Cover Of Exotic Species Research Articles

Effect of exotic species management on the recovery of relict forests through citizen participation

In the cities, remnants of the vegetation that originally occupied these areas can be found. Exotic species have established in these relicts, and therefore these spaces require management aimed at recovering native vegetation. In this study, we evaluated the impact of the removing exotic species on the floristic composition of the remnants of an oak forest. We compared the abundance/cover and composition of exotic plants in a forest that has been managed through citizen participation with that of reference forests (both with remnants, but the former without protection and the latter with protection). We established 48 sampling sites to characterize the vegetation in three strata (upper, middle, and low) and monitored changes in composition in managed forest sites for a period of 3 years. The results indicate that the managed forest hosts different species than those found in the reference forests, with lower abundance and cover of exotic plants in the upper and lower strata. Regarding temporal changes, we observed shifts in floristic composition before and after management activities, and a reduction in the cover of exotic species before and after management, with consistently low cover three years later. This underscores the effectiveness of citizen-led management activities, offering a viable alternative for restoring native vegetation in urban environments.

Functional composition of plant communities mediates biomass effects on ecosystem service recovery across an experimental dryland restoration network

Abstract Land degradation can result in a loss of critical ecosystem services that we often seek to restore through re‐establishment of desired plant communities. Trait‐based approaches have the potential to target specific ecosystem services based on associations between the functional composition of plant communities and ecosystem properties that serve as indicators of those services. The effect of functional composition on ecosystem recovery may depend on the amount of restored plant biomass, itself a supporting service frequently targeted in restoration efforts. Yet, interactions between functional composition and biomass are not formally integrated into trait‐based analytical frameworks. We tested the hypothesis that functional composition of plant communities both drives, and interacts with, biomass production to influence indicators of soil functioning and weed suppression across a network of degraded dryland restoration experiments. This networked approach allowed us to identify generalized effects of functional composition on ecosystem recovery across a range of dryland climate conditions. Climate had a substantial effect on ecosystem indicators, with weed cover and soil surface stability increasing in more arid climates, water infiltration increasing with precipitation, and aggregate structure increasing with less freezing. After accounting for climate effects across study sites, we found significant effects of community‐weighted mean (CWM) trait values on biomass, particularly a positive effect of leaf carbon‐to‐nitrogen ratio, and of CWM‐biomass interactions on other ecosystem indicators. Cover of exotic species was reduced in restored communities with a combination of low leaf dry matter content and high biomass, soil water infiltration increased with lower specific root length and high biomass, and soil aggregate stability increased with higher root dry matter content and high biomass, among other effects. Functional diversity had no significant effects on any ecosystem indicators. Synthesis: The influence of community functional composition on ecosystem properties increases with community biomass, particularly in disturbed or low productivity systems. This suggests that active management should not only focus on trait values that optimize individual ecosystem indicators but also how those functional strategies are complementary or counter to those that increase biomass. Read the free Plain Language Summary for this article on the Journal blog

Simultaneous effect of habitat remnancy, exotic species, and anthropogenic disturbance on orchid diversity in South Australia

AbstractOrchids are potentially useful as ecological indicators because of their sensitivity to habitat fragmentation and anthropogenic disturbance. While many studies explore the effect of single factors on orchid diversity, few investigate how the extent, configuration, and condition of surrounding habitat affect whole orchid communities. Here, we unravel the effect of biological invasions, anthropogenic disturbance (i.e., grazing pressure, ecological condition), and habitat fragmentation on an Australian orchid community. We sampled 39 plots across nine sites in the Mount Lofty Ranges, Australia. We recorded the number of orchid species and number of individuals per species in mid‐winter, early‐spring, and late‐spring to account for the effect of season on species visibility, with 115 surveys in total. We ranked grazing intensity and ecological condition, and estimated cover of exotic species. We analyzed the response of richness and diversity through generalized linear mixed models, and differences in species composition through non‐metric multidimensional scaling. Habitat configuration in the surrounding landscape had different effects at increasing radii, explaining 29%–87% of variance. Patch‐level orchid diversity was positively correlated with habitat edges in the immediate area, and with habitat cohesion at medium scales, whereas diversity was negatively correlated with increasing mean patch habitat area across larger surrounding areas. Orchids co‐existed with exotic species but were negatively affected once exotic cover exceeded 20%. Species composition was correlated with exotic cover. Our findings reveal a complex relationship between orchid communities and their surrounding environments suggesting orchids benefit from a somewhat disturbed environment at patch and landscape scales. These idiosyncratic responses suggest orchid diversity may be unreliable as early‐warning indicators of habitat disturbance.

Conservation of Sage‐grouse Critical Habitat and Implications for Prairie Songbirds

ABSTRACTA popular approach for conserving multiple organisms in a given region is to identify one or more umbrella species. Identifying and managing for species that have a high degree of overlap in their habitat requirements with a broader suite of organisms is expected to enhance the effectiveness of conservation actions when resources are limited. The greater sage‐grouse (Centrocercus urophasianus) has received considerable attention as a potential umbrella species for shrub‐steppe flora and fauna in the core of its range. However, the extent to which conserving sage‐grouse habitat benefits mixed‐grass prairie songbirds in the northern portion of its range is unclear. We compared abundance of grassland songbirds within grasslands designated as sage‐grouse critical habitat by the Canadian federal government, and within a 1.6‐km buffer of adjacent habitat in southwestern Saskatchewan, Canada in 2016 and 2017. We also quantified relationships between bird abundance and gradients of vegetation structure. Lark bunting (Calamospiza melanocorys) abundance was 3 times greater within sage‐grouse critical habitat, whereas chestnut‐collared longspur (Calcarius ornatus), Baird's sparrow (Centronyx bairdii), and Sprague's pipit (Anthus spragueii) abundance was 1.6–1.8 times greater outside of these areas. The abundance of grassland specialists was negatively associated with total shrub and silver sagebrush (Artemisia cana) cover, both of which characterize sage‐grouse critical habitat in Canada at both landscape and local scales. Furthermore, characteristics of vegetation structure associated with grassland generalists overlapped with characteristics of vegetation structure that defined greater sage‐grouse critical habitat. Conservation of greater sage‐grouse critical habitat in Canada is likely to have a positive influence on shrub‐dependent or shrub‐tolerant species, such as lark bunting. The conservation of open, native grasslands within greater sage‐grouse critical habitat could benefit grassland specialists, however, any changes in the current management that increases the cover of exotic species, silver sagebrush, and other shrubs will negatively affect grassland specialists. We recommend that protection and management activities be targeted towards greater sage‐grouse, Sprague's pipit, and chestnut‐collared longspur to maximize the conservation of mixed‐grass prairie songbirds and minimize the number of focal umbrella species required. © 2021 The Wildlife Society.

Legacy Effects of Russian Olive (Elaeagnus angustifolia L.) in a Riparian Ecosystem Three Years Post-Removal

Exotic invasive plants leave legacy effects when impacts persist following invader removal. Russian olive (Elaeagnus angustifolia L.) is an actinorhizal tree that is widespread along rivers in interior western North America. We monitored soil N and plant community response to E. angustifolia removal in a riparian ecosystem in eastern Colorado, USA. We collected pre-removal data for two years, and tracked post-removal response for three years. Soil N was generally higher in E. angustifolia plots than in reference plots (not under E. angustifolia) within each removal status treatment (No Removal, Removal, Control). Percent cover of native species was higher in reference plots than in E. angustifolia plots, within treatments in all years. Percent cover of exotic species was higher in E. angustifolia plots than in reference plots, except in the Removal and No Removal treatments immediately following tree removal. Underlying this pattern was a shift in exotic species dominance from Nepeta cataria to Bassia scoparia. The removal treatment successfully reduced E. angustifolia populations at the study sites, but three years afterwards riparian soils and plant communities showed little evidence of recovery towards reference conditions. However, E. angustifolia removal did not appear to worsen invasion impacts or produce novel management problems.

Frequent planned fire can prevent succession to woody plant dominance in montane temperate grasslands

AbstractNatural temperate grasslands are endangered throughout their range, largely because of their almost complete conversion to agriculture, and by changes in fire and grazing regimes. Woody encroachment by shrubs is a global threat to the structure, function and composition of grasslands. We wished to determine the characteristics of fire regimes that prevent or reverse woody plant encroachment, maintain native species richness and minimise exotic plant invasion in temperate grassland at the Surrey Hills Tasmania, where a fire management plan with variable prescriptions had been implemented for two decades. We collected floristic, fire regime and environmental data from 105 quadrats (1 × 10 m) in 2016/2017, and compared the data from 2016/2017 to that from a 1994 survey using the same methods. A high frequency and cover of native shrubs characterised areas unburned for at least 20 years before 2017, but not those unburned in the 20 years before 1994. Shrub cover began to strongly increase after a decade without fire and was greater on larger plains. Native species richness declined with an increasing minimum interval between fires and increased with elevation. It began to decline at the quadrat scale when shrub cover attained 40%. In the data set as a whole, 17 of the 67 most abundant taxa were absent from all quadrats unburned for 20 years before 2017, indicating a high potential for loss of species at a landscape scale in the absence of fire. Exotic species cover was randomly distributed in relation to fire regimes and environment. The current fire management regime has largely been at a sufficient frequency and minimum interval (approximately a decade between fires) to maintain the grassiness and native species richness of treated plains, which is fortunate given that recent land‐use change appears to have resulted in increased frequency of shrubs independent of their cover.

Effect of the invasive exotic herb Centaurea solstitialis on plant communities of a semiarid ecosystem

ABSTRACT Background The effects of many invasive species on invaded communities are still scarcely known. Centaurea solstitialis is an invasive Eurasian herb, widely distributed around the world, but its effects on recipient communities are not well known. Aims To evaluate the effect of C. solstitialis invasion on richness and cover of native and exotic resident species in naturally established communities. Methods We repeated a field experiment in three old-fields of central Chile where C. solstitialis was not present. The experiment simulated the invasion of C. solstitialis by adding 600 seeds per plots of 1 × 1 m in size and compared the richness and cover of all naturally growing species in plots with and without C. solstitialis. Results For exotics, initial species richness (average ca.18% and 20% per site), and cover (average ca. 20% and 34% per site) were significantly reduced by C. solstitialis in two out of the three sites. The abundances of four out of 17 exotic resident species were negatively affected by C. solstitialis. Native species were not affected by C. solstitialis. Conclusions C. solstitialis can outcompete resident species of communities where it invades, but local ecological factors influence its effects producing different impacts among species and localities.

Management Regime and Field Age Affect Species Richness and Cover of Native Forbs and Exotic Species in Virginia Grasslands

Joseph Ledvina Joseph Ledvina Conservation Ecology Center, Smithsonian Conservation Biology Institute, Front Royal, VA and Natural Resources Section, Environmental Division, Department of Transportation, Little Rock, AR.

Drought in Southern California coastal sage scrub reduces herbaceous biomass of exotic species more than native species, but exotic growth recovers quickly when drought ends

Semi-arid regions with Mediterranean-type climates harbor exceptional biodiversity, but are increasingly threatened by invading exotic annual species and climatic changes, including drought. In semi-arid ecosystems, antecedent conditions often influence plant growth, but the role of antecedent conditions for drought response and recovery of native versus exotic species remains largely unexplored. From 2013 to 2016, we imposed experimental rainfall treatments (average rainfall, moderate or severe drought) in plots under a native shrub canopy and in inter-spaces dominated by herbaceous vegetation, and quantified growth (peak biomass) and abundance (cover) of native and exotic herbaceous species. The following year, we quantified recovery from the drought treatments (2017). Exotic biomass was less resistant to drought (declined more than native biomass), but was more resilient (increased more than native biomass in the year following drought), especially in unshaded inter-spaces between shrubs. These responses were associated with life history; annual species responded more negatively to drought in the inter-spaces than perennial species. Current years' rainfall was a better predictor of biomass than prior rainfall, but antecedent factors were also important. After four years of rainfall treatments, exotic species had the highest growth recovery in the severe drought treatment, while growth of natives had the opposite response. In contrast, litter was positively associated with plant growth regardless of origin. This study demonstrates that when native and exotic species differ in life history, as they do in Mediterranean climate ecosystems, they may respond differently to antecedent factors, and hence differ in recovery from climate extremes such as drought.

The legacy of pasture improvement causes recruitment failure in grassy eucalypt woodland conservation reserves in the Midlands of Tasmania

Australia’s most fragmented and least reserved landscapes are the grassy eucalypt woodlands of the south-east. Two hundred years of agricultural disruption have transformed these landscapes, and agricultural enterprises continue to expand and develop, meaning the threats to these landscapes have not abated. The Tasmanian Midlands is primarily privately owned, with very little area devoted to conservation of biodiversity. In this landscape, conservation covenants have been enacted on many private properties with the intention of encouraging tree recruitment and conservation of threatened plant communities and rare species. Evidence of the effectiveness of these covenants in protecting overstorey tree population health is lacking. This study compared the demographic structures of overstorey Eucalyptus species and midstorey tree genera on public and private properties with contrasting land use histories. Reserves on private lands had little tree recruitment, probably because exotic pasture species were common, whereas tree recruitment was abundant in public reserves, where pasture improvement has not occurred. Active measures are needed to restore ecological structure and function in grassy woodland conservation reserves on private land by encouraging regeneration of Eucalyptus and Acacia species as well as returning the ground layer to a functionally native state. This will entail reinstating fire disturbance, reducing exotic pasture species cover and managing domesticated, feral and native herbivores.

A new approach for hydrologic performance standards in wetland mitigation

Wetland restoration performed as a requirement of compensatory mitigation does not always replace lost acreage or functions. Most new projects are required to identify performance standards to evaluate restoration outcomes. Current performance standards are primarily related to vegetation with little to no evaluation of wetland hydrologic regimes. Because of the agreement in the scientific literature about the role of hydrology in creating and maintaining wetland structure and function, hydrologic performance standards may be an ecologically meaningful way to evaluate restoration outcomes. This research tests the use of water level data from project specific reference sites to evaluate restored water levels for three distinct wetland types across the United States. We analyzed existing datasets from past and ongoing wetland mitigation projects to identify the number of years it took water levels in restored wetlands to match reference sites, and to test whether similar water levels between restored and reference sites leads to increased vegetation success. Wetland types differed in the number of years it took for water levels to match reference sites. Vernal pools in California took nine years to match reference sites, fens and wet meadows in Colorado took four years, and forested wetlands in the southeastern US were hydrologically similar to reference sites the first year following restoration. Plant species cover in all three restored wetland types was related to the water level similarity to reference sites. Native cover was higher when water levels were more similar to reference sites, and was lower in areas where water levels were different. Exotic species cover showed the opposite relationship in fens and wet meadows, where hydrologic similarity led to low cover of exotic species. Along with the general agreement of the importance of hydrology for wetland form and function, this research shows that hydrologic performance standards may also lead to increased vegetation success in some wetland types.

Vegetative Community Response to Landscape-Scale Post-fire Herbicide (Imazapic) Application

AbstractDisturbances such as wildfire create time-sensitive windows of opportunity for invasive plant treatment, and the timing of herbicide application relative to the time course of plant community development following fire can strongly influence herbicide effectiveness. We evaluated the effect of herbicide (imazapic) applied in the first winter or second fall after the 113,000 ha Soda wildfire on the target exotic annual grasses and also key non-target components of the plant community. We measured responses of exotic and native species cover, species diversity, and occurrence frequency of shrubs and forbs seeded before (1 to 2 or 9 to 10 mo) herbicide application. Additionally, we asked whether landscape factors, including topography, species richness, and/or soil characteristics, influenced the effectiveness of imazapic. Cover of exotic annual grass cover, but not of deep-rooted perennial bunchgrass, was less where imazapic had been applied, whereas more variability was evident in the response of Sandberg bluegrass (Poa secundaJ. Presl) and seeded shrubs and forbs. Regression-tree analysis of the subset of plots measured both before and after the second fall application revealed greater reductions of exotic annual grass cover in places where their cover was <42% before spraying. Otherwise, imazapic effects did not vary with the landscape factors we analyzed.

Short-term soil nutrient and plant community responses to the eradication of a nitrogen fixing tree, Lupinus arboreus

Invasive plant eradication aims to restore an original state by eliminating the plant and its alteration of ecosystem structure and function. The method by which an invasive plant is eradicated is known to influence restoration success; however, success may be impeded by a legacy effect. We sought to examine how the soil nutrient pool and plant community assembly responded to the eradication of an invasive nitrogen-fixing legume, Lupinus arboreus, using three different techniques. We applied herbicide to six mature plants and with a handsaw removed the above ground biomass of 12 mature plants; six of these were left in situ to decompose and six were removed from site. Soil nutrient concentrations were measured, and plant community composition was monitored, in the treatment and reference plots over the following 13 months. Eradication resulted in a significant increase in extractable ammonium-N and potassium, in contrast nitrate-N declined. The plant community changed as a result of felling L. arboreus, and the removal of all biomass initially resulted in an increase in native plant cover. By the end of the experimental period, however, L. arboreus seedlings were increasing in abundance and native cover was declining. The application of herbicide resulted in the slow decay of the mature plant and as such delayed an increase in exotic species cover and inhibited L. arboreus germination. Nutrient concentrations in the eradication plots were not significantly different from the reference plots at the end of the experimental period. The impact of L. arboreus on the soil nutrient pool remains after eradication, a legacy effect which will hinder restoration of the original structure and function in the long term.

Composición, diversidad y estructura de la vegetación de bosques ribereños en el centro sur de Chile

Riparian forests have been highlighted for their unique biological diversity and high importance in ecosystem services. However, they are also ecosystems threatened by forestry, livestock or agriculture. In Chile, studies evaluating the diversity and structure of riparian forest vegetation contrasting slope forests or other habitats are limited. Therefore, the aims of this study were: (1) to compare the composition and diversity of vascular plant species; (2) to compare tree species size structure; and (3) to compare the vertical stratification of plant communities between riparian forests and slope forests. Six southern Chilean watersheds (35-38° S) were studied, sampling the vegetation in plots of 40 m2. In each watershed, 10 plots were located in riparian forests and 10 in slope forests, on both sides of each river. We found important differences in species composition between the two forest types. In the riparian forest we found a higher diversity of tree and climbing species, higher diversity and cover of native species, higher density of tree species in adult state and higher cover of the strata 1-5 m. In the slope forest we found a larger richness and cover of exotic species, higher regeneration density and higher cover of strata 5 m. Results can be related to the higher moisture of riparian forests, showing the particularity of riparian forests regarding species composition, diversity and structure, compared to other habitats.

Removal of perennial herbaceous species affects response of Cold Desert shrublands to fire

AbstractQuestionsTwo of the primary global change factors that threaten shrublands worldwide are loss of native perennial herbaceous species due to inappropriate livestock grazing and loss of native shrubs due to altered fire regimes. We asked: (1) how do the separate and interacting effects of removal of perennial herbaceous species and burning influence relative abundance of plant functional groups over longer time frames; and (2) how do interactions between perennial herbaceous species removal and burning differ along environmental gradients? We discuss implications of our findings for ecosystem resilience to these disturbances.LocationShoshone Mountain Range, Nevada and East Tintic Range, Utah, USA.MethodsWe used a factorial experiment to test effects of perennial herbaceous species removal (0%, 50% and 100%) and burning (burned and not burned) on plant functional group cover along elevation gradients within watersheds characterized by Artemisia tridentata Nutt. vegetation types. The experiment was conducted in two locations (Nevada and Utah) with sites located at low (1960 and 1710 m), mid (2190 and 2085 m) and high (2380 and 2274 m) elevations and was repeated in 2 yr. Percentage cover of native and exotic species and canopy area, density and size of the shrub A. tridentata were evaluated 12 and 13 yr after study implementation.ResultsOver a decade later, removal treatments resulted in highly significant decreases (40–62%) in perennial native grass and forb cover across site elevations. Burning decreased overall shrub and A. tridentata cover, but effects on perennial native grass cover differed among elevations. Removal had strong positive effects on A. tridentata seedling recruitment and resulted in progressive increases in density and canopy area following burning. A. tridentata canopy areas on burned plots with 0%, 50% and 100% removal were 0.19, 0.40 and 0.84 m2, respectively. Annual invasive grass density also increased with degree of removal, and both density and cover decreased with elevation.ConclusionsOur results show that loss of perennial herbaceous species, which can result from inappropriate livestock grazing, and loss of shrubs, which often results from fire, interact to affect key functional groups. The implications are that ecosystem resilience to disturbance in Cold Desert shrublands decreases when competition from perennial native grasses and forbs for available resources no longer prevents dominance by A. tridentata and other shrubs and/or annual invasive grasses. Managing livestock grazing to maintain or increase perennial herbaceous species, especially deep‐rooted grasses, which contribute to resilience along elevation gradients, can help prevent threshold crossings to undesirable states and retain critical ecosystem services following disturbances such as wildfire.

Nutrient addition shifts plant community composition towards earlier flowering species in some prairie ecoregions in the U.S. Central Plains.

The distribution of flowering across the growing season is governed by each species’ evolutionary history and climatic variability. However, global change factors, such as eutrophication and invasion, can alter plant community composition and thus change the distribution of flowering across the growing season. We examined three ecoregions (tall-, mixed, and short-grass prairie) across the U.S. Central Plains to determine how nutrient (nitrogen (N), phosphorus, and potassium (+micronutrient)) addition alters the temporal patterns of plant flowering traits. We calculated total community flowering potential (FP) by distributing peak-season plant cover values across the growing season, allocating each species’ cover to only those months in which it typically flowers. We also generated separate FP profiles for exotic and native species and functional group. We compared the ability of the added nutrients to shift the distribution of these FP profiles (total and sub-groups) across the growing season. In all ecoregions, N increased the relative cover of both exotic species and C3 graminoids that flower in May through August. The cover of C4 graminoids decreased with added N, but the response varied by ecoregion and month. However, these functional changes only aggregated to shift the entire community’s FP profile in the tall-grass prairie, where the relative cover of plants expected to flower in May and June increased and those that flower in September and October decreased with added N. The relatively low native cover in May and June may leave this ecoregion vulnerable to disturbance-induced invasion by exotic species that occupy this temporal niche. There was no change in the FP profile of the mixed and short-grass prairies with N addition as increased abundance of exotic species and C3 graminoids replaced other species that flower at the same time. In these communities a disturbance other than nutrient addition may be required to disrupt phenological patterns.

Recovery of Basalt Substrate for Xeric Scrub Restoration in a Lava Field in Mexico City

The alteration of biophysical conditions in cities limits restoration and management of ecosystems in urban areas. The Pedregal de San Angel Ecological Reserve (PSAER) protects important remnants of a unique type of xeric scrub in a lava field embedded in Mexico City. Unearthing and replacing basalt rock have been carried out in PSAER to recover rocky basalt substrate, a physical feature in this type of location, in areas where it has been removed or buried and invading species have become predominant. To obtain an overview of the recovery of the plant community following restoration actions, we sampled and compared vegetation structure in terms of species richness, composition, relative coverage and diversity in 12 sites of PSAER: five conserved areas, three restored areas, and four disturbed areas. Restored areas presented intermediate values with respect to those in conserved and disturbed areas for total and non-ruderal native species richness, Simpson dominance index, and relative cover of non-ruderal and exotic species. Our results show that replacing and unearthing rock are still insufficient but are promising methods to promote restoration of this xeric scrub and similar plant communities that develop on lava fields.

The importance of travelling stock reserves for maintaining high-quality threatened temperate woodlands

Travelling stock reserves (TSRs) are thought to represent some of the highest-quality and least degraded remnants of threatened temperate woodland in south-eastern Australia. These public reserves have not had the same high levels of grazing pressure and other disturbances as woodland remnants on private land. Thus, TSRs are expected to be important for the protection of biodiversity in heavily cleared and modified landscapes. We tested the hypothesis that land tenure had significant effects on the quality of woodlands by comparing vegetation structural attributes between TSRs and remnant vegetation used for primary production purposes. Vegetation attributes were monitored in 155 permanent plots over 5 years in remnant temperate woodland sites in the Riverina bioregion of New South Wales. Overall, TSRs supported higher native plant species richness and were characterised by higher ground cover of native shrubs and less cover of exotic plant species than agricultural production areas. We found land tenure had significant effects on some vegetation attributes demonstrated to be important for threatened fauna. We attribute these results to TSRs having a history of lower grazing pressure compared with remnants managed for agricultural production. Our study provides empirical evidence to support the high conservation value of TSRs in formerly woodland-dominated, but now extensively cleared agricultural landscapes.

A century of ecosystem change: human and seabird impacts on plant species extirpation and invasion on islands

We used 116 years of floral and faunal records from Mandarte Island, British Columbia, Canada, to estimate the indirect effects of humans on plant communities via their effects on the population size of a surface-nesting, colonial seabird, the Glaucous-winged gull (Larus glaucescens). Comparing current to historical records revealed 18 extirpations of native plant species (32% of species historically present), 31 exotic species introductions, and one case of exotic introduction followed by extirpation. Contemporary surveys indicated that native species cover declined dramatically from 1986 to 2006, coincident with the extirpation of ‘old-growth’ conifers. Because vegetation change co-occurred with an increasing gull population locally and regionally, we tested several predictions from the hypothesis that the presence and activities of seabirds help to explain those changes. Specifically, we predicted that on Mandarte and nearby islands with gull colonies, we should observe higher nutrient loading and exotic plant species richness and cover than on nearby islands without gull colonies, as a consequence of competitive dominance in species adapted to high soil nitrogen and trampling. As predicted, we found that native plant species cover and richness were lower, and exotic species cover and richness higher, on islands with versus without gull colonies. In addition, we found that soil carbon and nitrogen on islands with nesting gulls were positively related to soil depth and exotic species richness and cover across plots and islands. Our results support earlier suggestions that nesting seabirds can drive rapid change in insular plant communities by increasing nutrients and disturbing vegetation, and that human activities that affect seabird abundance may therefore indirectly affect plant community composition on islands with seabird colonies.

Density-dependent grazing impacts of introduced European rabbits and sympatric kangaroos on Australian native pastures

Little information is available on relationships between pest animal density and damage in natural ecosystems. Introduced European rabbits, Oryctolagus cuniculus, cause severe damage to Australian native vegetation but density–damage relationships are largely unexplored. There are no recognized simple methods to estimate their impacts on native pastures, due in part to confusion with grazing impact of other herbivores. We tested simple quantitative sampling methods using multiple small quadrats to detect site differences in pasture cover, pasture species richness and dung pellet density of herbivores, from which rabbit density and relative abundance of larger herbivores were estimated. Native pasture cover and species richness declined exponentially with increasing rabbit density, within the range of 0–5 rabbits ha−1, while cover of unpalatable exotic pasture species increased. By contrast, kangaroo abundance was positively related to palatable native pasture cover and negatively related to cover of unpalatable weeds, and had no negative effect on native pasture cover or species richness that was discernable against a background of low to moderate rabbit densities. Perennial native forbs and perennial grasses replaced invasive Wards weed as the dominant ground cover at low rabbit densities. We conclude that, regardless of previous grazing history, contemporary kangaroo grazing pressure and weed invasion, the severely degraded state of native pastures was perpetuated by rabbits. The effect of rabbits on native pasture can be recorded in a simple manner that is suitable for identifying density–damage relationships in the presence of other herbivores and changes over time. This method is seen as particularly useful in setting target densities below which rabbits must be managed to maintain native plant communities and ecosystem function in southern Australia. It may also be useful to demonstrate rabbits’ impacts in other regions, including optimum densities for plant biodiversity benefits in their native European range.