Effects of urbanization on Neotropical wasp and bee assemblages in a Brazilian metropolis

Abstract

The rapid growth of cities in many parts of the world has stimulated an increasing number of ecological studies of urban environments. Here, 12 study sites in the city of Belo Horizonte (MG), southeastern Brazil, were compared to analyze the effects of habitat changes related to urbanization on the species richness and abundance of native wasps and bees. Two spatial scales were considered: large portions of the urban landscape (entire sites) and small areas within these sites (public squares). Overall, the abundance of advanced eusocial bees (i.e. stingless bees), which were the dominant species, was directly affected by the loss of vegetation cover and the increase of buildings associated with urbanization. The magnitude of this effect varied according to the nesting habits of each species. The loss of vegetation cover associated with urbanization also had a negative effect on the abundance and species richness of advanced eusocial wasps. Generalist species of bees and wasps, such as Trigona spinipes and Polybia occidentalis, were very abundant and not sensitive to the habitat changes related to urbanization. Advanced eusocial bees also responded to small-scale habitat changes (size and vegetation cover of public squares). No relation was found between the solitary and the primitive eusocial wasps and bees and the habitat changes considered here. Conservation strategies in urban environments need to consider different spatial scales in order to maintain or enhance the local diversity of wasps and bees.

Introduction

Urban environments are generally characterized as areas with a high density of buildings exposed to constant and intense human activity (McIntyre et al., 2001). Urbanization, the process by which these environments are generated, can be summarized as a local increase in the density of inhabitants coupled with increased per capita energy consumption and extensive modifications of the environment (including the microclimate) (Gilbert, 1991, Vitousek et al., 1997). This process generates unstable ecosystems that depend on large inputs of energy, and where great amounts of waste materials are accumulated (Stearns, 1970, McDonnell and Pickett, 1990). Despite covering a small fraction of the Earth’s surface (<5%), urban environments have a widespread influence on surrounding ecosystems (Vitousek et al., 1997, Goudie, 2000). Half of the world’s population currently lives in urban areas. This proportion is expected to increase to 60% in the year 2030 (United Nations, 1999). In Brazil, approximately 55% of the population was living in cities in 1970. Today, almost 80% of the population is living in urban areas (IBGE, 2001). This fast growth of cities is promoting an increasing interest in urban ecosystems and in the impact of urbanization on distinct biotas (Gilbert, 1991, Rebele, 1994, Grimm, 1997, Vitousek et al., 1997). Ecological studies of such ecosystems are essential to reduce local and regional impacts of urbanization (Gilbert, 1991, Bede et al., 1997, Niemela, 1999).

Different approaches have been used to study the fauna of urban environments (Moore, 1979, McDonnell and Pickett, 1990, Niemela, 1999). The theory of island biogeography, for instance, has been the foundation for many studies of insect and bird richness in “urban islands” (e.g. parks and public squares) (reviews in Davis and Glick, 1978, McIntyre, 2000). Despite being spatially heterogeneous, urban environments are structurally well defined (Turner, 1989). Generally, each part (or structural component) of the urban landscape presents clear limits (Wiens et al., 1993, Grimm, 1997). Public squares, for example, are well-defined habitat portions surrounded by a matrix of buildings equally well defined. Although “urban islands” may be easily perceived, this biogeographic approach has some limitations since the permeability of the urban matrix may be highly variable (Niemelä, 1999). Species with great dispersal ability, for example, may pass through the matrix; consequently they can use a portion of the urban habitat larger than a specific “island”. For these reasons different spatial scales have to be analyzed to study the effects of urbanization on a particular group of species (Hostetler, 1999). Additionally, it has to be considered that urbanization is a scale-dependent process. Urban planning and development are done in an hierarchical way (“from the mayor to the gardener”), and consequently, the effects of urbanization can not be fully appreciated by considering only one spatial scale (Savard et al., 2000).

Despite the ubiquity of insects, there are few studies especially concerned with the effects of urbanization on non-pest species, particularly in the Neotropical region (Laroca et al., 1982, Davis, 1982; Ruszczyk, 1986a, Ruszczyk, 1986b, Ruszczyk, 1986c, Ruszczyk, 1986d, Ruszczyk, 1996; Diefenbach and Becker, 1992, McIntyre, 2000). Earlier studies found a relatively high insect diversity in some cities (e.g. London and Rome), suggesting that these habitats may not be as restrictive as expected (Owen and Owen, 1975, Zapparoli, 1997). In addition, it has been observed that species richness and abundance can greatly vary within each city, indicating that different levels of urbanization have distinct effects on the local insect fauna (Owen and Owen, 1975, Frankie and Koehler, 1978, Zapparoli, 1997, McIntyre, 2000, McIntyre et al., 2001).

Information about the responses of wasps and bees to urbanization is important for a number of reasons. Many wasp species, especially the eusocial ones, are key predators in tropical ecosystems (LaSalle and Gauld, 1993, Raw, 1998b). Bees are the most important pollinators of angiosperms and consequently they are also key species in many terrestrial ecosystems (Neff and Simpson, 1993). Wasps and bees are sensitive to variations in abiotic conditions (e.g. temperature, luminosity and moisture), which may be related to changes in the urbanization level (Genise, 1981, Roubik, 1992, Morgan and Jeanne, 1992). In addition, wasps and bees are frequently found in urban environments, efficiently occupying different microhabitats (e.g. walls, roofs, ceilings, etc.) (Nogueira-Neto, 1970, Fowler, 1983, Martins and Pimenta, 1993, Knoll et al., 1994, West-Eberhard et al., 1995, Saure, 1996, Raw, 1998a). Despite their ecological importance and their potential as indicators of environmental conditions (Brown, 1991), few authors have analyzed the effects of urbanization on wasp and bee assemblages. The literature suggests considerable variation in the responses of these insects to urbanization (Skibinska, 1986, Gayubo et al., 1987, Gayubo and Torres, 1989, Gayubo and Torres, 1990, Gayubo and Torres, 1991, Torres and Gayubo, 1989, Saure, 1996). In Brazil, previous studies restricted to bees have found a great variation in species sensitivity to urbanization and a considerable reduction of diversity in highly urbanized areas (Laroca et al., 1982, Bortoli and Laroca, 1997, Knoll et al., 1994).

It is generally assumed that urbanization (not considering deserts), reduces habitat complexity, mostly by reducing natural vegetation cover. Consequently, it may be predicted that less urbanized areas are structurally more complex. For these reasons, it can also be predicted that less urbanized areas support a greater abundance and species richness of wasps and bees. Our main objective in this paper is to examine these predictions. Additionally, we have attempted to analyze the effects of urbanization at a smaller spatial scale, focusing on the microhabitats of public squares. Based on island biogeography theory, it was assumed that public squares surrounded by an urbanized matrix could be considered islands with different levels of isolation according to the structural complexity of their surroundings. Hence, the following predictions were also examined: (1) larger squares support a higher abundance and richness of wasps and bees, and (2) squares with more complex surroundings support a higher abundance and richness of wasps and bees.