Analysing the Role of Environmental Stresses on Species Richness and the Process of Hierarchical Structuring of Species Abundances in Marine Gastropods communities at Suva (Fiji Islands)

Abstract

Anthropogenic environmental stresses, especially physio-chemical pollution, are causing steadily increasing threat to many ecosystems, among which coastal marine communities in tropical shallow waters are especially sensitive. In particular, species-rich marine gastropod assemblages are doomed to bear sharp drops in species diversity when exposed to pollutants released offshore. Yet, the details of the process of decline in species diversity remain to be addressed and analysed more deeply. By addressing a series of previously reported inventories of marine gastropod communities along a sharp gradient of pollution along southern coast at Suva (Fiji Archipelago), I first confirm the already recognised trend towards both a severe decrease in species richness and a strong increase of the unevenness in species abundance distribution, as a response to incremental pollution. Yet, the last trend – increased unevenness – reveals being essentially the purely mathematical consequence of the concomitant decline in species richness. In fact, the genuine intensity of the process of hierarchical structuring of species abundances proves remaining virtually unaffected by environmental degradation, contrary to what has been generally thought so far. Also, another unexpected aspect of the decline in species richness with growing pollution is that this decline is far from being primarily restricted to the set of rarest species; in fact, the originally abundant species are also largely implied in this decline.
 Moreover, considering separately the two co-occurring feeding guilds, it is shown that herbivores and carnivores are substantially involved the same in the drop in species diversity; as a result, their relative contributions in the community do not seem markedly contrasted by growing pollution. In another respect, a recently proposed paradigmatic hypothesis is supported, according to which the herbivore-guild has quite less numerous species with more unevenly distributed abundances, as compared to the carnivore-guild. Yet, once again, this increased unevenness is essentially the purely mathematical consequence of the concomitant decline in species richness; the genuine intensity of the process of hierarchical structuring of species abundances does remain substantially unchanged. At last, a comparatively extremely high sensitivity to pollution is highlighted for the emblematic genus Conus, which suffers, here, a dramatic drop in species diversity.

The Full Hierarchical Structuration of Species Abundances Reliably Inferred from the Numerical Extrapolation of Still Partial Samplings: A Case Study with Marine Snail Communities in Mannar Gulf (India)

“…Interestingly, the same tendency is highlighted for the intertidal gastropod fauna of rocky shore in Andaman Islands [19] and for the gastropod fauna associated to coral-reefs in Fiji Islands [31]. In fact, the trend for higher unevenness in the guild of primary consumers might well be general for most marine and terrestrial ecosystems, as argued on both theoretical and empirical basis in [46].…”

“…To get rid of this spurious dependence, deprived from biological significance, it has been shown appropriate to cancel its influence by comparing the actual "S.A.D." to the corresponding "brokenstick" distribution [29], computed for the same species richness S t [19,30,31]. Accordingly, the genuine intensity, "I str ", of the hierarchical structuring process is relevantly defined by standardizing the unevenness of the actual "S.A.D."…”

Comparing the Complete Hierarchical Structuration of Species Abundances in Reef Fish Communities According to Coral Morphology, Using the Numerical Extrapolation of Only Incomplete Inventories

“…To get rid of this mathematical influence of species richness and, thus highlight the genuine intensity of the structuring process, it has been shown appropriate [39] to cancel this mathematical influence by comparing the slope of the actual "S.A.D." to the slope of the corresponding "broken-stick" distribution [40], computed for the same species richness S t [41][42][43]. Accordingly, the genuine intensity, "I str ", of the hierarchical structuring process is relevantly defined by standardising the degree of unevenness U of the "S.A.D."…”

Influence of Coral-reef Complexity on Species Richness and the Hierarchical Structuration of Species Abundances in Reef fish Communities: A Case Study in South-east Brazil

“…Previous studies have highlighted a generally positive influence of coral habitat complexity on the species richness of the associated reef fish communities [7,10,11,8,12,13,14,15,16,9,[60][61][62][63]]. Yet, most of these results suffered from the bias resulting from the (hardly avoidable) incompleteness of the samplings on which they are based [11,8,43,[64][65][66][67]. Thus, as in preceding reports dealing with coral reef-associated communities [11,8,43,[64][65][66][67], numerical extrapolations were implemented to compensate for the lack of exhaustivity of available samplings, thus providing least-biased estimates of the number of unrecorded species and their respective abundances.…”

“…Yet, most of these results suffered from the bias resulting from the (hardly avoidable) incompleteness of the samplings on which they are based [11,8,43,[64][65][66][67]. Thus, as in preceding reports dealing with coral reef-associated communities [11,8,43,[64][65][66][67], numerical extrapolations were implemented to compensate for the lack of exhaustivity of available samplings, thus providing least-biased estimates of the number of unrecorded species and their respective abundances. Thereby, the fullrange of the Species Abundance Distribution is derived, including the set of species that had remained undetected.…”