Habitat degradation increases functional originality in highly diverse coral reef fish assemblages

Abstract

Citation: Brandl, S. J., M. J. Emslie, D. M. Ceccarelli, and Z. T. Richards. 2016. Habitat degradation increases functional originality in highly diverse coral reef fish assemblages. Ecosphere 7(11):e01557. 10.1002/ecs2.1557Abstract. As anthropogenic and natural disturbances intensify, there is mounting concern about the loss of functionally important or unique species. Functional redundancy, or the presence of several different species occupying similar functional niches, can provide insurance against diversity loss, but evidence for this effect is rare. Likewise, the ways in which functional redundancy patterns respond to disturbances are poorly known, impeding a thorough understanding of community-level dynamics post disturbance. Here, we use an extensive reduction of hard coral cover following a tropical cyclone to explore the response of a highly diverse reef fish assemblage to habitat degradation. We demonstrate that despite clear trait valuespecific susceptibility of fishes to the disturbance, five of six functional indices (including functional richness and evenness) showed no relationship with habitat degradation. In contrast, functional originality, which quantifies the average functional uniqueness of species within an assemblage, increased post disturbance, exhibiting a negative, albeit weak, relationship with decreasing coral cover. The increase in functional originality is simultaneously driven by the loss of functionally similar species in susceptible groups (predominantly small planktivorous and omnivorous species that associate with live coral habitat) and the addition of functionally unique species in groups that benefit from the disturbance (large, non-territorial species feeding on algal turfs, detritus, and invertebrates). Our findings suggest that coral reefs with high coral cover can foster fish assemblages with low functional originality (i.e., high functional redundancy), therefore preventing detectible changes in some of the most commonly applied functional indices post disturbance. However, we caution that the limited resolution of trait-based approaches may mask the loss of functionally unique species and that, with an increase in functional originality, post disturbance assemblages may be less suited to adequately maintain certain ecosystem functions in the face of future disturbances. Thus, there is an urgent need for further exploration of the dynamics between disturbances, functional redundancy, and ecosystem functioning.

A test of trophic cascade theory: fish and benthic assemblages across a predator density gradient on coral reefs

“…(Sweatman et al 2011). Declines in acroporid cover, and thus habitat complexity, are associated with the loss of small-bodied coral-associated fishes, such as territorial damselfishes Brandl et al 2016), which may explain the lower density and biomass of territorial grazers in the Swains compared to the Ribbons. Bottom-up effects that shape habitat complexity and populations of small-bodied fishes likewise drive prey availability to large piscivores (Alvarez-Filip et al 2011a), and in Fiji, the loss of habitat complexity and thus prey availability is a more important driver of piscivore assemblages than fishing pressure (Wilson et al 2008).…”

The meaning of the term ‘function’ in ecology: A coral reef perspective

“…carnivore), schooling behaviour (solitary, pairing, schooling), mobility (sedentary, mobile), position in water column (benthic or pelagic) and activity patterns (nocturnal/diurnal). These traits have revealed useful insights from global distributions of trait combinations (Mouillot et al, ; Stuart‐Smith et al, ) to local changes in community composition (Brandl et al, ; Richardson, Graham, Pratchett, Eurich, & Hoey, ).…”

“…Wismer, Tebbett, Streit, & Bellwood, )) but the impact on ecosystem functions remains to be determined. Some functions may be robust to changes in coral biodiversity, or fish biodiversity, with some major functions such as herbivory appearing to operate at high levels even in low‐coral cover locations (Bellwood et al, ; Brandl et al, ). This resilience to change may not be permanent or apply to other functions.…”

Coral reef ecosystem functioning: eight core processes and the role of biodiversity

“…; Brandl et al . ), often with the implicit or explicit assumption that higher functional diversity (ie a larger area of occupied niche space) translates to higher functioning. Similarly, many studies on coral reefs have focused on quantifying functional redundancy (eg Burkepile and Hay ; Brandl et al .…”

“…Similarly, many studies on coral reefs have focused on quantifying functional redundancy (eg Burkepile and Hay ; Brandl et al . ; McWilliam et al . 2018b), often revealing a lack of redundancy and therefore inferring that losses of species may equate to losses of functions, which compromises the resilience of reefs to anthropogenic disturbances (Nash et al .…”