Assessing the potential for demographic restoration and assisted evolution to build climate resilience in coral reefs

Abstract

Interest is growing in developing conservation strategies to restore and maintain coral reef ecosystems in the face of mounting anthropogenic stressors, particularly climate warming and associated mass bleaching events. One such approach is to propagate coral colonies ex situ and transplant them to degraded reef areas to augment habitat for reef-dependent fauna, prevent colonization from spatial competitors, and enhance coral reproductive output. In addition to such "demographic restoration" efforts, manipulating the thermal tolerance of outplanted colonies through assisted relocation, selective breeding, or genetic engineering is being considered for enhancing rates of evolutionary adaptation to warming. Although research into such "assisted evolution" strategies has been growing, their expected performance remains unclear. We evaluated the potential outcomes of demographic restoration and assisted evolution in climate change scenarios using an eco-evolutionary simulation model. We found that supplementing reefs with pre-existing genotypes (demographic restoration) offers little climate resilience benefits unless input levels are large and maintained for centuries. Supplementation with thermally resistant colonies was successful at improving coral cover at lower input levels, but only if maintained for at least a century. Overall, we found that, although demographic restoration and assisted evolution have the potential to improve long-term coral cover, both approaches had a limited impact in preventing severe declines under climate change scenarios. Conversely, with sufficient natural genetic variance and time, corals could readily adapt to warming temperatures, suggesting that restoration approaches focused on building genetic variance may outperform those based solely on introducing heat-tolerant genotypes.

Keywords: assisted evolution; climate change; coral bleaching; coral reef restoration; evolutionary rescue.

FIGURE 1

Schematics of the (a) ring lattice dispersal network without supplementation, the (b) temperature trajectories for each patch across the network during the temperature increase period, and (c) the dispersal network with supplementation to the four hottest sites. Each patch is connected to its four nearest neighbors as well as itself. Warmer and cooler colors are reefs that experience higher and lower temperatures, respectively. The optimal growing temperatures at the nursery reefs (Z _a=s ) were set depending on the degree of thermal trait enhancement.

FIGURE 2

Coral responses to climate change across levels of coral effective fecundity/larval production (β) and genetic variance (V) without supplementation or assisted evolution. Circles in panel (a) show the median average network‐wide proportion of total habitat area occupied by coral at the end of simulated climate change trajectories across levels of genetic variance (V). Error bars represent 80% quantiles, and different colors correspond to values of β. Panels (b–j) show a simulated time series of coral cover across the reef network during climate change trajectories. Solid lines and shaded borders represent the median and 50% quantiles (across simulations), respectively, of coral cover over time at each patch within the network. Red and blue lines represent warmer and cooler patches, respectively. Simulations shown here assume a competition matrix parameterized for alternative stable states (i.e. coral‐ and macroalgal‐dominated regimes).

FIGURE 3

Effects of supplementation and assisted evolution on final (year 500) network‐wide average coral cover across spatial strategies. The y‐axis represents the median average network‐wide coral cover in year 500. Circles and 80% quantile bars are grouped on the x‐axis by annual maximum supplementation level (βC _a=s ), which represents the proportion of target reef patch area that was added to target sites each year for 500 years. For a standard network composed 100 km² reefs, the values of βC _a=s on the x‐axis (0, 0.0000001, 0.000001, 0.00001, 0.0001) correspond to supplementation levels of 0, 0.001, 0.01, 0.1, 1 ha year⁻¹. Light green circles represent restoration scenarios with no assisted evolution (AE) (trait enhancement of 0°C), although dark green circles represent scenarios with assisted evolution (trait enhancement of 3°C). Each panel represents a different spatial design of supplementation efforts, where panel (a) shows a strategy targeting the coldest patches, panel (b) shows a strategy targeting the hottest patches and panel (c) shows a strategy targeting randomly selected patches. Simulations shown here assume a competition matrix parameterized for alternative stable states between corals and macroalgae with $V=0.05$ and β = 0.01.

FIGURE 4

(a–c) Effects of the supplementation and assisted‐evolution efforts on minimum coral cover. Details of this figure are identical to those of Figure 3 except that plotted values represent the minimum proportional amount of coral cover that occurred during the 500 year simulations across different spatial designs and varying levels of supplementation and assisted evolution. Please refer to Appendix S1: Figures S5–S10 for the full trajectories

FIGURE 5

Effects of the duration of supplementation and assisted‐evolution efforts on final average coral cover. Details of this figure are identical to those of Figure 3 except that each panel represents a different duration for which supplementation was sustained ranging from 50 years (panel a) up to 100 years (panel b) and 200 years (panel c). Results shown here are from a supplementation strategy targeting hot reefs.