Assessing Regional and Interspecific Variation in Threshold Responses of Forest Breeding Birds through Broad Scale Analyses

Abstract

BackgroundIdentifying persistence and extinction thresholds in species-habitat relationships is a major focal point of ecological research and conservation. However, one major concern regarding the incorporation of threshold analyses in conservation is the lack of knowledge on the generality and transferability of results across species and regions. We present a multi-region, multi-species approach of modeling threshold responses, which we use to investigate whether threshold effects are similar across species and regions.Methodology/Principal FindingsWe modeled local persistence and extinction dynamics of 25 forest-associated breeding birds based on detection/non-detection data, which were derived from repeated breeding bird atlases for the state of Vermont. We did not find threshold responses to be particularly well-supported, with 9 species supporting extinction thresholds and 5 supporting persistence thresholds. This contrasts with a previous study based on breeding bird atlas data from adjacent New York State, which showed that most species support persistence and extinction threshold models (15 and 22 of 25 study species respectively). In addition, species that supported a threshold model in both states had associated average threshold estimates of 61.41% (SE = 6.11, persistence) and 66.45% (SE = 9.15, extinction) in New York, compared to 51.08% (SE = 10.60, persistence) and 73.67% (SE = 5.70, extinction) in Vermont. Across species, thresholds were found at 19.45–87.96% forest cover for persistence and 50.82–91.02% for extinction dynamics.Conclusions/SignificanceThrough an approach that allows for broad-scale comparisons of threshold responses, we show that species vary in their threshold responses with regard to habitat amount, and that differences between even nearby regions can be pronounced. We present both ecological and methodological factors that may contribute to the different model results, but propose that regardless of the reasons behind these differences, our results merit a warning that threshold values cannot simply be transferred across regions or interpreted as clear-cut targets for ecosystem management and conservation.

Threshold detection: matching statistical methodology to ecological questions and conservation planning objectives

“…First, thresholds are, by definition, values where the response, e.g., species abundance or occupancy, rapidly changes into a less desirable state. However, ecological degradation can occur before the threshold is reached, and a single threshold value may not be valid across all landscapes (Lindenmayer and Luck 2005, Johnson 2013, van der Hoek et al 2013. Therefore, conservation targets should include a safe buffer away from these critical values (Bennett and Radford 2009).…”

What you find depends on where you look: responses to proximate habitat vary with landscape context

“…Amount of habitat in the landscape has long been understood to influence species occurrence, with landscape-scale effects operating simultaneously with proximate-scale effects (e.g., Cunningham et al 2006, Desrochers et al 2010, Zitske et al 2011. Landscape-scale habitat amount also can influence local-scale edge effects (Thompson et al 2002). We examine whether landscapescale habitat amount influences habitat occupancy at the proximate scale.…”

“…Studies of habitat responses of bird species have sometimes produced inconsistent findings regarding sensitivity to habitat area, edges, and other aspects of habitat composition or fragmentation (Villard 1998, Thompson et al 2002, Bayard and Elphick 2010, van der Hoek et al 2013, Vetter et al 2013). This inconsistency limits our ability to address both basic and applied questions, such as differences in population vulnerability in the face of landscape change, variation among species in habitat area requirements, plasticity in habitat use, and the likely effectiveness of conserving one habitat area versus another.…”

“…We do not focus here on the relative importance of habitat amount and habitat configuration, a question that has been explored extensively elsewhere (Thompson et al 2002, Fahrig 2013, Villard and Metzger 2014 and that continues to be debated (Fahrig 2015, Hanski 2015. Indeed, Lindenmeyer and Fischer (2007) and Didham et al (2012) have argued that while this dichotomy has become entrenched in our understanding of habitat change, it has not always been useful for furthering conservation goals (see also Villard and Metzger 2014).…”

“…The BBS shows the value of incorporating data from throughout the continent to gain a broader understanding of population trends. Further studies that compare multiple study areas are important for improving our understanding of fundamental species ecology (van der Hoek et al 2013, Vetter et al 2013. For example, if one particular landscape factor, such as edge density or habitat area, is important in one region, is its influence similar in others?…”