Effects of Bird Predation on Grasshopper Densities in an Arizona Grassland

Abstract

In a 4—yr field experiment, we tested the hypotheses that insectivorous birds (1) controlled densities of herbivorous grasshoppers in an ungrazed semiarid grassland in southeastern Arizona, and (2) functioned as keystone predators, by limiting abundances of grasshoppers that otherwise might change vegetation cover and species composition, and/or by mediating the effects of otherwise competitively superior members of the grasshopper assemblage. We measured grasshopper densities and vegetation on 32 464—m2 grassland plots for 1 yr, then enclosed 16 of these plots with bird exclosures and continued data collection for 3 yr. Eight of the 16 experimental plots were further modified in the last 2 yr of the study by installing fine—mesh 1 m high barriers designed to retard grasshopper dispersal. Microclimates of caged plots differed only slightly from open plots. Lizards and rodents increased inside the exclosures, but they were removed and released elsewhere such that their average abundances did not differ among treatments. By the final year of the study, mean annual adult grasshopper density was >2.2 times higher on plots from which birds were excluded, and where grasshoppers were enclosed by dispersal barriers, than on unmanipulated control plots. Mean nymph density was >3.0 times higher in the same comparison. Grasshoppers were significantly more abundant in bird exclosures with insect dispersal barriers, indicating that experimental plots were dispersal sources rather than sinks. Seven of 12 common grasshopper species were more abundant inside the bird exclosures, while none was less abundant. Among the more abundant taxa, those responding most positively were grass feeders: Eritettix simplex, Opeia obscura, Paropomala wyomingensis, and Phoetaliotes nebrascensis. We found no evidence that grasshoppers competed with one another under increased densities inside the bird exclosures. Although the amount of insect herbivory was somewhat higher inside the bird exclosures, and was positively correlated with grasshopper density across all 32 plots (r = 0.87), overall vegetation cover and species composition did not differ among treatments by the end of the study. Dactylotum variegatum, an aposematic species apparently immune to avian predation, showed no significant responses to the experiment. Birds clearly limited grasshoppers in this grassland ecosystem, but they failed to qualify as keystone predators, at least in the short term, for two reasons: (1) in their absence, increased grasshopper densities had no appreciable impact on vegetation cover or species composition; and (2) there was no evidence that birds mediated competition among grasshoppers.

Quantifying pest control services by birds and ants in Kenyan coffee farms

“…Birds are highly mobile and, with over 50% of all species primarily insectivorous, are ideal providers of pest control services (Wenny et al 2011). Birds have been shown to decrease insects in a variety of systems (Bock et al 1992, Mols and Visser 2002, Kellermann et al 2008, Jedlicka et al 2011, Karp et al 2013. Furthermore, studies have shown that birds not only reduce pest populations, but that plants respond with higher levels of productivity (Whelan et al 2008, Mooney et al 2010.…”

Seasonal variation in top-down and bottom-up processes in a grassland arthropod community

“…Grasslands have received a relatively large amount of attention with regard to top-down and bottom-up control (Bock et al 1992;Schmitz 1994Schmitz , 1998Chase 1996Chase , 1998Moran et al 1996;Beckerman et al 1997;Schmitz et al 1997;Moran and Hurd 1998;Ritchie 2000;Schmitz and Suttle 2001;Moran and Scheidler 2002), and these studies have demonstrated that both top-down and bottom-up processes are prevalent.…”

Bird predation on herbivorous insects: indirect effects on sugar maple saplings

“…In terrestrial ecosystems, the indirect effects of firstand second-order predators on plant performance has been studied through field experiments in a number of systems: carnivores through predation on grazing mammals (McLaren and Peterson 1994), spiders (Carter and Rypstra 1995) and mantids (Moran and Hurd 1998) through predation on herbivorous insects, Anolis lizards through predation on Homoptera (Spiller and Schoener 1990;Dial and Roughgarden 1995;Spiller and Schoener 1997), beetle larvae through predation on ants (in an antplant mutualism system; Letourneau and Dyer 1998), and birds through predation on herbivorous insects (Loyn et al 1983;Atlegrim 1990;Bock et al 1992). Assessment of plant performance has primarily been through quantification of leaf damage to the plants as mediated by predation pressure on the herbivores.…”