Oskar Kindvall

Individual mobility prevents an Allee effect in sparse populations of the bush cricket Metrioptera roeseli: an experimental study

The bush cricket, Metrioptera roeseli (Orthoptera, Tettigoniidae), occurs in patchy and heterogeneous agricultural landscapes. Such a mosaic of different types of grassland habitats causes spatial variation in local population density. Low population densities may result in fewer mating opportunities that can give rise to an Allee effect, possibly leading to a population decline or extinction. This experimental study shows that individuals can avoid an Allee effect by adjusting their movement behaviour in sparse populations. Even at a population density of five individuals per hectare, i.e. approximately one percent of normal population densities, no reduction of mating frequencies was detected. Observed net displacements made by different individuals in high and low population densities could successfully be predicted by a simple model of animal movement.

The impact of extreme weather on habitat preference and survival in a metapopulation of the bush cricket Metrioptera bicolor in Sweden

The bush cricket, Metrioptera bicolor Philippi (Orthoptera, Tettigoniidae), lives in an archipelago of grassland fragments within pine forest in southernmost Sweden. In 1992, an exceptional drought forced this bush cricket to use other habitats including the pine forest, which is normally an effective barrier for dispersal. This behaviour may increase the gene flow between separate habitat patches. In most years, tall and low grassland are roughly equally good for M. bicolor, but in 1992 local populations living on patches with a high proportion of tall grassland decreased less than populations living on patches dominated by low grassland. This study shows that we cannot expect dispersal rates and habitat preferences to remain constant over time. In order to preserve endagered species it may be important to save a wide range of habitats with different local climates.

Reduced population control of an insect pest in managed willow monocultures.

Background There is a general belief that insect outbreak risk is higher in plant monocultures than in natural and more diverse habitats, although empirical studies investigating this relationship are lacking. In this study, using density data collected over seven years at 40 study sites, we compare the temporal population variability of the leaf beetle Phratora vulgatissima between willow plantations and natural willow habitats. Methodology/Principal Findings The study was conducted in 1999–2005. The density of adult P. vulgatissima was estimated in the spring every year by a knock-down sampling technique. We used two measures of population variability, CV and PV, to compare temporal variations in leaf beetle density between plantation and natural habitat. Relationships between density and variability were also analyzed to discern potential underlying processes behind stability in the two systems. The results showed that the leaf beetle P. vulgatissima had a greater temporal population variability and outbreak risk in willow plantations than in natural willow habitats. We hypothesize that the greater population stability observed in the natural habitat was due to two separate processes operating at different levels of beetle density. First, stable low population equilibrium can be achieved by the relatively high density of generalist predators observed in natural stands. Second, stable equilibrium can also be imposed at higher beetle density due to competition, which occurs through depletion of resources (plant foliage) in the natural habitat. In willow plantations, competition is reduced mainly because plants grow close enough for beetle larvae to move to another plant when foliage is consumed. Conclusion/Significance To our knowledge, this is the first empirical study confirming that insect pest outbreak risk is higher in monocultures. The study suggests that comparative studies of insect population dynamics in different habitats may improve our ability to predict insect pest outbreaks and could facilitate the development of sustainable pest control in managed systems.

Movement behaviour of the pine weevil Hylobius abietis in relation to soil type: an arena experiment.

Pine weevil, Hylobius abietis (L.) (Coleoptera: Curculionidae), movement behaviour on different substrates was studied in an indoor arena using a video camera and digital image processing technique. We analysed individual variation in movement characteristics, i.e. turning angles, movement directions, and distance moved per unit time on the bare and level arena surface which consisted of mineral soil (sand) and/or humus sections in various spatial configurations. Pine weevils on humus did not turn back when they came to the border with the sand. However, most individuals moved faster on sand than on humus. Thus, the results suggest that interactions between substrate differences and individual movement behaviour may to some extent explain why pine weevils have been observed to feed less frequently on coniferous seedlings planted on mineral soil than on those planted on humus.

Consequences of modelling interpatch migration as a function of patch geometry when predicting metapopulation extinction risk

Abstract We have constructed a simulation model with demographic stochasticity for a generalised two-patch system. With the model, it was possible to vary the effect of patch geometry on interpatch migration from no effect, as in traditional models, to a gradually more pronounced effect. In systems with a constant amount of available habitat but with different allocation of habitat among patches we generally found lower metapopulation extinction risks if interpatch migration was adjusted to current sizes of local patches than if interpatch migration was assumed to be independent of patch sizes. In systems with equal patches, but with a different total amount of available habitat we found that simulations based on the assumption of patch-independent interpatch migration rates generate far too optimistic predictions of how well a metapopulation will persist when decreasing the total amount of habitat compared with simulations based on an adjustment of interpatch migration to current patch sizes.

Predator foraging strategy influences prey population dynamics: arthropods predating a gregarious leaf beetle

We examined whether behavioural variation within an enemy complex attacking the willow leaf beetle, Phratora vulgatissima, influences the population dynamics of this gregarious prey. The most common enemies are three species of heteropteran arthropods: the two mirids Orthotylus marginalis and Closterotomus fulvomaculatus and the anthocorid Anthocoris nemorum. When attacking egg clusters on plants in the laboratory, the two mirids consumed a greater proportion of eggs within egg clusters than the anthocorid. The anthocorid visited and ate eggs from more egg clusters than both the mirids. The two foraging strategies have been characterized as ‘find and stay’ for the mirids and ‘run and eat’ for the anthocorid. By using a stochastic exponential growth model we showed that model prey experienced different temporal dynamics when exposed to predators that differ in the probabilities of finding prey aggregations and of consuming prey within aggregations. Model prey exposed to the find and stay type of predator was less likely to become established and to increase in abundance than model prey exposed to the run and eat type. In a field study, we found a correspondence between high abundance of find and stay mirids and low densities of leaf beetles. The results suggest that, even when average predation rate is constant, the foraging strategy of the predator can have population level consequences for the prey. The consumption of prey in dense patches seems to be important in the control of gregarious prey, especially in the early phase of prey population establishment.

Modelling mating success of saproxylic beetles in relation to search behaviour, population density and substrate abundance

Abstract We compared the efficiency of two mate-finding strategies exploited by representatives of the beetle families Cisidae and Anobiidae (genus Dorcatoma ) that live inside fruiting bodies of wood-decaying fungi. In the Cisidae both sexes are attracted to host odour, but no pheromones seem to be present (nonpheromone strategy). In the Dorcatoma species only the females are attracted to host odour, but having found a host they attract males with a sexual pheromone (pheromone strategy). With a simulation model, we compared the efficiency of the two strategies at four densities of trees hosting fungal fruiting bodies and at three relative densities of insects. We found only small differences in efficiency between the two strategies at high relative densities of conspecific individuals, regardless of host tree density. The pheromone strategy was relatively more efficient when the relative density of insects or the density of host trees decreased. Thus, species adopting the nonpheromone strategy are probably more sensitive to habitat fragmentation and more likely to decline and go extinct at low population densities (because of Allee effects) than species using the pheromone strategy. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

High Temperature Triggers Latent Variation among Individuals : Oviposition Rate and Probability for Outbreaks

Background It is anticipated that extreme population events, such as extinctions and outbreaks, will become more frequent as a consequence of climate change. To evaluate the increased probability of such events, it is crucial to understand the mechanisms involved. Variation between individuals in their response to climatic factors is an important consideration, especially if microevolution is expected to change the composition of populations. Methodology/Principal Findings Here we present data of a willow leaf beetle species, showing high variation among individuals in oviposition rate at a high temperature (20°C). It is particularly noteworthy that not all individuals responded to changes in temperature; individuals laying few eggs at 20°C continued to do so when transferred to 12°C, whereas individuals that laid many eggs at 20°C reduced their oviposition and laid the same number of eggs as the others when transferred to 12°C. When transferred back to 20°C most individuals reverted to their original oviposition rate. Thus, high variation among individuals was only observed at the higher temperature. Using a simple population model and based on regional climate change scenarios we show that the probability of outbreaks increases if there is a realistic increase in the number of warm summers. The probability of outbreaks also increased with increasing heritability of the ability to respond to increased temperature. Conclusions/Significance If climate becomes warmer and there is latent variation among individuals in their temperature response, the probability for outbreaks may increase. However, the likelihood for microevolution to play a role may be low. This conclusion is based on the fact that it has been difficult to show that microevolution affect the probability for extinctions. Our results highlight the urge for cautiousness when predicting the future concerning probabilities for extreme population events.