Abstract
Since typically there are many predators feeding on most herbivores in natural communities, understanding multiple predator effects is critical for both community and applied ecology. Experiments of multiple predator effects on prey populations are extremely demanding, as the number of treatments and the amount of labour associated with these experiments increases exponentially with the number of species in question. Therefore, researchers tend to vary only presence/absence of the species and use only one (supposedly realistic) combination of their numbers in experiments. However, nonlinearities in density dependence, functional responses, interactions between natural enemies etc. are typical for such systems, and nonlinear models of population dynamics generally predict qualitatively different results, if initial absolute densities of the species studied differ, even if their relative densities are maintained. Therefore, testing combinations of natural enemies without varying their densities may not be sufficient. Here we test this prediction experimentally. We show that the population dynamics of a system consisting of 2 natural enemies (aphid predator Adalia bipunctata (L.), and aphid parasitoid, Aphidius colemani Viereck) and their shared prey (peach aphid, Myzus persicae Sulzer) are strongly affected by the absolute initial densities of the species in question. Even if their relative densities are kept constant, the natural enemy species or combination thereof that most effectively suppresses the prey may depend on the absolute initial densities used in the experiment. Future empirical studies of multiple predator – one prey interactions should therefore use a two-dimensional array of initial densities of the studied species. Varying only combinations of natural enemies without varying their densities is not sufficient and can lead to misleading results.
Highlights
Most studies of predator-prey interactions have considered relationships between a single prey species and a single predator species [1]
Understanding multiple predator effects is critical for both community [4,5] and applied ecology – e.g., in biological control programs, predicting outcomes of multiple predator – single prey interactions is especially important [6,7,8], since interactions between introduced predators or parasitoids and other natural enemies may even inadvertently increase their prey populations [9,10,11,12,13]
Whether nonlinear outcomes are present in the form of risk enhancement or risk reduction will influence whether the prey’s population growth rates are higher or lower than those predicted by linear predator effects (e.g., [20,21])
Summary
Most studies of predator-prey interactions have considered relationships between a single prey species and a single predator species [1]. Discussion so far has not yielded any clear-cut results, mainly because the observed population dynamics in multiple predator – single prey communities are often quite complicated due to several types of nonlinear effects, which, generally predict qualitatively different results, if initial absolute densities of the species studied differ, even if their relative densities are maintained [14,15,16,17]. Most commonly mentioned in this context are nonlinear predator effects (interactions among predators), which can raise (risk enhancement) or lower (risk reduction) a prey’s risk of predation in the presence of multiple predator species [18,19]. Whether nonlinear outcomes are present in the form of risk enhancement or risk reduction will influence whether the prey’s population growth rates are higher or lower than those predicted by linear predator effects (e.g., [20,21]). There is still uncertainty about their effect on population densities of the species in question [25,28]
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