Understanding and predicting species extinctions and coextinctions is a major goal of ecological research in the face of a biodiversity crisis. Typically, models based on network topology are used to simulate coextinctions in mutualistic networks. However, such topological models neglect two key biological features of species interactions: variation in the intrinsic dependence of species on the mutualism, and variation in the relative importance of each interacting partner. By incorporating both types of variation, we developed a stochastic coextinction model capable of simulating extinction cascades far more complex than those observed in previous topological models. Using a set of empirical mutualistic networks, we show that the traditional topological model may either underestimate or overestimate the number and likelihood of coextinctions, depending on the intrinsic dependence of species on the mutualism. More importantly, contrary to topological models, our stochastic model predicts extinction cascades to be more likely in highly connected mutualistic communities.
Summary1. Many game-theoretical models assume that the outcome of agonistic interactions depends on morphological and physiological asymmetries between rivals (the RHP-asymmetry hypothesis). However, some empirical studies fail at identifying traits linked to fighting capacity and are thus unable to support the role of RHP in contest resolution. Perhaps the role of RHP asymmetries in contest resolution is less general than previously thought. 2. If RHP asymmetries are indeed important, then the actual suite of relevant traits should depend on how costs are accrued during disputes (the functional hypothesis). In species in which contests involve physical contact at some stage, strength-related traits should be important, whereas in species in which disputes do not involve physical contact, persistence-related traits should determine individual fighting ability. 3. We performed a meta-analysis to investigate the generality of morphological and physiological asymmetries between winners and losers in arthropod contests. We also investigated whether the suite of traits determining fighting ability differs between disputes involving physical contact and disputes in which physical contact is absent. 4. We show that RHP asymmetries are a general property of contest settlement among arthropods. However, strength-related traits did not have a greater effect than persistence-related traits in determining the settlement of disputes involving physical contact. Neither did persistence-related traits have a greater role in disputes without physical contact than in those involving physical contact. 5. We provide the first quantitative assessment of the generality and magnitude of RHP asymmetries in contest resolution among arthropods. As different suites of traits were important regardless of the type of dispute, we highlight the importance of considering functionally diverse morphological and physiological traits when trying to identify determinants of fighting ability in agonistic interactions, instead of focusing on a few functionally similar traits. 6. The assumption of fighting capacity differences among rivals in game-theoretical models was supported by our data. Consequently, models predicting contest settlement based on uncorrelated asymmetries are incompatible with the observation that winners and losers consistently differ in certain attributes.
Human land use tends to decrease the diversity of native plant species and facilitate the invasion and establishment of exotic ones. Such changes in land use and plant community composition usually have negative impacts on the assemblages of native herbivorous insects. Highly specialized herbivores are expected to be especially sensitive to land use intensification and the presence of exotic plant species because they are neither capable of consuming alternative plant species of the native flora nor exotic plant species. Therefore, higher levels of land use intensity might reduce the proportion of highly specialized herbivores, which ultimately would lead to changes in the specialization of interactions in plant-herbivore networks. This study investigates the community-wide effects of land use intensity on the degree of specialization of 72 plant-herbivore networks, including effects mediated by the increase in the proportion of exotic plant species. Contrary to our expectation, the net effect of land use intensity on network specialization was positive. However, this positive effect of land use intensity was partially canceled by an opposite effect of the proportion of exotic plant species on network specialization. When we analyzed networks composed exclusively of endophagous herbivores separately from those composed exclusively of exophagous herbivores, we found that only endophages showed a consistent change in network specialization at higher land use levels. Altogether, these results indicate that land use intensity is an important ecological driver of network specialization, by way of reducing the local host range of herbivore guilds with highly specialized feeding habits. However, because the effect of land use intensity is offset by an opposite effect owing to the proportion of exotic host species, the net effect of land use in a given herbivore assemblage will likely depend on the extent of the replacement of native host species with exotic ones.
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