Why species matter: an experimental assessment of assumptions and predictive ability of two functional‐group models

Abstract

Abstract. Community ecologists use functional groups based on the rarely tested assumption that within-group responses to ecological processes are similar and thus members are functionally equivalent. However, recent research suggests that functional equivalency may break down with human impacts. We tested the equivalency assumption and model predictions of responses to simulated human alterations in nutrients and large herbivores for two models of coral reef algae, the Relative Dominance Model (RDM) and the Functional Group Model (FGM). Results of both mesocosm and field experiments using assembled communities were compared to model predictions, and within-and between-group variability were assessed. Both models' predictions of group response to herbivory matched experimental outcomes, but only the RDM predicted response to nutrients. However, within-group variability was dramatic, because the RDM grouped species with opposite responses to herbivory and the FGM grouped species with unique responses to nutrients. These heterogeneous responses resulted in loss of information and masked strong interactions between herbivory and nutrients that were not included in the models. As humans continue to impact major ecological processes in ecosystems globally, we postulate that functional-group models may need to be reformulated to account for shifting baselines.

Sediments influence accumulation of two macroalgal species through novel but differing interactions with nutrients and herbivory

“…Galaxaura is a calcified branching red alga that is unpalatable to herbivores (Mantyka and Bellwood 2007) and rapidly becomes nutrient limited (Clausing and Fong 2016), likely because of a low internal nutrient storage capacity (Aisha et al 1995). Padina, in contrast, is a lightly calcified foliose brown alga that is readily consumed by herbivorous fish (Mantyka and Bellwood 2007;Fong and Fong 2014) and responds more variably to water nutrient inputs (Clausing and Fong 2016). In Mo'orea, sediment adheres to fine hairs on the thallus surface in both of these species on the reef flat ( Fig.…”

An experimental assessment of herbivory and nutrient effects on a small-scale in a coral reef macroalgal community

“…Fast growing macroalgae are heavily implicated in shifts from coral to algal dominance (Hughes, 1994). Given the rapid response of some macroalgae (this study, Fong et al, 1998;Fong and Fong, 2014), it is important to understand temporal and spatial variation in the processes regulating these groups.…”

“…I assembled communities of the 4 most common species of macroalgae on this reef, putatively Padina boryana (Thivy), Halimeda opuntia (L. J.V.Lamouroux), Galaxaura divaricate (synonym to G. fasciculata) (L. Huisman & R.A.Townsend), and Dictyota bartayresiana (J.V.Lamouroux). These four species have been successfully used in artificial assemblages in mesocosms to determine effects of nutrient additions (Fong and Fong, 2014). I collected macroalgae from the same location in the field, sorted them by species, and cleaned algal thalli of all sediment and epiphytes.…”

Seasonal recruitment and survival strategies of Palisada cervicornis comb. nov. (Ceramiales, Rhodophyta) in coral reefs

“…Although species‐level identification of algae is difficult and often requires specialized methods that go beyond morphological characterization, proper identification of species provides important information about many characteristics of organisms that clarify their role in the ecosystem (Knowlton and Jackson , De Clerck et al. , Fong and Fong ).…”