Quantifying predation on folivorous insect larvae: the perspective of life‐history evolution (original) (raw)
Related papers
Journal of Animal Ecology, 2009
1. Body size is positively correlated with fecundity in various animals, but the factors that counterbalance the resulting selection pressure towards large size are difficult to establish. Positively sizedependent predation risk has been proposed as a selective factor potentially capable of balancing the fecundity advantage of large size. 2. To construct optimality models of insect body size, realistic estimates of size-dependent predation rates are necessary. Moreover, prey traits such as colouration should be considered, as they may substantially alter the relationship between body size and mortality risk. 3. To quantify mortality patterns, we conducted field experiments in which we exposed cryptic and conspicuous artificial larvae of different sizes to bird predators, and recorded the incidence of bird attacks. 4. The average daily mortality rate was estimated to vary between 4% and 10%. In both cryptic and conspicuous larvae, predation risk increased with prey size, but the increase tended to be steeper in the conspicuous group. No main effect of colour type was found. All the quantitative relationships were reasonably consistent across replicates. 5. Our results suggest that the size dependence of mortality risk in insect prey is primarily determined by the probability of being detected by a predator rather than by a size-dependent warning effect associated with conspicuous colouration. Our results therefore imply that warningly coloured insects do not necessarily benefit more than the cryptic species from large body size, as has been previously suggested.
What keeps insects small?—Size dependent predation on two species of butterfly larvae
Insect size usually increases greatly in the latter stages of development, while reproductive value increases strongly with adult size. Mechanisms that can balance the benefits associated with increased growth are poorly understood, raising the question: what keeps insects from becoming larger? If predation risk was to increase with juvenile size, it would make an extension of development very risky, favouring smaller final sizes. But field measures of juvenile mortality seldom show any general patterns of size dependence. We here therefore try to estimate a mechanistic relationship between juvenile size and predation risk by exposing the larvae of two closely related butterflies to a generalist invertebrate predator in a laboratory experiment. Predation risk increased with larval size but was not affected by the species-specific growth rate differences. These results indicate that predation risk may increase with the size of the juvenile even when predators are relatively small. By basing a model simulation on our data we also show that size dependent predation of the kind found in this study has potential to stabilise selection on body size in these species. Thus, these findings suggest that more detailed studies of the size dependence of predation risk on juvenile instars will increase the understanding of what it is that keeps insects small.
Size dependent predation risk in cryptic and conspicuous insects
Evolutionary Ecology, 2007
It is not clear which selective pressures balance the strong fecundity advantage associated with large female body size in insects. A positively sizedependent mortality risk could provide a solution. In aviary experiments with artificial larvae, we studied if larger larvae of folivorous insects are more readily found (= detectability) and/or attacked (= acceptability) by birds. As size and colouration are likely to interact in determining birds' responses, both cryptic and conspicuous prey items were used. As detectability is likely to be context-dependent, both simple (smooth) and complex (plants) backgrounds were used in respective experiments. In the conspicuous larvae, acceptability correlated negatively with prey size. However, their detectability was context dependent, being positively correlated with size on the simple background, whereas no significant effect was found on the complex background. Surprisingly, cryptic larvae showed no correlation between detectability and size, and there was only a weak tendency for birds to attack large larvae more readily. On the basis of a quantitative model, we conclude that the effect of positively size dependent bird predation, as a single factor, is not likely to counterbalance the fecundity advantage in cryptic species, and may thus not be crucial in determining the optimum for body sizes in these insects. In conspicuous species, there is a potential for different outcomes, because detectability and acceptability affect survival in different directions. The net outcome is, therefore, likely to be highly context-dependent. Furthermore, our results provide an explanation for the recently reported absence of systematic body-size differences between cryptic and conspicuous Lepidopteran larvae: although conspicuous larvae benefit from increasing their warning signal when growing larger, they also suffer a much sharper rise in detectability.
Joint evolution of predator body size and prey-size preference
Evolutionary Ecology, 2007
We studied the joint evolution of predator body size and prey-size preference based on dynamic energy budget theory. The predators' demography and their functional response are based on general eco-physiological principles involving the size of both predator and prey. While our model can account for qualitatively different predator types by adjusting parameter values, we mainly focused on 'true' predators that kill their prey. The resulting model explains various empirical observations, such as the triangular distribution of predator-prey size combinations, the island rule, and the difference in predator-prey size ratios between filter feeders and raptorial feeders. The model also reveals key factors for the evolution of predator-prey size ratios. Capture mechanisms turned out to have a large effect on this ratio, while prey-size availability and competition for resources only help explain variation in predator size, not variation in predator-prey size ratio. Predation among predators is identified as an important factor for deviations from the optimal predator-prey size ratio.
Journal of Animal Ecology, 2010
1. Two major theories underpin our understanding of how predation risk shapes life history. The first is centred around predator induced changes in activity that subsequently reduce food intake and thus growth. The second is centred around size selective, predator induced changes in development.2. Here, we challenge these theories using experiments and probabilistic models of maturation reaction norms to investigate predator induced life history in the water flea Daphnia pulex facing two different predators.3. We combine this reaction norm investigation with an assessment of growth rate, development rate, moult number and moult duration to uncover the mechanisms controlling predator induced life history plasticity when D. pulex face either large or small size selective predators.4. The probabilistic reaction norms reveal predator specific norms of reaction in size and age along a food gradient. Fish cues reduce age and size, with a bias in age, and do so by reducing moult number and duration. Midge cues increase age and size, with a bias in size, and do so by fine scale modulation of early growth rates.5. These data contribute towards developing a unified view of how predation risk from multiple predators shapes life history evolution.
Predicting Predation through Prey Ontogeny Using Size-Dependent Functional Response Models
The American Naturalist, 2011
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Behavioural and physiological responses to food availability and predation risk
2005
Several empirical studies have demonstrated the existence of intraspecific variation in age and size at reproductive maturity for organisms experiencing different food environments and predation risk. For some species, these changes have been shown to arise primarily through changes in foraging activity. Theoretically, changes in age and size at maturity can arise through either behavioural or physiological responses. Here we analyse two models. The first is a conventional life-history model with no explicit recognition of the physiology of energy utilization by the organism -growth (i.e. weight gain) is simply the difference between assimilation and respiration, and there are no physiological restrictions on the timing of maturation. The changes in age and size at maturity in response to food availability and predation risk predicted by this model are consistent with published experimental data for one particular species, the midge Chironomus tentans. Numerical calculations with parameters appropriate to this species suggest that the optimal response is purely behavioural. The second model is a general, dynamic energy budget model that takes account of the energetic costs associated with development to reproductive maturity. With that model, we prove that the optimum partitioning of energy between growth and development is independent of predation risk and food availability, thereby demonstrating the generality of the previous finding with the life-history model. On the basis of the combined insight from the two models, we propose that fixed allocation to growth and development, despite variation in food availability and predation risk, is optimal for a broad class of life histories. Consequently, the absence of an allocation response to experimental manipulation of food or predators should not necessarily be taken as evidence for physiological or other constraints on life-history adaptation.
The reaction norm of size and age at maturity under multiple predator risk
Journal of Animal …, 2010
1. Two major theories underpin our understanding of how predation risk shapes life history. The first is centred around predator induced changes in activity that subsequently reduce food intake and thus growth. The second is centred around size selective, predator induced changes in development.