Bigger is Not Always Better in Habitat Conservation

Whilst Island Biogeography Theory originally led many to believe that larger, more connected patches of habitat are more important for species conservation, new research suggests that overlooking smaller patches could be dangerous (Image Credit: LuxTonnerre, CC BY 2.0, Image Cropped)

Global synthesis of conservation studies reveals the importance of small habitat patches for biodiversity (2019) Wintle et al., PNAS, https://doi.org/10.1073/pnas.1813051115

The Crux

Human land use over the past millenia has divided species habitats into smaller and smaller patches – a practice which often leaves conservationists with the tough choice of which remaining patches they should focus their efforts on. Traditional practice has seen the prioritisation of large patches that are well connected to other, with this preference often meaning that smaller more isolated patches are neglected, and often cleared.

This week’s paper authors wanted to check whether this was really the best way of doing things, by looking at the relative conservation value of a variety of habitat patches.

What They Did

Using 31 conservation studies from 28 different countries, the researchers ranked different habitat patches by their conservation value, using the number of species that were thought to be present in that cell as their metric. They determined the cell values by removing the least valuable first, then recalculating cell conservation values according to how many species were still remaining, with the most valuable cells being removed last.

They then analysed the spatial characteristics of high value patches by looking at four spatial parameters, including patch shape complexity, patch border complexity, patch size and relative isolation.

Did You Know: Island Biogeography Theory

The thought that large, well-connected patches are likely to be more biodiverse comes from Island Biogeography Theory (IBT), a revolutionary model created in the 1970s. It suggested that species richness of an island increased with its size, and decreased with its isolation. Whilst it was very influential, it was very simplistic, and many researchers, including one of the original authors, have gone onto find many exceptions and adjustments to IBT. However, it still remains one of the first examples of a pattern that is present on a global scale.

What They Found

The study found that small, isolated patches with complex shapes tend to have higher conservation value than equally small, simply shaped patches that are close to other intact patches. However, conservation value can decline if the perimeter of the border is simpler. Simpler borders often occur as a product of a road or deliberate clearing nearby.

Problems

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Species like the Western Ringtail Possum are dependent on small fragments of habitat along Australia’s west coast (Image Credit: S J Bennett, CC BY 2.0)

The researchers admit that a major problem here is the poor explanatory power of the variables they have to work with. Often when working the statistical modelling, we may get a few variables which are significant, but that doesn’t mean that the overall model explains our data very well. The good news here is that there are a lot more studies out there that can be integrated into this one, and can potentially create a better picture of which patches are more valuable.

So What?

This paper needs to be picked up by policy-makers quickly (it came out early last year, so let’s hope it already has). Some of the countries mentioned in this study (in particular Australia and the USA) have a less than stellar record when it comes to land clearance, and a number of species that depend on smaller patches. The automatic assumption that larger, better-connected patches are first priority needs to be put to rest.

Sam Perrin is a freshwater ecologist currently completing his PhD at the Norwegian University of Science and Technology who has had way too much coffee today to make up for a lack of sleep. 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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