Indirect effects on other trophic levels and food webs
Specific recommendations in this area are extremely difficult, and studies need to be designed on a case-by-case basis. In general, the ecology of the system within which non-target parasitism is occurring needs to be very well known if realistic indirect effects are to be measured. The ultimate goal of the release of biological control agents is the restoration of invaded ecosystems. Uninvaded reference sites or long-term documentation of communities before release of biological control agents would provide useful benchmarks (Blossey 1999). The current poor availability of biological inventories will make true assessments of indirect impacts on food webs and species difficult. Monitoring protocols need to be able to detect the extent to which the release of biological control agents can drive population fluctuations or changes in ecosystem function. Natural ecosystems are immensely complex, although invaded systems may have lost a degree of their original complexity. However, the prevalence of organism interactions makes it difficult to predict the response of even well-understood systems to environmental change or perturbations (Yodzis 1988, Polis and Strong 1996). Consequently, arguing with confidence that conditions have ecologically improved or deteriorated, or are simply different due to changes associated with spread of biological control agents, is impossible unless such impacts can reliably be distinguished from natural oscillations or plant succession. Lag effects make the detection and mitigation of impacts even more challenging (Byers and Kendall 1982, Parker et al. 1999). Laboratory and small-scale field experiments can not adequately replicate interactions that occur in the field. The only way to capture the full range of ecological effects of the release of biological control agents is by detailed observations in actual ecosystems.
Blossey B. (1999). Before, during and after: the need for long-term monitoring in invasive plant species management. Biological Invasions 1: 301-311.
Byers R.A. and Kendall W.A. (1982). Effects of plant genotypes and root nodulation on growth and survival of Sitona spp. larvae. Environmental Entomology 11: 440-443.
Parker I.M., Simberloff D., Lonsdale W.M., Goodell K., Wonham M., Kareiva P.M., Williamson M.H., Von Holle B., Moyle P.B., Byers J.E. and Goldwasser L. (1999). Impact: toward a framework for understanding the ecological effects of invaders. Biological Invasions 1: 3-19.
Polis G.A. and Strong D.R. (1996). Food web complexity and community dynamics. The American Naturalist 147: 813-846.
Yodzis P. (1988). The indeterminancy of ecological interactions as perceived through perturbation experiments. Ecology 69: 508-515.
Direct effects on non-target, beneficial or valued species