Designing High-Tide Refuge Islands for the California Ridgway’s Rail

California Ridgway’s rail (Rallus obsoletus obsoletus) is endangered in part becauseof increased predation resulting from a lack of high-tide refuge habitat in SanFrancisco Estuary marshes (Figure 1). This problem will worsen with sea level rise.

To decrease predation on Ridgway’s rail, the California Coastal Conservancy hasconstructed 62 “high-tide refuge islands” (refuge islands) in 13 tidal marshes in theestuary. These islands are intended to augment existing high-tide refuge habitatand are part of the Invasive Spartina Project’s efforts to improve habitat for the rail.

AcknowledgmentsThanks to the California Coastal Conservancy,Don Edwards San Francisco Bay National WildlifeRefuge (DENWR), Olofson Environmental, U.S.Geological Survey, Point Blue ConservationScience, and Brian Fulfrost and Associates fordata and planning support, and to DENWR,California Department of Fish and Wildlife,Midpenninsula Regional Open Space District,East Bay Regional Park District, and the City ofPalo Alto for making land available for islandconstruction.

Gavin Archbald,*1 Max Busnardo,1 Ron Duke,1 and Marilyn Latta.2 1H. T. Harvey & Associates, 983 University Avenue, Building D, Los Gatos, CA 95032; *gavinarchbald@harveyecology.com; www.harveyecology.com. 2California Coastal Conservancy, 1330 Broadway, 13th Floor, Oakland, CA 94612-2530

Figure 6. Annual monitoring of refuge islands shows a progressive increase in the quality of extreme high-tide refuge habitat on islands as gumplant on islands has matured.

Figure 4. Refuge island conceptual cross section.

Conclusions and Next StepsRefuge islands provide a cost-effective way to rapidly establish high-tiderefuge habitat for Ridgway’s rail within existing tidal salt marshes. Islandscan provide high-quality extreme high-tide refuge habitat 1 year afterconstruction when built to 1.7 feet above MHHW and revegetated withgumplant container plants. Point Blue Conservation Science is currentlyconducting a study to determine the effects of refuge islands on theRidgeway’s rail population. The study involves statistical analysis ofannually collected Ridgeway’s rail call-count survey data.

H. T. Harvey & Associates restoration ecologistsmonitored island elevations; gumplant height,survival, and cover; and soil properties on theconstructed islands over a 4-year period (2013–2016). We used monitoring results to iterativelyimprove the refuge island design and assesswhether islands are providing:

• high-quality extreme high-tide refuge habitat(dense gumplant extends 2 feet above thepredicted highest astronomical tide [HAT]);

• minimum-quality extreme high-tide refugehabitat (dense gumplant extends 1 foot aboveHAT); or

• high-tide refuge habitat (marsh vegetationdoes not extend 1 foot above HAT).

Our key findings from 2016 monitoring aresummarized in Figures 6 and 7 and below:

• Islands built to elevations of 1.0 foot and 1.3feet above mean higher high water (MHHW)provided, on average, about 1 vertical foot ofgumplant canopy above HAT, the minimalcover needed to hide rails from predatorsduring most extreme high tides.

• Islands built to 1.7 feet above MHHW hadgumplant canopy on average 3 feet aboveHAT and provided high-quality extreme high-tide refuge for rails.

• Island soil derived from marsh sedimentsremained horticulturally suitable afterconstruction, but gumplant canopy wasenhanced by adding terrestrial soil to plantingholes, which reduced transplant shock.

State of the San Francisco Estuary Conference

October 10–11, 2017

Refuge Island Design and

ConstructionRefuge islands are smallearthen features designedand constructed to mimic therefuge habitat function ofnatural slough channels linedwith gumplant (Grindeliastricta) (Figure 3). The islandswere designed andconstructed on a pilot scale,then the design was modifiediteratively over 3 subsequentconstruction years based onannual habitat monitoringresults. The goal of the islanddesign is to allow robustgumplant and salt marshvegetation growth above theelevation of extreme hightides (Figure 4).

Crews of six people manuallyconstructed each refugeisland during the winters of2012–2015; each island took1–2 days to build (Figure 5).Sediment excavated from anearby slough channel edgewas used to build most refugeislands. In one marsh,imported clean terrestrial soilwas used.

Figure 1. Gumplant growing on natural berms along slough channels provides high-

tide refuge cover in mature salt marshes.

Refuge Island Locations

A technical advisorycommittee of stakeholdersand rail biologists, led byH. T. Harvey & Associates,selected marshes across theestuary in which toconstruct refuge islands(Figure 2). Locations wereselected to have thegreatest positive effect onRidgway’s rail survival.

Marsh selection criteriaincluded relatively high railuse, evidence of railpopulation decline, andinsufficient existing refugehabitat. Figure 2. Refuge island location map.

Figure 3. Refuge island plan view. After islands were shaped, islands and excavation areas were covered with marsh sod harvested during construction. Islands were then planted with

nursery-grown gumplant and saltgrass (Distichlis spicata) container stock.

Figure 7. Refuge islands constructed in 2013 at Cogswell Marsh (top), in 2014 at Muzzi Marsh (center), and at Dumbarton Marsh in 2015

(bottom) during monitoring in fall 2016.

Introduction

Habitat Monitoring Methods and Results

Figure 5. The refuge island construction process.

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