Amy K. Hahs

The use of gradient analysis studies in advancing our understanding of the ecology of urbanizing landscapes: current status and future directions

Over the past decade, the urban–rural gradient approach has been effectively used to study the ecology of cities and towns around the world. These studies have focused on understanding the distribution of plants and animals as well as ecosystem processes along gradients of urbanization that run from densely urbanized inner city to more rural exurban environments. We reviewed 300 papers investigating urbanization gradients that were published in peer-reviewed journals between 1990 and May 2007. Sixty-three percent of the papers investigated the distribution of organisms along urbanization gradients. Only five papers addressed the measures used to quantify the urbanization gradient itself. Within the papers addressing the distribution of organisms, 49% investigated the responses of birds to urbanization gradients, and <10% of the papers investigated more cryptic organisms. Most of these studies utilized a variety of broad measures of urbanization, but future advances in the field will require the development of some standardized broad measures to facilitate comparisons between cities. More specific measures of urbanization can be used to gain a mechanistic understanding of species and ecosystem responses to urbanization gradients. While the gradient approach has made a significant contribution to our understanding of the ecology of cities and towns, there is now a need to address our current knowledge gaps so that the field can reach its full potential. We present two examples of research questions that demonstrate how we can enhance our understanding of urbanization gradients, and the ecological knowledge that we can obtain from them.

A global synthesis of plant extinction rates in urban areas

Plant extinctions from urban areas are a growing threat to biodiversity worldwide. To minimize this threat, it is critical to understand what factors are influencing plant extinction rates. We compiled plant extinction rate data for 22 cities around the world. Two-thirds of the variation in plant extinction rates was explained by a combination of the citys historical development and the current proportion of native vegetation, with the former explaining the greatest variability. As a single variable, the amount of native vegetation remaining also influenced extinction rates, particularly in cities > 200 years old. Our study demonstrates that the legacies of landscape transformations by agrarian and urban development last for hundreds of years, and modern cities potentially carry a large extinction debt. This finding highlights the importance of preserving native vegetation in urban areas and the need for mitigation to minimize potential plant extinctions in the future.

Ecology of cities and towns : a comparative approach

The unprecedented growth of cities and towns around the world, coupled with the unknown future effects of global change, has created an urgent need to increase ecological understanding of human settlements, in order to develop inhabitable, sustainable cities and towns in the future. Although there is a wealth of knowledge regarding the understanding of human organisation and behaviour, there is comparatively little information available regarding the ecology of cities and towns. This book brings together leading scientists, landscape designers and planners from developed and developing countries around the world, to explore how urban ecological research has been undertaken to date, what has been learnt, where there are gaps in knowledge, and what the future challenges and opportunities are.

A DISPERSAL-CONSTRAINED HABITAT SUITABILITY MODEL FOR PREDICTING INVASION OF ALPINE VEGETATION

Developing tools to predict the location of new biological invasions is essential if exotic species are to be controlled before they become widespread. Currently, alpine areas in Australia are largely free of exotic plant species but face increasing pressure from invasive species due to global warming and intensified human use. To predict the potential spread of highly invasive orange hawkweed (Hieracium aurantiacum) from existing founder populations on the Bogong High Plains in southern Australia, we developed an expert-based, spatially explicit, dispersal-constrained, habitat suitability model. The model combines a habitat suitability index, developed from disturbance, site wetness, and vegetation community parameters, with a phenomenological dispersal kernel that uses wind direction and observed dispersal distances. After generating risk maps that defined the relative suitability of H. aurantiacum establishment across the study area, we intensively searched several locations to evaluate the model. The highest relative suitability for H. aurantiacum establishment was southeast from the initial infestations. Native tussock grasslands and disturbed areas had high suitability for H. aurantiacum establishment. Extensive field searches failed to detect new populations. Time-step evaluation using the location of populations known in 1998-2000, accurately assigned high relative suitability for locations where H. aurantiacum had established post-2003 (AUC [area under curve] = 0.855 +/- 0.035). This suggests our model has good predictive power and will improve the ability to detect populations and prioritize areas for ongoing monitoring.

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.

Habitat complexity influences fine scale hydrological processes and the incidence of stormwater runoff in managed urban ecosystems.

Urban ecosystems have traditionally been considered to be pervious features of our cities. Their hydrological properties have largely been investigated at the landscape scale and in comparison with other urban land use types. However, hydrological properties can vary at smaller scales depending upon changes in soil, surface litter and vegetation components. Management practices can directly and indirectly affect each of these components and the overall habitat complexity, ultimately affecting hydrological processes. This study aims to investigate the influence that habitat components and habitat complexity have upon key hydrological processes and the implications for urban habitat management. Using a network of urban parks and remnant nature reserves in Melbourne, Australia, replicate plots representing three types of habitat complexity were established: low-complexity parks, high-complexity parks, and high-complexity remnants. Saturated soil hydraulic conductivity in low-complexity parks was an order of magnitude lower than that measured in the more complex habitat types, due to fewer soil macropores. Conversely, soil water holding capacity in low-complexity parks was significantly higher compared to the two more complex habitat types. Low-complexity parks would generate runoff during modest precipitation events, whereas high-complexity parks and remnants would be able to absorb the vast majority of rainfall events without generating runoff. Litter layers on the soil surface would absorb most of precipitation events in high-complexity parks and high-complexity remnants. To minimize the incidence of stormwater runoff from urban ecosystems, land managers could incrementally increase the complexity of habitat patches, by increasing canopy density and volume, preserving surface litter and maintaining soil macropore structure.

Composition of the plant community in remnant patches of grassy woodland along an urban–rural gradient in Melbourne, Australia

The composition of the plant community in remnant patches of open grassy woodlands with an overstorey of Eucalyptus camaldulensis was investigated along an urban–rural gradient in Melbourne, Australia. The plant community showed very little difference between patches along the gradient, particularly in terms of the indigenous plant species. Average annual rainfall was the main factor contributing to patterns of indigenous plant species richness, while the level of urbanization in the surrounding landscape had a strong influence on the number of non-indigenous species recorded in the remnant plant community. Patterns of species richness were largely influenced by landscape-scale factors, while the percent cover of indigenous and non-indigenous plant species were more strongly influenced by patch scale factors. The findings of this study suggest that the plant communities investigated during this study appear to be relatively resilient to changes in the landscape associated with urbanization, but the plant community may be affected by predicted changes in average annual rainfall associated with climate change.

Increasing biodiversity in urban green spaces through simple vegetation interventions

Cities are rapidly expanding world-wide and there is an increasing urgency to protect urban biodiversity, principally through the provision of suitable habitat, most of which is in urban green spaces. Despite this, clear guidelines of how to reverse biodiversity loss or increase it within a given urban green space is lacking. We examined the taxa- and species-specific responses of five taxonomically and functionally diverse animal groups to three key attributes of urban green space vegetation that drive habitat quality and can be manipulated over time: the density of large native trees, volume of understorey vegetation and percentage of native vegetation. Using multi-species occupancy-detection models, we found marked differences in the effect of these vegetation attributes on bats, birds, bees, beetles and bugs. At the taxa-level, increasing the volume of understorey vegetation and percentage of native vegetation had uniformly positive effects. We found 30-120% higher occupancy for bats, native birds, beetles and bugs with an increase in understorey volume from 10% to 30%, and 10-140% higher occupancy across all native taxa with an increase in the proportion of native vegetation from 10% to 30%. However, increasing the density of large native trees had a mostly neutral effect. At the species-specific level, the majority of native species responded strongly and positively to increasing understorey volume and native vegetation, whereas exotic bird species had a neutral response. Synthesis and applications. We found the probability of occupancy of most species examined was substantially reduced in urban green spaces with sparse understorey vegetation and few native plants. Our findings provide evidence that increasing understorey cover and native plantings in urban green spaces can improve biodiversity outcomes. Redressing the dominance of simplified and exotic vegetation present in urban landscapes with an increase in understorey vegetation volume and percentage of n

Soil Carbon and Carbon/Nitrogen Ratio Change under Tree Canopy, Tall Grass, and Turf Grass Areas of Urban Green Space.

Soils in urban green spaces are an important carbon (C) store, but urban soils with a high carbon to nitrogen (C/N) ratio can also buffer N eutrophication from fertilizer use or atmospheric deposition. The influence of vegetation management practices on soil C cycling and C/N ratios in urban green spaces is largely unknown. In 2013, we collected replicate ( = 3) soil samples from tree canopy, tall grass, and short turf grass areas ( = 3) at four random plot locations ( = 4) established in 13 golf courses ( = 13). At each sample point, soil was separated into 0- to 0.1-, 0.1- to 0.2-, and 0.2- to 0.3-m depths (total = 1404). Linear mixed models investigated the relationships between soil properties, vegetation attributes, and green space age. Tree canopy soil was less compacted (1.07 g cm) than grassy areas (1.32 g cm). Similarly, tree canopy soil had mean C/N ratios of 17.2, as compared with between 14.2 and 15.3 in grassy areas. Soil properties in tree canopy areas were best explained by tree basal area and understory vegetation volume. Soil C/N increased with increasing understory vegetation, and the difference in soil C/N between tree canopy and short turf grass areas increased over time. The soil properties in tree canopy areas of urban green space mean they can increasingly buffer the localized use of N fertilizers and atmospheric N deposition. Managers of urban green spaces concerned about N pollution of groundwater and waterways could consider planting trees in suitable topographic locations and promoting understory vegetation and surface litter accumulation.

Expanding fundamental ecological knowledge by studying urban ecosystems

The expansion, densification and proliferation of urban areas around the world is currently occurring at a rate that is unprecedented in human history. It is predicted that global urban land cover will triple between 2000 and 2030, with some regions (including biodiversity hotspots) experiencing a ninefold increase in urban land cover over the same time period (Seto, G€ uneralp & Hutyra 2012). Accompanying the expansion of urban landscapes, it is anticipated that the human population living in cities and towns globally will increase from 3 5 to 5 billion people within the next 20 years (Fragkias et al. 2013). Thus, the demands of an expanding and urbanizing human population are one of the pressing ecological problems our world is facing (Sanderson et al. 2002), alongside, and in combination with, global climate change and changes to biodiversity at local and global scales (Pimm et al. 2014). Yet, urban environments also present a unique opportunity to expand our fundamental knowledge related to ecology and evolution due to the presence of intense and often novel selection pressures. In the inaugural issue of this journal, Calow (1987) defined functional ecology as the sum of three interactive processes: (i) those occurring between organisms and their environment, (ii) biotic interactions between organisms and (iii) adaptive processes driven by natural selection. The same three processes were highlighted 1 year earlier by Jared Diamond in Nature, when he called for biologists to pay more attention to the potential of using the unprecedented environmental conditions that exist within towns and cities to develop and test evolutionary and ecological theory (Diamond 1986). There is thus a natural synergy between functional ecology and urban ecology, as exemplified by some of the classic papers that have appeared in this journal, such as Rydell (1992) who demonstrated that the form of echolocation system determined the impact of light pollution on bat foraging behaviour. The potential of combining functional ecological research with urban ecology is, however, a long way from being fully realized. This is in part explained by the youth of urban ecology as a discipline. Scientific enquiry into the ecological consequences of urban environments has been underway for over half a century, although most of the momentum emerged after the mid 1990s (McDonnell 2011; Wu 2014). Thus, the focus of much urban research to date has involved describing patterns along environmental gradients (Gagn e 2013; McDonnell & Hahs 2013) rather than investigating the mechanistic processes that lie at the heart of functional ecology. To be effective in addressing the global challenges of urbanization, a much better understanding of how the urban environment affects the ecology and evolution of organisms needs to be developed (Grimm et al. 2008; Marzluff 2012; Gil & Brumm 2014; McDonnell & Hahs 2015). The purpose of this special feature is to draw attention to the plethora of opportunities that await researchers investigating the ecology and evolution of organisms in urban environments. The combination of environmental stressors and conditions within urban areas provides a novel opportunity to test and expand our theories related to ecology and evolution of organisms, and some intriguing insights are already beginning to emerge. For example, the detailed understanding of the molecular, genetic and developmental mechanisms of beak evolution that has arisen from studying Galapagos finches has been significantly advanced by studying beak evolution in the house finch Carpodacus mexicanus in response to novel urban food sources and its consequences for acoustic communication (Badyaev 2010, 2014). Thus, urban ecology has the potential to extend our understanding of extremely well-studied ecological and evolutionary problems.