Land‐use legacies and present fire regimes interact to mediate herbivory by altering the neighboring plant community

Abstract

Past and present human activities, such as historic agriculture and fire suppression, are widespread and can create depauperate plant communities. Although many studies show that herbivory on focal plants depends on the density of herbivores or the composition of the surrounding plant community, it is unclear whether anthropogenic changes to plant communities alter herbivory. We tested the hypothesis that human activities that alter the plant community lead to subsequent changes in herbivory. At 20 sites distributed across 80 300 hectares, we conducted a field experiment that manipulated insect herbivore access (full exclosures and pseudo‐exclosures) to four focal plant species in longleaf pine woodlands with different land‐use histories (post‐agricultural sites or non‐agricultural sites) and degrees of fire frequency (frequent and infrequent). Plant cover, particularly herbaceous cover, was lower in post‐agricultural and fire suppressed woodlands. Density of the dominant insect herbivore at our site (grasshoppers) was positively related to plant cover. Herbivore access reduced biomass of the palatable forb Solidago odora in frequently burned post‐agricultural sites and in infrequently burned non‐agricultural woodlands and increased mortality of another forb (Pityopsis graminifolia), but did not affect two other less palatable species (Schizachyrium scoparium and Tephrosia virginiana). Herbivory on S. odora exhibited a hump‐shaped response to plant cover, with low herbivory at low and high levels of plant cover. Herbivore density had a weak negative effect on herbivory. These findings suggest that changes in plant cover related to past and present human activities can modify damage rates on focal S. odora plants by altering grasshopper foraging behavior rather than by altering local grasshopper density. The resulting changes in herbivory may have the potential to limit natural recovery or restoration efforts by reducing the establishment or performance of palatable plant species.

Spatial arrangement of canopy structure and land‐use history alter the effect that herbivores have on plant growth

“…Nonagricultural and post-agricultural habitats each support a different assemblage of plants (Flinn andVellend 2005, Brudvig et al 2013) and grasshoppers (Hahn and Orrock 2015b). In a previous experiment conducted in non-experimental patches with intact canopies we found that herbivory rates were greater in post-agricultural sites and this was driven largely by the amount of alternative resources (plant material) available for herbivores to consume, with the highest rates of herbivory at intermediate levels of plant cover (Hahn and Orrock 2015a). Within our experimental landscapes, we use herbivore exclosures to quantify herbivory by grasshoppers on the growth of four species of perennial herbs to evaluate possible changes in herbivory at different distances from the resulting habitat edges .…”

“…The opposite pattern can also occur, where herbivory rates are greater in shade habitats (Muth et al 2008, Hakes andCronin 2012), presumably because plant defenses are lower than in shady habitats while herbivores are still present (Muth et al 2008). Alternatively, shady habitats can reduce overall herbaceous cover making the rare herbaceous plants that do persist in a shaded habitat more vulnerable to herbivory Cronin 2012, Hahn andOrrock 2015a). The varied results of these studies may be because studies generally only examine a single edge type (but see Wolf and Batzli 2004), include only one or a few focal species, and rarely experimentally manipulate both the edges and the quality of the adjacent patch.…”

“…The varied results of these studies may be because studies generally only examine a single edge type (but see Wolf and Batzli 2004), include only one or a few focal species, and rarely experimentally manipulate both the edges and the quality of the adjacent patch. Other widespread factors, such as historical agricultural land-use, that can alter plant communities (Flinn and Vellend 2005, Vellend et al 2007, Baeten et al 2014, insect communities (Debinski et al 2011, Hahn andOrrock 2015b), and the strength of biotic interactions (Hahn and Orrock 2015a) could potentially influence edge responses. However, one important limitation to studying edge effects in existing landscapes is v www.esajournals.org that patch type and edges are not manipulated experimentally, and thus potentially confounded by non-random land-use practices, limiting robust tests of edge effects in multiple habitat types .…”

Ontogenetic responses of four plant species to additive and interactive effects of land‐use history, canopy structure and herbivory

“…Grasshoppers are common invertebrate herbivores in our system (Evans et al . ; Hahn & Orrock ) and include the most likely insect consumers of the focal plant species. The two most common species of grasshoppers at our site are Melanoplus angustipennis (subfamily: Melanoplinae) and Schistocerca alutacea (subfamily: Cyrtacanthacridinae; Hahn & Orrock ), both species are mixed feeders that can eat a variety of grasses and forbs (Joern ).…”

“…We also hypothesized that, (ii) because plants in thinned patches grow faster compared to plants in unthinned patches (Harrington ), herbivory should have less of an effect on plant performance in thinned patches. Based on previous findings (Hahn & Orrock ), we predicted that (iii) herbivory would have greater effects where alternative plant resources are low, particularly in post‐agricultural sites. Collectively, we expected herbivory to have the greatest effect on juvenile plant survival in unthinned, post‐agricultural habitats.…”