@Biotropica 47(3): Editor’s Choice: Paleoclimate, Environmental Grain, & Species Distributions in Africa & S. America
I am pleased to announce the May 2015 Editor’s Choice Article: Nakazawa, Y. and Peterson, A. T. (2015). Effects of Climate History and Environmental Grain on Species’ Distributions in Africa and South America. Biotropica. 47: 292–299.
How climate change during the Pleistocene influenced the distribution of biomes and biota has long been contentious. THis study is unique in that it used advanced modeling techniques to explore how different levels of environmental restriction in analogues for niche breadth would influence species’ distributions, and in explicitly comparing the responses of African and South American environments. It turns out I’m going to have to revise the session in my tropical ecology class on the topic, because their results suggest biodiversity patterns relate closely to historical patterns of environmental grain and their stability through time in ways I hadn’t previously appreciated. It indicates melding contemporary analytical techniques with ‘classical’ theories can provide important insights, and is a reminder that to understand the present and future of tropical biodiversity we must continually look to the past. Congratulations to Yoshinori and Andrew, who explain the motivation and insights in their essay below.
Global patterns of species diversity have been the subject of many commentaries and much speculation, and numerous hypotheses of action of different ecological and evolutionary processes have been offered to account for those patterns (Pianka 1966). This subject, however, has remained particularly opaque to any experimentation, or even data-driven analyses, such that the many hypotheses remain just that: multiple competing explanations for a complex phenomenon. This situation begs, then, an alternative approach, if any progress is to be made in understanding global species diversity patterns and the external and internal drivers that create them.
The question at the heart of this issue is a simple one: how much “process” is needed to explain the pattern of species diversity? The beginnings of such an analysis were explored by Rangel et al. (2007), who created a simple, but instructive, simulation environment for exploring the evolution of species richness across landscapes. Rangel et al. focused on identifying the suite of evolutionary and biogeographic conditions that together produce realistic diversity patterns across South America in their simulations.
Yoshinori Nakazawa focused his doctoral dissertation on the topic of joint influences of climate and landscape on species diversity. In a first exploration (Nakazawa 2013), he also assessed present-day factors of climate, seasonality, and landscape that contribute to diversity patterns across South America. He simulated a very process-free world: species had fixed ecological niches, which were keyed to climate characteristics of random points across the continent; Nakazawa assessed climatic connectivity of these ‘niches’ to posit full geographic ranges for each ‘species,’ and analyzed which landscape features had to be incorporated for true diversity patterns to be approximated. He found that only by incorporating both the effects of large Amazonian rivers as barriers to dispersal and the effects of seasonality in constraining or facilitating connectivity of areas could he replicate current diversity patterns.
That first exploration, however, was a single snapshot in time, yet we know that species’ distributions and diversity respond to the summation of history over thousands or even millions of years. As a consequence, in a second effort, we added the effects of a single cycle of warm-cool-warm transition in global climates at the end of the Pleistocene (135,000 years ago until present), taking advantage of outputs from the Community Climate System Model. The result is the paper that is being published in Biotropica: a view of how species diversity would respond if those very simple species had to pass through a filter of a Pleistocene climate cycle. Indeed, in our simulations, the only requirements were that species’ distributional areas could not (1) disappear entirely in any time period, nor (2) move so fast between time periods that distributional areas do not overlap.
The result was a novel view of drivers of species diversity patterns across South America and Africa: our simulations included no competition, evolution, or speciation whatsoever, and rather reflected only the effects of climate, landscape, and dispersal. Areas highly conserved environmentally over rather dramatic climate change events matched diversity hotspots and proposed refugia. However, and perhaps most interesting, African environments were far less conserved than those of South America, matching forest species diversity differences between the two continents. Clearly, our simulations suggested that biodiversity patterns are a function of environmental granularity and its stability through time, and that other processes are not necessarily required to explain these patterns.
This study, however, hinted at answers to some still-larger questions. Our interest is in the genesis of global diversity patterns. Our results suggest that it may be unnecessary to invoke differences in species interactions like competition or predation, or productivity responses to energy input, or any other complex explanation. Perhaps, rather, simple differences in climate history and how it interacts with landscape can explain the major features of biodiversity pattern worldwide. This topic could be referred to as ‘neutral biogeography,’ although it does incorporate dispersal and climate fluctuations, and a recent body of literature has co-opted that term for a rather process-laden set of ideas that involve competition (Hubbell 2001).
The challenge, however, remains open: how much “process” is necessary to explain global biodiversity patterns? To approach the question in greater depth, the University of Kansas group (now dispersed across four institutions and two countries) is developing a next generation of such simulations that incorporate more realistic ecological niches, longer-term climate change, and even speciation. The results, to be reported in papers in preparation, are highly promising: biodiversity patterns appear to emerge from the actions of a very simple suite of drivers. That is, no one would dispute that climates fluctuated during the Pleistocene, or that landscapes present complex stages on which the biogeographic play is acted, and considerable empirical evidence is accumulating that suggests that ecological niches are rather stable through time (Peterson 2011): perhaps this set of rather obvious points is enough to explain most of global biodiversity patterns? The Nakazawa Biotropica paper is a first step in many that will be required to develop a synthetic answer to this question.
Andrew Peterson, University of Kansas
Hubbell, S. T. 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press, Princeton, N.J.
Nakazawa, Y. 2013. Niche breadth, environmental landscape, and physical barriers: Their importance as determinants of species distributions. Biological Journal of the Linnean Society 108:241-250.
Peterson, A. T. 2011. Ecological niche conservatism: A time-structured review of evidence. Journal of Biogeography 38:817-827.
Pianka, E. R. 1966. Latitudinal gradients in species diversity: A review of concepts. American Naturalist 100:33-46.
Rangel, T. F. L. V. B., J. A. F. Diniz-Filho, and R. K. Colwell. 2007. Species richness and evolutionary niche dynamics: A spatial pattern-oriented simulation experiment. American Naturalist 170:602-616.