Influence of prescribed burning on reindeer winter pastures at landscape scale in northern Sweden: A modelling approach.

Whilst the re-introduction of fire can contribute to biodiversity conservation in Fennoscandian forests, the effects on reindeer herding remain uncertain. To assess the short- and long-term effects of prescribed burning on lichen supply in a productive forest landscape, we developed a model simulating lichen biomass available for reindeer grazing, covering 300 years and 1500 pine stands, under different soil preparation scenarios, including different prescribed burning regimes and mechanical scarification. Our simulations revealed that burning 25-50% of yearly clear-cuts has the potential to stop, or even reverse, reindeer lichen decline at landscape scale after 70 years, greatly surpassing the short-term losses caused by burning. No burning or burning 5% of yearly clear-cuts, as required by the FSC certification, compounded the negative effects of fire suppression and scarification on lichen. Compared to the scenario with no soil preparation, all our simulations resulted in a continuous decrease of lichen supply in Lichen-type stands, indicating that any form of disturbance in these habitats can strongly limit future gains.

Trophic interactions may reverse the demographic consequences of inbreeding.

Extinctions have no simple determinism, but rather result from complex interplays between environmental factors and demographic-genetic feedback that occur at small population size. Inbreeding depression has been assumed to be a major trigger of extinction vortices, yet very few models have studied its consequences in dynamic populations with realistic population structure. Here we investigate the impact of Complementary Sex Determination (CSD) on extinction in parasitoid wasps and other insects of the order Hymenoptera. CSD is believed to induce enough inbreeding depression to doom simple small populations to extinction, but we suggest that in parasitoids CSD may have the opposite effect. Using a theoretical model combining the genetics of CSD and the population dynamics of host-parasitoid systems, we show that CSD can reduce the risk of parasitoid extinction by reducing fluctuations in population size. Our result suggests that inbreeding depression is not always a threat to population survival, and that considering trophic interactions may reverse some pervasive hypotheses on its demographic impact.

Host-parasitoid dynamics and the success of biological control when parasitoids are prone to allee effects.

In sexual organisms, low population density can result in mating failures and subsequently yields a low population growth rate and high chance of extinction. For species that are in tight interaction, as in host-parasitoid systems, population dynamics are primarily constrained by demographic interdependences, so that mating failures may have much more intricate consequences. Our main objective is to study the demographic consequences of parasitoid mating failures at low density and its consequences on the success of biological control. For this, we developed a deterministic host-parasitoid model with a mate-finding Allee effect, allowing to tackle interactions between the Allee effect and key determinants of host-parasitoid demography such as the distribution of parasitoid attacks and host competition. Our study shows that parasitoid mating failures at low density result in an extinction threshold and increase the domain of parasitoid deterministic extinction. When proned to mate finding difficulties, parasitoids with cyclic dynamics or low searching efficiency go extinct; parasitoids with high searching efficiency may either persist or go extinct, depending on host intraspecific competition. We show that parasitoids suitable as biocontrol agents for their ability to reduce host populations are particularly likely to suffer from mate-finding Allee effects. This study highlights novel perspectives for understanding of the dynamics observed in natural host-parasitoid systems and improving the success of parasitoid introductions.

Spatial heterogeneity and functional response: an experiment in microcosms with varying obstacle densities.

Spatial heterogeneity of the environment has long been recognized as a major factor in ecological dynamics. Its role in predator-prey systems has been of particular interest, where it can affect interactions in two qualitatively different ways: by providing (1) refuges for the prey or (2) obstacles that interfere with the movements of both prey and predators. There have been relatively fewer studies of obstacles than refuges, especially studies on their effect on functional responses. By analogy with reaction-diffusion models for chemical systems in heterogeneous environments, we predict that obstacles are likely to reduce the encounter rate between individuals, leading to a lower attack rate (predator-prey encounters) and a lower interference rate (predator-predator encounters). Here, we test these predictions under controlled conditions using collembolans (springtails) as prey and mites as predators in microcosms. The effect of obstacle density on the functional response was investigated at the scales of individual behavior and of the population. As expected, we found that increasing obstacle density reduces the attack rate and predator interference. Our results show that obstacles, like refuges, can reduce the predation rate because obstacles decrease the attack rate. However, while refuges can increase predator dependence, we suggest that obstacles can decrease it by reducing the rate of encounters between predators. Because of their opposite effect on predator dependence, obstacles and refuges could modify in different ways the stability of predator-prey communities.

An implicit approach to model plant infestation by insect pests.

Various spatial approaches were developed to study the effect of spatial heterogeneities on population dynamics. We present in this paper a flux-based model to describe an aphid-parasitoid system in a closed and spatially structured environment, i.e. a greenhouse. Derived from previous work and adapted to host-parasitoid interactions, our model represents the level of plant infestation as a continuous variable corresponding to the number of plants bearing a given density of pests at a given time. The variation of this variable is described by a partial differential equation. It is coupled to an ordinary differential equation and a delay-differential equation that describe the parasitized host population and the parasitoid population, respectively. We have applied our approach to the pest Aphis gossypii and to one of its parasitoids, Lysiphlebus testaceipes, in a melon greenhouse. Numerical simulations showed that, regardless of the number and distribution of hosts in the greenhouse, the aphid population is slightly larger if parasitoids display a type III rather than a type II functional response. However, the population dynamics depend on the initial distribution of hosts and the initial density of parasitoids released, which is interesting for biological control strategies. Sensitivity analysis showed that the delay in the parasitoid equation and the growth rate of the pest population are crucial parameters for predicting the dynamics. We demonstrate here that such a flux-based approach generates relevant predictions with a more synthetic formalism than a common plant-by-plant model. We also explain how this approach can be better adapted to test different management strategies and to manage crops of several greenhouses.