Species-area relationships as indicators of human impact on fish communities

Background/Question/Methods

Species-area relationships are well-known macro-ecological properties often used as indicators of human impact on natural communities. The structural forms of these relationships depend on ecological properties of the sampled communities, as well as on the statistical nature of the data used to construct these functions; aspects that are not often easy to disentangle. Bottom trawl surveys are used worldwide to monitor the status of marine communities. Such surveys generate biodiversity datasets across a broad range of spatial and temporal scales. Hence they offer great opportunities for testing macro-ecological functions designed to characterize natural communities and their response to human impact. Here we analyze a unique dataset of historical trawl surveys carried out in southeastern Australia and spanning the entire period of commercial exploitation. Using data on fish, we built species accumulation curves along spatial and temporal gradients of community exploitation. We tested different models commonly used to describe species-area relationships and used the slope (z) of the log-linear logarithmic function to characterize community structure. Computer-based simulations were used to tease apart the statistical and biological properties of the derived species-area relationships.

Results/Conclusions

Results showed that z increased as the cumulative period of fishing increased. Increases in z were marked on the continental shelf and slope of New South Wales, and on the upper slope of Tasmania, where trawling has been exploiting fish communities since its development. Conversely, z remained constant on the continental shelf of Tasmania, an area characterized by low historical and current level of trawling activity. Simulations showed that a range of factors influence z for a given community such as: species richness, absolute numbers of individuals, community evenness, intra- and interspecific aggregation of individuals, and surveys spatial extent. Our results indicate that southeastern Australian demersal communities have structurally changed over the period of exploitation. However, our patterns are in contrast with the findings of similar studies on more exploited regions of the world, where z decreases with increasing level of community depletion. By analyzing a unique dataset of trawl surveys monitoring fish communities of southeastern Australia from the onset of commercial exploitation, we detected stages of community change missed in studies unable to consider the entire history of exploitation, as exploitation begun prior to data being available.