Detecting Ecological Traps in Human-Altered Landscapes: A Case Study of the Thick-billed Longspur Nesting in Croplands
Amber E. Swicegood, Department of Animal and Range Sciences, Montana State University, Bozeman, MT, USA
Kevin S. Ellison, Program Manager, Northern Great Plains, American Bird Conservancy, Bozeman, MT, USA
Marisa Sather, Partners for Fish and Wildlife Program, U.S. Fish and Wildlife Service, Glasgow, MT, USA
Scott G. Somershoe, Division of Habitat Conservation, Land Bird Coordinator U.S. Fish and Wildlife Service Migratory Birds Program, Denver, CO, USA
Lance B. McNew, Department of Animal and Range Sciences, Montana State University, Bozeman, MT, USA
Correspondence : Lance McNew, Montana State University, 103 Animal Bioscience Building, Bozeman, MT 59717, USA. Email: lance.mcnew@montana.edu
ABSTRACT Conversion of the North American prairies to cropland remains a prominent threat to grassland bird populations. Yet, a few species nest in these vastly modified systems. The thick-billed longspur (Rhynchophanes mccownii ) is an obligate grassland bird whose populations have declined 4% annually during the past 50 years. Thick-billed longspurs historically nested in recently disturbed or sparsely vegetated patches within native mixed-grass prairie, but observations of longspurs in crop fields during the breeding season suggest such fields also provide cues for habitat selection. Maladaptive selection for poor quality habitat may contribute to ongoing declines in longspur populations, but information on thick-billed longspur breeding ecology in crop fields is lacking. We hypothesized that crop fields may function as ecological traps; specifically, we expected that crop fields may provide cues for territory selection but frequent human disturbance and increased exposure to weather and predators would have negative consequences for reproduction. To address this hypothesis, we compared measures of habitat selection (settlement patterns and trends in abundance) and productivity (nest density, nest survival, and number of young fledged) between crop fields and native sites in northeastern Montana, USA. Settlement patterns were similar across site types and occupancy ranged from 0.52 ± 0.17SE to 0.99 ± 0.01 on April 7 and 30, respectively. Early season abundance differed by year and changes in abundance during the breeding season were associated with precipitation-driven vegetation conditions, rather than habitat type. Standardized nest density (0.19 ± 0.27SD nests/plot/hour), the number of young fledged per successful nest (2.9 ± 0.18SE), and nest survival (0.24 ± 0.03 SE; n=222 nests) were similar for crop and native sites. Collectively, the data did not support our hypothesis that crop fields are ecological traps: longspurs did not exhibit a clear preference for cropland and reproductive output was not significantly reduced. Our results indicate that crop fields provide alternative breeding habitat within a human-dominated landscape.
KEYWORDS Agricultural landscapes, ecological trap, grassland birds, maladaptive selection, Rhynchophanes mccownii , thick-billed longspur

1 | INTRODUCTION

In North America, grassland birds have experienced steeper long-term declines than any other avian guild during the past 50 years (Rosenberg et al., 2019; Sauer, 2020) and more than 79% of grasslands have been lost since the early 1800s (Samson and Knopf, 1994; White, 2000). While factors such as fire suppression, overgrazing, desertification, and the introduction of non-native plant species have contributed to degradation of native prairies, conversion to large-scale, row-crop agriculture remains one of the greatest threats to grassland ecosystems (Knapp et al., 1999; Blann, 2006; Ellis et al., 2010; Wright and Wimberly, 2013). Indeed, intensification of agricultural practices is considered a leading driver of grassland bird population declines worldwide (Wilson et al., 2005; Quinn et al., 2017; Davis et al., 2020). While row-crop monocultures support lower avian diversity than native grasslands (Best et al., 1997; Davis et al., 2020), the few species that nest in crop fields face a myriad of hazards. Farming operations (e.g., tilling, discing, harvest) may result in nest destruction (Devries et al., 2008; Santangeli et al., 2018). Soil instability in crop fields may lead to nest failures during floods and heavy rains (Van Pelt et al., 2017). Herbicide and pesticide applications may directly harm adults and nestlings or may indirectly affect bird populations by reducing invertebrate food resources (Pimentel et al., 1995; Loss et al., 2015). In addition, most crops grow rapidly into dense monocultures, with vegetation conditions changing from short-stature (5–15 cm tall) with large amounts of bare ground to tall (60–70 cm), closed-canopy conditions within 2–3 months of seeding (Wilson et al., 2005).
Ecological traps occur when there is a mismatch between habitat selection cues and habitat quality (Battin, 2004) and are most commonly identified where human activities produce novel environmental cues or alter habitat quality associated with a particular cue (Robertson et al., 2013; Hale and Swearer, 2016; Simon and Fortin, 2019). An ecological trap differs from a demographic sink in that animals often exhibit strong preference for trap habitat (Pulliam, 1988; Gilroy et al., 2011). Such maladaptive selection leads to negative fitness consequences and reduced population viability (Schlaepfer et al., 2002; Battin, 2004; Gilroy et al., 2011). The ideal free distribution theory that underpins source-sink population models assumes animals exhibit optimal habitat selection when distributing themselves among habitat patches and that the fittest individuals obtain the highest-quality territories. In reality, individuals likely select habitat according to evolutionarily predisposed cues and ecological traps are attractive because they provide such cues (Delibes et al., 2001; Abrams et al., 2012; Fletcher Jr et al., 2012; Hale et al., 2015; Hale and Swearer, 2016).
The thick-billed longspur (Rhynchophanes mccownii ; hereafter “longspur”) is a grassland songbird endemic to the short- and mixed-grass prairies of North America (Knopf, 1996). Like most obligate grassland birds, populations of thick-billed longspurs have declined precipitously (4% per year on average; Figure 1) since the advent of the North American Breeding Bird Survey (Rosenberg et al., 2019; Sauer, 2020), but mechanisms driving the decline are poorly understood. Habitat for longspurs is patchy within native mixed-grass prairie, limiting distributions at regional scales and making this a focal species for federal conservation efforts (Somershoe, 2018). Unlike many other grassland birds, longspurs have a unique preference for recently disturbed or sparsely vegetated habitats, and historically relied on large-scale disturbance regimes to maintain suitable habitat patches through spatial-temporal interactions of soil, precipitation, fire, and intensive periodic defoliation by native herbivores (e.g., bison [Bison bison ], locusts [chiefly Melanoplus spretus ]) (Mickey, 1943; Felske, 1971; Samson et al., 2004; McLachlan, 2007; Shaffer et al., 2019; With, 2021). However, these dynamic processes that once shaped prairie ecosystems are largely absent in today’s Northern Great Plains (Samson and Knopf, 1996; Fuhlendorf and Engle, 2004; Samson et al., 2004; Hovick et al., 2015). Modern rangeland management is focused on preventing overgrazing, maintaining desirable plant communities, and homogenizing defoliation at consistent and moderate levels to create sustainable and profitable livestock production (Briske et al., 2005; Toombs et al., 2010; Holechek, 2011; Fuhlendorf et al., 2012). The resulting landscapes often support habitat for a narrower suite of wildlife species (Samson and Knopf, 1996; Samson et al., 2004; Fuhlendorf et al., 2012) and those that require habitat found at the extremes of the disturbance-succession gradient, including longspurs, likely do not benefit from modern rangeland management.
Row-crop agriculture is the dominant disturbance factor producing extensive bare ground at large scales in the Northern Great Plains. Sporadic reports of longspurs occurring in crop fields during the breeding season (Felske, 1971; Shaffer et al., 2019) suggest that crop fields may resemble suitable nesting habitat as longspurs arrive on the breeding grounds in April. However, the selection of crop fields during territory establishment could contribute to population declines if reproductive success is depressed through 1) destruction of nests by discing, seeding, and herbicide/pesticide application, and 2) abandonment of nests or territories in response to rapid changes in vegetation conditions or disturbance by farming activity. If crop fields provide attractive nesting habitat where longspurs experience low reproductive success, modern agricultural landscapes may be operating as ecological traps for this short-grass prairie specialist.
Identifying an ecological trap involves demonstrating habitat preference (i.e., disproportionate selection) that results in reduced vital rates (e.g., nest survival). Importantly, high population densities do not necessarily equate to preference for that habitat type (Van Horne, 1983; Vickery et al., 1992) and comparisons of relative selection metrics between habitat types are needed to discern preferences. A demonstrated preference for crop fields coupled with reduced reproductive success relative to native prairie would indicate an ecological trap with significant implications for longspur conservation (Lloyd and Martin, 2005; Robertson and Hutto, 2006). Delayed settlement in crop fields after native sites are occupied, coupled with lower reproductive success, would suggest crop fields simply provide spill-over habitat, which is characteristic of source-sink population dynamics (Gilroy and Sutherland, 2007).
We conducted a 2-year field study to evaluate whether crop fields operate as ecological traps for nesting longspurs within the core of their breeding distribution in northeastern Montana, USA (Figure 2). We compared settlement patterns, temporal trends in abundance, and nest density between crop and native sites, assuming that earlier settlement and higher use of crop fields indicated that either longspurs preferred crop sites for nesting or that native sites were limited (Robertson and Hutto, 2006). We measured reproductive success in both habitat types to quantify the relative quality of crop fields as breeding habitat for longspurs. Our objectives were to compare the following in crop and native sites: 1) settlement patterns of territorial male longspurs, 2) longspur abundance and nest densities, as well as changes in abundance over the breeding season, 3) nest survival and fledging success, and 4) structural changes in vegetation during the breeding season.

2 | MATERIALS AND METHODS

2.1 | Study Area

The study area was located within the core of the remaining breeding distribution of longspurs in northern Valley County, Montana, USA (Figure 2). The climate in this region is semi-arid with long, cold winters and short, hot summers producing frequent thunderstorms, hail, and flash floods (Cooper et al., 2001). Average daily temperatures range seasonally from below 0° C to 20–25° C. Annual precipitation averages 25–35 centimeters and typically comes as rain in late May and early June (Lenard et al., 2006; PRISM 2022). The region is at about 915 meters in elevation. Clay shale is the most abundant substrate and the landscape is dominated by glacial till (Cooper et al., 2001). The study area incorporated cultivated crop fields in the eastern portion and native mixed-grass prairie in the western portion. Federal lands and private ranches occurred west of Opheim and Baylor, and the Bitter Creek Wilderness Study Area, characterized by its undeveloped badlands, was located at the center of the study area.
Native grassland in this region is classified as northern mixed-grass prairie (Coupland, 1961; Charboneau et al., 2013). Cool season grasses dominated and common species included western wheatgrass (Pascopyrum smithii ), needle-and-thread (Hesperostipa comata ), prairie junegrass (Koeleria macrantha ), green needlegrass (Nassella viridula) , Sandberg bluegrass (Poa secunda ) and threadleaf sedge (Carex filifolia ). One warm season grass, blue grama (Bouteloua gracilis ), was present at some sites. Spikemoss (Selaginella densa ) was locally abundant as well. Shrub cover was low-moderate across most of this region (Charboneau et al., 2013).
Northern Valley County is characterized by large expanses of poor soils unsuitable for cultivation and more productive areas used for row-crop production. Arid patches within large tracts of native mixed-grass prairie provide habitat for longspurs, known to be locally abundant in such areas. These native sites consist of aridic, well-drained glacial soils of the Elloam series and have relatively low (<1000 kg ha-1) vegetation production potential (Lenard et al., 2006; Lipsey, 2015). Primary crops in the region included spring wheat, barley, lentil, pea, flax, and canola. In some areas wheat farming was conducted on a 50:50 rotation with 50% of the acreage fallowed each year to conserve soil moisture and nutrients (M. Sather, USFWS, pers. comm.). Pulse crops such as peas and lentils are often planted between wheat rotations to add nitrogen, conserve soil moisture, and disrupt weeds, pests, and diseases (Miller et al., 2002; Long et al., 2014). Pulse crop rotations have replaced summer fallow over most of the region (M. Sather, USFWS, pers. comm.).