Biodiversity, broadly the term that describes the variety of life, is variably distributed across the Earth. Scientists have long been interested in the patterns of biodiversity distribution, the processes that drive these patterns, and the implications of these patterns. Mounting concern for the future of biodiversity means it has remained a priority research area and has resulted in the development of new tools and methods for assessment at both the micro and macro scale, for example laboratory studies of genetic diversity through to the use of remote sensing for landscape-level exploration (Gaston 2010). At the global scale a few well-described distribution patterns, which hold true for many taxa, are evident. These patterns have given rise to four dominant areas of inquiry which seek to describe and explain the macroecology of global species distribution. These areas of inquiry include (1) work on latitudinal gradients which seeks to explain the primary pattern of high levels of species richness at the tropics; (2) work on species—energy relationships which seeks to explain the humped relationship where species richness (that is numbers of different types of species) peaks with moderate available energy; (3) understanding of the informing role of regional species richness on local species richness where, if the initial pool of species is already high, this could possibly result in higher local richness; and (4) unraveling relationships of taxonomic covariance which suggests that a high diversity of some taxa would probably also mean a high diversity in other taxa.

Gaston (2010) cautions that the patterns of biodiversity distribution are unlikely to be explained by one single mechanism, and that scale is likely to confound both mechanisms and observed pattern, and as a result there will likely be variation and exception to any pattern. Cities and their development have a significant and determining effect on ecology' at local, regional, and global scales. However, any city, irrespective of its age, is always a more recent influence on the local and regional biodiversity and ecology than the factors that informed the original biodiversity of the region. This original template must always be understood as the primary point of departure for understanding contemporary urban biodiversity. In many instances this original template can be seen to be both dynamic and often not fully understood, adding an additional layer of complexity to urban biodiversity research.

Urbanization has been described as catastrophic for the diversity of native species (Garrard et al. 2018). The establishment of cities typically reduces the density of flora and homogenizes ecosystems, with urban ecosystems becoming more akin to one another than to their adjacent native counterparts. Global assessments of birds in 54 cities and plants in 110 cities suggests that while the density of species is dramatically reduced in urban landscapes, species native to their region still predominate (Aronson et al. 2014). Cities and their evolution profoundly impact ecological processes and patterns, degrade and destroy natural habitat, breaking it up into fragmented and disconnected pieces, and change species assemblages. Aronson etal. (2014) demonstrate how profound these effects are, calculating a species per km* density of just 8 percent of the original native bird species, and 25 percent for plants. Whilst cities are often home to threatened species, they present a significant opportunity, to reconnect people with nature, through which they can receive multiple acknowledged health and well-being benefits (Garrard et al. 2018). These combined factors of critical biodiversity and persistent and growing extinction threats, at the points of highest human population density, all combine to make cities important sites for biodiversity conservation. This chapter looks at biodiversity in cities with an emphasis on how we measure it; current trends in urban biodiversity; and trends in urbanization in relation to global biodiversity. It concludes by considering possible future developments in measuring biodiversity and ways of actively enhancing it in cities.

Units, measures, and scale

Measuring biodiversity

The units of biodiversity can be anything from genes to ecosystems, but species still tend to be the most common unit of measurement (Purvis and Hector 2000). Basic biodiversity measures for an agreed unit can be divided into: measures of richness, which give counts at a given scale; evenness, which describes the level of variation within a community; and a combination of these two which results in measures of diversity. There is also a myriad of other measures, such as rarity and dominance, and multiple indices that bring together elements of richness and evenness. Some measures invoke a landscape view such as alpha, beta, and gamma diversity which position biodiversity and change at various scales across the landscape. Species themselves can be categorized in many ways that reflect their contribution to biodiversity, for example as native to an area where they have evolved and occurred over long periods of time, endemic to an area meaning they occur only there, or non-native to an area meaning they are recent introductions from some other area, or alternatively according to their conservation concern, such as through their status in the IUCN Red List of Threatened Species. These different units of consideration will result in the emergence of different patterns, where for example endemism and richness are not necessarily correlated (McDonald et al. 2008).

The importance of biodiversity

There is a strong, and growing, argument for conserving biodiversity based on its intrinsic value, but alongside this our interest in biodiversity relates strongly to ecosystem functioning, that is the joint effects of all processes that sustain an ecosystem (Naeem and Wright 2003), and the instrumental and relational benefits that flow from this (Pascual et al. 2017). We know that living things, from microscopic organisms to the megafauna, working in varying combinations and at different scales, underpin the ecosystem processes that regulate several large-scale life-supporting processes, such as nutrient cycling, carbon sequestration, and oxygen production (Reiss et al. 2009). Biodiversity has also been shown as critical to system resilience where higher species richness links to ecological systems that can continue to function in the face of change (Oliver et al. 2015). This is particularly relevant in cities which are highly dynamic and rapidly changing environments, and ones that must also face predicted global change with shifts in climate (Seto et al. 2012). While the link between biodiversity and ecosystem functioning has been agreed, proving this relationship remains something of a holy grail in ecology (Gaston 2010). Findings so far are not unequivocal.

Tying biodiversity, or associated biodiversity metrics or proxies, irrefutably to ecosystem functioning in its various guises is an ongoing and highly valuable area of research. In response to some of the challenges presented in joining the dots between biodiversity and functioning, and no doubt in relation to general trends in ecology, there has been a simultaneous shift towards research that seeks to establish how biodiversity matters to ecosystem functioning and ecological processes (Naeem and Wright 2003; Reiss et al. 2009). McDonnell and Hahs (2013) draw this debate into the urban ecology literature, urging researchers to pursue more functional and mechanistic understandings.

The importance of scale

In all biodiversity studies the issue of time and space scales is completely accepted as being central to understanding biodiversity metrics; their contributions to functioning; and how external pressures affect them. Scale affects whether a species is labeled native, endemic, or nonnative. Similarly, what constitutes native can be a matter of temporal scale where naturalized species, given enough time, might be described as native, and similarly human-created, but self-sustaining ecosystems are recognized as novel ecosystems (Hobbs et al. 2006). Cities present dynamic environments where changes tend to be rapid over both time and space. The lack of temporal data presents a hindrance to understanding rates of change and restoration efforts. All contemporary urban biodiversity measures need to be able to cope with the future ecological responses to environmental changes.

Connections between biodiversity and ecosystem functioning

Calls for more mechanistic understandings have resulted in the development of several measures of diversity. Ecosystem services provide one such example that makes the connection between the role of a species, or suite of species, in human life-support systems. A second example is the functional guilds of basic units of biodiversity which are grouped in relation to the particular role they play in an ecosystem, such as insectivorous birds whose feeding habits result in the pollination of certain species (Conole and Kirkpatrick 2011). The links here to ecosystem service provision are evident. These more outcome-related measures are useful as they allow for intercity comparison, from which we can build global understandings of system responses and start to develop generalized agreed principles (McDonnell et al. 2009). In addition to the standard metrics emerging from ecology, a particularly well-used method that is regularly adopted in urban ecology is gradient analysis, with measurement of biodiversity along the rural-to-urban gradient (Muller and Werner 2010).

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