Insect herbivory and vertebrate grazing impact food limitation and grasshopper populations during a severe outbreak

Abstract

1. Competition between herbivores often plays an important role in population ecology and appears strongest when densities are high or plant production is low. Phytophagous insects are often highly abundant, but relatively few experiments have examined competition between vertebrates and phytophagous insects.2. In grassland systems worldwide, grasshoppers are often the dominant phytophagous insect, and livestock grazing is a dominant land use. For this study, a novel experiment was conducted examining competition between vertebrates and invertebrates, where both grasshopper densities and sheep grazing were manipulated inside 10-m 2 caged mesocosms during a grasshopper outbreak. We examined how grasshopper densities and the timing of vertebrate herbivory affected grasshopper densities, if the effects of vertebrates on survival and reproduction changed with grasshopper density, and how a naturally occurring grasshopper outbreak affected grasshopper populations in the following year.3. Densities of grasshoppers at the site peaked at 130 m -2 . Food-limited competition was stronger in treatments with higher grasshopper densities and repeated or late livestock herbivory, leading to reduced survival, femur length, and functional ovarioles, a measure of future reproduction. Strong food-limited density-dependent reproduction and survival led to reduced hatching densities in 2001.4. As competition was typically stronger with high grasshopper densities than with livestock grazing, competition from vertebrates could be relatively less important for phytophagous insect population dynamics during outbreaks. The experiment provides insights into how competition between insect and vertebrate herbivores influences insect population dynamics, and indicates that severe outbreaks can rapidly subside with strong competition from vertebrate and insect herbivores.

Ecosystem carbon exchange in response to locust outbreaks in a temperate steppe

“…By contrast, Dyer et al (1993) have found that low-level herbivory under non-outbreak conditions could increase plant production, whereas extremely high-intensity herbivory under outbreak conditions results in substantial productivity reduction. At outbreak density, grasshopper herbivory could reduce approximately 70-95 % of the aboveground biomass in short-and mixed-grass prairies (Burleson and Hewitt 1982;Chase 1996;Thompson et al 1996;Carson and Root 2000;Branson 2010;Branson and Haferkamp 2014). Therefore, clipping 80 % of aboveground biomass in this study could well have simulated biomass losses caused by locust feeding under outbreak conditions.…”

“…On the one hand, insects that feed on plant aboveground tissues can substantially suppress plant growth (Hoogesteger and Karlsson 1992;Jardon et al 1994;Coupe and Cahill 2003;Esper et al 2007;Cobb 2010) and biomass (Schowalter et al 1986;Brown 1994;Chase 1996; Thompson et al 1996;Carson and Root 2000;Branson 2010;Branson and Haferkamp 2014) under outbreak conditions, consequently leading to significant declines in ecosystem function. For instance, the depressed plant growth and productivity caused by the outbreaks of mountain pine beetle can suppress forest ecosystem carbon (C) uptake in North America (Kurz et al 2008a, b;Brown et al 2010;Clark et al 2010;Dymond et al 2010;Stinson et al 2011).…”

Grasshopper species composition shifts following a severe rangeland grasshopper outbreak

“…Above average August precipitation in 1998 and 1999 led to elevated grass nitrogen content , Table 1) and P. nebrascensis reproduction (Branson 2008). The 97% decline in P. nebrascensis densities in 2001 were associated with low late season food availability and nitrogen content in 2000 ( Table 1) that reduced survival and reproduction (Branson and Haferkamp 2014). The larger proportional reduction in P. nebrascensis in 2001 compared to M. sanguinipes and A. deorum (Table 1) at least partially resulted from its later phenology.…”

“…Cattle were the only mammal excluded from the site and insects were not controlled. The site consisted of mixed grass prairie, with Western Wheatgrass (Pascopyrum smithii) initially comprising over 90% of vegetation (Branson 2008, Branson andHaferkamp 2014). In the area of the study site, greater than 90% of plant production typically occurs by July 1 (Heitschmidt and Vermeire 2005), with annual precipitation highly variable but averaging ~34 cm (Heitschmidt and Vermeire 2006).…”

“…Even less is known about shifts in the relative abundance of species during and following outbreak periods, as sampling efforts either fail to monitor species composition or end when outbreaks or chemical control efforts subside (Onsager 2000). Little, if any, data exist where grasshopper densities, species composition and vegetation were sampled during and after a severe outbreak, which is needed to better understand the cause of outbreaks and population declines in the western U.S. (Branson et al 2006, Branson andHaferkamp 2014). Given species differences in food preference and phenology, grasshopper species should differentially respond to abiotic and biotic conditions (Joern 2000, Onsager 2000, Branson et al 2006.…”