The scope and value of extensive ecological studies in the broom biological control programme.

Abstract

AbstractBiological control programmes have varied in the extent of ecological studies carried out on potential agents and target organisms. At one extreme, studies were limited to surveys for potential biological control agents, which were then shipped to the target country. The biological control programme against Scotch broom (Cytisus scoparius) is at the other extreme: we list 29 significant studies on the ecology of broom in its native and introduced ranges, probably representing at least 100 years of work by scientific personnel. These studies have proved useful for predicting the success and safety of the biological control programme against broom as an alien weed. Three studies highlight the significance of insect herbivores reducing broom longevity and seed production: an 11-year insecticide exclusion experiment in the UK; replicated experiments examining recruitment of broom as a native plant in the UK/France and as an alien weed in New Zealand/Australia; and simulation modelling of broom abundance. However, predicting the contribution of each biological control agent to broom suppression is difficult, and may be required for risk-benefit analyses under new regulations. The safety of broom agents to non-target plant species has been addressed using normal host range testing procedures, and extensive field surveys of broom and related plants in Europe. There are concerns over adverse, indirect effects that released biological control agents may have on non-target organisms. For insect herbivores released against weeds, adverse indirect effects could occur because the agent acts as a new food source, competitor or disease vector in existing food webs; or because there are adverse effects on existing biota via a reduction in the abundance of the target weed (or non-target plant). Ecological studies of broom enable us to identify some of these potential effects, but predicting their magnitude prior to release of an agent is a major ecological challenge. Equally important to emphasize are the potential benefits to indigenous species from suppression of alien weeds. A balance is needed, because the time and resources required to make accurate ecological predictions of potential adverse effects from released agents could impede biological control of key environmental weeds, resulting in worse net impacts on indigenous species.