Reciprocal subsidies to arthropod food webs in and around fishless ponds

The movement of allochthonous input from high to low productivity food webs is well documented. However, the importance of allochthonous inputs simultaneously moving between habitats of similar productivity has received much less attention, even though it may be the more common scenario. Here I present the results of a experiment designed to test the importance of reciprocal subsidies of allochthonous prey to arthropod predators (wolf spiders: Lycosidae and dragonfly nymphs: Libellulidae and Aeshnidae) in adjacent aquatic and terrestrial food webs of similar productivity. I used  field enclosures arrayed along the margin of fishless ponds to manipulate both the abundance of aquatic insects subsidies to terrestrial food webs and terrestrial arthropod subsidies to the aquatic food web and then monitored predator distribution, size, diet and reproductive output over a four month period. Enclosures successfully reduced insect subsidies to terrestrial habitats by 50% and subsidies to aquatic habitats by 33%.  Reductions in terrestrial subsidies reduced per capita mass and the proportion of reproductive female wolf spiders. Species level response to the manipulation differed: a highly mobile species (Pardosa milvina) occurred in smaller numbers and size where aquatic insects were reduced. Another species with high moisture requirements (Pirata cantralli) instead showed a trend towards reduced allocation to reproduction. Reductions in aquatic subsidies shifted the diet of dragonfly nymphs toward more aquatic prey, as evidenced by isotopic data, and altered the relative abundance of newts and crayfish and dragonfly larvae, suggesting a shift in intraguild interactions, but did not influence the abundance or mass of dragonfly larvae. These results suggest that reciprocal subsidies between habitats of similar productivity may be important, and that their impacts are likely to depend upon consumer and food web characteristics.