Functional biogeography of coastal marine invertebrates along the south-eastern Pacific coast

Characterizing the spatial structure of taxonomic and functional diversity (FD) of marine organisms across regional and latitudinal scales is essential for improving our understanding of the processes driving species richness and those that may constrain or enhance the set of species traits that define the functional structure of communities. Here, we present the functional diversity of coastal invertebrate macrofaunal species along the south-eastern Pacific, from 7°N to 56°S, we describe spatial variation of species traits, and examine the relationship with environmental variables. We define the functional traits and the distribution range of 2350 marine macroinvertebrates to calculate eight metrics of FD. Random forest regression was applied to identify significant relationships between FD and six environmental variables. Finally, functional ß-turnover was estimated to detect alongshore shifts in functional structure and their coincidence with biogeographical domains. In contrast with taxonomic richness, measures of trait differences, functional space and functional specialisation increase with latitude, while functional evenness exhibits a humpback shape, peaking at mid-latitudes. Functional redundancy decreases significantly poleward, while indications of vulnerability increase. In contrast to taxonomic richness, FD was tightly connected to variables indicative of stress and productivity, such as dissolved oxygen and nutrients. Sea surface temperature and coastal area best explained the increased FD redundancy towards the tropics. The high spatial correlation between taxonomic and functional ß-turnover suggests environmental filters play an important role in the functional structure of the seascape. Our findings suggest that processes favouring taxonomic richness are latitudinally divergent from those favouring functional diversity. Correlations with environmental variables suggest that increased sea surface temperature and measures of stability increase redundancy, while variation in dissolved oxygen and nutrients positively affect functional diversification. Moreover, the functional diversity patterns suggest low resilience of high-latitude coastal ecosystems, which are heavily exploited and threatened by climate change, hence highlighting the urgent need for effective conservation policies.

Methods
Based on the geographical ranges of 2350 species of marine macroinvertebrates belonging to seven phyla found in coastal shallow ecosystems, we built a presence-absence matrix with one latitudinal degree resolution. The dataset was compiled using records obtained from specialized literature, scientific expeditions (SARCE 2013, CONAMA 2008), data platforms (Ocean Biogeographic Information System, OBIS, and Global Biodiversity Information Facility, GBIF) and museum collections. Species known to occur only in waters deeper than 30m were not included. Further taxonomic detail can be found in Appendix 2. Given that the only source of variation is the presence or absence of species by latitudinal band, we grouped the species in functional entities (FEs) before the calculation of the FD indices, implementing the species number per FE as a proxy of abundance. Each species was assigned to one of 247 FEs based on 31 ecological traits nested into eight ecological attributes (Table S2). Trait selection was based on the potential to reflect ecological processes, for instance, body size and morphology, which have been related to vulnerability to disturbance and ecological roles, and that could readily and unequivocally be assigned to species for which there are no ecological studies. Trait compilation was based on information from online repositories, scientific journals and specialist texts. The chosen traits maintain the balance between redundancy and uniqueness in order to diminish under- or over-estimation of functional attributes. To capture variation and trait affinity of a given taxon, a fuzzy coding technique was implemented to codify in categorical terms each species (or assign to one trait combination) depending on the presence and level of the trait (see Table S2). The relative abundances of FE were calculated as the number of species in a given trait combination, divided by the total richness recorded by latitudinal bin. Finally, a FE × latitude matrix was generated to quantify the functional changes along the SEPC.