Extending trait dispersion across trophic levels: predator assemblages act as top-down filters on prey communities

Description

Notes

Methods

We sampled fish and epifaunal communities in 6 eelgrass beds in Bodega Harbor and Tomales Bay in the summers of 2019 and 2021. We sampled fishes in 6 sets of a custom beach seine net when the water level was at or below 1 m above the seafloor. The seine sampled a circular area of 11 m². We counted, identified to the lowest possible taxonomic level (typically species), and released animals retained in the seine.

We sampled epifauna at 12 locations within each site separated by at least 10m and spanning a depth gradient of intertidal to shallow subtidal. We collected each sample by everting an open-mouth drawstring mesh bag (500 µm mesh size) over a clump of shoots in the eelgrass bed so that the mouth of the bag was flush with the sediment surface, cutting the shoots, and closing the drawstring to capture shoots, macroalgae, and associated animals. We transferred the shoots to the laboratory on ice, rinsed, and hand-inspected them to dislodge the epifauna, which we then passed through a 500 µm sieve and ultimately transferred into 70% ethanol. We then identified epifauna to the lowest possible taxonomic level (typically species). We also quantified the biomass of macroalgae in each epifaunal sample. We also measured water temperature, total eelgrass shoot density m^-2, flowering shoot density m^-2, percent cover, canopy height, and epiphyte dry weight mm^-2 eelgrass as described by Aoki et al. (2022).

For the 23 most abundant species of peracarids in our surveys, we assigned values for 11 traits putatively related to predator avoidance and microhabitat niche. We collected three of these traits (maximum body size, shape, and living habit) from the literature. We determined the tube fidelity for each species according to observations of living and preserved specimens along a four-point ordered scale as follows: none (species lacks silk glands to build tubes), low (species has silk glands but was never observed in a tube alive or preserved in ethanol), medium (species has silk glands and was observed in tubes when alive but readily flees tube when exposed to ethanol), and high (species has silk glands, is tubicolous when alive, and is regularly found inside tubes after preservation in ethanol). We measured mean body size (length from rostrum to telson), relative eye diameter, and relative antenna lengths from 10-20 preserved individuals collected across sites and years. We measured activity levels as fractions of time spent swimming, walking, and still (unmoving) from one-minute video recordings of 10-20 live individuals per species across sites and years. We log-transformed peracarid traits where appropriate to conform to a normal distribution.

We assigned two categorical (vertical position and foraging mode) and one continuous trait (trophic level) to the 16 most abundant fishes based on the literature. We fuzzy-coded vertical position and foraging mode among 5 and 3 levels, respectively, to accommodate species that could be classified among multiple levels. We collected linear morphometric measurements of fishes (body and head dimensions, fin lengths, eye size and position, and mouth height and protrusion) from 2-26 specimens per species and size class collected from seines, and standardized them for ease of comparison across species.

This dataset includes average trait data for both peracarids and fishes.

To address the potential effects of evolutionary history on peracarid community responses to predators, we built a phylogeny of our species by subsetting from the ultrametric peracarid supertree published by Ashford et al. (2018). For species in our dataset that were not included in the supertree we substituted congeners or confamilials as needed.

Tables S1-S3 (on Zenodo) show modeled responses of residual peracarid community trait and phylogenetic dispersion to fish community-weighted mean trait values (Table S1), total fish community trait dispersion (Table S2) and residual fish community trait dispersion after accounting for habitat filters (Table S3).