Spatial and temporal variation in the structure of stream food webs: Investigating the effects of shifting basal resources and predation from a top predator, the river otter (Lontra canadensis).

摘要

Predation is an important factor structuring the abundance and distribution of organisms in aquatic communities. However, the relative contribution of biotic interactions and environmental conditions, in terms of the structure and function of ecosystems, is highly variable. In addition, the availability and source of energy resources will impact trophic dynamics and interaction strengths among community components. For example, large detrital reservoirs may stabilize consumer-resource dynamics in harsh environments. My objectives were to quantify spatial and temporal variation in stream food webs and predator-prey dynamics in the Mulberry River Drainage of northwest Arkansas, U.S.A. I examined shifts in the trophic base of stream food webs using stable isotope ratio analysis (13C/12C and 15 N/14N, circular statistics, and mixing models. I predicted that communities would shift from autochthonous resources in the spring and summer to autochthonous resources in the winter due to increased leaf inputs in the autumn. In addition, I predicted that autochthonous resources would be more important at sites with larger watersheds and greater canopy openness. In terms of predator-prey dynamics, I examined how an apex predator, the river otter (Lontra canadensis), impacted fish and crayfish populations. In particular, I integrated data on prey availability, diet, and field metabolic rate to estimate otter consumption in a bioenergeties framework. I trapped crayfish monthly as an index of crayfish availability to otter predation. Otter diet was assessed from scat samples collected seasonally and by stable isotope analysis of whole-prey signatures and otter plasma samples. Muscle samples from carcasses were also collected for stable isotope analysis during the winter trapping season. In addition, I estimated daily energy expenditure in wild otters inhabiting a pond enclosure using the doubly labeled water method. I predicted that otters would consume a greater proportion of fish during the winter and spring corresponding to low crayfish availability. Finally, I conducted mesocosm (predator exclusions) and tethering experiments in pool habitats during low flows to examine the effects of aquatic and terrestrial predators on central stoneroller (Campostoma anomalum) and crayfish (Orconectes meeki) survival in intermittent streams. I tested the hypothesis that large fish are at greater risk from terrestrial predators in shallow habitats compared to small fish. In addition, I tested the hypothesis that crayfish survival is maximized by small and large individuals in shallow and deep habitats, respectively. Results demonstrated substantial spatial and temporal variation in the trophic base of intermittent stream food webs and detritus was a major energy pathway that likely contributed to community resistance and resilience to disturbance. Prey availability and diet varied seasonally and otters demonstrated elevated metabolic demand and consumption rates suggesting potentially strong effects on prey populations. The mesocosm experiment demonstrated a negative effect of terrestrial predators on central stoneroller survival. In terms of size effects, predation risk was greater for large individuals. Finally, I did not detect a depth effect with the crayfish tethering experiment and small crayfish demonstrated increased mortality compared to large crayfish in both shallow and deep treatments. Although predator effects were demonstrated experimentally, more studies are needed to examine how multiple predators and environmental heterogeneity interact to structure aquatic ecosystems. Identifying factors that regulate community structure will be critical for predicting predator-prey dynamics and ecosystem responses to natural and anthropogenic perturbations.