Food webs, risks of alien enemies and reform of biological control.

Abstract

As ilwasions of alien species mount, biological control ''''ill become an increasingly important tool of conservation and agriculture. In an effort to understand indirect interactions in biological control, we review food web ecology in terms of resource competition, trophic cascades, intra-guild predation, apparent competition, omnivory and a diverse set of tri-trophic interactions. The most inclusive study suggests that food webs in biological control are simpler than in natural communities. Risks to non-target species created by biological control have been studied seriously for only about 20 years, and knowledge of these risks is incomplete. The greatest risks are known to be posed by the organisms with the broadest diets, such as vertebrates and the snail Euglandina rosea, which has probably caused the extinction of an entire genus of native snails in Polynesia. Some parasitoid species have been introduced that are sufficiently polyphagous to attack native insects, and cases of serious harm to non-target populations are now coming to light. However, polyphagous organisms continue to be imported for biological control. One case in point is the campaign against the Russian wheat aphid, in which over 8.5 million individual invertebrates, including more than 1 million individuals of 12 species of ladybird beetles new to North America, were released over the past 15 years, with little study of potential non-target effects, direct or indirect. Another case is the new use of the polyphagous black carp for suppression of pest snails in industrial catfish ponds. This fish poses great risks to the high native diversity of molluscs in the Mississippi drainage. We argue that risk to native flora in © CAB International 2001 . Evaluating Indirect Ecological Effects of Biological Control (eds E. Wajnberg, J.K. Scott and P.c. Quimby) 57 58 O.R. Strong and R. W. Pemberton biological control of weeds can be judged before introduction. For the New Vorld, the lowest non-target risk comes from stenophagous insects released against weeds with no native congeners. vVhen weeds have native congeners, introductions of even relatively stenophagous insects have led to the use of non-target, native plants. Restraint is key to safe biological control. First must come judicious winnowing of potential targets. Not every alien species is a threat. Biological control is not the appropriate response to every pest, especially to native species perceived as pests. Second, not every available enemy promises relief. Importing multiple agents in a lottery search for one that might do the job increases the probability of non-target attacks upon the native biota. Restraint can come only from open discussion of risks versus benefits of biological control. What was the basis for the choice of the large number of imported enemy species in the campaign against Russian wheat aphid? 'bat is the calculus of risk versus benefit in the dissemination of the black carp in the Mississippi delta? Regulation of biological control in the USA is archaic. Oversight derives from a hotchpotch of old legislation designed to serve agriculture, and protection of native species under the current regulatory frame,,,'ork is deficient. Native invertebrates, terrestrial, aquatic and marine, are at greatest risk in the current structure, while native plants have had some, but not full, protection from foreign herbivores imported and disseminated for biological control. Although the ecological and economic value of invertebrates is not widely appreciated, these species are crucial to ecological integrity of our wild, urban and agricultural landscapes. Indirect interactions among native invertebrates can be threatened by alien species, and these contribute to the integrity of natural food webs. The sensible course is to extend Federal protection to minimize the risk to all native organisms. There is also a great need to pay attention to biological control organisms after they are released, and to restrict the propagation and re-release of those that can damage non-target organisms. ,lith invasive species just as with many environmental issues, doing nothing is not neutral. Imported natural enemies are the last best hope to parry some of the most damaging exotic pests in natural areas as well as in agriculture. In the absence of reform, rational as well as irrational opposition to biological control will grow. Only sensible reform will maintain public support for this powerful tool.