Biological control in a fragmented landscape

We know that spatial heterogeneity can stabilize predator-prey interactions (e.g. Chapter 10). However, the dynamics of pests and their biological control agents may become destabilized, resulting in pest outbreaks, if habitat change occurs at a scale that these uncertainties make ecologists all the more valuable

'dataless management' where no estimates are available?

natural enemy success may depend on predation efficiency in a patchy habitat

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0 10 20 30 40 50 60 Habitat abundance (% clover)




e d







u c






10 20 30 40 50 60 Habitat abundance (% clover)

Figure 15.19 The distribution pattern (lacunarity index - a measure of aggregation) of (a) clover (i.e. habitat) and populations of (b) pest aphids, (c) an introduced ladybird beetle control agent (Harmonia axyridis) and (d) a native ladybird beetle (Coleomegilla maculata). In these experiments, clover plants were clumped together as opposed to being dispersed through the landscape. Error bars are ±1 SE. (After With et al., 2002.)

o o o o interferes with the search behavior of a control agent (Kareiva, 1990).

With et al. (2002) created replicate landscapes (plots) of red clover (Trifolium pratense), each 16 X 16 m, that differed in terms of clover abundance (10, 20, 40, 50, 60 and 80% T. pratense). Their aim was to explore whether thresholds in landscape structure precipitate similar thresholds in the distribution of a pest aphid, Acyrthosiphon pisum, and to discover how landscape structure affects the search behavior of two ladybird beetle predators of aphids, one an introduced biocontrol agent, Harmonia axyridis, the other a native species, Coleomegilla maculata. Colonization by the aphids and beetles was by natural immigration to the outdoor plots.

Lacunarity is an index of aggregation derived from fractal geometry that quantifies the variability in the distribution of gap sizes (distances among clover patches in the landscape). The distribution of clover in the experimental landscapes showed a threshold at 20% habitat, indicating that gap sizes became greater and more variable below this level (Figure 15.19a). This threshold was mirrored by the aphids (Figure 15.19b) and was strongly tracked by the exotic control agent (H. axyridis) but not the native predator (C. maculata) (Figure 15.19c, d).

Although the native ladybird foraged more actively among stems within the clover cells, overall it was less mobile and moved less between clover cells in the landscape than the introduced ladybird, which showed a greater tendency to fly (Table15.5). With its greater mobility, the introduced species was more effective at tracking aphids when they occurred at low patch occupancy, a prerequisite for successful biological control (Murdoch & Briggs, 1996).

Findings such as these have implications both for the selection of effective biological control agents and for the design of agricultural systems, which may need to be managed to preserve habitat connectivity and thus enhance the efficiency of natural enemies and/or biological control agents (Barbosa, 1998).

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