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Behavioral responses across a mosaic of ecosystem states restructure a sea otter-urchin trophic cascade

Citation

Smith, Joshua et al. (2021), Behavioral responses across a mosaic of ecosystem states restructure a sea otter-urchin trophic cascade, Dryad, Dataset, https://doi.org/10.7291/D1566H

Abstract

Consumer and predator foraging behavior can impart profound trait-mediated constraints on community regulation that scale-up to influence the structure and stability of ecosystems. Here we demonstrate how the behavioral response of an apex predator to changes in prey behavior and condition can dramatically alter the role and relative contribution of top-down forcing, depending on the spatial organization of ecosystem states. In this study, a rapid and dramatic decline in the abundance of a meso-predator (Pycnopodia helianthoides) and primary producer (Macrocystis pyrifera, ‘kelp’) coincided with a fundamental change in purple sea urchin (Strongylocentrotus purpuratus) foraging behavior and condition, resulting in a spatial mosaic of kelp forests interspersed with patches of sea urchin barrens. We show that this mosaic of adjacent alternative ecosystem states led to an increase in the number of sea otters (Enhydra lutris nereis) specializing on urchin prey, a population-level increase in urchin consumption, and an increase in sea otter survivorship. We further show that the spatial distribution of sea otter foraging effort for urchin prey was not directly linked to high prey density, but rather was predicted by the distribution of energetically profitable prey. Therefore, we infer that spatially explicit sea otter foraging enhances the resistance of remnant forests to overgrazing, but does not directly contribute to the resilience (recovery) of forests. These results highlight the role of consumer and predator trait-mediated responses to resource mosaics that are common throughout natural ecosystems and enhance understanding of reciprocal feedbacks between top-down and bottom-up forcing on the regional stability of ecosystems.

Usage Notes

This file explains all of the variables in each of the datasets required to conduct the analyses used in the manuscript, "Behavioral responses across a mosaic of ecosystem states restructure a sea otter-urchin trophic cascade." These data were generated from: (1) Benthic subtidal surveys led by Joshua G. Smith and (2) observations of sea otter foraging behavior led by Tim Tinker. In this file we also provide the source code used to analyze a 20-year time series of published sea otter census data collected by the United States Geological Survey (available at: https://doi.org/10.3133/ds1097), and subtidal data collected by the Partnership for Interdisciplinary Studies of Coastal Oceans (available at: https://doi.org/10.6085/AA/PISCO_SUBTIDAL.151.2).

Smith_2017-19_benthic_data.csv
This file contains data used to test the hypotheses that (1) a shift in sea urchin behavior from passive to active foraging is associated with a decline in kelp availability, and (2) active sea urchin foraging behavior is associated with a decline in prey quality (gonad volume). The data file contains information on the counts of purple sea urchins, gonad index, and kelp stipe counts. Sea urchin dissections to determine gonad condition were conducted in 2017 and 2018; kelp stipe counts were conducted in 2018 and 2019; and estimates of urchin density were recorded from 2017-2019 within 1x1m quadrats (n=16 per site). These benthic surveys were conducted along the Monterey Peninsula, CA, USA.

See associated JMP script “Urchin_behavior_script” for the construction of Figure 2.

Otter_counts.csv
This file contains published sea otter census data collected by the United States Geological Survey (available at: https://doi.org/10.3133/ds1097). These data were used to analyze the numerical and functional responses of sea otters (hypotheses 3 and 4). *See associated files ’State Space_model_tables.csv’ and ’StateSpace_trend.stan’ files below

Smith_focalpatch_data.csv
This file contains the data used to test the hypothesis that the spatial distribution of sea otter foraging effort for urchin prey is not directly linked to high prey density, but rather is predicted by the distribution of energetically profitable prey patches (hypothesis 5).

See associated R script entitled, “Focal_patch_probability_plot.R”.

Trophic_dynamics_timeseries.csv
This file contains published data from the Partnership for Interdisciplinary Studies of Coastal Oceans (available at: https://doi.org/10.6085/AA/PISCO_SUBTIDAL.151.2) combined with sea otter abundance data from the United States Geological Survey available at: https://doi.org/10.3133/ds1097). These data were used to generate Figure 1 and to analyze the dynamics that initiated the formation of the kelp forest-urchin barrens mosaic along the Monterey Peninsula, CA, USA.

State Space_model_tables.csv
This file contains the initial stable stage distribution (ssd) and the switch vector (sw - for assigning sets of demographic conditions to time period) for the use with the file ‘StateSpace_trend.stan’

Table 1: provides the best-fit ssd vectors for each region (named accordingly).

Table 2: provides the best-fit sw vectors for each region.

Funding

National Science Foundation, Award: OCE-1538582