Towards a mechanistic understanding of urbanization's impacts on fish

Abstract

Human population is increasing at a rate of 1.8% per year and urbanization is a global trend. In 2005, there were 3.2 billion urban residents worldwide, representing 49% of the global population (UN 2006). The urban population is projected to increase to 4 billion in 2018 and to more than 5 billion in 2030 (UN 2006). However, in the developed regions 75% of the population lived in urban settlements in 2005. This proportion is expected to increase in Europe from 72% in 2005 to 78% in 2030 and in North America from 81% at present to 87% in 2030 (UN 2006). Urban areas cover only 2.4% on the terrestrial surface of Earth, but their average population density is 52 times that of rural areas (MA 2005). Except coastal areas and island states, the highest average urban population density was found along inland waters with 817 people per km2 (MA 2005). Indeed, most people believe that urban environmental conditions are deteriorating, and the condition of urban waters is high on their list of worries. Features of urbanization have been reviewed by Paul & Meyer (2001): impervious surface covers, alteration of drainage density and flow dynamics, decreasing groundwater renewing and sediment supply, and increases in surface runoff, water temperature, pollutants, and nutrients. The cumulative effect of various human activities in urban areas profoundly influence urban waters and their biota, either directly by channel modification and habitat degradation or indirectly by land use change and runoff (Booth et al. 2004). The percentage of impervious surface cover has been commonly suggested as the best single predictor of the response of stream biota to urbanization (e.g. Karr & Chu 2000, Allan 2004, Booth et al. 2004, Miltner et al. 2004). However, the threshold values for demonstrated significant biological degradations at the catchment level of 10-15% total impervious area are much below the commonly observed >50% impervious cover in metropolitan areas at the regional level (Booth et al. 2004). Karr & Chu (2000) considered biological communities as irreparably damaged if the impervious cover within a watershed ranged between 25-60%. Urbanization is highly positively correlated with both the endangerment of native and the invasion of non-native fish within watersheds and thus, considered as major cause of biotic homogenization (Marchetti et al. 2006). Urbanization tends to favour the persistence of relatively few intolerant, generalist native species, the introduction and establishment of widespread non-natives, and the extinction and extirpation of specialized, intolerant native species (Marchetti et al. 2006). But how do urbanized areas and impervious covers impact fish and promote non-native species? What are the basic mechanisms? Non-linear relations between impervious cover and biological communities have been observed, in particular if instream habitats or hydrodynamics were also considered. For example, in Ohio watersheds at sites with relatively undeveloped riparian buffers, the biological integrity was maintained despite high levels of urban land use (Miltner et al. 2004). Accordingly, habitat destruction caused by urban development seemed the primary force driving common species to decline due to resource limitations and enabling different or new species to increase when benefiting from the changed habitats. To analyse this question, between 1992 and 2002 the fish assemblages of 27 Federal waterways have been extensively studied: more than 470 sites were surveyed, 2,100 samples collected, and 336,500 fish recorded representing 35 species. This data set revealed substantial findings on environmental factors structuring local fish assemblages (Wolter & Vilcinskas 1997a, 1998a, Wolter 2000, 2003), environmental pressures (Wolter 2001a, Arlinghaus et al. 2002), urban gradients (Wolter 1997b, 2000, Wolter 1999a), the impact of urbanization on fish abundance (Wolter & Vilcinskas 1996,Wolter 2001b) and population dynamics (Wolter 1998, 1999b, Wolter & Vilcinskas 1998b), and the ecological performance of species (Wolter & Vilcinskas 1997b, Arlinghaus & Wolter 2003). This paper refers briefly to three main results of the mentioned studies to illustrate the specifics of fish communities in urban waters for the example of the water system of Berlin, Germany, where 3.5 million people reside. Detailed information on methods used, sampling design, environmental variables, species lists etc. are presented in the original papers. © 2008 Springer Science+Business Media, LLC.