Migratory connectivity in the context of differential migration

Abstract

Understanding how breeding populations are spatially and temporarily associated with one another over the annual cycle has important implications for population dynamics. Migratory connectivity typically assumes that populations mix randomly; yet, in many species and populations, sex-, age-or other subgroups migrate separately, and/or spend the non-breeding period separated from each other-a phenomenon coined differential migration. These subgroups likely experience varying environmental conditions, which may carry-over to affect body condition, reproductive success and survival. We argue that environmental or habitat changes can have disproportional effects on a population's demographic rates under differential migration compared to random mixing. Depending on the relative contribution of each of these subgroups to population growth, environmental perturbations may be buffered (under-proportional) or amplified (overproportional). Thus, differential migration may result in differential mortality and carry-over effects that can have concomitant consequences for dynamics and resilience of the populations. Recognizing the role of differential migration in migratory connectivity and its consequences on population dynamics can assist in developing conservation actions that are tailored to the most influential demographic group(s) and the times and places where they are at peril.

All routes are not created equal: An ungulate's choice of migration route can influence its survival

“…Similar to research that has shown variation in seasonal‐range quality can influence individual fitness (Searle, Rice, Anderson, Bishop, & Hobbs, ), our work shows some migration routes may confer greater fitness benefits than others (sensu Briedis & Bauer, ). Regardless of summer range destination, we found the survival of deer was 27%–33% lower for animals that migrated along routes categorized as exterior compared to interior.…”

“…In contrast to ungulates that exhibit flexible migratory behaviours (Peters et al, ), mule deer may be further predisposed to continue utilizing non‐optimal routes because of their rigid migratory behaviours characterized by high fidelity to routes, irrespective of age, geography or reproductive status (Sawyer, Merkle, et al, ). Regardless of whether certain species are more prone to maladaptive migratory behaviours than others, the variable mortality risk we observed among migratory routes represents another form of differential migration – where the demographic consequences of migratory connectivity can be markedly different for distinct groups of individuals within the same population (Briedis & Bauer, ).…”

“…Understanding how vital rates or demography may vary within a migratory population can help identify specific places (e.g. migratory routes or seasonal ranges) where certain groups are at most risk (Briedis & Bauer, ). Our study provides a novel link between migration and demography by revealing that large herbivores may occupy the same general seasonal ranges, but increase mortality risk by nearly three times simply by using a different migratory route.…”

Effects of Natal Dispersal and Density-Dependence on Connectivity Patterns and Population Dynamics in a Migratory Network

“…Other possible future extensions to the model could introduce other processes that might affect connectivity or population dynamics including seasonal interactions (e.g., when quality of winter habitat affects breeding success (Norris and Taylor, 2006;Harrison et al, 2010), orientation processes, i.e., how animals orient themselves or choose routes (Thorup and Rabøl, 2001;Willemoes et al, 2014), and differential migration by sex (Briedis and Bauer, 2018), which might require coupled but separate networks for males and females. The ecological model presented here could be adapted to explore evolutionary dynamics, e.g., evolution of natal dispersal.…”

What Do Ecology, Evolution, and Behavior Have in Common? The Organism in the Middle

“…People currently studying migratory species emphasize the importance of studying the full annual cycle (Marra et al 2015). An important next step to studying differential migration in the 1980s would have been to connect wintering birds to their breeding locations, but the ability to do that was far off in the future (Briedis and Bauer 2018). Another approach would have been to study heritability of migratory behavior, but most songbirds are ill suited for genetic studies because so few breed successfully in captivity (but see Pulido et al 1996).…”