Gastropods alien to South Africa cause severe environmental harm in their global alien ranges across habitats

Alien gastropods have caused extensive harm to biodiversity and socioeconomic systems like agriculture and horticulture worldwide. For conservation and management purposes, information on impacts needs to be easily interpretable and comparable, and the factors that determine impacts understood. This study aimed to assess gastropods alien to South Africa to compare impact severity between species and understand how they vary between habitats and mechanisms. Furthermore, we explore the relationship between environmental and socioeconomic impacts, and both impact measures with life‐history traits. We used the Environmental Impact Classification for Alien Taxa (EICAT) and Socio‐Economic Impact Classification for Alien Taxa (SEICAT) to assess impacts of 34 gastropods alien to South Africa including evidence of impact from their entire alien range. We tested for correlations between environmental and socioeconomic impacts per species, and with fecundity and native latitude range using Kendall's tau tests. Kruskal–Wallis tests were used to compare impact magnitude among mechanisms and habitats, respectively. This study presents the first application of EICAT and SEICAT for invertebrates. There was no correlation between environmental impacts and socioeconomic impacts. Habitats did not differ regarding the severity of impacts recorded, but impacts via disease transmission were lower than other mechanisms. Neither fecundity nor native range latitude was correlated with impact magnitude. Despite gastropods being agricultural and horticultural pests globally, resilience of socioeconomic systems makes high impacts uncommon. Environmental systems may be vulnerable to gastropod impacts across habitats, having experienced multiple local extinctions of wetland island snail fauna. South Africa stands out as the only continental country that follows this trend. The knowledge gained on severity and nature of gastropod impacts is useful in risk assessment, which can aid conservation management. To make impact assessments more realistic, we suggest alternative ways of reporting impacts classified under EICAT and SEICAT.     

Determinants of data deficiency in the impacts of alien bird species

To identify the factors that influence the availability of data on the negative impacts of alien bird species, in order to understand why more than 70% are currently classified as Data Deficient (DD) by the Environmental Impact Classification of Alien Taxa (EICAT) protocol. Information on factors hypothesised to influence the availability of impact data were collated for 344 alien bird species (107 with impact data and 237 DD). These data were analysed using mixed effects models accounting for phylogenetic non‐independence of species (MCMCglmm). Data deficiency in the negative impacts of alien birds is not randomly distributed. Residence time, relative brain size and alien range size were found to be strongly related to the availability of data on impacts. The availability of data on the negative impacts of alien birds is mainly influenced by the spatial and temporal extents of their alien ranges. The results of this study suggest that the impacts of some DD alien birds are likely to be minor (e.g. species with comparatively long residence times as aliens, such as the common waxbill Estrilda astrild and the Java sparrow Padda oryzivora). However, the results also suggest that some DD alien birds may have damaging impacts (e.g. species from orders of alien birds known for their impacts to biodiversity but with comparatively small alien ranges, such as the New Caledonian crow Corvus moneduloides). This implies that at least some DD alien birds may have impacts that are being overlooked. Studies examining the traits that influence the severity of alien bird impacts are needed to help to predict which DD species are more likely to impact upon biodiversity.     

Multiple stressors on biotic interactions: how climate change and alien species interact to affect pollination

Global change may substantially affect biodiversity and ecosystem functioning but little is known about its effects on essential biotic interactions. Since different environmental drivers rarely act in isolation it is important to consider interactive effects. Here, we focus on how two key drivers of anthropogenic environmental change, climate change and the introduction of alien species, affect plant–pollinator interactions. Based on a literature survey we identify climatically sensitive aspects of species interactions, assess potential effects of climate change on these mechanisms, and derive hypotheses that may form the basis of future research. We find that both climate change and alien species will ultimately lead to the creation of novel communities. In these communities certain interactions may no longer occur while there will also be potential for the emergence of new relationships. Alien species can both partly compensate for the often negative effects of climate change but also amplify them in some cases. Since potential positive effects are often restricted to generalist interactions among species, climate change and alien species in combination can result in significant threats to more specialist interactions involving native species.     

How repeatable is the Environmental Impact Classification of Alien Taxa (EICAT)? Comparing independent global impact assessments of amphibians

The magnitude of impacts some alien species cause to native environments makes them targets for regulation and management. However, which species to target is not always clear, and comparisons of a wide variety of impacts are necessary. Impact scoring systems can aid management prioritization of alien species. For such tools to be objective, they need to be robust to assessor bias. Here, we assess the newly proposed Environmental Impact Classification for Alien Taxa (EICAT) used for amphibians and test how outcomes differ between assessors. Two independent assessments were made by Kraus (Annual Review of Ecology Evolution and Systematics, 46, 2015, 75‐97) and Kumschick et al. (Neobiota, 33, 2017, 53‐66), including independent literature searches for impact records. Most of the differences between these two classifications can be attributed to different literature search strategies used with only one‐third of the combined number of references shared between both studies. For the commonly assessed species, the classification of maximum impacts for most species is similar between assessors, but there are differences in the more detailed assessments. We clarify one specific issue resulting from different interpretations of EICAT, namely the practical interpretation and assigning of disease impacts in the absence of direct evidence of transmission from alien to native species. The differences between assessments outlined here cannot be attributed to features of the scheme. Reporting bias should be avoided by assessing all alien species rather than only the seemingly high‐impacting ones, which also improves the utility of the data for management and prioritization for future research. Furthermore, assessments of the same taxon by various assessors and a structured review process for assessments, as proposed by Hawkins et al. (Diversity and Distributions, 21, 2015, 1360), can ensure that biases can be avoided and all important literature is included.     

Invasive aliens on tropical East Asian islands

Tropical East Asia (TEA) has numerous islands, both continental and oceanic. This study uses information on invasive aliens in terrestrial habitats on these islands to test the generality of the continental-oceanic contrast in invasibility, assess the conservation impacts of invasive species, and suggest ways to mitigate these. The continental islands of Hong Kong and Singapore are worst-case scenarios for continental invasibility and alien species often dominate in chronically disturbed sites, but very few have successfully invaded closed forests, with the exception of birds in Hong Kong. On other, less densely populated, continental islands, closed-canopy forests appear to resist invasions by all taxa, with few known exceptions. Forests on oceanic islands isolated by <100 km during the last glacial maximum appear no more susceptible to plant and invertebrate invasions than those on continental islands, but invasions by mammals are widespread. Snake invasions may be under-recognized. The remote oceanic Ogasawara (Bonin) Islands, >1000 km from the nearest continent, have a native biota of largely tropical East Asian origin and are suffering from alien forest invasions across the taxonomic spectrum. These patterns of invasibility are consistent with the idea that alien invasion is facilitated by the absence of native species in the same functional group. Alien invasives are not yet a major conservation problem in TEA, except on remote islands, but their dominance on disturbed sites may slow or prevent recovery of native biodiversity. Strict quarantine is impractical in TEA, although some major introduction routes could be blocked. Management efforts should focus on early recognition and immediate control of potential problem species.     

A new comprehensive database of alien plant species in Chile based on herbarium records

There is an urgent need for comprehensive national databases on alien plant species, especially in developing countries. Despite the fact that plant invasions are considered a major threat to biodiversity, they have been poorly studied or not considered a conservation priority in South America. We aim to assess alien plant distribution in Chile, using the first comprehensive public alien plant database, and discuss the implications of using herbarium records to develop national databases of alien plants. We used herbarium records to assemble a comprehensive national database of alien plants. We calculated the number of alien and native species and specimens recorded in each 10 × 10 km cell. We evaluated sampling efforts and tested for relationships between alien and native species collections, as well as other spatial patterns along the latitudinal gradient. Alien and native species richness was positively correlated. Alien plants were mostly collected in central Chile, with few species collected in both the extreme north and south. However, native plants were strongly collected in central Chile, as well as in both extremes of the country. Alien and native plants followed the same pattern of accumulation along the latitudinal gradient, with native plants being relatively more collected than alien plants. Herbarium records provide valuable baseline information to evaluate plant species distribution. However, there are important gaps in this database, (e.g. variable sampling effort for alien and native plants, incomplete information on life-history traits). Given scientists and land managers increasing demand for baseline information and the high cost of collecting such data in developing countries, herbarium records should be used more frequently for research and management of plant invasions.     

Environmental gradients shift the direction of the relationship between native and alien plant species richness

Aim

To assess how environmental, biotic and anthropogenic factors shape native–alien plant species richness relationships across a heterogeneous landscape.

Location

Banks Peninsula, New Zealand.

Methods

We integrated a comprehensive floristic survey of over 1200 systematically located 6 × 6 m plots, with corresponding climate, environmental and anthropogenic data. General linear models examined variation in native and alien plant species richness across the entire landscape, between native‐ and alien‐dominated plots, and within separate elevational bands.

Results

Across all plots, there was a significant negative correlation between native and alien species richness, but this relationship differed within subsets of the data: the correlation was positive in alien‐dominated plots but negative in native‐dominated plots. Within separate elevational bands, native and alien species richness were positively correlated at lower elevations, but negatively correlated at higher elevations. Alien species richness tended to be high across the elevation gradient but peaked in warmer, mid‐ to low‐elevation sites, while native species richness increased linearly with elevation. The negative relationship between native and alien species richness in native‐dominated communities reflected a land‐use gradient with low native and high alien richness in more heavily modified native‐dominated vegetation. In contrast, native and alien richness were positively correlated in very heavily modified alien‐dominated plots, most likely due to covariation along a gradient of management intensity.

Main conclusions

Both positive and negative native–alien richness relationships can occur across the same landscape, depending on the plant community and the underlying human and environmental gradients examined. Human habitat modification, which is often confounded with environmental variation, can result in high alien and low native species richness in areas still dominated by native species. In the most heavily human modified areas, dominated by alien species, both native and alien species may be responding to similar underlying gradients.

     

近十年植物入侵生态学重要研究进展

外来植物入侵对本地生物多样性、生态安全、社会经济发展和人类健康造成了严重威胁。因此, 探究植物入侵机制及其生态效应对我国生态可持续发展具有重要意义, 也是植物入侵生态学的主要研究内容。过去10年, 生态学家开展了大量研究, 取得了丰硕成果, 为推动入侵植物防控与生物多样性保护提供了理论指导。为深入解析外来植物入侵机制, 为其防控管理提供依据, 本文从以下3个方面综述了植物入侵生态学重要进展: 首先是外来植物生物学特性、生物和非生物环境对植物入侵的调控作用; 其次是外来植物入侵对本地生态系统的影响; 最后介绍了外来植物入侵的类比研究范式——本地植物入侵, 以及多组学技术在入侵生态学研究中的应用。基于这些研究进展, 本文展望了该领域的未来发展趋势, 包括: (1)研究对象从单一物种扩展到多物种比较; (2)研究地理范围从局域尺度扩大到纬度梯度格局; (3)入侵机制从单一理论验证到综合阐释多个假说。     

Alien molluscs affect the composition and diversity of native macroinvertebrates in a sandy flat of Lake Neuchâtel,Switzerland

The spread of alien molluscs is a serious threat to native biodiversity in fresh waters. Alien freshwater molluscs may deplete the resources of native species and alter the physical structure of the habitat through their shell mass. These changes might have both positive and negative effects on native community members. We investigated the native macroinvertebrate community in relation to the densities of four alien mollusc species (Corbicula fluminea, Dreissena polymorpha, Potamopyrgus antipodarum and Lithoglyphus naticoides) in a sandy flat of Lake Neuchatel, Switzerland. The habitat examined was dominated by these alien mollusc species. The abundance of the alien molluscs did not directly impact the native community assembly. However, C. fluminea and D. polymorpha influenced the composition and diversity of native macroinvertebrates by transforming the sandy substratum into a partly hard substratum habitat. Substantial differences in community composition between shallow (<3.5 m) and (≥5 m) deep sites were recorded. At shallow sites, the abundance of D. polymorpha was significantly reduced as a result of depth-selective feeding of ducks. A controlled shell decay study revealed that shells of alien molluscs (C. fluminea, D. polymorpha) persist for a longer period in the sediment than those of native molluscs. Consequently, shells of alien molluscs have a long-lasting impact by modifying the sandy habitat. This form of ecosystem engineering favours the occurrence of several native taxa, but is disadvantageous for other taxa with specific habitat requirements, and thus can be regarded as an indirect impact of competition.     

The role of species traits and taxonomic patterns in alien bird impacts

Aim To test whether the distribution of alien bird impacts varies across bird families and regions of origin, and to investigate whether species traits associated with successful introductions can predict which species will have negative impacts in the new area of introduction. Location Europe and the Mediterranean Basin. Methods Combining historical information and published literature about negative economic, biological and human health impacts, we compared the distribution of impacts among bird families and native origins of bird species for three major types of impact (economic, biodiversity and human health). We examined the relationships between ecological, biological and reproductive characteristics of species and the severity of the impacts. Results The majority of alien species with reported impacts originated from the Afrotropical, Indo‐Malayan and Palaearctic biogeographical regions. The distribution of alien bird species in Europe with reported impacts shows a taxonomic bias and largely mirrors patterns of establishment. While most species had primarily either economic or biodiversity impacts, several species in the Anatidae, Corvidae, Passeridae, Phasianidae and Sturnidae families were associated with moderate to serious negative impacts on both economic resources and native biodiversity. After controlling for taxonomic effects, species with the greatest overall impacts were habitat generalists and multi‐brooded, while species with smaller bodies and the tendency to form large feeding or roosting flocks were linked with greater impacts on native biodiversity. Main conclusions This study presents the first synthesis of published impact data for alien birds and provides a broad‐scale perspective on factors that contribute to their impacts. The results show that accounting for both species traits and taxonomy improves our ability to predict the impacts of alien bird species. Because several species are currently in the early stages of establishment in Europe, there may be an opportunity to limit negative impacts with efforts that promote proactive strategies against species and families possessing the above characteristics.     

Little,but increasing evidence of impacts by alien bryophytes

Based on data of bryophyte invasions into 82 regions on five continents of both hemispheres, we aim here at a first comprehensive overview of the impacts that bryophytes may have on biodiversity and socio-economy. Of the 139 bryophytes species which are alien in the study regions seven cause negative impacts on biodiversity in 26 regions, whereas three species cause negative impacts on socio-economic sectors in five regions. The vast majority of impacts stem from anecdotal observations, whereas only 14 field or experimental studies (mostly on Campylopus introflexus in Europe) have quantitatively assessed the impacts of an alien bryophyte. The main documented type of impact on biodiversity is competition (8 alien bryophytes), with native cryptogams being most affected. In particular, C. introflexus (9 regions) and Pseudoscleropodium purum (7 regions) affect resident species composition. The few socio-economic impacts are caused by alien bryophytes which form dense mats in lawns and are then considered a nuisance. Most negative impacts on biodiversity have been recorded in natural grasslands, forests, and wetlands. Impacts of alien bryophytes on biodiversity and socio-economy are a recent phenomenon, with >85 % of impacts on biodiversity, and 80 % of impacts on socio-economy recorded since 1990. On average, 40 years (impacts on biodiversity) and 25 years (impacts on socio-economy) elapsed between the year a bryophyte species has been first recorded as alien in a region and the year impacts have been recorded first. Taking into account the substantial time lag between first record and first recorded impact in a region, it seems to be likely that the currently moderate impacts of alien bryophytes will continue to increase. As quantitative studies on impacts of alien bryophytes are rare and restricted to few environments and biogeographic regions, there is a need for addressing potential impacts of alien bryophytes in yet understudied settings.     

The hitchhiker’s guide to alien ant invasions

Alien ants are among the most deleterious predatory invertebrates causing havoc to native biodiversity including negative effects on other ant species, other invertebrates and vertebrates, particularly birds and lizards, ecosystem function, economy, animal and human health. The patterns of alien ant invasions and reasons for their success are among the most intensively studied facets of invasion ecology, with feedback to a general understanding of ant ecology. Alien ant management can intervene at any step during the invasion process, with action for preventive measures being the most efficient. Beside standard chemical treatments, new methods and technologies for mitigation and control of ant invasions are mostly in a trial stage. A brief outlook on the most promising offensive lines is given with particular attention paid to Wolbachia-symbiosis. Ultimately, an integrative alien species management strategy is imperative to cope with accelerating biodiversity losses due to biological invasions.     

Observed and dark diversity of alien plant species in Europe: estimating future invasion risk

Invasion by alien species is a growing concern for nature conservation. We estimated the level of invasion by alien plant species and future invasion risks at the European scale. We used a pan-European atlas and eight regional plant atlases to determine the distribution of alien and native plant richness. In addition, we estimated alien and native dark diversity (species currently absent from a site but present in the surrounding region and able to colonize the site). We used relative diversity metrics to indicate current and future risks by alien species: relative alien richness (compared to native species), alien and native completeness (log-ratio of observed to dark diversity) and completeness difference between alien and native species. Observed and relative richness of alien species were greatest in NW Europe; this suggests that sites in NW Europe could be more disturbed. Observed alien and native species richness show clear regional hotspots; the distribution of completeness values is dispersed, indicating local hotspots. Northern Europe has relatively lower alien completeness, likely because potential invaders inhabit the region but have not yet reached many localities, thereby suggesting a risk of future invasion. A greater number of potential alien species in the region increases the probability that some alien species could have detrimental impacts. Both alien richness and completeness are positively correlated with native richness and completeness, respectively, indicating that both groups share similar distribution patterns. Alien species diversity metrics in Europe are related positively to human population density and agricultural land-use. We suggest that the dark diversity concept can broaden our understanding of alien species diversity and future invasion risks.     

Current status of alien vertebrates in the Galápagos Islands: invasion history, distribution, and potential impacts

Human activity has promoted the invasion of the Galápagos Islands by alien species from each of the five classes of vertebrates. We review the current distribution of alien vertebrates in the archipelago, their impacts on native species, and management efforts aimed at alien vertebrates. A total of 44 species have been reported in the archipelago, with 20 species establishing feral populations. Mammals were the first group arriving in the archipelago and remain the most numerous, with 10 established species. Alien birds invaded after mammals and four species have established populations. Reptiles, amphibians, and fish invaded later and are represented by three, one, and two species, respectively. Alien mammals are the most injurious to native biota, contributing to the decline or extinction of several species. Aside from mammals, no other class of alien vertebrate has had documented impacts on native species. Several populations of large and medium-sized mammals and birds have been eradicated.     

Alien plant species coexist over time with native ones in Chilean Mediterranean grasslands

Aims Alien species are commonly considered as harmful weeds capable of decreasing native biodiversity and threatening ecosystems. Despite this assumption, little is known about the long-term patterns of the native–alien relationships associated with human disturbed managed landscapes. This study aims to elucidate the community dynamics associated with a successional gradient in Chilean Mediterranean grasslands, considering both native and alien species.Methods Species richness (natives and aliens separately) and life-form (annuals and perennials) were recorded in four Chilean post-agricultural grazed grasslands each covering a broad successional gradient (from 1 to 40 years since crop abandonment). A detrended correspondence analysis (DCA), mixed model effects analyses and correlation tests were conducted to assess how this temporal gradient influenced natives and aliens through community dynamics.Important findings Our results show different life-form patterns between natives and aliens over time. Aliens were mainly represented by annuals (especially ruderals and weeds), which were established at the beginning of succession. Annual aliens also predominated at mid-successional stages, but in old grasslands native species were slightly more representative than alien ones within the community. In the late successional states, positive or no correlations at all between alien and native species richness suggested the absence of competition between both species groups, as a result of different strategies in occupation of the space. Community dynamics over time constitute a net gain in biodiversity, increasing natives and maintaining a general alien pool, allowing the coexistence of both. Biotic interactions including facilitation and/or tolerance processes might be occurring in Chilean post-agricultural grasslands, a fact that contradicts the accepted idea of the alien species as contenders.     

Growth‐form and spatiality driving the functional difference of native and alien aquatic plants in Europe

Trait‐based approaches are widely used in community ecology and invasion biology to unravel underlying mechanisms of vegetation dynamics. Although fundamental trade‐offs between specific traits and invasibility are well described among terrestrial plants, little is known about their role and function in aquatic plant species. In this study, we examine the functional differences of aquatic alien and native plants stating that alien and native species differ in selected leaf traits. Our investigation is based on 60 taxa (21 alien and 39 native) collected from 22 freshwater units of Hungarian and Italian lowlands and highlands. Linear mixed models were used to investigate the effects of nativeness on four fundamental traits (leaf area, leaf dry matter content, specific leaf area, and leaf nitrogen content), while the influence of growth‐form, altitude, and site were employed simultaneously. We found significantly higher values of leaf areas and significantly lower values of specific leaf areas for alien species if growth‐form was included in the model as an additional predictor.We showed that the trait‐based approach of autochthony can apply to aquatic environments similar to terrestrial ones, and leaf traits have relevance in explaining aquatic plant ecology whether traits are combined with growth‐forms as a fixed factor. Our results confirm the importance of traits related to competitive ability in the process of aquatic plant invasions. Alien aquatic plants can be characterized as species producing soft leaves faster. We argue that the functional traits of alien aquatic plants are strongly growth‐form dependent. Using the trait‐based approach, we found reliable characteristics of aquatic plants related to species invasions, which might be used, for example, in conservation management.     

A Unified Classification of Alien Species Based on the Magnitude of their Environmental Impacts

Species moved by human activities beyond the limits of their native geographic ranges into areas in which they do not naturally occur (termed aliens) can cause a broad range of significant changes to recipient ecosystems; however, their impacts vary greatly across species and the ecosystems into which they are introduced. There is therefore a critical need for a standardised method to evaluate, compare, and eventually predict the magnitudes of these different impacts. Here, we propose a straightforward system for classifying alien species according to the magnitude of their environmental impacts, based on the mechanisms of impact used to code species in the International Union for Conservation of Nature (IUCN) Global Invasive Species Database, which are presented here for the first time. The classification system uses five semi-quantitative scenarios describing impacts under each mechanism to assign species to different levels of impact—ranging from Minimal to Massive—with assignment corresponding to the highest level of deleterious impact associated with any of the mechanisms. The scheme also includes categories for species that are Not Evaluated, have No Alien Population, or are Data Deficient, and a method for assigning uncertainty to all the classifications. We show how this classification system is applicable at different levels of ecological complexity and different spatial and temporal scales, and embraces existing impact metrics. In fact, the scheme is analogous to the already widely adopted and accepted Red List approach to categorising extinction risk, and so could conceivably be readily integrated with existing practices and policies in many regions.     

外来入侵植物对本地植物菌根真菌的影响及其机制

菌根真菌共生是植物吸收养分的一个重要策略。外来植物可以干扰本地植物与菌根真菌的共生关系从而抑制本地植物生长, 这是近年来被发现的一种重要入侵机制, 在研究中得到日益广泛的关注。该文从以下几个方面着重综述这种入侵机制: 1)外来植物对本地植物菌根真菌的影响, 包括菌根真菌侵染率、菌根内部结构、根外菌丝的量、菌根真菌的群落组成、非菌根真菌的影响及网络结构; 2)外来植物对本地植物菌根真菌上述影响的机制, 包括资源竞争、化感作用和土壤肥力等生态机制以及相关的分子机制; 3)上述两个方面随入侵时间的变化格局。尽管干扰本地植物菌根真菌是一种重要的入侵机制, 但相对其他的入侵机制(例如天敌逃逸、新武器假说等)来说, 这类机制的研究目前仍很匮乏。鉴于此, 该文提出了未来需要重点关注的几个方面: 1)全球变化背景下, 入侵植物对本地植物菌根真菌的影响如何变化; 2)包括这种菌根机制在内的多种入侵机制之间的关系; 3)深入探究入侵的这种菌根机制在大的时空尺度上的变化规律。     

A review on the effects of invasive plants on mycorrhizal fungi of native plants and their underlying mechanisms

菌根真菌共生是植物吸收养分的一个重要策略。外来植物可以干扰本地植物与菌根真菌的共生关系从而抑制本地植物生长, 这是近年来被发现的一种重要入侵机制, 在研究中得到日益广泛的关注。该文从以下几个方面着重综述这种入侵机制: 1)外来植物对本地植物菌根真菌的影响, 包括菌根真菌侵染率、菌根内部结构、根外菌丝的量、菌根真菌的群落组成、非菌根真菌的影响及网络结构; 2)外来植物对本地植物菌根真菌上述影响的机制, 包括资源竞争、化感作用和土壤肥力等生态机制以及相关的分子机制; 3)上述两个方面随入侵时间的变化格局。尽管干扰本地植物菌根真菌是一种重要的入侵机制, 但相对其他的入侵机制(例如天敌逃逸、新武器假说等)来说, 这类机制的研究目前仍很匮乏。鉴于此, 该文提出了未来需要重点关注的几个方面: 1)全球变化背景下, 入侵植物对本地植物菌根真菌的影响如何变化; 2)包括这种菌根机制在内的多种入侵机制之间的关系; 3)深入探究入侵的这种菌根机制在大的时空尺度上的变化规律。     

Livestock reduces juvenile tree growth of alien invasive species with a minimal effect on natives: a field experiment using exclosures

Many alien invasive tree species were originally introduced to their non-native ranges for use in forestry and as urban trees. These alien species were selected for their fast growth and not necessarily for possessing mechanisms which deter browsing. Instead, many tree species native to semiarid areas of the world evolved mechanisms which deter browsing, presumably at the cost of slower growth. In a semiarid rangeland we observed that livestock exclusion greatly promoted the growth of juveniles of several alien species but not of native species, and we hypothesized that this increase in growth of aliens was due to livestock preference for alien and not native trees. With the objective of quantifying our observations and understanding the mechanism underlying the increased growth rates of alien juvenile trees under livestock exclusion, we assessed growth and browsing levels in juveniles of two alien invasive and four abundant native tree species within three parcels where livestock was excluded and three parcels with livestock at 0.20 cattle equivalents.ha^?1. Alien species grew around four-fold faster under livestock exclusion than with livestock and, as predicted, received five times more browsing than natives. Instead, native species did not significantly increase their growth rate with livestock exclusion. The results support our hypothesis and the implications for management would be that stocking paddocks with livestock to browse existing alien juveniles and re-growth of felled adults should be effective in delaying invasions of trees used for forestry without significantly affecting the growth of the most abundant native trees.