A Non-Native Prey Mediates the Effects of a Shared Predator on an Ecosystem Service

James E Byers,Heidi W Weiskel,Rachel S Smith,Charles Y Robertson,John F Valentine

doi:10.1371/journal.pone.0093969

James E Byers, Heidi W Weiskel + Show 3 more

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https://doi.org/10.1371/journal.pone.0093969

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Abstract

Non-native species can alter ecosystem functions performed by native species often by displacing influential native species. However, little is known about how ecosystem functions may be modified by trait-mediated indirect effects of non-native species. Oysters and other reef-associated filter feeders enhance water quality by controlling nutrients and contaminants in many estuarine environments. However, this ecosystem service may be mitigated by predation, competition, or other species interactions, especially when such interactions involve non-native species that share little evolutionary history. We assessed trophic and other interference effects on the critical ecosystem service of water filtration in mesocosm experiments. In single-species trials, typical field densities of oysters (Crassostrea virginica) reduced water-column chlorophyll a more strongly than clams (Mercenaria mercenaria). The non-native filter-feeding reef crab Petrolisthes armatus did not draw down chlorophyll a. In multi-species treatments, oysters and clams combined additively to influence chlorophyll a drawdown. Petrolisthes did not affect net filtration when added to the bivalve-only treatments. Addition of the predatory mud crab Panopeus herbstii did not influence oyster feeding rates, but it did stop chlorophyll a drawdown by clams. However, when Petrolisthes was also added in with the clams, the clams filtered at their previously unadulterated rates, possibly because Petrolisthes drew the focus of predators or habituated the clams to crab stimuli. In sum, oysters were the most influential filter feeder, and neither predators nor competitors interfered with their net effect on water-column chlorophyll. In contrast, clams filtered less, but were more sensitive to predators as well as a facilitative buffering effect of Petrolisthes, illustrating that non-native species can indirectly affect an ecosystem service by aiding the performance of a native species.

Highlights

Non-native species often negatively affect ecosystem services in their introduced ranges (e.g., [1]), they can positively influence ecosystem services by complementing the roles of native service providers or mitigating their loss [2,3,4,5,6]
Chlorophyll a drawdown by filter feeders Chlorophyll a drawdown differed among the individual species (Tables 2 and 3), with oysters extracting significantly greater quantities of chlorophyll a compared to the clam and Petrolisthes treatments (Figure 1)
The relationship between the oyster and the clam treatments was consistent in an additional trial set that compared species with population sizes determined by equivalent dry tissue weights, rather than total wet weights [Mean standardized percent chlorophyll a loss (6 standard deviations (SD)) over three hours: oysters 41.261.6; clams 4.163.3]

Summary

Introduction

Non-native species often negatively affect ecosystem services in their introduced ranges (e.g., [1]), they can positively influence ecosystem services by complementing the roles of native service providers or mitigating their loss [2,3,4,5,6]. Overview To compare filtration abilities we quantified chlorophyll a drawdown by three abundant oyster reef species: the oyster Crassostrea virginica, the hard clam Mercenaria mercenaria, and the invasive crab Petrolisthes armatus.

Results

Conclusion