Spatial heterogeneity can create biodiversity – all you need are species with different dispersal abilities

ARC fellow and qaecologist Mike Bode had a paper recently published in PNAS. It’s about how different dispersal abilities allow reef fish to coexist. Read on for Mike’s summary of this research.

The Great Barrier Reef contains more than 1,500 species of reef fish (which seems a bit greedy, given the North Sea has about 230 species in total). Ecologists aren’t really sure what processes create and maintain this staggering diversity, but they’ve proposed an almost equally varied set of explanations. Our search has recently turned to the role played by the process of dispersal – the exchange of juvenile fish larvae between reefs. This paper identifies a novel theory of coexistence: species that differ only in their dispersal abilities can coexist, if habitat patches are distributed heterogeneously.

It works like this. Imagine a simple metacommunity of three reefs:

Reef metacommunity

where the distance between the first two reefs (d1) is different to the distance between the second and third reef (d2). Although this is a caricature of a real coral reef system, it does share one fundamental property: some regions contain dense aggregations of reefs, while other regions have sparse reef distributions. When species have different dispersal abilities, this spatial heterogeneity means that different species are suited to particular regions. For example, a species that disperses effectively over short distances will be most successful around reefs 1 and 2. In contrast, a species with longer-distance dispersal abilities will disperse more effectively around reefs 2 and 3. If the distribution of patches varies in different locations, species with particular dispersal abilities will reproduce more effectively in some regions than in others. This simple fact creates a robust and general mechanism for coexistence. It turns out that if you scale up to a much larger system like the Great Barrier Reef, then a large number of species can coexist, even if they differ only in the average time they spend dispersing as larvae (and different fish species do spend different amounts of time dispersing). Take the reefs just south of Cairns (excuse the terrible colour scheme). If we allow species with different dispersal distances to compete in this system, eventually the fish community settles down to a stable pattern of coexistence. Each species finds a set of reefs that corresponds to their dispersal abilities, and they keep almost all the other individuals out. I’ve colour coded each reef to reflect the species that dominates the population there. It’s important to note, however, that although we think we understand in general how a species might keep out other species, we don’t yet know why the purple species happens to like those reefs in particular. The distribution of reefs is altogether too complex for our usual techniques to easily apply.

Distribution of Reefs

Distribution of Reefs south of Carins

One final point: this type of dispersal-mediated coexistence is most interesting because the mechanism is more or less invisible. Normally, to understand why we find certain species in particular places, we look for relationships between the species’ characteristics and the habitat we find them in. So, we tend to find polar bears at high latitudes, and brown bears at lower latitudes. We might argue then that a white coat makes a species a superior hunter in arctic environments, but is a liability in regions with more vegetation and less frequent snow. With fish, this doesn’t seem to be true. If we look at how different species compete for resources, they’re often indistinguishable. In the figure above, if you tried to work out what’s the difference between a purple reef and a red reef, you’d be eternally frustrated. And that’s how many coral reef fish specialists have felt when trying to identify niche differences between species: frustrated. For example, Morgan Pratchett (2005, Marine Biology 148:373-382) showed that 11 different species of coexisting butterfly fish pretty much target the same resources. It could be that the important differences and trade-offs that allow species to coexist are only apparent when they’re dispersing as juveniles, which is just when it’s hardest to observe them.

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