Recent ecological studies have shown a strong relation between temperature, echinoids and their grazing effects on macro-algal communities. In this study, we speculate that climate warming may result in an increasingly favourable environment for the reproduction and development of the sea urchin Arbacia lixula. The relationship between increased A. lixula density and the extent of barren grounds in the Mediterranean Sea is also discussed.Keywords: ocean warming; overgrazing; sea urchin fertilisation and development Factors influencing the functioning of ecosystems, such as climate change, inputs of nutrients and/or toxic chemicals, groundwater reduction, habitat fragmentation, harvest of biomasses or loss of biodiversity, are never constant. The state of some ecosystems may respond gradually to such changes, whereas others might remain unresponsive over time (or ranges of conditions), and then respond abruptly when conditions reach a critical level. This implies that, for certain environmental conditions, the ecosystem might have two or more alternative stable states, separated by an unstable equilibrium that marks the border between the 'basins of attraction' of the states (the theory of alternative stable states; ASS) [1,2]. This theory also hypothesised that natural systems are often in persistent, resilient, alternative states: alternative combinations of ecosystem states and environmental conditions that may persist at a particular spatial extent and temporal scale.These states have been shown to be maintained by intrinsic mechanisms involving biotic and abiotic interactions (grazing or predation intensity, storm frequency, pollution, local extinction, invasion, nutrient loading, etc.) which inhibit reversal to the previous community state [3]. In combination with other factors, the loss of a keystone species [4], which results from changes in top-down interactions between (1) predators and herbivores and (2) herbivores and macroalgae, may cause changes in grazing intensity and consequent switches between two or more alternative states in marine ecosystems.For example, in temperate seas, several species of echinoids play a crucial role, and are even more important than other herbivores (i.e. fishes) as controllers of benthic communities [5][6][7][8][9][10][11][12][13].
