Trait convergence and plasticity among native and invasive species in resource-poor environments

? Premise of study: Functional trait comparisons provide a framework with which to assess invasion and invasion resistance. However, recent studies have found evidence for both trait convergence and divergence among coexisting dominant native and invasive species. Few studies have assessed how multiple stresses constrain trait values and plasticity, and no study has included direct measurements of nutrient conservation traits, which are critical to plants growing in low-resource environments. ? Methods: We evaluated how nutrient and water stresses affect growth and allocation, water potential and gas exchange, and nitrogen (N) allocation and use traits among a suite of six codominant species from the Intermountain West to determine trait values and plasticity. In the greenhouse, we grew our species under a full factorial combination of high and low N and water availability. We measured relative growth rate (RGR) and its components, total biomass, biomass allocation, midday water potential, photosynthetic rate, water-use efficiency (WUE), green leaf N, senesced leaf N, total N pools, N productivity, and photosynthetic N use efficiency. ? Key results: Overall, soil water availability constrained plant responses to N availability and was the major driver of plant trait variation in our analysis. Drought decreased plant biomass and RGR, limited N conservation, and led to increased WUE. For most traits, native and nonnative species were similarly plastic. ? Conclusions: Our data suggest native and invasive biomass dominants may converge on functionally similar traits and demonstrate comparable ability to respond to changes in resource availability.     

Image spectroscopy and stable isotopes elucidate functional dissimilarity between native and nonnative plant species in the aquatic environment

? Nonnative species may change ecosystem functionality at the expense of native species. Here, we examine the similarity of functional traits of native and nonnative submersed aquatic plants (SAP) in an aquatic ecosystem. ? We used field and airborne imaging spectroscopy and isotope ratios of SAP species in the Sacramento-San Joaquin Delta, California (USA) to assess species identification, chlorophyll (Chl) concentration, and differences in photosynthetic efficiency. ? Spectral separability between species occurs primarily in the visible and near-infrared spectral regions, which is associated with morphological and physiological differences. Nonnatives had significantly higher Chl, carotene, and anthocyanin concentrations than natives and had significantly higher photochemical reflectance index (PRI) and δ(13) C values. ? Results show nonnative SAPs are functionally dissimilar to native SAPs, having wider leaf blades and greater leaf area, dense and evenly distributed vertical canopies, and higher pigment concentrations. Results suggest that nonnatives also use a facultative C(4) -like photosynthetic pathway, allowing efficient photosynthesis in high-light and low-light environments. Differences in plant functionality indicate that nonnative SAPs have a competitive advantage over native SAPs as a result of growth form and greater light-use efficiency that promotes growth under different light conditions, traits affecting system-wide species distributions and community composition.     

Increase in nonnative understorey vegetation cover after nonnative conifer removal and passive restoration

Nonnative conifers are widespread in the southern hemisphere, where their use as plantation species has led to adverse ecosystem impacts sometimes intensified by invasion. Mechanical removal is a common strategy used to reduce or eliminate the negative impacts of nonnative conifers, and encourage native regeneration. However, a variety of factors may preclude active ecological restoration following removal. As a result, passive restoration – unassisted natural vegetation regeneration – is common following conifer removal. We asked, ‘what is the response of understorey cover to removal of nonnative conifer stands followed by passive restoration?' We sampled understorey cover in three site types: two‐ to ten‐year‐old clearcuts, native forest and current plantations. We then grouped understorey species by origin (native/nonnative) and growth form, and compared proportion and per cent cover of these groups as well as of bare ground and litter between the three site types. For clearcuts, we also analysed the effect of time since clearcut on the studied variables. We found that clearcuts had a significantly higher average proportion of nonnative understorey vegetation cover than native forest sites, where nonnative vegetation was nearly absent. The understorey of clearcut sites also averaged more overall vegetation cover and more nonnative vegetation cover (in particular nonnative shrubs and herbaceous species) than either plantation or native forest sites. Notably, 99% of nonnative shrub cover in clearcuts was the invasive nonnative species Scotch broom (Cytisus scoparius). After ten years of passive recovery since clearcutting, the proportion of understorey vegetation cover that is native has not increased and remains far below the proportion observed in native forest sites. Reduced natural regeneration capacity of the native ecosystem, presence of invasive species in the surrounding landscape and land‐use legacies from plantation forestry may inhibit native vegetation recovery and benefit opportunistic invasives, limiting the effectiveness of passive restoration in this context. Abstract in Spanish is available with online material.     

Effects of plant litter diversity,species, origin and traits on larval toad performance

Decreased plant diversity is expected to reduce ecosystem function. Although many studies have examined effects of plant species on trophic interactions, information regarding effects of native or non‐native plant diversity on performance of individuals of higher trophic levels is limited. We reared larval American toad Anaxyrus americanus tadpoles in outdoor mesocosms containing litter of 1, 3, 6 or 12 plant species drawn randomly from a pool of 24 (15 native, 9 nonnative) species. Tadpole performance varied significantly among litter types in single litter treatments and pH and litter C:N were significant predictors of tadpole performance. Metamorphs were larger in mixtures than expected based on performance in single species treatments, suggesting a non‐additive effect of diversity. Litter diversity did not affect probability of survival or probability of metamorphosis. Plant origin (native or non‐native) had no significant effect on amphibian performance. Our study suggests some benefits to tadpole development at low levels of plant diversity, but questions assumed benefits of increased plant diversity and assumed detrimental effects of nonnative plant species for a common larval amphibian. Presence of specific plant species with strong negative effects on tadpole performance may outweigh diversity benefits in brown food webs.     

Native or nonnative host plants: What is better for a specialist moth?

The enemy release hypothesis (ERH) predicts that the lack of natural enemies, such as herbivores, contributes to the success of nonnative plants as colonizers. Larvae of the Neotropical specialist moth Utetheisa ornatrix (Erebidae: Arctiinae) can feed on unripe seeds and leaves of both native and nonnative Crotalaria species (Fabaceae). Despite some species being able to eat nonnative plants, such behavior can impair the herbivore, as they are not adapted to the alien plant, and still contribute to the success of the nonnative species via enemy release. We tested the performance of the moth from hatching to adulthood fed on two native (C. micans and C. paulina) and two nonnative (C. pallida, C. juncea) host plants. Utetheisa ornatrix performed better (lower development time, heavier pupae and more eggs) on the native host plants than in the nonnative. However, larva performance in nonnative C. pallida was similar to that in the native host plants. Using the larval weight 7 days after hatching from the eggs as a proxy for performance in twelve Crotalaria species (five Neotropical natives, four nonnatives from Afrotropical region, and three nonnatives from India), we found similar results. Crotalaria nutritional compounds, the defensive pyrrolizidine alkaloids and Crotalaria phylogeny did not explain moth performance. Our results give some support to the ERH. The good moth performance in nonnative C. pallida may be related to its high availability as host plant for U. ornatrix, and its longer time since their introduction in Neotropics which would provide opportunity for the moth to adapt.     

Plant invaders outperform congeneric natives on amino acids

As a key nitrogen (N) source, soil amino acids play an important role in plant N nutrition. However, how amino acids differentially influence the N use strategies of native and invasive plants remains unclear. We performed a potted experiment using five pairs of native and invasive plant congeners, which were subject to 23 N treatments (i.e., 20 protein primary amino acids, nitrate, ammonium, and control), each with 10 replicates. We determined their growth, biomass allocation, N use efficiency, and the growth advantage of plant invaders over their natives. Native and invasive plants used the same 18 amino acid N sources (i.e., a similar amino acid economics spectrum). The growth of plant invaders was invariably better than the growth of native plants, and this superior growth of invaders was linked to their higher root biomass allocation and greater N use efficiency. Additionally, invasive plants had a greater growth advantage on amino acid N than on inorganic N, so was this advantage greater on neutral amino acids than on acidic amino acids. These findings suggest that the differences in amino acid use strategies between invasive and native congeners could help to explain plant invasiveness, as indicated by a growth advantage.     

High N,dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought

Hotter, longer, and more frequent global change‐type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought‐induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean‐type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water‐use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation‐type conversion.     

Breeding system of the critically endangered Lakela’s Mint and influence of plant height on pollinators and seed output

Understanding reproductive systems of rare plants is critical for conservation efforts. Lakela's Mint, Dicerandra immaculata Lakela var. immaculata, is an endangered plant endemic to an approximately 4.8-km long area in Florida, USA. We used an experimental garden and three populations of Lakela's Mint to determine: (1) what is the breeding system (autonomous, asexual, self-fertile, cross-fertile) and are insects necessary for reproduction; (2) which native and nonnative insect species visit flowers and is the frequency of visits to a plant influenced by its height; (3) does the number of flowers visited within a plant by individual insects differ among native and nonnative insect species and due to plant height; and (4) is seed output influenced by plant height? Our results indicate that the breeding system of Lakela's Mint was facultative outcrossing. Insect-pollinated flowers produced more seeds than flowers that reproduced autonomously or asexually. The honey bee Apis mellifera L., a nonnative species, was the most frequent visitor to plants and visited more flowers within plants than native pollinators, but its behavior was not influenced by plant height. Native pollinators such as Bombus impatiens Cresson were attracted more frequently to shorter plants, but visited fewer flowers than on taller plants. Despite having fewer total and pollinated flowers, shorter plants had a higher output of intact seeds than taller plants, which could be due to differences in efficiency between native and nonnative pollinators or other factors. Our results add insight into factors influencing seed output and interactions between pollinators and rare plants.     

Historic land use influences contemporary establishment of invasive plant species

The legacy of agricultural land use can have widespread and persistent effects on contemporary landscapes. Although agriculture can lead to persistent changes in soil characteristics and plant communities, it remains unclear whether historic agricultural land use can alter the likelihood of contemporary biological invasions. To understand how agricultural land-use history might interact with well-known drivers of invasion, we conducted factorial manipulations of soil disturbance and resource additions within non-agricultural remnant sites and post-agricultural sites invaded by two non-native Lespedeza species. Our results reveal that variation in invader success can depend on the interplay of historic land use and contemporary processes: for both Lespedeza species, establishment was greater in remnant sites, but soil disturbance enhanced establishment irrespective of land-use history, demonstrating that contemporary processes can help to overcome legacy constraints on invader success. In contrast, additions of resources known to facilitate seedling recruitment (N and water) reduced invader establishment in post-agricultural but not in remnant sites, providing evidence that interactions between historic and contemporary processes can also limit invader success. Our findings thus illustrate that a consideration of historic land use may help to clarify the often contingent responses of invasive plants to known determinants of invasibility. Moreover, in finding significantly greater soil compaction at post-agricultural sites, our study provides a putative mechanism for historic land-use effects on contemporary invasive plant establishment. Our work suggests that an understanding of invasion dynamics requires knowledge of anthropogenic events that often occur decades before the introduction of invasive propagules.     

Differential modulation of host plant delta13C and delta18O by native and nonnative arbuscular mycorrhizal fungi in a semiarid environment

Native, drought-adapted arbuscular mycorrhizal fungi (AMF) often improve host-plant performance to a greater extent than nonnative AMF in dry environments. However, little is known about the physiological basis for this differential plant response. Seedlings of Olea europaea and Rhamnus lycioides were inoculated with either a mixture of eight native Glomus species or with the nonnative Glomus claroideum before field transplanting in a semiarid area. Inoculation with native AMF produced the greatest improvement in nutrient and water status as well as in long-term growth for both Olea and Rhamnus. Foliar delta18O measurements indicated that native AMF enhanced stomatal conductance to a greater extent than nonnative AMF in Olea and Rhamnus.delta13C data showed that intrinsic water-use efficiency in Olea was differentially stimulated by native AMF compared with nonnative AMF. Our results suggest that modulation of leaf gas exchange by native, drought-adapted AMF is critical to the long-term performance of host plants in semiarid environments. delta18O can provide a time-integrated measure of the effect of mycorrhizal infection on host-plant water relations.     

Importance of plant traits and herbivory for invasiveness of Phragmites australis (Poaceae)

Biological invasions change native plant communities, but theory predicting whether introductions create naturalized or invasive species is lacking. Focusing on either plant traits or interactions of introduced plants with native biota creates unreliable results, and improvements may require integration of trait- and interaction-based approaches. To assess the importance of plant traits and herbivory on invasiveness, we incorporated herbivore effects in comparisons of growth and phenology of invasive Phragmites australis and its native congener P. australis subsp. americanus. Our results were influenced by venue (field or common garden), with extended life span and optimized leaf-age structure of introduced P. australis indicating greater potential for resource capture. Attack by introduced gallflies affected expression of plant traits, but we found no consistent effect of aphid attack. Origin did not affect leaf emergence or stem height, but preferential gallfly attack stunted native P. australis and delayed senescence. Greater resource capture and lower attack by nonnative herbivores could give introduced P. australis an advantage over the native subspecies. Our results demonstrating the importance of plant traits as well as their modification by interactions with natural enemies questions whether the outcome of plant introductions can be predicted.     

Dynamics of energy and nutrient concentration and construction cost in a native and two alien C4 grasses from two neotropical savannas

In Venezuela, the alien grasses Melinis minutiflora Beauv. and Hyparrhenia rufa (Nees.) Stapf tend to displace the native savanna plant community dominated by Trachypogon plumosus (Humb. and Bonpl.) Nees. This occurs in either relatively wetter and fertile highland savannas or in drier and less fertile lowland savannas. Although the native and aliens are perennial C₄ grasses, higher net assimilation leaf biomass per plant and germination rate of the latter are some causes for their higher growth rates and for their competitive success. The objective of this study is to compare seasonal tissue energy, N, P and K concentrations and the calculated construction costs (CC) between the native grass and either one of the alien grasses from lowland and highland savannas. We predict that, in order to out-compete native plants, alien grasses should be more efficient in resource use as evidenced by lower tissue energy and nutrient concentrations and CC.Tissue energy and nutrient concentration were measured throughout the year and compared between M. minutiflora and the co-occurring local population of T. plumosus in a highland savanna and between H. rufa and its neighbor local population of T. plumosus in a lowland savanna. CC was calculated from energy, N and ash concentrations considering ammonium as the sole N source. Differences between co-occurring species, T. plumosus populations, seasons, and organs were analyzed with ANOVA.Highland and lowland grasses differed in concentration and allocation of energy and nutrients whereas the differences between alien and native grasses were specific for each pair considered. Highland grasses had higher energy, N, P and CC than lowland grasses. These variables were always lowest in the culms. In the more stressed lowland site, tissue energy and nutrient concentrations decreased significantly during the dry season except in the roots of both grasses which had the highest energy and nutrients concentrations during the drought. This seasonal response was more marked in the local lowland population of T. plumosus in which maximum CC alternated seasonally between leaves and roots. Energy and nutrient concentrations and CC were the lowest in H. rufa. In the lowland savannas, the higher efficiency of resource use in the invader grass contributes to its higher competitive success through increased growth rate. In the highlands, overall tissue energy concentration and CC, but not N nor P concentration, were lower in the fast growing M. minutiflora but seasonal differences were lacking. The higher leaf CC in T. plumosus can be attributed to the higher proportion of sclerenchyma tissue which is more expensive to construct. Considering CC, both fast growing alien grasses are more efficient in resource use than the co-occurring native grass. However, the role of CC explaining the competitive success of the former, through higher growth rates, is more evident in the more stressful environment of the lowland savanna.     

Traits of an invasive grass conferring an early growth advantage over native grasses

Aims Invasive species often have higher relative growth rates (RGR) than their native counterparts. Nutrient use efficiency, total leaf area and specific leaf area (SLA) are traits that may confer RGR differences between natives and invasives, but trait differences are less prominent when the invasive species belongs to the same plant functional type as the dominant native species. Here, we test if traits displayed soon after germination confer an early size advantage. Specifically, we predicted that invasive species seedlings grow faster than the natives because they lack trade-offs that more strongly constrain the growth of native species.Methods We quantified plant morphological and physiological traits and RGR during early seedling growth at high and low nutrient levels in three dominant perennial native C₄ grasses: Panicum virgatum L. (switchgrass), Schizachyrium scoparium (Michx.) Nash (little bluestem) and Andropogon gerardii Vitman (big bluestem); and a perennial C₄ exotic invasive grass, Sorghum halepense (L.) Pers. (Johnsongrass).Important findings After 2 weeks of growth, Johnsongrass seedlings had greater biomass, SLA and photosynthetic nitrogen use efficiency, but lower leaf N concentrations (% leaf N) and root:shoot ratio than natives. As growth continued, Johnsongrass more quickly produced larger and thicker leaves than the natives, which dampened the growth advantage past the first 2 to 3 weeks of growth. Investment in carbon gain appears to be the best explanation for the early growth advantage of Johnsongrass. In natives, growth was constrained by an apparent trade-off between allocation to root biomass, which reduced SLA, and production of leaves with high N content, which increased carbon gain. In Johnsongrass, root:shoot ratio did not interact with other traits, and % leaf N was decoupled from RGR as a result of a trade-off between the positive indirect association of % leaf N with RGR and the negative direct association of % leaf N with RGR.     

Invasive plants in Minnesota are “joining the locals”: A trait‐based analysis

Questions

Predicting which newly arrived species will establish and become invasive is a problem that has long vexed researchers. In a study of cold temperate oak forest stands, we examined two contrasting hypotheses regarding plant functional traits to explain the success of certain non‐native species. Under the “join the locals” hypothesis, successful invaders are expected to share traits with resident species because they employ successful growth strategies under light‐limited understorey conditions. Instead, under the “try harder” hypothesis, successful invaders are expected to have traits different from native species in order to take advantage of unused niche space.

Location

Minnesota, USA.

Methods

We examined these two theories using 109 native and 11 non‐native plants in 68 oak forest stands. We focused on traits related to plant establishment and growth, including specific leaf area (SLA), leaf carbon‐to‐nitrogen ratio (C:N), wood density, plant maximum height, mycorrhizal type, seed mass and growth form. We compared traits of native and non‐native species using ordinations in multidimensional trait space and compared community‐weighted mean (CWM) trait values across sites.

Results

We found few differences between trait spaces occupied by native and non‐native species. Non‐native species occupied smaller areas of trait space than natives, yet were within that of the native species, indicating similar growth strategies. We observed a higher proportion of non‐native species in sites with higher native woody species CWM SLA and lower CWM C:N. Higher woody CWM SLA was observed in sites with higher soil pH, while lower CWM C:N was found in sites with higher light levels.

Conclusions

Non‐native plants in this system have functional traits similar to natives and are therefore “joining the locals.” However, non‐native plants may possess traits toward the acquisitive end of the native plant trait range, as evidenced by higher non‐native plant abundance in high‐resource environments.

     

Vegetation Recovery 16 Years after Feral Pig Removal from a Wet Hawaiian Forest

Nonnative ungulates can alter the structure and function of forest ecosystems. Feral pigs in particular pose a substantial threat to native plant communities throughout their global range. Hawaiian forests are exceptionally vulnerable to feral pig activity because native vegetation evolved in the absence of large mammalian herbivores. A common approach for conserving and restoring forests in Hawaii is fencing and removal of feral pigs. The extent of native plant community recovery and nonnative plant invasion following pig removal, however, is largely unknown. Our objective was to quantify changes in native and nonnative understory vegetation over a 16 yr period in adjacent fenced (pig‐free) vs. unfenced (pig‐present) Hawaiian montane wet forest. Native and nonnative understory vegetation responded strongly to feral pig removal. Density of native woody plants rooted in mineral soil increased sixfold in pig‐free sites over 16 yr, whereas establishment was almost exclusively restricted to epiphytes in pig‐present sites. Stem density of young tree ferns increased significantly (51.2%) in pig‐free, but not pig‐present sites. Herbaceous cover decreased over time in pig‐present sites (67.9%). In both treatments, number of species remained constant and native woody plant establishment was limited to commonly occurring species. The nonnative invasive shrub, Psidium cattleianum, responded positively to release from pig disturbance with a fivefold increase in density in pig‐free sites. These results suggest that while common native understory plants recover within 16 yr of pig removal, control of nonnative plants and outplanting of rarer native species are necessary components of sustainable conservation and restoration efforts in these forests.     

Vegetation removal and seed addition contribute to coastal sandplain grassland establishment on former agricultural fields

Creating native‐species‐rich grasslands to replace agricultural grasslands can be an important strategy for supplementing the area of grasslands, which are in decline in many regions. In the northeastern United States, sandplain grasslands support a diverse plant community and rare plant and animal species that are declining because of reductions in historical disturbances such as fire and grazing. We designed an experiment on Martha's Vineyard, Massachusetts, to test methods of establishing native‐species‐rich coastal sandplain grassland on former agricultural land. We tested the efficacy of: (1) tilling, herbicide, hot foam, and plastic cover in removing initial nonnative vegetation, and (2) combinations of tilling and seeding for establishing native species. We measured native and nonnative species richness and percent cover before and for 5 years after treatment. Herbicide, plastic cover, and spring, summer, and fall tilling were about equally effective in reducing nonnative species cover and promoting native species cover. Tilling and seeding each increased native species richness and percent cover, and seeding and tilling together increased native species richness and cover more than either treatment alone. Combined seeding and disturbance also reduced the cover of nonnative species, but nonnative species cover remained higher than in adjacent reference sandplain grassland. Results indicated that native species establishment was enhanced by the availability of seeds and by reduction of initial nonnative plant cover. The most efficient method of converting coastal agricultural grasslands to sandplain grassland with a higher number and proportion of native species is a single season of plant removal and seeding.     

Evaluation of strategies to conserve and restore intraspecific biodiversity of brown trout: outcomes from genetic monitoring in the French Alps

Introduction of nonnative cultured fish is one of the most important threats to native salmonid populations. In brown trout, more than a century of stocking practices has led to a large hybridization between initially geographically isolated lineages, threatening native populations and thereby intraspecific diversity. In the French region of Haute-Savoie, managers and scientists implemented together three management strategies (genetic refuge, direct translocation of wild spawners and stocking with native fry) on 19 test sites for more than 15 years, in the aim to recover pure or nearly pure native populations. Here we propose an assessment of the different management strategies based on a synthetic analysis of the evolution of the introgression rate. While none of the implemented strategies completely achieves the initial objective to restore pure native populations, they differ in their efficiency: introgression rates tend to decrease quickly when direct translocation of native spawners of stocking with native fry strategies are used. The genetic refuge strategy shows slower and more heterogeneous changes of introgression rates. In general, pure nonnative fish are efficiently removed but at the cost of an increased presence of hybrids. Our results imply that intraspecific dynamics react quickly to management practices and that these changes are probably fueled by evolutionary feedbacks that are not yet well understood.     

Effects of nonnative invertebrates on two life stages of the native eastern oyster <Emphasis Type="Italic">Crassostrea virginica</Emphasis>

Since their recent introductions into Florida waters, three sessile invertebrates [Perna viridis (Asian green mussel), Mytella charruana (charru mussel) and Megabalanus coccopoma (pink titan acorn barnacle)] have expanded their range along the Atlantic coast in estuarine waters. Little research has been done to understand how these nonnative species interact with the ecologically and economically important eastern oyster Crassostrea virginica. To assess the potential effects of P. viridis, M. charruana and M. coccopoma on C. virginica, the following questions were addressed in manipulative experiments. (1) Does the presence of nonnative species decrease oyster larval settlement? (2) Do oyster larvae avoid settling on nonnative species? (3) Do nonnative species decrease survival of juvenile oysters (spat)? (4) Do nonnative species hinder spat growth? We included two controls: absence of nonnative species and presence of the native mussel Geukensia demissa. The nonnative species influenced settlement, growth and survival of C. virginica in different ways. M. coccopoma and P. viridis negatively influenced larval settlement, whereas M. charruana had no influence on the total number of settled larvae. Oyster larvae avoided settling on all three nonnative species and the native G. demissa. Both nonnative mussels negatively affected survival of juvenile oysters but only M. charruana also reduced spat growth. The native mussel, G. demissa, had no negative impacts on total settlement, survival and growth of C. virginica; in fact, it increased larval settlement in some trials. These three nonnative species should be classified as invasive because all had negative effects on native C. virginica.     

No evidence of resource limitation to aboveground growth of blue grama (<Emphasis Type="Italic">Bouteloua gracilis</Emphasis>) on 1 ky-old semi-arid substrate

Biogeochemical theory and a substantial body of empirical data show that nitrogen (N), an atmospherically derived nutrient, limits plant growth on young substrates, while phosphorus (P), a rock-derived nutrient, limits plant growth on old substrates. In arid regions, water is also often a limiting resource to plant growth. We applied resource amendments of N, P, N + P, and water to blue grama (Bouteloua gracilis) growing on a 1 ky-old basaltic cinder substrate to test the hypothesis that N and water limit aboveground net primary production (ANPP) in a semi-arid climate, early in soil development. Contrary to our hypothesis, ANPP did not differ among treatments, suggesting that none of the resource amendments were limiting to blue grama growth. Unamended aboveground tissue N and P concentrations were three to five times lower at the 1 ky-old site than on older (55–3000 ky-old) substrates, suggesting differences in nutrient use efficiency across the substrate age gradient.     

Island biogeography extends to small-scale habitats: low competitor density and richness on islands may drive trait variation in nonnative plants

Previous island biogeography studies have quantified species richness on the scale of entire islands rather than smaller scales relevant to plant-to-plant competitive interactions. Further, they have not accounted for density compensation. Using mainland and island sites along the New England coast, we asked two questions. First, are both richness and density lower in small-scale habitats within islands than in similar mainland habitats? Second, do differences in competitor richness and density drive post-establishment trait variation in nonnative plant species? We used field surveys and individual-based rarefaction to estimate richness and density in 100-m² plots and demonstrated that island sites have significantly fewer species and individuals per unit area than mainland sites. We then conducted a field study in which we removed competing neighbors from nonnative plant individuals and found that when competitors were removed, individuals in low-competition environments demonstrated a lesser increase in vegetative growth but a greater increase in reproductive effort and herbivore tolerance relative to mainland individuals whose neighbors were also removed. We found that the central concept of island biogeography, i.e., that islands host fewer species than comparable mainland habitats, can be extended to smaller-scale habitats and that this difference in competitive pressure between mainland and island habitats can act as a driver of trait variation in nonnative plants.