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PNZ1 – Host variety enhances diversity: the role of multiple secondary habitat-forming seaweeds in facilitating estuarine invertebrate communities

This project describes the simplest habitat cascade system documented during my PhD, in soft-bottom shells-bed estuaries. Here, my aim was to test if habitat cascades are strongly affected by the biomass of the secondary habitat formers and if different secondary habitat former species (i.e., ecologically and morphologically different) facilitate clients differently. Feature of this project was the strong ecological and morphological difference between the primary and the secondary habitat formers (respectively a bivalve and two species of seaweeds). This difference makes the seaweeds relevant in creating a strong habitat cascade, providing a large number of features absent in the primary habitat formers and providing, therefore, additional opportunities of facilitation (such as sheltering within the fronds, moisture retention in the soft tissues for intertidal stress buffering, disproportionate increase of settlement space, etc.). Then, I expected that seaweeds increase enormously the benefits provided by the cockles for the local invertebrate community and, therefore, that habitat cascades are of paramount importance in these habitats.

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The aim of this study was to quantify the variability in habitat cascades supported by two ecologically and morphologically different secondary habitat formers, the sheet-forming green seaweed Ulva sp. and the coarsely branched red seaweed Gracilaria chilensis, through the following hypotheses:

(1) the invertebrates’ abundance, diversity and community structure depend on the biomass and/or the identity of the secondary habitat former, and that Gracilaria typically have higher diversity and abundances because it has a more structural complex form;

(2) the invertebrates have different host-specificity for Ulva and Gracilaria in virtue of their ecological differences, and that herbivorous invertebrates generally prefer the simpler Ulva whereas small species that could be susceptible to high predation (e.g., juveniles and slow moving soft crabs) generally prefer the more complex Gracilaria;

(3) that these effects (see point 1 and 2) are stronger in northern regions and summer months because metabolic processes, feeding and predation rates are higher in warm and cold temperate conditions;

(4) the secondary habitat former morphology (foliose vs branched) and type (living vs mimic) are more ecologically relevant than the primary habitat former type (living vs dead vs mimic) in driving the invertebrates’ assemblage (where ecological relevance are calculated from sum of squares explained in Anova models);

(5) the seaweeds have an important role in reducing predation pressure providing shelter for gastropods and the predation rate is dependent on the biomass and morphology of the seaweeds.

PNZ2 – High level habitat cascades: a comparison in estuarine environments

As complementary to the previous project, here I described a higher-level habitat cascade, again in soft-bottom estuarine communities, testing how a higher order habitat cascade affects client diversity. Here, the primary habitat former was again a bivalve while both the secondary and tertiary habitat formers were seaweeds but different in species and morphologically features (see previous project). To date, there are only two cases of ‘long habitat cascades’ described but I believe that they are common and widespread even if not enough investigated.

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In this study I hypothesized the existence of a 4-level habitat cascade based on the successive interaction between the bivalve Austrovenus stutchburyi, the green seaweed Ulva sp. and the red seaweed Gracilaria chilensis, testing the following hypothesis:

(1) the 4-level habitat cascade is more stable than the corresponding 3-level one previously described (AustrovenusGracilaria-invertebrates), i.e., it supports larger abundance and richness of invertebrates as a result of a more structurally complex interaction;

(2) this condition is consistent across season, with more noticeable effects in summer;

(3) the contribution of Ulva as 3rd habitat former is relevant across latitudes, with stronger effects in northern regions, irrespective of the condition of the second habitat former (here, artificial mimic);

(4) similar effects are reported when the habitat formers are non-living (here, artificial mimic), as a result of the contribution of Ulva’s mimics to the morphological features of the habitat cascade.