Japanese stilt grass (Microstegium vimineum), a nonnative invasive grass, provides alternative habitat for native frogs in a suburban forest (original) (raw)
2009, Urban Habitats
Japanese stilt grass (Microstegium vimineum) is an invasive grass in the eastern and midwestern United States. It tolerates a wide range of light and moisture conditions and has readily replaced native herbaceous vegetation in many areas. Despite its detrimental effects, Japanese stilt grass does provide some benefit, serving as habitat for ground amphibians such as frogs and toads (anurans) in areas where populations of white-tailed deer (Odocoileus virginianus) have depleted native herbaceous cover. We investigated relative abundances of common anuran species both inside and outside of a stilt grass invasion front in a Northeastern mixed hardwood forest. We used pitfall trap arrays to sample anuran species during the summers of 2006 and 2007 and we captured four species: wood frog (Rana sylvatica); pickerel frog (Lithobates palustris); spring peeper (Pseudacris crucifer); and American toad (Bufo americanus). We captured more individuals from each of these species in stilt grass plots. Too few spring peepers were captured for analysis, so we modeled the captures of the three remaining species—wood frog, pickerel frog, and American toad—with negative binomial regression against stilt grass presence/absence, soil moisture, soil temperature, and relative humidity. We compared models containing these parameters using AIC (Akaike's Information Criterion), a common information criterion used in model selection. All three species selected stilt grass plots, but appeared to do so for different reasons. Pickerel and wood frogs seemed to select primarily areas of high soil moisture, which was consistently greater in stilt grass plots. American toads selected stilt grass areas and areas of high humidity, though humidity did not vary according to stilt grass presence or absence. For all three species, stilt grass seemed to provide habitat value beyond any causal or correlative relationship with microclimate. These results suggest that some invasive species of herbaceous cover provide alternative habitat for native wildlife in degraded communities. Managers need to consider the effect on wildlife when considering removal of invasives, particularly when there is little native habitat and when removal would be destructive.
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Dissertation
Plants play a major role in the construction and maintence of ecosystems, supporting green food webs through herbivory and brown food webs through detrital inputs, affecting abiotic habitat variables, and influencing energy flow through these systems by changing the structural context in which trophic transfers take place. Alterations in these communities, such as those following plant invasion, may therefore affect consumers within impacted habitats. However, the multifaceted role of plants makes the nature and strength of these effects difficult to predict. This dissertation explores detritally and structurally-mediated effects through which invasion by the Asian grass Microstegium vimineum influences forest floor consumers through investigating belowground C dynamics, abiotic parameters, energy flow, and the growth and survival of 3200 American toads, 800 southern leopard frogs, and 200 eastern newts across eight independent invasion fronts. The mechanisms behind observed effects were further explored through manipulations of the presence of lycosid spider predators and short-term foraging and survival trials within paired invaded and uninvaded enclosures. Belowground, this plant was found to influence abiotic habitat parameters and carbon dynamics, potentially contributing to observed effects on the production of microarthropods. However, it also serves as an important basal resource, making it likely that bottom-up influences of this plant are a consequence of changes in detrital communites in invaded habitats rather than resource sequestration of nonindigenous plant tissues. Investigation of the mechanisms through which stilt grass influences the American toad demonstrated that structural changes initiated a contextually-mediated dampening of intraguild predation rates among lycosid spiders, increasing their survival, and thereby augmenting predation pressure on metamorphic toads. Interspecific comparisons of the effects of invasion on three amphibians utilizing disparate metamorphic strategies revealed that the nature of the influence of invasion may be predicted by the metamorphic strategy of the species in question, as small, actively foraging toads are suceptible to top-down pressures while larger, sit-and-wait leopard frogs are sensitive to bottom-up effects and cryptic, toxic eastern newts did not respond to trophic effects. This research demonstrates that invasion indirectly affects consumers through alterations in detrital inputs and habitat structure, although the nature and strength of these effects varies interspecifically.
Invasive species can induce shifts in habitat use by native taxa: either by modifying habitat availability, or by repelling or attracting native species to the vicinity of the invader. The ongoing invasion of cane toads (Rhinella marina) through tropical Australia might affect native frogs by affecting refuge-site availability, because both frogs and toads frequently shelter by day in burrows. Our laboratory and field studies in the wet-dry tropics show that native frogs of at least three species (Litoria tornieri, Litoria nasuta and Litoria dahlii) preferentially aggregate with conspecifics, and with (some) other species of native frogs. However, the frogs rarely aggregated with cane toads either in outdoor arenas or in standardized experimental burrows that we monitored in the field. The native frogs that we tested either avoided burrows containing cane toads (or cane toad scent) or else ignored the stimulus (i.e. treated such a burrow in the same way as they did an empty burrow). Native frogs selected a highly non-random suite of burrows as diurnal retreat sites, whereas cane toads were less selective. Hence, even in the absence of toads, frogs do not use many of the burrows that are suitable for toads. The invasion of cane toads through tropical Australia is unlikely to have had a major impact on retreat-site availability for native frogs.
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