Is island biogeography a poor fit to shallow Minnesota lakes?

Background/Question/Methods

Classic island biogeography predicts that species richness will be highest in larger, better interconnected patches of habitat. Island biogeography has proven useful in predicting species richness in many habitats and serves as the foundation for many conservation strategies. However, it was recently hypothesized that island biogeography patterns may be reversed in shallow lakes for organisms other than fish because fish favor turbid states with high nutrient levels and low abundance of submerged aquatic plants. Accordingly, species richness of invertebrates, plants, and other organisms would be highest in small isolated lakes. We tested this hypothesis in 2009 by sampling species richness of aquatic invertebrates, submerged aquatic plants, and fish in 104 shallow lakes in five ecoregions of Minnesota, USA.  GIS and air photos were used to measure lake surface area, assess whether lakes were seasonally connected to other aquatic habitat, and to measure downstream distance to other surface waters under extreme floods. We then tested whether species richness of aquatic invertebrates, fish and submerged plants were influenced by lake size, connectivity, and isolation using linear regression and t-tests.

Results/Conclusions

Fish species richness was positively related to lake size (P < 0.001) and connectivity to other aquatic habitats (P < 0.001), and showed a marginally negative relationship with distance to aquatic habitat (P = 0.067). In contrast, species richness of submerged aquatic plants was negatively related to lake size (P = 0.003) and was unrelated to connection (P = 0.671) or distance to other waters (P = 0.222). Similarly, aquatic invertebrate species richness showed no relationship to lake size (P = 0.130), connectivity (P = 0.090), or distance to other aquatic habitat (P = 0.129).  Moreover, biomass of planktivores and benthivores was positively related to lake size (P < 0.001) and was higher in connected lakes (P < 0.001). Our results indicate island biogeography is useful for predicting fish species richness in shallow lakes, but poorly predicts richness of aquatic invertebrates and submerged plants. We suspect this is due to large lake size and connectivity increasing biomass of benthivorous and planktivorous fish, and negative influences of fish mitigating benefits of habitat size and connectivity for aquatic invertebrates and plants. Our results highlight the need for managers to consider food web interactions in efforts to conserve species richness in shallow lakes.