3.20.1 Allotment invertebrates and comparative studies
The composition of the taxa found in the current study was similar to that of the BUGS gardens studies discussed in more detail below, but the actual
proportions of some of the taxa were quite different. For example, whilst Smith et al. (2006b) found that the three most abundant taxa of the pitfall traps were woodlice (45%), beetles (25%) and slugs (19%) respectively, they constituted 24%, 38% and 9% respectively in the current study. In addition, spiders contributed 17%., compared to less than 5% in the BUGS study. The reasons for these differences are likely to be many. However, in the case of the slugs, it is more likely that this species would be much more actively discouraged from allotments, due to their primary raison d’etre as a means of growing food crops (see Chapter 2, Section 2.10.9) and as shown by the prevalent use of slug pellets (see Chapter 2, Section 2.10.14).
There are very few published studies specifically on allotment invertebrates (as discussed in Section 3.1), partly because, like private householders’ gardens, allotment plots are outside the immediate control and management remit of local government and administrative authorities (Loram et al., 2007). This means there has been little interest in funding research and, as found by the Sheffield gardens (BUGS) project detailed at the end of Section 3.1, may reflect the difficulties in systematically obtaining data from a fragmented, rather
inaccessible resources with multiple owners and tenants (Thompson et al., 2003; Thompson et al., 2004; Smith et al., 2005; Loram et al., 2007). Fortunately for the current study, one local authority, Hull City Council, along with the University of Hull, saw the value of such a study of allotments using the interdisciplinary approach advocated by Grimm et al. (2000) (see Chapter 1, Section 1.3). This was due in part to their close links to Local Biodiversity Action Plans and the potential not only to collect previously un-gathered scientific data, but for community engagement and participation, across local authority boundaries.
There were however some urban garden and wider urban greenspace studies that could be used for some comparisons. For example, Smith et al. (2006a,b) studied a range of environmental and ecological factors, including the
abundance and diversity of pitfall trapped invertebrates, in urban gardens in Sheffield, Yorkshire to determine their biodiversity value (BUGS project). They found that a broad range of factors operated at the individual garden scale, across geographic scales and per garden management styles, but different factors were important for the relative abundance of different species (Smith et al., 2006a,b).
3.20.2 Exploring invertebrate abundance and diversity along the urban- rural gradient
Blair (1999) found in a study of butterflies and birds along an urban-rural
gradient that the two groups differed in abundance. Whilst butterfly abundance did decrease as the sites become more urbanized, the birds were most
abundant at the suburban sites. Similarly, a study by Eremeeva and Sushchev (2005) found that the city centre plots in Kemerovo, Russia, contained a higher
abundance (but lower diversity) of butterflies than the suburbs. The trend was similar in the current study as that for the butterflies in the two discussed above, as the urban areas had the highest invertebrate abundance, followed by the suburban sites.
With regard to diversity, Zapparoli (1997) found a rich invertebrate fauna in the heavily urbanized city of Rome, which was partly explained by the mosaic pattern of urban environments caused by urbanization over the last 140 years, river modification (which caused flooding), land reclamation and disease vector control. He found that the main invertebrate reservoirs were parks and
historical villas, urban and semi-natural greenspaces and archaeological sites, some of which formed ‘green corridors’. However, researchers such as Kuhnelt (1982) have found that in general, the number of invertebrate species (species richness) diminishes from the outskirts to the centre of the city. In the current study, the results were rather mixed, with an overall trend towards highest diversity on most suburban sites (see Figure 3.7). In a review of the effects of urbanization on species richness, McKinney (2008) found that for invertebrates, about 30% of the studies had shown moderate levels of urbanization related to the intermediate disturbance hypothesis (IDH) (Connell, 1978). Thus, the current results, whilst not fully agreeing, do provide some evidence for the intermediate-disturbance hypothesis (Connell, 1978; Collins et al., 1997; Eckert and Walz, 1998), which suggests that areas with a moderate degree of
disturbance will be more diverse than either an undisturbed or a highly disturbed area. The IDH and other factors of disturbance (see Section 3.19 above) will be examined in greater detail in relation to individual species found in Chapter 5 and discussed further in Chapter 6.
According to the theory of MacArthur and Wilson (1963), the number of species on islands is the result of an equilibrium between immigration and extinction rates, which are determined by, among other things, the size of the island (Schaefer, 1982). Faeth and Kane (1978) have suggested that the same may be true for green islands in the urban area. Logically therefore, that is what one would expect to find on urban patches (effectively islands surrounding by hard surface) due to the increasing range of anthropogenic pressures (as discussed in the previous Chapters and summarised in Section 3.1) as one gets closer to a city centre. Although patch size is a somewhat simplistic explanation, it does go some way to help explain the current results found. These aspects will be more fully explored in Chapter 5 in relation to individual species.
The epigeal invertebrate abundance and diversity found on the Yorkshire
allotments, especially in the city centre, would seem to compare favourably with other urban invertebrate studies in that the sites were relatively abundant and diverse. This is particularly true for the Hull sites, Newland and Bude, as Hull was built mainly on the floodplain, with the allotments forming a greenspace refuge matrix, similar to the findings of Zapparoli (1997) discussed above. The following Chapters will explore the abundance and diversity of the invertebrates in relation to allotment management style (Chapter 4) and any effects at
individual species level for selected taxa (Chapter 5), along with management and climatic implications (Chapter 6) therefore will not be discussed in further detail here.
3.21 Summary
• From a range of measured environmental and biological variables, a pattern emerged showing groupings of allotment sites which could be classed as urban, suburban or rural. The key urban determinants were the amount of surrounding hard surface, site area and human population, whilst the main rural determinants were the amount of surrounding farmland and number of surrounding trees. The suburban sites were mainly characterized by site area and the number of plots on site.
• Beetles, woodlice and spiders constituted almost 79% of the total catch, with slugs, millipedes, opilione, snails and centipedes making up the other 21% respectively.
• The urban site contained the highest proportion of epigeal invertebrate abundance, which may not have been the expected result due to the higher anthropogenic pressures on such sites. Reasons for this are likely to be complex, but may be related to the lack of alternative suitable habitat, allowing invertebrates to congregate in urban allotment patches with limited option for dispersal. Invertebrates on more rural sites have greater dispersal opportunities due to greater availability of suitable habitat. The suburban sites appeared to be intermediate.
• Cottingham, a suburban site, had both the highest diversity and evenness. Beverley, also a suburban site, had the lowest diversity and Driffield, a rural site, had the lowest evenness. All other sites were intermediate in diversity and evenness.
CHAPTER 4: MORPHO-SPECIES ABUNDANCE AND DIVERSITY IN