Jasmonic acid treatment and mammalian herbivory differentially affect chemical defenses and growth of wild mustard (Brassica kaber)

Summary. Jasmonic acid (JA) is a wound-related hormone found in most plants that, when applied exogenously, can induce increases in levels of chemical defenses in patterns similar to those induced by mechanical damage or insect feeding. Relative to responses to insect and pathogen attack, chemical responses of herbaceous plants to mammalian herbivore attack have been little studied. In a field experiment, we compared the effects of JA treatment and naturally occurring mammalian herbivory on the expression of trypsin inhibitors, glucosinolates, peroxidase activity and growth of wild mustard (Brassica kaber). Exogenous JA significantly increased trypsin inhibitor activity and glucosinolate concentration, and moderately increased peroxidase activity in the eighth true leaves of five-week-old plants, relative to untreated controls. In contrast, levels of these chemical defenses in the eighth true leaves or in regrowth foliage of plants that had ∼80% of their leaf area removed by groundhogs (Marmota monax) did not differ from that in undamaged and untreated controls. Although exogenous JA significantly elevated levels of chemical defenses, it did not affect height of plants through the season and only slightly reduced time to first flower. Groundhog herbivory significantly reduced height and delayed or abolished flowering, but these effects were not substantial unless coupled with apical meristem removal. We hypothesize that the lack of effect of groundhog herbivory on chemical defenses may be due in part to the speed and pattern of leaf area removal by groundhogs, or physiological constraints caused by leaf area loss. Despite having no effect on chemical defense production, leaf area loss by groundhogs was more costly to growth and fitness than the effects of JA application in this study, but only substantially so if coupled with apical meristem removal. We suggest that in general, costs of defense production in plants are likely to be minimal when compared to the risk of losing large amounts of leaf area or primary meristematic tissue. Thus, if they are effective at deterring herbivory, the benefits of inducible defense production likely outweigh the costs in most cases. Received 20 December 2000; accepted 3 May 2001     

Facilitation across stress gradients: the importance of local adaptation

While there is some information on genetic variation in response to competition in plants, we know nothing about intraspecific variation in facilitation. Previous studies suggest that facilitation should increase fitness in stressful environments. However, whether a plant experiences an environment as stressful may depend on prior adaptive responses to stressors at a site. Local adaptation to stress at a site may reduce the likelihood of facilitation. Seeds of Plantago erecta from stressful (serpentine soil) and non-stressful (non-serpentine soil) edaphic environments were reciprocally planted into these two soil types. Although competition did not differ significantly among seed sources, there was evidence for a local adaptation effect on facilitation. Non-serpentine seeds planted into serpentine soil exhibited greater individual plant biomass at higher densities. The interaction between population source and growth environment indicates a role for evolutionary processes such as local adaptation in the expression of facilitation in plants.     

Effects of Population Size on Seed Production and Germinability in an Endangered, Fragmented Grassland Plant

Abstract: Fragmentation and isolation of plant populations are thought to affect demographic processes such as seed production and cause reductions in fitness. I followed seed set over a 3-year period in eight populations of the endangered Rutidosis leptorrhynchoides (Asteraceae) that differed in population size from 13 to over 5000 flowering plants. Germinability of the resultant seed was also examined to determine whether small populations had lower fitness than large populations. Seed set was significantly associated with population size in 2 of the 3 years. Small populations (<30 flowering plants) produced significantly fewer seeds per head in 1994 and 1995 than did large populations (500 to over 5000 flowering plants), which did not differ significantly from one another. There was, however, substantial variation within populations. In 1993 seed production did not follow any simple relationship with population size, possibly because environmental stress from low rainfall had an overriding impact. Differences in seed germinability between populations were largely not evident, suggesting that this aspect of fitness has not declined substantially in small populations relative to large populations. This study suggests that nongenetic, demographic factors are of immediate importance to the persistence of small populations of R. leptorrhynchoides because of their potential impacts on seedling recruitment.     

Inbreeding Depression, Environmental Stress, and Population Size Variation in Scarlet Gilia (Ipomopsis aggregata)

Despite a large body of theory, few studies have directly assessed the effects of variation in population size on fitness components in natural populations of plants. We conducted studies on 10 populations of scarlet gilia, Ipomopsis aggregata , to assess the effects of population size and year-to-year variation in size on the relative fitness of plants. We showed that seed size and germination success are significantly reduced in small populations (those 100 flowering plants) of scarlet gilia. Plants from small populations are also more susceptible to environmental stress. When plants from small and large populations were subjected to an imposed stress (combined effects of transplanting and experimental clipping, simulating ungulate herbivory) in a common garden experiment, plants from small populations suffered higher mortality and were ultimately of smaller size than plants from large populations. In addition, experimental evidence indicates that observed fitness reductions are genetic, due to the effects of genetic drift and/or inbreeding depression. When pollen was introduced from distant populations into two small populations, seed mass and percentage of germination were bolstered, while pollen transferred into a large population had no significant effect. Year-to-year variation in population size and its effects on plant fitness are also discussed. In one small population, for example, a substantial increase in size from within did not introduce sufficient new (archived) genetic material to fully overcome the effects of inbreeding depression.     

Nonadditive effects of floral display and spur length on reproductive success in a deceptive orchid

Pollinators may mediate selection on traits affecting pollinator attraction and effectiveness, and while nonadditive effects of traits influencing the two components of pollination success are expected when seed production is pollen limited, they have been little studied. In a factorial design, we manipulated one putative attraction trait (number of flowers) and one putative efficiency trait (spur length) previously shown to be subject to pollinator-mediated selection in the deceptive orchid Dactylorhiza lapponica. Removal of half of the flowers reduced pollen removal, proportion of flowers receiving pollen, fruit set, and fruit mass compared to unmanipulated plants, while spur-tip removal increased fruit set and fruit mass but did not affect pollen removal or proportion of flowers receiving pollen. The effect of spur-tip removal on fruit mass was stronger among plants with intact number of flowers compared to plants with experimentally reduced number of flowers. The results demonstrate that number of flowers and spur length are direct targets of selection and may affect female fitness nonadditively. More generally, they show that the adaptive value of a given trait can depend on floral context and illustrate how experimental approaches can advance our understanding of the evolution of trait combinations.     

Differing Effects of Cattle Grazing on Native and Alien Plants

Abstract:   Habitat managers use cattle grazing to reduce alien plant cover and promote native species in California grasslands and elsewhere in the western United States. We tested the effectiveness of grazing as a restoration method by examining the effects of herbivory on native and alien plants. At Carrizo Plain National Monument, California, we surveyed native and alien species cover in adjacent grazed and ungrazed areas. We also established experimental plots in which plants were clipped or mulch (dead biomass) was removed. In addition, we clipped plants grown in pots and plants in the field that grew with and without competitors. Native species were negatively affected by clipping in 1999, 2000, and 2001, whereas alien species were unaffected. In the experimental field plots, the European annual forb Erodium cicutarium compensated in growth and reproduction following simulated herbivory. In contrast, growth and reproduction of the native perennial bunchgrass Poa secunda were reduced 1 year after clipping. In pots, E. cicutarium overcompensated and grasses undercompensated. In the field, European grasses were unaffected by the removal of competitors. It is unclear by what mechanism E. cicutarium was able to compensate, but the ability may be related to its basal rosette growth form and indeterminately growing inflorescences. The native California grassland community assembled in the absence of grazing herds, whereas invasive European species have been exposed to grazing for centuries. It may be that these invaders have adaptations that better enable them to recover from grazing. In the grassland we studied, the strategy of livestock grazing for restoration is counterproductive. It harms native species and promotes alien plant growth.     

Predispersal seed herbivores, not pollinators, exert selection on floral traits via female fitness

Herbivores that oviposit in flowers of animal-pollinated plants depend on pollinators for seed production and are therefore expected to choose flowers that attract pollinators. This provides a mechanism by which seed herbivores and pollinators could impose conflicting selection on floral traits. We measured phenotypic selection on floral traits of Lobelia siphilitica (Lobeliaceae) via female fitness to determine the relative strength of selection by pollinators and a specialist predispersal seed herbivore. We were able to attribute selection on flowering phenology to the herbivores. However, no selection could be attributed to pollinators, resulting in no conflicting selection on floral traits. Unlike pollinators, whose preference for certain floral traits does not always translate into higher fitness, any discrimination by seed herbivores is likely to decrease fitness of the preferred floral phenotype. Thus predispersal seed herbivores may be a significant agent of selection on floral traits.     

Mutualism as reciprocal exploitation: African plant-ants defend foliar but not reproductive structures

The foundation of many plant-ant mutualisms is ant protection of plants from herbivores in exchange for food and/or shelter. While the role of symbiotic ants in protecting plants from stem- and leaf-feeding herbivores has been intensively studied, the relationship between ant defense and measures of plant fitness has seldom been quantified. We studied ant aggression, damage by herbivores and seed predators, and fruit production among Acacia drepanolobium trees occupied by four different acacia-ant species in an East African savanna. Levels of ant aggression in response to experimental disturbance differed strongly among the four species. All four ant species recruited more strongly to new leaf growth on host plants following disturbance, while recruitment to developing fruits was on average an order of magnitude lower. Host plants occupied by more aggressive ant species suffered significantly less vegetative damage from leaf-feeding insects, stem-boring beetles, and vertebrate browsers than host plants occupied by less aggressive ant species. However, there were no differences among fruiting host plants occupied by different ant species in levels of seed predation by bruchid seed predators. Fruit production on host trees was significantly correlated with tree stem diameter but not with the identity of resident ants. Our results demonstrate that defense of host plants may differ substantially among ant species and between vegetative and reproductive structures and that fruit production is not necessarily correlated with high levels of aggression by resident ants.     

Bottom-up multitrophic effects in resprouting plants

Severe damage often provokes compensatory resprouting of plants, which commonly modify plant morphological and phenological traits. Rapid plant growth often results in poorly defended nutrient-rich foliage, which is more susceptible to foliar-chewing herbivores. It is less known how other guilds of arthropods are affected by plant regrowth. We tested the hypotheses that clipping-induced resprouting and nutrient availability, separately and in combination, would (1) influence plant traits, (2) benefit chewing herbivores, sap-suckers, gallers, and pre-dispersal seed predators, and (3) cascade up to the third trophic level by positively affecting herbivores. Resprouted plants were morphologically and phenologically different from undamaged plants; as a result, seed predation, infestation rate, richness, and diversity of seed predators increased, and species composition was altered. Leaf consumption by chewing herbivores was four times higher on resprouted plants. The number of galls decreased, whereas the abundance of sap-sucking and leaf-chewing insects was not affected. The incidence of predators and parasitoids was also higher on resprouted plants and on plants with nutrients added, but the increase was less pronounced compared to the herbivores they feed on. Thus, the effects of resprouting, contingent on nutrient availability, can propagate simultaneously through two independent tri-trophic level pathways.     

Herbivory-mediated pollinator limitation: negative impacts of induced volatiles on plant-pollinator interactions

Although induced plant responses to herbivory are well studied as mechanisms of resistance, how induction shapes community interactions and ultimately plant fitness is still relatively unknown. Using a wild tomato, Solanum peruvianum, native to the Peruvian Andes, we evaluated the disruption of pollination as a potential ecological cost of induced responses. More specifically, we tested the hypothesis that metabolic changes in herbivore-attacked plants, such as the herbivore-induced emission of volatile organic compounds (VOCs), alter pollinator behavior and consequentially affect plant fitness. We conducted a series of manipulative field experiments to evaluate the role of herbivore-induced vegetative and floral VOC emissions as mechanisms by which herbivory affects pollinator behavior. In field surveys and bioassays in the plants' native habitat, we found that real and simulated herbivory (methyl jasmonate application) reduced attractiveness of S. peruvianum flowers to their native pollinators. We show that reduced pollinator preference, not resource limitation due to leaf tissue removal, resulted in reduced seed set. Solitary bee pollinators use floral plant volatiles, emitted in response to herbivory or methyl jasmonate treatment, as cues to avoid inflorescences on damaged plants. This herbivory-induced pollinator limitation can be viewed as a general cost of induced plant responses as well as a specific cost of herbivory-induced volatile emission.     

Moss harvest truncates the successional development of epiphytic bryophytes in the Pacific Northwest.

We evaluated the impact of commercial moss harvest on the development of an understory epiphyte community in the Pacific Northwest by characterizing natural development stages using data from both a long-term regrowth study and demographic sampling. First, experimentally stripped 1 m long cylindrats on 46 shrub stems in the Oregon Coast Range were monitored for species composition and abundance annually during the first five years of recovery and again in year 10. Second, a pathway of community development was inferred by examining the relative species composition and abundance of epiphytic species present in moss mats in a four-stage chronosequence. We (1) characterized the change in richness and composition from year 1 through 10 of regrowth following experimental disturbance, (2) quantified the proportion of approximately 1-, 10-, 25-, and 50-year-old moss mats of commercially harvestable species that were monodominant, diverse, and late successional, and (3) contrasted these proportions with estimates from a compositional transition matrix derived from long-term monitoring. Roughly half of the observed moss mats demonstrated neutral dynamics and were composed of a mixture of readily dispersed acrocarps and pleurocarps. The remaining half exhibited positive dynamics and were dominated by aggressively growing pleurocarpous species such as Isothecium myosuroides. Following structural developmental pathways well established for vascular plants, moss mats shift with time from high diversity and evenness in the initial colonization and extended establishment phases to increasing Isothecium dominance during a presumed competitive-exclusion phase. Old mats exist in alternate states of either Isothecium dominance or mixed composition, either of which may have late-successional species. Patchy historic commercial moss harvest likely facilitated high diversity by increasing the simultaneous occurrence of all moss mat age classes, while modern strip harvesting methods are predicted to promote predominantly even-aged mats that are unlikely to reach "old-growth" conditions before re-harvest.     

Modelling forest–savanna mosaic dynamics in man-influenced environments: effects of fire,climate and soil heterogeneity

Forests and savannas are the major ecotypes in humid tropical regions. Under present climatic conditions, forest is in a phase of natural expansion over savanna, but traditional human activities, especially fires, have strongly influenced the succession. We here present a new model, FORSAT, dedicated to the forest–savanna mosaic on a landscape scale and based on stochastic modelling of key processes (fire and succession cycle) and consistent with common field data. The model is validated by comparison between the qualitative emergent behaviour of the model and results of biogeographical field studies. Three types of forest succession are shown: progression of the forest edge, formation and coalescence of clumps in savanna and global afforestation of savanna. The parameters (frequency of savanna fires, climate and soil fertility) appear to have comparable effects and there is a sharp threshold between a forest edge progression scenario and the cluster formation one. Moreover, pioneer seed dispersal pattern and recruitment are determinant: peaked curves near a seed source and far dispersal combine to increase the fitness of the pioneers.     

Neighborhood effects on seed dispersal by frugivores: testing theory with a mistletoe-marsupial system in Patagonia

The outcome of the dispersal process in zoochorous plants is largely determined by the behavior of frugivorous animals. Recent simulation studies have found that fruit removal rates and mean dispersal distances are strongly affected by fruiting plant neighborhoods. We empirically tested the effects of conspecific fruiting plant neighborhoods, crop sizes, and plant accessibility on fruit removal rates and seed dispersal distances of a mistletoe species exclusively dispersed by an arboreal marsupial in Northern Patagonia. Moreover, in this study, we overcome technical limitations in the empirical estimation of seed dispersal by using a novel 15N stable isotope enrichment technique together with Bayesian mixing models that allowed us to identify dispersed seeds from focal plants without the need of extensive genotyping. We found that, as predicted by theory, plants in denser neighborhoods had greater fruit removal and shorter mean dispersal distances than more isolated plants. Furthermore, the probability of dispersing seeds farther away decreased with neighborhood density. Larger crop sizes resulted in larger fruit removal rates and smaller probabilities of longer distance dispersal. The interplay between frugivore behavioral decisions and the spatial distribution of plants could have important consequences for plant spatial dynamics.     

Associations of plant fitness, leaf chemistry, and damage suggest selection mosaic in plant-herbivore interactions

The geographic mosaic theory of coevolution states that variation in species interactions forms the raw material for coevolutionary processes, which take place over large geographic scales. One key assumption underlying the process of coevolution in plant-herbivore interactions is that herbivores exert selection on their host plants and that this selection varies among plant populations. We examined spatial variation in the existence and strength of phenotypic selection on host plant resistance exerted by specialist herbivores in 17 archipelago populations of the perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). In these highly fragmented populations, V. hirundinaria is consumed by the larvae of two specialist herbivores: the folivorous moth Abrostola asclepiadis and the seed predator Euphranta connexa. Selection imposed on host plants by these herbivores was examined by analyzing the associations between levels of herbivory, plant fitness, and contents of a number of leaf chemicals reflecting plant resistance to and quality for the herbivores. We found extensive spatial variation in the levels of herbivory and in plant fitness. More importantly, the impact of both leaf herbivory and seed predation on plant fitness varied among plant populations, indicating spatial variation in phenotypic selection. In addition, leaf chemistry varied widely among plant populations, reflecting spatial variation in plant quality as food for the herbivores. However, leaf compounds influenced folivory similarly in all the studied plant populations, and interestingly, some of the compounds were associated with the intensity of seed predation. Finally, some of the leaf compounds were associated with plant fitness, and the strength and direction of these associations varied among plant populations. The observed spatial variation in the strength of the interactions between V. hirundinaria and its specialist herbivores suggests a geographic selection mosaic. Because the occurrence and strength of spatial variation varied between the two specialist herbivores, our results highlight the importance of considering multiple enemies when trying to understand evolution of interactions between plants and their herbivores.     

Intraspecific Chromosome Number Variation: a Neglected Threat to the Conservation of Rare Plants

Abstract: The effectiveness of rare plant conservation will increase when life history, demographic, and genetic data are considered simultaneously. Inbreeding depression is a widely recognized genetic concern in rare plant conservation, and the mixing of genetically diverse populations in restoration efforts is a common remedy. Nevertheless, if populations with unrecognized intraspecific chromosome variation are crossed, progeny fitness losses will range from partial to complete sterility, and reintroductions and population augmentation of rare plants may fail. To assess the current state of cytological knowledge of threatened and endangered plants in the continental United States, we searched available resources for chromosome counts. We also reviewed recovery plans to discern whether recovery criteria potentially place listed species at risk by requiring reintroductions or population augmentation in the absence of cytological information. Over half the plants lacked a chromosome count, and when a taxon did have a count it generally originated from a sampling intensity too limited to detect intraspecific chromosome variation. Despite limited past cytological sampling, we found 11 plants with documented intraspecific cytological variation, while 8 others were ambiguous for intraspecific chromosome variation. Nevertheless, only one recovery plan addressed the chromosome differences. Inadequate within‐species cytological characterization, incomplete sampling among listed taxa, and the prevalence of interspecific and intraspecific chromosome variation in listed genera, suggests that other rare plants are likely to have intraspecific chromosome variation. Nearly 90% of all recovery plans called for reintroductions or population augmentation as part of recovery criteria despite the dearth of cytological knowledge. We recommend screening rare plants for intraspecific chromosome variation before reintroductions or population augmentation projects are undertaken to safeguard against inadvertent mixtures of incompatible cytotypes.     

Seed trait changes in dispersers' guts and consequences for germination and seedling growth

The effectiveness of a frugivore as a disperser of a plant is greatly determined by how fruits and seeds are handled in its mouth and its digestive tract. Although a number of studies have investigated the effect of avian ingestion on germination, we still know very little about the modifications to seeds during ingestion and the specific consequences on plant fitness. Here we investigate for the first time the different mechanisms by which germination patterns of seeds are modified following ingestion by frugivores. Specifically, we examine changes in seed mass, water content, permeability, seed coat thickness, texture, and resistance in two common Mediterranean fleshy-fruited plants, Phillyrea angustifolia and Myrtus communis, after ingestion by Eurasian Blackbirds, Turdus merula. We found a number of differences between the plant species: Phillyrea seeds lost mass, mainly due to water loss, and had thinner coats after gut passage, but Myrtus seeds did not. Seeds of both species showed increased permeability, while Myrtus seeds in particular became less resistant to breakage. No quantifiable changes in seed coat texture were detected in either species, although this trait was partly associated with differences in germination rate in Phillyrea. High intraspecific plant variation was found for most seed traits measured. Seed passage through birds' guts sped up germination in both species, especially in Myrtus. Increased permeability in seeds of both species following ingestion resulted in a higher germination rate. Moreover, seeds with thick coats (and in the case of Phillyrea, harder coats) germinated at a slower rate and produced seedlings that also grew more slowly, indicating a cost of coat thickness and/or hardness for seedling emergence. Results obtained here contribute to explaining the great heterogeneity in germination responses among and within plant species and the large variety of factors, both intrinsic and extrinsic to the plants, that influence such responses.     

Pre-dispersal seed predation: the role of fruit abortion and selective oviposition

Oviposition sites of phytophagous insects should correlate with plant traits that maximize survival of the progeny. Plants, on the other hand, should benefit from traits and developmental patterns that complicate oviposition decisions. In the antagonistic interaction between plant and pre-dispersal seed predator the time lag between egg laying and seed development may allow for abortion of fruits in plants, potentially reducing fitness loss through predation. We studied the perennial herb Lathvrus vernus and the beetle pre-dispersal seed predator Bruchus atomarius in Sweden to determine the fitness consequences of nonrandom fruit abortion in the plant and oviposition patterns of the beetle. The beetle had a sophisticated ability to locate fruits with high probability of retention, partly by fruit position and phenology but also by some additional unidentified cue. Mortality of eggs was density dependent, but still the egg-laying pattern was clumped. We found no defensive strategy in the plant; instead the predictable fruit abortion pattern was associated with decreased plant fitness. We discuss how interactions may pose simultaneous selection pressures on plant and insect traits and how life history traits and other selective forces may shape the adaptive outcome of the interaction in plant and insect, respectively.     

Stochastic population dynamics in populations of western terrestrial garter snakes with divergent life histories

Comparative evaluations of population dynamics in species with temporal and spatial variation in life-history traits are rare because they require long-term demographic time series from multiple populations. We present such an analysis using demographic data collected during the interval 1978-1996 for six populations of western terrestrial garter snakes (Thamnophis elegans) from two evolutionarily divergent ecotypes. Three replicate populations from a slow-living ecotype, found in mountain meadows of northeastern California, were characterized by individuals that develop slowly, mature late, reproduce infrequently with small reproductive effort, and live longer than individuals of three populations of a fast-living ecotype found at lakeshore locales. We constructed matrix population models for each of the populations based on 8-13 years of data per population and analyzed both deterministic dynamics based on mean annual vital rates and stochastic dynamics incorporating annual variation in vital rates. (1) Contributions of highly variable vital rates to fitness (lambda(s)) were buffered against the negative effects of stochastic variation, and this relationship was consistent with differences between the meadow (M-slow) and lakeshore (L-fast) ecotypes. (2) Annual variation in the proportion of gravid females had the greatest negative effect among all vital rates on lambda(s). The magnitude of variation in the proportion of gravid females and its effect on lambda(s) was greater in M-slow than L-fast populations. (3) Variation in the proportion of gravid females, in turn, depended on annual variation in prey availability, and its effect on lambda(s) was 4 23 times greater in M-slow than L-fast populations. In addition to differences in stochastic dynamics between ecotypes, we also found higher mean mortality rates across all age classes in the L-fast populations. Our results suggest that both deterministic and stochastic selective forces have affected the evolution of divergent life-history traits in the two ecotypes, which, in turn, affect population dynamics. M-slow populations have evolved life-history traits that buffer fitness against direct effects of variation in reproduction and that spread lifetime reproduction across a greater number of reproductive bouts. These results highlight the importance of long-term demographic and environmental monitoring and of incorporating temporal dynamics into empirical studies of life-history evolution.     

Challenges to Introducing and Managing Disturbance Regimes for Holocarpha macradenia, an Endangered Annual Grassland Forb

Abstract:  Introducing rare plants to new sites for conservation to offset effects of habitat destruction requires detailed knowledge of habitat requirements, plant demography, and management needs. We conducted a factorial experiment replicated at three coastal prairie sites to test the effects of clipping frequency and litter accumulation on seed germination, seedling survival, reproduction, and seedling recruitment of introduced populations of the endangered, tall-stature, annual forb, Holocarpha macradenia (DC.) E. Greene. Clipping favored H. macradenia , primarily by enhancing seed germination and flower production. Litter accumulation had no effect on seed germination, even after 5 years of treatments. Seedling recruitment was highly site specific with large numbers of recruits recorded at only one of three sites. Although recruitment of seedlings was higher in clipped plots for 2–3 years, by 4–5 years after introduction very few seedlings survived to reproduction in any treatment. We attribute this result to a combination of poor habitat quality, small population size, and lack of a seed bank. We were unsuccessful in introducing this relatively well-studied species of concern to apparently suitable habitat at multiple sites in multiple years, which suggests that translocating rare plant populations to mitigate for habitat destruction is an expensive and highly uncertain endeavor.     

Two sides of the same coin? Rare and pest plants native to the United States and Canada

Plant biodiversity is at risk, with as many as 10% of native species in the United States being threatened with extinction. Habitat loss has led a growing number of plant species to become rare or threatened, while the introduction or expansion of pest species has led some habitats to be dominated by relatively few, mostly nonindigenous, species. As humans continue to alter many landscapes and vegetation types, understanding how biological traits determine the location of species along a spectrum from vulnerability to pest status is critical to designing risk assessment protocols, setting conservation priorities, and developing monitoring programs. We used boosted regression trees to predict rarity (based on The Nature Conservancy global rankings) and pest status (defined as legal pest status) from data on traits for the native vascular flora of the United States and Canada including Hawaii, Puerto Rico, and the Virgin Islands (n approximately = 15,000). Categories were moderately to highly predictable (AUCpest = 0.87 on 25% holdout test set, AUCrarity = 0.80 on 25% holdout test set). Key predictors were chromosome number, ploidy, seed mass, and a suite of traits suggestive of specialist vs. generalist adaptations (e.g., facultative wetland habitat association and phenotypic variability in growth form and life history). Specifically, pests were associated with high chromosome numbers, polyploidy, and seed masses ranging from 0.1 to 100 mg, whereas rare species were associated with low chromosome numbers, low ploidy, and large (>1000 mg) seed masses. In addition, pest species were disproportionately likely to be facultatively associated with wetlands, and variable in growth form and life history, whereas rare species exhibited an opposite pattern. These results suggest that rare and pest species contrast along trait axes related to dispersal and performance in disturbed or novel habitats.