Regional environmental variation and local species interactions influence biogeographic structure on oyster reefs

Abstract

Kimbro. 2020. Regional environmental variation and local species interactions influence biogeographic structure on oyster reefs. Ecology 101(2):Abstract. Although species interactions are often assumed to be strongest at small spatial scales, they can interact with regional environmental factors to modify food web dynamics across biogeographic scales. The eastern oyster (Crassostrea virginica) is a widespread foundational species of both ecological and economic importance. The oyster and its associated assemblage of fish and macroinvertebrates is an ideal system to investigate how regional differences in environmental variables influence trophic interactions and food web structure. We quantified multiple environmental factors, oyster reef properties, associated species, and trophic guilds on intertidal oyster reefs within 10 estuaries along 900 km of the southeastern United States. Geographical gradients in fall water temperature and mean water depth likely influenced regional (i.e., the northern, central and southern sections of the SAB) variation in oyster reef food web structure. Variation in the biomass of mud crabs, an intermediate predator, was mostly (84.1%) explained by reefs within each site, and did not differ substantially among regions; however, regional variation in the biomass of top predators and of juvenile oysters also contributed to biogeographic variation in food web structure. In particular, region explained almost half (40.2%) of the variation in biomass of predators of blue crab, a top predator that was prevalent only in the central region where water depth was greater. Field experiments revealed that oyster mortality due to predation was greatest in the central region, suggesting spatial variation in the importance of trophic cascades. However, high oyster recruitment in the middle region probably compensates for this enhanced predation, potentially explaining why relatively less variation (17.9%) in oyster cluster biomass was explained by region. Region also explained over half of the variation in biomass of mud crab predators (55.2%), with the southern region containing almost an order of magnitude more biomass than the other two regions. In this region, higher water temperatures in the fall corresponded with higher biomass of fish that consume mud crabs and of fish that consume juvenile and forage fish, whereas biomas of their prey (mud crabs and juvenile and forage fish, respectively) was generally low in the southern region. Collectively, these results show how environmental gradients interact with trophic cascades to structure food webs associated with foundation species across biogeographic regions.

Environmental gradients influence biogeographic patterns of nonconsumptive predator effects on oysters

“…Nevertheless, this trait response was only detected at one of our six sites, with substantially more variation (94%) in recruitment explained by a biogeographic gradient in larval supply (Fig. 4B;Byers et al 2014, Byers et al 2015, Grabowski et al 2020. Furthermore, this regional recruitment pattern appears to be a consistent feature of the SAB given that a similar unimodal pattern of oyster recruitment across latitude was also demonstrated by Byers et al (2015) and Grabowski et al (2020).…”

“…Reefs assigned to three trophic levels were stocked with one toadfish (length >150 mm) and one male blue crab (carapace width >100 mm), as well as with the 35 mud crabs and 12 oyster drills. These organismal densities reflect mean values observed on natural reefs throughout the SAB (Kimbro et al 2014, Grabowski et al 2020). Because of the anticipated arrival of Hurricane Irene, we ended this experiment after 90 d.…”

“…In this study, we quantified oyster recruitment to the experimental tiles to contemporaneously measure the pattern in recruitment. Lastly, unpublished results from the Grabowski et al (2020) biogeographic assessment of oyster reef communities revealed a north–south gradient in benthic chlorophyll a (Fig. 1; see Appendix for full methods and results), which was used as a proxy for benthic microalgae, an important contributor to oyster diet (Fukumoria et al 2008, Abeels et al 2012).…”

“…(B) Polynomial regression fit between the recruitment of natural oysters to experimental reefs and the latitude of experimental sites in this study. (C) Linear regression fit between phytoplankton concentration (chlorophyll a) and latitude of monitoring locations(Grabowski et al 2020) and experimental sites in this study.v www.esajournals.org 11 October 2020 v Volume 11(10) v Article e03260…”

“…(A) Polynomial regression fit between estimated volumetric water flow rate (m 3 /tidal cycle) and latitude of 50 oyster reefs throughout the SAB. These reefs are part of a biogeographic assessment of oyster reef communities conducted byGrabowski et al (2020). (B) Polynomial regression fit between the recruitment of natural oysters to experimental reefs and the latitude of experimental sites in this study.…”