A cross-taxonomic comparison of insect responses to grassland management and land-use legacies

Abstract

Many species of plants and animals associated with grasslands are rare or declining due to habitat loss and degradation. Although grassland plants and insects evolved in the context of both grazing and fire, the appropriate use of grazing and fire has been debated among those concerned with protecting insect communities. We established an experiment to test insect responses to three grassland management treatments: (1) patch‐burn graze (burning of spatially distinct patches and free access by cattle), (2) graze‐and‐burn (burning of entire tract with free access by cattle), and (3) burn‐only. Because we expected that land‐use legacies could also affect insect abundance and diversity, we evaluated effects of time since fire, grazing history, remnant history (remnant or reconstructed grassland) and pre‐treatment vegetation characteristics, which were assumed to be a legacy of prior land‐use. Butterflies (Lepidoptera), ants (Hymenoptera: Formicidae), and leaf beetles (Coleoptera: Chrysomelidae) were surveyed for three years to compare their responses to each of these treatments as measured by abundance, richness and species diversity. Each of these taxa is relatively diverse and was expected to have the potential to have strong negative responses to grazing and burning, but we predicted more positive responses to patch‐burn grazing. Our results showed that land‐use legacies affected insect abundance, richness and diversity, but treatments did not. Ant abundance was lower in tracts with a history of heavy grazing. Ant species richness was positively associated with pre‐treatment time since fire and vegetation height and negatively associated with pre‐treatment proportion native plant cover. Butterfly abundance was positively associated with pre‐treatment litter cover. Leaf beetle diversity was positively associated with pre‐treatment native plant cover, and leaf beetle abundance was negatively associated with time since fire. Our results indicate that land‐use legacies can exert more influence on grassland insect community composition than current management, but the particular aspects of these land‐use legacies that are important vary across insect taxa. The implications of these finding are that (1) land‐use legacies should garner more attention in grassland management and (2) conservation of grassland insect communities will be improved by taxon‐specific analysis of land‐use legacy variables.

Exotic-Dominated Grasslands Show Signs of Recovery with Cattle Grazing and Fire

“…Prior to the experiment, all degraded sites appeared to have experienced extended periods of intense grazing, applications of chemical fertilizer (likely N-based), and seeding of exotic forage species in the past (M. Moe, personal communication). Further details on the land use history at these sites can be found in Debinski et al [43] and McGranahan et al [48]. We perceive the diversity of land-use histories as a benefit to our analysis in this study because this provides a greater representation of degraded grasslands on the landscape and because the difficulty of detecting trends in less controlled studies makes a significant result more robust.…”

“…Because the pastures used in this experiment were prairie remnants [43], we expected that the addition of fire and grazing could provide conditions that would release some of the native plant species. We hypothesized that butterfly community composition on degraded pastures would become more similar to reference sites (native-dominated tallgrass prairie remnants) with the application of fire and moderate grazing treatments because 1) these processes were central in structuring and maintaining the tallgrass prairie ecosystem historically, and 2) both grazing and fire are required for maintaining diversity in many grassland systems.…”

Effects of grassland management practices on ant functional groups in central North America

“…Land-use history of each tract was determined by interviewing landowners and agency land managers who owned/managed the tracts. Debinski et al (2011). All ants were identified to species-level in the laboratory.…”

“…As reported in Debinski et al (2011), we initiated an experiment in tallgrass prairies of Iowa and Missouri in 2006 to compare the effects of three different management regimes (patch-burn graze, grazeand-burn, and burn-only) on abundance, species richness, and diversity of key invertebrate taxa, namely ants, butterflies and chrysomelid beetles). We also examined these response variables in remnant grasslands and grassland restorations.…”

“…If the species within a functional group are similar in their response to disturbance, the greater abundance values obtained from pooling can increase the potential of detecting a response. Here, using data from the same experiment as the Debinski et al (2011) study, we report on the response of ant functional groups to 1) three grassland management regimes, 2) remnant status [remnant versus restoration], 3) time since fire within patch-burn graze tracts, 4) pre-existing habitat characteristics, and 5) treatment-induced habitat characteristics. Given the anticipated effects of disturbance regimes on amount of bare ground, vegetation composition and vegetation structure, we hypothesized that grazing, burning and combinations thereof would alter ant functional group abundance, and that functional groups would differ in their responses.…”