Organisms, all along the phyla of the animal kingdom, are believed to migrate as a result of detecting and responding to factors governing resource availability. The foundational theory considers each migrating individual as “‘information processing units’, with interactions amongst them providing collective benefits”, such as improved migratory direction. Should an individual commit an error in the information processing, the aforementioned grouping would average the individual measurements, so as to deduce the mean migratory direction.
These migrating populations have two types of individuals:
-Leaders, who have a higher ability to detect and respond to directional gradient from the environment, but with weak (or none) social skills. They tend to occupy frontal or peripheral positions and expend more energy in trotting off the beaten track and facing dangers such as predators.
-Social individuals, who have strong social skills but weak ability to detect and respond to gradient. However, they utilise the strengths of the leaders for a free ride.
Population density, according to the model, is a crucial leverage factor. Extremely low-density populations (ergo, lesser probability of encountering others) comprises of leaders, thus resulting in solitary migration. Extremely high-density populations results in resident population due to a lack of migration (attributed to frequent ‘collisions among individuals’). It is when leaders and social individuals coexist that collective migration ensues. The bottom-line is that ‘the evolution of the migratory strategy (resident, solitary, or collective) is determined by the ecology of the species (i.e population density, habitat structure, costs and benefits of migration)’. Presumably, there could be other regressors as well....
Anthropogenic factors have been exerting pressures (such as habitat fragmentation and changes in population density) on the existence of many migratory species (examples cited in the paper includes: American bison and its steep decline in its population density; extinction of passenger pigeon) and migratory patterns (Blackcaps becoming resident; Eastern house finch exhibiting the reappearance of lost migration). As habitat fragmentation increased, the individuals adapt their migratory strategy by travelling longer distances to find an appropriate habitat. The researchers’ model predicted that in such cases, paradoxically, the population’s migration ability reduces relatively gradually with increasing habitat fragmentation. The reasoning is that: ‘at high levels of habitat fragmentation, no individuals evolve to be leaders, and therefore, the population loses its migratory ability. Even after restoring the habitat, a population’s migratory ability does not recover at the same habitat quality at which it declined due to the relatively short time scale of these changes’.
Guttal V, & Couzin ID (2010). Social interactions, information use, and the evolution of collective migration. Proceedings of the National Academy of Sciences of the United States of America, 107 (37), 16172-7 PMID: 20713700
Image Source: The Wandering Angel