Abstract
ABSTRACTIncreasingly imperative objectives in ecology are to understand and forecast population dynamic and evolutionary responses to seasonal environmental variation and change. Such population and evolutionary dynamics result from immediate and lagged responses of all key life‐history traits, and resulting demographic rates that affect population growth rate, to seasonal environmental conditions and population density. However, existing population dynamic and eco‐evolutionary theory and models have not yet fully encompassed within‐individual and among‐individual variation, covariation, structure and heterogeneity, and ongoing evolution, in a critical life‐history trait that allows individuals to respond to seasonal environmental conditions: seasonal migration. Meanwhile, empirical studies aided by new animal‐tracking technologies are increasingly demonstrating substantial within‐population variation in the occurrence and form of migration versus year‐round residence, generating diverse forms of ‘partial migration’ spanning diverse species, habitats and spatial scales. Such partially migratory systems form a continuum between the extreme scenarios of full migration and full year‐round residence, and are commonplace in nature.Here, we first review basic scenarios of partial migration and associated models designed to identify conditions that facilitate the maintenance of migratory polymorphism. We highlight that such models have been fundamental to the development of partial migration theory, but are spatially and demographically simplistic compared to the rich bodies of population dynamic theory and models that consider spatially structured populations with dispersal but no migration, or consider populations experiencing strong seasonality and full obligate migration. Second, to provide an overarching conceptual framework for spatio‐temporal population dynamics, we define a ‘partially migratory meta‐population’ system as a spatially structured set of locations that can be occupied by different sets of resident and migrant individuals in different seasons, and where locations that can support reproduction can also be linked by dispersal. We outline key forms of within‐individual and among‐individual variation and structure in migration that could arise within such systems and interact with variation in individual survival, reproduction and dispersal to create complex population dynamics and evolutionary responses across locations, seasons, years and generations. Third, we review approaches by which population dynamic and eco‐evolutionary models could be developed to test hypotheses regarding the dynamics and persistence of partially migratory meta‐populations given diverse forms of seasonal environmental variation and change, and to forecast system‐specific dynamics. To demonstrate one such approach, we use an evolutionary individual‐based model to illustrate that multiple forms of partial migration can readily co‐exist in a simple spatially structured landscape. Finally, we summarise recent empirical studies that demonstrate key components of demographic structure in partial migration, and demonstrate diverse associations with reproduction and survival. We thereby identify key theoretical and empirical knowledge gaps that remain, and consider multiple complementary approaches by which these gaps can be filled in order to elucidate population dynamic and eco‐evolutionary responses to spatio‐temporal seasonal environmental variation and change.
Highlights
Despite the ubiquity of complex spatial variation in seasonal environmental conditions, increasingly widespread evidence of diverse forms of partial migration (Fig. 1), and increasing general interest in rapid eco-evolutionary responses to environmental change and resulting ‘evolutionary rescue’ (e.g. Pelletier et al, 2009; Bonte et al, 2012; Gonzalez et al, 2013; Chevin & Hoffmann, 2017), we still lack any general understanding of key forms of spatio-temporal demographic variation, covariation, structure and heterogeneity that can arise in partially migratory systems, and of the short-term and longer-term population dynamic and evolutionary consequences
They illustrate how simple rules regarding seasonal movement decisions can generate diverse individual strategies that span classical definitions of dispersal and migration without requiring such strategies to be explicitly specified. Such models offer exciting opportunities to address numerous key questions concerning how different regimes of spatio-temporal environmental variation foster different interacting strategies of migration and dispersal; to identify general rules spanning different stage-structured life histories, spatial structures of seasonality and forms of density-dependence, plasticity, carry-over effects and costs; and to assess eco-evolutionary and population dynamic consequences of diverse forms of spatio-temporal environmental variation including local and global persistence, transient and cyclic dynamics and range-shifting (Table 1). To achieve these aims our basic illustrative individual-based models (IBMs) could readily be extended in multiple ways to include: more zones and/or patches with different structures of seasonality and zone-specific survival and/or reproductive success; different forms of density-dependence and costs of migration and dispersal; different life histories, including iteroparity and age, sex, stage, state and/or cohort-structure in any demographic rate; different forms of temporal variation in seasonality; different forms of plasticity, canalisation, social learning and assortative mating in migration versus residence; and different underlying genetic architectures
(1) Key links between environmental and demographic variation, and resulting population and evolutionary dynamics, have not yet been elucidated in spatially structured populations where individuals vary in their expression of seasonal migration, even though such migration constitutes one pre-eminent means by which individuals can respond to seasonal environmental variation and change
Summary
Progress in understanding and forecasting the dynamics of populations inhabiting spatially structured seasonally varying environments requires new models and empirical studies that coalesce attributes of the major existing bodies of work that consider complex structure, variation and micro-evolution in key demographic rates in non-migratory systems (Section I.2), or consider seasonal demography given obligate seasonal migration (Section I.3). By fully encompassing demographic complexity involving seasonality and among-individual and within-individual variation in seasonal migration, such work could provide a holistic framework for population dynamic theory and forecasting To facilitate this goal, we first review fundamental scenarios where the occurrence and form of seasonal migration varies among individuals within populations, creating different forms of spatio-temporal population structure (Sections II and III). To provide an overarching general framework that encompasses all these scenarios, we outline the concept of a ‘partially migratory meta-population’ (PMMP) We draw on this framework to hypothesise numerous ways in which complex forms of spatio-temporal structure and variation in migration within and among individuals could arise and interact with variation in reproduction, survival and dispersal to shape population dynamics across locations, seasons, years and generations (Section IV).
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