Island biodiversity theory doesn’t apply to metaphorical islands

The equilibrium theory of island biogeography is three things. One, it’s a mouthful of jargon. Two, it’s long been one of the theoretical pillars of conservation science. Three, it might be completely useless, at least for those islands not surrounded by water.

That’s according to new research published last week in Nature. To understand just what the researchers, led by Stanford conservation biologist Chase D. Mendenhall, mean by all that, we’ll address each of those three things in order.

Thing One: The Jargon.
Just what is the equilibrium theory of island biogeography? It’s an idea first described in the late 1960s by researchers Robert MacArthur and E.O. Wilson to describe species diversity in islands, and how the isolated terrestrial ecosystems found on islands evolve over time. The two main processes that alter an island’s diversity are immigration and extinction. Immigration is the colonization of new organisms to an island from elsewhere: another island or the mainland. That has the potential to increase diversity. If a species goes extinct, diversity decreases. Smaller islands with less space available increases the chances of extinction, and the reverse is also true.

To prove that this is how island biodiversity worked in the real world, Wilson turned to some mangrove islands in the Florida Keys. The islands were fumigated to completely eradicate the arthropods; all the insects, spiders, and crustaceans were wiped out. Then he waited to see how the island ecosystems would recover. Within a year, all the islands had been recolonized. As expected, those islands closer to the mainland rebounded faster than those farther away.

Thing Two: The Pillar.
Like the arthropods in the mangrove swamps, the theory quickly colonized the field of evolutionary biology. The driving idea – that there was a relationship between land area and species diversity – led to the implementation of wildlife corridors, national parks, nature reserves, and more. According to the theory, national parks that were too small for some species would “relax towards equilibrium,” or lose species. That led some to advocate for fewer, larger reserves. Others pointed out that the idea, while useful, was not comprehensive, and that habitat diversity also mattered, not just total land surface area. Still, the idea took hold and proliferated.

While originally used to describe island communities in the actual sense of the word (masses of land surrounded by water) the theory has been used to predict biodiversity in all manner of metaphorical islands. Sky islands are mountaintops that are different enough from the surrounding areas to host their own unique ecosystems. Isolated springs are islands within the desert, supporting their own biological communities. City parks may be islands of green within our concrete jungle of paved roads and highways, homes and businesses. And in the increasingly deforested tropics, remaining patches of forest may be islands in a sea of agricultural croplands. Call them pseudo-islands. Their island-ness is but a metaphor.

While conservation science continues to innovate, the theoretical pillars at its nucleus have not changed much in the last half century. It isn’t that those foundational theories are never useful anymore, but that new knowledge has demonstrated that their utility has become much more limited. “Theories from simple island ecosystems are still used in ways that incorrectly estimate rates of species extinction and distort projections of ecological risk in human-dominated ecosystems,” Mendenhall writes, “further exhausting an environmental, apocalyptic narrative.”

It turns out that the ominous rhetoric surrounding the Anthropocene may be just a hair alarmist. “Confusion and controversy surrounding the biodiversity crises are substantially related to the overextension of the theory of island biogeography to human-dominated ecosystems, exacerbating a rift between conservation theory and real-world practices,” he adds. In large part that’s because we now know that anthropogenic systems, from cities to farmland, are not the biological wastelands they were once feared to be.

Thing Three: New Insights.
To demonstrate that not all pseudo-islands are islands, Mendenhall and his colleagues turned to the bats of two different “island” formations. They compared bat biodiversity in a Panamanian island ecosystem, in which the islands were of the traditional sort, to those in the Costa Rican countryside, in which the pseudo-islands were patches of forest surrounded by coffee plantations. The two sites were picked because the ecosystems are of similar ages, they are nearby one another, they have homologous bat diversity from an evolutionary perspective, and they have historically hosted a similar diversity of bat species.

bat ecosystems

If the Costa Rican pseudo-islands really aren’t properly described by the traditional island biogeography theory, then the researchers expected three patterns. First, that the Costa Rican forest fragments would support more species and have lower rates of extinction because the agricultural landscape itself could aid in promoting biodiversity, even if less efficiently than the original habitat it replaced.

Second, that the species in the pseudo-islands would be more evenly distributed than species in the true islands, also because the human-made croplands could support individual bats while water can’t.

Third, that some Costa Rican species we think of as forest dwellers would expand into the surrounding agricultural habitat, suggesting that some species respond to habitat change in a more adaptive manner.

Consistent with the first hypothesis, as forest area declined, the rate of species loss was “substantially and significantly” higher for the true island ecosystem than for the fragmented forest ecosystem.

The second hypothesis was also supported by the data. The relative abundance of each individual bat species was more likely to be uneven in the Panamanian island system, with some species having high abundance and others having low abundance on each individual island. On the other hand, individual species abundance within the Costa Rican forest fragments was far more regular, because the boundaries of the reserves were far more permeable. Life on a plantation may be sub-optimal for a bat, but they can make a life there; they can’t make a life at sea.

When it came to the third hypothesis – that species in the forest fragments would have an opportunity to respond to habitat changes in a way that those in the true islands don’t – the data were striking. Of thirty bat species living in the Costa Rican countryside, fourteen were actually more abundant outside of the reserves. Nearly half of the bat species there appear to have adapted efficiently to habitat changes.

The results couldn’t be more straightforward. The researchers write that they “found strong evidence that bat biodiversity patterns follow predictions of island biogeographic theory on true islands – both natural and human-made – but countryside ecosystems are more complex.”

Taken together, the findings reconfirm the equilibrium theory of island biogeography as it was originally intended: to describe islands. But when it comes to metaphorical islands, the theory falls considerably short. That’s because when humans dominate a landscape, they aren’t simply destroying a habitat, they’re changing the overall composition of the land. They’re replacing one habitat with another, and even if the new land-use patchwork mosaic is sometimes sub-optimal for the species with which we share that landscape, some critters have demonstrated remarkable adaptability.

This research shows that the effects of human activity on biodiversity aren’t intuitive or obvious. As Mendenhall and his colleagues conclude, “approaches for estimating biodiversity that explicitly account for human-made habitats are critical for predicting biodiversity change and avoiding its collapse in the agricultural landscapes that will increasingly dominate the Earth.” – Jason G. Goldman | 23 April 2014

Source: Mendenhall C.D., Karp D.S., Meyer C.F.J., Hadly E.A. & Daily G.C. (2014). Predicting biodiversity change and averting collapse in agricultural landscapes, Nature, DOI:

Header image: The Orange nectar bat, L. robusta, a species that has begun to colonize the coffee plantations of Costa Rica, via Hans Hillewaert/Wikimedia Commons. Map graphic via Mendenhall et al. (2014).