Causes and consequences of individual variation in animal movement

Abstract

Animal movement comes in a variety of 'types' including small foraging movements, larger one-way dispersive movements, seasonally-predictable round-trip migratory movements, and erratic nomadic movements. Although most individuals move at some point throughout their lives, movement patterns can vary widely across individuals within the same species: differing within an individual over time (intra-individual), among individuals in the same population (inter-individual), or among populations (inter-population). Yet, studies of movement (theoretical and empirical alike) more often focus on understanding 'typical' movement patterns than understanding variation in movement. Here, I synthesize current knowledge of movement variation (drawing parallels across species and movement types), describing the causes (what factors contribute to individual variation), patterns (what movement variation looks like), consequences (why variation matters), maintenance (why variation persists), implications (for management and conservation), and finally gaps (what pieces we are currently missing). By synthesizing across scales of variation, I span across work on plasticity, personality, and geographic variation. Individual movement can be driven by factors that act at the individual, population, community and ecosystem level and have ramifications at each of these levels. Generally the consequences of movement are less well understood than the causes, in part because the effects of movement variation are often nested, with variation manifesting at the population level, which in turn affects communities and ecosystems. Understanding both cause and consequence is particularly important for predicting when variation begets variation in a positive feedback loop, versus when a negative feedback causes variation to be dampened successively. Finally, maintaining standing variation in movement may be important for facilitating species' ability to respond to future environmental change.

Keywords: Context-dependent; Dispersal kernel; Environmental change; Foraging ecology; Movement ecology; Nomadism; Partial migration; Personality; Plasticity; Population dynamics; Range expansion; Sex-biased dispersal.

Fig. 1

Schematic of the causes, patterns and consequences of movement variation. a External factors (environment) are perceived by an individual, and taken in combination with its genotype, internal state and history to determine the movement response, (b) movement can vary along three ‘axes’ (whether to move, when to move, and where to move), and (c) movement first impacts the individual before potentially scaling up to affect the population, community and ecosystem. While causes often act in parallel, consequences are typically nested. Variation in any of the causes (or their interaction) can contribute to variation in movement, and moving in turn can feed back to affect variation if a consequence of moving is increasing variation in the causes of movement (positive feedback, solid arrow) or decreasing said variation (negative feedback, dashed arrow)

Fig. 2

Individual differences in movement can occur at several scales. Each line represent one individual that displays one of two different movement behaviors (A or B) across each of two years, for two different populations. Differences can occur (a) within the same individual over time, (b) among individuals within a population, or (c) among populations of individuals