Community Invasibility Research Articles

COMPOSITIONAL AND FUNCTIONAL STABILITY OF ARTHROPOD COMMUNITIES IN THE FACE OF ANT INVASIONS

There is a general consensus that the diversity of a biotic community can have an influence on its stability, but the strength, ubiquity, and relative importance of this effect is less clear. In the context of biological invasions, diversity has usually been studied in terms of its effect on a community's invasibility, but diversity may also influence stability by affecting the magnitude of compositional or functional changes experienced by a community upon invasion. We examined the impacts of invasive ants on arthropod communities at five natural area sites in the Hawaiian Islands, and assessed whether differences among sites in community diversity and density variables were related to measures of stability. Ant invasion was usually associated with significant changes in overall community composition, as measured by Bray-Curtis distances, particularly among endemic subsets of the communities. Changes in mean species richness were also strong at three of the five sites. Among sites, diversity was negatively related to stability as measured by resistance to overall compositional change, but this effect could not be separated from the strong negative effect of invasive ant density on compositional stability. When compositional stability was measured as proportional change in richness, the best predictor of stability among endemic community subsets was endemic richness, with richer communities losing proportionately more species than species-poor communities. This effect was highly significant even after controlling for differences in invasive ant density and suggested that communities that had already lost many endemic species were resistant to further species loss upon ant invasion, while more intact communities remained vulnerable to species loss. Communities underwent strong but idiosyncratic functional shifts in association with ant invasion, both in terms of trophic structure and total arthropod biomass. There were no apparent relationships, however, between functional stability and community diversity or density measures. Instead, invasive ant density was the best among-site predictor of the magnitude of functional change. Overall, diversity appeared to be a poor predictor of stability in the face of ant invasion in these communities, possibly because any actual diversity effects were overshadowed by community-specific factors and variation in the magnitude of the ant-mediated perturbation.

Invasibility of experimental grassland communities: the role of earthworms, plant functional group identity and seed size

Invasions of natural communities by non‐indigenous species threaten native biodiversity and are currently rated as one of the most important global‐scale environmental problems. The mechanisms that make communities resistant to invasions and drive the establishment success of seedlings are essential both for management and for understanding community assembly and structure. Especially in grasslands, anecic earthworms are known to function as ecosystem engineers, however, their direct effects on plant community composition and on the invasibility of plant communities via plant seed burial, ingestion and digestion are poorly understood.In a greenhouse experiment we investigated the impact of Lumbricus terrestris, plant functional group identity and seed size of plant invader species and plant functional group of the established plant community on the number and biomass of plant invaders. We set up 120 microcosms comprising four plant community treatments, two earthworm treatments and three plant invader treatments containing three seed size classes.Earthworm performance was influenced by an interaction between plant functional group identity of the established plant community and that of invader species. The established plant community and invader seed size affected the number of invader plants significantly, while invader biomass was only affected by the established community. Since earthworm effects on the number and biomass of invader plants varied with seed size and plant functional group identity they probably play a key role in seedling establishment and plant community composition.Seeds and germinating seedlings in earthworm burrows may significantly contribute to earthworm nutrition, but this deserves further attention. Lumbricus terrestris likely behaves like a ‘farmer’ by collecting plant seeds which cannot directly be swallowed or digested. Presumably, these seeds are left in middens and become eatable after partial microbial decay. Increased earthworm numbers in more diverse plant communities likely contribute to the positive relationship between plant species diversity and resistance against invaders.

A Two‐Part Measure of Degree of Invasion for Cross‐Community Comparisons

Invasibility is a critical feature of ecological communities, especially for management decisions. To date, invasibility has been measured in numerous ways. Although most researchers have used the richness (or number) of exotic species as a direct or indirect measure of community invasibility, others have used alternative measures such as the survival, density, or biomass of either a single or all exotic species. These different measures, even when obtained from the same communities, have produced inconsistent results and have made comparisons among communities difficult. Here, we propose a measure of the degree of invasion (DI) of a community as a surrogate for community invasibility. The measure is expressed as 2 independent components: exotic proportion of total species richness and exotic proportion of total species abundance (biomass or cover). By including richness and abundance, the measure reflects that the factors that control invasibility affect both of these components. Expressing exotic richness and abundance relative to the richness and abundance of all species in a community makes comparisons across communities of different sizes and resource availability possible and illustrates the importance of dominance of exotic species relative to natives, which is a primary management concern associated with exotic species.

Invasion of Broom Snakeweed (Gutierrezia sarothrae) Following Disturbance: Evaluating Change in a State-and-Transition Model

Broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. & Rusby) is an aggressive native invasive species that thrives after disturbance in semiarid rangelands of the western United States. A 5-yr (2002–2006) study was initiated following grazing and fire disturbances on an Upland Gravelly Loam ecological site in the sagebrush steppe of northern Utah, to evaluate broom snakeweed invasion in different plant communities. The study site originally had two plant communities: a sagebrush/bunchgrass community that received alternate-year, fall cattle grazing, and was dominated by bluebunch wheatgrass (Elymus spicatus) and an open stand of Wyoming big sagebrush (Artemisia tridentata subsp. wyomingensis), and a sagebrush community that received continuous, annual, spring cattle grazing that removed the bunchgrasses, leaving a dense stand of Wyoming big sagebrush with an understory of Sandberg bluegrass (Poa secunda). Portions of these two plant communities were burned in a wildfire in 2001, removing the sagebrush, and creating two additional communities. The burned portion of the sagebrush/bunchgrass community became a bluebunch wheatgrass–dominated community, and the burned portion of the sagebrush community became a snakeweed-dominated community. Foliar cover, aboveground biomass, and sagebrush and snakeweed densities were compared among the four communities. Mature snakeweed plants that existed in the sagebrush/bunchgrass community were eliminated in 2003, because of drought conditions. Snakeweed was eliminated in the bluebunch wheatgrass community by the wildfire in 2001, and there was no reestablishment. Snakeweed density and cover remained constant in the sagebrush community. Snakeweed cover increased from 2% to 31% in the snakeweed community, despite the presence of Sandberg bluegrass. The data were used to evaluate and update the current Upland Gravelly Loam (Wyoming big sagebrush) ecological site description in the Great Salt Lake Major Land Resource Area and its state-and-transition model to reflect vegetation changes associated with snakeweed invasion.

Biotic interactions and their consequences for macroevolution: learning from the fossil record and beyond.

Every organism interacts with a host of other organisms of the same and different species throughout its life. These biotic interactions have varying influences on the reproduction and dispersal of the organism, and hence also the population and species lineage to which the organism belongs. By extension, biotic interactions must contribute to the macroevolutionary patterns that we observe in the fossil record, but exactly how, when and why are research questions we have been asking before the start of the journal Paleobiology. In this contribution for Paleobiology's 50th anniversary, we present a brief overview of how paleobiologists have studied biotic interactions and their macroevolutionary consequences, recognizing paleontology's unique position to contribute data and insights to the topic of interspecies interactions. We then explore, in a semi free-form manner, what promising avenues might be open to those of us who use the fossil record to understand biotic interactions. In general, we emphasize the need for an increased effort in the understanding of ecological details, the integration of different types of information, and to strive for model-based approaches.

Assessing an exotic plant surveying program in the Mojave Desert, Clark County, Nevada, USA

Exotic species can threaten native ecosystems and reduce services that ecosystems provide to humans. Early detection of incipient populations of exotic species is a key step in containing exotics before explosive population growth and corresponding impacts occur. We report the results of the first three years of an exotic plant early detection and treatment program conducted along more than 3,000 km of transportation corridors within an area >1.5 million ha in the Mojave Desert, USA. Incipient populations of 43 exotic plant species were mapped using global positioning and geographic information systems. Brassica tournefortii (Sahara mustard) infested the most soil types (47% of 256) surveyed in the study area, while Nicotiana glauca (tree tobacco) and others currently occupy less than 5% of soil types. Malcolmia africana (African mustard) was disproportionately detected on gypsum soils, occurring on 59% of gypsum soil types compared to 27% of all surveyed soils. Gypsum soils constitute unique rare plant habitat in this region, and by conventional wisdom were not previously considered prone to invasion. While this program has provided an initial assessment of the landscape-scale distribution of exotic species along transportation corridors, evaluations of both the survey methods and the effectiveness of treating incipient populations are needed. An exotic plant information system most useful to resource mangers will likely include integrating planning oriented coarse-scale surveys, more detailed monitoring of targeted locations, and research on species life histories, community invasibility, and treatment effectiveness.

Effects of experimental manipulation of light and nutrients on establishment of seedlings of native and invasive woody species in Long Island, NY forests

While earlier studies on the process of invasion often focused on single factors or on the general explanation of ‘disturbance,’ recent work has attempted to move towards a more mechanistic understanding of the factors that promote plant community invasion. Manipulative experiments provide a means for discerning causal relationships and interactive effects of environmental factors in promoting invasion; such experiments have been conducted in a number of grassland and shrub ecosystems. This study extends multifactor manipulative experiments into forest communities to compare factors influencing early seedling establishment for native and invasive woody plants. In Long Island, NY, invasion patterns are correlated with forest community type (pine barrens or hardwood), light availability, and soil N and Ca. We conducted manipulative field experiments in two different years to determine the relative importance and interaction of experimental gaps and N and Ca addition in pine barrens and hardwood forests in promoting invasion. We used seedlings of seven common native and invasive species in the first experiment, and 16 native and invasive species paired phylogenetically in the second experiment. Light had the strongest effect on plant growth; all plants grew more in gaps. We found no difference in the average growth rates of native and invasive species. Invasives responded more to high resources than did natives, with highest relative growth rates in gaps in the more fertile soils of the hardwood forests. Opportunities for invasion may differ from year to year, with differential success of invaders only in some years and under some environmental conditions. Clearly, to understand the complex interactions between resources and invasion in forests will require many manipulative experiments across a range of environments and using suites of invasive and native species.

Effect of plant species richness on invasibility of experimental plant communities

Invasion of unsown species to artificially created assemblages of grassland species was inves- tigated in a 3-year field experiment. In the experiment, assemblages varying in species richness (1, 2, 4, 8, and 16 species) and functional group richness (1-4, grasses, legumes, rosette forbs, and creeping forbs) were grown in control and fertilized plots, without any attempt to prevent the invasion of unsown species or to weed them. The relationship between species and functional group diversity and above-ground biomass was positive for sown species in all study years (2003, 2004 and 2005). In the latter 2 years, weed invader biomass decreased significantly with increasing bio- mass of sown species and their functional group richness, but not with number of species. However, no suppressive effect of species or functional group richness beyond that by increased biomass of residents was found. In fact, slight but significant positive partial effect of species richness was found, suggesting that the negative effect of the same amount of resident biomass on invaders is stronger when the biomass is composed of fewer species. The negative relationship between the number of functional groups of residents and invader biomass suggested that better coverage of functional trait space could be a mechanism promoting the resistance to invasion. In Addition, species com- position of invaders were significantly related to initial composition of sown residents.

Does fall prescribed burning Artemisia tridentata steppe promote invasion or resistance to invasion after a recovery period?

Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh-bunchgrass communities were used to analyze the influence of disturbances on invasibility after a recovery period. These communities evolved with periodic fires shifting dominance from shrubs to herbaceous species. However, fire can facilitate Bromus tectorum L. invasion of these plant communities. We evaluated the invasibility of A. tridentata ssp. wyomingensis-bunchgrass communities 4 years after prescribed fall burning at six sites by comparing burned to unburned (control) communities. These communities did not have B. tectorum present prior to introduction. B. tectorum was introduced at 1, 10, 100, 1000, and 10,000seedsm−2 in burned and unburned communities. B. tectorum individuals established only when introduced at 10,000seedsm−2. In the areas seeded at 10,000seedsm−2, B. tectorum density and cover were more than three-fold higher in the control than burned treatments (P=0.04 and 0.08, respectively). Total herbaceous vegetation cover, density, and production increased with burning (P<0.01, 0.02, and <0.01, respectively). Bare ground and inorganic nitrogen were higher in the control than the burned treatment (P=0.02 and <0.01, respectively). Prescribed fall burning of late seral A. tridentata ssp. wyomingensis-bunchgrass communities stimulated the herbaceous component and increased the resistance of the communities to B. tectorum invasion 4 years post-burn. However, we do not suggest the use of prescribed burning in communities where invasive annual grasses are present or in close proximity. We acknowledge that our results would probably have been drastically different if B. tectorum or other invasive annual grasses had been a component of the plant communities prior to prescribed burning or became a component immediately after burning.

Declines in plant species richness and endemic plant species in longleaf pine savannas invaded by Imperata cylindrica

Despite the widespread perception that non-native species threaten biodiversity, there are few documented cases of non-native species displacing rare or specialized native species. Here, I examined changes in plant species composition over 5 years during patch expansion of a non-native grass, Imperata cylindrica, in longleaf pine flatwoods in Mississippi, USA. I used a multivariate approach to quantify the degree of habitat specialization and geographic range of all species encountered. I examined losses of species collectively as a function of plant height (controlling for initial frequency) and then the relationship between height and the degree of association with longleaf pine flatwoods, disturbed habitats, and the outer Gulf Coastal Plain of the southeastern USA. Patch expansion resulted in dramatic declines in species richness and increases in ground-level shade at both sites in just 3 years. Most tall saplings, shrubs, and vines were not endemic to longleaf pine communities and were less likely to be displaced than short herbs, most of which were indicative of longleaf pine communities. These results suggest that invasion of longleaf pine communities by I. cylindrica will likely cause significant losses of short, habitat-specialists and reduce the distinctiveness of the native flora of these threatened ecosystems.

DIVERSITY–INVASIBILITY ACROSS AN EXPERIMENTAL DISTURBANCE GRADIENT IN APPALACHIAN FORESTS

Research examining the relationship between community diversity and invasions by nonnative species has raised new questions about the theory and management of biological invasions. Ecological theory predicts, and small-scale experiments confirm, lower levels of nonnative species invasion into species-rich compared to species-poor communities, but observational studies across a wider range of scales often report positive relationships between native and nonnative species richness. This paradox has been attributed to the scale dependency of diversity-invasibility relationships and to differences between experimental and observational studies. Disturbance is widely recognized as an important factor determining invasibility of communities, but few studies have investigated the relative and interactive roles of diversity and disturbance on nonnative species invasion. Here, we report how the relationship between native and nonnative plant species richness responded to an experimentally applied disturbance gradient (from no disturbance up to clearcut) in oak-dominated forests. We consider whether results are consistent with various explanations of diversity-invasibility relationships including biotic resistance, resource availability, and the potential effects of scale (1 m2 to 2 ha). We found no correlation between native and nonnative species richness before disturbance except at the largest spatial scale, but a positive relationship after disturbance across scales and levels of disturbance. Post-disturbance richness of both native and nonnative species was positively correlated with disturbance intensity and with variability of residual basal area of trees. These results suggest that more nonnative plants may invade species-rich communities compared to species-poor communities following disturbance.

Contrasting effects of resource availability and plant mortality on plant community invasion by Bromus tectorum L.

The positive effect of disturbance on plant community invasibility is one of the more consistent results in invasion ecology. It is generally attributed to a coincident increase in available resources (due to the disturbance) that allows non-resident plant species to establish (Davis MA, Grime JP Thompson K, J Ecol 88:528–534, 2000). However, most research addressing this issue has been in artificial or highly modified plant communities. Our goal in this study was to investigate the interactive effects of resource availability and plant mortality disturbance on the invasion of natural plant communities. We conducted a series of experiments that examined the response of Bromus tectorum L., a highly invasive annual grass, to experimentally created gradients of resource availability [nitrogen (N) and water] and resident plant species mortality. We found that B. tectorum biomass was co-limited by N and water. Biomass at the end of the growing season was a saturating function (i.e., increased to a maximum) of water, which determined maximum biomass, and N, which determined the rate at which maximum biomass was attained. Despite that fact that plant mortality increased N availability, it had a negative impact on invasion success. Plant mortality also decreased foliar cover, standing dead biomass, and soil cover by litter. In harsh environments, removing foliar and soil cover may increase germination and seedling stress by increasing soil temperatures and water loss. Across all treatments, B. tectorum success decreased with decreasing foliar cover and standing dead biomass. This, in combination with the strong limitation of B. tectorum biomass by water in this experiment, suggests that our plant mortality disturbance removed soil cover that may have otherwise aided B. tectorum invasion into this semi-arid plant community by reducing water stress.

Short‐ and long‐term effects of disturbance and propagule pressure on a biological invasion

1 Invading species typically need to overcome multiple limiting factors simultaneously in order to become established, and understanding how such factors interact to regulate the invasion process remains a major challenge in ecology. 2 We used the invasion of marine algal communities by the seaweed Sargassum muticum as a study system to experimentally investigate the independent and interactive effects of disturbance and propagule pressure in the short term. Based on our experimental results, we parameterized an integrodifference equation model, which we used to examine how disturbances created by different benthic herbivores influence the longer term invasion success of S. muticum. 3 Our experimental results demonstrate that in this system neither disturbance nor propagule input alone was sufficient to maximize invasion success. Rather, the interaction between these processes was critical for understanding how the S. muticum invasion is regulated in the short term. 4 The model showed that both the size and spatial arrangement of herbivore disturbances had a major impact on how disturbance facilitated the invasion, by jointly determining how much space-limitation was alleviated and how readily disturbed areas could be reached by dispersing propagules. 5 Synthesis. Both the short-term experiment and the long-term model show that S. muticum invasion success is co-regulated by disturbance and propagule pressure. Our results underscore the importance of considering interactive effects when making predictions about invasion success.

Pinus radiata invasion in Australia: Identifying key knowledge gaps and research directions

Abstract Early recognition of plant invaders has been widely identified as the key to their successful management and yet too often species are only noticed and receive adequate attention once they have become widespread and control has become difficult and costly. Slow growing species are at particular risk of being overlooked, despite their ability to cause significant ecological damage. One such species, Pinus radiata (Monterey pine), has spread from large commercial plantations into native vegetation across the southern hemisphere. Here we review the status of P. radiata invasion in Australia, a country where the species has successfully naturalized but remains an invader of only low‐level concern. Patterns of spread in Australia mirror those in New Zealand and South Africa, two countries where invasive pines are considered major threats to biodiversity. While many areas adjacent to plantations remain free from invasion, dense infestations have occurred at several sites in areas of high conservation value demanding the implementation of adequate control measures. Expansion of Australia's plantation estate and increasing human disturbance of natural areas surrounding plantations will increase the likelihood and extent of invasion. Continued monitoring of wildling populations will determine the ability of pines to dominate native eucalypt forest and will provide insight into broader ideas of community invasibility.

Evolutionary cause of the vulnerability of insular communities

Communities on oceanic islands are considered to be vulnerable to biological invasion. However, because the detailed structures of such communities have not yet been revealed, the relationship between their vulnerability and structure is not clear. Because such communities evolved without biological invasion, they are expected to have structures different from those of mainland communities, and this difference is expected to affect their vulnerability to invasion. I conducted computer simulations based on a food web model and investigated the difference in structure between mainland and insular model communities, the former of which evolved with frequent invasion and the latter without invasion. In addition, by conducting computer simulations of invasion of these model communities, I investigated the relationship between community structure and vulnerability to biological invasion. The insular model community evolved to have an unstable structure, in that a small number of plant species supported a large number of animal species, and each species in the community had a small biomass. When a plant species invaded and disturbed the base of the insular model community, many animal species relying on the plants easily became extinct. In addition, when a carnivorous species invaded, animal species with small biomass tended to become extinct. Community collapses caused by biological invasion occurred more frequently in the insular model community than in the mainland model community. These results indicated that those communities that evolved without invasion were vulnerable to invasion. The available data on real insular communities suggest that some have reached the endangered state predicted by this model.

Limiting similarity between invaders and dominant species in herbaceous plant communities?

Summary 1 Limiting similarity theory predicts that successful invaders should differ functionally from species already present in the community. This theory has been tested by manipulating the functional richness of communities, but not other aspects of functional diversity such as the identity of dominant species. Because dominant species are known to have strong effects on ecosystem functioning, I hypothesized that successful invaders should be functionally dissimilar from community dominants. 2 To test this hypothesis, I added seeds of 17 different species to two different experiments: one in a natural oldfield community that had patches dominated by different plant species, and one in grassland mesocosms that varied in the identity of the dominant species but not in species richness or evenness. I used indicator species analyses to test whether invaders had higher establishment success in plots with functionally different dominant species. 3 A large percentage of invader species (47–71%) in both experiments showed no difference in affinity across the different dominant treatments, although one-third of species did show some evidence for limiting similarity. Exotic invaders had much higher invasion success than native invaders, and seemed to be inhibited by dominant species that were functionally similar. However, even these invasion patterns were not consistent across the two experiments. 4 The results from this study show that there is some evidence that dominant species suppress invasion by functionally similar species, beyond the effect of simple presence or absence of species in communities, although it is not the sole factor affecting invasion success. Patterns of invasion success were inconsistent across species and experiments, indicating that other studies using only a single species of invader to make conclusions about community invasibility should be interpreted with caution.

Species evenness and invasion resistance of experimental grassland communities

Concern for biodiversity loss coupled with the accelerated rate of biological invasions has provoked much interest in assessing how native plant species diversity affects invasibility. Although experimental studies extensively document the effects of species richness on invader performance, the role of species evenness in such studies is rarely examined. Species evenness warrants more attention because the relative abundances of species can account for substantially more of the variance in plant community diversity and tend to change more rapidly and more frequently in response to disturbances than the absolute numbers of species. In this study, we experimentally manipulated species evenness within native prairie grassland mesocosms. We assessed how evenness affected primary productivity, light availability and the resistance of native communities to invasion. The primary productivity of native communities increased significantly with species evenness, and this increase in productivity was accompanied by significant decreases in light availability. However, evenness had no effect on native community resistance to invasion by three common exotic invasive species. In this study, niche complementarity provides a potential mechanism for the effects of evenness on productivity and light availability, but these effects apparently were not strong enough to alter the invasibility of the experimental communities. Our results suggest that species evenness enhances community productivity but provides no benefit to invasion resistance in otherwise functionally diverse communities.

Habitat stress, species pool size and biotic resistance influence exotic plant richness in the Flooding Pampa grasslands

Summary Theory and empirical evidence suggest that community invasibility is influenced by propagule pressure, physical stress and biotic resistance from resident species. We studied patterns of exotic and native species richness across the Flooding Pampas of Argentina, and tested for exotic richness correlates with major environmental gradients, species pool size, and native richness, among and within different grassland habitat types. Native and exotic richness were positively correlated across grassland types, increasing from lowland meadows and halophyte steppes, through humid to mesophyte prairies in more elevated topographic positions. Species pool size was positively correlated with local richness of native and exotic plants, being larger for mesophyte and humid prairies. Localities in the more stressful meadow and halophyte steppe habitats contained smaller fractions of their landscape species pools. Native and exotic species numbers decreased along a gradient of increasing soil salinity and decreasing soil depth, and displayed a unimodal relationship with soil organic carbon. When covarying habitat factors were held constant, exotic and native richness residuals were still positively correlated across sites. Within grassland habitat types, exotic and native species richness were positively associated in meadows and halophyte steppes but showed no consistent relationship in the least stressful, prairie habitat types. Functional group composition differed widely between native and exotic species pools. Patterns suggesting biotic resistance to invasion emerged only within humid prairies, where exotic richness decreased with increasing richness of native warm‐season grasses. This negative relationship was observed for other descriptors of invasion such as richness and cover of annual cool‐season forbs, the commonest group of exotics. Our results support the view that ecological factors correlated with differences in invasion success change with the range of environmental heterogeneity encompassed by the analysis. Within narrow habitat ranges, invasion resistance may be associated with either physical stress or resident native diversity. Biotic resistance through native richness, however, appeared to be effective only at intermediate locations along a stress/fertility gradient. We show that certain functional groups, not just total native richness, may be critical to community resistance to invasion. Identifying such native species groups is important for directing management and conservation efforts.

DOMINANT SPECIES IDENTITY, NOT COMMUNITY EVENNESS, REGULATES INVASION IN EXPERIMENTAL GRASSLAND PLANT COMMUNITIES

While there has been extensive interest in understanding the relationship between diversity and invasibility of communities, most studies have only focused on one component of diversity: species richness. Although the number of species can affect community invasibility, other aspects of diversity, including species identity and community evenness, may be equally important. While several field studies have examined how invasibility varies with diversity by manipulating species identity or evenness, the results are often confounded by resource heterogeneity, site history, or disturbance. We designed a mesocosm experiment to examine explicitly the role of dominant species identity and evenness on the invasibility of grassland plant communities. We found that the identity of the dominant plant species, but not community evenness, significantly impacted invasibility. Using path analysis, we found that community composition (dominant species identity) reduced invasion by reducing early-season light availability and increasing late-season plant community biomass. Nitrogen availability was an important factor for the survival of invaders in the second year of the experiment. We also found significant direct effects of certain dominant species on invasion, although the mechanisms driving these effects remain unclear. The magnitude of dominant species effects on invasibility we observed are comparable to species richness effects observed in other studies, showing that species composition and dominant species can have strong effects on the invasibility of a community.

Experimental Light Treatments Affect Invasion Success and the Impact of Microstegium vimineum on the Resident Community

In forests of the eastern United States, exotic plant invasions can reduce native plant diversity and change ecosystem processes. Many invasive plants colonize forest edge habitat, but little is known about the ability of the exotic annual grass Microstegium vimineum to invade edges compared to interior forest. In particular, the extent of invasion and impact on resident plants might change along the environmental gradient from edge to interior sites. A common garden study was used to test how Microstegium invasion affected six forest understory graminoid species across three experimental light treatments. Contamination of the Microstegium seed stock with seed of the dominant native perennial grass Dichanthelium clandestinum provided the opportunity to study the interaction between the two species across the light treatments. The light treatments shifted the competitive balance between Microstegium and Dichanthelium, with Dichanthelium dominating in full sun, Microstegium dominating in full shade, and the two species producing more similar biomass in part shade. Invasion of resident communities by these two species resulted in a significant reduction in resident community biomass in part shade but not in full sun or full shade. Three of the six resident graminoid species produced significantly less biomass under invaded conditions. Successful management of Microstegium will require that land managers monitor partly shaded and fully shaded forests to locate invading populations. Invasions should be treated quickly to minimize impacts on resident community species.