Recovering populations of the southern sea otters suppress a global marine invader
Data files
Oct 25, 2024 version files 56.07 KB
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Fig3a.xlsx
18.54 KB
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Fig3b.csv
8.23 KB
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Fig3c.xlsx
21.31 KB
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Fig5.csv
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README.md
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Abstract
Understanding the role of apex predators on ecosystems is essential for designing effective conservation strategies. Supporting recovery of apex predators can have many benefits; one that has been rarely examined is control of invasive prey. We investigated whether a recovering apex predator, the southern sea otter (Enhydra lutris nereis), can exert local control over a global marine invader, the green crab (Carcinus maenas). We determined that southern sea otters in Elkhorn Slough estuary in California can consume large numbers of invasive green crabs and found strong negative relationships in space and time between otter and green crab abundance. Green crabs persisted at highest abundance in this estuary at sites with artificial tidal restriction that were not accessible to otters. Green crab abundance remained lower in this estuary than in all other estuaries in the region, which lack resident sea otters. Conservation organizations and agencies have invested heavily in recovery of southern sea otters, increasing their numbers in this estuary. Restoration of natural tidal exchange and lost marshes and seagrass beds further supports their populations. We have demonstrated that these investments in top predator recovery and habitat restoration have reduced the impacts of a global invader. Our investigation highlights that investment in recovery of top predators can increase beneficial food web interactions and resilience of the entire ecosystem.
Most of the crab data were collected as part of the long-term crab monitoring program at Elkhorn Slough National Estuarine Research Reserve. The field protocol we used was the same as the protocol used by researchers/collaborators, who collected the data at the other estuaries in this study.
The majority otter data were collected by USGS, during their annual surveys. Protocol publicly available online. The only otter data not collected by USGS, were collected by Elkhorn Slough Safari, as described in the manuscript.
All data were QAQC’ed by multiple people, and multiple times. We eliminated crab data where a trap was broken or lost during deployment.
The green crab data are reported in catch per unit effort, which was calculated as number of crabs per trap, at a site, on a given day.
The otter data in Figure 3a, 3b, 4, 5 are reported in density, otters/km2. Densities are calculated as described in Supporting Information.
The otter data in Figure 3c are counts of individuals on a given boat tour, and averages calculated per month as total numbers for that month divided by number of boat tours.
Description of the data and file structure
If a cell has the value “null” it means there were no data for that day/year/site etc. The same sites were used for the different analyses, but may have different groupings. For example VM was a site in the lower estuary, without tidal restriction.
File Fig. 3a
Column names
Site: identifier for study site (APM = Azevedo Pond; HI = Hummingbird Island; HL = Hudson Landing; KP = Kirby Park; NMF = North Marsh Full; NMM = North Marsh Mute; SM = South Marsh, VB = Vierra Beach; WS = Whistlestop Lagoon). All sites and abbreviations are also listed in the SI, Table S1
Year: year in which data in a given row were collected
Sea otter density [otters/km^2]: calculated kernel densities of otters within the USGS survey cell closest to the site
Green crab CPUE [crabs/trap]: average number of individual crabs in traps set, at a site, in one day
Section of estuary: estuary area category (lower, middle, upper)
Dist. from mouth [km]: distance of “section of estuary” from the mouth of the estuary to that section (e.g. lower, middle, upper)
Dist. category: numerical category of estuary area (0-6 = lower, 6-8 = middle, 8-10=upper estuary)
File Fig. 3b
Column names
Site: identifier for study site (APM = Azevedo Pond; HI = Hummingbird Island; HL = Hudson Landing; KP = Kirby Park; NMF = North Marsh Full; NMM = North Marsh Mute; SM = South Marsh, VB = Vierra Beach; WS = Whistlestop Lagoon). All sites and abbreviations are also listed in the SI, Table S1
Year: year in which data in a given row were collected
Sea otter density [otters/km^2]: calculated kernel densities of otters within the USGS survey cell closest to the site
Green crab CPUE [crabs/trap]: average number of individual crabs in traps set, at a site, in one day
Hydrology: site category identifying whether the site is tidally restricted or not (restriction, no tidal restriction)
File Fig. 3c
Column names
Site: identifier for study site (APM = Azevedo Pond; HI = Hummingbird Island; HL = Hudson Landing; KP = Kirby Park; NMF = North Marsh Full; NMM = North Marsh Mute; SM = South Marsh, VB = Vierra Beach; WS = Whistlestop Lagoon). All sites and abbreviations are also listed in the SI, Table S1
Year: year in which data in a given row were collected
Green crab CPUE [max # crabs/trap/day]: of all the traps set at a site, in one day, this is the maximum number of green crabs found in one trap, at that site, in one day
Month: Month during which the Elkhorn Slough Safari counted otters seen during each excursion (Jan-Dec)
Sea otter abundance [individuals/trip]: average number of individual otters per trip, counted during excursions within the month of the prior column
File Fig. 5
Column names
Year: year in which data in a given row were collected
Estuary: identifier for which west coast estuary in which the green crab trapping data came from (Bodega Bay, Drakes Estero, Elkhorn Slough, San Francisco Bay, Tomales Bay)
Transformed green crab CPUE: For cross-estuary comparison we transformed the raw CPUE data using the equation: LN(CPUE) + 0.05
Sharing/Access information
Upon request to rikke@elkhornslough.org, we can share our data with you directly via Microsoft Excel
Crab data were derived from the the dissertation of Rikke Jeppesen, and Elkhorn Slough Reserve long-term crab monitoring data base, in addition to otter data supplied by Tim Tinker, UGSG annual otter surveys.
Code/Software
Code used for the population modeling is found the in Supplemental Information of the manuscript, in Appendix 1.
Most of the crab data were collected as part of the long-term crab monitoring program at Elkhorn Slough National Estuarine Research Reserve. The field protocol we used was the same as the protocol used by researchers/collaborators, who collected the data at the other estuaries in this study. Briefly, we used standard trapping protocols for crabs, soaking baited traps (minnow and Fukui) for 24 hrs, at a tide level of zero feet. All crabs were measured, sexed, and identified to species level.
The majority otter data were collected by USGS, during their annual surveys. Protocol publicly available online. The only otter data not collected by USGS, were collected by Elkhorn Slough Safari, as described in the manuscript.
All data were QAQC'ed by multiple people, and multiple times. We eliminated crab data where a trap was broken or lost during deployment.
Microsoft Excel or Apache Open Office Calc (free)