Exotic Plant Richness Research Articles

Housing is positively associated with invasive exotic plant species richness in New England, USA

Understanding the factors related to invasive exotic species distributions at broad spatial scales has important theoretical and management implications, because biological invasions are detrimental to many ecosystem functions and processes. Housing development facilitates invasions by disturbing land cover, introducing nonnative landscaping plants, and facilitating dispersal of propagules along roads. To evaluate relationships between housing and the distribution of invasive exotic plants, we asked (1) how strongly is housing associated with the spatial distribution of invasive exotic plants compared to other anthropogenic and environmental factors; (2) what type of housing pattern is related to the richness of invasive exotic plants; and (3) do invasive plants represent ecological traits associated with specific housing patterns? Using two types of regression analysis (best subset analysis and hierarchical partitioning analysis), we found that invasive exotic plant richness was equally or more strongly related to housing variables than to other human (e.g., mean income and roads) and environmental (e.g., topography and forest cover) variables at the county level across New England. Richness of invasive exotic plants was positively related to area of wildland-urban interface (WUI), low-density residential areas, change in number of housing units between 1940 and 2000, mean income, plant productivity (NDVI), and altitudinal range and rainfall; it was negatively related to forest area and connectivity. Plant life history traits were not strongly related to housing patterns. We expect the number of invasive exotic plants to increase as a result of future housing growth and suggest that housing development be considered a primary factor in plans to manage and monitor invasive exotic plant species.

Exotic vs. native plant dominance over 20 years of old-field succession on set-aside farmland in Argentina

Exotic plants are major constituents of species pools in modern landscapes. Managing succession for restoration of degraded ecosystems thus requires an understanding of novel trajectories unfolding in mixed, native/exotic plant assemblages. We examined trends in native and exotic species abundance over 20 years of old-field succession on set-aside farmland in the Inland Pampa, Argentina. Changes in plant cover and species richness were annually monitored on adjacent permanent plots established in different years (1978–1989). Both native and exotic species occurred in early, mid and late successional stages, exhibiting similar life-form replacement patterns, from annual forbs, through annual to perennial grasses. Exotic plant richness declined with plot age. Yet, four exotic grasses remained dominant through succession (50–70% cover), with plots initiated in later years showing increased exotic cover. While native perennial grasses occurred from the onset of succession, increasing from 5 to 12 spp/plot, they only showed transient peaks below 30% cover. Cluster analysis of 113 plot-year samples identified alternative community states for early, mid and late successional stages, which were connected by a complex network of interweaving dynamic pathways. Depending on the plot, vegetation dynamics comprised directional temporal trajectories as well as nondirectional pathways, and arrested community states dominated by exotic grasses. Our results illustrate the overwhelming role of exotic species in modern old-field succession, and their potential to hinder recovery of native communities on former agricultural land. Community states with novel, native/exotic plant mixtures could be managed to deliver specific ecosystem services (e.g. forage production, carbon sequestration). However, meeting conservation goals may require active restoration measures, including exotic plant removals and native grass seeding.

Land‐use and isolation interact to affect wetland plant assemblages

Different management regimes imposed on similar habitat types provide opportunities to investigate mechanisms driving community assembly and changes in species composition. We investigated the effect of pasture management on vegetation composition in wetlands with varying spatial isolation on a Florida cattle ranch. We hypothesized that increased pasture management intensity would dampen the expected negative effect of wetland isolation on native species richness due to a change from dispersal‐driven community assembly to niche‐driven assembly by accentuated environmental tolerance. We used native plant richness, exotic plant richness and mean coefficient of conservatism (CC) to assess wetland plant assemblage composition. Sixty wetlands were sampled, stratified by three levels of isolation across two pasture management intensities; semi‐native (less intensely managed; mostly native grasses, never fertilized) and agronomically improved (intensely managed, planted with exotic grasses, and fertilized). Improved pasture wetlands had lower native richness and CC scores, and greater total soil phosphorus and exotic species coverage compared to semi‐native pasture wetlands. Increased wetland isolation was significantly associated with decreases in native species richness in semi‐native pasture wetlands but not in improved pasture wetlands. Additionally, the species–area relationship was stronger in wetlands in improved pastures than semi‐native pastures. Our results indicate that a) native species switch from dispersal‐based community assembly in semi‐native pastures to a species‐sorting process in improved pastures, and b) recently‐introduced exotic species already sorted for more intensive management conditions are primarily undergoing dispersal‐based community assembly. That land‐use may alter the relative importance of assembly processes and that different processes drive native and exotic richness has implications for both ecosystem management and restoration planning.

HOUSING IS POSITIVELY ASSOCIATED WITH INVASIVE EXOTIC PLANT SPECIES RICHNESS IN NEW ENGLAND, USA

Understanding the factors related to invasive exotic species distributions at broad spatial scales has important theoretical and management implications, because biological invasions are detrimental to many ecosystem functions and processes. Housing development facilitates invasions by disturbing land cover, introducing nonnative landscaping plants, and facilitating dispersal of propagules along roads. To evaluate relationships between housing and the distribution of invasive exotic plants, we asked (1) how strongly is housing associated with the spatial distribution of invasive exotic plants compared to other anthropogenic and environmental factors; (2) what type of housing pattern is related to the richness of invasive exotic plants; and (3) do invasive plants represent ecological traits associated with specific housing patterns? Using two types of regression analysis (best subset analysis and hierarchical partitioning analysis), we found that invasive exotic plant richness was equally or more strongly related to housing variables than to other human (e.g., mean income and roads) and environmental (e.g., topography and forest cover) variables at the county level across New England. Richness of invasive exotic plants was positively related to area of wildland–urban interface (WUI), low-density residential areas, change in number of housing units between 1940 and 2000, mean income, plant productivity (NDVI), and altitudinal range and rainfall; it was negatively related to forest area and connectivity. Plant life history traits were not strongly related to housing patterns. We expect the number of invasive exotic plants to increase as a result of future housing growth and suggest that housing development be considered a primary factor in plans to manage and monitor invasive exotic plant species.

Impacts of erosion control treatments on native vegetation recovery after severe wildfire in the Eastern Cascades, USA

Slope stabilisation treatments like mulching and seeding are used to increase soil cover and reduce runoff and erosion following severe wildfires, but may also retard native vegetation recovery. We evaluated the effects of seeding and fertilisation on the cover and richness of native and exotic plants and on individual plant species following the 2004 Pot Peak wildfire in Washington State, USA. We applied four seeding and three fertilisation treatments to experimental plots at eight burned sites in spring 2005 and surveyed vegetation during the first two growing seasons after fire. Seeding significantly reduced native non-seeded species richness and cover by the second year. Fertilisation increased native plant cover in both years, but did not affect plant species richness. Seeding and fertilisation significantly increased exotic cover, especially when applied in combination. However, exotic cover and richness were low and treatment effects were greatest in the first year. Seeding suppressed several native plant species, especially disturbance-adapted forbs. Fertilisation, in contrast, favoured several native understorey plant species but reduced tree regeneration. Seeding, even with native species, appears to interfere with the natural recovery of native vegetation whereas fertilisation increases total plant cover, primarily by facilitating native vegetation recovery.

Patterns of native and exotic vascular plant richness along an elevational gradient from sea level to the summit of the Appalachian Mountains, U.S.A

Abstract We used floristic studies from the state of North Carolina to compare the patterns of total, native and exotic plant species richness from sea level (Atlantic coast) to the summit of the Appalachian Mountains. Few studies have investigated how patterns of native and exotic species richness differ along environmental gradients, and these studies have yielded contrasting results. We compare our results with those few published studies, and demonstrate that there is a need for future studies examining exotic richness along gradients. We modeled the effects of size of study area, year of study, and elevation on species richness using a dataset of sixty-eight floristic studies. Native and exotic species richness showed a positive relationship with area and year. Exotic species showed a steeper slope than native species for the species-area relationship. Richness of both groups was positively but weakly related to year of study. After accounting for area and year, native species displayed a hump-shaped...

Factors affecting exotic annual plant cover and richness along roadsides in the eastern Mojave Desert, USA

Roads are often considered a critical point of entry for invasive exotic plant species. We evaluated the cover and richness of exotic plants relative to road type (gravel or paved) and distance from roads in the eastern Mojave Desert where exotic annual species are increasing the flammability of the desert. Exotic and native annual plant cover and richness were sampled from five to 45m from roads in three prevalent microsite types for this system: Larrea tridentata microsites, Ambrosia dumosa microsites, and interspaces between shrubs. Six exotic and 58 native annual species were detected during sampling. There was no distinction between the behavior of exotics and natives relative to road type or distance from the roads. A. dumosa and L. tridentata microsites had more exotic cover than interspaces. Likewise, exotic richness was significantly higher in the shrub microsites. A taxonomically controlled analysis involving three plant families revealed no significant differences in cover of native or exotic species relative to their distance from the road. Our results suggest that exotic plant species in the Mojave Desert are not necessarily more prevalent near roadsides than in adjacent undisturbed desert. Therefore, roadside surveys alone may not be adequate to detect exotic species presence.

The distribution of native and exotic plants in a naturally fragmented sagebrush-steppe landscape

We tested two general hypotheses for the diversity of native and exotic plants in an undisturbed, naturally fragmented sagebrush-steppe landscape in SE Idaho, USA, evaluating whether the MacArthur–Wilson hypothesis of island biogeography or a suite of environmental variables explained the distributions of native and exotic plants. We also tested a third hypothesis, which incorporated assumptions about the origin of exotic plants and their interaction with native plants. Of the three hypotheses we tested, the hypothesis that included exotic species best explained the diversity of the native plant community. The MacArthur–Wilson model of island biogeography did not explain the diversity of native (R 2 = 0.13) or exotic plants well (R 2 = 0.11), and the model fit the data poorly. A model of environmental variables better explained the diversity of native (R 2 = 0.48) and exotic plants (R 2 = 0.57), but it also fit the data poorly. Instead, proximity to a railroad explained the cover (R 2 = 0.59) and richness of exotic plants (R 2 = 0.63), which then explained the species richness of native plants (R 2 = 0.34), and the model fit was adequate and had the lowest AIC value. This suggests that the transportation corridor had a significant, though indirect, effect on the native plant community, even in this undisturbed area. Moreover, explained variance, model fit, and the AIC model selection criteria favored the model with the railroad and exotic species over the M–W and environmental models. Since the habitat patches we studied were largely undisturbed by people and their activities, our results further suggest that the transportation corridor influenced the distribution of exotic plants by serving as a vector for colonization, rather than as a source of disturbance. Additionally, the results suggest that exotic plant species have had a negative effect on the diversity of the native plant community and have changed its composition. The results also support the inference that the nascent exotic plant community is influenced by source-sink (Pulliam in Am Nat 132:652–661, 1988) and assembly dynamics. In contrast, the native plant community appears to be more strongly influenced by environmental conditions associated with an elevational gradient, but there is evidence that the native community also has undergone directional change in species composition, associated with the invasion by non-native species.

Land use history alters the relationship between native and exotic plants: the rich don’t always get richer

Observational studies of diversity have consistently found positive correlations between native and exotic species, suggesting that the same environmental factors that drive native species richness also drive exotic species richness, i.e., “the rich get richer”. We examined patterns of native and exotic plant species richness in temperate forests that have been undergoing reforestation since the turn of the twentieth century to test the influence of disturbance arising from land-use history on this relationship. Overall, we found no relationship between native and exotic plant species richness. Instead, we found a positive relationship between native and exotic richness in older but not younger-growth forests, suggesting that the same processes that drove exotic plant richness in older forests also facilitated native plants. In contrast, younger forests had similar numbers of native species relative to older forests, but 41% more exotic species and 24% more compacted soils. Moreover, exotic but not native species richness was positively correlated with increasing soil compaction across all sites. Overall, our results suggest that elevated exotic plant invasions in younger forests are a legacy of soil disturbance arising from agricultural practices at the turn of the century, and that native and exotic plants may respond differentially to disparate environmental drivers.

Linear declines in exotic and native plant species richness along an increasing altitudinal gradient in the Snowy Mountains, Australia

Abstract Declines in plant species richness with increasing altitude are common, but the form of the relationship can vary, with both monotonic decreasing relationships and humped relationship recorded. However, these different richness to altitude relationships may be due to methods that used different plot sizes/areas and survey efforts. To explore native and exotic plant richness along an altitudinal gradient in the Snowy Mountains of Australia, we consistently surveyed plots that were 120 m2 in area at 39 sites ranging from 540 to 2020 m. To relate exotic plant richness to disturbance, we surveyed plots at 16 sites along main roads and 23 sites along minor roads and also compared these 39 roadside plots to 120‐m2 plots located in undisturbed vegetation adjacent to the roadside (native plant richness was only surveyed in 25 of these 39 adjacent plots). We found a negative linear relationship between total, exotic and native species richness and altitude for plots on the side of main roads (16 sites) and minor roads (23 sites). For adjacent plots negative linear relationships were significant for all measures of species richness except for native species adjacent to major roads. As the pattern occurred for exotics it is less likely to be due to historical constraints on the species pools. The pattern could be influenced by difference in levels of disturbance along the gradient, although any such gradient in disturbance would have to apply to roadside and adjacent plots on major and minor roads. Therefore, it may be due to other factors such as changes in climate along the altitudinal gradient, although additional sampling including direct measures of climatic conditions, soil and disturbance factors would be needed to determine if this was the case.

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

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

PLANT SPECIES INVASIONS ALONG THE LATITUDINAL GRADIENT IN THE UNITED STATES: COMMENT

Few biogeographic patterns have been as well documented as the latitudinal gradient in species richness. For North American plants, the decrease in number of species from the subtropics to the arctic is well established (Stevens 1989, Currie 1991, Qian 1999), but becomes less defined as the spatial grain of species richness decreases (e.g., Monk 1967, Glenn-Lewin 1977). There is considerable interest among biogeographers and community ecologists as to whether clear latitudinal patterns are manifest at relatively fine spatial resolutions, where the signature of local processes such as historical disturbances or fine-scale environmental variation could obscure broad-scale geographic drivers of richness (Rahbek and Graves 2001, Whittaker et al. 2001). Because adequate data have generally been lacking, however, there remained until recently no systematic attempt to address the latitudinal gradient of plant richness for the full extent of the United States using relatively fine-scale data. Stohlgren et al. (2005) examined native and exotic vascular plant species richness at the county level for the 48 conterminous U.S. states in relation to latitude, climate, topographic and biotic factors, and human disturbance. By comparing a variety of multifactor models explaining native and exotic plant richness, Stohlgren et al. (2005) concluded that: (1) there is no relationship between native plant richness and latitude at the county level; (2) the strongest single predictor of exotic species richness is native species richness; and (3) bird species richness, a surrogate for habitat heterogeneity, is the best predictor of native plant richness. If true, these observations would represent a major advance in the study of plant diversity patterns, as they would suggest scale thresholds where broad-scale geographic patterns are no longer manifest due to the increasing importance of other factors such as local habitat heterogeneity. However, we argue that these conclusions are inaccurate, stemming from uncritical evaluation of completeness in the data set and from artifacts deriving from inappropriate data transformation. Furthermore, we analyze a more appropriate data set to the issue of county-level patterns of native and exotic species richness, and obtain different results than that of Stohlgren et al. (2005). A major conclusion of Stohlgren et al. (2005:2301) is that ‘‘regression analyses showed no relationship between latitude and native plant species density (richness).’’ We assert that this result is an artifact of two major deficiencies in their study; namely, that (1) their data set of county richness values underestimates true county richness, and (2) richness values were incorrectly transformed to account for variation in survey area. Plant data used by Stohlgren et al. (2005) come from the Biota of North America Program (BONAP), a collection of taxon occurrences from the literature and from herbarium surveys recorded by county (Stohlgren et al. 2003). These data are widely regarded as the ‘‘standard plant data set for many government and non-government agencies’’ (Stohlgren et al. 2005:2299), and we agree that this is the most comprehensive collection of plant occurrence records for North America. However, records for many counties were not themselves collected with the aim of providing complete floristic lists of counties (see Palmer 1995). Thus, although some counties are well collected, others remain less well described for a variety of reasons (such as low survey effort). In addition to adding error to the assessment of county-level richness patterns (thereby undermining the signal of geographic pattern in the data), variation in floristic completeness among counties could cause spurious correlations between native and exotic richness; as well-surveyed counties are more likely to contain more occurrences of both native and exotic species. The use of such data for the analysis of richness patterns is appropriate if researchers make sufficient attempts to correct for, or are at least be able to quantify, the variation in floristic completeness among counties. Stohlgren et al. (2005) give no assessment of data quality in their analysis. Moreover, the only attempt made to reduce the bias associated with including poorly surveyed counties is the removal of 114 counties with fewer than 100 occurrences. This procedure is both insufficient (114 is ,4% of the data set) and flawed, in that occurrences are a function of both survey effort and Manuscript received 3 February 2006; accepted 6 March 2006. Corresponding Editor: H. Hillebrand. 1 Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599 USA. 2 Research and Collections Center, Illinois State Museum, 1011 East Ash Street, Springfield, Illinois 62703 USA. 3 Botany Department, Oklahoma State University, Stillwater, Oklahoma 74078 USA. 4 Present address: Department of Biology, Syracuse University, 130 College Place, Syracuse, New York 13244 USA. E-mail: fridley@syr.edu

Successful biological control of mist flower (Ageratina riparia) in New Zealand: Agent establishment, impact and benefits to the native flora

The white smut fungus (Entyloma ageratinae) and the gall Xy (Procecidochares alani) were released in New Zealand in 1998 and 2001 respectively to suppress mist Xower (Ageratina riparia). The fungus established and spread rapidly, crossing 80 km of sea to Great Barrier Island within 2 years. The mean number of P. alani galls increased exponentially to 1.96/stem at release sites, but dispersal was slow. The impact of the biocontrol agents was monitored once annually from 1998/99 to 2003/04, at up to 51 sites in the North Island. The mean percentage of live leaves infected with fungus rapidly reached nearly 60%. Maximum plant height declined signiWcantly. In heavy infestations, mean percentage cover of mist Xower declined from 81 to 1.5%. Galls were only recorded towards the end of the impact study, and at low mean numbers. As mist Xower declined, the species richness and mean percentage cover of native plants increased. In contrast, the species richness and mean percentage cover of exotic plants (excluding mist Xower) did not change signiWcantly. Many plant species colonizing the plots were important native mid- or late-successional shrubs or trees. With few exceptions, the exotic plant species common in the plots were not weeds that appeared to threaten native forest habitats. There was only a weak “replacement weed eVect” from the potentially serious invader African club moss (Selaginella kraussiana). These data, together with reports of reduced threats to rare endemic plants from mist Xower, suggest this rapid, well-monitored weed biocontrol program was very successful.

Opposing Effects of Native and Exotic Herbivores on Plant Invasions

Exotic species are widely assumed to thrive because they lack natural enemies in their new ranges. However, a meta-analysis of 63 manipulative field studies including more than 100 exotic plant species revealed that native herbivores suppressed exotic plants, whereas exotic herbivores facilitated both the abundance and species richness of exotic plants. Both outcomes suggest that plants are especially susceptible to novel, generalist herbivores that they have not been selected to resist. Thus, native herbivores provide biotic resistance to plant invasions, but the widespread replacement of native with exotic herbivores eliminates this ecosystem service, facilitates plant invasions, and triggers an invasional "meltdown."

Anthropogenic impacts upon plant species richness and net primary productivity in California

AbstractWe assess the importance of anthropogenic land‐use, altered productivity, and species invasions for observed productivity–richness relationships in California. To this end, we model net primary productivity (NPP) c. 1750 AD and at present (1982–1999) and map native and exotic vascular plant richness for 230 subecoregions. NPP has increased up to 105% in semi‐arid areas and decreased up to 48% in coastal urbanized areas. Exotic invasions have increased local species diversity up to 15%. Human activities have reinforced historical gradients in species richness but reduced the spatial heterogeneity of NPP. Structural equation modelling suggests that, prior to European settlement, NPP and richness were primarily controlled by precipitation and other abiotic variables, with NPP mediating richness. Abiotic variables remain the strongest predictors of present NPP and richness, but intermodel comparisons indicate a significant anthropogenic impact upon statewide distributions of NPP and richness. Exotic and native species each positively correlate to NPP after controlling for other variables, which may help explain recent reports of positively associated native and exotic richness.

Applicability of landscape and island biogeography theory to restoration of riparian understorey plants

Summary Ecosystem recovery is influenced by processes at different spatial scales, yet land managers lack specific predictions on the relative importance of such processes that might guide management decisions. We tested whether ideas from landscape ecology (local vs. landscape scales) and island biogeography theory (patch size and isolation) predict restoration success for understorey plant communities in a highly fragmented riparian landscape, in an effort to provide guidance on how to allocate scarce restoration resources. We sampled naturally colonizing riparian forest understorey plant communities in 15 riparian forests restored by planting native woody species along a 150‐km stretch of the Sacramento River in central California. We analysed native and exotic understorey species richness and cover as a function of biotic and abiotic local and landscape variables. Cover and species richness of exotic understorey plants decreased strongly with increasing overstorey cover, and were lower in quadrats closer to river base flow. Native understorey species richness and cover were negatively related to exotic cover and positively related to connectivity with remnant forest. Cover of native wind‐dispersed species was best explained by higher percentage forest cover surrounding a site within a 1000‐m buffer, whereas cover of native water‐dispersed species was higher closer to the river. Neither patch size nor time since restoration explained a significant amount of native or exotic species richness or cover. Synthesis and applications. Local factors explained more of the variance in understorey plant communities, but much of the variance remained unexplained. Our results provide weak support for the predictions of island biogeography theory and the importance of landscape‐scale variables. These theories did not have strong predictive power in this applied restoration context at this temporal scale. Given limited resources, efforts to restore understorey plant communities in this highly fragmented system should focus on local‐scale restoration methodologies, such as increasing cover of native overstorey species and reducing cover of exotic plants.

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.

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 >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.

Promotion of exotic weed establishment by endangered giant kangaroo rats (Dipodomys ingens) in a California grassland

Giant kangaroo rats (Dipodomys ingens) continually modify their burrow precincts by digging tunnels, clipping plants, and other activities. In the valley grasslands of the Carrizo Plain Natural Area (San Luis Obispo County, California), this chronic disturbance to soil and vegetation promoted the establishment of exotic ruderal and early successional plant species. Erodium cicutarium, Bromus madritensis ssp. rubens, and other Mediterranean annuals were found to constitute a very large proportion of the vegetation on giant kangaroo rat precincts. When vegetation on precincts was compared with the vegetation in less disturbed intermediate areas located between precincts, species richness, cover and frequency of exotic plants were significantly greater on precincts. The reverse was found for native species. In addition, exotic species encountered in this study had significantly larger seeds than did native species, suggesting that these granivorous kangaroo rats preferentially cache large weed seeds on their precincts. Since the kangaroo rats depend on exotic plants for food and the exotic plants depend upon the kangaroo rats to disturb their habitat continually, the weed-kangaroo rat relationship is mutualistic. This strong relationship may also inhibit population growth of native grassland plants which occupy disturbed habitats but have difficulty competing with exotic weeds for resources. From a conservation perspective, this mutualism presents an intractable management dilemma. Restoration of valley grasslands where endangered giant kangaroo rats occur, to conditions where native species dominate, may be impossible.

Predicting Richness of Native, Rare, and Exotic Plants in Response to Habitat and Disturbance Variables across a Variegated Landscape

Species richness of native, rare native, and exotic understorey plants was recorded at 120 sites in temperate grassy vegetation in New South Wales. Linear models were used to predict the effects of environment and disturbance on the richness of each of these groups. Total native species and rare native species showed similar responses, with richness declining on sites of increasing natural fertility of parent material as well as declining under conditions of water enrichment (resulting from human‐induced changes in drainage characteristics, leading to increased run‐off), severe livestock grazing, and soil disturbance. The response of rare native species to water enrichment, however, was significantly greater than that of all native species. Exotic species richness varied in reverse to that of native species with positive responses to water enrichment and soil disturbance. The contrasting behaviors are attributed to differences in the evolutionary history of native and exotic assemblages and their resulting preadaptations to a landscape recently subjected to agricultural settlement. It would appear that for exogenous disturbances, the intermediate disturbance hypothesis is not supported by our data. In the sampled region, pastures represent the major land‐use in terms of area, but have relatively low densities of native and rare species compared with more lightly grazed areas. However, their management is considered to be essential to the maintenance of diversity on a regional scale.