Abstract
Demographic processes exert different degrees of control as individuals grow, and in species that span several habitats and spatial scales, this can influence our ability to predict their population at a particular life-history stage given the previous life stage. In particular, when keystone species are involved, this relative coupling between demographic stages can have significant implications for the functioning of ecosystems. We examined benthic and pelagic abundances of the sea urchin Paracentrotus lividus in order to: 1) understand the main life-history bottlenecks by observing the degree of coupling between demographic stages; and 2) explore the processes driving these linkages. P. lividus is the dominant invertebrate herbivore in the Mediterranean Sea, and has been repeatedly observed to overgraze shallow beds of the seagrass Posidonia oceanica and rocky macroalgal communities. We used a hierarchical sampling design at different spatial scales (100 s, 10 s and <1 km) and habitats (seagrass and rocky macroalgae) to describe the spatial patterns in the abundance of different demographic stages (larvae, settlers, recruits and adults). Our results indicate that large-scale factors (potentially currents, nutrients, temperature, etc.) determine larval availability and settlement in the pelagic stages of urchin life history. In rocky macroalgal habitats, benthic processes (like predation) acting at large or medium scales drive adult abundances. In contrast, adult numbers in seagrass meadows are most likely influenced by factors like local migration (from adjoining rocky habitats) functioning at much smaller scales. The complexity of spatial and habitat-dependent processes shaping urchin populations demands a multiplicity of approaches when addressing habitat conservation actions, yet such actions are currently mostly aimed at managing predation processes and fish numbers. We argue that a more holistic ecosystem management also needs to incorporate the landscape and habitat-quality level processes (eutrophication, fragmentation, etc.) that together regulate the populations of this keystone herbivore.
Highlights
The population dynamics of keystone species can have farreaching consequences
Our results indicate that different processes acting at different spatial scales are influencing the demographic
While regional scale factors determine larval availability and settlement patterns of the pelagic stages, once in the benthos, processes linked to local-scale habitat features become crucial in controlling the population outcome
Summary
The population dynamics of keystone species can have farreaching consequences. Population outbreaks, of herbivores, have been observed to cause important ecosystem shifts in terrestrial, freshwater, and marine environments [1,2,3]. While top-down factors like predation are often strong enough to explain population dynamics in a multiplicity of ecosystems [4], when a species has a life history that spans multiple spatial scales and habitats, it is often difficult to explain such dynamics with a single factor. This is true in the case of marine benthic organisms with planktonic larval stages, which depend both upon factors controlling the arrival of new individuals and on the structural and functional properties of the habitats in which they recruit [5,6]. Most processes affecting different life-stages are scale-dependent, and the identification of certain, prevailing mechanisms for population control will depend on the study’s spatio-temporal scale
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