The effects of the red imported fire ant on seed fate in the longleaf pine ecosystem

The natural patterns of myrmecochory are disrupted by the dominance of red imported fire ants (Solenopsis invicta) in the southeastern United States. This leads to questions about the role of fire ants as seed dispersers. We examined the fate of ant-dispersed seed in the longleaf pine ecosystem. First, we determined removal rates for a suite of common ground cover species. Then, we verified the final location of removed seeds by using a wax cast to examine nest contents, and locating dyed seeds deposited in trash piles on the ground surface. Finally, we determined if the germination rate of seeds deposited by fire ants was affected by ant dispersion. Fire ants were most attracted to elaiosome-bearing seed and collected nonelaiosome-bearing seed at a much lower rate. No seeds were found in the contents of wax castings of fire ant nest chambers, suggesting that seed is not stored within the nest. Of the dyed seeds that we presented to fire ants, 30–40 % were recollected in surface trash piles in the mound vicinity within 1 week following removal. Undiscovered seeds were considered destroyed or buried in foraging tunnels. A small percentage of the deposited seeds were able to germinate, but there was no difference in the percent germination between seeds manipulated by fire ants and the control. Low germination was likely due to a high percentage of immature seeds used in the study. Our findings support a growing body of evidence that fire ants facilitate the movement of seeds in the longleaf pine ecosystem.     

Contingent fire effects on granivore removal of exotic woody plant seeds in longleaf pine savannas

Prescribed fire is increasingly used to inhibit woody encroachment into fire-dependent ecosystems, yet its effects on other processes influencing invasion are poorly understood. We investigated how fire influences exotic woody invasion through its effects on granivore activity, and whether these effects depend on the habitat in which seed predation takes place. We quantified seed removal for four species of exotic woody plants (Albizia julibrissin, Elaeagnus umbellata, Melia azedarach and Triadica sebifera) in 17 sites in longleaf pine savanna that varied in time since fire (one or three growing seasons post-fire) in the sandhills region of North Carolina, USA. Within each site, we established paired plots in upland and upland-wetland ecotone communities and presented seeds in depots that allowed either arthropod, or arthropod and small vertebrate access. We found that differences in seed removal with time since fire were contingent on habitat and granivore community. In ecotones, three of four species had higher proportions of seeds removed from plots that were three growing seasons post-fire than plots one growing season post-fire, whereas only T. sebifera showed this pattern in upland habitat. Allowing vertebrate granivores access to seeds enhanced seed removal, and this effect was strongest in ecotone habitat. While granivores removed seeds of all four plant species, removal of E. umbellata was significantly higher than that of the other species, suggesting that granivores exhibited seed selection. These findings suggest that ecotone habitats in this system experience greater seed removal than upland habitats, particularly as time since fire increases, and differences are mainly due to the activity of vertebrate granivores. Such differences in seed removal, together with seed selection, may contribute to variation in exotic woody invasion of longleaf pine savannas.     

Demographic effects of large, introduced herbivores on a long-lived endemic plant

The introduction of alien ungulates is a major threat for the survival of endangered plants, especially in island ecosystems. However, very few studies have investigated the potential damage of large herbivores on plant diversity in Mediterranean protected areas. In this study, we describe the population structure and the long-term dynamics of the main populations of the Sardinian narrow endemic Centaurea horrida Bad. (Asteraceae), by means of permanent plots where individual plants were tagged and monitored through 6?years (2004?C2009). We monitored this endangered plant at three sites: two were protected areas where introduced and feral ungulates are present, and the other one was a non-protected site without introduced ungulates. We found that adults and saplings were more abundant at the non-protected site. Through matrix models, we also highlighted that the non-protected population showed the highest population growth rate. Finally, by means of an exclusion experiment for ungulates at one protected site, we demonstrated that herbivores had a negative effect on the survival of seedlings and adult plants, and reduced the stochastic population growth rate. An LTRE analysis showed that differences in the survival, especially of adult individuals, had the highest responsibility in explaining the higher population growth rate when herbivores are excluded. Our study constitutes a clear example on how the protection of alien large herbivores can have opposite effects on the conservation of an endangered plant. Some management options are proposed, and the urgent need of manipulative experiments on species-specific interactions between protected plants and alien herbivore species is invoked.     

Measuring the effects of invasive species on the demography of a rare endemic plant

The impacts of invasive species are among the greatest threats to the persistence of native species and communities. Yet most work on rare plants has focused on issues such as habitat fragmentation and genetic diversity, while few studies have quantified the impacts of invasive plants on native ones or investigated the underlying mechanisms of those impacts. I used removal experiments to assess the effects of invasive grasses on the seedling and adult demography of an endangered California endemic, Oenothera deltoidesssp. howellii. Invasive plant removal significantly increased O. deltoides seedling recruitment, but had no effect on adult plants. Differences in seedling recruitment were primarily driven by greatly increased seedling emergence rates in removal plots, although there was also some evidence of higher seedling survivorship with invasives removal. Differences in habitat type strongly influenced both the effects of removal treatments and O. deltoides demography, with areas that support natural recruitment showing weaker treatment effects and higher overall recruitment, but lower adult survivorship, compared to those under restoration through planting. These results indicate that inhibition of germination due to reduced soil disturbance, rather than resource competition, appears to be the strongest impact of invasive plants on this rare endemic. Although previous work has documented the importance of changed disturbance regimes in generating invasion impacts, invader effects on rare plants are generally presumed to result from resource competition. Studies like this one highlight the need for a greater emphasis on understanding the mechanisms by which invasive plants impact native ones, and the importance of such information in designing conservation and management strategies.     

Competitive abilities of three narrowly endemic plant species in experimental neighborhoods along a fire gradient

We conducted field experiments manipulating lichens, shrubs, and herbs along a time-since-fire gradient and assessing effects on three endemic herbaceous species of Florida scrub: Eryngium cuneifolium, Hypericum cumulicola, and Polygonella basiramia. Responses included seed germination, survival, biomass, and fecundity. Transplants into recently burned patches generally had higher survival, larger biomass, and greater reproductive output than transplants into long-unburned patches. Open areas and sites near oaks frequently were more favorable than sites near Florida rosemary. Ground lichens did not affect germination but increased mortality rate of seedlings. Neighboring small shrubby and herbaceous species did not affect the performance of these species. Of the three species, naturally occurring E. cuneifolium were farthest from large shrubs, and their microhabitats had the least ground lichens and shrubs. Eryngium cuneifolium and H. cumulicola are capable of forming persistent seed banks and their recruitment after fire depends mostly on these dormant seeds. Polygonella basiramia relies on seed dispersal and immediate seed germination to colonize recently burned patches. Management for these species should involve variable fire regimes to allow all three species to persist along with many other scrub endemics.     

Declines in plant species richness and endemic plant species in longleaf pine savannas invaded by <Emphasis Type="Italic">Imperata cylindrica</Emphasis>

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

Differential effects of elevated ultraviolet-B radiation on plant species of a dune grassland ecosystem

In a greenhouse study, plants of three monocotyledonous and five dicotyledonous species, which occur in a Dutch dune grassland, were exposed to four levels of ultraviolet-B (UV-B) radiation. UV-B levels simulated up to 30% reduction of the stratospheric ozone column during summertime in The Netherlands. Six of the plant species studied in the greenhouse were also exposed to enhanced UV-B irradiance in an experimental field study. In the field experiment plants either received the ambient UV-B irradiance (control) or an enhanced UV-B level simulating 15–20% ozone depletion during summertime in The Netherlands. The purpose of both experiments was to determine the response of the plant species to UV-B radiation and to compare results obtained in the greenhouse with results of the field experiment. Large intraspecific differences in UV-B sensitivity were observed in the greenhouse study. Total dry matter accumulation of monocotyledons was increased, while dry matter accumulation of dicotyledons remained unaffected or decreased. The increase in biomass production of monocotyledons at elevated UV-B was not related to the rate of photosynthesis but to alterations in leaf orientation. In the greenhouse study, UV-B radiation also affected morphological characteristics. Shoot height or maximum leaf length of five out of eight species was reduced. In the field study only one species showed a significantly decreased maximum leaf length at enhanced UV-B. Possible reasons for this discrepancy are discussed. The absorbance of methanolic leaf extracts also differed between species. UV absorbance of field-grown plants was higher than greenhouse-grown plants. In the greenhouse study, the highest UV-B level increased UV-B absorbance of some species. In the field study however, this stimulation of UV absorbance was not observed. In general, results obtained in the greenhouse study were similar to results obtained in the field study. Difficulties in extrapolating results of UV-B experiments conducted in the greenhouse to the field situation are discussed.     

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m^–2 month^–1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.     

Pyrrolizidine alkaloids in two endemic capeverdian Echium species

Echium hypertropicum Webb and Echium stenosiphon Webb subsp. stenosiphon are capeverdian shrubs used in folk medicine for the treatment of cough and gastrointestinal diseases. Acid-base extraction was used to obtain alkaloid-rich fractions from both species. GC–MS and ESI-MS/MS analysis indicated the presence of pyrrolizidine alkaloids (PAs) and purified substances were also analyzed by 1D and 2D NMR experiments. A total of ten alkaloids were detected, eight of which were identified by comparing their molecular masses and mass fragmentation patterns with the NIST database and data in the literature. The hepatotoxic diesters echimidine and 7-(2-methylbutyryl)-9-echimidinylretronecine were identified in both species. Echimidine was the major component in the diethyl ether fraction from leaves of E. hypertropicum, whereas the 7-(2-methylbutyryl)-9-echimidinylretronecine was the major component in the dichloromethane fraction from leaves of E. stenosiphon. To our knowledge, this is the first report on the occurrence of pyrrolizidine alkaloids in capeverdian species of Echium. This study on Echium led to the identification of constituent pyrrolizidine alkaloids, serving to assist taxonomists with the complex taxonomy of this genus.     

Inferring plant ecosystem organization from species occurrences

In this paper, we present an approach capable of extracting insights on ecosystem organization from merely occurrence (presence/absence) data. We extrapolate to the collective behavior by encapsulating some simplifying assumptions within a given set of constraints, and then examine their ecological implications. We show that by using the mean occurrence and co-occurrence of species as constraints, one is able to capture detailed statistics of a plant community distributed across a vast semiarid area of the United States. The approach allows us to quantify the species’ effective couplings: Their frequencies exhibit a peak at zero and the minimal pairwise model is able to capture about 80% of the ecosystem structure. Our analysis reveals a relatively stronger impact of the species network on uncommon species and underscores the importance of species pairs experiencing positive couplings. Additionally, we study the associations among species and, interestingly, find that the frequencies of groups of different species, which the approach is able to capture, exhibit a power-law-like distribution.     

Short-term effects of two aphid species on plant growth and root respiration of three leguminous species

The short-term effects of cowpea aphids ( Aphis craccivora Koch) and pea aphids ( Acyrthosiphon pisum Harris), both Homoptera: Aphididae, on plant growth and respiration of excised, intact roots of cowpea [ Vigna unguiculata (L.) Walp. cv. Caloona], broadbean ( Vicia faba L. cv. Aquadulce) and garden pea ( Pisum sativum L. cv. Victory Freezer) seedlings were investigated, but not all plant-aphid combinations were used. Plant and root mean relative growth rates were significantly reduced within 10 days in the infested plants. Rates of total root respiration were was also significantly reduced in all infested plants within 10 days, presumably because of the reduced availability of translocate to the roots. The contribution of the cytochrome pathway to root respiration was significantly greater in control than in infested plants. The activity and engagement of the alternative respiratory pathway was also greater in control plants, and was absent in infested plants after 10 days infestation in all cases but one. These data indicate that the roots of aphid-infested plants were more efficient, in terms of energy conversion, than their respective controls.     

Performance of two alpine plant species along environmental gradients in an alpine meadow ecosystem in central Tibet

Understanding how biotic interactions and abiotic conditions affect plant performance is important for predicting changes in ecosystem function and services in variable environments. We tested how performances of Astragalus rigidulus and Potentilla fruticosa change along gradients of biotic interactions (represented by plant species richness, abundance of the dominant plant species Kobresia pygmaea, and herbivory intensity) and abiotic conditions (represented by elevation, aspect, and slope steepness) across a semi-arid landscape in central Tibet. Redundancy analyses showed that the biotic variables explained 30 and 39 % of the variation in overall performance of A. rigidulus (P = 0.03) and P. fruticosa (P = 0.01), respectively. Abiotic variables did not contribute significantly to variation among A. rigidulus populations. Plant size decreased with species richness in both species and was larger on south- rather than north-facing slopes. Reproductive effort for both species was significantly negatively related to the abundance of K. pygmaea and both species had larger reproductive effort on south- rather than north- and west-facing slopes. The proportion of biomass allocated to sexual reproduction in P. fruticosa was negatively correlated with K. pygmaea abundance and herbivory intensity. The population density of P. fruticosa was positively related to elevation, species richness, and K. pygmaea abundance. We conclude that plant performance at a local scale was more strongly related to biotic than abiotic conditions, but different components of plant performance responded differently to predictor variables and the responses were species-specific. These findings have important implications for rangeland management under changing environmental conditions.     

Nitrogen supply effects on leaf dynamics and nutrient input into the soil of plant species in a sub-arctic tundra ecosystem

Global warming will lead to increased nitrogen supply in tundra ecosystems. How increased N supply affected leaf production, leaf turnover and dead leaf N input into the soil of Empetrum nigrum and Andromeda polifolia (evergreens), Eriophorum vaginatum (graminoid) and Betula nana (deciduous) in a sub-arctic tundra in northern Sweden between 2003 and 2007 was experimentally investigated. There was considerable interspecific variation in the response of leaf production to N addition, varying from negative, no response to a positive response. Nitrogen addition effects on leaf turnover also showed considerable variation among species, varying from no effect to increased leaf turnover (up to 27% in Eriophorum). Nitrogen addition resulted in a four to fivefold increase in N content in the dead leaves of both evergreens and a 65% increase in Eriophorum. Surprisingly, there was no increase in Betula. The response of dead leaf P contents to N addition was rather species specific. There was no response in Empetrum, whereas there were significant increases in Andromeda (+214%) and Eriophorum (+32%), and a decrease of 47% in Betula. As an overall result of the changes in leaf production, leaf turnover and dead leaf N and P contents, nitrogen addition increased in all species except Betula the amount of N and, for Andromeda and Eriophorum the amount of P transferred to the soil due to leaf litter inputs. However, the way in which this was achieved differed substantially among species due to interspecific differences in the response of the component processes (leaf production, leaf turnover, dead leaf nutrient content).     

Effects of fire season on flowering of forbs and shrubs in longleaf pine forests

Summary Effects of variation in fire season on flowering of forbs and shrubs were studied experimentally in two longleaf pine forest habitats in northern Florida, USA. Large, replicated plots were burned at different times of the year, and flowering on each plot was measured over the twelve months following fire. While fire season had little effect on the number of species flowering during the year following fire, fires during the growing season decreased average flowering duration per species and increased synchronization of peak flowering times within species relative to fires between growing seasons. Fires during the growing season also increased the dominance of fall flowering forbs and delayed peak fall flowering. Differences in flowering resulting from variation in fire season were related to seasonal changes in the morphology of clonal forbs, especially fall-flowering composites. Community level differences in flowering phenologies indicated that timing of fire relative to environmental cues that induced flowering was important in determining flowering synchrony among species within the ground cover of longleaf pine forests. Differences in fire season produced qualitatively similar effects on flowering phenologies in both habitats, indicating plant responses to variation in the timing of fires were not habitat specific.     

Contrasting effects of plant invasion on pollination of two native species with similar morphologies

Invasive plants may decrease native plant density and disrupt interactions between native plants and their pollinators. We hypothesized that invasive Solidago canadensis (Asteraceae) competes for pollination services with two confamilial species, Ixeris chinensis and Sonchus arvensis. Breeding-system studies revealed that both native species are self-incompatible. In plots with all three species we found that Solidago received the highest visitation rates. To test the hypothesis of competition for pollination in the context of reduced native density, we established 3 plots for both native species with three Solidago densities (uninvaded, 50 and 75 % invaded) and corresponding decreases in native density. We investigated the effects of varying densities of Solidago on honeybee visitation rates, number of successive visits within individual ramets, pollen-load composition on bees, and seed set. For both native species, increasing Solidago density and decreasing native density resulted in bees carrying higher ratios of Solidago pollen and in bees visiting fewer capitula prior to departing from a plant. However, for other aspects of pollination, the native species responded very differently to Solidago. With increasing Solidago and decreasing native density, Ixeris received fewer honeybee visits and produced fewer seeds, demonstrating competition for pollination, but Sonchus attracted more honeybee visits and showed a non-significant trend toward setting more seeds, suggesting facilitation. These opposing effects occurred despite similarities in the native species’ floral morphology, suggesting that the effects of invasive plants are difficult to predict. In this case the different effects may relate to Sonchus being a taller plant with larger flowers.     

Shrubs as ecosystem engineers across an environmental gradient: effects on species richness and exotic plant invasion

Ecosystem-engineering plants modify the physical environment and can increase species diversity and exotic species invasion. At the individual level, the effects of ecosystem engineers on other plants often become more positive in stressful environments. In this study, we investigated whether the community-level effects of ecosystem engineers also become stronger in more stressful environments. Using comparative and experimental approaches, we assessed the ability of a native shrub (Ericameria ericoides) to act as an ecosystem engineer across a stress gradient in a coastal dune in northern California, USA. We found increased coarse organic matter and lower wind speeds within shrub patches. Growth of a dominant invasive grass (Bromus diandrus) was facilitated both by aboveground shrub biomass and by growing in soil taken from shrub patches. Experimental removal of shrubs negatively affected species most associated with shrubs and positively affected species most often found outside of shrubs. Counter to the stress-gradient hypothesis, the effects of shrubs on the physical environment and individual plant growth did not increase across the established stress gradient at this site. At the community level, shrub patches increased beta diversity, and contained greater rarified richness and exotic plant cover than shrub-free patches. Shrub effects on rarified richness increased with environmental stress, but effects on exotic cover and beta diversity did not. Our study provides evidence for the community-level effects of shrubs as ecosystem engineers in this system, but shows that these effects do not necessarily become stronger in more stressful environments.     

Features and distribution patterns of Chinese endemic seed plant species

We compiled and identified a list of Chinese. endemic seed plant species based on a large number of published References and expert reviews. The characters of these seed plant species and their distribution patterns were described at length. China is rich in endemic seed plants, with a total of 14 939 species (accounting for 52.1%of its total seed plant species) belonging to 1584 genera and 191 families. Temperate families and genera have a significantly higher proportion of endemism than cosmopolitan and tropical ones. The most primitive and derived groups have significantly higher endemism than the other groups. The endemism of tree, shrub, and liana or vine is higher than that of total species; in contrast, the endemism of herb is lower than that of total species. Geographically,these Chinese endemic plants are mainly distributed in Yunnan and Sichuan provinces, southwest China. Species richness and proportion of these endemic plants decrease with increased latitude and have a unimodal response to altitude. The peak value of proportion of endemism is at higher altitudes than that of total species and endemic species richness. The proportions of endemic shrub, liana or vine, and herb increase with altitude and have a clear unimodal curve. In contrast, the proportion of tree increases with altitude, with a sudden increase at～4000 m and has a completely different model. To date, our study provides the most comprehensive list of Chinese endemic seed plant species and their basic composition and distribution features.     

Complexity of multitrophic interactions in a grassland ecosystem depends on plant species diversity

1.?We studied the theoretical prediction that a loss of plant species richness has a strong impact on community interactions among all trophic levels and tested whether decreased plant species diversity results in a less complex structure and reduced interactions in ecological networks. 2.?Using plant species-specific biomass and arthropod abundance data from experimental grassland plots (Jena Experiment), we constructed multitrophic functional group interaction webs to compare communities based on 4 and 16 plant species. 427 insect and spider species were classified into 13 functional groups. These functional groups represent the nodes of ecological networks. Direct and indirect interactions among them were assessed using partial Mantel tests. Interaction web complexity was quantified using three measures of network structure: connectance, interaction diversity and interaction strength. 3.?Compared with high plant diversity plots, interaction webs based on low plant diversity plots showed reduced complexity in terms of total connectance, interaction diversity and mean interaction strength. Plant diversity effects obviously cascade up the food web and modify interactions across all trophic levels. The strongest effects occurred in interactions between adjacent trophic levels (i.e. predominantly trophic interactions), while significant interactions among plant and carnivore functional groups, as well as horizontal interactions (i.e. interactions between functional groups of the same trophic level), showed rather inconsistent responses and were generally rarer. 4.?Reduced interaction diversity has the potential to decrease and destabilize ecosystem processes. Therefore, we conclude that the loss of basal producer species leads to more simple structured, less and more loosely connected species assemblages, which in turn are very likely to decrease ecosystem functioning, community robustness and tolerance to disturbance. Our results suggest that the functioning of the entire ecological community is critically linked to the diversity of its component plants species.     

An invasive plant provides refuge to native plant species in an intensely grazed ecosystem

Invasion by exotic plant species and herbivory can individually alter native plant species diversity, but their interactive effects in structuring native plant communities remain little studied. Many exotic plant species escape from their co-evolved specialized herbivores in their native range (in accordance with the enemy release hypothesis). When these invasive plants are relatively unpalatable, they may act as nurse plants by reducing herbivore damage on co-occurring native plants, thereby structuring native plant communities. However, the potential for unpalatable invasive plants to structure native plant communities has been little investigated. Here, we tested whether presence of an unpalatable exotic invader Opuntia ficus-indica was associated with the structure of native plant communities in an ecosystem with a long history of grazing by ungulate herbivores. Along 17 transects (each 1000 m long), we conducted a native vegetation survey in paired invaded and uninvaded plots. Plots that harboured O. ficus-indica had higher native plant species richness and Shannon–Wiener diversity H′ than uninvaded plots. However, mean species evenness J was similar between invaded and uninvaded plots. There was no significant correlation between native plant diversity and percentage plot cover by O. ficus-indica. Presence of O. ficus-indica was associated with a compositional change in native community assemblages between paired invaded and uninvaded plots. Although these results are only correlative, they suggest that unpalatable exotic plants may play an important ecological role as refugia for maintenance of native plant diversity in intensely grazed ecosystems.