The effect of leaf litter species on vernal pool invertebrates and communities
Vernal pools form during the spring when snowmelt and rainwater gather in depressions in the ground. These systems are isolated from streams and therefore have isolated nutrient cycles. Decaying leaf litter replenishes nutrient levels in these wetlands for use by other organisms. Fallen leaf litter also is both a shelter and a food source for many invertebrates. Differences in leaf chemistry (phytochemicals, lignin content) among plant species alters nutritional value of detritus to invertebrates. In this experiment, invertebrate communities on leaf litter from three tree species were compared to determine whether leaf species affected invertebrate colonization rates or densities. Eastern cottonwood (Populus deltoides), Red maple (Acer rubrum), and Pin oak (Querqus palustris) were chosen to reflect common leaf litter types found in vernal pools. We hypothesized that red maple leaves would have the most diverse and abundant community of invertebrates because they have less tannins and decompose faster. We collected leaf litter bags 2, 7, 14, and 30 days after deployment, measured remaining leaf dry weight, and identified invertebrates to family. Nonmetric Multidimensional Scaling, PermANOVA, and Indicator Species Analysis was used to assess community patterns.
Our results show that litter decomposition was fastest in red maple, then cottonwood, and slowest in oak litter. On all sample dates, the red maple leaf litter remaining was significantly lower than pin oak and cottonwood. Leaf litter in litter bags were quickly colonized by detritivorous invertebrates, but there were relatively few taxa during the study. Aquatic Asellidae (Crustacea, Isopoda) were the most abundant invertebrate taxa in all leaf species, accounting for 56% of the total invertebrate individuals. The next most abundant taxa were Chironomidae (O. Diptera) midge larvae (26% of total). Other taxa included Planorbidae (Gastropoda) and Sphaeriidae (Bivalvia). Surprisingly, abundance of invertebrate communities did not differ among leaf litter types or dates. However, when leaf types were compared on individual sampling dates, there was a difference among diversity. The invertebrate community in cottonwood leaves was less diverse than either red maple or pin oak. A multivariate NMDS ordination found that cottonwood was different from the other two leaf types. Our data also implies that vernal pools with a canopy of red maple or pin oak trees might have higher secondary productivity than cottonwood dominated forests. In red maple forests, decomposition by invertebrates may quickly release nutrients from leaf litter into vernal pools and promote higher primary productivity.