Allee effects limit coral fertilization success

Abstract

Coral populations are being progressively thinned by climate change, which elevates the risk of reproductive failure from Allee effects during fertilization. Studies have shown that fertilization success improves during more intense and synchronized spawning, but the local dependence of fertilization on coral density remains unknown in wild populations. Here, we measure the fertilization success of individual colonies of the common table coral, Acropora hyacinthus in Palau, Micronesia. We find strong evidence of spatial Allee effects such that fertilization averaged 30% when colonies were in close proximity (<0.5 m), but this declined rapidly to less than 10% at a separation of 10 m and virtually zero by 15 to 20 m. The distance of a nearest neighbor conspecific, gravid colony is a better predictor of observing Allee effects than local colony density measured at several alternative scales (2 to 10 m). Spawning synchrony increases when corals are in close proximity, which may reinforce Allee effects as reproductive colonies become further separated. Fertilization success declined nearly threefold on a second night of spawning with higher wind despite there being more intensive spawning of colonies, highlighting the potential importance of prevailing weather in driving the outcome of mass spawning events. Overall, we find that the local population of A. hyacinthus achieved 71% of its fertilization potential. Further studies of spatial patchiness in coral populations are needed to generalize the likelihood of Allee effects, how climate change impacts reproductive function on reefs, and opportunities for restoration to safeguard reproductive success by reconnecting depleted coral populations.

Keywords: coral reef; reproduction; resilience; spawning; threshold.

Fig. 1.

Spawning behavior of A. hyacinthus on the 26th (night 1) and 27th (night 2) of March 2024 showing (A) collection points of floating egg containers after ~1 h of drifting, (B) trends in spawning intensity and (C) use of fluorescein dye to track sperm direction at equivalent tide and wind to night 2. Orange circles mark the release point of tagged colonies. Sperm direction also shown in (A).

Fig. 2.

Key results of study showing the relationships between spawning time and fertilization success on successive nights (A), Allee effects of distance between colonies on fertilization success (B), the effects of colony distance on spawning synchrony (C), and the probability of encountering a damaged gamete in the egg container (D). All CI are 95%. Impacts on colony-level fertilization success on nights 1 (calm) and 2 (windy) revealing the weak positive impact of spawning synchrony on night 1 (A) and strong negative effects of the windy night and distance to the nearest neighboring colony (B). Error bars denote 95% CL.

Fig. 3.

Representation of the metric for the risk of Allee effects on A. hyacinthus at our study reef. Colonies are ordered from that with the closest nearest neighbor to the most isolated. The y-axis represents colony-level fertilization success expressed as a percentage of the maximum achievable fertilization where nearest neighbor distance is zero. Fertilization potential is the proportional area represented by the colored polygon (or equivalently the average of individual colony fertilization success as a percentage of the max).

Fig. 4.

Study design showing Uchelbeluu Reef with the study site marked as a square (A), the positions of individual colonies within the study site (B), collection hood above A. hyacinthus (C) just after the egg bundle container (D) was released.