Migratory Animals Couple Biodiversity and Ecosystem Functioning Worldwide
Authors: Bauer S. , Hoye B. J. 
Published: April 2014

Background
Every year, billions of migratory animals cross the planet in pursuit of increased foraging opportunities, improved safety, and higher reproductive output. In so doing, these migrants transport nutrients, energy, and other organisms (including seeds, mollusks, parasites, and pathogens) between disparate locations. Migrants also forage and are preyed upon throughout their journeys, thereby establishing transport and trophic interactions with resident communities. Migratory animals thus couple ecological communities across the globe and may mediate their diversity and stability. However, as yet, the influence of migrants and their services on these communities is often overlooked, and as a consequence of global changes, migrations are threatened worldwide.

Advances
We review several examples in which interactions between migratory animals and resident communities have been quantified, illustrating the processes by which migrants may uniquely alter energy flow, food-web topology and stability, trophic cascades, and the structure and dynamics of (meta-)communities. For example, the inputs of nutrients and energy originating from distant localities by migrants can dramatically increase resource availability, with rippling consequences for productivity at various trophic levels and the potential to drive the transition between alternative stable states. Migrant-mediated transport of propagules of other organisms can lead to the establishment of new or lost species, as well as influencing gene flow and genetic mixing among resident populations. Similarly, migrants can alter parasite transmission, diversity, and evolution by harboring a broader range of parasites than residents and by either facilitating or hindering the long-distance dispersal of parasites.
Foraging by migrants can also have profound effects on community processes and ecosystem functions. For instance, grazing by migratory animals can alter nutrient cycling, primary productivity, biomass of edible plants, competitive interactions between plant species, and ultimately, the composition and long-term persistence of the entire plant community. The most striking difference between migrant and resident consumers is, however, the pulsed nature of migrant utilization and the timing of their interactions. Together, these fundamentally define the relationship between migrant abundance and primary production (in the case of migrant herbivores) or the stability of food webs (in the case of migratory predators).

Outlook
Our Review demonstrates that the highly predictable, seasonally pulsed nature of animal migration, together with the spatial scales at which it operates and the immense number of individuals involved, not only set migration apart from other types of movement, but render it a uniquely potent, yet underappreciated, dimension of biodiversity that is intimately embedded within resident communities. Given the potential for migration to influence ecological networks worldwide, we suggest integrative network approaches, through which studies of community dynamics and ecosystem functioning may explicitly consider animal migrations, understand the ramifications of their declines, and assist in developing effective conservation measures.