Indirect legacy effects of an extreme climatic event on a marine megafaunal community

Abstract

While extreme climatic events (ECEs) are predicted to become more frequent, reliably predicting their impacts on consumers remains challenging, particularly for large consumers in marine environments. Many studies that do evaluate ECE effects focus primarily on direct effects, though indirect effects can be equally or more important. Here, we investigate the indirect impacts of the 2011 “Ningaloo Niño” marine heatwave ECE on a diverse megafaunal community in Shark Bay, Western Australia. We use an 18‐year community‐level data set before (1998–2010) and after (2012–2015) the heatwave to assess the effects of seagrass loss on the abundance of seven consumer groups: sharks, sea snakes (multiple species), Indo‐pacific bottlenose dolphins (Tursiops aduncus), dugongs (Dugong dugon), green turtles (Chelonia mydas), loggerhead turtles (Caretta caretta), and Pied Cormorants (Phalacrocorax spp.). We then assess whether seagrass loss influences patterns of habitat use by the latter five groups, which are under risk of shark predation. Sharks catch rates were dominated by the generalist tiger shark (Galeocerdo cuvier) and changed little, resulting in constant apex predator density despite heavy seagrass degradation. Abundances of most other consumers declined markedly as food and refuge resources vanished, with the exception of generalist loggerhead turtles. Several consumer groups significantly modified their habitat use patterns in response to the die‐off, but only bottlenose dolphins did so in a manner suggestive of a change in risk‐taking behavior. We show that ECEs can have strong indirect effects on megafauna populations and habitat use patterns in the marine environment, even when direct effects are minimal. Our results also show that indirect impacts are not uniform across taxa or trophic levels and suggest that generalist marine consumers are less susceptible to indirect effects of ECEs than specialists. Such non‐uniform changes in populations and habitat use patterns have implications for community dynamics, such as the relative strength of direct predation and predation risk. Attempts to predict ecological impacts of ECEs should recognize that direct and indirect effects often operate through different pathways and that taxa can be strongly impacted by one even if resilient to the other.

A Systematic Review of How Multiple Stressors From an Extreme Event Drove Ecosystem-Wide Loss of Resilience in an Iconic Seagrass Community

“…The "Ningaloo Niño" MHW in 2011 pushed the temperate persistent meadow-forming seagrass species A. antarctica and P. australis past their capacity to resist high temperatures in Shark Bay, Western Australia. This drove a change in state where extensive leaf defoliation in A. antarctica (Fraser et al, 2014) and subsequent death of shoots and whole meadows resulted in bed erosion, sediment resuspension and movement (Thomson et al, 2015;Nowicki et al, 2017), and major losses to seagrass-dependant biota (Caputi et al, 2016;D'Anastasi et al, 2016;Nowicki et al, 2019) (Figures 6, 7). The breakdown in resistance is among the largest observed in Australia (Figure 7).…”

“…Effects of seagrass loss were assessed on all major species of airbreathing megafauna that occur in Shark Bay via visual transect surveys at the surface (Nowicki et al, 2019). These surveys, running continuously since 1998, have been part of a wider community research project on the seagrass banks immediately north of Monkey Mia (see Heithaus et al, 2012 for descriptions).…”

“…In addition to air-breathing fauna, Nowicki et al (2019) also quantified changes to the large shark community via standardized drumline fishing. ∼4 days per month (mostly between Feb-Oct), 10 drumlines baited with ∼1.5 kg of fish each were set at dawn.…”

“…Fast local scale (individual to population, weeks to months) responses of temperate seagrass to the MHW included FIGURE 6 | Generalized timeline of change in seagrass associated biota before to after the 2011 heatwave. Red, population decline; Yellow, other change to population; Green, no decline in population; Gray, no data; "X", fishery closure (see Nowicki et al, 2019 for details).…”

“…The slower 1-2 year system-wide seagrass loss after the 2011 MHW demonstrate how indirect, biotic legacies of MHWs can be significant, even for organisms that are resistant to the direct abiotic effects (Figure 7) (Nowicki et al, 2019). This multi-faceted nature of resistance needs to be considered in the context of ECEs, including MHWs.…”