Environmental gradients influence biogeographic patterns of nonconsumptive predator effects on oysters

Abstract

When prey alter behavioral or morphological traits to reduce predation risk, they often incur fitness costs through reduced growth and reproduction as well as increased mortality that are known as nonconsumptive effects (NCEs). Environmental context and trophic structure can individually alter the strength of NCEs, yet the interactive influence of these contexts in natural settings is less understood. At six sites across 1000 km of the Southeastern Atlantic Bight (SAB), we constructed oyster reefs with one, two, or three trophic levels and evaluated the traits of focal juvenile oysters exposed to predation risk cues. We monitored environmental variables (water flow velocity, microalgal resources, and oyster larval recruitment) that may have altered how oysters respond to risk, and we also assessed the cost of trait changes to oyster mortality and growth when they were protected from direct predatory loss. Regardless of trophic structure, we found that oyster shell strength and natural oyster recruitment peaked at the center of the region. This high recruitment negated the potential for NCEs by smothering and killing the focal oysters. Also independent of trophic structure, focal oysters grew the most at the northernmost site. In contrast to, and perhaps because of, these strong environmental effects, the oyster traits of condition index and larval recruitment were only suppressed by the trophic treatment with a full complement of risk cues from intermediate and top predators at just the southernmost site. But at this same site, statistically significant NCEs on oyster growth and mortality were not detected. More strikingly, our study demonstrated environmental gradients that differentially influence oysters throughout the SAB. In particular, the results of our trophic manipulation experiment across these gradients suggest that in the absence of predation, environmental differences among sites may overwhelm the influence of NCEs on prey traits and population dynamics.

Variation in the density and body size of a threatened foundation species across multiple spatial scales

“…Our results support previous studies showing that oyster characteristics can differ among estuaries (Powers et al 2009; McAfee et al 2016; Kimbro et al 2020). The larger oyster sizes and densities observed in Crookhaven River and Port Hacking could be linked to better water quality and greater wave action at these sites, which can increase recruitment, food supply, and filtration rates, driving greater survival and growth (Dove & Sammut 2007; Diggles 2013; Theuerkauf et al 2017; Vozzo et al 2020).…”

Oyster abundance on subtidal reefs depends on predation, location, and experimental duration

“…They report that either mud crabs (Grabowski et al, 2008; Hill & Weissburg, 2013; Johnson et al, 2014; Rindone & Eggleston, 2011) or blue crabs (Carroll et al, 2015; O'Connor et al, 2008) reduced oyster abundance. Additionally, on intertidal oyster reefs, mud crabs occur in high abundances across broad geographic ranges (Grabowski et al, 2020) and can have significant consumptive effects on juvenile oysters (Kimbro et al, 2020). Our multisite, subtidal field experiment suggests that mud crabs were the only predator to have a measurable top‐down effect on oyster abundance, although differences were only significant at one of the three oyster sanctuaries.…”

“…We utilized a combination of mensurative surveys and field experiments to assess whether the abundance of eastern oysters (Crassostrea virginica) was affected by manipulating access of different predators within a natural subtidal marine food web at three different locations within an estuary. Although numerous studies have quantified the effect of multiple predators on intertidal oyster reefs across a range of environmental conditions (e.g., Grabowski et al, 2008Grabowski et al, , 2020Johnson et al, 2014;Kimbro et al, 2020), predation of oysters can vary significantly between intertidal and subtidal locations (Johnson et al, 2014). Specifically, we monitored oyster abundance over 16 months within different cage designs to exclude predators based on their size and morphology, and surveyed predator abundance at all locations (Figure 1b).…”

Nonconsumptive Predator Effects on Prey Demography: Recent Advances Using Intertidal Invertebrates

“…Finally, as for all interspecific interactions (Menge and Sutherland, 1987;Kondoh, 2001;Silliman and He, 2018), the intensity of predator NCEs on prey demography likely depends greatly on the abiotic context and food supply (Kimbro et al, 2020;Wirsing et al, 2021). In fact, as noted above, the intensity of dogwhelk NCEs on barnacle recruitment was found to depend on wave exposure and prey food supply.…”