5.0 INTRODUCTION
5.1 Invertebrate species distributions
Data on the individual species composition of epigeal invertebrate fauna of allotments are not widely available, but in some respects can be compared to agricultural land. For example, Wallin (1988) noted that cereal fields are unstable habitats in which survival of a particular carabid species should be related to its dispersal tendencies. Thus species associated with unstable habitats (e.g. allotments) are generally considered to have a higher dispersal capacity than those from more stable habitats (e.g. woods) (Wallin, 1988 and refs therein). It is likely therefore that allotments would provide largely unstable habitat, with small oases of stable habitat in the form of compost heaps,
boundary hedges etc.
For non-flying epigeal insects in agricultural landscapes, dispersal between local populations that maintains a metapopulation is dependent upon field boundary permeability and has been shown to have a theoretical optimum depending on the frequency and intensity of pesticide use on the farm (Sherratt and Jepson 1993 in Holland et al., 2005). This is likely to hold true for insects in urban/suburban situations albeit perhaps with different boundary types. In addition, taxa with limited mobility are less able to exploit spatially varying resources and avoid local disturbance (Hilty and Merenlender 2000) and thereby provide a clear signal of local changes. Allotments would therefore make the ideal study habitat to explore these issues.
When conducting invertebrate sampling to species level, it is often necessary to examine only a proportion of the species caught as even limited sampling of invertebrates can yield an enormous number of specimens and an immense array of species (Ward and Lariviere, 2004; New, 2005). Ward and Lariviere (2004) also note this sub-sampling is necessary due to time, cost and expertise limitations but the detailed study of a limited number of carefully chosen
taxonomic groups is more productive and realistic than attempting to evaluate a larger number of groups superficially (New, 1996). Ideally, a suite of taxa would represent, among other things, major functional guilds and would convey as much information as possible on the sampled sites (New, 1998).
The opportunistic species hypothesis (Gray, 1989; Magura et al., 2004) states that at high levels of disturbance, opportunist species are predicted to gain dominance. This hypothesis would therefore suggest that allotments would be dominated by generalist species. In addition, as allotments are usually regularly disturbed habitats due the nature of their purpose and management i.e. regular digging over and harvesting of crop (see Chapter 1, Section 1.9). This could cause changes in invertebrate species composition, population dynamics, breeding success and a range of ecological and ecosystem processes (Donald and Evans, 2006). Smaller allotment sites may be at greater risk of species’ extinctions due to the smaller population size of any given species as the dynamics of small populations are governed by a high level of uncertainty (Begon et al., 1996). At an even smaller scale, each allotment plot is managed individually, therefore may act as a factor for further habitat fragmentation, especially for species that only travel a few metres in their lifetime and
see Margules and Milkovits, 1994). The theory of island biogeography (MacArthur and Wilson, 1967) predicts an equilibrium number of species on ‘islands’ and this theory has been used to predict that fewer species will persist in small more remote habitat remnants compared to larger, well connected ones (Margules and Milkovits, 1994). However, as noted in Chapter 1, Section 1.9, the effects of disturbance on any community, regardless of position along the urban-rural gradient, are more complex than any single theory can combine. A variety of abiotic and biotic factors may act as agents of disturbance (Svensson et al., 2007). Whilst Grime (1977) defined disturbance as partial or total
destruction of biomass, Pickett and White (1985) have a broader definition where disturbance is “any relative discrete event in them that disrupts ecosystems, community or population structure and changes resources, substrate availability or the physical environment” (in Svensson et al., 2007). Thus, epigeal invertebrates on an urban-rural gradient would also be likely to conform to the intermediate disturbance hypothesis (Connell, 1978; Roxburgh et al., 2004) which states that species diversity is highest at intermediate levels of disturbance. It would therefore be predicted that diversity would be highest on suburban allotments.
If a clear picture of the actual species composition of allotment epigeal
invertebrates could be built up it could serve a number of useful purposes. For example, predictions could be made about their resilience to issues such as climate change and invasion by non-native species (e.g. Harlequin ladybird), more appropriate management choices could be made and the biodiversity value of allotments could be increased.
5.2 Aims of this Chapter
The first aim of this Chapter was to determine which individual species of spiders, woodlice and beetles were present on three Yorkshire allotment sites. Next, the species abundance and diversity of these species were compared across an urban-rural gradient, in relation to two different allotment
management styles: traditional and wildlife-friendly. The aims were defined in relation to the second research question:
Q2: What is the composition of the epigeal invertebrate communities on allotments and do they vary in relation to position along an urban-rural gradient and/or to individual plot management style?
5.2.1. Invertebrate species abundance
Specifically, the aims were addressed by the following objectives to: