Community driven alternative density states in lake ecosystems

Background/Question/Methods

Community structure can have important implications for population dynamics. One of the possible dynamical outcomes of community structure is alternative states in population densities under the same environmental conditions.  While theory often predicts this, whether it be transient dynamics or stable equilibruim, caused by community assembly history or ongoing community interactions,  there are few examples in the literature where this has been found in nature. Most examples of alternative states that exist are from laboratory experiments or represent presence/absence of species or functional groups.  A large data-set of fish densities and lake data was collected in Ontario from 2009 to 2012 with the Broad Scale Monitoring Program by the Ontario Ministry of Natural Resources. We performed cluster analyses in fish density along PCA component axes that represented abiotic attributes of the lakes. Cluster analysis was performed to determine if the fish densities showed a consistent change with environmental gradients, or if there were distinct clusters representing alternative states. A second PCA analysis was performed on the fish densities to provide a representation of fish community structure and t-tests were performed to determine whether the fish community differed between clusters.

Results/Conclusions

The first principle component axis from the PCA on lake data represented lake morphology and water clarity, and the second axis represented lake chemistry. The PCA on fish densities split the community into littoral predators (e.g. sunfish, smallmouth bass, and pike) and pelagic predators (e.g. lake trout, cisco, and whitefish). For several fish species, including walleye and smallmouth bass, alternative states were found (2 clusters fit better than one), and in most cases, the fish communities were significantly different between the states. The split of the states was always vertical when the axis represented lake morphology, however when the axis represented lake chemistry, half of the fish densities split horizontally, suggesting that there may be a different reaction in density to gradients in stress (lake chemistry) than to gradients in accessibility (lake morphology).