Transient amplification enhances the persistence of tropicalising coral assemblages in marginal high‐latitude environments

Abstract

Predicting the viability of species exposed to increasing climatic stress requires an appreciation for the mechanisms underpinning the success or failure of marginal populations. Rather than traditional metrics of long-term population performance, here we illustrate that short-term (i.e. transient) demographic characteristics, including measures of resistance, recovery and compensation, are fundamental in the poleward range expansion of hard corals, facilitating the establishment of coral populations at higher latitudes. Through the annual census of subtropical and temperate Acropora spp. colonies in Japan between 2017 and 2019, we show how enhanced transient amplification (i.e. short-term increases in population growth following disturbance) supports the persistence of coral assemblages within more variable high-latitude environments. The transient dynamics of both the subtropical and temperate assemblages were strongly influenced by their corresponding recruitment patterns. However, we demonstrate that variation in colony survival and fragmentation patterns between the two assemblages determines their relative capacities for transient amplification. This latitudinal variation in the transient dynamics of Acropora spp. assemblages emphasizes that coral populations can possess the demographic plasticity necessary for exploiting more variable, marginal conditions.

Recent exposure to environmental stochasticity does not determine the demographic resilience of natural populations

“…Resilience remedying data biases was beyond the scope of this study, we must also acknowledge geographic and taxonomic biases implicit within our sources of demographic data (see Paniw et al, 2021;Römer et al, 2021;Salguero-Gómez et al, 2021; Figure 1a). Environmental stochasticity plays a crucial role in regulating species range boundaries (Benning et al, 2022;Feldman et al, 2015), and indeed, the transient dynamics of coral assemblages in southern Japan and eastern Australia mediate their capacity for persisting outside of their core ranges (Cant et al, 2021;Cant, Cook, et al, 2022). Yet coral populations represent a taxonomic group for which there is not yet extensive demographic data available (Edmunds & Riegl, 2020).…”

“…Following these disturbance events, characteristics that inhibit transient declines in population growth, or those promoting short-term increases, are likely more beneficial than those maximizing long-term growth (Cant, Cook, et al, 2022;McDonald et al, 2016). Logically, therefore, one would expect increased environmental stochasticity to select for populations with increased resistance (i.e.…”

High-latitude marginal reefs support fewer but bigger corals than their tropical counterparts

“…Despite some uncertainty in compositional functional regression results, it is plausible that the high coral cover in Australian high-latitude coral communities (Harriott et al ., 1994) is created by few large coral colonies. The lower growth rates and higher fission rates of larger corals (Dornelas et al ., 2017) could be the main driver of coral persistence in marginal reefs (Cant et al ., 2022). As ongoing climate change leads to more variable and extreme environmental conditions (Spady et al ., 2022), we hypothesise that tropical accreting reefs will increasingly exhibit a population size structure that resembles those now observed at marginal reefs; namely, one with a higher proportion of larger corals.…”

“…The copyright holder for this preprint this version posted October 21, 2022. ; https://doi.org/10.1101/2022.10.20.513025 doi: bioRxiv preprint classically favoured by coral reef ecologists, and 2) a novel compositional functional regression approach (Talská et al, 2018) that has not been used in this context despite its potential. At higher latitudes, where conditions are harsh due to extremes in temperature, light levels and storm events, we expect fewer small coral colonies, because coral mortality rates are generally highest for the smallest corals (Connell, 1973), and sexual recruitment rates are low (Harriott & Banks, 1995;Abrego et al, 2021;Cant et al, 2022). Potential differences in population size structure of corals along this environmental gradient might indicate the effect of stress on coral population dynamics, providing a lens to the future, where reefs might be affected by increased disturbances as a result of climate change.…”

Coral assemblages at higher latitudes favor short‐term potential over long‐term performance

“…Exploring the performance of populations exposed to environmental stochasticity requires a consideration of their transient (i.e., short‐term) dynamics (Cant, Cook, et al, 2022; Ezard et al, 2010; Hastings, 2004; Hastings et al, 2018). Asymptotic (i.e., long‐term) population growth rate (λ), which describes temporal changes in population size at stationary equilibrium (Caswell, 2001), is the predominant metric used to quantify population performance (Caswell, 2001; Crone et al, 2011).…”