Food web analysis of macrobenthic riffle communities

Date

1992-12

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

The cascade model is an ecological theory that predicts many of the structural features of real food webs. The food web for the macroinvertebrate benthic community inhabiting riffles in the South Llano River (Junction, Texas) was developed to test specific hypotheses that are derived from the cascade model. The empirical food web was constructed based on feeding trials, stomach content analyses, published dietary records of stream insects, and field observations. Parameters of the web (e.g., connectance, mean food chain length and standard deviation) were estimated to address questions concerning food web structure. Monte Carlo simulations were used to test whether chain length distribution of the web could be explained solely by the prey choices of each predator, or if optimal foraging constraints determined chain length. Twenty-six trophic spedes and 68 links were identified. The empirical food web comprised 159 food chains with a mean (SD) of 5.06 (1.01) links. Omnivory was prevalent at all consumer trophic levels. The top predator, Corydalus comutus (L.), consumed prey at all consumer levels and appeared to be constrained only by prey size. The food web adheres to four of the five laws of the cascade model. The only law that did not meet was the predator to prey ratio, with a greater than expected value of 1:1.61. Chain length distribution was significantly different (skewed and with more long chains) from distributions generated at random from a species pool equal to that observed in the South Llano River macrobenthic community. Similar results were obtained when food chains were generated with the probability of taxa being consumed by predators equal to the relative abundance of the prey species. These results suggest that chain length distribution of the macroinvertebrate riffle community is not random, and that optimal foraging constraints do not influence this distribution.

A simulation model, based on a series of Holling type II differential equations, was adapted from the literature to investigate the effects of omnivory on the dynamics of individual populations at different trophic levels in the macroinvertebrate riffle community, as well as the stability and persistence of food chains. Initial population densities were equal to actual field estimates, whereas predation rates and population growth rates were obtained from the literature for the top predator (C. comutus), intermediate (Ambrysus circumcinctus). and basal (Petrophila sp.) spedes. Population density fluctuations depended on the trophic level at which each population feeds. Demography of primary and secondary consumers numicked predator-prey dynamics; predator population lagged behind the prey in a cyclic manner. The tertiary consumer exhibited stable limit cydes or chaos, depending on the amount of connectance. The model without omnivore (minimal connectance), the range in population fluctuation was the greatest for the secondary consumer, followed by the primary and then tertiary consumer. Low levels of omnivory (between 1 and 4%), decreased minimum densities for all consumer populations. Higher onmivory levels produced wide oscillations and populations quickly became extinct. Sensitivity analysis revealed that the model was highly sensitive to initial conditions. In addition, the system exhibited chaotic behavior. This implies that pervasive fluctuations in population density that are characteristic of many aquatic insects could be the product of deterministic rather than stochastic processes.

Description

Keywords

Food chains (Ecology) -- Texas -- Llano River, Biotic communities -- Texas -- Llano River

Citation