Community structure is often different in human-altered habitats compared to un-altered habitats. However, changes may not be apparent for all trophic levels because positive and negative effects cancel each other. Separating these confounding effects is important for understanding the extent of anthropogenic activities on community structure. For example, predators (negative) and nutrients (positive) may trade-off effects on prey growth resulting in no net change between altered and unaltered habitats as long as the rate of change for both effects is similar in human altered habitats. We characterized aquatic communities at sites near (altered) and far (un-altered) from a canal in an oligotrophic subtropical wetland with throw traps, tethering experiments, and a reciprocal transplant experiment to examine the interactive effects of nutrients and predators on pulmonate snail growth. The transplant experiment was conducted at each site in mesh bags to which we added periphyton and snails. Periphyton was moved between sites near and far from the canal during experimental set-up and after twenty days; snail growth was measured after twenty and forty days.
Results/Conclusions
Aquatic communities near the canal were different from communities far from the canal and crayfish, a snail predator, were more numerous near canals. Tethering experiments revealed that snail mortality was generally higher near canals. Periphyton chlorophyll-a content, a measure of food quality, was higher at one site near the canal compared to sites far from the canal. We found that snail growth at a site depended on periphyton origin. Snails grew fastest consuming periphyton that originated at sites near the canal but, that was placed at sites distant from the canal. Experimental research has demonstrated that crayfish cues slow snail growth, while nutrient additions speed growth. The combination of these results suggests interplay between the negative effects of predators and the positive effects of nutrients on snail growth resulting in no net effect of growth near the canal. Results also reveal that mechanisms underlying the similar growth patterns between altered and un-altered sites were different. Snails at sites near the canal enjoyed higher quality resources at the cost of more predators and their cues, while snails at sites far from the canal encountered relatively low quality resources with few predators. Understanding the extent of anthropogenic effects on community structure requires monitoring efforts to establish their effects and experimentation to tease apart the confounding effects of abiotic and biotic factors that human activities alter.
