The importance of core habitat for a threatened species in changing landscapes

Abstract

Habitat loss, fragmentation, and alteration of the landscape matrix are interdependent processes, collectively responsible for most recent species extinctions. Thus, determining the extent to which these landscape processes affect animals is critical for conservation. However, researchers have often assumed that interdependent effects are independently related to animals’ responses, underestimating the importance of one or several landscape processes in driving species declines. We demonstrate how to disentangle the interdependent effects of habitat amount, fragmentation, and edge context on population size by assessing abundance of a rapidly declining grassland songbird species (grasshopper sparrow Ammodramus savannarum) in eastern Kansas (USA). We conducted >7,000 point count surveys at >2,000 sites over two breeding seasons, then modelled the direct, interactive, and indirect effects of landscape factors on abundance within spatial scales (200‐, 400‐, 800‐, and 1,600‐m radii) relevant to our focal species’ dispersal behaviour. Sparrow abundance correlated most strongly with landscape structure within 400‐m radii, increasing nonlinearly with grassland area and decreasing with the proportion of grassland near cropland or woody edges. Sparrows’ negative response to cropland edges was mostly an added, indirect consequence of reduced grassland area, whereas sparrows’ stronger negative response to woody edges was not attributable to variation in grassland area. Fragmentation and edge context mattered most in landscapes comprising c. 50%–80% grassland. Synthesis and applications. In our research, abundance of a threatened grassland songbird was influenced more by core grassland area (a function of total grassland area, fragmentation, and edge context) than total grassland area per se. Moreover, a local extinction threshold of c. 50% grassland indicated that small amounts of habitat were unsuitable for our focal species regardless of habitat configuration or matrix type. Local extinction thresholds in response to habitat area provide clear baseline targets for land managers; above those thresholds, configuration and the matrix can be modified to increase abundance of edge‐sensitive animals. Conflicting evidence in the literature regarding the importance of fragmentation and matrix features could be partially explained by species‐level traits, or methodological issues such as defining landscapes at ecologically arbitrary spatial scales, assessing landscape quality using species richness, and ignoring interactive and indirect effects.

Herbaceous production lost to tree encroachment in United States rangelands

“…These findings are consistent with model-based projections of climate-driven woodland expansion (Klemm et al, 2020;Shafer et al, 2015) and impacts from a century of wildland fire suppression (Ratajczak et al, 2014;Scholes and Archer, 1997). Given that modest increases in tree density can have outsized impacts on ecosystem services and biodiversity in rangelands (Baruch-Mordo et al, 2013;Ge and Zou, 2013;Hamilton et al, 2019;Herse et al, 2018;Huxman et al, 2005), these results suggest that woody encroachment is a primary change agent across broad regions of U.S. rangelands.…”

Correlates of blood parasitism in a threatened marshland passerine: infection by kinetoplastids of the genus Trypanosoma is related to landscape metrics of habitat edge

“…We are not aware of other studies linking landscape features associated with edge to blood parasite infection in birds of open habitats. Core area of habitat (positively) and edge density (negatively) were found to be related to bird abundance in a grassland passerine (Herse et al ., 2018). Within-species research on blood parasite infection in relation to habitat edge and fragmentation all concerned forest habitats and showed negative (Pérez-Rodríguez et al ., 2018), positive (Laurance et al ., 2013) or no associations (Sebaio et al ., 2010; Gudex-Cross et al ., 2015).…”

Examining Land Use Changes to Evaluate the Effects of Land Management in a Complex, Dynamic Landscape

“…These are examined through the lens of area, density, edge, shape, proximity, interspersion, connectivity and diversity metrics, which include parameters such as number of patches, perimeter-area ratio, patch richness, etc. (Torbick et al 2006;Driezen et al 2007;Schetter et al 2013;Herse et al 2018). These metrics, specifically landscape and class-level, are reliable for assessing natural and/or anthropogenic LULC changes that disrupt abiotic or biotic landscape structure including management activities (Gottgens et al 1998;Lopez et al 2002;Houlahan and Findlay 2004;Johnston and Rejmánková 2005).…”