In line with expectations, we observed distinct species compositions between locations. BRUVS observations yielded 29 fish species and one mammal (family Delphinidae), with 18 species present in Malpelo and 19 species present in Cape Verde (Fig. 2A). Species ubiquitous to both sites were wahoo (Acanthocybium solandri), silky shark (Carcharhinus falciformis), mackerel scad (Decapterus sp.), amberjack (Seriola sp), and juveniles of family Carangidae. Cape Verde hosted fewer species (5.22 ± 2.11 SD per string) than Malpelo (6.25 ± 2.19 SD), albeit not significantly (PERMANOVA F1,15 = 0.3011, p-value = 0.773). Abundance was higher in Cape Verde (873 individuals total, mean 97 ± 128.9 SD per string), than in Malpelo (465 individuals total, mean 58.1 ± 59.9 SD per string), whereas biomass in Malpelo (7,838 kg total, mean 979.8 ± 506.3 SD per string) far exceeded the biomass observed in Cape Verde (3,558 kg total, mean 395.44 ± 563.1 SD per string, Fig. 2B). An ANOSIM test revealed significantly dissimilar clusters between locations (R = 0.5, p = 0.001, Fig. 3A and B). Fifteen species were responsible for 95% of the dissimilarity between sites (Table S3, SIMPER). Mackerel scad (Decapterus sp.), rainbow runner (Elagatis bipinnulata), yellowfin tuna (Thunnus albacares), and little tunny (Euthynnus alletteratus) cumulatively contributed 55% of the difference in abundance between Cape Verde and Malpelo, while rainbow runner, yellowfin tuna, scalloped hammerhead (Sphyrna lewini), and cornetfish (Fistularia sp.) contributed significantly too (p < 0.05).
In contrast with expectations, communities from the two locations filled the same amount of the functional space (functional richness Malpelo: 0.542, Cape Verde: 0.452), and species abundance were similarly distributed in the filled space (functional divergence: Malpelo: 0.833, Cape Verde 0.913) in both locations. While the volume of the overall space occupied by each location was relatively high, the majority of the species were densely distributed in one area of the space, displaying low functional evenness (0.425 for Cape Verde, and 0.346 for Malpelo). The first two dimensions showed most species clustered on the right-hand side (Fig. 3C), defined by mobility, whereas the outliers on the left side of the space had a more restricted range (Fig. S2A). Highly mobile species, clustered into two groups, one characterised by purely pelagic species of very small, and medium size, and with a planktivorous or omnivorous diet and one by large or very large, benthopelagic species living solitary or in small groups, and with either a piscivorous or invertivorous diet. The third and fourth dimensions of the functional space showed some of the same stratification observed on the first two dimensions. However, the species which diverged from the main group were planktivores and invertivores which fed on mobile prey (Fig. 3D), with loose grouping according to different daily activity patterns (Fig. S2B).
Functional divergence and functional evenness are indices of the variance and regularity of species’ distribution within the functional space, respectively, weighted by abundance (Villéger et al. 2008). A highly divergent and minimally even community is one in which some functional roles are much better represented and insured than others, which leaves points of exposure to disturbance, particularly when – as was the case here - dominant and common species are sensitive (McLean et al. 2019). Two of the most abundant species observed at either location are currently on the IUCN Red List Red, namely yellowfin tuna (Near Threatened) and Atlantic horse mackerel (Vulnerable; IUCN 2020). Furthermore, ubiquitous top predators like the silky shark and scalloped hammerhead, essential to ecosystem functioning, are Red Listed as Vulnerable and Critically Endangered, respectively (IUCN 2020).
Overall functional beta-diversity between the locations was 0.6, to which species turnover contributed 90.3%. The remaining proportion arose from distinct trait combinations, often between confamilials like the triggerfishes (mobile in Malpelo, and reef-associated in Cape Verde), surgeonfishes (large in Malpelo and small in Cape Verde) and jacks (large and piscivorous in Malpelo, and medium and invertivore in Cape Verde). Fishing remains the primary threat to pelagic elasmobranchs and teleosts (Pacoureau et al 2021). Although the bulk of the functional space was similar between locations, the presence of such unique trait combinations suggests that some nuanced differences in sensitivity may still render certain locations more or less resilient (Villéger et al. 2013). Future research should aim to identify pelagic systems which overperform compared to expectations, in order to identify unique resilient traits associated with either positive or negative ecosystem outcomes (Cinner et al., 2016).
Our survey, although limited in spatio-temporal scale, included different biogeographical provinces and covers contrasting ends of environmental and human pressure gradients. It presents evidence that pelagic ecosystems may share a common ‘backbone’ of functional traits related to mobility and predatory diet. Such a backbone of 21 common traits has already been documented to exist within global reef ecosystems (McLean et al. 2021). Since ecological disturbance is likely to affect species with identical functional traits in similar ways (Mouillot et al. 2013), low trait diversity within pelagic ecosystems may make the pelagic faunas particularly vulnerable to disturbance. We propose that further study specifically aims to determine whether this is a universal feature of the pelagic realm, and the degree to which it may affect the resilience of mid-water communities.