Billions of animals, including insects, mammals, fish, and birds, migrate through the planet every year, which uniquely influences the environment and the ecological communities along migration routes.
“The frequency of migrations and the immense number of individuals involved often mean that migrant inputs constitute “resource pulses,” defined as occasional, intense, brief episodes of increased resource availability that can profoundly alter demographic rates and abundances of interacting populations” [Bauer & Hoye 2014: 6]
Effect on nutrients, energy, and toxicants:
Migrants transport nutrients, energy, and other substances from one ecosystem to another, creating a net inflow of energy and nutrients into the destination ecosystem. For example, salmon increased the nitrogen and phosphorus in their spawning habitat by 190% and 390% when migrating from the ocean back to their natal lakes and streams. At the same time, migrants may also introduce and accumulate toxicants, such as heavy metals, to receiving communities.
Effect on propagule dispersal:
Migrants play an important role in dispersing propagules, such as seeds, suckers, or spores across the resident communities.
In light of the importance of dispersal for population structure, adaptive capabilities, and evolutionary trajectories in theoretical studies, such long-distance dispersal events may be highly important for the (re)colonization of unoccupied habitats, the recovery of lost populations, maintenance of gene flow, and gene mixing in metapopulations, even if they are relatively rare events [Bauer & Hoye 2014: 2].
Moreover, migrants could also disperse propagules within resident communities. For example, long-nosed bats are responsible for up to 100% of columnar cacti pollination when they migrate to western Mexico. It is important to note that the timing of migration is very important; the migrants can only serve as major pollinators when visiting the communities during peak flowering.
Effect on parasite dispersal:
Migrants may increase parasite dynamics by facilitating the long-distance dispersal of parasites (including zoonotic pathogens like Ebola that also affect humans) to resident species. A few key mechanisms are involved in migration-facilitated parasite dispersal. For example, migrating animals are likely exposed to a greater range of parasites than are resident species. Some migrant animals may have suppressed immune responses due to the high investment of energy into migration, increasing their susceptibility to infection. In addition, while migrating, animals tend to aggregate in larger groups, thus enhancing transmission rates, compared to other times of the year when they are stationary.
However, the role of migrants in transmitting parasites is complicated. Studies of monarch butterflies have shown that they have a shorter flying distance when infected with parasites, andinfected Bewick’s swans delay their departure and travel shorter distances. These findings suggest that migrants may reduce infection risk through infection-induced delays.
Effect of migratory herbivores (plant-eating species):
Migrants may alter the nutrient cycling, productivity, the biomass of edible plants, and ground cover of dead plant material. The grazing intensity of migrant herbivores is decoupled from the timing of plant growth so plants can grow when they are left, which substantially increases the primary productivity compared to an ecosystem with the equivalent number of resident herbivores.
The outcome of the interaction between migrants and residents differs depending on the food resources. During periods of plenty of food residents could share the excess resources with the migrants. However, during the dry season when food is scarcer, synergistic negative effects may be created.
Effects of migratory predators:
Migratory predators can positively influence the communities through prey population control. For example, birds and bats may control the insect population, which reduces damage to crops. Seasonal outmigration may also reduce pressure on prey in the places left behind by migrants, allowing those populations to regrow.
Effects of migratory prey:
Migratory prey could be an important resource for resident predators. Some predators even time their reproduction to coincide with migratory prey to increase their reproductive rate.
Migratory prey may also provide resident prey with a temporal refuge from predation. However, an abundant number of migrants may harm residents by boosting the abundance of resident predators, which then switch to resident prey after the migratory prey departs.
Many ecosystems have evolved to depend upon the activities of both resident and transitory migrating animals, and understanding these relationships is critical to preserving and restoring ecosystem complexity and resiliency.
Across the globe, migration is an increasingly threatened phenomenon as a consequence of habitat destruction, creation of barriers, over-exploitation, and climate change. The loss of migrants and migratory behavior also entails the loss of their ecosystem services—the manifold transport and trophic effects outlined above. Management strategies must therefore be designed to conserve not only migratory species but also their ecosystem functions. Yet, the conservation of migrants poses exceptional scientific and societal challenges, as events at each stage of the migratory cycle affect behavior and demographic rates and ecological interactions at other stages [Bauer & Hoye 2014: 9].
Bauer, S. & B. J. Hoye, 2014. Migratory animals couple biodiversity and ecosystem functioning worldwide. Science 344(6179), https://www.science.org/doi/10.1126/science.1242552