EFFECTS OF DENSITY-DEPENDENCE, FEEDBACK AND ENVIRONMENTAL SENSITIVITYON CORRELATIONS AMONG PREDATORS, PREY AND PLANT RESOURCES - MODELS AND PRACTICAL IMPLICATIONS

1. The methods of time averaging (Puccia & Levins 1985) are used to sh ow that a positive correlation between predator and prey populations i s neither necessary nor sufficient as evidence of density dependence. The sign of the correlation depends on which species is impacted by th e environment. This is a general property of systems with negative fee dback loops. Correlation patterns are useful for diagnosing the dynami cs of populations and for evaluating control strategies. Multiple pred ators are modelled, with or without negative feedback or self damping in their own numbers, along with prey and plant resources, and the imp lications of environmental change are examined. Literature is reviewed to see how these models may help explain observed patterns. 2. Many a uthors have defined density dependence in terms of correlations betwee n prey numbers and the rates of predation or parasitism. So defined, p ositive density dependence is less common than expected, which has led to much discussion about possible explanations and complicating facto rs, such as community interactions, time lags, and environmental distu rbances. It is suggested here that the term density dependence be rest ricted to the biological effects of population density on the reproduc tion and/or survival of members of that population. 3. The impact of a natural enemy on the average population of its prey varies directly w ith the predation rate and inversely with its self damping. However, a predator that reduces the correlation between a pest and its host pla nt can reduce resource or yield loss without affecting the average pes t population. 4. With self damped predators, environmental variability that acts directly on the plant resource generates a positive correla tion between resource and consumer. Variation affecting a predator gen erates a negative correlation, so that pest populations are low when t he resource is most available, favouring increases in that resource. T hus, for example, a biocontrol agent that is sensitive to environmenta l change may increase yields. A combination of predators which reduce pest numbers and also the correlation with the resource may be prefera ble, including predators which are not self damped but are sensitive t o the environment.