Variación ambiental y patrones di etarios del erizo negro Tetrapygus niger en costas intermareales rocosas del norte de Chile

Abstract

The role and impact of herb́vores on rocky intertidal food webs could be more complex and diverse than previously considered, particularly in the case of larger and more abundant consumers such as the black urchin Tetrapygus niger (Molina) in the coast of Chile. In this context, we analyzed the variation in diet richness and composition of the black urchin in four rocky intertidal communities over a 1,000 km stretch of coast in northern Chile, sampled seasonally between the austral winter of 2004 and spring of 2005. The 2004-2005 El Niño event took place during the study, and we incorporated its potential effects to this evaluation. For each community and season, we measured the abundance of species present in 45 quadrants (0.25 m 2) distributed between five permanent blocks, collecting about 10 individuals of T. niger to measure their body size (dry biomass and maximum diameter) and to determine the number, identity and occurrence frequency of dietary items in their gastric tract. We collected a total of 251 urchin individuals, which consumed 89 items of algae (64 %) and invertebrates (36 %; including other herbivores). A significant inverse relationship between the number of ingested animal items and body size suggests that T. niger changes its diet throughout the ontogeny. Dietary niche breadth was high but tended to decrease toward lower latitudes. The spectrum and occurrence frequency of dietary items were not significantly different among communities, despite their marked differences in upwelling intensity. Similarly, we did not find clear relationships between the natural abundance of resources and their frequency in the diet of T. niger, highlighting the green alga Ulva rígida which was the most frequently consumed item in all communities although not the most abundant in the field. However, the dietary richness of T. niger showed a significant negative relation with sea surface temperature, and was significantly lower throughout the El Niño period. Our data suggest that T. niger is a polyphagous gene alist whose consumption patterns are strongly modulated by environmental thermal fluctuations. We also hypothesize that T. niger could use both opportunistic and selective strategies on different resources, and its trophic position and connectedness would be more complex than those of a simple primary consumer.