Invasive Allochthonous Input: the Chinese Tallow Tree and Stream Food Webs
The invasive Chinese Tallow tree (Triadica sebifera or Sapium sebiferum) was heavily introduced to the southern United States from Asia at the turn of the 20th Century. A Tallow invasion can reduce richness within the plant community by direct competition and can decrease density of consumer communities by limiting basal food resources. Additionally, tallow leaches rapidly into aquatic systems, where a sharp increase in aerobic microbial decomposition can simultaneously drop dissolved oxygen and pH levels, thus causing mortality in macroorganisms. I predicted an input of solely Tallow leaves into mesocosms (artificial streams) would temporarily increase N & P concentration, algae concentration, invertebrate density, and fishes because of increased rates of rapid decomposition, but would fail to sustain long-term and overall growth for that same reason. In contrast, sycamore leaves would provide a more sustained, long-term allochthonous subsidy to the mesocosms, thus resulting in increased growth of fishes and other response variables relative to that of tallow leaf treatments. Cellulose paper was used as a no-leaf control to account carbon input. Bullhead minnows (Pimephales vigilax) were stocked in half of our experimental stream mesocosms, where benthic algae, invertebrate density, and nutrient content were sampled for 16 weeks. After 16 weeks, 50% of the original Sycamore leaves remained, whereas less than 10% of the cellulose paper (control) and Tallow remained. The concentration of benthic algae was highly dependent on time as values increased significantly after week 6, and tended to be higher in treatments with no-fish and cellulose/tallow. Invertebrate density generally iv remained higher in no-fish treatments; densities were highest in tallow treatments before the experimental halfway point, and were highest in sycamore treatments past the halfway point. Nitrogen and phosphorous concentrations fluctuated highly throughout the experimental runtime, and showed no significant interactions among treatment groups. Fish were largest in mesocosms with the Sycamore leaves where either the undecomposed leaves provided additional surface area for food resources, or growth was higher relative to tallow treatments because of possible physiological inhibition. Tallow treatment fish growth was significantly smaller than sycamore, and relatively equal to cellulose. Herein, we discuss the reasons for the decreased fish growth in the presence of tallow leaves such as: (1) rapid decomposition providing a short-term nutrient pulse that moved through the food web quickly, and (2) physiological inhibition from the chemical composition of tallow leaves.