Functional diversity schemes: are they useful?
Recent years have seen great interest in the importance of species richness for the functioning and stability of ecological communities (Ives and Carpenter 2007). Empirical examinations of richness effects typically vary the number of species in experimental treatments and measure resulting ecosystem functions such as biomass accumulation or resource uptake (Naeem et al. 2009). Across trophic levels and communities of many types, a clear pattern has emerged from these experiments: community processes (biomass accumulation, resource uptake, etc.) generally become more efficient when more species are present (Hooper et al. 2005; Cardinale et al. 2006). This pattern is generally attributed to resource partitioning among species, where species differ in ecologically significant ways such that they complement one another (Hooper et al. 2005). For example, in English meadow communities multiple plant species coexist, because different plant species exploit different hydrological conditions (Silvertown et al. 1999). The plants that dominate drought-prone areas are different from those that thrive in flood-prone areas and, presumably, total plant biomass is greatest when both plant groups (drought tolerant and flood tolerant) are present.
A remaining challenge is to effectively predict, a priori, the particular species (or groups of species) that will complement one another. One simplifying scheme that has received considerable attention is the lumping of species into ‘functional groups’. In this functional-group approach, species within a group are relatively similar to one another, and considered ecologically redundant, whereas species in different groups are distinct and complementary (Hillebrand and Matthiessen 2009). This approach gained support from studies suggesting that plant species can be classified into such functional groups (grasses, forbs, legumes and woody plants), and that the number of functional groups is a more effective predictor of ecosystem function than species richness (Diaz and Cabido 2001). For example, in savannah grasslands, plant communities that included C3 grasses, C4 grasses, forbs, legumes and woody plants had greater biomass and plant nitrogen accumulation, and reduced light penetration, than those communities lacking one or more of these groups (Tilman et al. 1997). These authors suggested that competition was greater within than between functional groups, consistent with niche similarity within, but niche differentiation among, groups.
Agricultural monocultures are often thought to be more prone to herbivore outbreaks than natural systems, and early agroecologists posited that the lack of biodiversity in agricultural systems contributes to their instability (Pimentel, 1961; van Emden & Williams, 1974). In contrast, some detailed reviews have concluded that perhaps one or two particularly effective natural enemies are all that is needed for effective pest control (Hawkins et al., 1999). Such issues come to the fore when a decision must be made in classical biological control about whether to introduce one or several natural enemy species in an effort to control exotic pests (Myers et al., 1989; Denoth et al., 2002), and when designing schemes to conserve indigenous natural enemies by modifying cultural practices (Landis et al., 2000; Tscharntke et al., 2005). Here, we first review the major classes of natural enemies – specialists and generalists – and the traits of each that are likely to contribute to (or detract from) their effectiveness as biological control agents. We then discuss interactions within diverse communities of natural enemies that are likely to affect biological control.
Specialist natural enemies: the best biological control agents?
Biological control practitioners have long debated the question: what are the traits of an effective biological control agent? General consensus seems to focus around a few traits that a successful agent will possess (see Chapter 9).
Email your librarian or administrator to recommend adding this to your organisation's collection.