Collective behavior as a driver of critical transitions in migratory populations

Background/Question/Methods

Mass migrations are one of the most striking examples of animal movement in the natural world. Such migrations are major drivers
of ecosystem processes and strongly influence the survival and fecundity of individuals. For animals undertaking these migrations,
a formidable challenge is to find their way over long distances and through complex, dynamic environments. Recent theoretical
and empirical work suggests that by traveling in groups, individuals are able to overcome these challenges and increase their
ability to navigate. Here, using analytic and numerical models, we explore the implications of collective navigation on migratory
and population dynamics, for both breeding migrations (travel to distinct, fixed, end-points) and feeding migrations (circuitous,
or nomadic, travel tracking favorable conditions).

Results/Conclusions

We show that while collective navigation does improve a population’s ability
to migrate accurately, it can lead to Allee effects, causing the sudden collapse of migratory populations if numbers fall below a
critical threshold. In some scenarios hysteresis prevents the migration from recovering even after the cause of the collapse has been
removed. For collectively navigating populations which are locally adapted to specific breeding sites, a slight increase in mortality
can cause a collapse of genetic population structure, rather than population size, making it much more difficult to detect and prevent.
Our theoretical results suggest that if collective behavior plays a significant role in a species’ migration, increased survival pressure
may result in sudden and nonlinear population declines.