Species-area relationships and heterogeneity

Loss of habitat is commonly identified as a reduction in area (or patch-size) and its effects are often investigated using species-area relationships to evaluate loss of biodiversity.  In many habitats, however, area alone is not sufficient to explain the decline of populations because natural habitats are rarely homogeneous.  In fact, reductions in diversity of habitats (e.g. destruction of microhabitats) might have even greater effects on the diversity of assemblages.  While these are two well-known attributes of habitats (i.e. area and habitat-diversity), there have been few empirical studies that have attempted to separate their effects by experimental manipulation.

I have done a series of experiments to separate the effects of number, relative proportion and identity of patches of habitat using additive designs.  Predictions were tested using assemblages of marine molluscs colonizing artificial mimics of macro-algal habitats on rocky intertidal platforms.  These experiments to disentangle different attributes of habitat-heterogeneity showed that these are complex and highly interactive.  In particular, not all heterogeneous habitats had the expected numbers of species from a simple species-area relationship.  Instead, the presence of more species in heterogeneous habitats could, to a great extent, be explained by the presence or absence of particular types of sub-habitats and not just by patch-size.

The complexity we found in responses of assemblages to composition of heterogeneous habitats was not predicted and could not have been observed without appropriate manipulations of composition of habitats.  A mechanistic understanding of the associations between species and particular types of habitats is essential for better predictions of species’ responses to loss of natural habitats.

References:

Matias M.G., Underwood, A. J., Hochuli, D. F. & Coleman, R. A. 2011. Habitat identity influences species-area relationships in heterogeneous habitats. Marine Ecology Progress Series, 437, 135-145.

Matias M.G., Underwood A.J, Hochuli, Coleman R.A. 2010. Independent effects of patch-size and structural complexity in diversity of benthic macro-invertebrates. Ecology. 91, 1908–1915.

Matias M.G., Underwood, A. J. & Coleman, R. A. 2010 Effects of structural diversity and identity of patches of habitat on diversity of benthic assemblages. Austral Ecology, 35, 743–751.

Matias M.G., Underwood A.J, Coleman R.A. 2007 Interactions of components of habitats alter composition and variability of assemblages. Journal of Animal Ecology 76, 986–994.

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