The effects of search costs on habitat selection by dispersers are largely unknown. We explore how habitat selection behavior is affected by the risk of mortality en route and by deferred search costs (i.e., costs incurred during search that reduce fitness after arrival in the new habitat), using a model designed for long‐distance natal dispersers searching for scarce patches of suitable habitat embedded within a matrix of unsuitable habitat. In this situation, increases in the risk of mortality during search reduce disperser selectivity, where selectivity is reflected by the period during search when dispersers are only willing to accept a high‐quality habitat. However, the effects of deferred costs on selectivity depend on other factors with pronounced effects on selectivity, including encounter rates with high‐quality habitats, relative habitat quality, and total search time. Surprisingly, under some sets of conditions, increases in deferred costs lead to increases in disperser selectivity. Overall, the effects of mortality and deferred costs on selectivity are small relative to the effects of other factors on selectivity. For instance, our model suggests that selectivity is much more strongly affected by total search time than by search costs, and it predicts a positive relationship between total search time and disperser selectivity across individuals in the same population, even in the face of considerable inter‐individual variation in risk of mortality or deferred search costs.
Summary1. Low internal energy reserves at the beginning of the breeding season may impose physiological constraints on an animal's reproductive investment and may alter the optimal trade-off between investment in reproduction and somatic condition. 2. Here we examine how the energetic condition of female Mountain Pine Beetles ( Dendroctonus ponderosae ) affects their reproductive investment. We starved beetles to simulate the decrease in energy that accompanies dispersal and tested whether starved beetles had decreased egg number and decreased egg size, or both. We further distinguished whether changes are due to physiological constraints or shifts in allocation between reproduction and somatic condition. 3. We found that starved beetles produced smaller eggs than non-starved beetles, but females were able to partially offset the energetic deficit by feeding at their breeding habitat. Starvation did not decrease the number of eggs beetles produced. 4. The number and size of eggs produced depended on whether females allocated energy to reproduction or to somatic condition. However, this life-history allocation decision was independent of the amount of energy beetles had at the beginning of reproduction. 5. Our results demonstrate the importance of assessing reproductive investment in the context of other life-history trade-offs. Specifically, since egg size in Mountain Pine Beetles was highly dependent on both the amount of energy remaining after dispersal and whether energy was allocated to reproduction or somatic maintenance, we expect both of these trade-offs to be under strong selection.
The ability of a herbivore to tolerate plant defensive chemicals may vary with the herbivore's energetic state. We investigated the effect of body condition on the survivorship of individual mountain pine beetles, Dendroctonus ponderosae, exposed to host monoterpenes at concentrations comparable to constitutive and induced levels of defence using fumigant exposure. Body condition index was calculated as the residual mass after fitting the relationship between fresh weight and body size. Differences in survivorship among the four monoterpenes tested (a-pinene, myrcene, terpinolene and limonene) were small. Beetles with a higher body condition index survived high monoterpene concentrations better than those in poorer condition. There was no direct effect of sex, but positive effects of body size and fat content on survivorship favoured females, the sex that pioneers attacks on live trees. Higher body condition index corresponded to both higher fat content and fat-free body mass; the same conclusions about monoterpene identity and size-dependent or energy-dependent tolerance of high monoterpene concentrations held if fat or fat-free body mass were used in place of body condition index. This study highlights the need to consider insect body condition in understanding insectplant interactions.
1. The pine engraver bark beetle Ips pini (Say) (Coleoptera: Scolytidae), aggregates primarily on dead or dying pine trees. In this study pine engravers were laboratory-reared on logs at a range of low densities to determine whether there was a fecundity advantage of breeding aggregations. 2. Mean reproductive success for both males and females declined exponentially with increasing density.3. Female pine engravers had shorter egg galleries at higher densities, suggesting that they left high-density breeding sites earlier. This would reduce the number of eggs that failed to survive due to larval competition.4. Some pine engravers colonized the logs voluntarily during the experiment. These volunteers settled independently of the original density.5. The fungus Ophiostoma sp. was present on the logs and may be competing with the pine engravers for limited bark area.6. Aggregation resulted in a considerable cost to pine engraver reproductive success even at low densities. Thus, it remains perplexing why pine engravers aggregate actively in nature.
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