The Impact of Fish Exclusion on the Abundance and Species Composition of Larval Odonates: Results of Short‐Term Experiments in a North Carolina Farm Pond

Under natural conditions in a man—made North Carolina farm pond, a single small species, Perithemis tenera, numerically dominates a littoral assemblage of larval dragonflies. Censuses of exuvia of larvae metamorphosing from the pond in three successive years demonstrated a consistent negative correlation between numerical dominance in the assemblage and species—specific size in the final larval instar. This pattern suggested that size—dependent processes, such as vertebrate predation, might structure the odonate assemblage. Exclusion of vertebrate predators from patches of emergent vegetation by screen exclosures for 3 mo increased total abundances of larval odonates by an order of magnitude. Dominance shifted from small species to species of intermediate size, especially Pachydiplax longipennis, where fish were excluded. The largest species remained rare and were unaffected by fish exclusion. Four odonate species were facultatively multivoltine, completing larval development within 1—2 mo after colonization of the fish exclosures. Fish exclusion also increased the abundance of large microcrustacea, especially the cladoceran Simocephalus serrulatus. large microcrustacea increased in abundance despite a corresponding increase in the abundance of their macroinvertebrate predators, where fish were excluded. The usual dominance of small Perithemis in this assemblage was a probable consequence of differentially heavy predation by fish on moderate—size species, and the unexplained failure of large species to recruit well under experimental or natural conditions. These results support the general importance of predation in structuring freshwater communities and document an important mode of population regulation operating during the aquatic larval phase of the odonate complex life cycle.