Biodiversity on urban roundabouts—Hemiptera, management and the species–area relationship
Introduction
With the exception of work on synanthropic pest species, the study of insects and other arthropods within urban environments has been a relatively neglected area of study, despite the large and ever increasing size of such areas (McIntyre, 2000; McIntyre, Rango, Fagan, & Faeth, 2001). However those studies that have been published have reported that considerable insect biodiversity exists within towns and cities, including a number of rare and notable species (Chudzicka, 1986; Zapparoli, 1997; McIntyre, 2000; Hostetler & McIntyre, 2001; Jones, 2003). Consequently it is important that consideration is given to the factors, which influence biodiversity within urban areas, particularly as this knowledge has the potential to modify management and planning practices, which would be beneficial to the enhancement of biodiversity.
Many factors have been suggested as important in determining the relative species richness of different urban habitats and habitat patches. These include the level of isolation and fragmentation, management practices, disturbance, and habitat type, age, area, diversity and connectivity, as well as the characteristics, such as relative mobility, of different taxonomic groups (Davis & Glick, 1978; Zapparoli, 1997; Denys & Schmidt, 1998; Fernández-Juricic, 2000; McIntyre, 2000; Savard, Clergeau, & Mennechez, 2000; Ganzhorn & Eisenbeiß, 2001; Hostetler & McIntyre, 2001; Rudd, Vala, & Schaefer, 2002; Whitmore, Crouch, & Slotow, 2002; Weller & Ganzhorn, 2004).
In this study the species richness of Hemiptera was investigated in relation to both habitat management and area. Habitat management has long been recognized as having a major effect on grassland Hemiptera communities (Morris, 2000). Both cutting and grazing, have been found to reduce overall abundance and species richness because they reduce vegetation height and structural complexity of the grassland, thus reducing the stratification that is so important in determining species diversity (Andrzejewska, 1965; Waloff & Solomon, 1973; Denno, 1977; Morris, 1979; Morris & Lakhani, 1979; Prestidge, 1982; Sedlacek, Barrett, & Shaw, 1988; Brown, Gibson, & Kathirithamby, 1992; Dennis, Young, & Gordon, 1998; Morris, 2000).
In urban areas grasslands are frequently under relatively intensive management, principally through mowing, which may be repeated at regular intervals throughout the growing season. In contrast, arboreal habitats are much less frequently or catastrophically managed. Management of arboreal habitats is likely to be restricted to tree planting, pruning and felling. In established habitats not undergoing development or other land use changes, felling is relatively uncommon, and pruning, in most cases, does little to alter habitat structure. Tree planting can be quite frequent and as such may be the most important form of arboreal habitat management in the short term. Thus there appears to be a contrast between grassland management with its dramatic alteration to the physical structure of grasslands, and consequent strong influence on Hemiptera populations, and the probably relatively small effect of management on arboreal Hemiptera communities.
Area has been related to biodiversity principally through the species–area relationship. The species–area relationship is a very frequently described pattern in which species richness increases with area (Lomolino, 2001). Such patterns have been found not only on oceanic and other very clearly delimited areas, but also terrestrial habitat patches and even using non isolated quadrats (Denno & Roderick, 1991; Crawley & Harral, 2001). This pattern has been recognized for a long time (Arrhenius, 1921) and much effort has been put to describing species–area relationships mathematically as well as proposing possible mechanisms that may give rise to them (MacArthur & Wilson, 1967; Connor & McCoy, 1979; McGuinness, 1984; Hill, Curran, & Foody, 1994; Lomolino, 2001).
In the work described here the interaction of area and habitat management was studied by looking for species–area relationships in two groups of Hemiptera, arboreal and grassland, as well as grassland plants. Of the very many previous studies of the species–area relationship, very few have been carried out in urban areas. Those that were, have mostly concerned areas such as parks or semi-natural fragments of habitats such as woodland (e.g. Faeth & Kane, 1978; Miyashita, Shinkai, & Chida, 1998; Fernández-Juricic, 2000). However urban areas contain many other open areas, which are potentially important reservoirs of urban biodiversity, including road verges, roundabouts and other road delimited areas. Whitmore et al. (2002) looked at road islands without finding significant species–area relationships. In the work presented here, roundabouts within the town of Bracknell, in south east England, were used to investigate both whether significant species–area relationships can be found in urban areas using relatively small road delimited sites, and whether two groups of Hemiptera exposed to contrasting management intensity showed differences in those species–area relationships.
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Study sites
The study was carried out within Bracknell, which is a town located in southeast England between latitude 51°23′ and 51°26′N, and longitude 0°43′ and 0°47′W. The 18 study sites were a series of 14 roundabouts and four other road enclosed sites. Further details of the names and locations of the sites are given in Table 1.
Site mapping
The area of the study sites (Table 1) was determined by using an aerial photograph of Bracknell, which enabled not only the total area to be assessed but also the area of
Species recorded
In total 561 individuals of 71 arboreal species, and 579 individuals of 43 grassland species of Hemiptera were recorded (Table 2), with only four species collected during both tree and grassland sampling. A summary of the number of species of trees and grassland plants is given in Table 2.
Species–area relationships: arboreal Hemiptera
There was a significant species–area relationship between the log species richness of arboreal Hemiptera and log area (, , ) (Fig. 1).
Multiple regression, using log area, the log number
Species–area relationships
The arboreal Hemiptera showed a significant species–area relationship, which was found to be the result of the increasing number of tree species, which was correlated with area. The arboreal Hemiptera show a high degree of host plant specialization, with many monophagous or oligophagous species, and consequently each tree species represents a different habitat with its own particular associated insect species (Southwood & Leston, 1959; Southwood, 1961; Le Quesne, 1965; Le Quesne & Payne, 1981;
Conclusions
This study has implications for the conservation and enhancement of urban biodiversity. It has demonstrated that given sympathetic management, small areas of urban grassland can support abundant and species rich Hemiptera communities. The road delimited areas studied are generally of fixed dimension and location, and therefore species richness cannot be enhanced by enlarging their area or increasing connectivity. However they have considerable potential to be altered in terms of their
Acknowledgements
We thank Dr. S. McNeill for use of the Vortis suction sampler. Dr. Bernard Nau provided draft copies of his key to the Miridae and Heteroptera checklist, and helped with the identification of some species. We are very grateful to Bracknell Forest Council and Bracknell Town Council for their advice and for allowing us to sample. We would also like to thank Terry Mawdesley and Jane Helden for their assistance.
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