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DISCUSSION One of the important findings of this experiment comes from the observation of early assemblages

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4. DISCUSSION

One of the important findings of this experiment comes from the observation of early assemblages. Even at the first stages of colonization variability among panels was large and assemblages were already diversified. These observations leaded to the consideration that the system analyzed showed several levels of richness, identity and density of species in early assemblages. This finding is in agreement with the large variability observed at these stages on other rocky shores (reviewed by Sousa 1985). Together with the known variability of this system (Menconi et al.1999, Benedetti-Cecchi 2000), heterogeneity during the early stages of colonization anticipated large variability also for later stages, stressing the necessity of an experimental approach in order to understand and predict recovery of assemblages into patches of open space (Benedetti-Cecchi 2000).

Differences in density of manipulated taxa affected the structure of later colonists already after 3 months from the start of the experiment. Whereas after 14 months and until the very end of the sampling period analyses highlighted also the role of initial Richness as an important structuring factor for later colonizers. In addition, results identified the importance of Identity as a factor influencing the number of later taxa, but only at intermediate levels of Richness.

Understanding the influence of early colonists on later ones, according to facilitation, tolerance and inhibition models (Connell and Slatyer, 1977), has been fundamental for the development of the issues of successions and especially for directing field studies towards experimental tests of hypotheses (McCook and Lubchenco 1994). In order to make predictions on the structure of mature assemblages, early experiments essentially involved the removal of one or two early species, and the same methodology was used also for testing later models of successions (e.g. Lubchenco 1986, Farrell 1991). The present work, incorporating current theories on biodiversity in a study of colonization, offers a unique opportunity to revisit the traditional models of Connell and Slatyer (1978) in terms of interactions within a rich assemblage of early organisms.

First of all this study underlined a facilitative effect of Density of early colonizers on the later assemblages, at least during the first months (August 2005). In fact, results not only indicate that different structures of assemblages (especially in terms of abundance of taxa) depended

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on the density of early colonizers (whatever their richness or identity), but they also suggested an unexpected positive effect of higher densities on later organisms (in particular on mean percentage covers of L. obtusa and J.rubens/H.virgatum). Until now, most of the studies conducted on rocky shores indicated inhibition or indirect effects as common traits of succession in this habitat (Lubchenco and Menge 1978, Sousa 1979, Lubchenco 1983, Underwood et al. 1983, Van Tamelen 1987, Bendetti-Cecchi 2000). Furthermore, inhibiting mechanisms related to density-dependent processes have been recently hypothesized in BEF experiments on this substrate. For example Stachowicz et al. (2002, 2006) suggested that consistent occupation of primary space in subtidal rocky habitats is key to enhancing invasion resistance in experimentally assembled native communities in southern England. And Arenas et al. (2006) stressed the importance of space as an available resource for invasive organisms in rockpools. Facilitation has rarely been documented in rocky shore succession (Turner 1983, Williams 1990, Bertness et al. 1999), while it has been proposed as the prominent driving mechanism of succession in salt marshes and sand dunes, where the effects of early colonists on later species include stabilization of the substratum, accumulation of nutrients or enhancement of pH (Olff et al. 1993, Van Adel et al. 1993). In the case of the present experiment, we can hypothesize that the presence of early organisms provided a more suitable environment for propagules or larvae of later species through lower rate of desiccation or water flow.

With the proceeding of colonization, however, the effect of Density on assemblages disappeared, and a positive effect was only evident on the mean percentage cover of J.rubens/H.virgatum after six months, while assemblages did not differ significantly until August 2006. This possibly indicated the replacement of facilitative mechanisms by competitive processes, as overall densities approach large values. Different processes and mechanisms acting at different stages is not a new concept in the field of successions (Farrell 1991), and space is known as one of the most important limiting factor for sessile organisms on rocky shores.

After 14 months (August 2006), however, significant differences emerged again between assemblages characterized by different levels of initial Density, but in interaction with Richness. Except for the treatment with the lowest level of Richness (initially composed only by encrusting corallines), all the other assemblages showed the same organisms characterizing the two levels of Density. In particular L. obtusa and red filamentous algae characterized the

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High and Low Density treatments respectively, with a coherent trend since the start of the experiment. Differently, J. rubens/H. virgatum (the other most abundant taxon, excluding those manipulated) has almost macroscopically disappeared in Winter and Spring (due to its annual life cycle, characterized by a main vegetative growth period during Autumn and a restart of the growth in Summer), and once reappeared it has changed its coverage trend being more abundant in the Low Density treatments. It is then possible that the colonizers present at Low Density after 14 months have now facilitated the growth of J. rubens/H. virgatum or, on the contrary, assemblages developed in treatments with initial High Density may have inhibited the growth of these algae.

Starting from May 2006 (Time 4, 11 months), some more later colonizers appeared in the treatments. C. boryana, thin tubular sheet-like algae and C. stellatus, even if present from the beginning, showed higher percentage covers only since this period; and in August 2006 (Time 5, the same month as Time 1) importantly characterized Density levels. This may be explained through competitively inferior features of settlers of these organisms compared to those of L. obtusa, J. rubens/H. virgatum and red filamentous algae. The percentage cover of these species, however, didn’t change significantly between Time 1 and Time5 (except J.

rubens/H. virgatum ); on the contrary the total available space in August 2006 was higher at High in comparison to Low Density, according with a general lowering of the total percentage cover of manipulated organisms since the start of the experiment. Given this considerations, we can suppose that the last colonizers were initially inhibited both by manipulated assemblages and competitive superiority of other settlers. But as early colonizers reached lower values, they were finally able to enter the succession.

Further, the analysis evidenced that at this same time (after 14 months) organisms characterizing both levels of density (especially in terms of higher percentage covers; red filamentous algae, J. rubens/H. virgatum and C. boryana for Low Density, L. obtusa and thin tubular sheet-like algae for High) were more abundant and mainly characterized higher levels of richness, evidencing a positive role of a greater number of early organisms on later ones, even after more than 1 year.

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;

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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. Even if a long term effect of early species is not new (Dickerson and Robinson 1985), our results highlighted again the role of density in this system, and an unexpected later positive effect of richness, suggesting complex interactions among colonizers as recovery proceeds.

A caveat in the interpretation of the results is that in two out of five dates assemblages in scraped plots differed from those occurring on unmanipulated panels. This indicates that the panels may not reflect natural patterns of recovery of assemblages at all stages. This issue requires further consideration in future studies.

Given the currently high rates of extinction of species, many studies have been conducted in order to understand the consequences of loss of biodiversity for the functioning of ecosystems. However few studies have taken into account the marine habitats and in particular rocky bottoms (but see Allison et al., Stachowicz et al. 1999 and 2002, Bruno et al.

2005, O’Connor and Crowe 2005, Arenas et al. 2006). The role of Identity of species (taxa) in these systems, that was evidenced by our results only marginally and only at intermediate levels of richness, has been highlighted in these previous studies. O’Connor and Crowe (2005), for example, found that diversity and productivity of intertidal algal diversity on rock pools differed significantly depending on the identity of gastropods species lost. The same results were shown by Bruno et al. (2005) on a study conducted on macroalgae communities, where species identity appeared to be much more important than species richness in controlling primary biomass production.

Moreover, according to Bruno et al. (2005) the mechanisms and effects of biodiversity in benthic marine macroalgal communities are very similar to those found in terrestrial and wetlands vascular plant systems. The results of their study showed for example strong similarities with those of the large-scale Biodepth experiment in European grasslands (Hector et al. 2002). However, most of these studies have not taken into account density of species, confounding its possible effects with those of richness or identity; this was stressed also in the only terrestrial work that directly manipulated density of species (He et al. 2003). More

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precisely the authors proposed that community density was a potentially confounding factor in the diversity-productivity relationship and may be responsible for some of the discrepancies observed in previous studies.

Our work, through the use of a novel experimental design, confirmed the potential role of density of species, at least in those habitats where density-dependent processes can operate (Benedetti-Cecchi 2004). While some studies conducted on macroalgal assemblages anticipated these effects in terms of availability of substratum for invaders responses (Stachowicz and Byrnes 2002 and 2006, Arenas et al. 2006), our results showed unexpected long term facilitative effects of densities of early colonizers; and in contrast with most of past studies evidenced a non confounded positive effect of richness.

Differently from past studies on succession, which focused on the effect of identity of a few early species on later colonizers, this complex scenario highlights the importance of experimentally testing the effects that different features of biodiversity may have on succession and the need for appropriate designs that can separate the effects of number, identity and density of species, and that will allow us to re-interpret old models of succession in the light of modern concepts of biodiversity.

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.

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