Performing Department
Environmental & Forest Biology
Non Technical Summary
The Snowy Plover ( Charadrius alexandrinus) is state-listed as threatened in Florida (Florida Fish and Wildlife Conservation Commission 2010), and thus understanding its status, trends, and threats is of interest to wildlife managers in the region. Beach nesting birds such as Snowy Plovers are often subject to intense pressure from human recreation and coastal development (Fleming et al. 1988, Burger 1990, Weston and Elgar 2007). Human impacts include habitat loss, disturbance, destruction of nests or chicks, and increased predation by domestic and human-commensal predators that can affect survival, dispersal, and birth rates (Ruhlen et al. 2003, Cohen et al. 2009, Engeman et al. 2010). Snowy Plovers, which breed and winter along the Gulf Coast of Florida, have likely decreased in Florida during the 1900's due to human activities (Himes et al. 2006). Estimates in recent decades have remained relatively stable but low, at between 200 and 225 pairs (Himes et al. 2006). Much of the historic nesting habitat on the southwest Florida coast has likely been lost to development, with the majority of the current population nesting on the Florida panhandle. While nest sites and broods are oftenprotected with symbolic fencing to prevent disturbance and direct mmiality by human recreation, the habitat itself remains threatened by further development and possibly by sea level rise.Furthermore, the adequacy of current management in preventing the impacts of disturbance has yet to be fully evaluated. A first step in addressing these data needs is to build a population model that will permit the prediction of population trends and the assessment of threats to population persistence and growth.Population Viability Analysis (PVA) uses mathematical modeling to estimate the probability of population persistence into the future (Beissenger et al. 2002). It entails iteratively projecting a population into the future thousands of times based on estimates of vital rates (bilih, survival, immigration, and emigration) and their variances. Commonly, a modeled population that is 95% likely to persist over 100 years (i.e., where the population fails to go extinct in 95% of the iterative model runs) is considered to be sustainable. In complex PVA's, the vital rates themselves are modeled as a function of natural and anthropogenic factors, which allows for the testing of the effects of particular threats or ecological correlates, and different management actions.We propose conducting a PVA to predict the persistence of Florida Snowy Plovers given currently available data on birth and survival rates. Because demographic data for Snowy Plovers in Florida is sparse, the predictions of this model will necessarily require refinement as more data on survival, movement, and birth rates are collected. However, this PVA will be an important starting point for understanding Snowy Plover population trends and the factors that affect them.
Animal Health Component
(N/A)
Research Effort Categories
Basic
100%
Applied
(N/A)
Developmental
(N/A)
Goals / Objectives
To predict the persistence of Florida Snowy Plovers given currently available data on birth and survival rates.
Project Methods
Model StructureWe will model the probability of Snowy Plover persistence in Florida as: (Equation 1)Nt+ 1 = Population size of adult females next year (year t+1),Nt = Population size of adult females this year (year t),a = apparent survival (probability of surviving, returning, and attempting to nest) of adult femaless = apparent survival of subadult femalesBt = bi1ih rate (fledglings per female, or productivity) in year t R = sex ratio of fledglings (assumed to be 0.5)I = immigration rate from other parts of the range (assumed to be 0, but included as a placeholder for later model refinement)The birth rate is the product of average clutch size, average nest survival, and average chick survival from hatch to fledge. With more specific productivity monitoring data, the model can be ultimately refined to include those parameters, and factors that affect them, including human disturbance and predation. Serial polyandry is known to be relatively common for Snowy Plovers (Warriner et al. 1986), and hence we will model B under different scenarios of the probability of double-brooding by females, based on local data and the literature.Apparent survival () is the product of true survival (S), emigration (1-F where F is site fidelity), and (in the case of subadults) age at first breeding (A). As data on those parameters becomes available, it will be possible to model S, F, and A individually, as well as the factors that affect them. However at this time, return rates of banded individuals are the best available data for Florida Snowy Plovers. Survival rates have not been studied for Florida Snowy Plovers over a long enough time period for estimates of annual variance to be computed. Thus we will use coefficients of variation from the literature, where it exists for Western Snowy Plovers (Stenzel et al. 2007) and if necessary other closely related species such as Piping Plovers ( Charadrhts melodus).We will calculate the birth rate needed for a stable population by setting N1 = N1+1 and solving for B1, as follows:(Equation 2)We will also use a model in which the southwest Florida and panhandle populations are estimated separately, with movement between them. The equations will be identical to Equation 1, with the addition of the terms '¥ps(a) and '¥ps(s) (breeding dispersal rates from the Panhandle to Southwest Florida for adults and subadults, respectively) and 'I'sp(a) and 'I'sp(s) (movement rates from Southwest Florida to the Panhandle). We will use best available estimates for those movement rates, but since movement has yet to be widely studied we will also test hypothetical scenarios of zero, low, and high movement rates.Population Viability AnalysisWe will simulate population growth over 100 years using Equation 1 in 100,000 iterations using SAS statistical software (Cary, NC). Initial inputs will include the current estimate of Snowy Plover population size, mean regional birth rates from monitoring data, and apparent survival estimates from banding studies on the Panhandle and Southwest Florida. Where the data are not sufficient for estimating apparent survival using mark-recapture models, we will use raw return rates and adjust them for detectability of banded birds based on expert opinion of Snowy Plover biologists in Florida. In each model run, we will randomly draw value for birth rate based on its regional mean and variance and assuming a normal distribution. We will randomly draw values for survival estimates based on normal distributions truncated at 0 and 1 (Melvin and Gibbs 1994), using either variances estimated in Florida or from other populations or closely related species. We will calculate the proportion of the 100,000 runs that the population went extinct and the average birth rate needed to sustain the population, given the means and variances for birth and survival used in the model.