Deficits in functional trait diversity following recovery on coral reefs

Abstract

The disturbance regimes of ecosystems are changing, and prospects for continued recovery remain unclear. New assemblages with altered species composition may be deficient in key functional traits. Alternatively, important traits may be sustained by species that replace those in decline (response diversity). Here, we quantify the recovery and response diversity of coral assemblages using case studies of disturbance in three locations. Despite return trajectories of coral cover, the original assemblages with diverse functional attributes failed to recover at each location. Response diversity and the reassembly of trait space was limited, and varied according to biogeographic differences in the attributes of dominant, rapidly recovering species. The deficits in recovering assemblages identified here suggest that the return of coral cover cannot assure the reassembly of reef trait diversity, and that shortening intervals between disturbances can limit recovery among functionally important species.

Keywords: coral reefs; disturbance; ecosystem function; functional traits; resilience; response diversity.

Figure 1.

Hypothetical patterns of resilience to disturbance through time. In each scenario, disturbances drive a loss of total abundance and a shift in taxonomic and functional composition caused by different susceptibilities among taxa. (a) The recovery of all parameters to their original state, indicating full resilience. (b) Taxonomic composition fails to recover, yet depleted taxa are replaced by recovering taxa with similar ecological functions, indicating response diversity. The dotted line indicates an alternate scenario where functions are maintained by taxa that survive the disturbance. (c) Following severe disturbance, declining taxa are replaced by a subset of taxa with depleted functions, indicating a deficit. The dotted line indicates a regime shift into an alternate ecological state, where parameters are persistently depleted. Other possible scenarios (e.g. loss of functions despite taxonomic consistency) are not shown.

Figure 2.

Disturbance–recovery cycles and loss of coral trait diversity. (a) Changes in coral cover on repeatedly surveyed reefs. Coloured lines are the mean trendline for three depths, which are shown individually in grey. Timing of original surveys varies between locations; Jamaica: 1977; Moorea: 1980; Lizard Island: 1995. Numbers indicate (1) pre-disturbance, (2) disturbed, and (3) recovering assemblages. Boxes indicate the timing of major disturbance events (B, bleaching; A, A. planci outbreak; S, storms; D, Diadema die-off). (b) Shifts in abundance-weighted trait diversity (FDis) at three depths between (1) pre-disturbance, (2) disturbed, and (3) recovering assemblages. Coloured lines connect median trait diversity across depths through time. Vertical grey lines show the differences in trait diversity between depths at each time point. (c) The percentage difference in coral cover and trait diversity between pre-disturbance and recovering reefs (assemblages 1 and 3) at three depths. Negative values indicate a deficit. Positive values indicate a gain.

Figure 3.

Shifts in abundance in coral trait space in three locations. (a) Coral trait space showing the positions of 44 taxonomic groups pooled across the three locations. Blue contour lines indicate the presence of distinct clusters of taxa. (b) Centroids of 12 morphological types in trait space: (1) complex-branching, (2) staghorn, (3) columnar, (4) corymbose, (5) digitate, (6) encrusting, (7) upright-encrusting, (8) laminar, (9) massive, (10) solitary, (11) submassive, and (12) tabular. Letters indicate seven vectors used to generate the trait space (table 1). (c) Abundances of taxa in trait space, comparing original, disturbed and recovering assemblages on reef slopes. The sizes of points indicate the abundance of each taxon at each time interval. Lines connect each taxon to the abundance-weighted means of trait space. (d) Changes in abundance in trait space following disturbance and recovery (between time points 1 and 3). The sizes of points indicate the increase (coloured) or decrease (grey) in abundance. Lines connect each taxon to the mean coordinates of winners and losers, weighted by the increase or decrease in abundance, respectively.

Figure 4.

Response diversity driven by differential survival and regeneration among taxa. (a) Response diversity shown by changes in the abundance of taxa after recovery in relation to changes in the abundance of their closest neighbours in trait space. Shifts in neighbour abundance for each taxon are calculated as the summed loss or gain in abundance of the four closest neighbours (scaled between 0 and 1 in each location). Labels show Spearman's rank correlation coefficients. (b) Changes in the abundance of taxa after recovery in relation to susceptibility to the initial disturbance. Change in cover between original and recovering assemblages (time points 1 and 3) is shown on the y-axis. On the x-axis, initial mortality is quantified as the decrease in cover relative to the total assemblage decrease. Labels are included for taxa with large increases in abundance following recovery.