Abstract
As demand for food increases, agricultural production is poised to
increase dramatically. Pesticides are commonly used to maintain high
crop yield, though they have several drawbacks, including reduced
efficacy over time and harmful effects to human and ecosystem health.
Bats are highly effective predators of crop pests and have great
potential to reduce crop damage resulting from insects. However, few
studies have investigated how pest control by bats might change over
large expanses of continuous row crop agriculture, which is common in
the Midwest. Agricultural landscapes offer few roosting opportunities
and could be difficult for bats to traverse, which might affect the
degree of ecosystem services provided by bats. We hypothesized that with
increasing distance into uninterrupted row crop agriculture 1) bat
activity would decrease and 2) bat species richness would decrease, but
that these effects would be buffered when insect pest abundance is high.
We deployed 50 acoustic bat detectors over 10 transects in east-central
Illinois from July to September 2021. In each transect, we placed
detectors on crop field edges at increasing distance from a large
riparian corridor assumed to be frequented by bats. Bat activity was
high across the landscape but declined by 56% from the forest edge to
4000m into row crop agriculture, while bat diversity decreased by 34%.
Pest abundance seemed to decrease overall bat activity but had no effect
on bat diversity. These results indicate that bats, although able to
have a large effect on crop pest reduction, might not be as efficient in
suppressing crop pests in vast uninterrupted agricultural landscapes
which offer scant roost availability. Our work will inform
recommendations to landowners and private lands managers on ways to
increase bat access to roosts and forest cover near agricultural areas,
thereby enhancing the potential for bats to provide pest control
services.
Keywords : Ecosystem services, Agricultural intensification,
Pest suppression, Diversity, Forest management
Introduction
As the human population grows and the demand for food increases, there
will be a greater need for more efficient agricultural land use (Godfray
et al., 2010). Tilman et al. (2001) predicted that by 2050 one billion
hectares of land would be converted to agriculture and anticipated a
2.4–2.7-fold increase in water eutrophication caused by the concomitant
increase in fertilizer and pesticide use. Despite intensification via
industrial monocropping, agricultural yields have not kept up with
demand (Ray et al., 2013). This is in part due to an increase in crop
pest abundance as a result of large-scale monocropping, which
contributes to so-called yield gaps (Chaplin-Kramer et al., 2011).
Harnessing the potential of natural predators to suppress crop pest
populations is an efficient and cost-effective strategy to increase
yields. Such a process of ecological intensification – increasing
agricultural yield without also increasing anthropogenic inputs – would
reduce our reliance on pesticides, which are less effective because many
pests have developed resistance over time (Bommarco et al., 2013). Bats
have been proposed as an important pest control species in agricultural
landscapes, potentially worth 3 billion USD to the US agricultural
industry alone (Boyles et al., 2011). In this study, we investigate how
bat activity patterns change over agricultural landscapes in relation to
distance from forested riparian corridors to better understand the
spatial scope of crop pest control by bats.
Agricultural pests have pervasive impacts on various forms of
agricultural production from animals to crops. Globally, an estimated
30% of crop loss is attributed to pests in both soybeans and corn, the
two main crops produced in the Midwest (Oerke, 2005). Protecting crops
against animal pests remains difficult and costly, and relies heavily on
synthetical chemicals (Oerke, 2005). Globally, the associated costs with
controlling for these pests can surpass $10 billion yearly, with an
additional $12 billion in external damages (Pimentel, 2009). These
estimated costs could be inflated due to forecasted increases in
agricultural pest abundance due to climate change (Diffenbaugh et al.,
2008). Therefore, efforts to enhance natural pest control are
imperative. Ecological intensification of agricultural landscapes
reestablishes the ecosystem services derived from natural systems,
thereby reducing costs of pest control while increasing crop yield
(Bommarco et al., 2013). Using these biodiversity-based methods has been
proven to sustainably increase crop yields (Maine and Boyles, 2015;
Maslo et al., 2022; Tschumi et al., 2016).
All bats in the midwestern region of the USA are insectivores (Whitaker
and Hamilton, 1998). These bats are voracious arthropod predators that
can consume up to their body weight in arthropods over the course of a
single night (Kunz et al., 1995). Bats may even exert a stronger
top-down effect on insect abundance than do birds, as has been found in
forest systems (Kalka et al., 2008). It is widely recognized that bats
have significant impacts on crop pest species and therefore provide
valuable ecosystem services (Boyles et al., 2011; Maslo et al., 2022;
Whitaker, 1995). The diet of common species such as the big brown bat
(Eptesicus fuscus ) includes 160 known agricultural pest or
disease vectors, including Helicoverpa zea , Spodoptera
frugiperda , and Chloridea virescens (Hughes et al., 2021; Maslo
et al., 2022). The effects of bats on pest control are multi-layered
because by foraging on the adult stages of pests like moths, bats
indirectly suppress egg production (McCracken et al., 2012). One study
found that bats not only suppressed pest larval densities in corn, they
furthermore contributed to the suppression of pest-associated mycotoxins
and fungal growth (Maine and Boyles, 2015). These agricultural ecosystem
services in the United States alone have been estimated to be worth
between 3.7 and 53 billion dollars annually (Boyles et al, 2011).
Ironically, bats are threatened by the very industry they help:
agricultural intensification is considered a major contributor to
pervasive bat declines in Europe and North America (Frick et al., 2020;
Wickramasinghe et al., 2003). Bats are further threatened by rampant
wind energy development (Davy et al., 2021; Friedenberg and Frick, 2021)
and, in North America, white-nose syndrome (Frick et al., 2020; Hoyt et
al., 2021). Urgent conservation action is needed to promote these
species in agricultural landscapes if they are to continue their
significant contribution to pest control.
Bats are not homogeneously distributed across the landscape. For
example, bat activity increases with increasing landscape heterogeneity
(Burgar et al., 2017; Chaplin-Kramer et al., 2011; Monck-Whipp et al.,
2018). Woodland interfaces are important predictors of bat activity
(Fill et al., 2022), and one study found that bat activity is 5 times
higher in landscapes containing a double row of trees compared to
standalone trees (Kalda et al., 2015). Bat species richness also varies
with landscape structure. Due to species-specific characteristics,
including wing aspect ratio and body size, some species prefer foraging
in cluttered habitat (Aldridge and Rautenbach, 1987). Although bats are
generalists, different species have specific diet preferences, and
larger bats tend to consume larger prey such as agricultural pests
(Hughes et al., 2021; Kunz et al., 2011). Because of their structural
complexity, woodlands and wooded corridors offer habitat to bats with
differing foraging behaviors and, therefore, often have higher bat
diversity than uninterrupted row crop agricultural landscapes (Kalda et
al., 2015). Indeed, bats prefer to forage along linear features in
agricultural landscapes, such as natural vegetation remnants and
farmland edges (Lentini et al., 2012). The sometimes surprisingly long
distances between potential roost locations (i.e., large trees in forest
patches) in the vast agricultural landscapes often found in the Midwest
likely presents a formidable challenge to bats foraging over
agricultural fields. Higher predation risk from raptors and owls in open
landscapes could also decrease the likelihood of bats venturing out into
agriculture (Lima and O’Keefe, 2013; Mikula et al., 2016; Murray and
Kurta, 2004). To better understand how bats contribute to agricultural
pest control, and how we can improve the efficacy, we require more
nuanced information on bat activity in relation to landscape
characteristics (Boyles et al., 2013).
We studied bat activity and diversity in central Illinois, USA in
relation to expansive, uninterrupted agriculture. We hypothesized that
both bat activity and bat diversity would decline with increasing
distance from forested corridors, but that this effect would be buffered
by insect pest abundance. With respect to overall bat activity, we
hypothesized that activity would decline with increasing distance from
large forested riparian corridors, which we assume to be favorable
roosting and foraging areas. However, we suspected this effect would be
buffered by pest insect species abundance, such that bats would be more
active further away from riparian corridors when pest abundance is high
than when pest abundance is low. Furthermore, we hypothesized that there
would be variable responses from bat species, such that larger bats
would be more active and persistent in areas further into uninterrupted
agriculture, while smaller species would be restricted to areas around
the riparian corridors, and that larger bat species would increase
activity rates in response to high pest abundance more so than smaller
bat species. With respect to bat diversity, we hypothesized that bat
diversity would decline with increasing distance from riparian
corridors, but that the magnitude of this effect would decrease with
increasing pest abundance.