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Community succession and seasonal onset of colonization in sublittoral hard and soft bottoms off northern Chile

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Citation:
Pacheco, V. and Aldo, S. (2009): Community succession and seasonal onset of colonization in sublittoral hard and soft bottoms off northern Chile , PhD thesis, University of Bremen.
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Abstract:

Community ecological succession consists of the sequence of changes in species composition through time that occurs after a site has been disturbed or on the appearance of a pristine substratum, and it is one of the key processes modulating patterns of diversity, structure and evolutionary history of benthic landscapes. I have used field experiments to describe the successional development of both sublittoral hard- and soft-bottom communities off northern Chile. The Humboldt Current System off northern Chile is an excellent natural laboratory to study succession because there are several sources of disturbances operating at different scales including the El Niño Southern Oscillation (ENSO), which drastically influences biodiversity patterns. This thesis specifically aimed (i) to describe the successional development of subtidal hard- and soft-bottom communities over 27 months and a two year period, respectively; (ii) to estimate the time necessary for the developing communities to resemble the surrounding natural community, and (iii) to evaluate the effect of the seasonal starting point of community succession over a one year period of development. Succession in hard-bottom habitats showed a progressive change in community structure and competition for space, which was identified as the most important factor during succession. After 27 months, the developing communities contained the same species as the natural community but did not yet fully resemble their surroundings. Seasonality had an effect on successful species settlement, but the final stages of the developing community were influenced by interspecific competition for space (i.e. hierarchical competition). Succession followed a non-rigid, but deterministic pattern, in which colonial suspension-feeding species were hierarchically dominant over solitary species. This dominance of suspension feeders appears to be favoured by high levels of primary production associated with upwelling. On hidden surfaces of the hard substratum early successional species survive better and can laterally spread over exposed habitats as strong interactions for space among species are alleviated in hidden habitats. Succession in soft-bottom habitats supported the “tolerance succession model” where components of the later successional stages can colonize at the same time with species generally associated to the early successional stages. Resemblance to natural sediment communities occurred after eighteen months. Seasonality had no evident effect on the resulting community composition and all communities starting in different seasons converged into a similar composition after one year of exposure. These results support the notion that benthic community can return to an almost identical faunal structure with the same dominants after disturbance events. Both, hard- and soft- bottom communities showed a high capability to return and to converge towards natural surrounding communities, which highlights the resistance of the benthic system to small scale disturbances. Furthermore, the finding of new species in both study locations suggest that the components and their potential colonizing role need to be surveyed in detail to understand the species turnover during succession. My results demonstrate that benthic communities from northern Chile present a high resistance capacity to small scale disturbance during cold conditions with hard bottoms responding slower compared to the relatively quick recovery detected in soft bottoms. The question to be resolved in the future is whether or not this resistance capacity is also valid during strong El Niño events. Enhancing the temporal and spatial scale of this type of experiments is therefore recommended not only for the understanding resistance and succession, but also other ecosystem stability properties such as resilience, persistence and elasticity.

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