Quantifying the importance of geographic replication and representativeness when estimating demographic rates, using a coastal species as a case study

Abstract

Demographic rates are rarely estimated over an entire species range, limiting empirical tests of ecological patterns and theories, and raising questions about the representativeness of studies that use data from a small part of a range. The uncertainty that results from using demographic rates from just a few sites is especially pervasive in population projections, which are critical for a wide range of questions in ecology and conservation. We developed a simple simulation to quantify how this lack of geographic representativeness can affect inferences about the global mean and variance of growth rates, which has implications for the robust design of a wide range of population studies. Using a coastal songbird, saltmarsh sparrow Ammodramus caudacutus, as a case study, we first estimated survival, fecundity, and population growth rates at 21 sites distributed across much of their breeding range. We then subsampled this large, representative dataset according to five sampling scenarios in order to simulate a variety of geographic biases in study design. We found spatial variation in demographic rates, but no large systematic patterns. Estimating the global mean and variance of growth rates using subsets of the data suggested that at least 10–15 sites were required for reasonably unbiased estimates, highlighting how relying on demographic data from just a few sites can lead to biased results when extrapolating across a species range. Sampling at the full 21 sites, however, offered diminishing returns, raising the possibility that for some species accepting some geographical bias in sampling can still allow for robust range‐wide inferences. The subsampling approach presented here, while conceptually simple, could be used with both new and existing data to encourage efficiency in the design of long‐term or large‐scale ecological studies.

Annual variation in the offspring sex ratio of Saltmarsh Sparrows supports Fisher's hypothesis

“…A male-biased offspring sex ratio would help to explain a male-biased adult sex ratio; however, we found neither a male-biased offspring sex ratio nor evidence for sex-specific nestling mortality that would lead to the observed differences in adult sex ratio. Further, studies have also found no sex differences in adult survival, with a mean survival rate of 0.44 for females and 0.49 for males (Field et al 2017).…”

“…To further evaluate our findings of variation in offspring sex ratio in relation to adult sex ratio, we used capture data from a separate ongoing study utilizing systematic and targeted netting of adults from the same 5-yr study period to estimate the adult sex ratio in our 4 study sites (Field et al 2017). By including targeted netting of adult females on nests, our methods minimized the capture bias that typically favors males in systematic netting studies.…”

Plasticity in nesting adaptations of a tidal marsh endemic bird

“…This plasticity, however, is likely insufficient in the face of sea‐level rise, which reduces high marsh habitat and modifies tidal regimes that disrupt synchronous breeding of sparrows with the 28‐day tidal cycle. The direct impacts of sea‐level rise are predicted to reduce the reproductive success of saltmarsh sparrows (Bayard & Elphick, ), which have already declined at a rate of 9% annually from 1998 to 2012 (Correll et al., ), leaving the species vulnerable to extinction within the next 50 years (Field et al., , ). The apparent adaptive capacity of saltmarsh sparrows, however, may enhance their ability to respond to management interventions targeted to mitigate nest flooding.…”

A test of a corollary of Allen's rule suggests a role for population density

“…The raw pattern of bill measurements, uncorrected for body size, shows maximum bill surface areas near 42°N (near the southern boundary of the hybrid zone and the peak in regional sparrow density: Field et al 2018) and declines both to the south and north (Fig. Across all samples and sexes, bill surface area was 5.3 mm 2 larger for saltmarsh (mean ± 95% CI = 70.8 ± 0.8 mm 2 ) versus Nelson's (65.5 ± 0.9 mm 2 ) sparrows.…”

“…They do, however, still possess larger bills on average than their closest non-tidal-marsh relative (Shriver et al 2011, Greenlaw et al 2018, which is the convergent pattern across a large suite of coastal birds (Grenier andGreenberg 2005, Luther andGreenberg 2011). Importantly, however, saltmarsh sparrows also possess variation in breeding density independent of the temperature cline, as density is highest in the range center (Wiest et al 2016, Field et al 2018. Importantly, however, saltmarsh sparrows also possess variation in breeding density independent of the temperature cline, as density is highest in the range center (Wiest et al 2016, Field et al 2018.…”