Native, insect herbivore communities derive a significant proportion of their carbon from a widespread invader of forest understories
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- Bradford, M.A., DeVore, J.L., Maerz, J.C. et al. Biol Invasions (2010) 12: 721. doi:10.1007/s10530-009-9480-x
Research on natural enemies demonstrates the potential for exotic plants to be integrated into foodwebs through the activities of native herbivores. The quantitative importance of exotics as a food resource to herbivores is more difficult to ascertain. In addition, some widespread invaders appear to have minimal herbivore loads. Microstegium vimineum is one example. It is an annual, C4 grass that invades forest understories and is widespread across the eastern US. Its invasion alters the structure and composition of forests. We sampled invertebrates in a tree-canopy gap and under canopy area, and used the unique carbon isotope value of M. vimineum to estimate the quantitative importance of the invader as a food resource relative to native plants. Seven of the eight invertebrate species derived on average >35% of their biomass carbon from M. vimineum, and some individuals representing both ‘chewing’ and ‘sucking’ feeding guilds derived their biomass carbon exclusively from M. vimineum. Our results show that M. vimineum can be a significant food resource for a multi-species, multi-guild, assemblage of native, invertebrate herbivores. Future work is required to assess whether M. vimineum is acquiring herbivores in other parts of its introduced range, and if so what might be the ecological consequences.
KeywordsEnemy release hypothesisExotic speciesGrass invasionGrasshoppersHardwood forestsHerbivoryInvertebrate herbivoresJapanese stilt grassMicrostegium vimineumNepalese browntop
Exotic plant species can be integrated into native foodwebs through feeding activities of native herbivores. The quantitative importance of exotic plant species as a food resource to herbivores is, however, difficult to ascertain. In addition, some widespread, invasive plant species appear to have minimal herbivore loads. Microstegium vimineum (Trin.) Camus is one example. There is no evidence that it is browsed by deer (Webster et al. 2008) or other vertebrates, and invertebrate herbivore damage rates on M. vimineum are low, ranging from 0.4 to <10% of leaf tissue removed (Morrison et al. 2007; Sanders et al. 2004). It invades forest understories across 25 US states (http:/plants.usda.gov/) and, given the extent of its distribution, understanding its impacts is important (Morrison et al. 2007). These include alterations to ecosystem structure (Baiser et al. 2008), plant and faunal communities (Baiser et al. 2008; Civitello et al. 2008; Oswalt et al. 2007; Vidra et al. 2006), soil properties (Kourtev et al. 2002), and success of other invasive plants (Belote and Weltzin 2006; Morrison et al. 2007). Whether its apparent lack of herbivores explains its expanding distribution and consequent impacts is unknown, as is its relevance to green foodwebs within its introduced range. Here, we report on an opportunistic study, based on the observation that two, visually abundant, species of Orthoptera resided on M. vimineum foliage in an invaded canopy gap. Given that M. vimineum is the only plant to utilize the C4-photosynthetic pathway at the site, we reasoned that if the Orthoptera were feeding on M. vimineum then they would have a stable carbon (C) isotope value distinct from that of the foodweb based on native plants, which all use the C3-photosynthetic pathway (see Fry 2006).
Materials and methods
A tree-canopy gap and an under canopy habitat were identified within a rapidly progressing M. vimineum invasion in a riparian forest within the Whitehall Experimental Forest (WEF), Athens, GA, USA (N 33°53.27′ W 83°21.93′). Anecdotal reports indicate M. vimineum established within the WEF ~15 years ago. The forest overstory is composed of Acer rubrum, Quercus nigra, Platanus occidentalis, and Liquidambar styraciflua. The uninvaded areas of the site are generally depauperate in understory plants (<5% cover) but M. vimineum forms a continuous lawn.
We spent 2 person hours per habitat, on September 15th 2008, using a sweep-net and hands to catch invertebrates located in the understories. Next, for M. vimineum we took five leaves from 10 individuals in each habitat and for native plants we collected five leaves from any native trees and herbs in the understory that exhibited signs of invertebrate leaf-chewers. Plant and invertebrate materials were dried at 65°C to constant mass, ball-milled to a fine powder and then weighed into tin cups for isotope determinations.
Microstegium vimineum has a C4-photosynthetic value and the difference in the C isotope composition from the natives, which use the C3-pathway, is sufficient to discriminate sources (see Fry 2006). The proportion of C derived from M. vimineum was calculated as: CM. vimineum derived = (δ13Cinvert − δ13Cnative)/(δ13CM. vimineum − δ13Cnative), where δ13Cinvert is the δ13C value of the biomass for each invertebrate, δ13Cnative is the mean δ13C value of the native leaf tissues and δ13CM. vimineum is the mean value for the M. vimineum tissues. The δ13C values were determined using continuous-flow isotope-ratio mass spectrometry (Thermo, San Jose, CA, USA).
Results and discussion
Identification and percentage biomass carbon derived from M. vimineum of invertebrate herbivores found on M. vimineum foliage in Whitehall Experimental Forest, Georgia, USA. Habitats are a canopy gap (CG) or under canopy (UC) patch
Invert. family, order, genus species
Common family name
# Indv. sampled
δ13C value of indv. (‰)
C derived from M. vimineum (%)
Orthoptera, Acrididae, Metaleptea brevicornis (L.)
Orthoptera, Tettigoniidae, Orchelimum sp.
Orthoptera, Tettigoniidae, Neoconocephalus sp.
Orthoptera, Tettigoniidae, Orchelimum sp.
Orthoptera, Gryllidae, Gryllus sp.
Hemiptera, Coreidae, Leptoglossus oppositus (Say).
Hemiptera, Largidae; Largus sp.a
Bordered plant bugs
Hemiptera, Pentatomidae, Dendrocoris sp.b
All invertebrate species recovered (except Leptoglossus oppositus) derived a substantial fraction of their biomass C from M. vimineum (Table 1). There was pronounced variation in the amount of biomass C derived from M. vimineum between species and, in the canopy gap habitat where >1 individual of the same species was sampled, within a species (Table 1). Notably, some species and/or individuals appeared to derive their biomass C exclusively from M. vimineum and this was true for multiple feeding guilds, taxonomic orders, and life stages (Table 1).
We have been working within the M. vimineum invasion described here since the 2005 growing season. The accumulation of an indigenous, invertebrate, herbivore community by an exotic, plant invader appears a recent occurrence (i.e., in 2008) at our site. Although our inferences are limited to a single invasion of M. vimineum within a hardwood forest, we cautiously venture outside the boundaries of our study site to discuss the potential ecological significance of our findings. First, M. vimineum has invaded forests over a large portion of the eastern and southern US and has wide-ranging impacts (e.g., Baiser et al. 2008). Its distribution and impact on forest ecosystems alone necessitate it receive the attention by the ecological community which is being afforded to other plant invaders (see Morrison et al. 2007). An important research focus will be to ask how the acquisition of herbivores might alter these impacts on forest ecosystems. If the herbivory we observed is sustained in future growing seasons, one might hypothesize that acquisition of enemies might reduce M. vimineum’s abundance and potentially distribution. However, non-invaded areas of our forested site have a highly depauperate understory and so M. vimineum might actually increase herbivore pressure on native plants by providing a resource for the native herbivores to increase their population sizes. This might then increase the apparent competitive advantage of M. vimineum. Future work is required to assess the extent and magnitude of herbivory on M. vimineum across its introduced range and, if significant, what the ecological consequences might be for the invaded ecosystems.
This research was supported by National Science Foundation funding to Coweeta LTER. Tom Maddox in the Odum School of Ecology performed isotope analyses.