Small invertebrate consumers produce consistent size spectra across reef habitats and climatic zones

Abstract

Changes in invertebrate body size-distributions that follow loss of habitat-forming species can potentially affect a range of ecological processes, including predation and competition. In the marine environment, small crustaceans and other mobile invertebrates ('epifauna') represent a basal component in reef food webs, with a pivotal secondary production role that is strongly influenced by their body size-distribution. Ongoing degradation of reef habitats that affect invertebrate size-distributions, particularly transformation of coral and kelp habitat to algal turf, may thus fundamentally affect secondary production. Here we explored variation in size spectra of shallow epifaunal assemblages (i.e. the slope and intercept of the linear relationship between log abundance and body size at the assemblage level) across 21 reef microhabitats distributed along an extensive eastern Australian climatic gradient from the tropical northern Great Barrier Reef to cool temperate Tasmania. When aggregated across microhabitats at the site scale, invertebrate body size spectra (0.125-8 mm range) were consistently log-linear (R 2 ranging 0.87-0.98). Size spectra differed between, but not within, major groups of microhabitats, and exhibited little variability between tropical and temperate biomes. Nevertheless, size spectra showed significant tropical/temperate differences in slopes for epifauna sampled on macroalgal habitats, and in elevation for soft coral and sponge habitats. Our results reveal epifaunal size spectra to be a highly predictable macro-ecological feature. Given that variation in epifaunal size spectra among groups of microhabitats was greater than variation between tropical and temperate biomes, we postulate that ocean warming will not greatly alter epifaunal size spectra directly. However, transformation of tropical coral and temperate macroalgal habitats to algal turfs due to warming will alter reef food web dynamics through redistribution of the size of prey available to fishes.

Production of mobile invertebrate communities on shallow reefs from temperate to tropical seas

“…the 'Hutchinsonian' niche), harkening back to seminal contributions on the organization of sessile organisms in rocky intertidal ecology [71,72]. Habitat resources, additional to food resources, appear responsible for large-scale patterns in epifaunal community structure [61,73]. This seems also to be the case in the current study with regards to their production, echoing a recent finding in communities of freshwater stream invertebrates in North America [14].…”

“…fish) to penetrate the habitat in order to extract epifaunal prey [93]. Hence, branching coral can provide refugia for larger epifaunal invertebrates that may be less susceptible to consumption by coral polyps [39,73].…”

Reef communities show predictable undulations in linear abundance size spectra from copepods to sharks

“…Firstly, the dominant energy pathways may vary with latitude, whereby a higher mean PPMR in temperate reefs leads to less energy lost through trophic inefficiencies and therefore a peak in abundance at larger body sizes. As described above, habitat composition also varies latitudinally and has been shown to play an important role in latitudinal variation in the body sizes of the smallest invertebrates studied here (Fraser et al, 2021;Yamanaka et al, 2012).…”

“…Data collection was performed using two distinct methods: (1) Collection of benthic habitat samples with associated invertebrate epifauna and (2) underwater visual surveys of fishes and large mobile macro‐invertebrates. Together, these datasets allowed construction of size spectra from small meiofaunal invertebrates (predominantly harpacticoid copepods, Fraser et al, (2021)) to the largest fishes including sharks (Edgar et al, 2014).…”

“…Together, these datasets allowed construction of size spectra from small meiofaunal invertebrates (predominantly harpacticoid copepods, Fraser et al, (2021)) to the largest fishes including sharks . Epifauna were sampled from 45 sites spanning the eastern coast of Australia, from tropical (Lizard Island; 14.7°S) to temperate (Southern Tasmania;43.3°S) reefs, between the years of 2015 and 2018.…”