Understanding the Ecology of Urban Green Space:Decreasing Habitat Quality and Increasing Isolation?

Emma Small
School of Geography and Environmental Sciences, University of Birmingham.

Introduction

Contrary to common perception, cities in the UK contain substantial areas of “green space” which therefore has conservation potential, but the factors affecting the ecology of these urban spaces needs to be understood.

Urbanisation involves changes in habitat quality, but also habitat isolation. The habitat quality of urban green space is affected by, for example, increasing pollution; intensive management and recreational use; soil disturbance and the presence of non-native plants. These factors influence the suitability of habitat patches for given invertebrate species. Planners will ask, what are the most important things to do to improve habitat quality?

Urbanisation also increases the isolation of habitat patches through (a) habitat fragmentation and (b) encapsulation in an ‘urban matrix’. If we find that populations are limited by isolation, the ecology of urban green spaces needs to be understood in terms of models of island biogeography. Planners will then need to ask, does this site have low conservation potential because it is isolated? Will building here significantly increase the isolation of nearby patches? Will “wildlife corridors” enhance biodiversity by reducing isolation?

Aims and hypotheses

The aim of this project is to investigate whether isolation can be shown to have a significant effect on urban invertebrate assemblages. Three initial hypotheses are tested here:

H₁: Increasing patch isolation affects habitat specialists more than habitat generalists

(Habitat generalists should be less affected as they survive more readily in the matrix between patches)

H₂: Increasing isolation will affect poor dispersers more than good dispersers

(Poor dispersers should be more affected because they find it harder to move between patches).

 H₃: Corridors of similar habitat act significantly to reduce isolation in an urban environment.

Methods

(A) Sixty-five sites were selected:

• 25 derelict sites across the conurbation in areas of low & high dereliction;

• 28 wetland sites located either on or off a river corridor that flows through the conurbation;

• 12 woodland sites located on four rural-urban gradients.

(B) Beetles and spiders were sampled at each site between April and October 1999:

The assemblages are then characterised in terms of:

• Species richness, diversity, dominance, rarity, & ‘urbanity’;

• Life traits, sex-ratio, habitat fidelity, morphology, wing morph, & dispersal ability.

In addition, allozyme analysis is used for a limited number of species.

(C) Measures of “habitat quality” were taken:

• Soil samples were tested for moisture, pH, and OM content;

• Soil penetrability, cover of litter, bare ground, moss; and degree of shading were quantified;

• Vegetation type, structure and density were recorded

(D) “Landscape metrics” describing the area surrounding each patch are derived:

A GIS is used to analyse the structure and nature (including ‘permeability’) of the urban matrix surrounding each site.

Preliminary results

Extremely preliminary results are as follows:

• Urbanisation appears linked to a decline in the species richness of carabids at wetland sites (particularly habitat specialists). Further analysis is required to establish the cause (H₁ and H₄);

• The R. Cole “wildlife corridor” does not appear to enhance the number of wetland specialist species but it may act to increase and stabilise the number of habitat generalists (H₄);

• The carabid assemblages of derelict sites appear unaffected by isolation (as measured by the amount of similar habitat in the vicinity). This is perhaps due to the high mobility of the habitat specialist species (H₂ and H₃).