Habitat and Scale Shape the Demographic Fate of the Keystone Sea Urchin Paracentrotus lividus in Mediterranean Macrophyte Communities

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.

Herbivore control in connected seascapes: habitat determines when population regulation occurs in the life history of a key herbivore

“…Urchin larvae have proved to be related to the number of recruits in both habitats by previous studies (Prado et al 2012). Here, larvae were most likely derived from the same regional pool, subject to larger oceanographic processes; settlement rates were similar in both habitats and the marginally higher settlement in macroalgal habitats did not amount to a clear demographic control between habitats.…”

“…This, despite the fact that settlement rates in seagrass meadows were relatively high in most locations, suggesting that recruitment limitation was not a factor. This adds to the growing consensus that the nearshore benthic systems of the Western Mediterranean may not be significantly recruitment limited (see Prado et al 2012).…”

“…This method has been successfully used to determine settlement rates in seagrasses and macroalgal substrates with the assumption that it reflects the natural settlement rates that occur in those habitats (Hereu et al 2004, Prado et al 2012. We placed one collector in each area and habitat within each location at two times in the peak settlement period (May-June, see Tomas et al 2004).…”

“…We fixed the filtered samples in 70% ethanol and counted urchin settlers in each sample with a binocular microscope. This method provides a robust estimate of pre-settlement larval availability in the water column (Prado et al 2012). …”

“…Individuals that survive to older size classes are subject to size-specific fish and invertebrate predators, which can be important controllers of urchin populations (Sala 1997, Bonaviri et al 2012. Additionally, there is some evidence that urchin movement and potential migration between macroalgal and seagrass habitats could drive population fluctuations within habitats (Ceccherelli et al 2009, Prado et al 2012.…”