Effects of disturbance on the diversity of hard-bottom macrobenthic communities on the coast of Chile

Abstract

The intermediate disturbance hypothesis (IDH) predicts maximal diversity at intermediate levels of disturbance, but the validity of this hypothesis is controversially discussed. In this study, results of a field experiment, which was conducted on the northern-central Chilean coast, are presented. Fouling communities on artificial settlement substrata were studied. A total of 7 disturbance frequencies were applied to previously established communities, and a single disturbance event resulted in a removal of ~20% of the biomass. Species cover was estimated at the end of the experimental period, and it was found that diversity was strongly affected by disturbance frequency. With high disturbance frequencies the composition of the community was changed, with a decrease in the solitary ascidian Pyura chilensis (Molina 1782). The decrease of P. chilensis resulted in an increase of the colonial ascidian Diplosoma sp. A unimodal relationship between disturbance frequency and species richness was found, supporting the IDH. The results suggest that disturbance sustains diversity by reducing the abundance of the dominant species (e.g. P. chilensis), preventing competitive exclusion of the subordinate species, thus allowing subordinate species to re-emerge when competition is alleviated by disturbance. The results also suggest that these species show a trade-off between competitive and colonizing abilities, pointing to the existence of a competitive hierarchy. Therefore, the presence of competitive exclusion and disturbance-induced suppression of the dominant species remains a crucial mechanism, permitting species coexistence in the context of the IDH in the system studied.

Soft bottom species richness and diversity as a function of depth and iceberg scour in Arctic glacial Kongsfjorden (Svalbard)

“…Therefore, species richness of the Kongsfjorden soft bottom community is highest at intermediate levels of scouring impact assuming that at medium depth the competition for space and food (Wilson 1991) of dominating species is moderated by the disturbance, which reduces the depression of sub-ordinate species. Thus, species of a lower level in the competitive hierarchy would reemerge (Valdivia et al 2005). This hypothesis is supported by observations from Barrow Strait along the exposed coast of Cornwallis Island (high Arctic Canada): Conlan and Kvitek (2005) analysed benthic communities of a chronically, although infrequently scoured, area and an ice-protected community comparatively and revealed that the former was more species-rich, abundant and massive.…”

Effects of regular and irregular temporal patterns of disturbance on biomass accrual and species composition of a subtidal hard-bottom assemblage

“…Under harsh environmental conditions, communities are kept at an early successional stage with low diversity due to a constant removal of species, while competitive exclusion of inferior competitors reduces diversity in the absence of disturbance (e.g. Valdivia et al 2005). In the present study, however, species richness increased monotonically and consistently in undisturbed assemblages, suggesting that competitive exclusion did not occur.…”

Predation contributes to invasion resistance of benthic communities against the non-indigenous tunicate Ciona intestinalis

“…C. intestinalis can reach sexual maturity and reproduce within 2-4 months (Dybern 1965;Carver et al 2003), allowing this species to reproduce successfully before aquaculture structures are retrieved from the sea and cleaned, which occurs about every 4 months (von Brand et al 2006). While C. intestinalis rapidly colonized new substrata and quickly covered all suspended substrata during our 3-month experiment (see also Uribe and Etchepare 2002), colonization and initial growth of the native ascidian P. chilensis is much slower and it only starts to dominate suspended structures after 4-6 months, and eventually outcompetes the short-lived C. intestinalis (Valdivia et al 2005;Cifuentes et al 2010). In the absence of predators (exclusion treatments) in both suspended and benthic communities, the invasive ascidian C. intestinalis was more successful in early-stage colonization of new substrata than the native P. chilensis in Tongoy Bay.…”

“…A more recent but similar introduction pattern was reported in northeastern Canada where C. intestinalis heavily colonized oyster cultures but was also not found on the natural substratum (rock, eelgrass) (Carver et al 2003). In contrast, the commercially exploited native ascidian P. chilensis, which also is a competitively dominant species in fouling communities in northern Chile (Valdivia et al 2005), forms dense aggregations on rocky bottoms in the low intertidal and shallow subtidal zones (Davis 1995). Using field experiments we evaluated whether native predators were able to eliminate non-indigenous species (C. intestinalis) from natural communities and compared the relative invasibility of benthic and fouling communities under different predation pressures in two bays that differ in the abundance of artificial structures.…”