Consumers Affect Prey Biomass and Diversity Through Resource Partitioning

Abstract

Consumer presence and nutrient availability can have contrasting and interactive effects on plant diversity. In a factorial experiment, we manipulated two levels of nutrient supply and the presence of two moderately specialized grazers in different combinations (no grazers, two species in monoculture, and both in combination). We tested how nutrients and grazers regulated the biomass of marine coastal epiphytes and the diversity of algal assemblages, based on the prediction that the effect of consumers on prey diversity depends on productivity and consumer specialization. Nutrient enrichment increased the epiphytic load, while monocultures of single grazer species partly prevented epiphyte growth. However, only the presence of two species with complementary feeding preferences effectively prevented epiphyte overgrowth. The epiphytes comprised micro- and macroalgal species, and the diversity of these algal assemblages differed, depending on grazer identity. For the microalgae, diversity was reduced by nutrient addition when grazer control was inefficient, but not when specialist microalgal grazers were present. Macroalgal diversity was reduced in ambient water with specialist macroalgal grazers compared to the treatment with inefficient ones. These results indicate that grazer composition and productivity are crucial in determining whether consumer pressure will have a positive or negative effect on algal diversity.

Consumer effects on ecosystem functioning in rock pools: roles of species richness and composition

“…This may have limited the strength of a species complementarity effect by offering limited functional diversity. Grazers from other phyla, such as the crustacean amphipods and isopods, may, for example, offer a greater potential for resource complementarity due to larger differences in feeding mode (Parker et al 1993, Råberg & Kautsky 2007. Secondly, the relatively homogeneous conditions afforded by these artificial pools (e.g.…”

“…Where interspecific resource partitioning occurs, for example, increasing species richness will allow a greater proportion of the resource spectrum to be used, potentially enhancing the associated ecosystem process above that of the single best-performing species (e.g. Råberg & Kautsky 2007, Griffin et al 2008.…”

Trophic cascades in seagrass meadows depend on mesograzer variation in feeding rates, predation susceptibility, and abundance

“…Differences in feeding niche among grazers have been demonstrated in several other seagrass and algae systems (Duffy & Hay 1991, Raberg & Kautsky 2007, Jaschinski & Sommer 2008, and there are several reasons to think that similar patterns of predation susceptibility may not be unique to our system. Below, we consider the generality of our predictions by examining patterns in grazer population growth, size, and habitat use that may be relevant to other seagrass systems.…”

“…In seagrass systems, invertebrate grazers vary in their feeding rates on microalgae, macroalgae, and seagrasses themselves (Duffy & Hay 1991, Raberg & Kautsky 2007, Jaschinski & Sommer 2008, Hughes et al 2010. Although much less is known about species-specific predation susceptibility, species do vary in ways that may affect their visibility and escape success, such as size, coloring, and behavior.…”

Multiple fish predators: effects of identity, density, and nutrients on lower trophic levels

“…Thus, herbivore traits, especially edibility (predator resistance), grazing efficiency, and feeding guild, are important mediators of multiple predator effects on lower trophic levels (Stachowicz et al 2007). The resulting biomass and species composition of primary producers is ultimately determined by the functional composition of the herbivores combined with nutrient availability (Råberg & Kautsky 2007). Accordingly, it has been shown that the loss of dominant fish predators in the coastal zone of the Baltic Sea has reduced grazing rates on algae by shifting the functional composition of the herbivore community (Eriksson et al 2009, Sieben et al 2011.…”