Productivity gradients cause positive diversity–invasibility relationships in microbial communities

Models predict that community invasibility generally declines with species diversity, a prediction confirmed by small‐scale experiments. Large‐scale observations and experiments, however, find that diverse communities tend to be more heavily invaded than simple communities. One hypothesis states that large‐scale environmental heterogeneity, which similarly influences native and invasive species, can cause a positive correlation between diversity and invasibility, overriding the local negative effects of diversity on invasibility. We tested this hypothesis using aquatic microbial communities consisting of protists and rotifers consuming bacteria and nanoflagellates. We constructed a productivity gradient to simulate large‐scale environmental heterogeneity, started communities with the same number of species along this gradient, and subjected equilibrial communities to invasion by non‐resident consumer species. Both invaders and most resident species increased their abundances with resource enrichment, resulting in a positive correlation between diversity and invasibility. Intraspecific interference competition within resident species and the positive effect of enrichment on the number of available resources probably accounted for the higher invasibility with enrichment. Our results provide direct experimental evidence that environmental heterogeneity in productivity can cause a positive diversity–invasibility relationship.     

Short-term invasibility patterns in burnt and unburnt experimental Mediterranean grassland communities of varying diversities

This paper reports the findings of a short-term natural invasibility field study in constructed Mediterranean herbaceous communities of varying diversities, under a fire treatment. Three components of invasibility, i.e. species richness, density and biomass of invaders, have been monitored in burnt and unburnt experimental plots with resident diversity ranging from monocultures to 18-species mixtures. In general, species richness, density and biomass of invaders decreased significantly with the increase of resident species richness. Furthermore, the density and biomass of invading species were significantly influenced by the species composition of resident communities. Although aboveground biomass, leaf area index, canopy height and percent bare ground of the resident communities explained a significant part of the variation in the success of invading species, these covariates did not fully explain the effects of resident species richness. Fire mainly influenced invasibility via soil nutrient levels. The effect of fire on observed invasibility patterns seems to be less important than the effects of resident species richness. Our results demonstrate the importance of species richness and composition in controlling the initial stages of plant invasions in Mediterranean grasslands but that there was a lack of interaction with the effects of fire disturbance.     

Components of short-term invasibility in experimental Mediterranean grasslands

The paper reports findings of a natural invasibility study in constructed Mediterranean grasslands, located in the Greek site of the BIODEPTH project. Three different components of invasibility, i.e. species richness, abundance and rarity of invaders, have been monitored in experimental plots where the resident diversity ranged from monocultures to 18-species mixtures. Invaders' richness and abundance were negatively correlated to the resident diversity, while the occurrence of rare invaders was strongly positively correlated to the resident diversity. Standing biomass explained also a significant part of the variation of communities' invasibility. Soil nutrients and structural traits of the constructed vegetation such as the space-filling showed no influence in the definition of the overall pattern of invasibility in these communities. A verbal model of selection of invaders at the neighbourhood scale is proposed to explain these diverging trends of invasibility.     

Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands

Aims Although biological invasions occur throughout the world, and some invaders are widespread in many habitats, few studies on the ecological impact of invaders have examined multiple sites. We tested how the impact of three widespread plant invaders changed depending on the identity of the species and the invaded island. We also tested whether relative species loss was lower in species‐rich communities than in species‐poor ones. Location We conducted floristic surveys and soil analyses in eight Mediterranean Basin islands: Crete and Lesbos (Greece), Sardinia (Italy), Corsica, Bagaud and Porquerolles (France), and Mallorca and Menorca (Spain). Methods We compared native species richness and diversity, proportion of life forms, soil percentage nitrogen, percentage organic carbon, C/N, and soil pH in nearby paired plots of 2 × 2 m: one control and one invaded by either the deciduous tree Ailanthus altissima, the succulent subshrubs Carpobrotus spp. or the annual geophyte Oxalis pes‐caprae, across eight Mediterranean Basin islands. Results On average, the presence of invaders reduced species diversity, Carpobrotus spp. exhibiting the largest impact and Oxalis the least. However, the relative impact was island‐dependent, and was positively but weakly associated with the species richness of the recipient community. Therophytes were the life form that experienced the largest decrease across islands. The effects of invasion on soil properties were very variable. Total N changed (increased) only in plots invaded by Ailanthus, significantly decreasing the C/N ratio. The presence of this tree increased soil pH, whereas the opposite was found in plots invaded by the other two species. Organic C increased in plots invaded by Ailanthus and Carpobrotus species. Main conclusions By conducting an analysis at multiple sites, we found that the three plant invaders had an impact on plant community structure not entirely concordant with changes in soil properties. The impacts depended on the identity of the species and of the invaded island, suggesting that impact of invaders is context‐specific. The impact in terms of species loss was not lower in species‐rich than in species‐poor communities.     

What drives invasibility? A multi‐model inference test and spatial modelling of alien plant species richness patterns in northern Portugal

Understanding and predicting biological invasions can focus either on traits that favour species invasiveness or on features of the receiving communities, habitats or landscapes that promote their invasibility. Here, we address invasibility at the regional scale, testing whether some habitats and landscapes are more invasible than others by fitting models that relate alien plant species richness to various environmental predictors. We use a multi‐model information‐theoretic approach to assess invasibility by modelling spatial and ecological patterns of alien invasion in landscape mosaics, and by testing competing hypotheses of environmental factors that may control invasibility. Because invasibility may be mediated by particular characteristics of invasiveness, we classified alien species according to their C‐S‐R plant strategies. We illustrate this approach with a set of 86 alien species in northern Portugal. We first focus on predictors influencing species richness and expressing invasibility, and then evaluate whether distinct plant strategies respond to the same or different groups of environmental predictors. We confirmed climate as a primary determinant of alien invasions, and as a primary environmental gradient determining landscape invasibility. The effects of secondary gradients were detected only when the area was sub‐sampled according to predictions based on the primary gradient. Then, multiple predictor types influenced patterns of alien species richness, with some types (landscape composition, topography and fire regime) prevailing over others. Alien species richness responded most strongly to extreme land management regimes, suggesting that intermediate disturbance induces biotic resistance by favouring native species richness. Land‐use intensification facilitated alien invasion, whereas conservation areas hosted few invaders, highlighting the importance of ecosystem stability in preventing invasions. Plants with different strategies exhibited different responses to environmental gradients, particularly when the variations of the primary gradient were narrowed by sub‐sampling. Such differential responses of plant strategies suggest using distinct control and eradication approaches for different areas and alien plant groups.     

Landscape‐scale determinants of non‐native fish communities

Aim Predicting and preventing invasions depends on knowledge of the factors that make ecosystems susceptible to invasion. Current studies generally rely on non‐native species richness (NNSR) as the sole measure of ecosystem invasibility; however, species identity is a critical consideration, given that different ecosystems may have environmental characteristics suitable to different species. Our aim was to examine whether non‐native freshwater fish community composition was related to ecosystem characteristics at the landscape scale. Location United States. Methods We described spatial patterns in non‐native freshwater fish communities among watersheds in the Mid‐Atlantic region of the United States based on records of establishment in the U.S. Geological Survey’s Nonindigenous Aquatic Species Database. We described general relationships between non‐native species and ecosystem characteristics using canonical correspondence analysis. We clustered watersheds by non‐native fish community and described differences among clusters using indicator species analysis. We then assessed whether non‐native communities could be predicted from ecosystem characteristics using random forest analysis and predicted non‐native communities for uninvaded watersheds. We estimated which ecosystem characteristics were most important for predicting non‐native communities using conditional inference trees. Results We identified four non‐native fish communities, each with distinct indicator species. Non‐native communities were predicted based on ecosystem characteristics with an accuracy of 80.6%, with temperature as the most important variable. Relatively uninvaded watersheds were predicted to be invasible by the most diverse non‐native community. Main conclusions Non‐native species identity is an important consideration when assessing ecosystem invasibility. NNSR alone is an insufficient measure of invasibility because ecosystems with equal NNSR may not be equally invasible by the same species. Our findings can help improve predictions of future invasions and focus management and policy decisions on particular species in highly invasible ecosystems.     

Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities

Biological invasions are a major threat to natural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a community to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the characteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbivore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of herbivory (e.g., productivity or edibility of primary producers) indirectly influence invasions.     

Giant invasive Heracleum persicum: Friend or foe of plant diversity?

The impact of invasion on diversity varies widely and remains elusive. Despite the considerable attempts to understand mechanisms of biological invasion, it is largely unknown whether some communities’ characteristics promote biological invasion, or whether some inherent characteristics of invaders enable them to invade other communities. Our aims were to assess the impact of one of the massive plant invaders of Scandinavia on vascular plant species diversity, disentangle attributes of invasible and noninvasible communities, and evaluate the relationship between invasibility and genetic diversity of a dominant invader. We studied 56 pairs of Heracleum persicum Desf. ex Fisch.‐invaded and noninvaded plots from 12 locations in northern Norway. There was lower native cover, evenness, taxonomic diversity, native biomass, and species richness in the invaded plots than in the noninvaded plots. The invaded plots had nearly two native species fewer than the noninvaded plots on average. Within the invaded plots, cover of H. persicum had a strong negative effect on the native cover, evenness, and native biomass, and a positive association with the height of the native plants. Plant communities containing only native species appeared more invasible than those that included exotic species, particularly H. persicum. Genetic diversity of H. persicum was positively correlated with invasibility but not with community diversity. The invasion of a plant community by H. persicum exerts consistent negative pressure on vascular plant diversity. The lack of positive correlation between impacts and genetic diversity of H. persicum indicates that even a small founder population may cause high impact. We highlight community stability or saturation as an important determinant of invasibility. While the invasion by H. persicum may decrease susceptibility of a plant community to further invasion, it severely reduces the abundance of native species and makes them more vulnerable to competitive exclusion.     

Small island biogeography in the Gulf of California: lizards, the subsidized island biogeography hypothesis, and the small island effect

Aim We used insular lizard communities to test the predictions of two hypotheses that attempt to explain patterns of species richness on small islands. We first address the subsidized island biogeography (SIB) hypothesis, which predicts that spatial subsidies may cause insular species richness to deviate from species–area predictions, especially on small islands. Next, we examine the small island effect (SIE), which suggests small islands may not fit the traditional log‐linear species–area curve. Location Islands with arthropodivorous lizard communities throughout the Gulf of California. Methods To evaluate the SIB hypothesis, we first identified subsidized and unsubsidized islands based on surrogate measures of allochthonous productivity (i.e. island size and bird presence). Subsequently, we created species–area curves from previously published lizard species richness and island area data. We used the residuals and slopes from these analyses to compare species richness on subsidized and unsubsidized islands. To test for an SIE, we used breakpoint regression to model the relationship between lizard species richness and island area. We compared results from this model to results from the log‐linear regression model. Results Subsidized islands had a lower slope than unsubsidized islands, and the difference between these groups was significant when small islands were defined as < 1 km². In addition to comparing slopes, we tested for differences in the magnitude of the residuals (from the species–area regression of all islands) for subsidized vs. unsubsidized islands. We found no significant patterns in the residual values for small vs. large islands, or between islands with and without seabirds. The SIE was found to be a slightly better predictor of lizard species richness than the traditional log‐linear model. Main conclusions Predictions of the SIB hypothesis were partially supported by the data. The absence of a significant SIE may be a result of spatial subsidies as explained by the SIB hypothesis and data presented here. We conclude by suggesting potential scenarios to test for interactions between these two small island hypotheses. Future studies considering factors affecting species richness should examine the possible role of spatial subsidies, an SIE, or a synergistic effect of the two in data sets with small islands.     

Vulnerability of riparian zones to invasion by exotic vascular plants

We compared the invasibility of riparian plant communities high on river banks with those on floodplain floors for four South African rivers. Analyses of abundant and significant riparian species showed that the floors have 3.1 times more exotic plants than the banks. The percent exotics ranges from 5% to 11% of total species richness for the banks, and from 20% to 30% for the floors. Species richness and percent exotics are negatively correlated for the banks, but not correlated for the floors.Despite great differences in climate, species richness, and landuse history, the percentages of exotic plants in three rivers in the Pacific Northwest of the USA and one river in southwestern France are similar to those in South Africa (24-30% vs. 20-30%, respectively). Furthermore, the high proportions of exotic species in these riparian plant communities are comparable to those reported for vascular plant communities on islands. We conclude that the macro-channel floor regions of the riparian zones of South African rivers are highly vulnerable to invasion by exotic vascular plants.     

Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation

Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi‐dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle‐bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores.     

Environmental Productivity and Biodiversity Effects on Invertebrate Community Invasibility

Productivity influences the availability of resources for colonizing species. Biodiversity may also influence invasibility of communities because of more complete use of resource types with increasing species richness. We hypothesized that communities with higher environmental productivity and lower species richness should be more invasible by a competitor than those where productivity is low or where richness is high. We experimentally examined the invasion resistance of herbivorous meiofauna of Jamaican rock pools by a competitor crustacean (Ostracoda: Potamocypris sp. (Brady)) by contrasting three levels of nutrient input and four levels of species richness. Although relative abundance (dominance) of the invasive was largely unaffected by resource availability, increasing resources did increase the success rate of establishment. Effects of species richness on dominance were more pronounced with a trend towards the lowest species richness treatment of 2 resident species being more invasible than those with 4, 6, or 7 species. These results can be attributed to a ‘sampling effect associated with the introduction of Alona davidii (Richard) into the higher biodiversity treatments. Alona dominated the communities where it established and precluded dominance by the introduced ostracod. Our experimental study supports the idea that niche availability and community interactions define community invasibility and does not support the application of a neutral community model for local food web management where predictions of exotic species impacts are needed.     

Diversity reduces invasibility in experimental plant communities: the role of plant species

Several studies have presented experimental evidence that diversity reduces invasibility in grassland communities. The interpretation of these results has been disputed recently and it was proposed that sampling effects were responsible for the observed decrease of invasibility with diversity. The experiments performed to date were not designed to adequately separate sampling from diversity effects. Using the establishment of native plant species in experimental plant communities as a model of invasibility, we show that the number of invaders decreased with increasing diversity. When the presence of particular species is included, their effects are dominant. Centaurea jacea showed a strong effect at low diversity, whereas Leucanthemum vulgare showed a very strong negative impact at each diversity level. The negative effect of the latter might be related to root‐feeding nematodes that showed far higher abundance in plots with Leucanthemum. However, diversity remained a significant factor in determining the number of invading species and the numbers of an abundant invader.     

Community-level birth rate: a missing link between ecology, evolution and diversity

We propose a conceptual model to explain the variation in species richness in local communities and in build-up of regional species pools over time. The idea is that the opportunity for new species to enter a community (its invasibility) determines the present richness of that community as well as the long-term build-up of a species pool by speciation and migration. We propose that a community's invasibility is determined by the turnover rate of reproductive genets in the community, which we call the 'community-level birth rate'. The faster the turn-over, the more species will accumulate per unit time and per unit community size (number of genets) at a given per-birth rate of immigration and speciation. Spatially discrete communities inhabiting similar environments sum up to metacommunities, whose inhabitant species constitute the regional species pool. We propose that the size of a regional species pool is determined by the aggregate community-level birth rate, the size of the metacommunity through time and age of the metacommunity. Thus, the novel contribution is our proposal of a direct effect of local environment on the build-up rate of species pools. The relative importance of immigrating species and neospecies originating locally will change with the temporal and spatial scale under consideration. We propose that the diversification rate specific to evolutionary lineages and the build-up rate of species pools are two sides of the same coin, and that they are both depending on mean generation time. The proposed model offers a reconciliation of two contrasting paradigms in current community ecology, viz. one focussing on present-time ecological processes and one focussing on historical events governing the size of species pools which in turn determines local richness.     

Ecological filtering of exotic plants in an Australian sub‐alpine environment

We investigated some of the factors influencing exotic invasion of native sub‐alpine plant communities at a site in southeast Australia. Structure, floristic composition and invasibility of the plant communities and attributes of the invasive species were studied. To determine the plant characteristics correlated with invasiveness, we distinguished between roadside invaders, native community invaders and non‐invasive exotic species, and compared these groups across a range of traits including functional group, taxonomic affinity, life history, mating system and morphology. Poa grasslands and Eucalyptus‐Poa woodlands contained the largest number of exotic species, although all communities studied appeared resilient to invasion by most species. Most community invaders were broad‐leaved herbs while roadside invaders contained both herbs and a range of grass species. Over the entire study area the richness and cover of native and exotic herbaceous species were positively related, but exotic herbs were more negatively related to cover of specific functional groups (e.g. trees) than native herbs. Compared with the overall pool of exotic species, those capable of invading native plant communities were disproportionately polycarpic, Asteracean and cross‐pollinating. Our data support the hypothesis that strong ecological filtering of exotic species generates an exotic assemblage containing few dominant species and which functionally converges on the native assemblage. These findings contrast with those observed in the majority of invaded natural systems. We conclude that the invasion of closed sub‐alpine communities must be viewed in terms of the unique attributes of the invading species, the structure and composition of the invaded communities and the strong extrinsic physical and climatic factors typical of the sub‐alpine environment.     

Island biogeography of bats in Baja California, Mexico: patterns of bat species richness in a near-shore archipelago

Aim We studied the relationship between the size and isolation of islands and bat species richness in a near‐shore archipelago to determine whether communities of vagile mammals conform to predictions of island biogeography theory. We compared patterns of species richness in two subarchipelagos to determine whether area per se or differences in habitat diversity explain variations in bat species richness. Location Islands in the Gulf of California and adjacent coastal habitats on the Baja California peninsula in northwest Mexico. Methods Presence–absence surveys for bats were conducted on 32 islands in the Gulf of California using acoustic and mist‐net surveys. We sampled for bats in coastal habitats of four regions of the Baja peninsula to characterize the source pool of potential colonizing species. We fitted a semi‐log model of species richness and multiple linear regression and used Akaike information criterion model selection to assess the possible influence of log₁₀ area, isolation, and island group (two subarchipelagos) on the species richness of bats. We compared the species richness of bats on islands with greater vegetation densities in the southern gulf (n = 20) with that on drier islands with less vegetation in the northern gulf (n = 12) to investigate the relationship between habitat diversity and the species richness of bats. Results Twelve species of bats were detected on islands in the Gulf of California, and 15 species were detected in coastal habitats on the Baja peninsula. Bat species richness was related to both area and isolation of islands, and was higher in the southern subarchipelago, which has denser vegetation. Log₁₀ area was positively related to bat species richness, which increased by one species for every 5.4‐fold increase in island area. On average, richness declined by one species per 6.25 km increase in isolation from the Baja peninsula. Main conclusions Our results demonstrate that patterns of bat species richness in a near‐shore archipelago are consistent with patterns predicted by the equilibrium theory of island biogeography. Despite their vagility, bats may be more sensitive to moderate levels of isolation than previously expected in near‐shore archipelagos. Differences in vegetation and habitat xericity appear to be associated with richness of bat communities in this desert ecosystem. Although observed patterns of species richness were consistent with those predicted by the equilibrium theory, similar relationships between species richness and size and isolation of islands may arise from patch‐use decision making by individuals (optimal foraging strategies).     

Invasion by Non-Native Annual Grasses: The Importance of Species Biomass, Composition, and Time Among California Native Grasses of the Central Valley

The Central Valley of California is noted for its dearth of remnant native grass populations and for low native grass seedling establishment within grasslands now dominated by non‐native annual species. In contrast, remnant populations are common along the coast, and studies have shown an ability for seedlings and adults to compete with non‐native annual grasses. The invasibility of well‐established populations of native grasses in the Central Valley remains unclear. The objectives of this study were to compare the invasibility of native grasses differing in density and species composition and, given the species in this study, to assess the ability of mixes with greater species richness to resist invasion relative to their abilities in monoculture. In the Sacramento Valley of California, six species of native grasses were planted at three densities in monospecific and mixed‐species plots. Percent cover of native perennial and non‐native annual grasses was measured in years 2 and 3, and biomass was sampled in year 5. Native grass biomass and, to a lesser extent, species composition were important in explaining variation in non‐native grass invasibility in the fifth year. Species‐rich treatments did not experience less invasion than would be expected by the proportional invasibility of each species in monoculture. However, invasibility of plots consisting of slower growing, shorter statured species decreased over time, suggesting a successional benefit to diverse communities. This study demonstrates that established stands of native grasses in the Sacramento Valley can resist invasion by non‐native annual grasses and that stand biomass is a particularly important factor in determining invasibility.     

Gap pattern and colonization opportunities in plant communities: effects of species richness, mortality, and spatial aggregation

We investigate how perturbations that induce mortality transform original spatial patterns in plant communities into binary spatial patterns of survivors and perished individuals. By means of computer simulation, we analyse effects of average mortality, interspecific variation of mortality around the mean, spatial distribution of the species (clumping degree), and species richness. Gap spatial pattern is quantified by four spatial indices or landscape metrics (gap area, density, shape and coherence). In single‐species communities, the emerging gap patterns are subject to critical phenomena: opportunities for colonizers to establish increase with mortality, but more rapidly at specific mortality thresholds. In multi‐species communities, neither species richness nor interspecific variation of mortality influences gap spatial pattern when community assembly is random. Colonization opportunities would therefore not be affected by local species extinction in such a system, nor by the presence of species with divergent sensitivities to perturbation. In a community that is highly spatially aggregated, increases in interspecific mortality variation shift the pattern towards fewer gaps that are larger and more isodiametric, which suggests increased establishment chances for colonizers. Similar changes are induced in communities characterized by large interspecific mortality differences if clumping degree is increased. Loss of species richness only modifies gap spatial pattern to a substantial extent if mortality variation is high: in this case, depauperate communities exhibit a wider variety of colonization opportunities (more gaps which are on average smaller, but the largest gap is larger) than species‐richer ones. These findings may explain the contrast between the negative diversity‐invasibility relationship often found in small‐scale experimental studies and the positive diversity‐invasibility relationship found in observational studies at larger scale. They also demonstrate that the pre‐disturbance spatial structure of a community significantly affects colonization opportunities for alien species, and is therefore a likely determinant of the trajectory of secondary succession following perturbation.     

Propagule Pressure: A Null Model for Biological Invasions

Invasion ecology has been criticised for its lack of general principles. To explore this criticism, we conducted a meta-analysis that examined characteristics of invasiveness (i.e. the ability of species to establish in, spread to, or become abundant in novel communities) and invasibility (i.e. the susceptibility of habitats to the establishment or proliferation of invaders). There were few consistencies among invasiveness characteristics (3 of 13): established and abundant invaders generally occupy similar habitats as native species, while abundant species tend to be less affected by enemies; germination success and reproductive output were significantly positively associated with invasiveness when results from both stages (establishment/spread and abundance/impact) were combined. Two of six invasibility characteristics were also significant: communities experiencing more disturbance and with higher resource availability sustained greater establishment and proliferation of invaders. We also found that even though ‘propagule pressure’ was considered in only ~29% of studies, it was a significant predictor of both invasiveness and invasibility (55 of 64 total cases). Given that nonindigenous species are likely introduced non-randomly, we contend that ‘propagule biases’ may confound current paradigms in invasion ecology. Examples of patterns that could be confounded by propagule biases include characteristics of good invaders and susceptible habitats, release from enemies, evolution of ‘invasiveness’, and invasional meltdown. We conclude that propagule pressure should serve as the basis of a null model for studies of biological invasions when inferring process from patterns of invasion. An erratum to this article can be found at