Sand dynamics in coastal dune landscapes constrain diversity and life-history characteristics of spiders.
In fragmented landscapes where natural disturbance acts at the local level, community composition can be altered either by 'species-by-species matching', with community-wide character displacement, or by 'constrained species sorting', whereby disturbance favours species with distinct ecological traits. It is necessary to understand such species shifts when aiming to detect bottlenecks associated with changing disturbance dynamics at different spatial levels. Such an understanding also provides insight into the underlying ecological mechanisms in operation and allows identification of species of conservation concern. This approach to understanding species' shifts was applied to a study of spider diversity and life-history variation in relation to sand dynamics in stabilized and dynamic grey dune landscapes located along the coasts of France, Belgium and the Netherlands. Local (α) diversity appeared to decrease with increasing local sand dynamics in both stable and dynamic landscapes. In dynamic dune landscapes, β-diversity decreased significantly and approached regional (γ) diversity under local grey dune stabilization; in stabilized landscapes, β-diversity was not affected by local sand dynamics. Comparative analyses of ecological traits revealed shifts in life-history patterns, suggesting that patterns in local diversity resulted from species sorting. Species from fragments characterized by high sand dynamics showed narrower niche breadths, larger body sizes and longer generation times, while summer-active species tended to become residential after sand stabilization. This provides evidence for constrained species sorting in which natural disturbance (through local sand dynamics) allows only species with distinct ecological traits to persist. Synthesis and applications. Species with larger body sizes, longer generation times and a higher degree of habitat specialization (i.e. related to dispersal ability) would be expected to be more vulnerable at dynamic sites and more prone to extinction than their counterparts from stabilized fragments. In particular, species with a burrowing lifestyle would be expected to go extinct locally as a result of increasing soil development and soil hardness. Shifts in species composition were found to be more pronounced in dynamic landscapes. Hence ensuring conservation of sand dynamics at a landscape level rather than at a local level is of prime importance when aiming to conserve typical psammophilous spider species within local assemblages in grey dune habitats.