Predators and connected habitats (2): are ecosystems in round lakes more stable?

Posted on March 5, 2011

Recent ecosystem analyses suggest that the stability of natural communities depends critically on the coupling of different compartments within the food web.  In this context, mobile consumers play a key role by transferring energy and nutrients, and it’s noteworthy that dramatic drops in community diversity and stability have often been recorded following the loss of top predators, for example as a result of overexploitation.  These findings have led to investigations into ecosystem linkages,  such as those between the near-shore and deep-water habitats of lakes. The results of several studies have supported the prediction that habitat coupling should be strongest in small lakes, where the larger perimeter : area ratio means that predators based in deep water have access to relatively extensive areas of inshore habitat.  However, the amount of edge habitat is dependent on lake shape as well as on lake size, and the effect of lake shape (specifically, the degree of shoreline convolution) on habitat coupling has now been examined.  Canadian researchers hypothesised that habitat coupling should be greater in convoluted lakes because of the increase in shoreline edge.  Based inAlgonquin Provincial Park,Ontario, the study focussed on seven lakes with similar glacial history, productivity, oxygen status and species assemblages.  Tissue samples were taken from fish and invertebrates and subjected to stable isotope analysis. Because food items from near-shore and deep-water habitats had different carbon signatures, it was possible to assess the importance of prey from these two areas in the diets of predatory trout.  Although the relative area of edge habitat was significantly higher in convoluted lakes, shallow-water food sources made up only 11% of lake trout diets, compared with 24% in circular lakes. Therefore the “shoreline space” hypothesis wasn’t supported.  The results were better explained by the fact that in convoluted lakes trout predators based in deep water have less access to shoreline habitats.  Access is more restricted because in summer water temperatures near the shallow lake margins are above the preferred range for cold-water trout.  It seems that, in these Canadian systems at least, habitat coupling is stronger in round lakes, and the extent of coupling is controlled by the accessibility, rather than the amount, of near-shore production. 

Reference:  Dolson, R., McCann, K., Rooney, N. & Ridgway, M.  2009.  Lake morphometry predicts the degree of habitat coupling by a mobile predator. Oikos 118, 1230-1238.