SUMMARY
Human alteration of the global environment and the high loss of biodiversity during the last decade have caught the attention of the general public and influenced the scientific world.
While initially the issues was faced from an ethical point of view, soon the attention of the scientific community shifted on the role that biodiversity has on ecosystem functioning.
The debate on biodiversity and ecosystem functioning (BEF hereafter) has developed animated debates and has generated a vast production of theoretical, observational and experimental studies. Together with the already assessed role of species richness, it has also been evidenced the importance of the functional characteristics of species for the functioning and stability of ecosystems, for example through the effects of dominant species, keystone species or ecological engineers. At the same time, the role of biological interactions has also been highlighted.
Studies revealed that ecosystem functioning could be influenced by richness and identity of species through two different mechanisms not mutually exclusive, called the “selection probability effect” (or “sampling effect”) and the complementarity effect”. The first is related to the fact that, increasing the levels of richness, there is a major chance for particularly important species to be present in a fixed area. Whereas the complementarity effect is associated with interactions among species, such as facilitation and resource partitioning.
Even if several experimental designs have been proposed in the BEF debate, their use has often been confusing in separating the effects of richness from those of density. Traditionally,
“substitutive designs” on biodiversity reduced the relative abundance of species as richness levels increased, maintaining overall density constant across levels of species richness.
However this experimental design may not be correct to separate the effect of species richness per se.
Ecological succession has been recognised as an important framework for theoretical development, empirical testing and, in some cases, successful prediction in ecology. As life- history, richness and density of species are essential in determining the transition among successional stages, the analysis of succession could to be investigated from the point of view
of BEF relationships. Variable patterns in richness, identity and density of early colonizers could in fact affect subsequent patterns of colonization of species.
The present study focused on assemblages of algae and invertebrates of rocky shore habitats on the coast of Calafuria (LI). An alternative design was used, which explicitly manipulates the density of organisms as a factor crossed with richness, and where a randomly constrained selection of identities of assemblages was used. This enabled us to analyze the effect of richness, identity and relative abundance of taxa characterizing early assemblages on subsequent patterns of colonization and on the structure of mature ones. Different biodiversity treatments were synthetically created assembling the early colonizers on the experimental units in the field, according to different levels of richness, identity and density. Percentage covers of later colonizers were sampled 6 times across 18 months. A second phase of the experiment involved the repetition of the previous experiment, with higher level of replication.
Preliminary results of the study showed that even at the first stages of colonization, assemblages were already diversified. After 3 months from the beginning of the experiment, density of early colonizers had a significant effect on the development of later assemblages.
Whereas from 11 months until the very end of the sampling period, richness of early assemblages was highlighted as an important structuring factor for later colonizers. The role of identity of early assemblages was also evidenced in influencing later ones.
The results showing a significant effect of density, indicate that different structures of assemblages depends on the density of early colonizers, and they also suggest a facilitative effect on later organisms. Analysis suggest that the presence of early organisms provided a more suitable environment for the settlement and growth of propagules or larvae.
After 14 months (August 2006), however, significant differences emerged again between assemblages characterized by different levels of initial Density, but in interaction with Richness. After 14 months, when assemblages could be considered mature, richness in interaction with density, diversified the later assemblages. However the assemblages present after 11 months could not be considered equal to the assemblages formed at the beginning of the experiment. It is then impossible that the initial levels of richness and density could influence assemblages after 11 months. On the other hand the results could be explained by
the fact that the first colonizers, settled after the density and richness treatments, facilitated or inhibited the colonization of the other organisms after 11 months.
In general these results showed interesting and complex aspect of successions in those cases in which recovery is started by an assemblage of early colonists, not just by few species.
Density of early colonizers can in fact have a strong facilitative effect at the beginning;
however, as succession proceeds, a lessening of the strength of this positive density- dependent effect can occur, suggesting a replacement by competitive processes. But, more interestingly, density and richness of early colonizers can have an effect even after a long period of time as more than 1 year.
Although a vast knowledge exists on the patterns of biodiversity changes and on ecosystem processes, conclusions and observations have often been confounded by difficulties in experimental design and interpretation of data. Incorporating concepts of biodiversity in studies of recovery and succession will enhance our ability to predict recovery scenarios in the face of increasing frequency of extreme events of disturbance, as predicted by models of climate change.
