Movement behaviour of a large marine predator – a complex systems approach (#64)
Spatial patterns in animal movement are a key determinant of trophic relationships, nutrient cycling and energy flows in ecosystems. In marine environments, considerable research effort has been focused on describing movement patterns of marine megafauna such as seals using satellite telemetry. However, new statistical techniques generated by the study of human mobility, such as the complex systems approach, now offer a novel means to analyse these data. Here, we use these approaches to explore the movement behaviour of a large marine predator, the southern elephant seal, from satellite telemetry data spanning five years and 176 deployments in the Southern Ocean.
The distribution of displacements of elephant seals was described by a power law, encompassing several spatial and temporal scales. When normalized by the average displacement for each temporal scale, the distribution of displacements follows a universal function, with a power-law which decays for displacements below the average (P(r) ~ r′-γ; where γ = 0.78) and a fast exponential-like decay for larger displacements. This scale-free behaviour of the distribution of displacements indicates that elephant seals use the same dispersal strategy, irrespective of spatial scale. The distribution of seal residencies also displayed a power law decay that combined frequent, low residency sites with infrequent, high residency sites. High residency was correlated with low average speed, possibly indicating area restricted search behaviour.Finallywe also tested community detection algorithms to identify particular marine areas of importance and inform on function.
Scale-free movement behaviour has been reported in other species as optimal strategies to find resources (e.g. Levy walk). However, we report a lower exponent than previously reported for this strategy. The displayed scaling confers an optimal strategy to explore a larger number of foraging locations and at the same time transit between focal locations. This supports a strategy of a highly directional pattern of movement to known areas where resources are plentiful, which then shifts to a searching mode once they have arrived.