Several network properties have been identified as determinants of the stability and complexity of mutualistic networks. However, it is unclear which mechanisms give rise to these network properties. Phenology seems important, because it shapes the topology of mutualistic networks, but its effects on the dynamics of mutualistic networks have scarcely been studied. Here, we study these effects with a general dynamical model of mutualistic and competitive interactions where the interaction strength depends on the temporal overlap between species resulting from their phenologies. We find a negative complexity-stability relationship, where phenologies maximising mutualistic interactions and minimising intraguild competitive interactions generate speciose, nested and poorly connected networks with moderate asymmetry and low resilience. Moreover, lengthening the season increases diversity and resilience. This highlights the fragility of real mutualistic communities with short seasons (e.g. Arctic environments) to drastic environmental changes.

Plant-soil feedbacks can have important implications for the interactions among plants. Understanding these effects is a major challenge since it is inherently difficult to measure and manipulate highly diverse soil communities. Mathematical models may advance this understanding by making the interplay of the various processes affecting plant-soil interaction explicit and by quantifying the relative importance of the factors involved. The aim of this paper is to provide a complete analysis of a pioneering plant-soil feedback model developed by Bever and colleagues (J Ecol 85: 561-573, 1997; Ecol Lett 2: 52-62, 1999; New Phytol 157: 465-473, 2003) to fully understand the range of possible impacts of plant-soil feedbacks on plant communities within this framework. We analyze this model by means of a new graphical method that provides a complete classification of the potential effects of soil communities on plant competition. Due to the graphical character of the method, the results are relatively easy to obtain and understand. We show that plant diversity depends crucially on two key parameters that may be viewed as measures of the intensity of plant competition and the direction and strength of plantsoil feedback, respectively. Our analysis provides a formal underpinning of earlier claims that plant-soil feedbacks, especially when they are negative, may enhance the diversity of plant communities. In particular, negative plant-soil feedbacks can enhance the range of plant coexistence by inducing competitive oscillations. However, these oscillations can also destabilize plant coexistence, leading to low population densities and extinctions. In addition, positive feedbacks can allow locally stable forms of plant coexistence by inducing alternative stable states. Our findings highlight that the inclusion of plant-soil interactions may fundamentally alter the predictions on the structure and functioning of above-ground ecosystems. The scenarios presented in this study can be used to formulate hypotheses about the ways soil community effects may influence plant competition that can be tested with empirical studies. This will advance our understanding of the role of plant-soil feedback in ecological communities.

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Many mutualisms involve inter-specific resource exchanges, making consumer-resource approaches ideal for studying their dynamics. Also in many cases these resources are short lived (e.g. flowers) compared with the population dynamics of their producers and consumers (e.g. plants and insects), which justifies a separation of time scales. As a result, we can derive the numerical response of one species with respect to the abundance of another. For resource consumers, the numerical responses can account for intra-specific competition for mutualistic resources (e.g. nectar), thus connecting competition theory and mutualism mechanistically. For species that depend on services (e.g. pollination, seed dispersal), the numerical responses display saturation of benefits, with service handling times related with rates of resource production (e.g. flower turnover time). In both scenarios, competition and saturation have the same underlying cause, which is that resource production occurs at a finite velocity per individual, but their consumption tracks the much faster rates of population growth characterizing mutualisms. The resulting models display all the basic features seen in many models of facultative and obligate mutualisms, and they can be generalized from species pairs to larger communities.

Climate change can alter species phenologies and therefore disrupt species interactions. Habitat destruction can damage biodiversity and population viability. However, we still know very little about the potential effects of these two factors on the diversity and structure of interaction networks when both act simultaneously. Here we developed a mutualistic metacommunity model to explore the effects of habitat destruction and phenological changes on the diversity and structure of plant-pollinator networks. Using an empirical data set of plant and pollinator interactions and their duration in days, we simulated increasing levels of habitat destruction, under projected scenarios of phenological shifts as well for historically recorded changes in phenologies. On one hand, we found that habitat destruction causes catastrophic collapse in global diversity, as well as inducing alternative states. On the other hand, phenological shifts tend to make interactions weaker, increasing local extinction rates. Together, habitat destruction and phenological changes act synergistically, making metacommunities even more vulnerable to global collapse. Metacommunities are also more vulnerable to collapses under scenarios of historical change, in which phenologies are shortened, not just shifted. Furthermore, connectance and nestedness tends to decrease gradually with habitat destruction before the global collapse. Small phenological shifts can raise connectance slightly, due novel interactions appearing in a few generalist species, but larger shifts always reduce connectance. We conclude that the robustness of mutualistic metacommunities against habitat destruction can be greatly impaired by the weakening of positive interactions that results from the loss of phenological overlap.

We use the optimal foraging theory to study coexistence between two plant species and a generalist pollinator. We compare conditions for plant coexistence for non-adaptive vs. adaptive pollinators that adjust their foraging strategy to maximize fitness. When pollinators have fixed preferences, we show that plant coexistence typically requires both weak competition between plants for resources (e.g., space or nutrients) and pollinator preferences that are not too biased in favour of either plant. We also show how plant coexistence is promoted by indirect facilitation via the pollinator. When pollinators are adaptive foragers, pollinator’s diet maximizes pollinator’s fitness measured as the per capita population growth rate. Simulations show that this has two conflicting consequences for plant coexistence. On the one hand, when competition between pollinators is weak, adaptation favours pollinator specialization on the more profitable plant which increases asymmetries in plant competition and makes their coexistence less likely. On the other hand, when competition between pollinators is strong, adaptation promotes generalism, which facilitates plant coexistence. In addition, adaptive foraging allows pollinators to survive sudden loss of the preferred plant host, thus preventing further collapse of the entire community.

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