Joint effects of species traits and environmental preferences on range edge shifts of British birds

Aim

Despite the strong evidence of species range shifts as a response to environmental change, attempts to identify species traits that modulate those shifts have been equivocal. We investigate the role of species traits and environmental preferences on birds' range shifts in Great Britain, an island where dispersal is limited by the English Channel and the North Sea.

Location

Great Britain (England, Scotland and Wales).

Taxa

Birds (Aves).

Time Period

1968–2011.

Methods

Using 404,949 occurrence records from two time periods, we investigated the potential drivers of leading and rear range edge shifts of breeding birds using phylogenetic linear mixed models. We hypothesized that shifts are influenced by species' trophic and morphological traits, dispersal abilities and environmental preferences, but also by the geographical boundaries of Great Britain.

Results

Geographical boundaries—the distance from the northern or southern boundaries of Britain—accounted for most of the variability in range edge shifts. Species traits and environmental preferences emerged as relevant drivers of range shifts only for northern and Passeriform species. Northern habitat specialist, those with more predators and those sensitive to precipitation were more likely to shift their rear edge poleward. For Passeriformes, habitat generalists, species with smaller dispersal capabilities, under higher predatory pressure or associated with forest and grassland were more likely to shift their rear edge poleward.

Main Conclusions

While geographical boundaries impose constraints on range shifts in British birds, the subtle effects of species traits and environmental preferences emerge as relevant predictors for Northern and passeriform species' rear edge shifts. This highlights the importance of accounting for geographical boundaries when predicting species responses to global change. Differential range shifts of species across different trophic levels could result in the reorganization of biotic interactions, with consequences for ecosystem structure and stability.     

Plant species loss in an urban area (Turnhout, Belgium) from 1880 to 1999 and its environmental determinants

Information from four archival literature sources from the late 19^th century was matched to present-day plant species distribution data for the region of Turnhout (Belgium) and for 15 smaller sub-regions within this region. In the entire study area 25% of the species recorded in the late 19^th century went extinct during the 20^th century and the extinction rate doubled at the more detailed sub-region level. Binary survival-extinction data and continuous residuals from a linear regression between historical and present-day abundance categories were used to investigate underlying ecological factors of change including habitat preference, ecological amplitude and life strategy. Species increasing relative to the overall trend were generally correlated with nutrient-rich habitats while declining species were more associated with nutrient-poor situations. Generalist species have become relatively more common whilst habitat specialists have strongly declined, resulting in a flora with many ‘losers’ and a few tolerant ‘winners’. The winners are often competitive species while the losers are mainly stress-tolerating species and species with combined life strategies (e.g. SC, SR). Correlations between the decline of historically present habitats and extinction rates of related habitat specialist species show clear trends. We suggest the most important factors involved in changes in flora diversity and vegetation composition are habitat loss due to urbanization and habitat deterioration, mainly due to agricultural intensification.     

Climate warming and land‐use changes drive broad‐scale floristic changes in Southern Sweden

Land‐use changes, pollution and climate warming during the 20th century have caused changes in biodiversity across the world. However, in many cases, the environmental drivers are poorly understood. To identify and rank the drivers currently causing broad‐scale floristic changes in N Europe, we analysed data from two vascular plant surveys of 200 randomly selected 2.5 × 2.5 km grid‐squares in Scania, southernmost Sweden, conducted 1989–2006 and 2008–2015, respectively, and related the change in frequency (performance) of the species to a wide range of species‐specific plant traits. We chose traits representing all plausible drivers of recent floristic changes: climatic change (northern distribution limit, flowering time), land‐use change (light requirement, response to grazing/mowing, response to soil disturbance), drainage (water requirement), acidification (pH optimum), nitrogen deposition and eutrophication (N requirement, N fixation ability, carnivory, parasitism, mycorrhizal associations), pollinator decline (mode of reproduction) and changes in CO₂ levels (photosynthetic pathway). Our results suggest that climate warming and changes in land‐use were the main drivers of changes in the flora during the last decades. Climate warming appeared as the most influential driver, with northern distribution limit explaining 30%–60% of the variance in the GLMM models. However, the relative importance of the drivers differed among habitat types, with grassland species being affected the most by cessation of grazing/mowing and species of ruderal habitats by on‐going concentration of both agriculture and human population to the most productive soils. For wetland species, only pH optimum was significantly related to species performance, possibly an effect of the increasing humification of acidic water bodies. An observed relative decline of mycorrhizal species may possibly be explained by decreasing nitrogen deposition resulting in less competition for phosphorus. We found no effect of shortage or decline of pollinating lepidopterans and bees.     

Habitat preferences and distribution characteristics are indicative of species long-term persistence in the Estonian flora

Large-scale changes in regional floras provide direct information about changes in biodiversity through time and enable the evaluation of conservation targets. We compared the distribution ranges in 2004 of Estonian native terrestrial flora with the distribution ranges before 1970, using the Atlas of Estonian Flora. Relative persistence was related to species endemism, commonness, occurrence at its border of the global distribution range, main habitat type, sensitivity to human impact, life-form, conservation category, and Red List category. A literature-based database of the flora of Estonian habitat types was used to evaluate relative persistence of the flora of different habitats. Changes in the flora are largely dependent on human activities. The decrease in mire and grassland habitats and the increase in forests are reflected in the persistences of related species. Flora of mire habitats decreased the most. The fact that an almost ten-fold decrease of grasslands has not resulted in as large a decrease in the ranges of grassland species could serve as evidence of the extinction debt of these habitats. We also found a greater decrease among habitat specialists than habitat generalists and lower average persistence of the species of species-rich habitats. Our data show that current prioritization of species for conservation is in concordance with needs, as reflected in the changes in the range of species. However, conservation has not been entirely successful: the decrease of protected species continues. Our simple method for summarizing large databases was effective for the evaluation of large scale effects of conservation actions.     

内蒙古昆虫物种多样性分布格局及其机制

物种多样性的地理分布格局及其机制是宏生态学和生物地理学的核心问题之一。区域尺度与局域尺度的影响因素, 如温度、降水、海拔变化、生境过滤、捕食、竞争与互惠等, 共同影响昆虫物种多样性的分布格局。然而, 迄今为止少有研究同时讨论不同尺度驱动因子对昆虫多样性地理分布格局的影响。本文基于内蒙古自治区86个旗县的昆虫多样性数据, 结合各地年平均气温、年降水量、古气候变化、海拔变化及植物多样性, 探讨昆虫物种多样性分布格局及其主要驱动因子。结果发现内蒙古昆虫多样性主要受到植物多样性与海拔变化的影响, 而气候因子对昆虫物种多样性的影响并不大。这一结果表明种间关系(食物多样性)与生境异质性可能对内蒙古昆虫多样性的分布格局起着主导作用。     

Wildlife Recovery During Tropical Forest Succession: Assessing Ecological Drivers of Community Change

Despite the high proportion of secondary forests in the tropics, their conservation value remains poorly understood, particularly with regard to animals. Most theoretical studies of succession have focused on plants, linking life history trade‐offs to well‐known patterns of community change. However, the same trade‐offs proposed for plants should apply to animals, and indeed, animal studies show a change in community dominance from habitat generalist to forest specialist species during succession. Focusing on the diverse terrestrial small mammals of the endangered Atlantic Forest, we assessed which ecological drivers (habitat structure and food availability) affect community changes during succession. If the change in community dominance is driven by trade‐offs between productivity and efficiency, it should be mainly associated with a decrease in food availability. As expected, from younger to older forest, habitat generalists decreased in richness and total abundance, concurrent with a decrease in arthropod biomass. By contrast, the increase in richness and total abundance of forest specialists was not clearly supported by the data; however, this group was not affected by food availability. These results are congruent with a trade‐off between competitive ability and ability to use abundant resources, and indicate that the major community change during succession involves habitat generalists. Secondary forests may thus be valuable for conservation, at least where habitat loss and fragmentation are not high, and old growth forest is available.     

Guild‐specific shifts in visitation rates of frugivores with habitat loss and plant invasion

Habitat loss and plant invasions are two major drivers of global change in subtropical and tropical ecosystems. Both lead to a loss of biodiversity and alter species interactions, which may imperil vital ecosystem processes such as seed dispersal by frugivores. Reponses of frugivores to disturbance are often linked to their specialization on certain habitats or resources. Yet, it is poorly understood how habitat loss and plant invasion structure interactions between plants and different habitat or feeding guilds. Here we investigated whether visitation rates of frugivores change guild‐specifically with increasing habitat loss and invasion level in a heterogeneous subtropical landscape. In 756 h of observations, we recorded 1446 plant–frugivore interactions among 18 plant species and 42 avian frugivore species. Visitation rates of forest specialists decreased with increasing habitat loss, but not with changes in invasion level. In contrast forest generalists and forest visitors were unaffected by either driver. Similarly, obligate frugivores that overall showed a generalized fruit choice were unaffected by habitat loss and changes in invasion level. Contrary, visitation rates of specialized partial and opportunistic frugivores decreased with higher invasion level. Importantly, the negative effect of plant invasion on partial frugivores was more pronounced as habitat loss in the same study site increased, indicating a synergistic effect of the two drivers. The implications of our study are twofold: first, frugivores respond guild‐specifically to habitat loss and plant invasion. Thereby forest dependency is mainly related to habitat loss, and degree of frugivory mainly related to plant invasion. Forest generalists and obligate frugivores in turn may play a key‐role for forest regeneration in disturbed forest landscapes. Second, particularly frugivores with a specialized fruit choice may be threatened by synergistic effects between habitat loss and plant invasion.     

The use of fluctuating asymmetry as a measure of farming practice effects in rodents: A species-specific response

The effects of agricultural intensification on vertebrate populations could vary depending on whether species are habitat specialists or habitat generalists. Organic farming practices are generally considered to be less intensive and more environmental friendly than conventional farming practices and, as a result, these two managements may impact on habitat specialists and habitat generalists in different ways. The effect of environmental and/or genetic stress on populations can be assessed using fluctuating asymmetry (FA) and body condition of animals. We predicted that populations of a specialist species, the Pampean grassland mouse (Akodon azarae) would have higher levels of FA and poorer body condition on conventional farms compared to populations of A. azarae on organic farms. In contrast, we predicted that populations of generalist species, the corn mouse (Calomys musculinus) and the small vesper mouse (Calomys laucha) would not show differences in FA or body condition between conventional and organic farms. We examined the expression of FA in the hind foot and used the scaled mass index as a surrogate for body condition. As predicted, we found higher FA in the habitat specialist (A. azarae) on conventional farms compared to organic farms, and found no differences in FA among the two generalist species (C. musculinus and C. laucha). However, we found no differences in body condition for the three studied species between the two managements. Our results suggest that the effect of farming practices on small mammals varies between habitat specialists and habitat generalists. The results of this study provide important insights for the study of asymmetries, both from biological and methodological perspectives. Our results support the idea that the level of FA may be used as an index to assess the effects of farming practices on vertebrate populations.     

Post‐glacial migration lag restricts range filling of plants in the European Alps

Aim To evaluate the effect of post‐glacial migration lags on the current distribution of Alpine plants and the factors responsible for possible range‐filling differences among species. Location Austrian Alps. Methods We used species distribution models to predict environmentally suitable sites for 183 Alpine plants at a fine spatial resolution (100 × 100 m²). We overlaid these predictions with independent mapping data (3′× 5′) and calculated the extent to which species fill their potential ranges at this coarser grain based on several different approaches. Moreover, we correlated range‐filling estimates with the magnitude of improvement of distribution models when using the distance to putative glacial refugia as an additional independent variable. Finally, we compared species‐specific range‐filling estimates with traits related to dispersal capacity and competitive ability of these species as well as with characteristics of their habitats. Results Even under a conservative approach, incomplete range filling appears common, with 46% and 31% of the species studied occurring in less than 75% and 50% of their predicted suitable ranges, respectively. Proximity to glacial refugia generally accounts for a lower percentage of the deviance in species distribution data (0–20%, mean 4%) than environmental variables (9–57%, mean 27%). However, its importance correlates closely and negatively with the calculated range‐filling estimates. Range filling significantly increases with the dispersal capacity of a species' propagules and the breadth of its altitudinal niche. Calcicolous species have lower range filling than silicicolous plants and substrate generalists. Conclusions Our results suggest that the current ranges of many Alpine plants are still shaped by delayed Holocene recolonization of suitable sites. Hence, long‐term migration lags also affect plant distribution in mountainous areas, at least on regional scales. These findings question whether high mountain floras will be able to track the expected rapid, climate change driven shifts in habitat.     

A meta-analysis of the effects of global environmental change on plant-herbivore interactions

Global environmental changes are hypothesized to affect herbivores indirectly via changes in plant defenses, and many studies have been conducted to explore effects of environmental change on plant chemistry and herbivory. We quantitatively synthesized data from these studies to produce generalities about the effects of a broad array of environmental changes on herbivores. Since conversion of natural habitat to agriculture has been one of the most profound environmental changes over the past century, the effects of global change variables on plant defense were also compared between natural and agricultural systems. We found evidence that increasing CO₂, light availability, and nutrients all consistently increase herbivory, particularly by generalists. No significant differences in chemistry and herbivory response variables were found between natural and managed systems. Overall, these results are consistent with recent predictions of a disruption of natural trophic interactions with global change.     

Two Species with an Unusual Combination of Traits Dominate Responses of British Grasshoppers and Crickets to Environmental Change

There are large variations in the responses of species to the environmental changes of recent decades, heightening interest in whether their traits may explain inter-specific differences in range expansions and contractions. Using a long-term distributional dataset, we calculated range changes of grasshoppers and crickets in Britain between the 1980s and the 2000s and assessed whether their traits (resource use, life history, dispersal ability, geographic location) explain relative performance of different species. Our analysis showed large changes in the distributions of some species, and we found a positive relationship between three traits and range change: ranges tended to increase for habitat generalists, species that oviposit in the vegetation above ground, and for those with a southerly distribution. These findings accord well with the nature of environmental changes over this period (climatic warming; reductions in the diversity and increases in the height of vegetation). However, the trait effects applied mainly to just two species, Conocephalus discolor and Metrioptera roeselii, which had shown the greatest range increases. Once they were omitted from the analysis, trait effects were no longer statistically significant. Previous studies on these two species emphasised wing-length dimorphism as the key to their success, resulting in a high phenotypic plasticity of dispersal and evolutionary-ecological feedback at their expanding range margins. This, combined with our results, suggests that an unusual combination of traits have enabled these two species to undertake extremely rapid responses to recent environmental changes. The fact that our results are dominated by two species only became apparent through cautious testing of the results’ robustness, not through standard statistical checks. We conclude that trait-based analyses may contribute to the assessment of species responses to environmental change and provide insights into underlying mechanisms, but results need to be interpreted with caution and may have limited predictive power.     

Retaining biodiversity in intensive farmland: epiphyte removal in oil palm plantations does not affect yield

The expansion of agriculture into tropical forest frontiers is one of the primary drivers of the global extinction crisis, resulting in calls to intensify tropical agriculture to reduce demand for more forest land and thus spare land for nature. Intensification is likely to reduce habitat complexity, with profound consequences for biodiversity within agricultural landscapes. Understanding which features of habitat complexity are essential for maintaining biodiversity and associated ecosystem services within agricultural landscapes without compromising productivity is therefore key to limiting the environmental damage associated with producing food intensively. Here, we focus on oil palm, a rapidly expanding crop in the tropics and subject to frequent calls for increased intensification. One promoted strategy is to remove epiphytes that cover the trunks of oil palms, and we ask whether this treatment affects either biodiversity or yield. We experimentally tested this by removing epiphytes from four‐hectare plots and seeing if the biodiversity and production of fruit bunches 2 months and 16 months later differed from equivalent control plots where epiphytes were left uncut. We found a species‐rich and taxonomically diverse epiphyte community of 58 species from 31 families. Epiphyte removal did not affect the production of fresh fruit bunches, or the species richness and community composition of birds and ants, although the impact on other components of biodiversity remains unknown. We conclude that as they do not adversely affect palm oil production, the diverse epiphyte flora should be left uncut. Our results underscore the importance of experimentally determining the effects of habitat complexity on yield before introducing intensive methods with no discernible benefits.     

Factors affecting plant species richness in field boundaries in the Mediterranean region

Field boundaries are expected to support the maintenance of biodiversity in agroecosystems, since they provide the habitat for a range of plant species. However, plant diversity in field boundaries has decreased substantially in recent decades. This pattern is generally linked with the intensification of agricultural land use at field and landscape level. Therefore, we aimed to test the effect of farming management (field and boundary management), boundary structure (width and habitat assemblage considering the Mediterranean grassland element), and landscape heterogeneity on plant species richness of field boundaries. Plants were recorded along 30 field boundaries next to organic fields and 30 next to conventional fields located in 15 agrarian localities of NE Iberian Peninsula along a gradient of landscape complexity. A total of 517 plant species were identified in the 60 field boundaries. We recorded 162 species (31%) catalogued as rare, very rare or extremely rare in the flora of the Catalan Countries. Our results showed the importance of landscape heterogeneity, field management and habitat assemblage, since they were found to be the most influential variables for plant species richness; whereas boundary width and boundary management were seen to contribute less to explaining plant diversity. Accordingly, agri-environmental schemes should be designed to promote organic farming and maintain the structure of the landscape mosaic in order to benefit plant diversity in field boundaries in the Mediterranean region.     

Coexistence of Habitat Specialists and Generalists in Metapopulation Models of Multiple-Habitat Landscapes

In coarse-grained environments specialists are generally predicted to dominate. Empirically, however, coexistence with generalists is often observed. We present a simple, but previously unrecognized, mechanism for coexistence of a habitat generalist and a number of habitat specialist species. In our model all species have a metapopulation structure in a landscape consisting of patches of different habitat types, governed by local extinction and colonization. Each specialist is limited to its specific type of habitat. The generalist can use more types of habitat, has a lower local competitive ability but can exploit patches left open by the specialists. Our modeling shows that coexistence is easily possible. The mechanism amounts to a colonization/competition trade-off at the landscape level, where the colonization advantage of the inferior competitor does not arise from a higher colonization rate but from its ability to use more types of habitat. Habitat availability has to be intermediate: when there are few patches of each habitat, only the generalist is able to maintain itself and when there are many patches, high propagule pressure of the specialists excludes the generalist. Habitat selection or temporal variations in relative habitat quality are not necessary for coexistence. Increased niche-width, colonization rate or local competitive ability of the generalist enhances its performance compared to the specialists. Various types of habitat degradation favour generalism. When able to use a broad range of habitats, generalists can generate so much propagule pressure that only a low level of local competitive ability is needed to globally exclude the specialists. Hence, in a reversal of the original problem, the question is why there are so many specialist metapopulations?     

Introduced plants as novel Anthropocene habitats for insects

Major environmental changes in the history of life on Earth have given rise to novel habitats, which gradually accumulate species. Human‐induced change is no exception, yet the rules governing species accumulation in anthropogenic habitats are not fully developed. Here we propose that nonnative plants introduced to Great Britain may function as analogues of novel anthropogenic habitats for insects and mites, analysing a combination of local‐scale experimental plot data and geographic‐scale data contained within the Great Britain Database of Insects and their Food Plants. We find that novel plant habitats accumulate the greatest diversity of insect taxa when they are widespread and show some resemblance to plant habitats which have been present historically (based on the relatedness between native and nonnative plant species), with insect generalists colonizing from a wider range of sources. Despite reduced per‐plant diversity, nonnative plants can support distinctive insect communities, sometimes including insect taxa that are otherwise rare or absent. Thus, novel plant habitats may contribute to, and potentially maintain, broader‐scale (assemblage) diversity in regions that contain mixtures of long‐standing and novel plant habitats.     

Disentangling drivers of spatial autocorrelation in species distribution models

Species distribution models (SDMs) are frequently used to understand the influence of site properties on species occurrence. For robust model inference, SDMs need to account for the spatial autocorrelation of virtually all species occurrence data. Current methods do not routinely distinguish between extrinsic and intrinsic drivers of spatial autocorrelation, although these may have different implications for conservation. Here, we present and test a method that disentangles extrinsic and intrinsic drivers of spatial autocorrelation using repeated observations of a species. We focus on unknown habitat characteristics and conspecific interactions as extrinsic and intrinsic drivers, respectively. We model the former with spatially correlated random effects and the latter with an autocovariate, such that the spatially correlated random effects are constant across the repeated observations whereas the autocovariate may change. We tested the performance of our model on virtual species data and applied it to observations of the corncrake Crex crex in the Netherlands. Applying our model to virtual species data revealed that it was well able to distinguish between the two different drivers of spatial autocorrelation, outperforming models with no or a single component for spatial autocorrelation. This finding was independent of the direction of the conspecific interactions (i.e. conspecific attraction versus competitive exclusion). The simulations confirmed that the ability of our model to disentangle both drivers of autocorrelation depends on repeated observations. In the case study, we discovered that the corncrake has a stronger response to habitat characteristics compared to a model that did not include spatially correlated random effects, whereas conspecific interactions appeared to be less important. This implies that future conservation efforts should primarily focus on maximizing habitat availability. Our study shows how to systematically disentangle extrinsic and intrinsic drivers of spatial autocorrelation. The method we propose can help to correctly identify the main drivers of species distributions.     

Biogeography of the Primates of Guyana: Effects of Habitat Use and Diet on Geographic Distribution

Species that exploit a wide range of resources or habitats (generalists) tend to be widely distributed, whereas species that exploit a narrow range of resources or habitats (specialists) often have a limited distribution. The distribution patterns are thought to result from specialists using relatively smaller habitats than those exploited by generalists. I used data from 1,725 km of primate surveys that I conducted in Guyana to test these hypotheses. Habitat breadth is the total number of different habitat types occupied by each species. I used the total number of different food categories exploited by each species to measure dietary breadth. Geographic range size is correlated with habitat breadth but not with dietary breadth or body size for the 8 primate species in Guyana. Habitat generalists—red howlers and wedge-capped capuchins—range into all habitats. Habitat specialists—spider monkeys, brown bearded sakis, and golden-handed tamarins—range only into large habitats. Habitat generalists tend to be dietary type specialists in Guyana. I suggest that only habitat generalists can subsist on the low-quality foods in small habitats in Guyana. Conversely, habitat specialists tend to be dietary type generalists in Guyana. They must feed on a variety of food types in large habitats. However, using the number of food categories exploited as a measure of dietary breadth may be only a weak aspect of multidimensional niche. Researchers testing biogeographic hypotheses associated with dietary breadth should consider including multivariate indicators of both the types of food categories eaten and the number of plant species exploited.     

Scavenger community structure along an environmental gradient from boreal forest to alpine tundra in Scandinavia

Scavengers can have strong impacts on food webs, and awareness of their role in ecosystems has increased during the last decades. In our study, we used baited camera traps to quantify the structure of the winter scavenger community in central Scandinavia across a forest–alpine continuum and assess how climatic conditions affected spatial patterns of species occurrences at baits. Canonical correspondence analysis revealed that the main habitat type (forest or alpine tundra) and snow depth was main determinants of the community structure. According to a joint species distribution model within the HMSC framework, species richness tended to be higher in forest than in alpine tundra habitat, but was only weakly associated with temperature and snow depth. However, we observed stronger and more diverse impacts of these covariates on individual species. Occurrence at baits by habitat generalists (red fox, golden eagle, and common raven) typically increased at low temperatures and high snow depth, probably due to increased energetic demands and lower abundance of natural prey in harsh winter conditions. On the contrary, occurrence at baits by forest specialists (e.g., Eurasian jay) tended to decrease in deep snow, which is possibly a consequence of reduced bait detectability and accessibility. In general, the influence of environmental covariates on species richness and occurrence at baits was lower in alpine tundra than in forests, and habitat generalists dominated the scavenger communities in both forest and alpine tundra. Following forecasted climate change, altered environmental conditions are likely to cause range expansion of boreal species and range contraction of typical alpine species such as the arctic fox. Our results suggest that altered snow conditions will possibly be a main driver of changes in species community structure.     

Plant invasions on small Mediterranean islands: An overview

Biological invasions have become one of the main drivers of habitat degradation and a leading cause of biodiversity loss in island ecosystems worldwide. The spread of invasive species poses a particular environmental threat on the islands of the Mediterranean Basin, which are hot spots of biodiversity and contain rare habitats and endemic species, especially on small islands, which are highly vulnerable to biodiversity loss. Following a recent survey, in this paper we aim to provide an overview of the present-day non-native vascular flora of small Mediterranean islands based on a sample of 37 islands located in the middle of the Mediterranean Sea, off the coast of Italy. By comparing the current data with those gathered during a previous survey conducted in the same study area, we also aim to highlight the main changes that have occurred in non-native plant species diversity, establishment and distribution in recent years and to present a first general overview of the most prominent plant taxa in the island’s introduced flora, focusing on those most responsible for these changes and those that pose the greatest environmental threats. We recorded 203 non-native plant species, 147 of which have established on at least one of the islands investigated. Overall, we detected a sharp increase in the number of species, in their levels of establishment and in the extent of their distribution within the study area in recent years. This may be explained by the intensification of research on plant invasions, as well as to new introduction, escape, establishment and invasion events on the islands in recent decades. The most remarkable plants detected include acacias and succulents, two groups that appear to be emerging very rapidly and to be posing new threats to the conservation of the islands’ natural environment, especially the genus Carpobrotus, whose spread into natural habitats containing rare and endemic taxa is seriously threatening biodiversity on both a local and global scale. On the whole, our results show that the plant invasion phenomenon in the study area has in recent years intensified considerably. As this process seems likely to continue, we should expect more establishment events in the future and the further spread of species that are already present. This is of particular conservation concern on the islands investigated in this survey, which are rich in endemisms, but have been facing deep socio-economic and environmental transformations in these last decades as a consequence of the abandonment of traditional management practices and the development of tourism. Our study thus confirms that plant invasions on Mediterranean islands are a serious environmental problem that threatens biodiversity conservation not only in the Mediterranean biogeographic region, but also on the global scale, and highlights the need to further increase efforts aimed at preventing, controlling or mitigating the effects of plant invasions in island ecosystems.     

Influence of past land use and current human disturbance on non-native plant species on small Italian islands

Biological invasions are regarded as one of the main drivers of habitat degradation in island ecosystems. Mediterranean islands have been subjected to a high degree of land conversion over the past 60 years, resulting in a massive reduction in the amount of rural land and the sprawl of tourist activities. The aims of this paper are to evaluate the current level of invasion of alien plant species in semi-natural vegetation types that have developed after the abandonment of agriculture and to analyze the relationships between non-native species, native flora, and environmental characteristics. Two Italian islands (Ponza and Ventotene) were surveyed using a random-stratified sampling. The occurrence and relative cover of alien plant species were compared and separate analyses were performed for the native flora. Abundance patterns of both native and alien species were then studied in the light of the environmental and anthropogenic features. Although we found that some non-native species are extremely widespread, their relative cover at the plot level is low. Permutational Multivariate Analysis of Variance and Indicator Species Analysis revealed dissimilarities in the native species composition, while Mann–Whitney and Kruskal–Wallis tests showed differences in the ecological requirements (moisture, soil reaction, and nitrogen) of the native species pool. Canonical Correspondence Analysis pointed to the importance of the proximity to agricultural areas, human disturbance, and past land management, particularly residual terraces, in determining the difference between plant communities on the two islands. The results of our study suggest that traditional forms of agriculture may represent a key element for countering the establishment and spread of non-native plants in Mediterranean areas.