Investigating the Small Island Effect

Image Credit: Dmitry Teslya, CC BY 2.0, Image Cropped

Species-area relationships on small islands differ among plant growth forms (2020) Schrader et al., Global Ecology and Biogeography, https://doi.org/10.1111/geb.13056

The Crux

We’ve talked a lot about Island Biogeography Theory (IBT) in the last couple of weeks. One of the key tenets, established way back in the 60s, is that as an island’s area decreases, its species richness tends to as well. Yet since IBT was conceptualised, there have been a number of amendments made to it. The Small Island Effect (SIE) is one of them.

SIE essentially means that below a certain threshold (called a ‘breakpoint’), species stop obeying that species richness to area relationship. This week’s researchers wanted to test whether that breakpoint was different between species groups, and whether the species area relationship changed below that breakpoint, or simply disappeared.

What They Did

The authors managed to acquire data from 700 islands on 17 different archipelagoes around the world, narrowed down from a much larger dataset to ensure that all archipelagoes included at least ten islands. They looked at the species richness of three major plant groups: herbs, shrubs and trees.

The species richness of these different groups was then modelled against island area. Two different models were used, one which accounted for a breakpoint below which the species area relationship simply disappeared, and one which accounted for a breakpoint below which the species area relationship changed (perhaps the relationship became stronger or weaker). Whichever model fit the data best was used to determine the nature of the breakpoint.

They then modelled the species richness of the islands below the breakpoint against several different environmental variables to see what was affecting the species richness of those islands.

Did You Know: Island Communities

Islands tend to have lower species richness that mainland areas, with their isolation filtering out many potential inhabitants. Because of this filtering, species that occur at low abundances on the mainland will often occur in much higher abundances on islands, as they are released from competition or predation by other species. Naturally, birds and bats occur at much higher densities on islands, whilst there are generally fewer amphibian species.

What They Found

The SIE for all the groups combined was present in 71% of all the archipelagoes. For herbs, it was also present in 71% of archipelagoes, and for 59% and 53% for shrubs and trees. The breakpoint occurred at larger areas for trees and shrubs than for herbs (ie. the island area below which the species-area relationship (SAR) changed or disappeared was much lower for herbs).

Below the breakpoint, the SAR simply disappeared for trees, with island area no longer affecting species richness. However for herbs and shrubs, the relationship was maintained below the breakpoint, but it was much weaker.

Archipelagoes with more isolated islands had much larger breakpoints. Species richness on islands which fell below the breakpoints were affected by different factors, however island perimeter and shape complexity, which are thought to increase species richness, didn’t in this case.

So What?

Predicting species richness and community dynamics on islands is of keen interest to conservationists, as we saw two weeks ago. Identifying a widespread phenomena like this is a valuable addition to island biogeography, showing that we need to consider species growth forms and other traits when making predictions about island communities.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who is currently suffering from conference fatigue and just had a coffee so bad it nearly made him puke. You can read more about his research on his Ecology for the Masses profile here, and follow him on Twitter here.

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