Kristine O. Evans

Improving distance sampling: accounting for covariates and non-independency between sampled sites

Summary 1. There is currently much interest in replacing the design-based component of conventional distance sampling methods by a modelling approach where animal densities are related to environmental covariates. These models allow identification of relationships between density and covariates. One of the uses of such models is to assess the effects of some intervention on numbers for species of conservation interest in designed distance sampling experiments. 2. In this context, we use an integrated likelihood approach for modelling sample counts, adopting a Poisson model and allowing imperfect detectability on the sample plots. We use the method of Royle, Dawson & Bates (2004, Ecology, 85, 1591), extended to model heterogeneity in detection probabilities using either multiple covariate distance sampling methods or stratification. Moreover, we include a random effect for site in the plot abundance model to accommodate correlation in repeat counts at a single site. 3. These developments were motivated by a large-scale experimental study to assess the effects of establishing conservation buffers along field margins on indigo buntings in several US states. We analyse the data using an integrated likelihood and include model selection for both the Poisson rate of counts and detection probabilities. We assess model performance by comparing our results with those using a two-stage approach (Buckland et al. 2009, Journal of Agricultural, Biological, and Environmental Statistics, 14, 432) which we extended by including a random effect for site in the plot abundance model. 4. The two methods led to the same selected models and gave similar results for parameters, which revealed significant beneficial effects of buffers on indigo bunting densities. Densities on buffered fields were on average 35% higher than on unbuffered fields. Using a detection function stratified by state captured some of the heterogeneity in detection probabilities between the nine states included in the analyses. 5. Synthesis and applications. We develop and compare two methods for analysing data from large-scale distance sampling experiments with imbalanced repeat measures. By including a random site effect in the plot abundance model, we relax the assumption of independent sample counts which is generally made for distance sampling methods, and we allow inference to be drawn for the wider region that the sites represent.

Assessing Multiregion Avian Benefits from Strategically Targeted Agricultural Buffers

Mounting evidence of wildlife population gains from targeted conservation practices has prompted the need to develop and evaluate practices that are integrated into production agriculture systems and targeted toward specific habitat objectives. However, effectiveness of targeted conservation actions across broader landscapes is poorly understood. We evaluated multiregion, multispecies avian densities on row-crop fields with native grass field margins (i.e., buffers) as part of the first U.S. agricultural conservation practice designed to support habitat and population recovery objectives of a national wildlife conservation initiative. We coordinated breeding season point transect surveys for 6 grassland bird species on 1151 row-crop fields with and without native grass buffers (9-37 m) in 14 U.S. states (10 ecoregions) from 2006 to 2011. In most regions, breeding season densities of 5 of 6 targeted bird species were greater in the 500-m surrounding survey points centered on fields with native grass buffers than in landscapes without buffers. Relative effect sizes were greatest for Northern Bobwhite (Colinus virginianus), Dickcissel (Spiza americana), and Field Sparrow (Spizella pusilla) in the Mississippi Alluvial Valley and Eastern Tallgrass Prairie regions. Other species (e.g., Eastern Meadowlark [Sturnella magna], Grasshopper Sparrow [Ammodramus savannarum]) exhibited inconsistent relative effect sizes. Bird densities on fields with and without buffers were greatest in the Central Mixed-grass Prairie region. Our results suggest that strategic use of conservation buffers in regions with the greatest potential for relative density increases in target species will elicit greater range-wide population response than diffuse, uninformed, and broadly distributed implementation of buffers. We recommend integrating multiple conservation practices in broader agricultural landscapes to maximize conservation effectiveness for a larger suite of species.

Response of non-grassland avian guilds to adjacent herbaceous field buffers: testing the configuration of targeted conservation practices in agricultural landscapes

Summary 1. A substantial part of the world’s land base is dominated by agriculture, and forest habitat often consists of discrete patches of forest and linear woody corridors. These natural components provide habitat for some forest birds, but make conservation of these species difficult. In-field practices applied outside forest patches, such as specific juxtapositions of herbaceous field buffers adjacent to forest habitat, could increase avian diversity contributions of existing forest without creation of additional forest habitat. Our prediction was that herbaceous field buffers would increase bird richness in adjacent forest, and we evaluated four potential mechanisms. 2. We used bird count data from a conservation buffer monitoring programme and hierarchical community models to estimate species richness of forest generalist, forest interior and shrubland (edge) species near forest edges with and without adjacent herbaceous field buffers. We accounted for heterogeneity in detection probabilities and forest cover in surrounding landscapes when estimating species- and guild-level responses. 3. Consistent with the drift fence hypothesis, adjacent herbaceous buffers were associated with a modest increase in richness of forest interior birds in woody corridors, but not in forest blocks. Consistent with resource complementation, adjacent herbaceous buffers were associated with modest increases in richness of shrubland (edge) birds in both woody corridors and forest blocks. 4. Across all species and guilds, adjacent buffers were generally associated with greater abundance (e.g. 28 of 39 species), but these increases were also relatively small and highly variable (i.e. overlapping 95% credible intervals). Corroborating existing research, effects of adjacent herbaceous buffers are likely real, but neither pervasive nor strong. 5. Synthesis and applications. Conservation practices targeted to grassland species often produce measurable conservation benefits for target species. However, biodiversity return for investment would be further increased if targeted practices could be deployed in ways that also produce benefits for non-target species in adjacent habitats. Our results suggest that additional benefits for non-target species using adjacent forest habitat are likely to be modest, so conservation planning should focus on species targeted by the conservation practices and avoidance of potential negative impacts on those species when positive benefits to adjacent habitat are weak or lacking.

Using hierarchical centering to facilitate a reversible jump MCMC algorithm for random effects models

Hierarchical centering has been described as a reparameterization method applicable to random effects models. It has been shown to improve mixing of models in the context of Markov chain Monte Carlo (MCMC) methods. A hierarchical centering approach is proposed for reversible jump MCMC (RJMCMC) chains which builds upon the hierarchical centering methods for MCMC chains and uses them to reparameterize models in an RJMCMC algorithm. Although these methods may be applicable to models with other error distributions, the case is described for a log-linear Poisson model where the expected value λ includes fixed effect covariates and a random effect for which normality is assumed with a zero-mean and unknown standard deviation. For the proposed RJMCMC algorithm including hierarchical centering, the models are reparameterized by modeling the mean of the random effect coefficients as a function of the intercept of the λ model and one or more of the available fixed effect covariates depending on the model. The method is appropriate when fixed-effect covariates are constant within random effect groups. This has an effect on the dynamics of the RJMCMC algorithm and improves model mixing. The methods are applied to a case study of point transects of indigo buntings where, without hierarchical centering, the RJMCMC algorithm had poor mixing and the estimated posterior distribution depended on the starting model. With hierarchical centering on the other hand, the chain moved freely over model and parameter space. These results are confirmed with a simulation study. Hence, the proposed methods should be considered as a regular strategy for implementing models with random effects in RJMCMC algorithms; they facilitate convergence of these algorithms and help avoid false inference on model parameters. We consider a hierarchical centering approach for reversible jump MCMC algorithms.We describe the case for a log-linear Poisson model with mixed effects.The zero-mean of the random effect is replaced with part of the linear predictor.We apply the methods to point transect data of indigo buntings and simulated data.Our methods improve model mixing and inference on parameters.

Effects of tissue collection methods on morphometrics and survival of captive neonatal northern bobwhite.

Abstract We assessed effects of tissue collection methods (i.e., patagial microbiopsy and down feathers) and chick age at sampling on morphometrics and 21-day survival of 600 captive neonatal northern bobwhite (Colinus virginianus). We observed minimal effects on morphometrics and no difference in survival among patagial microbiopsy (x ¯  =  0.96 ± 0.03), down feathers (x ¯  =  0.92 ± 0.04), and control (x ¯  =  0.86 ± 0.05) methods. DNA analysis from patagial microbiopsy, down feather, and egg tooth samples showed greater concentrations of DNA from patagial microbiopsy (x ¯  =  10.28 ± 1.74 µg/ml) than either down feather (x ¯  =  4.10 ± 1.74 µg/ml) or egg teeth (x ¯  =  2.35 ± 1.74 µg/ml).

Switchgrass Cultivation within Loblolly Pine Plantations Influences Invertebrate Community Composition and Resource Use

Abstract Global interest in biofuels has resulted in the development of novel land-use practices for the production of cellulosic biomass. One novel land-use practice that has recently been developed is intercropping switchgrass (Panicum virgatum) between rows of planted loblolly pines (Pinus taeda). However, our understanding of how intercropping switchgrass influences loblolly pine flora and fauna is limited. Therefore, we evaluated the influence of switchgrass cultivation within loblolly pine stands on invertebrate communities. We detected 2,913 individuals (n = 1,172 and 1,741 in 2014 and 2015, respectively), encompassing 13 orders. To examine invertebrate community composition among treatments, we conducted metric multidimensional scaling (MDS) in R. Multivariate analysis determined that treatment had a significant (Pr(>r) = 0.01) influence on invertebrate communities. Furthermore, stable isotope analysis suggests that Orthopterans are not assimilating cultivated switchgrass (C4 species), but are instead assimilating C3 plant species, such as Rubus argutus (sawtooth blackberry). Results indicate that switchgrass intercropping may be a viable land-use practice for the co-production of cellulosic biomass and forest products and the maintenance of invertebrate communities associated with loblolly pine plantations.