Propagule pressure hypothesis not supported by an 80-year experiment on woody species invasion

Spatio-Environmental Analysis of Vespula germanica Nest Records Explains Slow Invasion in South Africa

Simple Summary

Social wasp invasions can spread quickly and have serious impacts if they reach new regions with favourable climatic conditions. However, in areas less suitable to them, invasion patterns can show factors that may prevent their spread. We use nest records of the German wasp from the southern tip of South Africa, to map and analyse what habitats they seem to prefer. Factors investigated included temperature, rainfall, and moisture availability. We find that this invasive wasp prefers moister and cooler conditions, and because these habitats are patchy in the region they have invaded in South Africa, they can only spread slowly unassisted, and utmost between 50 and 200 km with the assistance of humans. This is the likely reason for the very slow invasion seen in South Africa. The spatial patterns we quantify here make it possible to use a remote-sensing approach to determine the suitability of an area for future invasions. Predicting the likelihood of invasions will greatly aid management actions. Public awareness around the potential accidental transport of the German wasp and similar species should also be undertaken.

Abstract

Investigating the distributions of invasive species in marginal habitats can give clues to the factors constraining invasive spread. Vespula germanica is the most widely distributed of all the invasive Vespids, which in the Southern Hemisphere typically have large extensive invasive populations. In contrast, the invasion into South Africa has been slow and is still confined to a small geographic area. Here we analyse the distribution of all recent nest records in South Africa (n = 405). The distance to main rivers, mean annual rainfall, summer normalised difference moisture index (NDMI) values, and mean annual temperatures (average, minimum, maximum, and summer maximum temperature) was measured for every nest. We find that value ranges of these variables are different between the value ranges recorded for nests, the general distribution area of the wasp, and the area of absence. Optimised Hot Spot Analysis was used to quantify spatial structure in the measured climatic variables. Generally, factors related to moisture stress set the environmental limits of V. germanica’s landscape distribution. Due to the strong preference of nesting sites close to river courses, for higher rainfall conditions, medium to medium-high NDMI values, and lower mean annual temperatures, it is unlikely that V. germanica will be able to spread uniformly where it is currently found in South Africa.

Biotic resistance and vegetative propagule pressure co-regulate the invasion success of a marine clonal macrophyte

Propagule pressure is considered a major driver of plant invasion success. Great propagule pressure would enable invasive species to colonize new areas overcoming the resistance of native species. Many highly invasive aquatic macrophytes regenerate from vegetative propagules, but few studies have experimentally investigated the importance of propagule pressure and biotic resistance, and their interaction, in determining invasion success. By manipulating both recipient habitat and the input of vegetative propagules of the invasive seaweed Caulerpa cylindracea in mesocosm, we examined whether higher propagule pressure would overcome the resistance of a native congeneric (Caulerpa prolifera) and influence its performance. With the native, C. cylindracea population frond number decreased irrespectively of pressure level. High propagule pressure did not increase stolon length and single plant size decreased due to the effects of intra- and interspecific competition. Native biomass decreased with increasing C. cylindracea propagule pressure. These results indicate that higher propagule pressure may fail in enhancing C. cylindracea invasion success in habitats colonized by the native species, and they suggest that biotic resistance and propagule pressure co-regulate the invasion process. These findings emphasize the need to preserve/restore native seaweed populations and may help to design effective management actions to prevent further C. cylindracea spread.

Dissecting the null model for biological invasions: A meta-analysis of the propagule pressure effect

A consistent determinant of the establishment success of alien species appears to be the number of individuals that are introduced to found a population (propagule pressure), yet variation in the form of this relationship has been largely unexplored. Here, we present the first quantitative systematic review of this form, using Bayesian meta-analytical methods. The relationship between propagule pressure and establishment success has been evaluated for a broad range of taxa and life histories, including invertebrates, herbaceous plants and long-lived trees, and terrestrial and aquatic vertebrates. We found a positive mean effect of propagule pressure on establishment success to be a feature of every hypothesis we tested. However, establishment success most critically depended on propagule pressures in the range of 10–100 individuals. Heterogeneity in effect size was associated primarily with different analytical approaches, with some evidence of larger effect sizes in animal rather than plant introductions. Conversely, no variation was accounted for in any analysis by the scale of study (field to global) or methodology (observational, experimental, or proxy) used. Our analyses reveal remarkable consistency in the form of the relationship between propagule pressure and alien population establishment success.

Alien species are a major contributor to human-induced global environmental change. The probability of whether or not an alien species will successfully establish in a novel environment is often related to the number of times a species is introduced and the number of individuals that are introduced each time, collectively termed ‘propagule pressure’. Despite this evidence, we don’t yet know whether this is a universal characteristic of species invasions, and the role of propagule pressure continues to be questioned. Here, we present a quantitative meta-analysis of the relationship between propagule pressure and establishment success across a broad range of species and geographies. We found that propagule pressure was consistently and positively associated with the establishment success of alien species. We conclude that propagule pressure is indeed the most consistent and strongest determinant of alien species establishment. No other factors suggested to explain establishment success can claim such universal support. Our results underpin a clear policy and management target for slowing invasion rates by reducing propagule pressure—ideally to single figures or zero—regardless of any other feature of the invasion.

Predicting island biosecurity risk from introduced fauna using Bayesian Belief Networks

Around the globe, islands are the last refuge for many threatened and endemic species. Islands are frequently also important sites for recreation, cultural activities, and industrial development, all of which facilitate the establishment of invasive species. Surveillance is employed on islands to detect the establishment of invasive species after their arrival, leading to decisions about follow-up actions. Unless surveillance is prioritised according to risk of establishment of invasives, it may be infeasible to implement efficiently over large tracts of publicly accessible land, especially in data-deficient areas. The key biosecurity problem for many regions is one of prioritizing sites for surveillance activities and identifying invasive species most likely to disperse to, and establish, and proliferate on those sites. We created a series of Bayesian Belief Networks (BBNs), linked by Java computing code and the freely available GeNIe application to automate the creation and computation of species- and site-specific biosecurity BBNs. The BBNs require data on island attributes, recreational or industrial visitor load, infrastructure, habitat availability, and animal behaviour and dispersal via swimming, flying, human movement, land bridges, or flood plumes. We used this biosecurity BBN to estimate the risk of 11 invasive faunal species arriving and establishing on 600 islands along the Pilbara coastline, Western Australia. Sensitivity analyses were conducted to identify nodes within the BBNs that required refined data inputs. Propagule pressure was the node with the greatest influence over the number of arrivals. Other nodes such as the number of visitors to islands and swimming capabilities of invasive animals greatly influenced the model results. Across the 11 species studied, our models predicted one arrival per 300 visitors. The biosecurity BBN can be used to identify the islands at highest risk from establishment of invasive species within any archipelago/s, and the invasive species most likely to establish on each island.

Leaf endophytic fungus interacts with precipitation to alter belowground microbial communities in primary successional dunes

Understanding interactions between above- and belowground components of ecosystems is an important next step in community ecology. These interactions may be fundamental to predicting ecological responses to global change because indirect effects occurring through altered species interactions can outweigh or interact with the direct effects of environmental drivers. In a multiyear field experiment (2010-2015), we tested how experimental addition of a mutualistic leaf endophyte (Epichloë amarillans) associated with American beachgrass (Ammophila breviligulata) interacted with an altered precipitation regime (±30%) to affect the belowground microbial community. Epichloë addition increased host root biomass at the plot scale, but reduced the length of extraradical arbuscular mycorrhizal (AM) fungal hyphae in the soil. Under ambient precipitation alone, the addition of Epichloë increased root biomass per aboveground tiller and reduced the diversity of AM fungi in A. breviligulata roots. Furthermore, with Epichloë added, the diversity of root-associated bacteria declined with higher soil moisture, whereas in its absence, bacterial diversity increased with higher soil moisture. Thus, the aboveground fungal mutualist not only altered the abundance and composition of belowground microbial communities but also affected how belowground communities responded to climate, suggesting that aboveground microbes have potential for cascading influences on community dynamics and ecosystem processes that occur belowground.

Successful population establishment from small introductions appears to be less common than believed

Although small populations are at high risk of extinction, there are regular reports in the scientific literature of purported small, isolated, persistent populations. One source of evidence of the viability of small populations comes from the alleged successful introduction of species to areas outside their original range from introductions of few individuals. We reviewed the examples from introduction compendia on deliberate translocations of birds, and the original sources, to identify and evaluate purported examples of successful establishments from small introductions. We found 23 purportedly successful introductions from few (<30) individuals. After assessing original sources, we found that two of the claims were substantiated; the rest were ambiguous or could be rejected as examples, primarily due to a lack of evidence in original sources of the number of birds released and because of supplemental individuals from other releases, releases in nearby regions, and the possibility of natural invasion. Our results suggest that reports of successful establishment of birds from introductions of few individuals have been overstated. These results strengthen the relationship previously reported between propagule pressure and likelihood of establishment, and support the lack of viability of small populations presumed by population theory. We suggest that analyses of introduction failure and success would benefit from excluding studies where introduction effort is unknown or unreliably documented.

Ectomycorrhizal fungal communities coinvading with Pinaceae host plants in Argentina: Gringos bajo el bosque

Coinvasive ectomycorrhizal (ECM) fungi allow Pinaceae species to invade regions otherwise lacking compatible symbionts, but ECM fungal communities permitting Pinaceae invasions are poorly understood. In the context of Pinaceae invasions on Isla Victoria, Nahuel Huapi National Park, Argentina, we asked: what ECM fungi are coinvading with Pinaceae hosts on Isla Victoria; are some ECM fungal species or genera more prone to invade than others; and are all ECM fungal species that associate with Northern Hemisphere hosts also nonnative, or are some native fungi compatible with nonnative plants? We sampled ECMs from 226 Pinaceae host plant individuals, both planted individuals and recruits, growing inside and invading from plantations. We used molecular techniques to examine ECM fungal communities associating with these trees. A distinctive subset of the ECM fungal community predominated far from plantations, indicating differences between highly invasive and less invasive ECM fungi. Some fungal invaders reported here have been detected in other locations around the world, suggesting strong invasion potential. Fungi that were frequently detected far from plantations are often found in early-successional sites in the native range, while fungi identified as late-successional species in the native range are rarely found far from plantations, suggesting a means for predicting potential fungal coinvaders.

Genetic diversity and floral width variation in introduced and native populations of a long-lived woody perennial

Populations of introduced species in their new environments are expected to differ from native populations, due to processes such as genetic drift, founder effects and local adaptation, which can often result in rapid phenotypic change. Such processes can also lead to changes in the genetic structure of these populations. This study investigated the populations of Rhododendron ponticum in its introduced range in Ireland, where it is severely invasive, to determine both genetic and flower width diversity and differentiation. We compared six introduced Irish populations with two populations from R. ponticum's native range in Spain, using amplified fragment length polymorphism and simple sequence repeat genetic markers. We measured flower width, a trait that may affect pollinator visitation, from four Irish and four Spanish populations by measuring both the width at the corolla tip and tube base (nectar holder width). With both genetic markers, populations were differentiated between Ireland and Spain and from each other in both countries. However, populations displayed low genetic diversity (mean Nei's genetic diversity = 0.22), with the largest proportion (76-93 %) of genetic variation contained within, rather than between, populations. Although corolla width was highly variable between individuals within populations, tube width was significantly wider (>0.5 mm) in introduced, compared with native, populations. Our results show that the same species can have genetically distinct populations in both invasive and native regions, and that differences in floral width may occur, possibly in response to ecological sorting processes or local adaptation to pollinator communities.

New pasture plants intensify invasive species risk

Agricultural intensification is critical to meet global food demand, but intensification threatens native species and degrades ecosystems. Sustainable intensification (SI) is heralded as a new approach for enabling growth in agriculture while minimizing environmental impacts. However, the SI literature has overlooked a major environmental risk. Using data from eight countries on six continents, we show that few governments regulate conventionally bred pasture taxa to limit threats to natural areas, even though most agribusinesses promote taxa with substantial weed risk. New pasture taxa (including species, subspecies, varieties, cultivars, and plant-endophyte combinations) are bred with characteristics typical of invasive species and environmental weeds. By introducing novel genetic and endophyte variation, pasture taxa are imbued with additional capacity for invasion and environmental impact. New strategies to prevent future problems are urgently needed. We highlight opportunities for researchers, agribusiness, and consumers to reduce environmental risks associated with new pasture taxa. We also emphasize four main approaches that governments could consider as they build new policies to limit weed risks, including (i) national lists of taxa that are prohibited based on environmental risk; (ii) a weed risk assessment for all new taxa; (iii) a program to rapidly detect and control new taxa that invade natural areas; and (iv) the polluter-pays principle, so that if a taxon becomes an environmental weed, industry pays for its management. There is mounting pressure to increase livestock production. With foresight and planning, growth in agriculture can be achieved sustainably provided that the scope of SI expands to encompass environmental weed risks.

The Human Release Hypothesis for biological invasions: human activity as a determinant of the abundance of invasive plant species

Research on biological invasions has increased rapidly over the past 30 years, generating numerous explanations of how species become invasive. While the mechanisms of invasive species establishment are well studied, the mechanisms driving abundance patterns (i.e. patterns of population density and population size) remain poorly understood. It is assumed that invasive species typically have higher abundances in their new environments than in their native ranges, and patterns of invasive species abundance differ between invaded regions. To explain differences in invasive species abundance, we propose the Human Release Hypothesis. In parallel to the established Enemy Release Hypothesis, this hypothesis states that the differences in abundance of invasive species are found between regions because population expansion is reduced in some regions through continuous land management and associated cutting of the invasive species. The Human Release Hypothesis does not negate other important drivers of species invasions, but rather should be considered as a potentially important complementary mechanism. We illustrate the hypothesis via a case study on an invasive rose species, and hypothesize which locations globally may be most likely to support high abundances of invasive species. We propose that more extensive empirical work on the Human Release Hypothesis could be useful to test its general applicability.

Exotic mammals disperse exotic fungi that promote invasion by exotic trees

Biological invasions are often complex phenomena because many factors influence their outcome. One key aspect is how non-natives interact with the local biota. Interaction with local species may be especially important for exotic species that require an obligatory mutualist, such as Pinaceae species that need ectomycorrhizal (EM) fungi. EM fungi and seeds of Pinaceae disperse independently, so they may use different vectors. We studied the role of exotic mammals as dispersal agents of EM fungi on Isla Victoria, Argentina, where many Pinaceae species have been introduced. Only a few of these tree species have become invasive, and they are found in high densities only near plantations, partly because these Pinaceae trees lack proper EM fungi when their seeds land far from plantations. Native mammals (a dwarf deer and rodents) are rare around plantations and do not appear to play a role in these invasions. With greenhouse experiments using animal feces as inoculum, plus observational and molecular studies, we found that wild boar and deer, both non-native, are dispersing EM fungi. Approximately 30% of the Pinaceae seedlings growing with feces of wild boar and 15% of the seedlings growing with deer feces were colonized by non-native EM fungi. Seedlings growing in control pots were not colonized by EM fungi. We found a low diversity of fungi colonizing the seedlings, with the hypogeous Rhizopogon as the most abundant genus. Wild boar, a recent introduction to the island, appear to be the main animal dispersing the fungi and may be playing a key role in facilitating the invasion of pine trees and even triggering their spread. These results show that interactions among non-natives help explain pine invasions in our study area.