Ecology

Neutral theory tested by birds

A continental-scale analysis of habitat and bird distribution in South America provides the latest challenge for neutral theory — a controversial idea in ecology about what determines the make-up of communities.

How do different species end up living together in communities? Do they coexist only when each finds a different niche, or simply when they happen to disperse to the same habitable region? Debate over these questions intensified not long ago with the introduction of ‘neutral theory’1, a stochastic theory of community properties whose predictions have proven stubbornly robust, despite its disregard of the niches that many ecologists hold dear. Writing in Proceedings of the National Academy of Sciences, however, Graves and Rahbek2 point out continental-scale patterns in the bird communities of South America that neutral theory may not be able to explain.

The dominant view in ecology is that species live together in communities only when they differ from one another. Species competing for the same nutrient or food source cannot coexist because one species will always be more efficient than the others and will quickly drive the rest to extinction3. Species that coexist must differ from one another in the resource they use most efficiently or in the environmental conditions to which they are best adapted — that is, they must have different niches. This view is often called ‘niche-assembly’.

The contrary viewpoint is that communities are primarily shaped by historical accidents that influence where species disperse (a beetle floating to a distant island on flotsam, for example, or the uplift of a mountain range that blocks the flight of seeds between nearby forests). This view has deeper roots in evolutionary biology, where history is at centre stage, than in ecology, which concentrates on short-term interactions between species. The idea behind it is that, rather than being quickly out-competed, species that are less efficient at using a resource evolve to be as efficient as their competitors. The main criterion for coexistence is dispersal to the same habitable region. This view is sometimes called ‘dispersal-assembly’.

The neutral theory tested by Graves and Rahbek2 is the modern synthesis of dispersal-assembly into a mathematical framework. It models a community as a finite collection of individuals that have identical probabilities of reproduction, death and dispersal. This yields predictions for community properties in terms of parameters that govern the stochastic changes that the community undergoes — for example the immigration or dispersal rate, the speciation rate, and the size of the community. Despite its simplicity, neutral theory's predictions have proven robust. Claims that it has been falsified4 have been followed by persuasive counter-arguments5.

Graves and Rahbek mount a new line of attack on neutral theory by testing it at the scale of an entire continent. Armed with an impressive bird-distribution database amassed from the collections of over 30 museums in 20 countries, as well as a global land-cover map created from satellite data, they quantify the correlation between the distribution of habitat (or land-cover type) and the distribution of birds across South America at the resolution of 1° latitude by 1° longitude grid cells.

Put simply, Graves and Rahbek find that birds in more widespread habitats tend to be more wide-ranging. In particular, birds present in the lowland regions of the continent to the east of the Andes, where elevation changes slowly and habitats are widespread, are on average an order of magnitude more wide-ranging than those on the western side of the Andes, where topographic relief is at a maximum and habitat changes quickly in space (Fig. 1a). Furthermore, as one looks outwards from various locations on the continent, the change in bird-community composition is asymmetric and mimics the underlying changes in habitat (Fig. 1b).

Figure 1: Summary of Graves and Rahbek's results2.
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NATL ACAD. SCI.

a, Bird species in the lowland regions of South America, where habitat types are more widespread, are more wide-ranging than those on the western edge of the continent, where the Andes create quick changes in elevation and habitat type. Colours indicate median range-size in units of 1° latitude × 1° longitude cells. b, The composition of bird communities changes asymmetrically as one looks outwards from a location in the Amazon basin (here 1–2° S, 69–70° W), mimicking the underlying distribution of riverside habitat. Colours indicate the number of species in common with the focal location whose coordinates are listed. Neutral theory may be consistent with a but Graves and Rahbek are correct that it cannot predict the ecological importance of habitat evident in b. The theory may still be relevant at smaller scales, however, and species differences in habitat requirements can evolve under dispersal-assembly on a heterogeneous landscape. (Figures reproduced from ref. 2.)

Graves and Rahbek conclude that there is a strong causal influence of birds' habitat requirements on their spatial distribution across South America. They argue that this influence contradicts neutral theory, which ignores species differences in habitat requirements.

It is worth pointing out that the first of Graves and Rahbek's results, a correlation between habitat extent and species' spatial extents, could arise from a source other than habitat influence, a source that is instead consistent with neutral theory. The Andes act as a physical barrier to the dispersal of both birds and the flora that cover the landscape. This barrier, combined with the very different land areas available to dispersing species to the west and east of it, could alone explain the observed correlation.

But there are no dispersal barriers to explain the relationship between community composition and the distribution of riverside habitat evident in Fig. 1b. And further study of the bird and land-cover databases would surely yield quantitative evidence of the influence of habitat. Practically all ecologists agree that species have habitat requirements that limit where they can live — tropical trees cannot survive on the Arctic tundra. Graves and Rahbek are correct that neutral theory cannot predict the resulting influence of habitat on community composition because it ignores species differences entirely.

But does the importance of habitat disagree with the letter or the intent of neutral theory? In other words, does it contradict the overall principle of dispersal-assembly?

Not necessarily. The idea of dispersal-assembly is not that differences between species do not exist — they are the inevitable result of disparate evolutionary histories. Rather, the idea is that species similarities, not their differences, lead them to find the same region habitable and to coexist. Neutral theory applies only in that realm of intermingling, where species are similar.

Habitat influence on species' distributions at any scale does indicate a role for niche-assembly, which has implications for ecological dynamics. The species that differ in the habitat they do best in cannot out-compete each other. Their differences allow them to coexist stably in the landscape.

However, unless habitat and species change in lock-step, habitat effects do not rule out a simultaneous role for dispersal-assembly. As Graves and Rahbek acknowledge, their observations limit only the spatial scale and groups of species within which neutral theory's unstable ecological dynamics may apply. Furthermore, differences between species in habitat requirements can arise from sources that are consistent with dispersal-assembly in a heterogeneous landscape over evolutionary timescales, such as from local selection for capabilities on a par with those of competitors. Selection for the avoidance of competition (or niche-assembly) may not be the evolutionary origin of these differences.

More empirical work is needed to distinguish between niche-assembly and dispersal-assembly on both ecological and evolutionary timescales. We also need to understand the implications of this distinction, and more refined ones, for judging the robustness and resilience of communities in the face of anthropogenic change.

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Ostling, A. Neutral theory tested by birds. Nature 436, 635–636 (2005). https://doi.org/10.1038/436635a

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