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

Herbivore outbreaks often trigger catastrophic overgrazing events in
marine macrophyte ecosystems. The sea urchin Paracentrotus lividus, the
dominant herbivore of shallow Mediterranean seascapes, is capable of
precipitating shifts to barrens when its populations explode. P. lividus
is found ubiquitously in rocky macroalgal communities and in sandy
seagrass meadows of Posidonia oceanica, two of the most important subtidal
habitats in the Mediterranean. We explored if habitat-specific regulation
across the principal stages of the urchin life cycle could help explain
the persistence of these populations in connected mosaics. We measured
each of three relevant ecological process (i.e. settlement,
post-settlement survival and predation) across a wide stretch of the
Mediterranean coast (ca. 600km). Our results show that habitat-specific
regulation is critical in determining urchin populations: each habitat
limited urchin sub-populations at different life stages. Settlement was
never limiting; urchins settled at similar rates in both habitats across
the coast. Post-settlement survival was a clear bottleneck, particularly
in seagrass meadows where no juvenile urchins were recorded. Despite this
bottleneck in seagrasses, adult urchin populations were very similar in
both seagrass and macroalgal habitats indicating that other processes
(potentially migration) could be key in determining adult distributions
across the mosaic. The fact that population regulation is clearly
habitat-specific suggests that sea urchin populations may be significantly
buffered from bottlenecks in mixed seascapes where both habitats co-occur.
Sea urchin populations can therefore persist across the seascape despite
strong habitat-specific regulation either by maintaining reproductive
output in one habitat or by migrating between them. By affording these
regulatory escapes to habitat-modifying species, patchy mosaics may be
much more prone to herbivore outbreaks and a host of cascading effects
that come in their wake.