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.).