Insect assemblages change along a gradient of invasion by a nonnative grass

Abstract

Because invasions by nonnative plants alter the structure and composition of native plant communities, invasions can alter the function of ecosystems for animals that depend on plants for food and habitat. We quantified effects of an invasion by a nonnative grass on the insect community in grasslands of southeastern Arizona. We sampled insects on 54 1-ha plots established across a gradient of invasion by Lehmann lovegrass (Eragrostis lehmanniana Nees), a perennial species native to southern Africa. Between 2000 and 2004, we captured 94,209 insects representing 13 orders, 91 families, and 698 morphospecies during 2,997 trap nights. Richness of families, richness of morphospecies, and overall abundance of insects decreased as dominance of nonnative grass increased. With every 100 g/m 2 increase in biomass of nonnative grass, the average number of insect families decreased by 5%, morphospecies decreased by 6%, and overall abundance decreased by 14%. In areas dominated by nonnative grass, 2 of 8 orders and 6 of 27 families of insects were present less frequently and one family was present more frequently; 5 of 8 orders and 6 of 27 families of insects were less abundant and 3 families were more abundant than in areas dominated by native grasses. As a result, this plant invasion altered the structure of the insect community, which has consequences for animals at higher trophic levels and for ecosystem processes, including decomposition and pollination. Because complete eradication of nonnative plants might be possible only rarely, maintaining stands of native vegetation in invaded areas may be an important practical strategy to foster persistence of animals in grasslands invaded by nonnative plants.

Phytophagous insect fauna tracks host plant responses to exotic grass invasion

“…According to this second hypothesis, we also would expect a unimodal variation in herbivore richness along the gradient of invasion by nonnative grasses through the effect of past disturbances on host plants. Why did we find a peak in insect richness at intermediate invasion levels, whereas several previous studies have reported a monotonic decline in insect richness in habitats invaded by nonindigenous plants (e.g., Litt and Steidl 2010;Yoshioka et al 2010)? A simple explanation is that endophagous herbivores are less susceptible to changes in vegetation structure (see Pearson 2009) and to harsher abiotic conditions (e.g., higher temperature and/or lower humidity promoting desiccation) compared to ectophagous herbivores, predators and other non-herbivorous insects.…”

“…Although some studies have shown that non-herbivorous insects are more susceptible to disturbances and changes in vegetation structure than herbivores (e.g., Pearson 2009;Litt and Steidl 2010), whether such responses also hold for non-herbivorous insects concealed within plant tissue, such as the parasitoids of endophages, remains to be determined. …”

Woody Plant Encroachment: Causes and Consequences

“…Although some species respond to changes in vegetation at broader scales, animals that function at smaller scales, such as small mammals and arthropods, are more likely to respond to changes in vegetation that alter local environmental characteristics (Wiens and Milne 1989). Consequently, some taxa, including birds, mammals, and reptiles, are more likely to respond to the structural changes in the plant community that accompany WPE, whereas other taxa, especially arthropods, are also likely to respond to changes in species composition that interfere with coevolved relationships with specific plant species (Litt and Steidl 2010). Relative to vertebrates, many arthropods are less mobile, depend on a narrower range of plants for food, cover, and sites for reproduction, and can have specialized relationships with specific plant species (Kremen et al 1993).…”

Impact of plant invasions on local arthropod communities: a meta‐analysis

“…Fourth, we examine the impact of invasive canopy cover on arthropod communities as suggested by Pawson et al (2010). The few studies that have explicitly looked into the effect of invader dominance on arthropod abundance and richness report mixed results (Heleno et al 2009;Litt & Steidl 2010;Parr, Ryan & Setterfield 2010;Pawson et al 2010;Spyreas et al 2010;Almeida-Neto, Prado & Lewinsohn 2011). Meta-analyses of a larger set of observational studies could potentially derive a more generalized view.…”