Loss of migratory pollinators shapes dynamics of plant populations at stopover sites

Global changes in environmental conditions are causing changes to animal migration patterns, including changes to migration timing and routes, as well as population declines or even extinction. These changes can have knock-on effects for communities and ecosystems both at the end points of the migratory route, as well as at the stopover sites that migrants visit during their journey. One key role that migrants can play within ecological communities is as mutualists, such as pollination. Pollination is critical for reproduction in many plants and thus for continued population growth and even spread of plant species. Migratory pollinators, in turn, derive key benefits from plants in the form of nectar. This mutualistic relationship sets up a potential feedback loop between abundance of flowering plants at stopover sites and the success of migrants passing through. Here, we develop a theoretical model to capture this feedback loop and explore how reductions in the abundance of migrants affects the population growth and spread of plants within a stopover site. We find that a small reduction in migrant abundance typically leads to a small reduction in plant population spreading speed while total loss of migrants visiting stopover sites can lead to abrupt changes for plants – either a decrease in the population spreading speed or the full collapse of the population. We also find combinations of parameter values where a decrease in migrant abundance counterintuitively increased plant population spreading speed. This result was especially notable when the plant population had an Allee effect and thus spread was ‘pushed’ forward by dynamics at the high-density population core.