Taking stock of exotic species in the new wild: Acknowledging the good and the bad.*

Are exotics good or bad? They are neither. They just are. But some exotics cause harm and impede certain priorities, and debates about exotics often ignore reality.

Book review: Fred Pearce. 2015. The New Wild: Why Invasive Species Will Be Nature’s Salvation. Beacon Press

There has been much soul-searching in invasion biology, with attacks, and subsequent rebuttals, on the very nature of why we study, manage and attempt prevent the spread of exotic species (Davis et al. 2011) (Alyokhin 2011, Lockwood et al. 2011, Simberloff 2011). What is needed at this juncture is a thoughtful and balanced perspective on the nature of the discipline of biological invasion. Unfortunately, the book “The New Wild” authored by Fred Pearce, is not that balanced treatment. What is presented in this book is a very one-sided view, where counter-evidence to the thesis that exotics will save nature is most often overlooked, straw men are erected to aid in this goal, and the positions of working ecologists and conservation biologists are represented as simplistic, anachronistic or just plain incorrect.

What Pearce has written is a book-long argument about why exotics shouldn’t be feared, but rather embraced as a partial solution to anthropogenic land use change. I do not wish to undermine the reality that exotics can play important roles in urban landscapes, or that some ecologists and conservation biologists do indeed harbour suspicions of exotics and subscribe to unrealistic notions of purely native landscapes. Exotic policy is at the confluence of culture, science, economics and politics, and this is why the science is so valuable (Sandiford et al. 2014). For Pearce, the truth of what most ecologists do and think seems like an inconvenient reality.  There are a number of pervasive, frustrating problems with Pearce’s book, where bad arguments, logical flaws and empirical slight-of-hand obfuscate issues that desperately need honest and reflective treatment.

A monoculture of the exotic plant Vincetoxicum rossicum that spans open and understory habitats near Toronto, Canada (photo by M. Cadotte). This is a species that interferes with other management goals and needs to be actively managed.

There are major problems with ‘The New Wild’ and these include:

1) A premise built on a non sequitur and wishful thinking. The general premise of the book, that exotics represent a way out of our environmental doldrums, is myopic. Pearce’s reasoning seems to be that he has conflated “the world is not pristine and restoration is difficult…” with the alternative being that exotics are positive and “we should bring them on”. Certainly we can question exotic control efficacy, costs and conservation goals, but that does not mean that exotics are necessarily the solution.

      2)   Underrepresenting the observed effects of some invasive non-indigenous species. Pearce’s book is not balanced. The perceived benefits of exotics in this ‘New Wild’ are extolled while dismissing some of the problems that invasive ones might cause. He says that exotics typically “die out or settle down and become model eco-citizens” (p. xii). But there is a third outcome that Pearce ignores –they move in and become unruly neighbours. When he must acknowledge extinctions, he minimizes their importance. For example when discussing Hawaiian bird extinctions: “The are only 71 known extinctions” (p. 12 –italics mine), or with California: “But only 30 native species are known to have become extinct as a result [of exotics]” (p. 64 –italics mine).

He also implies throughout the book that exotics increase diversity because “Aliens may find new jobs to do or share jobs with natives.” (p. 113). The available evidence strongly suggests that the numbers of species inhabiting communities has not increased over time (Vellend et al. 2013, Dornelas et al. 2014). Which on the surface seems like a good thing, except that many communities are now comprised of 20-35% exotics. This means that there have been losers. Vellend and colleagues (2013) show that the largest impact on native species diversity has been the presence of exotics. So, they do not necessarily find new jobs, but rather outcompete some natives.

      3)   Conservation biologists and ecologists in the crosshairs. Pearce continually lauds those like-minded, outspoken advocates of exotics while belittling ecologists and conservation biologists who don’t agree with him. His disrespect for the process of science comes in two forms. First, he seldom considers evidence or presents opinions counter to his thesis. He gives a partial reason about this bias; he says that ecologists (except for those few brave pioneering souls) ignore novel ecosystems and the functional contributions of exotics (for example on p. 13). This is demonstrably false (see next section). Pearce has little affection for conservation biologists and mainstream ecologists. Both groups are disparaged and dismissed throughout the book. Conservation biologists get a particularly rough ride, and he never acknowledges the difficulty of their task of balancing multiple priorities: extinction vs. ecosystem function, habitat preservation vs. socioeconomic wellbeing, etc. For example, Pearce states: “Conservation scientists are mostly blind to nature outside of what they think of as pristine habitats and routinely ignore its value” –again a demonstrably false assertion.

In a particularly galling example, Pearce resorts to ‘guilt by association’ as an ad hominem attack to undermine the validity of opposing views. He links conservation with eugenics: “Many conservationists of the first half of the twentieth century were prominent proponents of eugenics” (p. 141). It would be equally valid to state that most journalists were proponents of eugenics in the first half of the twentieth century. Pearce, being a journalist, should see this as a specious argument at best.

Ecologists share in this odd and unfair derision. “Ecologists are tying themselves in knots because they refuse to recognize that these novel, hybrid ecosystems are desirable habitats for anything.” (p. 156). Unfortunately for Pearce, there are more than 4000 papers on ‘novel ecosystems’.

      4)   Misrepresenting modern ecology and conservation. Pearce attacks ecological science throughout the book and as an example Pearce makes observations about the role of disturbance and refusal to acknowledge this by ecologists “intent on preserving their own vision of balanced nature” (p. 144). However, disturbance has been a central component of community ecology for the past five decades. Because of this balance-of-nature view, Pearce says ecologists are not studying degraded, disturbed or recovering systems. For example, with secondary forests, he says: “Yet the blinkered thinking persists. Degraded forests and forests in recovery are almost everywhere under-researched and undervalued.” (p. 157). Yet there are almost 9,500 papers on secondary forests –highlighting the ecological interest in these widespread systems. There are numerous such examples.

      5)   A black and white, either-or dichotomy.  What Pearce provides is a series of stark dichotomies with little room for subtle distinction. He ties resilience and ecosystem wellbeing to the arrival of exotics, without adequately assessing the drawbacks: “Nature’s resilience is increasingly expressed in the strength and colonizing abilities of alien species …we need to stand back and applaud” (p. xii).

Invariably in ecology, debates over ‘either/or’ dichotomies end up with the realization that these dichotomies are endpoints of a continuum. This is exactly the case with exotics. Are they bad or good? The answer is neither. They just are. Some exotics species provide economic opportunity, ecosystem services and help meet other management goals. Some exotics cause harm and impede certain priorities. Modern management needs to be, and in many cases is, cognizant of these realities.

 References

Alyokhin, A. 2011. Non-natives: put biodiversity at risk. Nature 475:36-36.

Davis, M. A., M. K. Chew, R. J. Hobbs, A. E. Lugo, J. J. Ewel, G. J. Vermeij, J. H. Brown, M. L. Rosenzweig, M. R. Gardener, and S. P. Carroll. 2011. Don't judge species on their origins. Nature 474:153-154.

Dornelas, M., N. J. Gotelli, B. McGill, H. Shimadzu, F. Moyes, C. Sievers, and A. E. Magurran. 2014. Assemblage Time Series Reveal Biodiversity Change but Not Systematic Loss. Science 344:296-299.

Lockwood, J. L., M. F. Hoopes, and M. P. Marchetti. 2011. Non-natives: plusses of invasion ecology. Nature 475:36-36.

Sandiford, G., R. P. Keller, and M. Cadotte. 2014. Final Thoughts: Nature and Human Nature. Invasive Species in a Globalized World: Ecological, Social, and Legal Perspectives on Policy:381.

Simberloff, D. 2011. Non-natives: 141 scientists object. Nature 475:36-36.

Vellend, M., L. Baeten, I. H. Myers-Smith, S. C. Elmendorf, R. Beauséjour, C. D. Brown, P. De Frenne, K. Verheyen, and S. Wipf. 2013. Global meta-analysis reveals no net change in local-scale plant biodiversity over time. Proceedings of the National Academy of Sciences 110:19456-19459.

 *This post is a synopsis of my book review in press at Biological Invasions

Can there be a periodic table of niches?

Are there a limited number of categories or groupings into which all niches can be classified?  I’ll 

admit that my first reaction is skepticism. For those ecologists who think of the similarities and generalities across systems, this may be an easier sell, compared to those who get caught up in the complexities of ecological systems. Classifying niches in this way is apparently a vision that distinguished ecologists have voiced: MacArthur: “I predict there will be erected a two- or three-way classification of organisms and their geometrical and temporal environments, this classification consuming most of the creative energy of ecologists.” 

From Winemiller et al. 2015.

Kirk O. Winemiller, Daniel B. Fitzgerald, Luke M. Bower, and Eric R. Pianka, takes on this rather ambitious goal in a new paper: “Functional traits, convergent evolution, and periodic tables of niches”. The periodic table, of course, is the foundation of chemistry – the predictive, descriptive arrangement of chemical elements based on their atomic number. Ecology may never achieve a similarly simple foundation, but the authors suggest that such a general classification of possible niches (and the species that are within them) is possible. A niche within a table would extend across taxa, habitats, and biomes, and would be seen repeatedly (i.e. periodically) across these.

Perhaps because they (and their reviewers) recognized the ambitious nature of this task, the paper helpfully acknowledges the reasons that a periodic table of niches might be a terrible idea right away. Unlike chemistry, ecology is strongly dependent on context, and stochasticity limits generality. The multi-dimensionality of the modern niche concept limits how few axes such a table could be reduced to. Evolution means that classifying a species’ niche is like trying to hit a moving target.

Examples of convergent evolution are common.

Still, even the chemical periodic table has some fuzzy matching going on – isotopes still group together under a given element, despite variation. “In the same way, elements can have different isotopes,…a niche category could have phenotypic variants but still have ecological properties or functions that are essentially the same.” In particular, the authors argue that convergent evolution has recreated particular suites of traits (niches) in different habitats and distantly related taxa. This has some connection to the idea that, perhaps, much like complex systems, complex arrays of traits may reoccur because they provide stability (e.g. are selected for).

How then to approach this task? Here the periodic table is rooted in a functional trait approach, where observable phenotypes capture niche information. The dimensions of the table are determined based on what must have been the result of long discussions and much difficulty, but the authors restricted themselves to five essential components: abiotic habitat, life history strategy, trophic position, defense mechanisms, and metabolic allocation strategies.

 

From Winemiller et al 2015.

From here, the use of various ordination approaches allow researchers to begin to identify species sharing trait combinations, allowing them to be classified within the table (see paper text for more detail). The combinations of these dimensions observed or unobserved in nature should inform us about the stability of certain niches, and perhaps provide predictions about which species to use for restoration approaches, which species may be invasive in a given system, or to predict shifting distributions.

If you had many different ecologists each develop a ‘periodic table of niches’, each table would be unique, evidence for how difficult drawing general principles and identifying the fundamental ecological dimensions is. Another person might consider dispersal its own dimension, for example, or dismiss defenses. This is especially true because the periodic table presented in this paper is phenomenological, lacking a clear connection with theoretical work, for example. The proof will be in its application and utility – do others adopt it, is it predictive, does it extend our understanding of the niche or improve applications? And I think there is a direction for functional ecology implicit in this work.

Their hearkening to MacArthur makes me wonder what MacArthur would think if he saw ecology today. His prediction that “there will be erected a two- or three-way classification of organisms and their geometrical and temporal environments, this classification consuming most of the creative energy of ecologists” falls short, but not in the ways he might have expected. Here then, is a classification system (and there have been other ideas and versions since his time), but even the 2 or 3 dimensions he generously offers aren't deemed nearly enough to capture ecological diversity. Is the simplicity that MacArthur mentions still considered possible? And I don't think the creative energy of ecologists has been focused on classifying niches in the way he mentions: it is more dispersed amongst topics, and human effects (climate change, fragmentation, habitat loss) have had a dominant role.

Winemiller, Kirk O., Fitzgerald, Daniel B., Bower, Luke M., Pianka, Eric R. 2015.  Functional traits, convergent evolution, and periodic tables of niches. Ecology Letters. DOI: 10.1111/ele.12462

The devil isn't always in the details: how system properties can inform ecology

Selection on stability across ecological scales. Jonathan J. Borrelli, Stefano Allesina, Priyanga Amarasekare, Roger Arditi, Ivan Chase, John Damuth, Robert D. Holt, Dmitrii O. Logofet, Mark Novak, Rudolf P. Rohr, Axel G. Rossberg, Matthew Spencer, J. Khai Tran, Lev R. Ginzburg. 2015. Trends in Ecology & Evolution, http://dx.doi.org/10.1016/j.tree.2015.05.001.

This paper in TREE  on selection at higher level systems has been on my must-read list since it came out a few weeks ago, and it was worth the wait. It does what the best TREE papers do - makes you think a bit more deeply about a common topic. In this case, it develops an approach to understanding complex ecological systems (communities, ecosystems) that is blind to the details that ecologists often focus on.

The search for generalities and commonalities drives modern ecology. In short (though this paper deserves an in-depth read), this paper argues that we can learn much by considering stability and feasibility in complex ecological systems. That is, we can also study community structure or trophic webs by considered whether specific configurations of the system are stable. This is in contrast to a context-centric study of a system, where the usual list of proximate causes (productivity, niche availability, connectivity, etc, etc) may be used to understand why the system looks as it does.

The authors' premise is that nonadaptive (e.g. unstable) ecological systems will be unfavourable and selected against, and the resulting selective process “can produce many of those recurrent ecological patterns that have been observed in nature over large scales of space and time.” This requires that you accept a few underlying concepts: first, that large scale systems also experience selection (whether one prefers selection be in parentheses is up to the reader), in that unstable systems will be lost at faster rates leading to greater frequency of stable systems; and second, that this process of selection is determined by the properties of the system alone, not the specific conditions ecologists often focus on.

As an illustration, consider four possible food webs depicting intraguild predation that vary in their interaction strengths. All configurations are possible, but A-C are likely to lead to exclusion of the intraguild predator. D is most likely to be stable since the strong interaction between the resource and prey results in negative feedbacks between the densities of all species (i.e. when the resource is low, the prey should also be low, reducing the predator density as well) and thus more likely to be observed in natural systems. 

From Borrelli et al 2015.

A more specific example looks at attack rates and handling times in predator-prey interactions. When stability is considered, it seems that although predator-prey cycles may occur, it should be uncommon to have such extreme oscillations that populations reach dangerously low levels where stochastic extinctions may occur. Data suggests that oscillatory dynamics are less common in predator-prey relationships, but do occur particularly for specialist predator/prey pairings. Theory (Rosenzweig-MacArthur predator-prey models) predict that such pairings should be most stable if prey are weakly self-limited and predators have high attach rates/long handling times. Empirical evidence for this prediction supports it surprisingly well.

From Borrelli et al 2015.

 
Other related approaches consider feasibility across food webs, communities, and ecosystems. A community perspective might consider interactions across all species, perhaps using a network approach. Networks should tend towards formations that are the most stable – e.g. short chains rather than long ones. The commonness of nested network structures may reflect these constraints. 

Such an approach to ecology is not entirely new (Robert May's weak interactions comes to mind). But it provides perhaps the best potential explanation I’ve seen for ‘generality’ focused approaches in ecology, including ecological allometric relationships, macroevolutionary patterns, and network approaches. Macroecological patterns have often captured, rather than tidy linear relationships, occupied versus unoccupied parameter space. Thinking about feasibility as a macroecological ‘mechanism’ for ecological patterns at the system scale might lead to new research directions. 

From the Archives: Conservation now and then

For the rest of the summer (until ESA!), we’re going to highlight some of the older topics and posts from the EEB & Flow. The blog has been around since December 2008, and so it has covered a lot of ground: 345+ posts with topics ranging from ecological history, to research advances, to work life balance, to the silly.

The interesting thing is that posts are like an archive of the various topics and directions ecological research has taken (or at least the research interests of the various post authors). And in many ways, papers from 2009 are frankly indistinguishable in topic and approach from today.

Take, for example, these posts from 2009 about conservation and climate change:

Salamanders and climate change – impending extinctions?

Fisheries and food webs: a whole system approach to cod recovery

The sushi of tomorrow… Jellyfish rolls?

Conserve now or wait for data?

The topics wouldn’t be out of place today. Risk assessments for specific species, fisheries and other applied questions, and consideration of the agony of conservation choices. 

(Not sure what this signifies - Maybe that 5 years isn't long in the grand scheme of research?)

The problem with collaboration in the electronic era...

E-communication has revolutionized every aspect of our lives. From how we shop, find love, watch movies and do science, the ability to interact with others globally has virtually eliminated barriers to the flow of ideas. I have fruitful collaborations with researchers in many different countries, which are greatly enhanced by e-mail and Skype. However, a new problem has emerged -scheduling people for meetings in multiple timezones!

Green = optional working time for researchers in different timezones; yellow = suboptimal; red = perhaps we allow people to sleep.

I routinely have Skype meetings with my editorial team in the UK at 5 or 6 am, but as the above graphic shows -scheduling a meeting amongst people in the UK, North America and Australia is virtually impossible.

Are scientists boring writers?

I was talking with an undergrad who is doing her honours project with me about the papers she’s reading, and she mentioned how difficult (or at least slow going) she’s found some of them. The papers are mostly reviews or straightforward experimental studies, but I remember feeling the same way as an undergraduate. Academic science writing uses its own language, and until you are familiar with the terms and phrases and article structure, it can be hard going. Some areas, for example theoretical papers, even have their own particular dialects (you don’t see the phrase “mean-field approximation” in widespread usage, for example). Grad school has the advantage of providing total immersion into the language, but for many students, lots of time/guidance and patience is necessary to understand the primary literature. But is science necessarily a boring language?

A recent blog piece argues that academic science writing needs to fundamentally change because it is boring, repetitive, and uninspired. And as a result, the scientific paper needs to evolve. The post quotes a biologist at University of Amsterdam, Filipe Branco dos Santos: he feels that the problem is rooted in the conservative nature of scientists, leading them to replicate the same article structure over and over again. Journals act as the gatekeeper for article style too – submission requirements enforce the inclusion of particular sections (Introduction, Methods, Results, Discussion, etc), and determine every thing from word counts, figure number, text size, and even title structure and length. Reviewers and editors are within their rights to require stylistic changes. The piece includes a few tips for better article writing: choose interesting titles, write in the active tense, use short sentences, avoid jargon, include a lay summary. It’s difficult to disagree with those points, but unfortunately the article makes no attempt to suggest what, precisely, we should be doing differently. Still, it suggests that consideration of the past, present and future of scientific writing is necessary.

One glaring issue with the post is that the argument that scientists are stuck in a pattern established hundreds of years ago ignores just how much science papers have changed, stylistically. Scientific papers are a very old phenomenon – the oldest, Philosophical Transactions of the Royal Society, was first published in 1665. The early papers were not formatted in the introduction / methods / results / discussion style of today, and were often excerpts from letters or reports.

From the first issue, “Of the New American Whale-fishing about the Bermudas” begins:

“Here follows a relation, somewhat more divertising, than the precedent Accounts; which is about the new Whale-fishing in the West Indies about the Bermudas, as it was delivered by an understanding and hardy Sea-man, who affirmed he had been at the killing work himself.”

Ecological papers written in the early 1900s are also strikingly different in style than those today. Sentences are long and complex, words like “heretofore”, “therefore”, and “thus” find frequent usage, and the language is rather flowery and descriptive.

From a paper in the Botanical Gazette in 1913, the first sentence:

“Plant geographers and climatologists have long been convinced that temperature is one of the most important conditions governing the distribution of plants and animals, but very little has as yet been accomplished toward finding out what sort of quantitative relationships may exist between the nature of floral and faunal associations and the temperature conditions that are geographically concomitant therewith.”

While this opening makes perfect sense and establishes the question to be dealt with in the paper, it probably wouldn’t make it past review without comment.

Some of my favourite examples that highlight how much ecological papers have changed come from R.H. Whittaker’s papers. He is clearly an avid (and verbose) naturalist and his papers are peppered with evocative phrases. For example, “If, for example, one stands on a viewpoint in the Southern Appalachian Mountains in the autumn, one sees a complex varicoloured mantle of vegetation covering the mountain topography” and “The student of vegetation seeks to construct systems of abstraction by which relationships in this mantle of vegetation may be comprehended.” Indeed!

Today, in contrast, academic science writing is minimalist – it is direct, focused, and clarity is prized. Sentences are typically shorter, with a single focal thought, and the aim is for a clear narrative without the peripatetic asides common in older work. These shifts in style reflect the prevailing thoughts about how to balance the role of scientific papers as a communication device versus as a contribution to the scientific record. It seems that science papers may be boring now because authors and editors would rather a paper be a little dry rather than be unclear or difficult to replicate. (Of course, some papers manage to be both boring and confusing, so this is not always successful….) Modern papers have a lot of modern bells and whistles too. The move away from physical copies of papers to pdfs and online only colour versions and supplementary information has made sharing results easier and more comprehensive than ever.

If there is going to be a revolution in academic science writing, it will probably be tied to the ongoing technological changes in science and publishing. The technology is certainly already present to make science more interactive to the reader, which might make it less boring? It is already possible to include videos or gifs in online supplements (a great example being this puppet show explaining Diversitree). More seriously, supplements can include data, computer code used for analyses or simulations, additional results. It’s possible to integrate GitHub repositories with articles tied to a paper’s analyses, or link markdown scripts for producing manuscripts. The one limitation is that these approaches is that they aren’t included in the main text and so most people never see them. It’s only a matter of time before we move towards a paper format that includes embedded elements (extending on current online versions that include links to reference papers). One could imagine plots that could be manipulated, or interactive maps, allowing you to explore the study site through satellite images of the vegetation and terrain.

Increasingly interactive papers might make it more fun to work through a paper, but a paper must stand alone without them. For me, the key to a well-written paper is that there is always a narrative or purpose to the writing. Papers should establish a focus and ensure connections between thoughts and paragraphs are always obvious to the reader. The goal is to never lose the reader in the details, because the bigger picture narrative can be read between the lines. That said, I rarely remember if a paper is boringly written: I remember the quality of the ideas and the science. I would always take a paper with interesting ideas and average writing over a stylish paper with no substance. So perhaps academic science writing is an acquired (or learned) taste, and certainly that taste could be improved, but it's clear that science writing is constantly evolving and will continue to do so.

I'll take 'things that have nothing to do with my research' for $400

I guess I do have a couple papers with the word fire in their titles?
And to Burns and Trauma's credit, this is a nicely formatted email and the reasons to publish with them are pretty convincing :-)

Is there a limit to how many species can the earth hold?

Counting species (bird lists, plant guides) is as old as ecology itself. And yet surprisingly, there are still different opinions on how many species the planet holds, and even, whether there are limits on how many species it can have. If the number of species has ecological limits, the assumptions ecologists often make – that species pools are limited and knowable, dynamics can reach equilibrium, competition should usually be important – would be stronger. Things would be more predictable. 

But is the production of diversity self-limited? There isn’t consensus but two recent articles in the American Naturalist (continuing a debate at the American Society of Naturalists meeting) provide some excellent debate of this question.

The debate is whether the majority of variation in continental-scale species richness is regulated by diversity-dependent feedbacks. In these papers, Dan Rabosky and Allen Hurlburt argue that species richness has ecological limits, while Luke Harmon and Susan Harrison take the contrary position, that species richness is dynamic. First, to define some terms: here, species richness is being considered at the largest spatial scale (e.g. terrestrial plants at the continental scale) so that dispersal limitation should be comparatively unimportant (because diversity changes are mostly driven by in situ speciation).

The crux of the Rabosky & Hurlburt argument is established in the Ecological Limits Hypothesis (ELH), which states that species richness will reach a dynamic (i.e. stochastic) equilibrium, where equilibrium richness reflects density dependence in speciation and/or extinction rates. Speciation and extinction rates are ultimately limited by total resource availability for the continent. Therefore variance in richness through time and between places should be driven these ecological limits, and richness should be predictable.

From Rabosky & Hurlburt 2015 - the Ecological Limit Hypothesis.

The evidence presented for the ELH comes from phylogenies and macroevolutionary models, the fossil record, and macroecological observations. First, there are well known patterns between species richness and energy, productivity, or habitat area, and these span multiple regions and groups of species (e.g. Jetz and Fine 2012). Further, Rabosky & Hurlburt argue that geological records suggest that changes in diversity are not unbounded or exponential, but instead rise and fall, correcting toward some equilibrium. Molecular phylogenies are often evaluated by looking at speciation rates over time, and the authors suggest that these frequently show declines, where speciation declines during adaptive radiations. One prediction that arises from the ELH is that perturbations will be followed by particular responses: “negative perturbations—mass extinctions, in particular—should lead to diversity recoveries. Second, positive perturbations—increases in the resource base available to a biota—predict increases in species richness to stable but greater equilibrial levels”.

The rebuttal article from Harmon & Harrison takes a strong and contrasting view, although it focuses mostly on poking holes into Rabosky & Hurlburt’s arguments, rather than laying out a competing hypothesis. If Rabosky & Hurlburt focused on evidence over huge evolutionary scales and spatial expanses, the Harmon & Harrison response has a particular interest in the temporal and spatial scales of interest to community ecologists (local, present day) and how these seem to disagree with Rabosky & Hurlburt's hypothesis.

First, Harmon & Harrison argue that that the macroevolutionary evidence (molecular phylogenies, fossil data) is not nearly so convincing as Rabosky & Hurlburt suggest. There are important limits to its utility resulting from issues of ambiguity in interpretation and methodological limitations. In addition, for most of the patterns Rabosky & Hurlburt highlight, there are other papers concluding that the pattern was not present in their data. With reference to the lack of relationship between clade age and diversity: “A common interpretation of these results is that a lack of a relationship between age and diversity is evidence for ecological limits.... However… this pattern is far from ubiquitous in real data and is compatible with other explanations”. They also take issue with the tendency for hand-wave-y interpretations of patterns in such data, and emphasize the need for better statistical analyses and consideration of alternate models. Fossil data has obvious limitations as well (hence the field of taphonomy), including the fact that fossils are rarely classified to the species-level, which means they do not represent species richness, but rather lineage richness.

But Harmon & Harrison's real disagreement is based on their view that ecological evidence from local communities does not at all suggest ecological limits. Energy-richness correlations, although common, may have alternative explanations: the tropics may have higher diversification rates for other reasons, or niche conservation means that more species niches suited to the tropics, confounding energy-diversity relationships. Further, local communities do not regularly show a positive energy-diversity relationship. In particular, Harmon & Harrison suggest that the logic from the ELH, if followed, predicts that if species richness is ultimately tied to the availability of energy, then competition should necessarily be very important in most ecological communities. They cite a stat from the invasion of California flora in which alpha diversity has risen by more than 1000 invasive species, with only 28 native extinctions (as of 2002), suggesting that local (or even regional) communities are not full. 

To this, Rabosky & Hurlburt rejoins that invasion is about dispersal changes, and not resources. Further, they believe that large evolutionary scales are most useful as evidence for the ELH, since they are most likely to show zero sum game, rather than temporary dynamics, and since confounding factors should become minimized.

The debate left me feeling a little unsatisfied (since expecting the authors solve the problem is a bit unreasonable), in part because the authors are really arguing from different scales and approaches. And both sides are clearly right in some cases (and in others, perhaps, clearly overreaching). And of course, proving whether or not there is an ecological limit on diversity is a rather difficult thing. When Harmon and Harrison argue that the ELH, which assumes that richness approaches some equilibrium value but varies about it in a stochastic fashion, isn’t parsimonious, they’re wrong – ecological processes are innately stochastic and it hardly seem un-parsimonious to assume as much. But they’re right that this view makes testing and model fitting very difficult since having high replication and good quality data is necessary (to capture accurately a distribution, rather than single value). Given the variety of issues with data representing diversification over evolutionary time, and frequently an inability to capture extinction rates with evolutionary data, having quality, replicated tests of the ELH is difficult.

On the other hand, at local scales over ecological time, observations may be less relevant. It’s not clear how to reconcile statements about saturation (or lack thereof) of local communities with richness at continental scales. Rabosky & Hurlburt suggest that local assemblages can be dynamic in diversity as long as there is a zero sum across all communities and through time. But a connection between continental scales and local scales is innate, and understanding how diversity relates over multiple spatial scales is an area of ecological research we need to continue to develop.

Given there are no easy tests of this sort of question (though bacterial microcosm provide some interesting results), we have been forced to draw conclusions based on weak tests and weak evidence. But ecologists do this because this is a truly important question, with huge implications across ecology and evolution. Ecological and evolutionary models make assumptions that implicitly or explicitly about carrying capacity, about determinants of rates of speciation and extinction, about invasion, about why global diversity changes, and these need to be confirmed. Further, if there is a strong ecological feedback of diversity, one of the most important implications is that major perturbations such as extinctions should be followed by major recoveries. In the Anthropocene, that’s an important implication. 

References:

Species Richness at Continental Scales Is Dominated by Ecological Limits. Daniel L. Rabosky, Allen H. Hurlbert. The American Naturalist, Vol. 185, No. 5 (May 2015), pp. 572-583.

Species Diversity Is Dynamic and Unbounded at Local and Continental Scales. Luke J. Harmon, Susan Harrison. The American Naturalist, Vol. 185, No. 5 (May 2015), pp. 584-593.