No abstract

A recent monograph by Hurlbert raised several problems concerning the appropriate design of sampling programs to assess the impact upon the abundance of biological populations of, for example, the discharge of effluents into an aquatic ecosystem at a single point. Key to the resolution of these issues is the correct identification of the statistical parameter of interest, which is the mean of the underlying probabilistic "process" that produces the abundance, rather than the actual abundance itself. We describe an appropriate sampling scheme designed to detect the effect of the discharge upon this underlying mean. Although not guaranteed to be universally applicable, the design should meet Hurlbert's objections in many cases. Detection of the effect of the discharge is achieved by testing whether the difference between abundances at a control site and an impact site changes once the discharge begins. This requires taking samples, replicated in time, Before the discharge begins and After it has begun, at both the Control and Impact sites (hence this is called a BACI design). Care needs to be taken in choosing a control site so that it is sufficiently far from the discharge to be largely beyond its influence, yet close enough that it is influenced by the same range of natural phenomena (e.g., weather) that result in long-term changes in the biological populations. The design is not appropriate where local events cause populations at Control and Impact sites to have different long-term trends in abundance; however, these situations can be detected statistically. We discuss the assumptions of BACI, particularly additivity (and transformations to achieve it) and independence.

"Switching" in predators which attack several prey species potentially can stabilize the numbers in prey populations. In switching, the number of attacks upon a species is disproportionately large when the species is abundant relative to other prey, and disproportionately small when the species is relatively rare. The null case for two prey species can be written: P1/P2 = cN1/N2, where P1/P2 is the ratio of the two prey expected in the diet, N1/N2 is the ratio given and c is a proportionality constant. Predators were sea—shore snails and prey were mussels and barnacles. Experiments in the laboratory modelled aspects of various natural situations. When the predator had a strong preference (c) between prey the data and the "null case" model were in good agreement. Preference could not altered by subjecting predators to training regimens. When preference was weak the data did not fit the model replicates were variable. Predators could be trained easily to one or other prey species. From a number of experiments it was concluded that in the weak—preference case no switch would occur in nature except where there is an opportunity for predators to become trained to the abundant species. A patchy distribution of the abundant prey could provide this opportunity. Given one prey species, snails caused a decreasing percentage mortality as prey numbers increased. This occurred also with 2 prey species present when preference was strong. When preference was weak the form of the response was unclear. When switching occurred the percentage prey mortality increased with prey density, giving potentially stabilizing mortality. The consequences of these conclusions for prey population regulation and for diversity are discussed.

Population regulation is a fundamental process related to most phenomena in ecology, including evolutionary ecology. I review the conceptual basis for defining regulation as bounded fluctuations in abundance, in contrast to the unbounded fluctuations of random—walk populations. Regulation arises as a result of potentially stabilizing density—dependent processes, even when brought about by “non—equilibrium” mechanisms. Although the phenomena is unambiguous in theory, detecting regulation by finding evidence for density dependence in a series of population estimates faces unsolved statistical problems. So, while there is growing evidence for widespread regulation, severe detection problems remain. I illustrate these with data from bird populations. Whether regulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent studies on a particularly well—regulated system–red scale (Aonidiella aurantii) and its controlling parasitoid, Aphytis melinus. We tested and failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rates, a refuge, and metapopulation dynamics. We also failed to find evidence for density—dependent parasitism, but such density dependence might be still be present. Recent laboratory and modeling studies have uncovered a number of other potentially stabilizing mechanisms centering on the response of individual Aphytis to their size—structured host. This plethora of size— and stage—dependent interactions leads naturally to a consideration of the factors controlling Aphytis' size—dependent behavioral decisions, and consequently to the elaboration of size—structured models. The latter provide a vehicle for bringing together investigations of selection of life histories, and population dynamics. This is illustrated by a model of Aphytis and red scale dynamics that can explain a dramatic case of competitive displacement. The red scale/Aphytis system exemplifies a particularly challenging problem in population regulation, namely to account for the co—occurrence of stability and severe suppression of the prey population. A potentially generic solution is to assume stabilizing density dependence in the parasitoid or predator population; however, this has the consequence of increasing the host or prey population equilibrium. My colleagues and I have shown that observed prey densities in a plankton system are too low for such a mechanism to be operating. Further work is needed to test this and other hypotheses.

Conservation priority-setting schemes have not yet combined geographic priorities with a framework that can guide the allocation of funds among alternate conservation actions that address specific threats. We develop such a framework, and apply it to 17 of the world's 39 Mediterranean ecoregions. This framework offers an improvement over approaches that only focus on land purchase or species richness and do not account for threats. We discover that one could protect many more plant and vertebrate species by investing in a sequence of conservation actions targeted towards specific threats, such as invasive species control, land acquisition, and off-reserve management, than by relying solely on acquiring land for protected areas. Applying this new framework will ensure investment in actions that provide the most cost-effective outcomes for biodiversity conservation. This will help to minimise the misallocation of scarce conservation resources.