Redundancy among three herbivorous insects across an experimental current velocity gradient

Abstract

We conducted streamside experiments to determine if the ability of herbivorous insects to remove algal periphyton varies with local current velocity. We used two mayfly species (Baetis bicaudatusand Drunella grandis) and one caddisfly species (Glossosoma verdona), which differ from one another in body morphology and mobility. Periphyton was grown for 30 days on ceramic tiles in constant velocity to create similar initial forage conditions for grazers. Tiles were transferred to three velocity regimes characteristic of the natural streambed: slow (3-5 cm s(-1)), medium (15-20 cm s(-1)) and fast (32-41 cm s(-1)). Four grazer treatments (Baetis, Drunella, and Glossosoma alone, and all species combined) were repeated for each velocity treatment to isolate the effect of local current on grazer ability to crop periphyton. Grazers differed in their abilities to remove periphyton across current treatments. Glossosoma removed significantly (P<0.05) more periphyton at fast versus either slow or medium velocities; Baetis showed a similar (but non-significant) trend; and, Drunella always removed about 75% of periphyton, irrespective of current. At fast current, periphyton removal was equivalent among the species. At medium current, Drunella removed significantly more than both Baetis and Glossosoma, whereas at slow current, Drunella removed more than Baetis, which removed more than Glossosoma. Periphyton removal under the combined three-grazer treatment was similar qualitatively to the combined effects of individual grazers. More periphyton tended to be removed as current increased, with the fast versus slow contrast showing marginal significance (P=0.10). Under all current regimes, the quantity of periphyton removed did not differ from the null model expectation of simple additive effects among individual grazers (i.e., no facilitation or inhibition). These experiments show that for some species, herbivory varies with current, which suggests that the herbivore "function" of cropping periphyton may vary with the environmental context of local current. Under some local velocities, however, different herbivore species "function" similarly and are potentially redundant with respect to periphytic removal. In naturally heterogeneous streams characterized by sharp gradients in local current velocity, we expect current-dependent species interactions to be common and at least partially contribute to intra-guild co-existence of species.

Macroinvertebrate response to stream restoration by large wood addition

“…The number of taxa in the best represented functional group, predators, was large (72 of 149 total taxa encountered) and representation of predators in each reach condition group used to plot NMS ordinations was also large (31 in rehabilitated reach condition subsets, 54 in downstream untreated subsets and 44 in the upstream site). However, even when functional guilds contain a large variety of taxa, they may be limited due to individual member taxa that vary in ability to function dependent on the experienced range of environmental conditions (Wellnitz and Poff, 2001;Poff et al, 2003). The ability of functional guilds to perform their anticipated functions, especially in an environment that is subject to wide variety of changing conditions as observed at our study reach, may crucially depend on this 'response diversity' aspect of functional redundancy (Laliberte et al, 2010).…”

Competition, predation and flow rate as mediators of direct and indirect effects in a stream food chain

“…(the dominant grazer group in our experiment) had a higher grazing efficiency at higher water velocity (Poff et al 2003). Although other abiotic factors no doubt influence the biomass of primary producers, we hypothesize that channels with higher water velocity supported a higher biomass of grazers because: (1) resource biomass was higher (the indirect effect of water velocity through the enhancement of the biomass of primary producers), (2) grazing efficiency was better (the direct effect of water velocity), and (3) the colonization rate was higher.…”

Algal blooms and the nitrogen‐enrichment hypothesis in Florida springs: evidence, alternatives, and adaptive management

“…Low flows and water control structures may exclude large mobile herbivores from springs, as in Silver Springs where fish biomass declined by .60% following the damming of the Oklawaha River downstream of its confluence with the Silver River (Munch et al 2006). Decreases in spring flows also could inhibit grazing rates of some invertebrates (Poff et al 2003). In addition, both invasive plant management and recreational use of springs can dramatically reduce standing crops of vegetation, potentially altering the quantity and quality of habitat available to grazers.…”