Quality vs. quantity in protecting habitat for birds

A new study from South Africa touches on an interesting conservation question about whether we should place greater importance on quality or quantity when it comes to protecting habitat to conserve biodiversity.

The theory of island biogeography predicts that larger and more connected habitat fragments will have greater biodiversity - with all other things being equal. This theory has strongly influenced conservationists leading many to place greater importance on the size and spatial configuration of conserved areas over their inherent quality.

However, as Simon Dures and Graeme Cumming have found, in certain cases, focusing on protecting a smaller amount of high-quality areas may lead to better outcomes for biodiversity.

The researchers looked at avian diversity in the Cape Flats Sand Fynbos in South Africa. A critically endangered biodiversity hotspot, this scrubland habitat comprises just 77 square kilometers of fragmented patches along the outskirts of Cape Town. 

The researchers conducted avian surveys and measured a number of environmental conditions at remnant patches. "We expected to find a strong influence of habitat amount and arrangement across a fragmented urban–rural gradient," the study authors write in their research article published in the journal Biological Conservation.

Instead they found that patch size and connectivity had no relationship with species richness. Meanwhile, the density of the highly invasive blue wattle tree (Acacia saligna) had a negative influence on species richness and was a more important factor in explaining the avian community composition.

The blue wattle tree homogenizes the landscape by crowding out native species and forming thick monocultures.  Two other habitat quality variables - vegetation structure and nearby urban density - were also correlated with avian diversity.

The researchers hypothesize that bird species richness may have been higher with greater urban density because the blue wattle tree, which requires fire to germinate, is much less common near the city where fire is more heavily controlled. 

The study findings challenge conventional wisdom about the universal importance of habitat size and connectivity in conservation - particularly in settings more affected by urbanization and other severe habitat disturbances.

However, the study does have some shortcomings - specifically in that its findings rest largely on the measure of species richness, which may not always be the best metric of the health of biodiversity. Despite this limitation, the study merits close attention from those working on conservation in urbanized settings. The authors write,

"Urban conservation biologists and urban biodiversity planners need to focus their efforts away from the size, shape and location of remnant intact areas of biodiversity, and instead focus on identifying those factors of the urban landscape that allow the persistence of species within a diverse matrix."   

--Reviewed by Rob Goldstein

Dures, S., & Cumming, G. (2010). The confounding influence of homogenising invasive species in a globally endangered and largely urban biome: Does habitat quality dominate avian biodiversity? Biological Conservation, 143 (3), 768-777 DOI: 10.1016/j.biocon.2009.12.019