Traits without boundaries: The environmental influence on biological traits across an intertidal region

Abstract

Trait-based methods can present better alternatives to species-centric methods when the objective is to understand the drivers of species distribution patterns across macro-spatial scales. We employed a trait-based approach to understand benthic macroinvertebrate species distributions along the coastline of South Africa, hypothesising that the operation of selection on species' traits would produce different distributional results from the already known patterns based on taxonomic identity. We selected eight lifestyle and reproduction trait categories for 150 intertidal macroinvertebrates from 52 sites spanning three main bioregions (East, South, and West) and two transition zones (South-west and South-east) across c. 3025 km of the South African coast. We used satellite derived near-shore sea surface temperature (SST), chlorophyll-a data estimates and site location to characterise environmental conditions that may influence intertidal species and traits distribution. An RLQ analysis indicated that intertidal species trait distribution did not conform to the biogeographic patterns identified using a taxonomic approach. Further, a combined fourth corner analysis further revealed strong significant interactions of four of the eight traits (reproductive type, body form and feeding type) with environment parameters (longitude, SST and chl-a), plus weakly significant effects of latitude. Two major currents dominate nearshore waters, the cool, eutrophic Benguela on the west and the warm, oligotrophic Agulhas on the east and south, yet trait distribution grouped cool (west coast) and warm (south coast) temperate sites separately from subtropical eastern sites rather than aligning with the three main bioregions defined by species taxonomic identity. These results support the habitat templet theory, suggesting that environmental influences on species distributions operates through their effects on species traits, with chl-a/nutrient availability a key determinant of trait distributions. This results in a transition in species' traits from warm, oligotrophic to cool eutrophic conditions. This coarse transition between two oceanic regimes is further refined by species with varying trait combinations colonising from either the Indian and Atlantic oceans, resulting in the bioregions defined by species identity.