Consumer mobility predicts impacts of herbivory across an environmental stress gradient

Abstract

Environmental stress impedes predation and herbivory by limiting the ability of animals to search for and consume prey. We tested the contingency of this relationship on consumer traits and specifically hypothesized that herbivore mobility relative to the return time of limiting environmental stress would predict consumer effects. We examined how wave-induced water motion affects marine communities via herbivory by highly mobile (fish) vs. slowmoving (pencil urchin) consumers at two wave-sheltered and two wave-exposed rocky subtidal locations in the Galapagos Islands. The exposed locations experienced 99th percentile flow speeds that were 2-5 times greater than sheltered locations, with mean flow speeds >33 cm/s vs. <16 cm/s, 2-7 times higher standing macroalgal cover and 2-3 times lower cover of crustose coralline algae than the sheltered locations. As predicted by the environmental stress hypothesis (ESH), there was a negative relationship between mean flow speed and urchin abundance and herbivory rates on Ulva spp. algal feeding assays. In contrast, the biomass of surgeonfishes (Acanthuridae) and parrotfishes (Labridae: Scarinae) was positively correlated with mean flow speed. Ulva assays were consumed at equal rates by fish at exposed and sheltered locations, indicating continued herbivory even when flow speeds surpassed maximum reported swimming speeds at a rate of 1-2 times per minute. Modeled variation in fish species richness revealed minimal effects of diversity on herbivory rates at flow speeds <40 cm/s, when all species were capable of foraging, and above 120 cm/s, when no species could forage, while increasing diversity maximized herbivory rates at flow speeds of 40-120 cm/s. Two-month herbivore exclusion experiments during warm and cool seasons revealed that macroalgal biomass was positively correlated with flow speed. Fish limited macroalgal development by 65-91% at one exposed location but not the second and by 70% at the two sheltered locations. In contrast, pencil urchins did not affect algal communities at either exposed location, but reduced macroalgae by 87% relative to controls at both sheltered locations. We propose an extension of the ESH that is contingent upon mobility to explain species-specific changes in feeding rates and consumer effects on benthic communities across environmental gradients.

Stargrazing: Trophic ecology of sea stars in the Galápagos rocky subtidal zone

“…Sandy habitats and patches of rock rubble occur at the base of ledges at some sites. Environmental conditions at the sites vary considerably in terms of the strength of upwelling, topography, and the diversity and abundance of sessile lter feeders and consumers such as sea urchins, whelks and sh (Witman et al 2010;Lamb et al 2018Lamb et al , 2020.…”

“…A total of seven cages were deployed at 15m depth for four days. Each cage was stocked with one P. cumingi individual (R = 15.0-16.5 cm), ve small E. galapagensis (2.0-3.0 cm test diameter), and ve large E. galapagensis (5.0-6.0 cm diameter), to simulate natural densities of both species (Brandt et al 2012;Witman et al 2017;Lamb et al 2020). Each sea urchin was tethered to a small, naturally occurring cobble to prevent escapes using a non-invasive tethering technique (Dee et al 2012;Witman et al 2017) to minimize procedural effects.…”

“…At the conclusion of the experiment, the plates were bagged, frozen, and subsequently processed for biomass by scraping organisms off the plates and separating them into categories of bleached CCA, healthy (non-bleached) CCA, sessile invertebrates and palatable macroalgae, such as Ulva spp., Polysiphonia spp., and Ceramium spp. (Lamb et al 2020). Each sample was sun-dried over three days and then dry biomass weighed to a precision of +/-0.001 g.…”

Intertidal estimates of sea urchin abundance reveal congruence in spatial structure for a guild of consumers

“…2 m, and sampling for sea urchins was done during spring low tides, however the minimum height of the tide during sampling differed among sites, ranging from 0.09 to 1.43 m (mean ± SD: 0.48 ± 0.22 m). For each site, tidal height at the start and end of sampling was determined based on predicted hourly tide charts (Kampfer, 2017(Kampfer, , 2018. Because high tidal height during sampling might bias species detection by making this difficult, we removed all visits from our analyses that involved sampling when tidal height was greater than 0.83 m (a total of eight visits).…”

“…Sea urchins generally undergo long-distance dispersal during their planktonic larval life stage, which lasts from weeks to several months (Cram, 1971;Dautov, 2020;Huggett et al, 2005;Strathmann, 1978). The influence of environmental filtering on sea urchin abundance has been documented along various environmental stress gradients including wave exposure and pH (Baggini et al, 2015;Lamb et al, 2020). These environmental parameters reduce the efficiency or ability of sea urchins to search for and consume food and, in turn, lower their growth rates, fecundity, and/or survival, hence contributing to the determination of their local and regional abundances and the limits of their ranges.…”