Myla F. J. Aronson

A global analysis of the impacts of urbanization on bird and plant diversity reveals key anthropogenic drivers.

Urbanization contributes to the loss of the worlds biodiversity and the homogenization of its biota. However, comparative studies of urban biodiversity leading to robust generalities of the status and drivers of biodiversity in cities at the global scale are lacking. Here, we compiled the largest global dataset to date of two diverse taxa in cities: birds (54 cities) and plants (110 cities). We found that the majority of urban bird and plant species are native in the worlds cities. Few plants and birds are cosmopolitan, the most common being Columba livia and Poa annua. The density of bird and plant species (the number of species per km2) has declined substantially: only 8% of native bird and 25% of native plant species are currently present compared with estimates of non-urban density of species. The current density of species in cities and the loss in density of species was best explained by anthropogenic features (landcover, city age) rather than by non-anthropogenic factors (geography, climate, topography). As urbanization continues to expand, efforts directed towards the conservation of intact vegetation within urban landscapes could support higher concentrations of both bird and plant species. Despite declines in the density of species, cities still retain endemic native species, thus providing opportunities for regional and global biodiversity conservation, restoration and education.

Fruit type, life form and origin determine the success of woody plant invaders in an urban landscape

The spread of alien plant species is a critical ecological event worldwide, but the forces that control this spread are not well documented. Alien plant species are well known to disrupt ecological services of native ecosystems, change the composition of native habitats, and often lead to the extirpation of native flora and fauna. Here, we report on life history patterns of plant species with rapidly spreading and declining ranges in North America’s major urban region. We tested for differences in life history traits between the 466 native and alien woody flora of the New York metropolitan area. We also examined the relationship between life history traits and change in distribution in the New York metropolitan area between 1900 and 2000. Native and alien species of the New York metropolitan area differ with respect to pollination vector and breeding system. However, pollination vector and breeding system are not associated with success, defined here as increasing range spread in the urban environment; instead, fruit type (dispersal), life form and origin are important determinants of success. Alien species that are deciduous trees, shrubs or vines with fleshy fruit are the most successful in increasing their distribution in this urban landscape. Newly introduced species with these characteristics are expected to have a better chance at establishing in similar urban landscapes and should be targets for intensive management. The ability to predict which alien species will become invasive is also a valuable tool for the prevention of invasions by newly introduced plant species.

Deer and Invasive Plant Species Suppress Forest Herbaceous Communities and Canopy Tree Regeneration

ABSTRACT: The loss of native biodiversity is a major ecological issue in human-dominated landscapes. In particular, the tree regeneration failure of deciduous forests remnants in suburban landscapes is of great concern to land managers and forestry associations. We tested the responses over two growing seasons of herbaceous plants and tree seedling survival and growth to the removal of invasive plant competition and deer herbivory. We first tested the response of understory vegetation to the removal of Microstegium vimineum (Japanese stiltgrass), a non-native invasive grass, and the removal of deer herbivory, via exclosures, in two forest fragments in central New Jersey. We then explored the restoration potential of planted seedlings of canopy tree dominants, Acer rubrum, Fraxinus americana, and Quercus rubra. The herbaceous community responded with an increase in species richness to the removal of M. vimineum. There was no response of the herbaceous community to the removal of deer herbivory, indicating that the herbaceous community will not recover rapidly from removal of these stressors alone. Survival and height growth of naturally regenerating tree seedlings increased with the removal of M. vimineum and the removal of deer herbivory, but these effects were not interactive. The survival and growth of planted seedlings of two tree species, F. americana and Q. rubra, were depressed by the presence of M. vimineum. Our results suggest that intensive management of M. vimineum and deer populations as well as active re-vegetation of herbaceous communities and tree seedlings are necessary to restore plant biodiversity in suburban deciduous forests.

Hierarchical filters determine community assembly of urban species pools

The majority of humanity now lives in cities or towns, with this proportion expected to continue increasing for the foreseeable future. As novel ecosystems, urban areas offer an ideal opportunity to examine multi-scalar processes involved in community assembly as well as the role of human activities in modulating environmental drivers of biodiversity. Although ecologists have made great strides in recent decades at documenting ecological relationships in urban areas, much remains unknown, and we still need to identify the major ecological factors, aside from habitat loss, behind the persistence or extinction of species and guilds of species in cities. Given this paucity of knowledge, there is an immediate need to facilitate collaborative, interdisciplinary research on the patterns and drivers of biodiversity in cities at multiple spatial scales. In this review, we introduce a new conceptual framework for understanding the filtering processes that mold diversity of urban floras and faunas. We hypothesize that the following hierarchical series of filters influence species distributions in cities: (1) regional climatic and biogeographical factors; (2) human facilitation; (3) urban form and development history; (4) socioeconomic and cultural factors; and (5) species interactions. In addition to these filters, life history and functional traits of species are important in determining community assembly and act at multiple spatial scales. Using these filters as a conceptual framework can help frame future research needed to elucidate processes of community assembly in urban areas. Understanding how humans influence community structure and processes will aid in the management, design, and planning of our cities to best support biodiversity.

Biodiversity in the City: Fundamental Questions for Understanding the Ecology of Urban Green Spaces for Biodiversity Conservation

&NA; As urban areas expand, understanding how ecological processes function in cities has become increasingly important for conserving biodiversity. Urban green spaces are critical habitats to support biodiversity, but we still have a limited understanding of their ecology and how they function to conserve biodiversity at local and landscape scales across multiple taxa. Given this limited view, we discuss five key questions that need to be addressed to advance the ecology of urban green spaces for biodiversity conservation and restoration. Specifically, we discuss the need for research to understand how green space size, connectedness, and type influence the community, population, and life‐history dynamics of multiple taxa in cities. A research framework based in landscape and metapopulation ecology will allow for a greater understanding of the ecological function of green spaces and thus allow for planning and management of green spaces to conserve biodiversity and aid in restoration activities.

Global Patterns and Drivers of Urban Bird Diversity

The rapid urbanization of the world has profound effects on global biodiversity, and urbanization has been counted among the processes contributing to the homogenization of the world’s biota. However, there are few generalities of the patterns and drivers of urban birds and even fewer global comparative studies. Comparable methodologies and datasets are needed to understand, preserve, and monitor biodiversity in cities. We explore the current state of the science in terms of basic patterns of urban birds in the world’s cities and lay out a research agenda to improve basic understanding of patterns and processes and to better inform conservation efforts. Urban avifaunas are often portrayed as being species poor and dominated by omnivorous and granivorous species that tend to be nonnative. Common families in cities include Accipitridae, Anatidae, and Scolopacidae, all of which have more species than expected in cities compared to the global distribution of species in these families. Recent research shows that cities support an avifauna dominated by native species and that cities are not homogenized at the global level. However, cities have lost substantial biodiversity compared to predicted peri-urban diversity, and 31 of the world’s most invasive bird species are found in cities. Future research is needed to better characterize the anthropogenic, environmental, and ecological drivers of birds in cities. Such mechanistic understanding is the underpinning of effective conservation strategies in a human dominated world.

Reconciling Scale in Paleontological and Neontological Data: Dimensions of Time, Space, and Taxonomy

Conserving biodiversity in the face of expanding human degradation of ecosystems is facilitated by understanding the natural state of communities prior to the impact of anthropogenic disruptions. Reconstructing communities and ecosystems as they existed in the past requires data from the fossil record on their species composition, richness, and abundance. Fossil data are potentially different from data collected from living communities in their spatial, temporal, and taxonomic scales and these differences must be understood so that accurate comparisons can be made between past and present states of living communities. Fifty-four long-term ecological studies of a wide range of taxon groups (mammals, invertebrates, plants, corals) and habitat types (marine, terrestrial, freshwater) were surveyed from the published ecological literature to determine the range of spatial, temporal and taxonomic scales at which data are commonly collected in ecological research. Long-term ecological studies encompass spatial scales from 50m2 to 100,000km2 and temporal scales from 5 to 100 years. Most studies resolve taxa to the species level and count individuals, although plant and coral studies sometimes quantify species by percent cover. All taxon groups and habitat types were studied across a wide range of spatial and temporal scales. Whether or not data from fossils can be collected and analysed at scales comparable to data from living communities depends on the type of organism, as well as the taphonomic circumstances of preservation, accumulation and deposition. Marine invertebrates can be sampled at comparable spatial and taxonomic scales to living invertebrates, but time averaging degrades the temporal resolution of the fossil deposits. Vertebrate fossils provide data at comparable taxonomic scales with some reduction in spatial and temporal resolution relative to live data. Plant fossils and pollen are capable of being sampled at temporal resolutions comparable to modern ecological studies, but pollen data are prone to spatial averaging and have much poorer taxonomic resolution than censuses of living communities. It is important to be mindful of the limitations that scale mismatches produce in the ability to use fossil data to resolve ecological events and to compare the details of ecological composition and structure between the present and the past.

Urban riparian systems function as corridors for both native and invasive plant species

Riparian areas are often the only green areas left in urban and suburban landscapes, providing opportunities for conservation and connectivity of both aquatic and terrestrial organisms. While city planners and land managers often tout the importance of riparian networks for these uses, it is not well established if urban riparian plant communities are actually functioning as connected assemblages. Furthermore, urban riparian zones are well known to be highly invaded by non-native plant species and may be functioning to increase the spread of non-native species across the landscape. Here we examine connectivity of plant assemblages in riparian networks within an extensively urbanized landscape. We sampled riparian plant communities at 13 sites along three second-order streams of the Rahway River watershed, New Jersey. We also characterized propagule dispersal at each site by sampling litter packs on the river banks five times between March–October 2011 and identifying germinants from litter packs after cold stratification. Species turnover of both riparian and litter vegetation was more strongly associated with flow distance, particularly for native species, indicating that riverine systems are important for promoting connectivity of native plant assemblages in urban landscapes. However, non-native germinants significantly dominated propagule dispersal along the stream reaches, particularly early in the growing season, suggesting spread utilizing the river system and preemption may be an important mechanism for invasion success in this system. Our data show that management of invasive species should be planned and implemented at the watershed scale to reduce spread via the river system.

Does Suburban Horticulture Influence Plant Invasions in a Remnant Natural Area

ABSTRACT: The horticultural trade is a well-known source of nonnative invasive plant species, yet urban and suburban developments are routinely planted with these species, creating high invasion pressure on adjacent natural areas. Understanding the spread of nonnative species and predicting invasions is critical for the management of natural habitats. Here, we examine the similarities in nonnative plant community composition between a remnant natural habitat at Fire Island National Seashore and the surrounding residential communities to assess the impact of nonnative invasive horticultural species on the natural area. In the natural area, we identified 16 locally listed invasive plant species. In residential areas we identified 21 locally listed invasive species. Of 162 properties surveyed, 144 appeared to be occupied and maintained by residents; 18 appeared unmaintained or abandoned. Unmaintained properties had significantly more invasive species than maintained properties. Nonnative species composition between the natural and residential areas was not significantly different. In the natural area, distance from suburban edge, native species richness, and soil moisture were important drivers of invasion. We show that in this particular natural area, invasive plants have not invaded farther than 25 m into the forest, indicating the strong role of edge effects in invasions. Additionally, we show that unmaintained properties in the residential areas may be the primary source of invasives to natural area. Homeowner education on the impacts of invasive species and active management of the nonnative invasive species in the unmaintained properties may be important for preventing further invasions to the forest.

Floristic response to urbanization: Filtering of the bioregional flora in Indianapolis, Indiana, USA

PREMISE OF THE STUDY Globally, urban plant populations are becoming increasingly important, as these plants play a vital role in ameliorating effects of ecosystem disturbance and climate change. Urban environments act as filters to bioregional flora, presenting survival challenges to spontaneous plants. Yet, because of the paucity of inventory data on plants in landscapes both before and after urbanization, few studies have directly investigated this effect of urbanization. METHODS We used historical, contemporary, and regional plant species inventories for Indianapolis, Indiana USA to evaluate how urbanization filters the bioregional flora based on species diversity, functional traits, and phylogenetic community structure. KEY RESULTS Approximately 60% of the current regional flora was represented in the Indianapolis flora, both historically and presently. Native species that survived over time were significantly different in growth form, life form, and dispersal and pollination modes than those that were extirpated. Phylogenetically, the historical flora represented a random sample of the regional flora, while the current urban flora represented a nonrandom sample. Both graminoid habit and abiotic pollination are significantly more phylogenetically conserved than expected. CONCLUSIONS Our results likely reflect the shift from agricultural cover to built environment, coupled with the influence of human preference, in shaping the current urban flora of Indianapolis. Based on our analyses, the urban environment of Indianapolis does filter the bioregional species pool. To the extent that these filters are shared by other cities and operate similarly, we may see increasingly homogenized urban floras across regions, with concurrent loss of evolutionary information.