Species richness change across spatial scales

Humans have elevated global extinction rates and thus lowered global scale species richness. However, there is no a priori reason to expect that losses of global species richness should always, or even often, trickle down to losses of species richness at regional and local scales, even though this relationship is often assumed. Here, we show that scale can modulate our estimates of species richness change through time in the face of anthropogenic pressures, but not in a unidirectional way. Instead, the magnitude of species richness change through time can increase, decrease, reverse, or be unimodal across spatial scales. Using several case studies, we show different forms of scale‐dependent richness change through time in the face of anthropogenic pressures. For example, Central American corals show a homogenization pattern, where small scale richness is largely unchanged through time, while larger scale richness change is highly negative. Alternatively, birds in North America showed a differentiation effect, where species richness was again largely unchanged through time at small scales, but was more positive at larger scales. Finally, we collated data from a heterogeneous set of studies of different taxa measured through time from sites ranging from small plots to entire continents, and found highly variable patterns that nevertheless imply complex scale‐dependence in several taxa. In summary, understanding how biodiversity is changing in the Anthropocene requires an explicit recognition of the influence of spatial scale, and we conclude with some recommendations for how to better incorporate scale into our estimates of change.     

Contrasting patterns of naturalized plant richness in the Americas: Numbers are higher in the North but expected to rise sharply in the South

With increasing availability of plant distribution data, the information about global plant diversity is improving rapidly. Recently, Ulloa Ulloa et al. (2017) presented the first comprehensive overview of the native vascular flora of the Americas, yielding a total count of 124,993 native species. Of these, 51,241 occur in North America and 82,052 in South America. By combining these data with the information in the Global Naturalized Alien Flora (GloNAF) database of naturalized alien floras, we point out that for a complete picture of the regional and continental plant richness, the naturalized alien species need to be considered. Ignoring this novel component of regional floras can lead to an inaccurate picture of overall change in biodiversity in the Anthropocene. We show that North and South America might face contrasting challenges in terms of potential threats to biodiversity posed by alien plant species, because of the different past and present dynamics of invasions and predictions of future development. In total, there are 7,042 naturalized alien plants occurring in the Americas, with 6,122 recorded in North America and 2,677 in South America; if only introductions from other continents are considered additions to the native continental flora make up 6.9 and 1.4 %, respectively. Nevertheless, predictions of naturalized plant trajectories based on global trade dynamics and climate change suggest that considerable increases in naturalized plant numbers are expected in the next 20 years for emerging South American economies, which could reverse the present state.     

Grazing, Environmental Heterogeneity, and Alien Plant Invasions in Temperate Pampa Grasslands

Temperate humid grasslands are known to be particularly vulnerable to invasion by alien plant species when grazed by domestic livestock. The Flooding Pampa grasslands in eastern Argentina represent a well-documented case of a regional flora that has been extensively modified by anthropogenic disturbances and massive invasions over recent centuries. Here, we synthesise evidence from region-wide vegetation surveys and long-term exclosure experiments in the Flooding Pampa to examine the response of exotic and native plant richness to environmental heterogeneity, and to evaluate grazing effects on species composition and diversity at landscape and local community scales. Total plant richness showed a unimodal distribution along a composite stress/fertility gradient ranging several plant community types. On average, more exotic species occurred in intermediate fertility habitats that also contained the highest richness of resident native plants. Exotic plant richness was thus positively correlated with native species richness across a broad range of flood-prone grasslands. The notion that native plant diversity decreases invasibility was supported only for a limited range of species-rich communities in habitats where soil salinity stress and flooding were unimportant. We found that grazing promoted exotic plant invasions and generally enhanced community richness, whereas it reduced the compositional and functional heterogeneity of vegetation at the landscape scale. Hence, grazing effects on plant heterogeneity were scale-dependent. In addition, our results show that environmental fluctuations and physical disturbances such as large floods in the pampas may constrain, rather than encourage, exotic species in grazed grasslands.     

Patterns of plant invasions in China: Taxonomic,biogeographic, climatic approaches and anthropogenic effects

This study was aimed to determine the patterns as well as the effects of biological, anthropogenic, and climatic factors on plant invasions in China. About 270 volumes of national and regional floras were employed to compile a naturalized flora of China. Habit, life form, origin, distribution, and uses of naturalized plants were also analyzed to determine patterns on invasion. Correlations between biological, anthropogenic and climatic parameters were estimated at province and regional scales. Naturalized species represent 1% of the flora of China. Asteraceae, Fabaceae, and Poaceae are the dominant families, but Euphorbiaceae and Cactaceae have the largest ratios of naturalized species to their global numbers. Oenothera, Euphorbia, and Crotalaria were the dominant genera. Around 50% of exotic species were introduced intentionally for medicinal purposes. Most of the naturalized species originated in tropical America, followed by Asia and Europe. Number of naturalized species was significantly correlated to the number of native species/log area. The intensity of plant invasion showed a pattern along climate zones from mesic to xeric, declining with decreasing temperature and precipitation across the nation. Anthropogenic factor, such as distance of transportation, was significantly correlated to plant invasions at a regional scale. Although anthropogenic factors were largely responsible for creating opportunities for exotic species to spread and establish, the local biodiversity and climate factors were the major factors shaping the pattern of plant invasions in China. The warm regions, which are the hot spots of local biodiversity, and relatively developed areas of China, furthermore, require immediate attentions.     

Landscape-scale patterns of biological invasions in shoreline plant communities

Little is known about the patterns and dynamics of exotic species invasions at landscape to regional spatial scales. We quantified the presence (identity, abundance, and richness) and characteristics of native and exotic species in estuarine strandline plant communities at 24 sites in Narragansett Bay, Rhode Island, USA. Our results do not support several fundamental predictions of invasion biology. Established exotics (79 of 147 recorded plant species) were nearly indistinguishable from the native plant species (i.e. in terms of growth form, taxonomic grouping, and patterns of spatial distribution and abundance) and essentially represent a random sub-set of the current regional species pool. The cover and richness of exotic species varied substantially among quadrats and sites but were not strongly related to any site-level physical characteristics thought to affect invasibility (i.e. the physical disturbance regime, legal status, neighboring habitat type, and substrate characteristics). Native and exotic cover or richness were not negatively related within most sites. Across sites, native and exotic richness were positively correlated and exotic cover was unrelated to native richness. The colonization and spread of exotics does not appear to have been substantially reduced at sites with high native diversity. Furthermore, despite the fact that the Rhode Island strandline system is one of the most highly-invaded natural plant communities described to date, exotic species, both individually and as a group, currently appear to pose little threat to native plant diversity. Our findings are concordant with most recent, large-scale investigations that do not support the theoretical foundation of invasion biology and generally contradict small-scale experimental work.     

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

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

The role of plant fidelity and land-use changes on island exotic and indigenous canopy spiders at local and regional scales

Understanding the processes that lead to successful invasions is essential for the management of exotic species. We aimed to assess the comparative relevance of habitat (both at local and at regional scale) and plant features on the species richness of local canopy spiders of both indigenous and exotic species. In an oceanic island, Azores archipelago, we collected spiders in 97 transects belonging to four habitat types according to the degree of habitat disturbance, four types of plants with different colonisation origin (indigenous vs. exotic), and four types of plants according to the complexity of the vegetation structure. Generalised linear mixed models and linear regressions were performed separately for indigenous and exotic species at the local and regional landscape scales. At the local scale, habitat and plant origin explained the variation in the species richness of indigenous spiders, whereas exotic spider richness was poorly correlated to habitat and plant structure. The surrounding landscape matrix substantially affected indigenous spiders, but did not affect exotic spiders, with the exception of the negative effect exerted by native forests on the richness of exotic species. Our results revealed that the local effect of habitat type, plant origin and plant structure explain variations in the species richness observed at a regional scale. These results shed light on the mechanistic processes behind the role of habitat types in invasions, i.e., plant fidelity and plant structure are revealed as key factors, suggesting that native forests may act as physical barriers to the colonisation of exotic spiders.     

Native vegetation structure,landscape features and climate shape non-native plant richness and cover in New Zealand native shrublands

Aim

Studies investigating the determinants of plant invasions rarely examine multiple factors and often only focus on the role played by native plant species richness. By contrast, we explored how vegetation structure, landscape features and climate shape non-native plant invasions across New Zealand in mānuka and kānuka shrublands.

Location

New Zealand.

Method

We based our analysis on 247 permanent 20 × 20-m plots distributed across New Zealand surveyed between 2009 and 2014. We calculated native plant species richness and cumulative cover at ground, understorey and canopy tiers. We examined non-native species richness and mean species ground cover in relation to vegetation structure (native richness and cumulative cover), landscape features (proportion of adjacent anthropogenic land cover, distance to nearest road or river) and climate. We used generalized additive models (GAM) to assess which variables had greatest importance in determining non-native richness and mean ground cover and whether these variables had a similar effect on native species in the ground tier.

Results

A positive relationship between native and non-native plant species richness was not due to their similar responses to the variables examined in this study. Higher native canopy richness resulted in lower non-native richness and mean ground cover, whereas higher native ground richness was associated with higher native canopy richness. Non-native richness and mean ground cover increased with the proportion of adjacent anthropogenic land cover, whereas for native richness and mean ground cover, this relationship was negative. Non-native richness increased in drier areas, while native richness was more influenced by temperature.

Main Conclusions

Adjacent anthropogenic land cover seems to not only facilitate non-native species arrival by being a source of propagules but also aids their establishment as a result of fragmentation. Our results highlight the importance of examining both cover and richness in different vegetation tiers to better understand non-native plant invasions.     

Combining environmental niche models,multi-grain analyses,and species traits identifies pervasive effects of land use on butterfly biodiversity across Italy

Understanding how species respond to human activities is paramount to ecology and conservation science, one outstanding question being how large-scale patterns in land use affect biodiversity. To facilitate answering this question, we propose a novel analytical framework that combines environmental niche models, multi-grain analyses, and species traits. We illustrate the framework capitalizing on the most extensive dataset compiled to date for the butterflies of Italy (106,514 observations for 288 species), assessing how agriculture and urbanization have affected biodiversity of these taxa from landscape to regional scales (3–48 km grains) across the country while accounting for its steep climatic gradients. Multiple lines of evidence suggest pervasive and scale-dependent effects of land use on butterflies in Italy. While land use explained patterns in species richness primarily at grains ≤12 km, idiosyncratic responses in species highlighted “winners” and “losers” across human-dominated regions. Detrimental effects of agriculture and urbanization emerged from landscape (3-km grain) to regional (48-km grain) scales, disproportionally affecting small butterflies and butterflies with a short flight curve. Human activities have therefore reorganized the biogeography of Italian butterflies, filtering out species with poor dispersal capacity and narrow niche breadth not only from local assemblages, but also from regional species pools. These results suggest that global conservation efforts neglecting large-scale patterns in land use risk falling short of their goals, even for taxa typically assumed to persist in small natural areas (e.g., invertebrates). Our study also confirms that consideration of spatial scales will be crucial to implementing effective conservation actions in the Post-2020 Global Biodiversity Framework. In this context, applications of the proposed analytical framework have broad potential to identify which mechanisms underlie biodiversity change at different spatial scales.     

Scaling Disturbance Instead of Richness to Better Understand Anthropogenic Impacts on Biodiversity

A primary impediment to understanding how species diversity and anthropogenic disturbance are related is that both diversity and disturbance can depend on the scales at which they are sampled. While the scale dependence of diversity estimation has received substantial attention, the scale dependence of disturbance estimation has been essentially overlooked. Here, we break from conventional examination of the diversity-disturbance relationship by holding the area over which species richness is estimated constant and instead manipulating the area over which human disturbance is measured. In the boreal forest ecoregion of Alberta, Canada, we test the dependence of species richness on disturbance scale, the scale-dependence of the intermediate disturbance hypothesis, and the consistency of these patterns in native versus exotic species and among human disturbance types. We related field observed species richness in 1 ha surveys of 372 boreal vascular plant communities to remotely sensed measures of human disturbance extent at two survey scales: local (1 ha) and landscape (18 km²). Supporting the intermediate disturbance hypothesis, species richness-disturbance relationships were quadratic at both local and landscape scales of disturbance measurement. This suggests the shape of richness-disturbance relationships is independent of the scale at which disturbance is assessed, despite that local diversity is influenced by disturbance at different scales by different mechanisms, such as direct removal of individuals (local) or indirect alteration of propagule supply (landscape). By contrast, predictions of species richness did depend on scale of disturbance measurement: with high local disturbance richness was double that under high landscape disturbance.     

Broad‐scale patterns in plant diversity vary between land uses in a variegated temperate Australian agricultural landscape

Plant diversity is threatened in many agricultural landscapes. Our understanding of patterns of plant diversity in these landscapes is mainly based on small‐scale (<1000 m²) observations of species richness. However, such observations are insufficient for detecting the spatial heterogeneity of vegetation composition. In a case‐study farm on the North‐West Slopes of New South Wales, Australia, we observed species richness at four scales (quadrat, patch, land use and landscape) across five land uses (grazed and ungrazed woodlands, native pastures, roadsides and crops). We applied two landscape ecological models to assess the contribution of these land uses to landscape species richness: (i) additive partitioning of diversity at multiple spatial scales, and (ii) a measure of habitat specificity – the effective number of species that a patch contributes to landscape species richness. Native pastures had less variation between patches than grazed and ungrazed woodlands, and hence were less species‐rich at the landscape scale, despite having similar richness to woodlands at the quadrat and patch scale. Habitat specificity was significantly higher for ungrazed woodland patches than all other land uses. Our results showed that in this landscape, ungrazed woodland patches had a higher contribution than the grazed land uses to landscape species richness. These results have implications for the conservation management of this landscape, and highlighted the need for greater consensus on the influence of different land uses on landscape patterns of plant diversity.     

Alien and native plants show contrasting responses to climate and land use in Europe

Aim We tested whether the distribution and cover of alien plant species in Europe was related to human disturbance and microclimate. Location Surveys were conducted at 13 sites across Europe, each containing a pair of landscapes with different land‐use intensities. Methods Sampling locations were chosen based on land use and microclimate at two scales: land use was characterized at the patch and landscape scale; climate was expressed as regional and local temperature. The slope of each sample location was derived from a digital elevation model. Cover of plant species was measured using point counts and analysed using mixed effect models. Species were classified as native, archaeophytes and neophytes (pre‐ versus post‐ad 1500 immigrants). Due to the zero inflation observed in the alien groups, their cover was analysed conditional on their presence. Results Anthropogenic disturbance was a significant explanatory variable, increasing the presence and cover of alien species and decreasing the cover of native species. Alien presence was increased in sites under agricultural management, while their cover responded to land use at both local and landscape scales (and to their interaction), such that only natural habitats in semi‐natural landscapes had low alien cover. Microclimate was important for neophytes, with presence concentrated around mesic conditions. Slope was relevant for archaeophytes and native species, suppressing the former group and promoting the latter one. Main conclusions We found that, at the European scale, the distribution of alien plants is related to anthropogenic disturbance more than to microclimatic differences. The presence of neophytes, however, was influenced by climate at local and regional scales, with the highest incidence under mesic conditions. The different patterns observed for the presence and cover of alien species suggest different mechanisms acting during their establishment and spread. They also suggest that to counteract the expansion of alien species natural habitats may need to be maintained at landscape scales.     

Reduced vertebrate diversity independent of spatial scale following feral swine invasions

Biological invasions often have contrasting consequences with reports of invasions decreasing diversity at small scales and facilitating diversity at large scales. Thus, previous literature has concluded that invasions have a fundamental spatial scale‐dependent relationship with diversity. Whether the scale‐dependent effects apply to vertebrate invaders is questionable because studies consistently report that vertebrate invasions produce different outcomes than plant or invertebrate invasions. Namely, vertebrate invasions generally have a larger effect size on species richness and vertebrate invaders commonly cause extinction, whereas extinctions are rare following invertebrate or plant invasions. In an agroecosystem invaded by a non‐native ungulate (i.e., feral swine, Sus scrofa), we monitored species richness of native vertebrates in forest fragments ranging across four orders of magnitude in area. We tested three predictions of the scale‐dependence hypothesis: (a) Vertebrate species richness would positively increase with area, (b) the species richness y‐intercept would be lower when invaded, and (c) the rate of native species accumulation with area would be steeper when invaded. Indeed, native vertebrate richness increased with area and the species richness was 26% lower than should be expected when the invasive ungulate was present. However, there was no evidence that the relationship was scale dependent. Our data indicate the scale‐dependent effect of biological invasions may not apply to vertebrate invasions.     

Processes at multiple scales affect richness and similarity of non‐native plant species in mountains around the world

Aim To investigate how species richness and similarity of non‐native plants varies along gradients of elevation and human disturbance. Location Eight mountain regions on four continents and two oceanic islands. Methods We compared the distribution of non‐native plant species along roads in eight mountainous regions. Within each region, abundance of plant species was recorded at 41–84 sites along elevational gradients using 100‐m² plots located 0, 25 and 75 m from roadsides. We used mixed‐effects models to examine how local variation in species richness and similarity were affected by processes at three scales: among regions (global), along elevational gradients (regional) and with distance from the road (local). We used model selection and information criteria to choose best‐fit models of species richness along elevational gradients. We performed a hierarchical clustering of similarity to investigate human‐related factors and environmental filtering as potential drivers at the global scale. Results Species richness and similarity of non‐native plant species along elevational gradients were strongly influenced by factors operating at scales ranging from 100 m to 1000s of km. Non‐native species richness was highest in the New World regions, reflecting the effects of colonization from Europe. Similarity among regions was low and due mainly to certain Eurasian species, mostly native to temperate Europe, occurring in all New World regions. Elevation and distance from the road explained little of the variation in similarity. The elevational distribution of non‐native species richness varied, but was always greatest in the lower third of the range. In all regions, non‐native species richness declined away from roadsides. In three regions, this decline was steeper at higher elevations, and there was an interaction between distance and elevation. Main conclusions Because non‐native plant species are affected by processes operating at global, regional and local scales, a multi‐scale perspective is needed to understand their patterns of distribution. The processes involved include global dispersal, filtering along elevational gradients and differential establishment with distance from roadsides.     

The roles of biotic resistance and nitrogen deposition in regulating non-native understory plant diversity

Understanding the mechanisms that allow for plant invasions is important for both ecologists and land managers, due to both the environmental and economic impacts of native biodiversity losses. We conducted an observational field study in 2008 to examine the relationship between native and non-native forest understory plant species and to investigate the influence of soil nitrogen (N) on plant community richness and diversity. In 2009, we conducted a companion fertilization experiment to investigate how various forms of N deposition (inorganic and organic) influenced native and non-native species richness and diversity. We found that native species richness and diversity were negatively correlated with 1) non-native species richness and diversity and 2) higher total soil inorganic N. In the deposition experiment, adding organic N fertilizers decreased native richness and diversity compared to inorganic N fertilizers. Together, these results indicate that increasing soil N can be detrimental to native species; however, native species richness and diversity may counteract the N-stimulation of non-native species. Furthermore, the negative effects of organic N deposition on native plants may be just as strong, if not stronger, than the effects of inorganic N deposition.     

Forest bees are replaced in agricultural and urban landscapes by native species with different phenologies and life‐history traits

Anthropogenic landscapes are associated with biodiversity loss and large shifts in species composition and traits. These changes predict the identities of winners and losers of future global change, and also reveal which environmental variables drive a taxon's response to land use change. We explored how the biodiversity of native bee species changes across forested, agricultural, and urban landscapes. We collected bee community data from 36 sites across a 75,000 km² region, and analyzed bee abundance, species richness, composition, and life‐history traits. Season‐long bee abundance and richness were not detectably different between natural and anthropogenic landscapes, but community phenologies differed strongly, with an early spring peak followed by decline in forests, and a more extended summer season in agricultural and urban habitats. Bee community composition differed significantly between all three land use types, as did phylogenetic composition. Anthropogenic land use had negative effects on the persistence of several life‐history strategies, including early spring flight season and brood parasitism, which may indicate adaptation to conditions in forest habitat. Overall, anthropogenic communities are not diminished subsets of contemporary natural communities. Rather, forest species do not persist in anthropogenic habitats, but are replaced by different native species and phylogenetic lineages preadapted to open habitats. Characterizing compositional and functional differences is crucial for understanding land use as a global change driver across large regional scales.     

Stream communities across a rural–urban landscape gradient

Rapid urbanization throughout the world is expected to cause extensive loss of biodiversity in the upcoming decades. Disturbances associated with urbanization frequently operate over multiple spatial scales such that local species extirpations have been attributed both to localized habitat degradation and to regional changes in land use. Urbanization also may shape stream communities by restricting species dispersal within and among stream reaches. In this patch-dynamics view, anthropogenic disturbances and isolation jointly reduce stream biodiversity in urbanizing landscapes. We evaluated predictions of stream invertebrate community composition and abundance based on variation in environmental conditions at five distinct spatial scales: stream habitats, reaches, riparian corridors and watersheds and their spatial location within the larger three-river basin. Despite strong associations between biodiversity loss and human density in this study, local stream habitat and stream reach conditions were poor predictors of community patterns. Instead, local community diversity and abundance were more accurately predicted by riparian vegetation and watershed landscape structure. Spatial coordinates associated with instream distances provided better predictions of stream communities than any of the environmental data sets. Together, results suggest that urbanization in the study region was associated with reduced stream invertebrate diversity through the alteration of landscape vegetation structure and patch connectivity. These findings suggest that maintaining and restoring watershed vegetation corridors in urban landscapes will aid efforts to conserve freshwater biodiversity.     

Filling in the gaps: modelling native species richness and invasions using spatially incomplete data

Detailed knowledge of patterns of native species richness, an important component of biodiversity, and non-native species invasions is often lacking even though this knowledge is essential to conservation efforts. However, we cannot afford to wait for complete information on the distribution and abundance of native and harmful invasive species. Using information from counties well surveyed for plants across the USA, we developed models to fill data gaps in poorly surveyed areas by estimating the density (number of species km⁻²) of native and non-native plant species. Here, we show that native plant species density is non-random, predictable, and is the best predictor of non-native plant species density. We found that eastern agricultural sites and coastal areas are among the most invaded in terms of non-native plant species densities, and that the central USA appears to have the greatest ratio of non-native to native species. These large-scale models could also be applied to smaller spatial scales or other taxa to set priorities for conservation and invasion mitigation, prevention, and control efforts.     

Temporal lags in observed and dark diversity in the Anthropocene

Understanding biodiversity changes in the Anthropocene (e.g. due to climate and land‐use change) is an urgent ecological issue. This important task is challenging because global change effects and species responses are dependent on the spatial scales considered. Furthermore, responses are often not immediate. However, both scale and time delay issues can be tackled when, at each study site, we consider dynamics in both observed and dark diversity. Dark diversity includes those species in the region that can potentially establish and thrive in the local sites’ conditions but are currently locally absent. Effectively, dark diversity connects biodiversity at the study site to the regional scales and defines the site‐specific species pool (observed and dark diversity together). With dark diversity, it is possible to decompose species gains and losses into two space‐related components: one associated with local dynamics (species moving from observed to dark diversity and vice versa) and another related to gains and losses of site‐specific species pool (species moving to and from the pool after regional immigration, regional extinction or change in local ecological conditions). Extinction debt and immigration credit are useful to understand dynamics in observed diversity, but delays might happen in species pool changes as well. In this opinion piece we suggest that considering both observed and dark diversity and their temporal dynamics provides a deeper understanding of biodiversity changes. Considering both observed and dark diversity creates opportunities to improve conservation by allowing to identify species that are likely to go regionally extinct as well as foreseeing which of the species that newly arrive to the region are more likely to colonize local sites. Finally, by considering temporal lags and species gains and losses in observed and dark diversity, we combine phenomena at both spatial and temporal scales, providing a novel tool to examine biodiversity change in the Anthropocene.     

Native versus exotic community patterns across three scales: Roles of competition,environment and incomplete invasion

Three fundamental, interrelated questions in invasion ecology are: (1) to what extent do exotic species outcompete natives; (2) are native and exotic communities functionally similar or different; and (3) are differences in biogeographic patterns in native and exotic communities due to incomplete invasions among exotics? These questions are analogous to general questions in community ecology regarding the relative roles of competition, environmental response and dispersal limitation in community assembly. We addressed each of these questions for plant communities in discrete meadow patches, using analyses at three scales ranging from the landscape to microsites. A weak positive relationship between native and exotic species richness in microsites, and a predominance of positive correlations in abundance among native and exotic species pairs suggest that competition has been less important than other factors in determining native versus exotic abundance and community composition. In contrast, models of species richness and community compositional change across scales suggest native versus exotic community patterns are largely determined by a mix of scale-dependent concordant (shared positive or negative) and discordant relationships with environmental variables. In addition, detailed analyses of species-area and species-abundance relationships suggest ongoing expansion of exotic species populations, indicating that the assembly of the exotic community is in its early stages. Thus, while competition does not appear to strongly affect native versus exotic abundances and compositions at present, it may intensify in the future. Our results indicate that synoptic patterns in native versus exotic richness that have been previously attributed to a single cause may in fact be due to a complex mix of concordant and discordant responses to environmental factors across scales. They also suggest that conservation efforts aimed at promoting natives and reducing exotics should focus on the factors and scales for which such a response (i.e., promotion of high native and low exotic richness) can be expected.