Litter arthropods in forest fragments across an urban-rural gradient: What's driving productivity?

Between 2010 and 2012 we conducted a number of tests and censuses to assess the quality of 30 urban forest fragments as part of the FRAME program (Forest Fragments in Managed Ecosystems). The FRAME was started, in part, to link soil chemistry and Ca-rich invertebrate prey to breeding bird success.  Our measurements also included 428 samples of leaf litter removed from ½ m² of forest floor throughout the sites; each of these were were run through Berlese funnels for 3 d. After all mobile invertebrates had been extracted, the leaf litter was sieved to 1 mm and additional invertebrates, including snails, were removed. We put these data into the context of site covariates including nonnative plant cover, soil chemistry, and the actual volume of litter at each point. We found that invertebrate abundance differed in relation to soil Ca availability. Gastropods and crustaceans were positively associated with soil Ca; in Ca-rich sites, snail abundance was 2.4 time greater. In contrast, the overall abundance of insect taxa was negatively associated with Ca. We hypothesize that insects may be negatively affected by increases in non-native plant cover associated with high soil Ca levels, as the majority of insects sampled were herbivorous species or detritivores.