Predator responses to gradients in prey density have important implications for population regulation and are a potential structuring force for subtidal marine communities, particularly on rocky reefs where herbivorous sea urchins can drive community state shifts. On rocky reefs in southern California where predatory sea otters have been extirpated, top-down control of sea urchins by alternative predators has been hypothesized but rarely tested experimentally. In laboratory feeding assays, predatory spiny lobsters (Panulirus interruptus) demonstrated a saturating functional response to urchin prey, whereby urchin proportional mortality was inversely density-dependent. In field experiments on rocky reefs near San Diego, CA, predators (primarily the labrid fish California sheephead, Semicossyphus pulcher) inflicted highly variable mortality on purple urchin (Strongylocentrotus purpuratus) prey across all density levels. However, at low to moderate densities commonly observed within kelp forests, purple urchin mortality increased to a peak at a density of ~11 m-2. Above that level, at densities typical of urchin barrens, purple urchin mortality was density-independent. When larger red urchins (Mesocentrotus franciscanus) were offered to predators simultaneously with purple urchins, mortality was density-independent. Underwater videography revealed a positive relationship between purple urchin density and both the number and richness of fish predators, but these correlations were not observed when red urchins were present. Our results demonstrate highly variable mortality rates across prey densities in this system and suggest that top-down control of urchins can occur only under limited circumstances. Our findings provide insight into the dynamics of alternate community states observed on rocky reefs.
ReadMe File describing all scripts and datasets
This text file provides a description and some metadata for each of the datasets and analysis scripts associated with the paper: Experiments reveal limited top-down control of key herbivores in Southern California kelp forests by Dunn & Hovel in Ecology
1 DunnHovel_Scripts_Data_ReadMe.txt
Spiny lobster functional response analysis
Functional response analysis for California spiny lobsters consuming purple and red sea urchins
Functional_Response_Analysis_DunnHovel.R
Spiny lobster functional response to purple urchins
California spiny lobsters consuming purple urchins in functional response experiments conducted in at San Diego State Coastal & Marine Institute Lab.
LobsFuncRespData.txt
Spiny lobster functional response to purple + red urchins
Cal. spiny lobsters consuming purple and red urchins simultaneously, experiments conducted at SDSU Coastal & Marine Institute Lab
LobsFuncRespDataBoth2.txt
Size-dependent mortality of urchins due to spiny lobster predation
California spiny lobsters consuming purple urchins and purple + red urchins in functional response experiments. Size-dependent mortality analysis script.
Functional_Response_SizeDependentMortality_DunnHovel.R
Purple urchin size-dependent survival
Purple urchin survival data following spiny lobster functional response experiments. Used to test size-dependent mortality.
Survival by TD Logistic Reg Data.txt
Purple + red urchin size-dependent survival
Purple urchin survival data following spiny lobster functional response experiments. Used to test size-dependent mortality.
Survival by TD Logistic Reg Data Both2.txt
Field experiment GLMM analyses of urchin mortality
GLMM analyses (binomial distribution) to test for density-dependent mortality of purple and purple + red sea urchins due to predation. Experiments conducted in Pt Loma and S. La Jolla kelp forests in 2014 and 2017.
Field_Experiment_Binomial_Analyses_DunnHovel.R
GLMM Binomial Analysis Data for Field Experiment
Data from field experiment testing for density-dependent mortality of urchins using a GLMM (Binomial distribution)
BinomialAnalysisData.txt
Field experiment, analysis to visualize urchin mortality with LOESS
Script to use loess models to visualize urchin mortality patterns in field experiment, conducted in Pt Loma and S La Jolla kelp forests in 2014 and 2017
Field_Experiment_LOESS_DunnHovel.R
Purple urchin mortality data for use with LOESS script
Data from field experiment, use to visualize mortality patterns of purple urchins with LOESS models
FieldDDdataPurpleAll.txt
Red + Purple urchin mortality data for loess analysis
Data set to visualize red + purple urchin mortality patterns using LOESS
FieldDDdataBoth.txt
Predator aggregation and species richness analyses
Script to analyze predator aggregation and species richness datasets from field experiment conducted in Pt Loma and S La Jolla in 2014 and 2017
Predator_Video_Analysis_DunnHovel.R
Predator video data
Data set from video recordings of predators foraging in field experiment conducted in Pt Loma and S La Jolla kelp forests in 2014 and 2017
PredVideoDataAll.txt