. Occasionally, when biodiversity and ecosystem functioning experiments are performed in a controlled, homogeneous setting, they can suffer from flaccid response curves. It’s perfectly normal, happens to lots of healthy microcosm communities, but it can be troubling and embarrassing nonetheless. After all, who doesn’t want a nice stiff linear response curve?

Bear with me here for a minute.

A few weeks ago, Bradley Cardinale published a study in which he tested the effects of algal biodiversity on water quality in streams. It’s a pretty classic diversity-function experiment; lots of artificial streams with different numbers of species of algae in them, and he measured productivity and nitrogen uptake. As is usually the case, the more species he put in each stream, the more these ecosystem functions increased.

But Cardinale did something else in this experiment that has never been done before, at least not on this scale. He added extra niche opportunities to some of the streams, so that they offered multiple different habitats for algae. He did this by introducing heterogeneity through flow and disturbance manipulations.

Figures from Cardinale 2011, Nature. a and b are the heterogeneous streams, d and e are the homogeneous ones.

You might be familiar with that saturating response curve that is typical of so many diversity-function experiments. It starts off with large increases in ecosystem functioning as species are added to communities, and then it levels off so that as additional species are added, they only increase ecosystem functioning by small amounts (figures d and e). The theory behind this is that there are only so many niches in an environment, and as more and more species are added some of them become redundant.

Well when Cardinale threw those extra habitats into his artificial streams, that floppy old saturating curve sprang up like a regressional jack-in-the-box (figures a and b).

What happened was the homogeneous streams became dominated by just a single species that was well adapted to that environment. The heterogeneous streams allowed different species to coexist and this let them make more efficient use of the resources in those streams.

This is a major finding for a few reasons. First, it confirms that one of the main mechanisms behind diversity-function relationships is niche partitioning. I’ve said in the past that knowledge of these mechanisms is sorely needed. Second, it links coexistence theory to ecosystem functioning, two fields that are closely related but often disconnected.

Finally, it means that biodiversity is even more valuable than we had previously thought. The natural world doesn’t contain very many homogeneous streams; it’s a complicated place. The real world is probably better represented by figures a and b than by figures d and e. So while controlled experiments have shown that some species are redundant for ecosystem functioning, there is no evidence here for any redundancy in more natural settings.

This paper also underlines the fact that these studies need to be done in nature as opposed to labs. Cardinale was able to simulate nature fairly realistically because he was using algae. That’s harder to do with more complex organisms. It’s difficult to recreate environmental heterogeneity in artificial ecosystems, and if ecosystem functioning depends on both biodiversity and heterogeneity, then it’s time to take this research outside. Manipulative field studies are a good start, but completely natural settings will probably reveal more of the true story.

So although it’s very common for artificial communities to suffer from Ecological Dysfunction, there is no reason that they can’t enjoy a healthy relationship with biodiversity like any other community. All they need is a little heterogeneity to spice things up and put that spring back in their step.

Andy Hector has written an excellent perspective on the study. I recommend reading it, particularly if you don’t want to read the entire original article.

The ecology blogosphere just got a little more crowded, and better (welcome Oikos blog)

A diversity of voices is why the internet is such a powerful intellectually democratizing form of communication. Ecology blogs, long the minority in scientific blogging just received an immense boost from the new Oikos blog, obviously associated with the journal, Oikos. While some of their content is dedicated to journal business, there have been great posts on ecological research and broader intellectual topics from Jeremy Fox, aka oikosjeremy.

Welcome.

Happy 10th birthday, neutral theory!

Rosindell, Hubbell, and Etienne. (2011). The unified neutral theory of biodiversity and biogeography at age ten.

I would argue that neutral theory is not only the most controversial idea, but also the most successful idea to permeate community ecology in the last ten years. A quick keyword search suggests that ~30 ecological papers related to the topic were published in the last year, including some with titles still reflecting the controversy; “Different but equal: the implausible assumption at the heart of neutral theory”. Neutral theory makes a seemingly unreasonable assumption—that species identity doesn’t matter—and yet seems to predict species-area relationships and species abundance distributions as well or better than niche theory does. This made it an infuriating challenge for many ecologists. The number and quality of papers that it inspired—both in support and opposition—are a reminder that disagreement is good for science.

It’s been a decade since the publication of “The Unified Neutral Theory of Biodiversity and Biogeography”, in which Steve Hubbell proposed a controversial model in which coexistence results from drift, dispersal and speciation, rather than ecological differences between species. To mark this anniversary, a review in TREE by James Rosindell, Stephen Hubbell, and Rampal Etienne reflects on neutral theory’s first ten years, and examine the influence neutral theory has had in many areas of community ecology. The authors also note that some of the limitations of neutral theory can be dealt with by extending the classic formulation of the model, so that unrealistic assumptions related to spatial structure, speciation rates, or the zero-sum assumption can be relaxed. The excessive interest in neutral theory’s species-abundance predictions left its other predictions unexamined, and there is still room for tests of how neutral theory informs species-time relationships, modes of speciation, and even conservation decisions.

Despite these accomplishments, the review is remarkably subdued, underlined by statements such as neutral theory is a “good starting point”, a “valuable null model”, and a “useful baseline”. However, it seems unnecessary to state, as some have, that "neutral theory is dead". Its legacy, captured in the final paragraphs, is still incredibly important: “…niches have dominated our attention and left less obvious, but still important processes forgotten… Perhaps the most important contribution of neutral theory has been to highlight the key roles of dispersal limitation, speciation and ecological drift, by showing how much can be explained by these processes alone...”

George Box said it best: “All models are wrong, but some are useful”.

Carnival three-five.

The 35th installment of the Carnival of Evolution is available from Lab Rat. Want to know who said what about evolution? Go to the Carnival.

Ecological interactions and evolutionary relatedness: contrary effects of conserved niches

Over the past several years a multitude of papers linking patterns of evolutionary relatedness to community structure and species coexistence. Much of this work has looked at co-occurrence patterns and looked for non-random patterns of relatedness. The key explanations of patterns has been that communities comprised of more distantly-related species is thought to be structured by competitive interactions, excluding close relatives. Alternatively, communities comprised of species that are closely related, are thought to share some key feature that allows them to persist in a particular set of environmental conditions or stress. This whole area of research is completely predicated on close relatives having more similar niche requirements then two distant relatives. This predication is seldom tested.In a recent paper in the Proceedings of the National Academy of Science, Jean Burns and Sharon Strauss examine the ecological similarity among 32 plant species and tested if evolutionary relationships offered insight into these similarities. The ecological aspects they examined were germination and early survival rates as well as interaction strengths among species. To assess how these were influenced by evolutionary relatedness, they planted each species in the presence of one of four other species varying in time since divergence from a common ancestor, creating a gradient of relatedness for each species. They found that germination and early survival decreased with increasing evolutionary distance. This surprising result means that species germinating near close relatives do better early on then if they are near distant relatives. The explanation could be that they share many of their biotic and abiotic requirements, and these conserved traits influence early success.

Conversely, when they examined interaction strengths over a longer period (measured as relative individual biomass with and without a competitor), they found that negative interactions were stronger among close relatives.

These two results reveal how evolutionary history can offer insight into ecological interactions, and that the mutually exclusive models of competitive exclusion versus environmental filtering do not capture the full and subtle influence of conserved ecologies. Evolutionarily conserved traits can explain both correlated environmental responses and competitive interactions.

Burns, J., & Strauss, S. (2011). More closely related species are more ecologically similar in an experimental test Proceedings of the National Academy of Sciences, 108 (13), 5302-5307 DOI: 10.1073/pnas.1013003108

The bellybutton, biodiversity reserve of the body

Although less recognized--and less glamourous--than most biodiversity hotspots, the human bellybutton harbours it own diverse collection of species, and these species tell us something about ourselves. That's the premise behind the Belly Button Biodiversity Project, which is getting some press for its large-scale sampling of bellybutton bacteria. For interesting discussion about where the data could lead, see Rob Dunn's (one of the researchers) website. His post, including the comments, hints at how much there is to learn about the ecology of human bacteria.

Update: Rob Dunn has now published a book "The Wild Life of Our Bodies", telling more stories of our changing relationships with other species.

Carnival time.

The 34th edition of the Carnival of Evolution is hosted at Quintessence of Dust. Everything from the evolution of perfection to the evolution of small importance will be found there.

White-nose syndrome and wind turbines: why biodiversity matters

Linking ecosystem services to economic benefits is a vital step in connecting ecological research to policy and political action. The UN Environmental Programme’s The Economics of Ecosystems and Biodiversity (TEEB) initiative represents a concerted effort to draw attention to the economic benefits of biodiversity and cost of ecosystem degradation, and to bring together scientists, economists and policy-makers.

Accordingly, Boyles et al. (Nature, 2011) paint a troubling picture about the value of economic benefits that insectivorous bats provide to the North American economy, and the degree of extinction risk they currently face. The authors point out that bats are “among the most overlooked, yet economically important, non-domesticated animals in North America”, and their loss would cost North Americans more than 3.7 billion dollars/year. Given rapid declines in populations due to white-nose syndrome (over 1 million bats killed) and wind turbine fatalities (projected to reach up to 30,000-100,000 fatalities/year as wind turbine installations increase), the authors suggest action can't wait.



Hopefully using the universal language of money helps translate scientific knowledge into political action. After all, bats are only one group of species: imagine the true cost of current rates of biodiversity loss and ecosystem destruction, from the smallest microorganism to the largest megafauna. The total must be staggering. And so, it seems, is the scale of action required to halt this decline.