Cang Hui

Impacts of alien plant invasions on species richness in Mediterranean-type ecosystems: a meta-analysis

Besides a general consensus regarding the negative impact of invasive alien species in the literature, only recently has the decline of native species attributable to biological invasions begun to be quantified in many parts of the world. The cause-effect relationship between the establishment and proliferation of alien species and the extinction of native species is, however, seldom demonstrated. We conducted a meta-analysis of studies in Mediterranean-type ecosystems (MTEs) to examine: (1) whether invasion of alien plant species indeed causes a reduction in the number of native plant species at different spatial and temporal scales; (2) which growth forms, habitat types and areas are most affected by invasions; and (3) which taxa are most responsible for native species richness declines. Our results confirm a significant decline in native species richness attributable to alien invasions. Studies conducted at small scales or sampled over long periods reveal stronger impacts of alien invasion than those at large spatial scales and over short periods. Alien species from regions with similar climates have much stronger impacts, with the native species richness in South Africa and Australia declining significantly more post-invasion than for European sites. Australian Acacia species in South Africa accounted for the most significant declines in native species richness. Among the different growth forms of alien plants, annual herbs, trees and creepers had the greatest impact, whereas graminoids generally caused insignificant changes to the native community. Native species richness of shrublands, old fields and dune vegetation showed significant declines, in contrast to insignificant declines for forest habitats.

Relative roles of climatic suitability and anthropogenic influence in determining the pattern of spread in a global invader

Because invasive species threaten the integrity of natural ecosystems, a major goal in ecology is to develop predictive models to determine which species may become widespread and where they may invade. Indeed, considerable progress has been made in understanding the factors that influence the local pattern of spread for specific invaders and the factors that are correlated with the number of introduced species that have become established in a given region. However, few studies have examined the relative importance of multiple drivers of invasion success for widespread species at global scales. Here, we use a dataset of >5,000 presence/absence records to examine the interplay between climatic suitability, biotic resistance by native taxa, human-aided dispersal, and human modification of habitats, in shaping the distribution of one of the worlds most notorious invasive species, the Argentine ant (Linepithema humile). Climatic suitability and the extent of human modification of habitats are primarily responsible for the distribution of this global invader. However, we also found some evidence for biotic resistance by native communities. Somewhat surprisingly, and despite the often cited importance of propagule pressure as a crucial driver of invasions, metrics of the magnitude of international traded commodities among countries were not related to global distribution patterns. Together, our analyses on the global-scale distribution of this invasive species provide strong evidence for the interplay of biotic and abiotic determinants of spread and also highlight the challenges of limiting the spread and subsequent impact of highly invasive species.

The harlequin ladybird, Harmonia axyridis: global perspectives on invasion history and ecology

The harlequin ladybird, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), is native to Asia but has been intentionally introduced to many countries as a biological control agent of pest insects. In numerous countries, however, it has been introduced unintentionally. The dramatic spread of H. axyridis within many countries has been met with considerable trepidation. It is a generalist top predator, able to thrive in many habitats and across wide climatic conditions. It poses a threat to biodiversity, particularly aphidophagous insects, through competition and predation, and in many countries adverse effects have been reported on other species, particularly coccinellids. However, the patterns are not consistent around the world and seem to be affected by many factors including landscape and climate. Research on H. axyridis has provided detailed insights into invasion biology from broad patterns and processes to approaches in surveillance and monitoring. An impressive number of studies on this alien species have provided mechanistic evidence alongside models explaining large-scale patterns and processes. The involvement of citizens in monitoring this species in a number of countries around the world is inspiring and has provided data on scales that would be otherwise unachievable. Harmonia axyridis has successfully been used as a model invasive alien species and has been the inspiration for global collaborations at various scales. There is considerable scope to expand the research and associated collaborations, particularly to increase the breadth of parallel studies conducted in the native and invaded regions. Indeed a qualitative comparison of biological traits across the native and invaded range suggests that there are differences which ultimately could influence the population dynamics of this invader. Here we provide an overview of the invasion history and ecology of H. axyridis globally with consideration of future research perspectives. We reflect broadly on the contributions of such research to our understanding of invasion biology while also informing policy and people.

The Impact of Land Abandonment on Species Richness and Abundance in the Mediterranean Basin: A Meta-Analysis

Land abandonment is common in the Mediterranean Basin, a global biodiversity hotspot, but little is known about its impacts on biodiversity. To upscale existing case-study insights to the Pan-Mediterranean level, we conducted a meta-analysis of the effects of land abandonment on plant and animal species richness and abundance in agroforestry, arable land, pastures, and permanent crops of the Mediterranean Basin. In particular, we investigated (1) which taxonomic groups (arthropods, birds, lichen, vascular plants) are more affected by land abandonment; (2) at which spatial and temporal scales the effect of land abandonment on species richness and abundance is pronounced; (3) whether previous land use and current protected area status affect the magnitude of changes in the number and abundance of species; and (4) how prevailing landforms and climate modify the impacts of land abandonment. After identifying 1240 potential studies, 154 cases from 51 studies that offered comparisons of species richness and abundance and had results relevant to our four areas of investigation were selected for meta-analysis. Results are that land abandonment showed slightly increased (effect size  = 0.2109, P<0.0001) plant and animal species richness and abundance overall, though results were heterogeneous, with differences in effect size between taxa, spatial-temporal scales, land uses, landforms, and climate. In conclusion, there is no “one-size-fits-all” conservation approach that applies to the diverse contexts of land abandonment in the Mediterranean Basin. Instead, conservation policies should strive to increase awareness of this heterogeneity and the potential trade-offs after abandonment. The strong role of factors at the farm and landscape scales that was revealed by the analysis indicates that purposeful management at these scales can have a powerful impact on biodiversity.

An interaction switch predicts the nested architecture of mutualistic networks

Nested architecture is distinctive in plant-animal mutualistic networks. However, to date an integrative and quantitative explanation has been lacking. It is evident that species often switch their interactive partners in real-world mutualistic networks such as pollination and seed-dispersal networks. By incorporating an interaction switch into a novel multi-population model, we show that the nested architecture rapidly emerges from an initially random network. The model allowing interaction switches between partner species produced predictions which fit remarkably well with observations from 81 empirical networks. Thus, the nested architecture in mutualistic networks could be an intrinsic physical structure of dynamic networks and the interaction switch is likely a key ecological process that results in nestedness of real-world networks. Identifying the biological processes responsible for network structures is thus crucial for understanding the architecture of ecological networks.

Tree invasions: patterns, processes, challenges and opportunities

Tree invasions have escalated in importance in the last few decades (more species, greater area invaded, more types of impacts, increasing complexity of management challenges), and are increasingly studied from many perspectives. This research spans many disciplines, including ecology, population biology, genetics, remote sensing, ecological modelling, risk analysis, resource economics and, increasingly, the humanities. There has been substantial progress in understanding patterns and processes, but many unanswered questions remain. Only a few invasive trees have been well studied, many of them in only a small part of their invasive range. Invasive trees often have substantial impacts, especially when they invade formerly treeless vegetation. Trees have several features that make them useful for understanding key aspects of biological invasions (the determinants of invasiveness and invasibility), but also the full spectrum of human perceptions and values that frames biological invasions as an environmental problem. This editorial provides background and summarizes the main outputs from a workshop held in Argentina in September 2012 that set out to summarize current knowledge on key topics and to determine the most important challenges facing researchers and managers. The sixteen papers in the special issue of Biological Invasions span disciplines, geographic regions and taxa and provide novel insights on pathways and historical perspectives, detection and monitoring, determinants of invasiveness, function and impact, and the many challenges that face managers.

Dynamical complexity and metapopulation persistence

Factors leading to chaotic dynamics in metapopulation are studied, including Allee effect, rescue effect and overcrowding effect. We use theoretical bifurcation diagram and lattice simulation to investigate the dynamics of metapopulation in time and space. The influence of habitat destruction on the complexity of dynamics, which can decrease the fraction of suitable patches and induce the change of colonization rate and extinction rate, is also discussed. The whole scene of the implicit relationship between dynamical complexity and metapopulation persistence is given, which has five primary results. First, metapopulation persistence reaches the maximum at moderate dynamical complexity, which can occur at moderate intensity of habitat destruction. Second, the intensity of habitat destruction can be approximated by the dynamical complexity because the habitat destruction regulates the dynamics. Third, Allee effects can amazingly regulate dynamics and improve the persistence slightly before extinction. Fourth, rescue effects stabilize dynamics and improve persistence. Finally, overcrowding effect is the key to incur chaos and increase the dynamical complexity, and to improve the persistence of metapopulations. Anyway, there is no uniform correlation between dynamical complexity and metapopulation persistence.

Extrapolating population size from the occupancy-abundance relationship and the scaling pattern of occupancy

The estimation of species abundances at regional scales requires a cost-efficient method that can be applied to existing broadscale data. We compared the performance of eight models for estimating species abundance and community structure from presence-absence maps of the southern African avifauna. Six models were based on the intraspecific occupancy-abundance relationship (OAR); the other two on the scaling pattern of species occupancy (SPO), which quantifies the decline in species range size when measured across progressively finer scales. The performance of these models was examined using five tests: the first three compared the predicted community structure against well-documented macroecological patterns; the final two compared published abundance estimates for rare species and the total regional abundance estimate against predicted abundances. Approximately two billion birds were estimated as occurring in South Africa, Lesotho, and Swaziland. SPO models outperformed the OAR models, due to OAR models assuming environmental homogeneity and yielding scale-dependent estimates. Therefore, OAR models should only be applied across small, homogenous areas. By contrast, SPO models are suitable for data at larger spatial scales because they are based on the scale dependence of species range size and incorporate environmental heterogeneity (assuming fractal habitat structure or performing a Bayesian estimate of occupancy). Therefore, SPO models are recommended for assemblage-scale regional abundance estimation based on spatially explicit presence-absence data.

Long-distance dispersal maximizes evolutionary potential during rapid geographic range expansion

Conventional wisdom predicts that sequential founder events will cause genetic diversity to erode in species with expanding geographic ranges, limiting evolutionary potential at the range margin. Here, we show that invasive European starlings (Sturnus vulgaris) in South Africa preserve genetic diversity during range expansion, possibly as a result of frequent long‐distance dispersal events. We further show that unfavourable environmental conditions trigger enhanced dispersal, as indicated by signatures of selection detected across the expanding range. This brings genetic variation to the expansion front, counterbalancing the cumulative effects of sequential founding events and optimizing standing genetic diversity and thus evolutionary potential at range margins during spread. Therefore, dispersal strategies should be highlighted as key determinants of the ecological and evolutionary performances of species in novel environments and in response to global environmental change.

Distribution patterns of metapopulation determined by Allee effects

The Allee effect, the social dysfunction and failure to mate successfully when population density falls below a certain threshold, is one of the most important phenomena in ecology that can profoundly affect metapopulation persistence. We have developed a continuous dynamic model by pair approximation and two derived spatial lattice models to describe the influences of Allee effects on the distribution and dynamics of metapopulation. Analytical results of pair approximation show that the initial global stable equilibrium of metapopulation size turns into a local stable equilibrium with Allee effects and sensitivity to the initial situations that can incur a threshold phenomenon in dynamics. When the intensity of the Allee effect varies within a certain range, a new positive local stable equilibrium appears. This new equilibrium has the same local intensity as the initial one and a smaller metapopulation size. However, a metapopulation with a too strong Allee effect is doomed. Simulation results from the lattice models reinforce these findings and show that the new equilibrium forms a static distribution border in space. Hence, an Allee effect with moderate intensity can incur three distribution patterns that are sensitive to the initial metapopulation size and the spatial configuration of local populations: aggregation, circumscription and extinction. The pattern of circumscription may be a new explanation for the species’ current distributional range. The relationships between distribution patterns (such as random, uniform, aggregation, circumscription and extinction) and other factors (such as mean-field assumption, local interaction, demographic stochasticity and Allee effect) are also discussed.