Trophic cascades in seagrass meadows depend on mesograzer variation in feeding rates, predation susceptibility, and abundance

Abstract

Seagrasses provide important habitat for fishes and invertebrates but are declining around the globe, often due to overgrowth by algae. One hypothesis for this overgrowth is that overfishing of top consumers has led to greater numbers of small predatory fishes that reduce the abundance of mesograzers. This trophic cascade hypothesis requires that the same species that control algal biomass are also susceptible to fish predation. While mesograzers are known to vary in their feeding rates on algae and seagrasses, much less is known about variation in predation susceptibility and how this is related to grazing abilities. For 6 common mesograzers from Bodega Harbor, California, USA, we assessed feeding rates on macroalgae (Ulva spp.), epiphytic microalgae, and eelgrass. We then assessed predation susceptibility using juvenile cabezon Scorpaenichthys marmoratus in tanks of eelgrass habitat with and without Ulva. We found that the fastest consumers of all 3 primary producers were the least susceptible to predation. This ap peared to be due to predator avoidance strategies; fish consumed visible caprellids at a higher rate than the larger consumers, which were either better camouflaged or able to avoid predation by building tubes within the macroalgae. Using our feeding and predation rates, along with relative abundances from field surveys, we calculated the expected trophic cascade effect with and without grazer species differences. Because fish predation was skewed towards the most abundant but least important (per capita) grazers, incorporating trait variation led to a 50 to 80% reduction in expected trophic cascade effects. Examining other seagrass communities for either similar grazer species or a similar mismatch between feeding rates and predation susceptibility may improve our understanding of the variation in trophic cascade effects across systems. KEY WORDS: Epiphytes · Overgrowth · Amphipod · EelgrassResale or republication not permitted without written consent of the publisher

Phylogeny as a Proxy for Ecology in Seagrass Amphipods: Which Traits Are Most Conserved?

“…We quantified microalgae consumption by offering grazers 9 cm 2 pieces of window screen covered with microalgae (grown in the field) and measured consumption as the reduction in chlorophyll a (hereafter chla) relative to no-grazer controls for that trial (for detailed chlorophyll measurement methods see [49]). …”

Predicting consequences of climate change for ecosystem functioning: variation across trophic levels, species and individuals

“…On the outer coast, three different species of amphipods are commonly found in Ulva and surfgrass (Phyllospadix spp. The three Ampithoids share an ability to build tubes within Ulva, a habit which reduces susceptibility to fish predation (Best & Stachowicz, 2012). 1a).…”

“…These mesograzers play a key role in controlling blooms of benthic algae and facilitating the growth of seagrasses, which are key habitat providers in coastal and estuarine systems (Duarte, 2002;Valentine & Duffy, 2006;Best & Stachowicz, 2012;Whalen et al, 2013). These mesograzers play a key role in controlling blooms of benthic algae and facilitating the growth of seagrasses, which are key habitat providers in coastal and estuarine systems (Duarte, 2002;Valentine & Duffy, 2006;Best & Stachowicz, 2012;Whalen et al, 2013).…”

Grazer Functional Roles, Induced Defenses, and Indirect Interactions: Implications for Eelgrass Restoration in San Francisco Bay

“…This has been supported by recent field experiments showing that mesograzers exert a strong positive effect on seagrass biomass through effective removal of epiphytic algae even under nutrient enriched conditions [10][11][12]. However, mesograzer species within an assemblage can have a range of effects on seagrass biomass, from beneficial to detrimental [13][14][15][16], even among closely related groups [17].…”