Cover Of Exotic Species Research Articles

HOW GRAZING AND SOIL QUALITY AFFECT NATIVE AND EXOTIC PLANT DIVERSITY IN ROCKY MOUNTAIN GRASSLANDS

We used multiscale plots to sample vascular plant diversity and soil characteristics in and adjacent to 26 long-term grazing exclosure sites in Colorado, Wyoming, Montana, and South Dakota, USA. The exclosures were 7–60 yr old (31.2 ± 2.5 yr, mean ± 1 se). Plots were also randomly placed in the broader landscape in open rangeland in the same vegetation type at each site to assess spatial variation in grazed landscapes. Consistent sampling in the nine National Parks, Wildlife Refuges, and other management units yielded data from 78 1000-m2 plots and 780 1-m2 subplots. We hypothesized that native species richness would be lower in the exclosures than in grazed sites, due to competitive exclusion in the absence of grazing. We also hypothesized that grazed sites would have higher native and exotic species richness compared to ungrazed areas, due to disturbance (i.e., the intermediate-disturbance hypothesis) and the conventional wisdom that grazing may accelerate weed invasion. Both hypotheses were soundly rejected. Although native species richness in 1-m2 subplots was significantly higher (P < 0.05) in grazed sites, we found nearly identical native or exotic species richness in 1000-m2 plots in exclosures (31.5 ± 2.5 native and 3.1 ± 0.5 exotic species), adjacent grazed plots (32.6 ± 2.8 native and 3.2 ± 0.6 exotic species), and randomly selected grazed plots (31.6 ± 2.9 native and 3.2 ± 0.6 exotic species). We found no significant differences in species diversity (Hill’s diversity indices, N1 and N2), evenness (Hill’s ratio of evenness, E5), cover of various life-forms (grasses, forbs, and shrubs), soil texture, or soil percentage of N and C between grazed and ungrazed sites at the 1000-m2 plot scale. The species lists of the long-ungrazed and adjacent grazed plots overlapped just 57.9 ± 2.8%. This difference in species composition is commonly attributed solely to the difference in grazing regimes. However, the species lists between pairs of grazed plots (adjacent and distant 1000-m2 plots) in the same vegetation type overlapped just 48.6 ± 3.6%, and the ungrazed plots and distant grazed plots overlapped 49.4 ± 3.6%. Differences in vegetation and soils between grazed and ungrazed sites were minimal in most cases, but soil characteristics and elevation were strongly correlated with native and exotic plant diversity in the study region. For the 78 1000-m2 plots, 59.4% of the variance in total species richness was explained by percentage of silt (coefficient = 0.647, t = 5.107, P < 0.001), elevation (coefficient = 0.012, t = 5.084, P < 0.001), and total foliar cover (coefficient = 0.110, t = 2.104, P < 0.039). Only 12.8% of the variance in exotic species cover (log10cover) was explained by percentage of clay (coefficient = −0.011, t = −2.878, P < 0.005), native species richness (coefficient = −0.011, t = −2.156, P < 0.034), and log10N (coefficient = 2.827, t = 1.860, P < 0.067). Native species cover and exotic species richness and frequency were also significantly positively correlated with percentage of soil N at the 1000-m2 plot scale. Our research led to five broad generalizations about current levels of grazing in these Rocky Mountain grasslands: (1) grazing probably has little effect on native species richness at landscape scales; (2) grazing probably has little effect on the accelerated spread of most exotic plant species at landscape scales; (3) grazing affects local plant species and life-form composition and cover, but spatial variation is considerable; (4) soil characteristics, climate, anddisturbances may have a greater effect on plant species diversity than do current levels of grazing; and (5) few plant species show consistent, directional responses to grazing or cessation of grazing.

EXOTIC PLANT SPECIES INVADE HOT SPOTS OF NATIVE PLANT DIVERSITY

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m2 subplots (20 1000-m2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m2 subplots (16 1000-m2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Exotic Plant Species Invade Hot Spots of Native Plant Diversity

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m2 subplots (20 1000-m2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m2 subplots (16 1000-m2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Exotic weed control treatments for conservation of fescue grassland in Montana

An analytical framework is described to evaluate exotic weed control treatments applied to native vegetation for conservation purposes. The analyses were used to assess the following responses to herbicide applications for control of an exotic forb Centaurea maculosa in fescue grassland: (1) efficacy on the target weed species, (2) similarity to reference stands defining the potential natural communities, (3) resistance and resilience of the nontarget vegetation, and (4) community structure in terms of life form proportions. Vegetation data were collected from replicated plots one year before spraying and for the first three years after spraying at two sites in western Montana. Low rates of selected herbicides effectively suppressed the target weed for three years after treatment. Grasses responded positively to release from exotic forb competition, while total nontarget forb standing crops were similar between treated plots and untreated control plots three years after spraying. The site with lower pretreatment cover of exotic species and higher pretreatment composition of native species had the largest positive response to spraying in terms of similarity to the potential natural vegetation. Although some treatments caused depressions after one year, all treatments had the same or higher per cent of pretreatment species present compared to the untreated control plots in the third year after spraying. Herbicide-sensitive taxa were identified by multivariate analysis of canopy cover data. Implications of the results for conservation management of high priority grassland sites are discussed.

Factors influencing the integrity of remnant bushland in subhumid Tasmania

The relationships between remnant integrity and size, age, shape, environment and disturbance are not well-established. One hundred of the remnants in relatively good condition ranging in size from 5 to 200 ha in the drier parts of Tasmania were sampled and the relationships between independent variables and the dependent variables of remnant native species richness, remnant exotic species richness, the ratio between the two, native species cover, exotic species cover, and the ratio of exotic/native cover were investigated. Age, shape and proximity of remnants had few or no effects on the dependent variables. Size appeared to affect the condition variables only through habitat heterogeneity. The nature of the surrounding vegetation and fire and grazing management were the critical independent variables. However, the different aspects of integrity had different relationships with these variables.

Riparian zones as havens for exotic plant species in the central grasslands

In the Central Grasslands of the United States, we hypothesized that riparian zones high in soil fertility would contain more exotic plant species than upland areas of low soil fertility. Our alternate hypothesis was that riparian zones high in native plant species richness and cover would monopolize available resources and resist invasion by exotic species. We gathered nested-scale vegetation data from 40 1 m2subplots (nested in four 1000 m2 plots) in both riparian and upland sites at four study areas in Colorado, Wyoming, and South Dakota (a total of 320 1 m2 subplots and 32 1000 m2 plots). At the 1 m2 scale, mean foliar cover of native species was significantly greater (P<0.001) in riparian zones (36.3% ± 1.7%) compared to upland sites (28.7% ± 1.5%), but at this small scale there were no consistent patterns of native and exotic species richness among the four management areas. Mean exotic species cover was slightly higher in upland sites compared to riparian sites (9.0% ± 3.8% versus 8.2% ± 3.0% cover). However, mean exotic species richness and cover were greater in the riparian zones than upland sites in three of four management areas. At the 1000 m2 scale, mean exotic species richness was also significantly greater (P<0.05) in riparian zones (7.8 ± 1.0 species) compared to upland sites (4.8 ± 1.0 species) despite the heavy invasion of one upland site. For all 32 plots combined, 21% of the variance in exotic species richness was explained by positive relationships with soil % silt (t =1.7, P=0.09) and total foliar cover (t = 2.4, P=0.02). Likewise, 26% of the variance in exotic species cover (log10 cover) was explained by positive relationships with soil % silt (t =2.3, P=0.03) and total plant species richness (t = 2.5, P=0.02). At landscape scales (four 1000 m2 plots per type combined), total foliar cover was significantly and positively correlated with exotic species richness (r=0.73, P<0.05) and cover (r=0.74, P<0.05). Exotic species cover (log10 cover) was positively correlated with log10% N in the soil (r=0.61, P=0.11) at landscape scales. On average, we found that 85% (±5%) of the total number of exotic species in the sampling plots of a given management area could be found in riparian zones, while only 50% (±8%) were found in upland plots. We conclude that: (1) species-rich and productive riparian zones are particularly invasible in grassland ecosystems; and (2) riparian zones may act as havens, corridors, and sources of exotic plant invasions for upland sites and pose a significant challenge to land managers and conservation biologists.

Vegetation, Soil, and Animal Indicators of Rangeland Health

We studied indicators of rangeland health on benchmark sites with long, well documented records of protection from stress by domestic livestock or histories of environmental stress and vegetation change. We measured ecosystem properties (metrics) that were clearly linked to ecosystem processes. We focused on conservation of soil and water as key processes in healthy ecosystems, and on maintenance of biodiversity and productivity as important functions of healthy ecosystems. Measurements from which indicators of rangeland health were derived included: sizes of unvegetated patches, cover and species composition of perennial grasses, cover and species composition of shrubs and herbaceous perennials, soil slaking, and abundance and species composition of the bird fauna. Indicators that provided an interpretable range of values over the gradient from irreversibly degraded sites to healthy sites included: bare patch index, cover of long-lived grasses, palatability index, and weighted soil surface stability index. Indicators for which values above a threshold may serve as an indicator of rangeland health include: cover of plant species toxic to livestock, cover of exotic species, and cover of increaser species. Several other indicator metrics were judged not sensitive nor interpretable. Examples of application of rangeland health indicators to evaluate the success of various restoration efforts supported the contention that a suite of indicators are required to assess rangeland health. Bird species diversity and ant species diversity were not related to the status of the sample site and were judged inadequate as indicators of maintenance of biodiversity.

Exotic plant invasion and understorey species richness: a comparison of two types of eucalypt woodland in agricultural Western Australia

Exotic plants are a major threat to native plant diversity in Australia yet a generic model of the invasion of Australian ecosystems by exotic species is lacking because invasion levels differ with vegetation/soil type and environmental conditions. This study compared relative differences in exotic species invasion (percent cover, spp. richness) and the species richness of herbaceous native plants in two structurally very similar vegetation types, Gimlet Eucalyptus salubris and Wandoo E. capillosa woodlands in the Western Australian wheatbelt. For each woodland type, plant variables were measured for relatively undisturbed woodlands, woodlands with &gt;30 years of livestock grazing history, and woodlands in road-verges. Grazed and road-verge Gimlet and Wandoo woodlands had significantly higher cover of exotic species, and lower species richness of native plants, compared with undisturbed Gimlet and Wandoo. Exotic plant invasion was significantly greater in Gimlet woodlands for both grazed (mean 78% cover) and road-verge sites (mean 42% cover) than in comparable sites in Wandoo woodlands (grazed sites 25% cover, road-verge sites 19% cover). There was no significant difference in the species richness of exotic plants between Wandoo and Gimlet sites for any of the three situations. Mean site richness of native plants was not significantly different between undisturbed Wandoo and undisturbed Gimlet woodlands. Undisturbed woodlands were significantly richer in plant species than grazed and road-verge woodlands for both woodland types. Grazed and road-verge Wandoo sites were significantly richer in plant species than communities in grazed and road-verge Gimlet. The percent cover of exotics was negatively correlated with total (native) plant species richness for both woodland types (Wandoo r = ?0.70, Gimlet r = ?0.87). Of the total native species recorded in undisturbed Gimlet, 83% and 61% were not recorded in grazed and road-verge Gimlet, respectively. This compared with 40% and 33% for grazed and road-verge Wandoo, respectively. Grazed Wandoo and grazed Gimlet sites had significantly fewer native plant species than did road-verge Wandoo and road-verge Gimlet sites. Ecosystem implications of differential invasions by exotic species, and the effects of grazing (disturbance) and other factors influencing susceptibility to exotic plant invasion (landscape, competition and allelopathy) on native species decline are discussed. Exclusion of livestock and adequate methods of control and prevention of further invasions by exotic plants are essential requirements for the conservation of these woodland systems.

The Spatial Configuration of Vegetation as an Indicator of Landscape Degradation Due to Livestock Enterprises in Argentina

1. The spatial configuration characteristics of vegetation were studied in order to develop ecological indicators of landscape degradation produced by livestock-related activities in Argentine subantarctic forest. This study was carried out in the Los Alerces National Park (LANP). 2. Two indices were devised. One, the 'D' index, was designed to express the variations in the vegetation types as related to the area occupied by each, the area-perimeter ratio, and the configuration regularity. The D index enabled determination of the degradation stage of different vegetation types, and the past and recent impact of livestock in a given area, providing some significant effects at landscape level could be detected. 3. The second index, the 'V' index, summarizes field information such as cover of annual species, cover of exotic species, cover of Rumex acetosella, total biomass and clonal biomass. This index was designed to evaluate the aptitude of the D index to monitor landscape degradation. 4. The identification and distribution of areas at risk, at a regional scale, facilitates the detection of those rural settlements that pose an imminent or potential hazard to the conservation of forest resources and require urgent changes in livestock management.

Grazing in remnant woodland vegetation: changes in species composition and life form groups

Abstract. Grazing by domestic livestock in native woodlands can have major effects on ecosystem functioning by the removal of plant species that form important functional groups. This paper documents the changes in floristics in a large group of remnants of native woodland left after agricultural clearing in southwestern Australia. Species richness and diversity were significantly reduced in remnants and the proportion of exotic species increased. Detrended Correspondence Analysis (DCA) was used to identify floristic and environmental patterns among plots and identified two distinct groups based on grazing intensity. This indicated that the significance of the relationship between grazing effects and DCA floristic axes was greater than edaphic characteristics that normally influence floristic patterns. Floristic characteristics of sites that were influencing the position of plots on the ordination diagram included proportion of exotic species and proportion of native perennial shrubs and herbs. Numbers of species of native shrubs and perennial herbs were significantly reduced in grazed plots and numbers of exotic annual grasses and herbs were significantly higher. Other life form groups such as native perennial grasses and geophytes were not significantly affected by grazing. Reproductive strategies of perennial species showed a significant decrease in numbers of resprouters and a significant increase in numbers of facultative seeder/sprouters. Exclosure plots showed increases in number and cover of perennial shrubs and herbs after three years whereas number and cover of exotic species did not change. Time series DCA showed that the floristic composition of exclosure plots in grazed sites became closer to that of the ungrazed sites.

Vegetation Responses along Edge‐to‐Interior Gradients in the Mixed Hardwood Forests of the Roanoke River Basin, North Carolina

Compared with forest interiors, forest edges typically have a different plant species composition and community structure, a phenomenon known as “edge effect.” Edge effects make the functional interior area of a forest smaller than its actual area. The objective of this study was to estimate how far the effects of agriculturally maintained edges penetrate the mixed hardwood forests of the Roanoke River Basin, North Carolina. I determined percentage cover for all vascular plant species in 10‐by‐100‐meter belt transects on north‐facing or south‐facing edges of four relatively undisturbed forests. Changes in the percentage cover of individual species, the relative cover of exotic species, and species richness all indicated that edge effects penetrate deeper on south‐facing edges (to 60 meters) than on north‐facing edges (to 20 meters). Analyses of species responses to the edge showed a number of species to be edge oriented, but no species was found to be interior oriented. The results of multivariate analyses (ordination and cluster analysis) suggested that edge effects could be detected to 50 meters on south‐facing edges and 10–30 meters on north‐facing edges. These results allow us to better understand the difference between a forest’s actual area and its functional interior area.

A multi-scale assessment of the occurrence of exotic plants on the Olympic Peninsula, Washington

. Exotic plants were surveyed in 208 plots within the Dungeness and Hoh river watersheds on the Olympic Peninsula, Washington, USA. Landscape patch types included uplands (clearcuts, young and mature forests) and riparian zones (cobble bars, shrub patches, riparian forests, and alder flats). Patterns of exotic plant invasions were assessed between watersheds, between riparian and upland areas, among patch types, and within clearcuts. 52 exotic plant species were encountered, accounting for 23% of the flora in each watershed. In both watersheds, exotic species richness was approximately 33% greater in riparian zones than in uplands, and mean number and cover of exotic species were > 50% greater in riparian zones than in uplands. Among landscape patch types, richness and mean number and cover of exotics was highest in young riparian patches, intermediate in clearcuts and riparian forests, and lowest in young and mature forests. The exception to this was Hoh alder flats, which had the highest mean cover of exotic plants. Cover of exotic plants peaked in uplands 3 to 7 yr after clearcutting, then decreased with increased canopy closure. Disturbance type and time since disturbance were major factors influencing invasibility. Landscape patch size, position within watershed (distance from patch to human population centers, major highway, or river mouth), and environmental variables (slope, aspect, and elevation) were not important indicators of landscape patch invasibility. Riparian zones facilitated movement of exotic plants through landscapes, but did not appear to act as sources of exotic plants for undisturbed upland areas.

Community Structure in Grazed and Ungrazed Grassland Sites in the Flooding Pampa, Argentina

-We compared the community structure and composition of two contiguous grasslands, one that had been continuously grazed and one that had remained ungrazed since 1972. Basal cover was measured at both sites in 1976 and 1985. The structure of the grazed area was affected by grazing during these 9 years, which was indicated by the reduction of the cover of grasses and the increase of the cover of forbs and exotic species. In the ungrazed site more than 95% of the basal cover was due to monocots, and the species that dominated this site were seldom found in the grazed site. Large tussocks replaced smaller ones in the absence of cattle. The grazed community was more diverse than the ungrazed one at a small scale (5 m), but less diverse at a larger scale (20 m). Cool season species were always dominant in the grazed site, but in the ungrazed site warm season species dominated some patches and cool season species, others. We concluded that in the absence of grazing, different competitive equilibria may be attained in different patches, probably due to subtle environmental differences. In the grazed area, the effects of cattle upon the community may override environmental heterogeneity and prevent the achievement of competitive equilibrium.