Beta diversity of urban floras among European and non-European cities.

Aim Cities represent an ideal study system for assessing how intensive land-use change and biotic interchange have altered beta diversity at broad geographic extents. Here we test the hypothesis that floras in cities located in disparate regions of the globe are being homogenized by species classified as invasive (naturalized species that have spread over a large area) or as a European archaeophyte (species introduced into Europe before ad 1500 from the Mediterranean Basin).We also test the prediction that the global influences of European activities (colonization, agriculture, commerce) have supported this outcome. Location One hundred and ten cities world-wide. Methods We examined the richness and composition of urban floras among European (n = 85) and non-European cities (n = 25) for species classified as native or non-native, or further classified as European archaeophyte or invasive. We modelled how geographic, climatic and anthropogenic factors were related to compositional similarity between European and non-European cities. Results We found that most plants in the cities we examined, particularly non- European cities, were native and unique to each city. Non-native species were similarly unique, but occurred in much lower proportions relative to natives. Although European archaeophytes and invasive species also occurred in lower proportions, they had similar compositions among cities. European archaeophytes were most prevalent in European cities, but were most similar among non-European cities. Contrasting European and non-European cities, geography and climate were most relevant for native and invasive species, whereas climate and agriculture were most relevant for European archaeophytes. Main conclusions Cities in disparate regions of the globe retain regionally distinct native and non-native plant assemblages, while invasive species, and especially European archaeophytes, were associated with lower beta diversity among cities. These findings suggest that intensive land-use change and biotic interchange, shaped through European influences, have had a world-wide effect on the beta diversity of urban plant assemblages

The compositional similarity of urban forests among the world's cities is scale dependent

Aim We examined species composition of urban forests from local to global scales using occurrence and abundance information to determine how compositional similarity is defined across spatial scales. We predicted that urban forests have become more homogeneous world-wide, which should result in minimal scale dependence that is more pronounced for non-native species, especially when considering abundance information. Location Thirty-eight cities world-wide. Methods We estimated compositional dissimilarities of urban forests, including both spontaneous and cultivated trees, from local to global spatial scales using six dissimilarity metrics. We used redundancy analysis to determine how climate, geographic distance and anthropogenic factors are related to compositional dissimilarity among cities. These analyses were implemented for all species combined and for native and non-native species separately. Results The 38 cities contained a median of 77 tree species, with a greater percentage of these classified as native (median = 58%). The similarity of urban forests was scale dependent, declining as the spatial scale increased – an outcome that did not differ when considering native and non-native species separately. Climate, geographic distance and city age were the main factors describing variation in tree species composition among cities. The addition of abundance information resulted in lower dissimilarity across spatial scales. Main conclusions Compositional similarity of urban forests is a scale dependent phenomenon that is not affected by the presence or absence of non-native species, suggesting a limited role for biotic interchange in promoting homogenization. However, compositional similarity across spatial scales increased uniformly with the addition of abundance information, suggesting that patterns of abundance may have greater biological relevance when homogenization trends among urban forests are considered

British plants as aliens in New Zealand cities: residence time moderates their impact on the beta diversity of urban floras

Anthropogenic activities have weakened biogeographical barriers to dispersal, thereby promoting the introduction, establishment and spread of alien species outside their native ranges. Several studies have identified a number of biological and ecological drivers that contribute to the establishment of plant species in the invaded range. One long-term factor that is generally accepted as a relevant determinant of invasion success is residence time, or time since first introduction into the new region. Residence time is often an important correlate of range extent in the invaded region, such that alien species with longer residence times in the novel environment tend to be more widely distributed. Plant species that were introduced in different regions at different times provide a unique opportunity to examine the effect of residence time on invasion success. In this paper, we examined how residence time affects the beta diversity of alien plants in selected urban floras of New Zealand and of English and Irish cities. We used an intercontinental plant exchange as a model system, comparing groups of species introduced to New Zealand and to the British Isles at different times (i.e., species native to the British Isles, British archaeophytes and British neophytes) and asked if differences in their beta diversity can be related to differences in their residence times. Our results suggest that observed patterns of beta diversity among the urban floras of New Zealand and of English and Irish cities can be attributed to a combination of residence time and of pre-adaptation to urban habitats that evolved, or were filtered in association with human activities, before the species were introduced into the invaded range

Bird strikes at commercial airports explained by citizen science and weather radar data

1. Aircraft collisions with birds span the entire history of human aviation, including fatal collisions during some of the first powered human flights. Much effort has been expended to reduce such collisions, but increased knowledge about bird movements and species occurrence could dramatically improve decision support and proactive measures to reduce them. Migratory movements of birds pose a unique, often overlooked, threat to aviation that is particularly difficult for individual airports to monitor and predict: the occurrence of birds vary extensively in space and time at the local scales of airport responses. 2. We use two publicly available datasets, radar data from the US NEXRAD network characterizing migration movements and eBird data collected by citizen scientists to map bird movements and species composition with low human effort expenditures but high temporal and spatial resolution relative to other large scale bird survey methods. As a test case we compare these results from weather radar distributions and eBird species composition with detailed bird strike records from three major New York airports. 3. We show that weather radar based estimates of migration intensity can accurately predict probability of bird strikes, with 80% of the variation in bird strikes across the year explained by the average amount of migratory movements captured on weather radar. We also show that eBird based estimates of species occurrence can, using species' body mass and flocking propensity, accurately predict when most damaging strikes occur. 4. Synthesis and applications: Our results highlight the power of federating datasets with movement and distribution data for developing better and more taxonomically and ecologically tuned models of likelihood of strikes occurring and severity of strikes. By better understanding when, and where, different species occur, airports across the world can predict seasonal periods of collision risks with greater temporal and spatial resolution; such predictions include potential to predict when the most severe and damaging strikes may occur

The varying role of population abundance in structuring indices of biotic homogenization

Aim: An important component of human-induced global change is the decrease or increase in community distinctiveness (taxonomic homogenization or differentiation, respectively) that follows the loss of native species and gain of non-native species. We use simulation approaches to assess the extent to which conclusions about the outcome of the homogenization process depend on whether or not abundance data are incorporated. Location: Data were produced through computer simulation. Methods: The frequency with which occurrence-based similarity indices and abundance-based similarity indices give different views of changes in community similarity, and the conditions under which such differences occurred were assessed using both deterministic and stochastic modelling approaches to simulate species assemblage states. Results: Occurrence-based and abundance-based indices were positively correlated across the set of simulations for both the deterministic and stochastic models. However, in both situations approximately one quarter (25%) of models resulted in contrasting outcomes for the two approaches of calculating changes in compositional similarity; that is, one data type showed a positive value (homogenization), whereas the other showed a negative value (differentiation). Main conclusions: In the majority of cases, species abundances will not change drastically enough after perturbation to produce large differences between homogenization scores measured using occurrence versus abundance information. However, in cases where these changes are large, it is important to recognize that the choice of metric to analyse homogenization trends will influence the qualitative and quantitative conclusions drawn. Studies of real assemblages are therefore necessary to evaluate the role of species abundance in defining the magnitude and direction of changes in community composition across space, and the implications of these changes for native biodiversity.Phillip Cassey, Julie L. Lockwood, Julian D. Olden and Tim M. Blackbur