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The biotic resistance hypothesis predicts that diverse native communities are more resistant to invasion. However, past studies vary in their support for this hypothesis due to an apparent contradiction between experimental studies, which support biotic resistance, and observational studies, which find that native and non‐native species richness are positively related at broad scales (small‐scale studies are more variable). Here, we present a novel analysis of the biotic resistance hypothesis using 24 456 observations of plant richness spanning four community types and seven ecoregions of the United States. Non‐native plant occurrence was negatively related to native plant richness across all community types and ecoregions, although the strength of biotic resistance varied across different ecological, anthropogenic and climatic contexts. Our results strongly support the biotic resistance hypothesis, thus reconciling differences between experimental and observational studies and providing evidence for the shared benefits between invasive species management and native biodiversity conservation. [ABSTRACT FROM AUTHOR]

Aim To determine whether one of the most invasive pine species introduced to the Southern Hemisphere, Pinus contorta, has changed plant species richness, composition, diversity, and litter depth where it has invaded into native open forest, shrub steppe and grassland communities and to assess whether changes were similar in its native and introduced ranges. Location Río Negro Province, Argentina; Aysén and Araucanía Regions, Chile; Greater Yellowstone Ecosystem, USA. Methods We measured changes in plant species richness, species composition and cover, diversity, and litter depth associated with increasing P. contorta tree cover along the invasion front at three sites in the introduced range (Argentina and Chile) and one in the native range (Montana, USA). Results Plant species richness and cover generally declined with increasing P. contorta canopy cover, at similar rates in both the introduced and native ranges. However, plant cover was not affected by P. contorta in a forested setting in the introduced range. P. contorta invasion explained more of the decline in species richness in the introduced than native range. Native species composition changed more strongly across the invasion gradient in the introduced than native range. Litter depth increased more rapidly with P. contorta cover in the native than introduced range. Main conclusions Our results highlight the potential of pines to alter plant communities whether encroaching from forests in the native range or from plantations in the introduced range. Species richness and plant cover declined in both settings; however, individual species abundance and species composition were more impacted in the introduced range than in the native range. We suggest that invading trees have a greater capacity to cause ecological impacts in their introduced than in their native range, particularly where they represent a novel life-form. [ABSTRACT FROM AUTHOR]

Theory predicts a positive relationship between biodiversity and stability in ecosystem properties, while diversity is expected to have a negative impact on stability at the species level. We used virtual experiments based on a dynamic simulation model to test for the diversity-stability relationship and its underlying mechanisms in Central European forests. First our results show that variability in productivity between stands differing in species composition decreases as species richness and functional diversity increase. Second we show temporal stability increases with increasing diversity due to compensatory dynamics across species, supporting the biodiversity insurance hypothesis. We demonstrate that this pattern is mainly driven by the asynchrony of species responses to small disturbances rather than to environmental fluctuations, and is only weakly affected by the net biodiversity effect on productivity. Furthermore, our results suggest that compensatory dynamics between species may enhance ecosystem stability through an optimisation of canopy occupancy by coexisting species. [ABSTRACT FROM AUTHOR]

Aim Conifers comprise an ancient and diverse group of plants showing a wide distribution range. To better understand the general patterns of species successfully established on islands, this review compiles information about the distribution, diversity, dispersal potential and conservation status of insular conifers, with special emphasis on those inhabiting remote oceanic islands. Location Global. Methods An exhaustive survey was made of world-wide databases and literature. We registered information on island distribution (including ocean region, extension and geological origin of the island), endemism and threat status for each insular conifer. Results 285 of the 547 conifer species considered in this review show an insular distribution (i.e. their distribution encompass insular territories). The family Podocarpaceae is best represented, with 40% of the insular species. The importance of endozoochory for long-distance dispersal is clear, because it was the most frequent dispersal syndrome among oceanic conifers. A high proportion of the total threatened conifers occur on islands (52%), and many of them are insular endemics (72%). Among conifer families, Araucariaceae is the most threatened in insular territories. Main conclusions Our results highlight the wide diversity of insular conifers, as well as the key role of oceanic islands in catalysing speciation mechanisms. Pacific islands in particular harbour the greatest diversity levels, constituting a major centre of diversification. The wide distribution of conifers reflects their great potential for dispersal and colonization, endozoochory being the most favourable dispersal syndrome for reaching remote islands. The general threat status of insular conifers highlights the fragility of island biota and the urgent need for policies focused on their preservation. [ABSTRACT FROM AUTHOR]

Although nutrient enrichment frequently decreases biodiversity, it remains unclear whether such biodiversity losses are readily reversible, or are critical transitions between alternative low- and high-diversity stable states that could be difficult to reverse. Our 30-year grassland experiment shows that plant diversity decreased well below control levels after 10 years of chronic high rates (95-270 kg N ha−1 year−1) of nitrogen addition, and did not recover to control levels 20 years after nitrogen addition ceased. Furthermore, we found a hysteretic response of plant diversity to increases and subsequent decreases in soil nitrate concentrations. Our results suggest that chronic nutrient enrichment created an alternative low-diversity state that persisted despite decreases in soil nitrate after cessation of nitrogen addition, and despite supply of propagules from nearby high-diversity plots. Thus, the regime shifts between alternative stable states that have been reported for some nutrient-enriched aquatic ecosystems may also occur in grasslands. [ABSTRACT FROM AUTHOR]

Differences between native and exotic species in competitive ability and susceptibility to herbivores are hypothesized to facilitate coexistence. However, little fieldwork has been conducted to determine whether these differences are present in invaded communities. Here, we experimentally examined whether asymmetries exist between native and exotic plants in a community invaded for over 200 years and whether removing competitors or herbivores influences coexistence. We found that natives and exotics exhibit pronounced asymmetries, as exotics are competitively superior to natives, but are more significantly impacted by herbivores. We also found that herbivore removal mediated the outcome of competitive interactions and altered patterns of dominance across our field sites. Collectively, these findings suggest that asymmetric biotic interactions between native and exotic plants can help to facilitate coexistence in invaded communities. [ABSTRACT FROM AUTHOR]

The success of invasive plants has been attributed to their escape from natural enemies and subsequent evolutionary change in allocation from defence to growth and reproduction. In common garden experiments withSenecio jacobaea, a noxious invasive weed almost worldwide, the invasive populations from North America, Australia, and New Zealand did indeed allocate more resources to vegetative and reproductive biomass. However, invasive plants did not show a complete change in allocation from defence to growth and reproduction. Protection against generalist herbivores increased in invasive populations and pyrrolizidine alkaloids, their main anti-herbivore compounds, did not decline in invasive populations but were higher overall compared with native populations. In contrast, invasive plants lost additional protection against specialist herbivores adapted to pyrrolizidine alkaloids. Hence, the absence of specialist herbivores in invasive populations resulted in the evolution of lower protection against specialists and increased growth and reproduction, but also allowed a shift towards higher protection against generalist herbivores. [ABSTRACT FROM AUTHOR]

Complementarity and sampling effects may both contribute to increased invasion resistance at higher diversity. We measured plant invader biomass across a long-term experimental plant diversity gradient. Invader species’ biomass was inhibited in more diverse plots, largely because of the presence of strongly competitive C4 bunchgrasses, consistent with a sampling effect. Invader biomass was negatively correlated with resident root biomass, and positively correlated with soil nitrate concentrations, suggesting that competition for nitrogen limited invader success. Resident root biomass increased and soil nitrate concentrations decreased with the presence of C4 grasses and also across the diversity gradient, suggesting that diverse plots are more competitive because of the presence of C4 grasses. In addition to this evidence for a sampling effect, we also found evidence for a complementarity effect. Specifically, the percentage of plots that had lower invader biomass than did the best resident monoculture (i.e. that had invader‘underyielding’) increased across the species richness gradient. This pattern cannot be explained by a sampling effect and is a unique signature of complementarity effects. Our results demonstrate the importance of multiple mechanisms by which diversity can increase invasion resistance. [ABSTRACT FROM AUTHOR]

Much confusion exists in the English-language literature on plant invasions concerning the terms 'naturalized' and 'invasive' and their associated concepts. Several authors have used these terms in proposing schemes for conceptualizing the sequence of events from introduction to invasion, but often imprecisely, erroneously or in contradictory ways. This greatly complicates the formulation of robust generalizations in invasion ecology. Based on an extensive and critical survey of the literature we defined a minimum set of key terms related to a graphic scheme which conceptualizes the naturalization/invasion process. Introduction means that the plant (or its propagule) has been transported by humans across a major geographical barrier. Naturalization starts when abiotic and biotic barriers to survival are surmounted and when various barriers to regular reproduction are overcome. Invasion further requires that introduced plants produce reproductive offspring in areas distant from sites of introduction (approximate scales: > 100 m over < 50 years for taxa spreading by seeds and other propagules; > 6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems). Taxa that can cope with the abiotic environment and biota in the general area may invade disturbed, seminatural communities. Invasion of successionally mature, undisturbed communities usually requires that the alien taxon overcomes a different category of barriers. We propose that the term 'invasive' should be used without any inference to environmental or economic impact. Terms like 'pests' and 'weeds' are suitable labels for the 50-80% of invaders that have harmful effects. About 10% of invasive plants that change the character, condition, form, or nature of ecosystems over substantial areas may be termed 'transformers'. [ABSTRACT FROM AUTHOR]