Extreme and complex variation in range‐wide abundances across a threatened Neotropical bird community

Abstract

Aim Understanding patterns and drivers of variation in abundance across full species ranges is crucial in conservation science, but our knowledge of these forms and processes is limited, especially in the tropics. This study aims to: (1) identify patterns in variation of abundance across sites; (2) examine congruence of abundance hotspots across species and spatial autocorrelation of abundance within species; and (3) assess the nature and strength of environmental correlates of abundance (topography, habitat and human pressure). Location Twenty‐six sites across the full ranges of 14 dry forest bird species in northern Peru. Methods Study sites in this patchy habitat were selected within strata derived from species distribution models, while also ensuring geographic representation. Species abundance data from variable‐width transects were compared across sites and across range core versus edge; relationships between abundance and environmental variables were examined using GAMs, and spatial autocorrelation was examined with multivariate Mantel tests. Results Although most species were recorded at most sites, local abundance varied by one or two orders of magnitude. Several species showed a humped rather than the classic skewed abundance distribution, with abundance not necessarily highest at the centre of species’ ranges. Spatial autocorrelation in species’ local abundance was evident only at distances less than 55 km. Sites of maximum abundance for individual species did not coincide—nine different sites held highest densities of at least one species. Relationships between local abundance and almost all environmental correlates were non‐linear. Main conclusions The extreme variation in species abundances and the complexity in their relationships with environmental variables have important implications, both for design of conservation‐motivated surveys for which we offer some recommendations, and for the need for multiple reserves to capture high local abundances of key species.

Estimating ecological metrics for holistic conservation management in a biodiverse but information‐poor tropical region

“…The seven species with their IUCN Red List status are Peruvian Plantcutter Phytotoma raimondii (Vulnerable; VU), Grey-and-white Elaenia Pseudelaenia leucospodia (Least concern; LC), Tumbes Tyrant Ochthoeca salvini (Near threatened; NT), Rufous Flycatcher Myiarchus semirufus (VU), Tumbes Sparrow Rhynchospiza stolzmanni (LC), Cinereous Finch Piezorina cinerea (LC), and Sulfur-throated Finch Sicalis taczanowskii (LC). We undertook surveys at 26 sites, selected randomly from two strata: the highest relative probability of obtaining species records (using a preliminary SDM) within each 50 × 50 km grid across the study area (see methods in Devenish, Buchanan, Smith, & Marsden, 2017; Figure 1). We undertook surveys at 26 sites, selected randomly from two strata: the highest relative probability of obtaining species records (using a preliminary SDM) within each 50 × 50 km grid across the study area (see methods in Devenish, Buchanan, Smith, & Marsden, 2017; Figure 1).…”

“…Survey design consisted of four 2.5 km parallel, straight transects (mainly oriented north-south), off tracks and paths, separated by 500 m at each site. The suitability of habitat variables as candidate covariates was assessed with respect to collinearity, effect on detection distance and variability across sites (see methods in Devenish et al, 2017). Metrics included tree species present; percentage vegetation cover at two vertical strata (<3 m, >3 m; estimated visually); diameter at breast height (dBH; with tape measure), height of lowest branch, and height (estimated visually) of three largest trees; total number of stems with dBH > 10 cm; and topographic measurements (slope, elevation).…”

“…They are endemic to the Tumbes dry forests and have almost their entire distributions within Peru below an elevation of 500 m. Habitat for the study species consists mainly of dry forest and scrub savannah amongst larger desert areas, located in a narrow band between the Andes and the coast (CDC, 1992). We undertook surveys at 26 sites, selected randomly from two strata: the highest relative probability of obtaining species records (using a preliminary SDM) within each 50 × 50 km grid across the study area (see methods in Devenish, Buchanan, Smith, & Marsden, 2017; Figure 1).…”

“…Bird density was estimated at the site level, using a global detection function, adjusted with the site-level covariates in the Distance 6.0 software (Thomas et al, 2010). The suitability of habitat variables as candidate covariates was assessed with respect to collinearity, effect on detection distance and variability across sites (see methods in Devenish et al, 2017).…”

Colonisation dynamics during range expansion is poorly predicted by dispersal in the core range

“…This may result in differences in (re) colonisation dynamics, and the spatial scale over which dispersal mediates such dynamics in different parts of the range. While the location of the range edge may emerge from these metapopulation dynamics (Lennon et al 1997), very little is known, even from the best studied species, about how local densities and/or regional occupancy might change from the core to the edge of the range (Devenish et al 2017), and what the consequences are for both patch dynamics and rate of onward expansion.…”