Volatility in coral cover erodes niche structure, but not diversity, in reef fish assemblages

Abstract

The world's coral reefs are experiencing increasing volatility in coral cover, largely because of anthropogenic environmental change, highlighting the need to understand how such volatility will influence the structure and dynamics of reef assemblages. These changes may influence not only richness or evenness but also the temporal stability of species' relative abundances (temporal beta-diversity). Here, we analyzed reef fish assemblage time series from the Great Barrier Reef to show that, overall, 75% of the variance in abundance among species was attributable to persistent differences in species' long-term mean abundances. However, the relative importance of stochastic fluctuations in abundance was higher on reefs that experienced greater volatility in coral cover, whereas it did not vary with drivers of alpha-diversity. These findings imply that increased coral cover volatility decreases temporal stability in relative abundances of fishes, a transformation that is not detectable from static measures of biodiversity.

Fig. 1.. Conceptual figure illustrating VPRSA.

(Top) Color-coded lines represent the log abundances of 80 species in two hypothetical communities over time. Both communities have the same total variance in log abundance. In the “niche-structured” community on the left, species differences in long-term average abundances explain most of the variance in log abundance in the community at any one time (i.e., high community determinism). By contrast, in the “stochastic” community on the right, long-term average abundances of the 80 species are very similar to each other such that the variance in abundance at any one time is driven instead by differences in species’ temporal patterns of stochastic fluctuations. The middle panel illustrates sampled species abundance distributions from these communities, which are obtained for each year of the time series. (Bottom) Similarity between samples is modeled as a function of time lag between each pair of sampled abundance distributions. The asymptotic similarity is equal to the proportion of variance in species’ log abundances that is due to difference in species’ long-term mean or equilibrium abundances (red); the distance from this asymptote to the intercept is the proportion due to stochastic fluctuations in relative abundance (blue), and the remainder is the proportion due to overdispersion (green). See Materials and Methods for details of the approach and the theoretical community dynamics model from which it was derived.

Fig. 2.. The map showing coral reefs included in all analyses, along with frequency distributions of explanatory and response variables across the Great Barrier Reef.

On the map, red circles show locations of the n = 40 reefs used in this study. (A and B) Explanatory variables related to coral cover fluctuations. Frequency distribution of the temporal mean and SD in living hard coral cover across reefs. (C and D) Static metrics of reef fish assemblage structure as response variables. Frequency distribution of (time-averaged) species richness and unevenness of reef fish assemblages across reefs. (E) Deterministic and stochastic variance components in relative species abundances of reef fishes as response variables. Frequency distribution of the variance in relative species abundances attributable to persistent species differences (red bars; n = 40), to stochastic fluctuations in species’ growth rates (blue bars; n = 40), and to overdispersion effects such as demographic and sampling variance (gray bars; n = 40). The solid line indicates the mean value (across reefs) of the variance component due to persistent species differences, and the dashed line separates the remaining variance explained by stochastic fluctuations (the larger portion) and overdispersion (the smaller portion).

Fig. 3.. Structural equation model for persistent species difference variance component, species richness, and unevenness of reef fish assemblages.

Solid arrows represent standardized effect estimates, where thick and thin lines represent significance levels of P < 0.01 and P < 0.05. Dashed arrows represent estimated effects that are not statistically significant. Symbols, plus and minus, indicate the effect direction. Tests of directed separation found no evidence of missing pathways. Model estimates are summarized in table S1.

Fig. 4.. Persistent species differences and stochastic fluctuation variance components in relative species abundance of reef fishes depend on coral cover volatility and average cover but not on environmental gradients.

(A and B) Relationships between the coral cover variables (log-SD and mean of coral cover) and the relative importance of variance components that underlie patterns of fish species commonness and rarity across reefs. The relationships are plotted using parameter estimates for the lowest-AIC models, with interactive effects of the coral cover volatility and mean coral cover as explanatory variables and variance components of fish assemblage as response variables (n = 40 reefs; table S2). To illustrate the interactive relationships, coral cover volatility (log-SD of coral cover) is plotted on the horizontal axis, with points color-coded according to levels of mean coral cover on the corresponding reef. The light-, medium-, and dark-colored lines represent the estimated relationship between the variance component and coral cover volatility for the first (21% cover), median (27% cover), and third (41% cover) quartiles of long-term average coral cover, respectively.

Fig. 5.. Alpha-diversity (species richness and unevenness) depend on environmental gradients but not on coral cover variables.

(A and D) Relationships between fish species richness or unevenness and the interaction of latitude with cross-shelf position. To illustrate the interactive relationships, latitude is plotted on the horizontal axis, with points color-coded according to the cross-shelf position of the corresponding reef. The light-, medium-, and dark-colored lines represent the estimated relationship between the variance component and the first, median, and third quartiles of cross-shelf position, respectively (darker, closer to shore). Fish species richness and unevenness do not depend on mean coral cover (B and E) nor coral cover volatility (C and F), whether in univariate relationships as plotted or in multiple regression (table S2).