Shifts in pollinator population structure may jeopardize pollination service.

Plant-pollinator interactions are among the best known and ubiquitous plant-animal mutualisms and are crucial for ecosystem functioning and the maintenance of biodiversity. Most pollinators are insects with several life-stages (e.g. egg, larva, pupa, adult) and the mutualistic interaction depends on the pollinator surviving these different life-stages. However, to our knowledge, pollinator population structure has been ignored in most theoretical models of plant-pollinator dynamics, and we lack understanding of the role of different life-stages in determining the stability of the mutualism. Here we therefore develop a simple plant-pollinator model with a facultative plant and an obligate pollinator with stage-structure. Our model predicts a globally stable equilibrium when pollinator demography is dominated by adults and a locally stable equilibrium when the plants are strongly dependent on pollination and pollinator demography is dominated by the larval stage. In the latter case, the mutualism is vulnerable to fluctuations in the pollinator population size or structure caused by external factors (e.g. pesticides) reducing larval development and increasing adult mortality. This may cause a sudden collapse rather than gradual decrease of the mutualism, after which the pollination service cannot be recovered by reducing these detrimental external factors, but must be accompanied by large increases in pollinator populations. This highlights the importance of considering population structure in plant-pollinator interactions.