Unequal flow of resource subsidies across aquatic-terrestrial ecosystems

Background/Question/Methods

Habitats are open systems linked by the flow of nutrients, detritus, and organisms, all of which have the potential to influence ecosystem functioning in the recipient system. Yet few studies have comprehensively quantified the resource subsidies across discrete habitats. Here we examine energy flow between mixed deciduous forests and temporary wetlands. We evaluated the influence of hydrology and ecosystem size on the bidirectional flow of energy in four vernal ponds by quantifying the abundance and biomass of emerging insects and amphibians, and influx of leaf litter. Aquatic emerging insects were collected with emergence traps from May through August, metamorphosing amphibians collected from pitfall traps from June through August (or until pond dried), and leaf litter was collected using litter traps from August through November 2010. All individuals collected were counted and identified to family for insects and to species for amphibians. We measured insect body length and used published length-mass regressions to calculate dry mass (DM). We took length (snout-to-vent) and weight data from all amphibian metamorphs and established length-mass regressions for each species to estimate dry weight for all individuals. Using these data, we tested the relationships between insect, amphibian, and litter dry mass to pond size and hydroperiod.

Results/Conclusions

Insect emergences were fairly continuous across a four-month period with the exception of a single peak during July. Alternatively amphibians had two major emergence pulses that were responsible for 46% of the total abundance. Interestingly, both amphibian pulses (totaling 544 individuals, 42 g DM) and peak insect emergence (365 individuals, 0.24 g DM) all occurred within a one month window from late June to late July. Peak litter fall occurred late September and early October (275 g DM). There was no relationship between insect emergent biomass and pond size (P > 0.1) or hydroperiod (P > 0.1). A strong but not significant relationship was found between amphibian subsidy and pond area (R² = 0.56, P > 0.1) and hydroperiod (R² = 0.52, P > 0.1). These data suggest litter inputs are substantially higher than the combined export of insects and amphibians in small woodland ponds, yielding a surplus of resources to the aquatic food web. Cross-habitat energy flow is not homogenously distributed across a gradient of pond hydroperiod and size, reinforcing the perspective that a landscape with a mosaic of wetlands varying in hydroperiod and size is necessary for optimal aquatic conservation and productivity.