A synergistic trio of invasive mammals? Facilitative interactions among beavers,muskrats, and mink at the southern end of the Americas

With ecosystems increasingly having co-occurring invasive species, it is becoming more important to understand invasive species interactions. At the southern end of the Americas, American beavers (Castor canadensis), muskrats (Ondatra zibethicus), and American mink (Neovison vison), were independently introduced. We used generalized linear models to investigate how muskrat presence related to beaver-modified habitats on Navarino Island, Chile. We also investigated the trophic interactions of the mink with muskrats and beavers by studying mink diet. Additionally, we proposed a conceptual species interaction framework involving these invasive species on the new terrestrial community. Our results indicated a positive association between muskrat presence and beaver-modified habitats. Model average coefficients indicated that muskrats preferred beaver-modified freshwater ecosystems, compared to not dammed naturally flowing streams. In addition, mammals and fish represented the main prey items for mink. Although fish were mink’s dominant prey in marine coastal habitats, muskrats represented >50 % of the biomass of mink diet in inland environments. We propose that beavers affect river flow and native vegetation, changing forests into wetlands with abundant grasses and rush vegetation. Thus, beavers facilitate the existence of muskrats, which in turn sustain inland mink populations. The latter have major impacts on the native biota, especially on native birds and small rodents. The facilitative interactions among beavers, muskrats, and mink that we explored in this study, together with other non-native species, suggest that an invasive meltdown process may exist; however further research is needed to confirm this hypothesis. Finally, we propose a community-level management to conserve the biological integrity of native ecosystems.     

Tidal and seasonal effects on survival rates of the endangered California clapper rail: does invasive Spartina facilitate greater survival in a dynamic environment?

Invasive species frequently degrade habitats, disturb ecosystem processes, and can increase the likelihood of extinction of imperiled populations. However, novel or enhanced functions provided by invading species may reduce the impact of processes that limit populations. It is important to recognize how invasive species benefit endangered species to determine overall effects on sensitive ecosystems. For example, since the 1990s, hybrid Spartina (Spartina foliosa × alterniflora) has expanded throughout South San Francisco Bay, USA, supplanting native vegetation and invading mudflats. The endangered California clapper rail (Rallus longirostris obsoletus) uses the tall, dense hybrid Spartina for cover and nesting, but the effects of hybrid Spartina on clapper rail survival was unknown. We estimated survival rates of 108 radio-marked California clapper rails in South San Francisco Bay from January 2007 to March 2010, a period of extensive hybrid Spartina eradication, with Kaplan–Meier product limit estimators. Clapper rail survival patterns were consistent with hybrid Spartina providing increased refuge cover from predators during tidal extremes which flood native vegetation, particularly during the winter when the vegetation senesces. Model averaged annual survival rates within hybrid Spartina dominated marshes before eradication (? = 0.466) were greater than the same marshes posttreatment (? = 0.275) and a marsh dominated by native vegetation (? = 0.272). However, models with and without marsh treatment as explanatory factor for survival rates had nearly equivalent support in the observed data, lending ambiguity as to whether hybrid Spartina facilitated greater survival rates than native marshland. Conservation actions to aid in recovery of this endangered species should recognize the importance of available of high tide refugia, particularly in light of invasive species eradication programs and projections of future sea-level rise.     

Cover thresholds for impacts of an exotic grass on the structure and assembly of a wet prairie community

Potential impacts of an exotic grass, Hemarthria altissima, on restoration of wet prairie community structure (species richness and cover of indicator species) and assembly processes (temporal turnover rates of plant species) on the Kissimmee River floodplain in Central Florida, USA, were evaluated over a 12-year period before and after restoration of hydrologic regimes (2001), and implementation of herbicide treatments (2006–2007) to control its spread. Thresholds for impacts were derived from comparisons of sample sites with variable levels of H. altissima cover. Prior to herbicide treatments, cover of H. altissima exhibited a logistic increase over time, with peak colonization and expansion occurring during major flood events. Mean post-restoration cover of three native wet prairie indicator species (Polygonum punctatum, Panicum hemitomon, and Luziola fluitans) increased to 37.8 ± 3.4 % in plots in which H. altissima cover was <12 %, and did not exceed 15 % in any plots with H. altissima cover >30 %. Prior to and after herbicide treatments, these indicator species largely accounted for observed differences in wet prairie community structure (i.e., cover of wetland forbs and grasses) between heavily infested sites and plots with low or no cover of H. altissima. The cover threshold at which H. altissima began to have these community-level effects was 40–50 %, but lower species richness was found only where H. altissima cover was >80 %. Increasing cover of H. altissima led to a significant decline in temporal turnover rates of plant species (P < 0.001, r² = 0.10), but also was largely due to plots with very high (>75 %) cover of H. altissima. Mean temporal turnover rates of plant species increased significantly (P = 0.03) after herbicide treatments and subsequently were highest during an ensuing flood pulse. However, 2–3 years after herbicide treatments, regrowth of H. altissima reestablished high cover (mean = 59 ± 9.5 %) in over half of the treated plots. The ability of H. altissima to establish dominant cover in restored hydrologic conditions on the Kissimmee River floodplain, and documented regrowth following herbicide treatments, increase the potential for this exotic grass species to be a pervasive threat to successful reestablishment of wet prairie community structure and assembly processes.     

Co-invasion of similar invaders results in analogous ecological impact niches and no synergies

Exotic species can cause changes to their invaded ecosystems, which can be large and long lasting. Despite most landscapes being invaded by multiple exotic plant species, >90 % of impact studies only characterize the impacts of single species. Therefore, our knowledge of invasive plant impacts does not reflect the co-invaded nature of most landscapes, potentially ignoring complex interactions among exotic species. Our objective was to characterize potential invader synergies (positive interactions) on biotic and abiotic ecological parameters among the important forest invaders Japanese stiltgrass (Microstegium vimineum) and wavyleaf basketgrass (Oplismenus undulatifolius), which co-invade eastern US deciduous forests. To characterize synergies, we used a factorial selective removal study, as well as an observational study to further explore invader cover-impact relationships. Although both invaders can reduce native plant richness by 70 % individually or in combination, there were no impact synergies. Total cover of any combination of the two invaders had a negative quadratic effect on total, exotic, and native plant richness; i.e., all community metrics were greatest at intermediate levels of total invader cover and lowest at maximum invader cover. Native richness was more greatly affected than exotic richness by the co-invasion. Soil metrics had no clear trend in either study. Japanese stiltgrass and wavyleaf basketgrass appear to have overlapping impact niches—the number, magnitude, and direction of biotic and abiotic changes to the invaded ecosystem—that only vary in impact magnitude, not breadth. As a result of their overlapping impact niches and non-synergies in this co-invaded system, the addition of the recent invader wavyleaf basketgrass has not resulted in additional changes to the invaded forests. Future impact studies should focus on multiple species and identifying synergies, especially as they relate to invader cover, which informs ecological interactions and management prioritization.     

Does restoration help the conservation of the threatened forest of Robinson Crusoe Island? The impact of forest gap attributes on endemic plant species richness and exotic invasions

Invasive plant species are major drivers of biodiversity losses, especially on islands which are prone to invasions and extinctions. In the “endemic montane forest” of Robinson Crusoe Island (Pacific Ocean, Chile) invasive exotic plant species threaten conservation efforts by establishing in gaps and outcompeting native tree species regeneration. We compared gap attributes and ground vegetation cover in three gap types: those dominated by native species (<5 % cover of invasive species), invaded gaps (>30 % cover by invasive species), and treated gaps (invasive species removed). We examined (a) which gap attributes favored native and exotic species, (b) the relationship between gap size and species richness, and (c) species responses to invasion and treatment. Gaps ranged in size from 46 to 777 m² caused mainly by uprooted and snapped trees. Multi response permutation procedures showed a different floristic composition between natural, invaded and treated gaps. The presence of Myrceugenia fernandeziana (native species) and Aristotelia chilensis (invasive species) as gap border trees was positively and negatively correlated with native species richness, respectively. New gaps had more native species than old gaps, and smaller gaps contained relatively more native species than larger ones. An increase in invasive species cover was related to a decline in native species cover and richness. 1–6 years after treatment gaps tended to recover their native floristic composition. Highly effective conservation management programs will concentrate on monitoring gap creation, early control of invasive species, and by treating smaller gaps first.     

Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds

Biological plant invasions pose a serious threat to native biodiversity and have received much attention, especially in terrestrial habitats. In freshwater ecosystems impacts of invasive plant species are less studied. We hypothesized an impact on organisms from the water column and from the sediment. We then assessed the impact of three aquatic invasive species on the plants and macroinvertebrates: Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum. Our research on 32 ponds in Belgium indicated that the reduction in the native plant species richness was a common pattern to invasion. However, the magnitude of impacts were species specific. A strong negative relationship to invasive species cover was found, with submerged vegetation the most vulnerable to the invasion. Invertebrate richness, diversity and abundance were measured in sediments of invaded and uninvaded ponds along a gradient of H. ranunculoides, L. grandiflora, and M. aquaticum species cover. We found a strong negative relationship between invasive species cover and invertebrate abundance, probably due to unsuitable conditions of the detritus for invertebrate colonization. Taxonomic compositions of aquatic invertebrate assemblages in invaded ponds differed from uninvaded ponds. Sensitive benthos, such as mayflies were completely absent in invaded ponds. The introduction of H. ranunculoides, L. grandiflora, and M. aquaticum in Belgian ponds has caused significant ecological alterations in the aquatic vegetation and the detritus community of ponds.     

Effects of Warming on Shrub Abundance and Chemistry Drive Ecosystem-Level Changes in a Forest–Tundra Ecotone

Tundra vegetation is responding rapidly to on-going climate warming. The changes in plant abundance and chemistry might have cascading effects on tundra food webs, but an integrated understanding of how the responses vary between habitats and across environmental gradients is lacking. We assessed responses in plant abundance and plant chemistry to warmer climate, both at species and community levels, in two different habitats. We used a long-term and multisite warming (OTC) experiment in the Scandinavian forest?Ctundra ecotone to investigate (i) changes in plant community composition and (ii) responses in foliar nitrogen, phosphorus, and carbon-based secondary compound concentrations in two dominant evergreen dwarf-shrubs (Empetrum hermaphroditum and Vaccinium vitis-idaea) and two deciduous shrubs (Vaccinium myrtillus and Betula nana). We found that initial plant community composition, and the functional traits of these plants, will determine the responsiveness of the community composition, and thus community traits, to experimental warming. Although changes in plant chemistry within species were minor, alterations in plant community composition drive changes in community-level nutrient concentrations. In view of projected climate change, our results suggest that plant abundance will increase in the future, but nutrient concentrations in the tundra field layer vegetation will decrease. These effects are large enough to have knock-on consequences for major ecosystem processes like herbivory and nutrient cycling. The reduced food quality could lead to weaker trophic cascades and weaker top down control of plant community biomass and composition in the future. However, the opposite effects in forest indicate that these changes might be obscured by advancing treeline forests.     

Effects of experimental rainfall manipulations on Chihuahuan Desert grassland and shrubland plant communities

Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C₃ shrubs into C₄-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass–shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C₄ grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C₃ shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland.     

The rehabilitation value of a small culturally significant island based on the arthropod natural capital

Small islands are particularly vulnerable to degradation by invasive species and often lack source populations to replace those lost during localized extinctions. Robben Island, a significant South African cultural heritage site, has a long history of anthropogenic impact. Introduced alien mammals and trees have resulted in numerous localized plant extinctions with their impact on arthropod biodiversity remaining unclear. We used Robben Island’s arthropod fauna as focal group to investigate the rehabilitation potential of this transformed island. Ground surface-living, foliage-inhabiting and flower-visiting arthropods were collected using five sampling techniques. Arthropod diversity was highest in the two ‘natural’ habitats compared to alien Eucalyptus plantations. Arthropod compositional diversity differed significantly between alien plantations and natural habitats. Nevertheless, a remarkable 17 % of all species were confined to plantations. However, when we corrected for the effect of rarity, only 15 species (4 %) were unique to alien plantations and all those identified to species level were either alien in origin or were widespread generalists only normally associated with transformed areas. This emphasizes the need to identify species that drive similarity indices to fully comprehend the conservation value of alien vegetation. Arthropods found in the remaining natural vegetation showed remarkable tolerance of long-term grazing pressures from introduced alien mammals. Only eight Orthoptera species were sampled, yet one was a rediscovery of a rare species and another a range extension. All indigenous species showed strong preferences for natural habitats. Thus, Robben Island could still hold conservation value if the natural habitats are rehabilitated and the Eucalyptus plantations removed.     

Changes in litter decomposition and soil organic carbon in a reforested tropical deciduous cover (India)

Soil organic carbon (SOC) up to 1 m depth originates from contemporary vegetation cover dating from past millennia. Deforestation and reforestation with economically important species is influencing soil carbon sequestration. An attempt has been made in this study to evaluate the impact of vegetation cover change (due to replacement of natural heterogeneous cover by teak and bamboo) on SOC using carbon isotopes (δ¹³C, ¹⁴C) in a tropical system (India). A litter decomposition study was carried out to understand the impact of differences in vegetation characteristics (specifically of leaves) on decomposition. Both experiments were carried out to look at the impact of changes in vegetation characteristics (specifically of leaves) on litter decomposition, and how these influence near term litter decomposition rates (k values) and long-term SOC content of the soil system beneath. Leaves of teak, bamboo and eight other species were selected for this study. The proportion of structural carbohydrates (lignin and cellulose) in leaves significantly (at 5 % level) influenced k values. The SOC and carbon isotope data collected in this study indicate that C₃ vegetation cover in the study area could be contemporary and dominant for the past few centuries. This can be extended up to ~2,200 years from the recorded ¹⁴C values of teak cover. The study confirms that k values of leaf litter influence SOC present beneath the vegetation cover at the decadal/century time scale.     

Habitat requirements, short-term population dynamics and coexistence of native and invasive Impatiens species: a field study

The genus Impatiens (Balsaminaceae) includes three widespread species in the Czech Republic, central Europe: the native I. noli-tangere, and two invasive species, I. parviflora and I. glandulifera, differing in the dynamics of invasion. They all occur in similar habitats and share basic life-history characteristics, which make them a suitable model for studying species traits associated with invasiveness. In this study we investigated differences in habitat requirements of these Impatiens species, their coexistence and short-term population dynamics in the field. We established 84 1 × 1 m permanent plots in five localities where all three species co-occurred. In each plot vascular plant species were determined, their cover estimated and all individuals of Impatiens species counted. Site characteristics including tree canopy cover, soil moisture, nitrogen and carbon content, and slope were measured directly. Nutrients, light, humidity and soil reaction were estimated using Ellenberg indicator values. The presence of I. noli-tangere was strongly correlated with high soil moisture, that of I. parviflora with high tree canopy cover and low soil moisture. Impatiens glandulifera exhibited a unimodal response to tree canopy cover, avoiding both very shaded and fully open sites. The current-year abundances of all species were negatively related to those of congeneric species. These results suggest that the coexistence of Impatiens species in the same habitat is due to microsite differentiation. Further spread of I. glandulifera to new habitats, and reduction of the native I. noli-tangere niche, can be expected in areas where the latter species co-occurs with competitively strong invasive congeners.     

Spatial variation of plant communities and shoot Cu concentrations of plant species at a timber treatment site

Plant species, spatial variability in plant diversity and vegetation cover were recorded at a French timber treatment site with Cu-contaminated soils (65–2600 mg/kg). Shoot biomass, shoot Cu concentration and accumulation were determined for each plant species found on 168 quadrats with increasing total Cu in soil and soil solution. A total of 91 species occurred on the site including four considered as invasive (Cyperus eragrostis, Phytolacca americana, Senecio inaequidens, and Sporobolus indicus). Species richness, Shannon index, vegetation cover and plant biomass decreased as soil Cu increased. At low soil Cu, members of the Poaceae were most frequent followed by Fabaceae, Rosaceae, and Asteraceae. At high soil Cu, Poaceae were again most frequent. Species known to form Cu-tolerant populations, i.e. Agrostis capillaris, A. stolonifera and Rumex acetosella were present. Shoot Cu concentration and accumulation were higher in plants growing in the most contaminated soils. At 2142 mg Cu/kg soil, shoot Cu accumulation peaked at 6 mg Cu/m² in A. capillaris, and its shoot Cu concentration (364 mg Cu/kg dry weight) exceeded the fodder Cu threshold for domestic livestock. In less Cu-contaminated soils some candidates were identified for sustainable phytoremediation with a potential financial return.     

Context- and density-dependent effects of introduced oysters on biodiversity

Pacific oysters, Crassostrea gigas, have been introduced throughout much of the world, become invasive in many locations and can alter native assemblage structure, biodiversity and the distribution and abundance of other species. It is not known, however, to what extent their effects on biodiversity change as their cover increases, and how these effects may differ depending on the environmental context. Experimental plots with increasing cover of oysters were established within two estuaries in two different habitats commonly inhabited by C. gigas, (mussel-beds and mud-flats) and were sampled after 4 and 15 months. Within mud-flat habitats, macroscopic species living on or in the substratum increased in richness, Shannon–Wiener diversity and number of individuals with oyster cover. In mussel-bed habitats, however, these indices were unaffected by the cover of oysters except at one estuary after 15 months when species richness was significantly lower in plots with the greatest cover of oysters. Assemblage structure differed with oyster cover in mud-flats but not in mussel-beds, except at 100 % cover in one location and at one time. Within mud-flats at one location and time (of four total tests), assemblages became more homogenous with increasing cover of oysters leading to a significant decrease in β-diversity. These responses were primarily underpinned by the facilitation of several taxa including a grazing gastropod (Littorina littorea), an invasive barnacle (Austrominius modestus) and a primary producer (Fucus vesiculosus) with increasing cover of oysters. Although there were consistent positive effects of C. gigas on mud-flat biodiversity, effects were weak or negative at higher cover on mussel-beds. This highlights the need for the impacts of invasive species to be investigated at a range of invader abundances within different environmental contexts.     

Measuring succession: methods for establishing long-term vegetation monitoring sites

Successional stages are often characterized by dominant plant species (species with the highest cover) and their effect on the structure and function of an area through time. However, the plant species determining the ecosystem properties and plant community dynamics may not be the dominant, especially when it is exotic. Understanding plant community dynamics in ecosystems that are uncharacterized and/or affected by invasive plant species requires a data-driven approach and proper placement of monitoring plots. To generate robust datasets on vegetation change through time, monitoring plot placement must consider the scale of ecological variation for both vegetation and soils and plots would ideally be replicated within similar ecological site types to quantify the consistency of successional transitions. We characterized soil and vegetation across and within seven floodplains affected by Russian olive (Elaeagnus angustifolia L.) along the Yellowstone River in southeastern Montana, USA. Using modern Classification and Regression Trees (CART) and multivariate net differentiation, we identified five distinct plant community types, or classes, characterized by their tertiary woody plant cover, not the primary species, Russian olive. Our findings indicate that Russian olive communities differ across space, and these riparian areas can be classified into distinct plant community types. Characterizing plant community types via this analytical approach should allow practitioners to modify management decisions and forecast succession within relevant plant communities.     

Declines in a ground-dwelling arthropod community during an invasion by Sahara mustard (Brassica tournefortii) in aeolian sand habitats

Sahara Mustard (Brassica tournefortii; hereafter mustard), an exotic plant species, has invaded habitats throughout the arid southwestern United States. Mustard has reached high densities across aeolian sand habitats of southwestern deserts, including five distinct sand habitats in the eastern Coachella Valley, California. We examined trends in ground-dwelling arthropod community structure concurrent with mustard invasion in 90 plots within those habitats from 2003 to 2011 (n = 773 plot·years). We expected arthropod communities to respond negatively to mustard invasion because previous work documented significant negative impacts of mustard on diversity and biomass of native plants, the primary resource base for many of the arthropods. Arthropod abundance and species richness declined during the study period while mustard cover increased, and arthropod metrics were negatively related to mustard cover across all plots. When controlling for non-target environmental correlates (e.g. perennial frequency and precipitation) and for potential factors that we suspected of mediating mustard effects (e.g. native cover and sand compaction), negative relationships with mustard remained statistically supported. Nevertheless, arthropod richness’s relationship decreased slightly in strength and significance suggesting that mechanistic pathways may be both direct (via habitat structure) and indirect (via native cover suppression and sand compaction). However, mechanistic pathways for mustard effects, particularly on arthropod abundance, remain unclear. Most arthropod taxa, including most detritivores, decreased through time and were negatively related to mustard cover. In contrast, many predators were positively related to mustard. In total, our study provides substantial evidence for a negative effect of Sahara mustard on the structure of a ground-dwelling arthropod community.     

Ecosystem functions in degraded riparian forests of southeastern Kenya

Species community structures shape ecosystem functions, which are mostly stronger pronounced in intact than in degraded environments. Riparian forests in semiarid Africa provide important habitats for endangered plant and animal species and provide various ecosystem functions, that is, services to people settling along these streams. Most of these riparian forests are severely disturbed by human activities and dominated by invasive exotic plant species in the meanwhile. Thus, ecosystem functions are negatively influenced. While most studies have analyzed a specific metric to measure the degree of ecosystem function, little is known about how strongly different ecosystem functions respond to anthropogenic disturbances in parallel. In this study, we analyzed a set of four proxies of ecosystem functions, ground‐dwelling arthropod abundances, pollination, seed dispersal, and predation, along a highly disturbed riparian forest in southeastern Kenya. We assessed the land cover and land use manually and with an Unmanned Aerial Vehicle. Our data show that ecosystem functions respond differently to vegetation cover, human disturbances, and the availability of the invasive exotic shrub Lantana camara. The occurrence of representatives from the groups Saltatoria and Formicidae profits from heterogeneous habitat structures and natural riparian forest, while representatives of the Araneae profit from high proportion of agricultural fields. In general, predation is higher in mixed land use and natural riparian forest, while pollination and seed dispersal showed no significant trend in regard on land coverage. Along with this, predation also increased with rising proportion of natural riparian forest, while the proportion of agricultural land negatively affects predation, but in parallel showed a slightly significant positive trend with seed dispersal. Human disturbances and the occurrence of the invasive exotic L. camara shrub did not significantly affect our metrics of ecosystem functioning, except of the negative impact of human disturbances on pollinators. In conclusion, our results underpin that ecosystem functions respond highly variable and individually to environmental changes.     

The impact of the invasive Ponto-Caspian amphipod <Emphasis Type="Italic">Pontogammarus robustoides</Emphasis> on littoral communities in Lithuanian lakes

The intentionally introduced Pontogammarus robustoides is the most successful amphipod invader of Lithuanian inland waters and has become established in large lakes. Its impact on littoral invertebrate communities was studied by comparing similar habitats across lakes that harbour or are devoid of the invader. In habitats where P. robustoides is well established and numerous, it significantly reduces species richness and community diversity. Moderate pontogammarid density in habitats that can sustain the native gammarid Gammarus lacustris, however, revealed no negative impact on diversity metrics. Among the lakes studied, the benthic biomass did not differ in invaded and uninvaded habitats. The biomass of indigenous invertebrates (excluding chironomids, which exhibited high lake-specific biomass variation) was lower in the places with well-established P. robustoides. A detrimental impact was observed upon the native isopod Asellus aquaticus and a negative correlation with most of the higher taxa of native invertebrates. In the invaded lake habitats that favour P. robustoides, a change in community structure and a decrease in diversity up to twofold or more are to be expected.     

Synergistic impact of water level fluctuation and invasion of Glyceria on Typha in a freshwater marsh of Lake Ontario

The effects of multiple stressors on the native Typha marsh community (mainly Typha latifolia) were examined using historical records of water levels, human census population, and field vegetation maps. Percent cover of the major plant species was estimated in a GIS, and the percent cover of Typha was related to changes in water level, human population growth, and percent cover of exotic Glyceria maxima and invasive Phragmites australis. Water level fluctuation was the major natural disturbance and it alone accounted for 88% of the variation in Typha. After partitioning out the effect of water level, both human population growth and the presence of exotic species were still significantly related to the decline of native Typha. We suggest that multiple stressors interact with each other to influence changes in native Typha community and cause greater detrimental impact. An important implication of our results is that projected water level decline due to climate change may not necessarily favor the restoration of a desirable native marsh because of the presence of other disturbances such as exotic and invasive species and altered nutrient regime.     

Functional identity versus species richness: herbivory resistance in plant communities

The resistance of a plant community against herbivore attack may depend on plant species richness, with monocultures often much more severely affected than mixtures of plant species. Here, we used a plant–herbivore system to study the effects of selective herbivory on consumption resistance and recovery after herbivory in 81 experimental grassland plots. Communities were established from seed in 2002 and contained 1, 2, 4, 8, 16 or 60 plant species of 1, 2, 3 or 4 functional groups. In 2004, pairs of enclosure cages (1 m tall, 0.5 m diameter) were set up on all 81 plots. One randomly selected cage of each pair was stocked with 10 male and 10 female nymphs of the meadow grasshopper, Chorthippus parallelus. The grasshoppers fed for 2 months, and the vegetation was monitored over 1 year. Consumption resistance and recovery of vegetation were calculated as proportional changes in vegetation biomass. Overall, grasshopper herbivory averaged 6.8%. Herbivory resistance and recovery were influenced by plant functional group identity, but independent of plant species richness and number of functional groups. However, herbivory induced shifts in vegetation composition that depended on plant species richness. Grasshopper herbivory led to increases in herb cover at the expense of grasses. Herb cover increased more strongly in species-rich mixtures. We conclude that selective herbivory changes the functional composition of plant communities and that compositional changes due to selective herbivory depend on plant species richness.     

Assessment of plant community restoration following Tree-of-Heaven (Ailanthus altissima) control by Verticillium albo-atrum

Tree-of-Heaven (TOH) is a highly invasive woody species incurring substantial investment in control efforts across its extensive adventive range. A recently isolated strain of the fungus Verticillium albo-atrum has been found to cause near 100 % mortality of TOH in laboratory and field tests. We assessed plant communities in experimentally infected TOH stands 5–6 years post treatment and compared them to uninfected control stands. We found no statistically significant differences in introduced or native species cover between treated and control stands. Healthy TOH stands often harbor substantial populations of native and introduced invasive species, and on average, successful control of overstory TOH by V. albo-atrum did not alter vegetation cover in these communities. V. albo-atrum appears to be a promising tool in targeted control of TOH that carries a relatively low risk of opportunistic weed replacement.