Habitat structural complexity increases age-class coexistence and productivity in fish populations

Abstract

AbstractStructurally-complex habitats harbour more taxonomically-diverse and more productive communities, a phenomenon generally ascribed to habitat complexity relaxing the strength of interspecific predation and competition. Here, we challenge this classical, community-centred view by showing that positive habitat complexity-productivity relationships may also emerge from between-age-class,intra-specific interactions at a single-population level. In replicated outdoor pond populations of the medaka fish (Oryzias latipes), structurally complex habitats provide refuges to newborns and relax the strength of cannibalism, resulting in increased survival of age-0+ individuals, in a 80 % increase in population growth rate, and in dampened negative density-dependence indicating elevated habitat carrying capacity. The resultant higher population density in complex habitats was associated with increased competition for food among age-0+ and age-1+ individuals, as revealed by their smaller and more variable body sizes. Positive habitat complexity-productivity relationships may thus be considered as a generally-emergent property of both size-structured communities and populations, in which a larger body size brings a predation advantage. Our results highlight that anthropogenic habitat simplification drives biodiversity loss not only from community- but also from population-level processes, and hence may further reduce population productivity in the surviving species. Enhancement of habitat structural complexity is therefore a pivotal action for biodiversity improvement, not only in the context of ecosystem management, but also for successful conservation of endangered populations.