Connectivity and metacommunity dynamics: insights from a lake network in Michigan, USA

Background/Question/Methods

Metacommunity theory stipulates that connectivity increases species diversity and similarity among species assemblages by increasing migration rates and reducing extinction rates (i.e., rescue effects).  We tested these predictions by examining fish communities in a network of coastal lakes directly connected to one another via Lake Michigan.  Our central question was: do connected lakes show high species diversity and community similarity relative to inland lakes?  To answer this question, we used fyke nets to sample littoral fish assemblages in six coastal lakes during 2009-2011 and compared their diversity and similarity to those reported for a set of inland lakes lacking connectivity.  Species diversity in both sets of lakes was evaluated using the Shannon-Weiner diversity index corrected for unequal sample sizes (i.e., rarefaction).  We used Morisita’s Index of similarity to assess community similarity and non-metric multidimensional scaling to visually compare coastal lake communities.  We also used canonical correspondence analysis to determine the influence of environmental factors on community structure.

Results/Conclusions

Rarified species diversity in our six coastal lakes ranged from 17.5 to 25.0.  We did not find a relationship between diversity and lake size.  The two largest lakes had diversity levels that were similar to inland lakes, while the three smallest lakes had 0.5-2x higher diversity than similarly-sized inland lakes.  We attribute the lack of a species-area relationship and high diversity in the small lakes to connectivity with Lake Michigan.  Communities were similar in four of the six coastal lakes.  The two distinct lakes were nearly 5x as turbid as the four similar lakes, suggesting that environmental factors may have contributed to differences in species composition.  By comparison, inland lakes often had distinct communities, an effect typically attributed to lake size, depth, predation, and competition.  Our findings in coastal lakes may be related to rescue effects from nearby source communities (i.e., Lake Michigan and/or other coastal lakes) to small lakes where predation and competition would otherwise reduce diversity.  We argue that the lack of a species-area relationship and the high diversity observed in small coastal lakes illustrate that connectivity can alter metacommunity dynamics by alleviating the influence of factors that control diversity in poorly connected systems.