Ocean warming and tropical invaders erode the performance of a key herbivore.

Climate change and bioinvasions are two facets of global change that can act in tandem to impact native species and ecosystems. However, their combined effects on key species have rarely been studied. The Mediterranean Sea is a hotspot of both ocean warming and bioinvasions, where their impact can be tested together. In recent years, the population of a key herbivore, the European purple sea urchin (Paracentrotus lividus), has virtually collapsed along the Israeli Mediterranean coast (south-eastern Levant). Here, we used field and lab experiments to test two complementary hypotheses that may explain the urchin population collapse: (a) resource competition that may lead to competitive exclusion by invasive grazers (two Red Sea rabbitfishes); and (b) reduced performance due to ocean warming. An inclusion-exclusion in-situ caging experiment revealed a strong negative impact of fish grazing on algal cover and on the urchin's gut content and gonado-somatic index (GSI). Laboratory experiments revealed a considerable negative impact of both elevated temperature and food deficiency on sea urchin respiration and GSI, and consequently on its energy budget and reproductive potential and, potentially, fitness. Such reduced reproductive capacity must have greatly lowered the sea urchin's population viability, contributing (and possibly even leading) to its collapse in the south-eastern Levant in the past two decades. Urchin population declines are expected to spread to the west and north of the Mediterranean Sea following further warming and rabbitfish expansion. This study is the first to demonstrate the potential additive effects of ocean warming and implied competitive exclusion by an invader on a native species at its warm biogeographic distribution edge.