To give insights into the mechanisms driving the structuration of the community, captured by the relative abundance distributions of species, the Niche Apportionment Models (NAM) were used.

The theoretical framework regarding the NAMs has been developed in early 90 s by Tokeshi44 who built on Sugihara’s niche-hierarchy models (1980), to support in the practical problems of fitting stochastic models to real data for the interpretation of species abundance patterns. The developed framework gives logical coherence to a range of niche apportionment models that can be categorized through the sequential breakage process of total niche. To describe how species break up the resource pool, determining the distribution of abundance of different species44,45, the theory proposed by Tokeshi 4547 takes into consideration six different models which are (sorted by decreasing evenness): Dominance Preemption (DP), Random Assortment (RA), Random Fraction (RF), Power Fraction (PF), MacArthur Fraction (MF) and Dominance Decay (DD); the abundance of a species is assumed to directly correspond to the amount of niche/resource apportioned to that species.

In these models of sequential breakage of the total niche, the dominance pattern depends on the probability p associated to each niche fraction. The resulting Species Abundance Distribution (SAD) vary from uneven patterns like the geometric series to extremely even ones similar to the broken-stick model48.

The power of this analysis ultimately relies on the robustness of the Species Abundance Distribution within a community in catching (in the shape of the SAD itself) the underlying mechanisms that drive the relations (coexistence, competition, etc.) among the species and therefore the structuration of the community. So that fitting one of the proposed stochastic models to the SAD (i.e. to find the model that fits better than others to the survey data, and this can be done with classical statistical methods) means that the probability of that specific sequential niche breakage (caught by the modelled scheme) is higher than others.

In order to highlight the mechanisms regulating the resource allocation within a community of related species, several authors suggest applying the NAMs only on that specific group of taxonomically related species or trophic guild4547. As copepods represent about 80% of the total zooplankton community in the VL38, we applied the NAMs to this dominant planktonic group in order to elucidate the mechanisms regulating the resource allocation in different areas and to understand if those mechanisms could explain the different distribution of O. davisae and O. nana. This is the first attempt to apply this approach to copepods communities. In fact, to date, these models have been used to explain and describe changes in relative abundance distributions of terrestrial and aquatic communities of both fresh and marine waters, like larval chironomid, stream fish assemblages, salt meadow vegetation, dragonfly community, terrestrial arthropods, marine macroalgae and phytoplankton community44,46,4951. The analyses have been performed using the R-package ‘nicheApport’ (available in GitHub, 2017).

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