Conspecific attraction boosts local density while causing lags in range expansion despite high dispersal ability: experiments with a reintroduced endangered mammal

Abstract

The potential range of many species is shifting, reflecting changing ecological conditions due to climate change or the disappearance of causes of decline. The length of any lag between new habitat becoming available and being colonised is likely driven by a complex interaction between individual decisions, habitat configuration and how population growth rate and persistence depends on local and neighbourhood density. Where such lags are lengthy the consequences for a species realised distribution may be profound. In fragmented landscapes, dispersers may preferentially settle in proximity of conspecifics, increasing persistence of small populations but restricting the colonisation of suitable but empty neighbourhoods. Thus during range expansion lags may occur when some equally suitable neighbourhoods become densely occupied while others remain vacant. Species reintroductions offer an ideal opportunity to answer fundamental questions on how patterns of occupancy emerge at the range edge. We performed and monitored experimental translocations of water voles, the UK’s fastest declining mammal, to quantify the influence of occupancy and habitat suitability on colonisation and local persistence. We used a novel statistical method to simultaneously consider the effect of occupancy across a range of spatial scales. Densely occupied neighbourhoods were highly persistent and frequently colonised. Persistence was more likely in highly suitable habitat, whereas colonists settled irrespective of habitat quality, suggesting conspecific attraction strongly influenced settlement decisions by dispersers. Distant, sparsely occupied neighbourhoods were colonised less frequently than expected from colonisation patterns and dispersal ability observed in established metapopulations of this species 1. The low persistence of such populations, far from the range edge, contributed to a lag in range expansion. There may be a mismatch between colonisation dynamics in the core and edge of the range, and dispersal potential of a species may not be realised where individuals preferentially settle in proximity to conspecifics. Where consolidation of extant neighbourhoods precedes colonisation of empty yet suitable range, the pool of potentially longer distance colonists is depleted causing lengthy lags. These findings will help optimise conservation delivery, especially during transient vulnerability post reintroduction before spatial equilibrium arises. Those seeking to speed up recolonisation by recovering species should consider reinforcing nascent populations at the range front, as well as reintroductions beyond the expanding edge. Under climate change, lags may lead to a mismatch between rates of movement of trailing and expanding edge of range shifting species, with detrimental consequences for a species realised distribution, conservation status and contribution to ecosystem function.

1. Sutherland, C. S., Elston, D. A., & Lambin, X. Ecology 95(11), 3149-3160