Contemporary ecological landscape planning is often based on the assumption that small isolated habitat patches sustain relatively few species. Here, we suggest that for shallow lakes and ponds, the opposite can be true for some groups of organisms. Fish communities tend to be poor or even absent in small isolated lakes. However, submerged vegetation is often more abundant in such waterbodies. As a consequence of low fish biomass and high vegetation abundance, the richness of aquatic birds, plants, amphibians and invertebrates is often relatively high in small, shallow, isolated lakes. Although the rarity of fish is in line with expectations from the ruling paradigms about effects of habitat fragmentation in landscape ecology, the relative richness of various other groups of organisms in small ponds is opposite to these expectations. The case of shallow lakes illustrates that incorporating ecological interactions is essential to understanding the potential effects of habitat fragmentation. Single‐species meta‐population approaches may be misleading if ecological interactions are strong. A meta‐community approach that explicitly incorporates biotic interactions, also those involving different trophic levels, is needed. Our diagnosis suggests that connection of isolated habitat fragments may in some cases reduce, rather than enhance, landscape‐level biodiversity, and implies that biodiversity at the regional level will be maximized if the local habitat patches vary widely in size and degree of connectivity.

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We examined the relationships between invertebrate community structure and a number of biotic and abiotic variables in 19 semipermanent prairie wetlands. We tested whether aquatic invertebrate communities differed (i) between wetlands with and without fathead minnows (Pimephales promelas) and (ii) according to drainage history of wetlands (restored versus natural, nondrained). We also evaluated influences of other environmental variables on invertebrate community structure, including abundance of aquatic macrophytes and amphibians and wetland depth and surface area. Invertebrate communities differed significantly between wetlands with and without fathead minnows, largely due to lower relative abundance of 19 invertebrate taxa (of 32 taxa analyzed) in wetlands with fathead minnows. In contrast, we found no differences in these taxa between natural and restored wetlands. Canonical correspondence analysis indicated that invertebrate community structure was affected by abundance of fathead minnows, abundance of aquatic macrophytes, and wetland depth, with fathead minnows the most influential variable measured. Many studies have documented the effects of fish predation on zooplankton communities, but our results show that fathead minnows in prairie wetlands affect a large number of diverse invertebrate taxa. The presence of these fish results in an invertebrate community distinctly different from that found in fishless wetlands.

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