Population Dynamics of Humboldt Bay Wallflower (Erysimum Menziesii) Over Three Decades on the North Spit of Humboldt Bay, California Andrea J
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Population Dynamics of Humboldt Bay wallflower (Erysimum menziesii) over three decades on the North Spit of Humboldt Bay, California Andrea J. Pickart, Annie Eicher, and Laurel Goldsmith U.S. Fish and Wildlife Service October 2018 INTRODUCTION Menzies’ wallflower (Erysimum menziesii) is a monocarpic perennial in the Brassicaceae restricted to early successional coastal dune habitats in Humboldt, Mendocino, and Monterey Counties, California. The current taxonomic treatment (Al-Shehbaz 2012) combines the different geographic populations as a single species, however, recent genetic work supports subspecies status for the Humboldt Bay population (Vorobik 2015), and it has been previously separated as Erysimum menziesii ssp. eurekense (Hickman 1993). Hereafter, we refer to the Humboldt Bay population as Humboldt Bay wallflower, by the genus name Erysimum or the common name “wallflower.” Humboldt Bay wallflower was listed by the U.S. Fish and Wildlife Service as an endangered species in 1992, with the recognized threats of invasive species displacement, recreational impacts, and lack of protected habitat (USFWS 1998). The wallflower is found only in the semi-stable foredune zone in the Abronia latifolia – Ambrosia chamissonis herbaceous alliance, also known as dune mat (Sawyer et al. 2005). This low-growing herbaceous community is characterized by relatively low cover (Pickart and Barbour 2007) in which wallflower assumes a patchy distribution. On the North Spit of Humboldt Bay, dune mat has been highly impacted by the spread of invasive species including: European beachgrass (Ammophila arenaria), ice plant (Carpobrotus edulis and C. edulis x C. chilensis), yellow bush lupine (Lupinus arboreus), and the annual grasses ripgut brome (Bromus diandrus) and rattlesnake grass (Briza maxima) (Pickart and Sawyer 1998). These species readily invade the sparse dune mat community, creating a dense canopy cover negatively correlated with wallflower presence (Duebendorfer 1992). Wallflower germinates in fall/winter following the onset of winter rains (Pickart et al. 2000) and then persists as a vegetative rosette for an average of three years before bolting (Pickart and Sawyer 1998) and flowering in late winter to early spring once it reaches a critical size threshold (Berg 1986). Fruits mature in summer, and the majority disperse close to the maternal plant in fall and winter. However, longer distance dispersal can also occur through a tumbleweed style of dispersal in which entire fruiting plants become dislodged and are transported by wind (Pickart and Sawyer 1998). There is no persistent seed bank, although some seeds remain viable in above-ground infructescence into a second year (Pickart 2004). Approximately 29% of dispersed seeds emerge and seedling mortality is very high, with only 05- 1.0% of seedlings reaching maturity (Pickart 2004). Mortality is highest during spring and summer months. Plants are subject to white rust disease caused by the biotrophic oomycete Albugo candida, which results in symptoms ranging from isolated white pustules to systemically infected plants in which reproduction is suppressed or prevented (Pickart 2004). A seasonal integral population model using demographic data from four cohorts of wallflower found that interannual variation in demographic rates was correlated with deviations from historical seasonal temperature averages (Schreiber 2015). Notably, fecundity of flowering individuals was lower during warmer winters, correlating with greater white rust disease severity. The model 1 predicted that warmer seasons contribute to lower population growth rates, with implications for a warming climate (Schreiber 2015). Management of Humboldt Bay dune habitats has been occurring for several decades, with the goal of restoration of invaded habitats to the early successional dune mat community through recovery of underlying processes (Pickart 2013), and in some cases with the stated goal of restoring habitat for Humboldt Bay wallflower (Pickart and Sawyer 1998). A survey of the wallflower over the entire North Spit was carried out in 1988 with the goal of establishing population levels, measuring geographic variation in population size, and to establish a baseline for measuring success of future management (Sawyer and André 1990). Since that time, population wide surveys have been carried out on approximately a decadal basis (Pickart et al. 2000). This paper documents results of the most recent (2015) survey and examines population trends over time. METHODS Methods in all surveys followed those established in the original 1988 baseline survey (Sawyer and André 1990). The North Spit was divided into 9 biopolitical strata representing geographically separate populations and/or those under separate management jurisdictions (Fig. 1). If a geographically continuous subpopulation was bisected by a management boundary, we created separate strata to allow us to detect differences due to management. Management entity and status for each strata is shown in Table 1. The 2015 survey occurred in the spring during the flowering season. Using a Trimble GPS, large, discrete clusters of wallflowers were mapped as polygons, and isolated occurrences or small clusters were mapped as points. As in previous surveys, plants less than 3 cm in diameter were excluded in order to reduce non-detection error and because plants this small in spring have a low likelihood of survival (Pickart and Sawyer 1998). In each stratum, selected polygons were sampled and others were censused, resulting in a stratified sampling design where the two strata consisted of sampled versus censused occurrences. Small polygons, and large polygons with relatively low plant density were censused, as were all points. We assigned numbers to polygons that were not censused and randomly selected enough polygons to yield a total sample size of 30 plots in each stratum. Circular plots of 1 m radius were then placed systematically within the polygon along east-west transects. Spacing between plots and between transects was consistent within a stratum but varied between strata in order to accommodate 30 plots per stratum. We measured reproductive status and disease incidence of every fifth censused plant (constituting a systematic sample) and of all plants in every other sample plot (constituting a cluster sample). To determine the amount of available habitat at the time of each survey, we used aerial imagery from within a year of each survey date, and heads-up digitized all dune mat. The most recent interval was field verified, which assisted with interpretation of older photographs as well. In order to explore the role of invasive species on available habitat, we heads-up digitized invasive species (Lupinus arboreus, Ammophila arenaria, Carpobrotus spp., and annual grasses) for the most recent time interval only. 2 Figure 1. Map of the North Spit Humboldt Bay showing location of strata. Table 1. Ownership and management status of strata. Stratum Name Ownership Management Status BLM Wetland Protection Area BLM Protected, unmanaged BLM Endangered Plant Protection Area BLM Protected, managed Eureka Dunes Protected Area City of Eureka Protected, unmanaged Eureka Airport City of Eureka Unprotected, unmanaged Samoa Industrial Private Unprotected, unmanaged Samoa Dunes Pending and current Protected, unmanaged public ownership Humboldt Coastal Nature Center Friends of the Dunes Protected, managed Ma-le’l Dunes CMA BLM, USFWS Protected, managed Lanphere Dunes USFWS Protected, managed Bair addition USFWS Protected, managed 3 ANALYSIS We used simple random sampling formulas (Arnab 2017) to estimate subpopulation totals and standard errors for sampled polygons by substrata (sampled portions of each stand). We then added the census totals to the sampling estimate to estimate the size of each subpopulation. Simple random sampling formulas for proportions were used to determine the proportions of reproductive and rust-infected plants for the subsample derived from censused strata. Cluster sampling formulas were used to estimate proportions for the subsample derived from sampled strata. Stratified random sampling formulas were used to combine the systematic and cluster estimates for each stratum, and to calculate an estimate for the entire population. RESULTS Population Size The population-wide total has increased exponentially since sampling started in 1988, from approximately 20,000 to more than 133,000 individuals (Fig. 2). However, there was substantial intrapopulation variability, with the Lanphere Dunes and Bair addition subpopulations exhibiting a much higher rate of increase than all other subpopulations (Fig. 3). As of 2015, these two geographically continuous populations represented 64% of the total population but only 30% of the occupied habitat. Density of Erysimum individuals at Lanphere/Bair was 2.7 times greater than the average density elsewhere. 180000 160000 140000 120000 100000 80000 60000 PopulationSize 40000 20000 0 1988 1997 2006 2015 Year Figure 2. Change in population size from 1988-2015 (error bars are standard error) The number of total individuals increased from 1988 to 2015 in all subpopulations other than the BLM Endangered Plant Area (BLM EPPA), the Samoa industrial site (“Mills”), and the Samoa Dunes. 4 Figure 3. Population size over time by stratum (subpopulation). Density The density of individuals has varied by site and over time (Fig. 4). In general, densities have declined over time, with the exception of the Lanphere Dunes and Bair subpopulations,