Mark J. McDonnell

Ecosystem processes along an urban-to-rural gradient

In order to understand the effect of urban development on the functioning of forest ecosystems during the past decade we have been studying red oak stands located on similar soil along an urban-rural gradient running from New York City ro rural Litchfield County, Connecticut. This paper summarizes the results of this work. Field measurements, controlled laboratory experiments, and reciprocal transplants documented soil pollution, soil hydrophobicity, litter decomposition rates, total soil carbon, potential nitrogen mineralization, nitrification, fungal biomass, and earthworm populations in forests along the 140 × 20 km study transect. The results revealed a complex urban-rural environmental gradient. The urban forests exhibit unique ecosystem structure and function in relation to the suburban and rural forest stands; these are likely linked to stresses of the urban environment such as air pollution, which has also resulted in elevated levels of heavy metals in the soil, the positive effects of the heat island phenomenon, and the presence of earthworms. The data suggest a working model to guide mechanistic work on the ecology of forests along urban-to-rural gradients, and for comparison of different metropolitan areas.

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.

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.

CH4 uptake and N availability in forest soils along an urban to rural gradient

Abstract Concern about increases in atmospheric CH 4 concentrations has resulted in investigations of the magnitude of and the factors that control aerobic soils as a sink for CH 4 . N additions decrease CH 4 consumption in temperate forest, prairie and agricultural soils, suggesting that low rates of CH 4 consumption are associated with high concentrations of available N. We have observed the opposite pattern in a series of oak-dominated forest sites of similar age on similar soils located along an urban to rural land-use gradient. Low rates of CH 4 consumption were observed in urban forest sites with low amounts of available N relative to rural sites that had high concentrations of available N. In situ consumption rates ranged from 6.9 to 2.1 mg CH 4 m −2 d −1 . Mean rates of consumption were 30% lower in urban than in rural and suburban forest sites, suggesting that factors associated with urbanization may decrease rates of CH 4 consumption in temperate forest soils. We hypothesize that low consumption rates of CH 4 in urban forest sites are a product of low rates of organic matter degradation and nutrient cycling caused by air pollution (especially ozone) damage to forest tree leaves. Reduced rates of decomposition may result in lower rates of C and N flow from litter to soil microbial populations responsible for consumption of CH 4 .

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.

Litter decomposition and nitrogen mineralization in oak stands along an urban-rural land use gradient

We investigated litter mass loss and soil nitrogen (N)-transformation rates in oak stands along a 130-km, urban-rural transect originating in New York City to examine the relationship between changes in these parameters and previously documented differences in soil temperature, heavy metal and total salt concentrations, and soil biota. Reference litter from a rural site was placed in litterbags, and rates of mass loss and changes in N concentration in litter residues were measured over a 6-month period. Net N-mineralization and nitrification rates were measured in A horizon soils using laboratory incubations under constant moisture and temperature. Both mass loss (76%) and N release (65%) from litterbags reached their maximum in urban stands, whereas net N-mineralization rates were 2.3-fold higher than in rural A horizon soils. Litter fragmentation by earthworms and higher soil temperatures are potential causes of the higher mass loss rates observed in urban stands. The higher releases of N measured in the urban litterbags could be a result of their faster mass loss rates, exogenous inputs of N from atmospheric deposition, a relatively low heterotrophic demand for N, or a combination of these factors. The results of this study suggest that in comparison with rural stands, urban forests are characterized by comparatively high rates of litter decomposition, and may also be characterized by comparatively high rates of N mineralization. Additional studies are needed to test whether these effects are common to urban environments in general.

LOCAL EXTINCTION OF GRASSLAND PLANTS: THE LANDSCAPE MATRIX IS MORE IMPORTANT THAN PATCH ATTRIBUTES

Past studies of local extinctions in fragmented habitats most often tested the influence of fragment size and isolation while ignoring how differences in the surrounding landscape matrix may govern extinction. We assessed how both the spatial attributes of remnant patches (area and isolation) and landscape factors (extent of urbanization and maximum inter-fire interval) influence the persistence of native plant species in grasslands in western Victoria, Australia. Persistence was determined in 2001 by resurveying 30 remnants first surveyed in the 1980s, and correlates of extinction were assessed using Bayesian logistic regression models. On average, 26% of populations of native species became locally extinct over two decades. Area and isolation had little effect on the probability of local extinction, but urbanization and longer maximum inter-fire intervals increased extinction risk. These findings suggest that the native grasslands studied are relatively insensitive to area- and isolation-based fragmentation effects and that short-term persistence of plant populations requires the maintenance of habitat quality. The latter is strongly influenced by the landscape matrix surrounding remnant patches through changes in fire regimes and increased exogenous disturbance.

Spatial Heterogeneity During Succession: A Cyclic Model of Invasion and Exclusion

The concept of heterogeneity is one of the most important and most widely applicable ideas in ecology. It is relevant at all scales and levels of organization and yet is expressed differently in each context (Hutchinson, 1953). One of the principal problems in understanding heterogeneity is to discover its patterns and, subsequently, its functional significance in its various manifestations. In this chapter we take heterogeneity to be the spatial pattern of elements of a biotic assemblage. Old fields, because of their commonness, variety, and manipulability, are ideal systems in which to investigate the pattern of heterogeneity and its ecological significance. Such studies can also contribute to understanding mechanisms of succession (Finegan, 1984; Pickett et al., 1987).

The ecological future of cities.

The discipline of urban ecology arose in the 1990s, primarily motivated by a widespread interest in documenting the distribution and abundance of animals and plants in cities. Today, urban ecologists have greatly expanded their scope of study to include ecological and socioeconomic processes, urban management, planning, and design, with the goal of addressing issues of sustainability, environmental quality, and human well-being within cities and towns. As the global pace of urbanization continues to intensify, urban ecology provides the ecological and social data, as well as the principles, concepts and tools, to create livable cities.