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Conserving pollinators in North American forests: A review

11 Oct 2016-Natural Areas Journal (Natural Areas Association)-Vol. 36, Iss: 4, pp 427-439
TL;DR: Efforts to improve forest conditions for pollinators should consider the needs of specialist species and vulnerable species with small scattered populations, and forests may serve as reservoirs of pollinators for recolonization of surrounding habitats.
Abstract: Bees and butterflies generally favor open forest habitats regardless of forest type, geographic region, or methods used to create these habitats. Dense shrub layers of native or nonnative species beneath forest canopies negatively impact herbaceous plant cover and diversity, and pollinators. The presence of nonnative flowers as a source of nectar, pollen, or larval food can have positive or negative effects on pollinators depending on the situation, but in cases where the nonnatives exclude native plants, the results are almost always negative. Roads and roadside corridors offer an opportunity to increase open, pollinator-friendly habitat even in dense forests by thinning the adjacent forest, mowing at appropriate times, and converting to native herbaceous plant communities where nonnative species have been planted or have invaded. Efforts to improve forest conditions for pollinators should consider the needs of specialist species and vulnerable species with small scattered populations. Conservation of bees and butterflies, as well as other pollinating species, in forested areas is important for most forest plant species, and forests may serve as reservoirs of pollinators for recolonization of surrounding habitats.

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Conserving Pollinators in North American Forests: A Review
Author(s): James L. Hanula Michael D. Ulyshen Scott Horn
Source: Natural Areas Journal, 36(4):427-439.
Published By: Natural Areas Association
DOI:
http://dx.doi.org/10.3375/043.036.0409
URL: http://www.bioone.org/doi/full/10.3375/043.036.0409
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Volume 36 (4), 2016 Natural Areas Journal 427
ABSTRACT: Bees and butterflies generally favor open forest habitats regardless of forest type, geographic
region, or methods used to create these habitats. Dense shrub layers of native or nonnative species beneath
forest canopies negatively impact herbaceous plant cover and diversity, and pollinators. The presence
of nonnative flowers as a source of nectar, pollen, or larval food can have positive or negative effects
on pollinators depending on the situation, but in cases where the nonnatives exclude native plants, the
results are almost always negative. Roads and roadside corridors offer an opportunity to increase open,
pollinator-friendly habitat even in dense forests by thinning the adjacent forest, mowing at appropriate
times, and converting to native herbaceous plant communities where nonnative species have been
planted or have invaded. Efforts to improve forest conditions for pollinators should consider the needs
of specialist species and vulnerable species with small scattered populations. Conservation of bees
and butterflies, as well as other pollinating species, in forested areas is important for most forest plant
species, and forests may serve as reservoirs of pollinators for recolonization of surrounding habitats.
Index terms: fire, forest management, invasive species, prescribed burning, verges
INTRODUCTION
Nearly 90% of the world’s flowering plants
rely on pollination by animals (Ollerton et
al. 2011), and of those, bees are consid-
ered to be the primary group responsible
(Winfree et al. 2011). Native pollinators
provide most of the pollination in forests
and grasslands of the United States (Mader
et al. 2011), where many wild forb and tree
species require their services. Additionally,
native pollinators from these natural areas
contribute substantially to the pollination
of adjacent crops, often without the need
for managed honey bees (Garibaldi et al.
2013; Morandin and Kremen 2013). The
consensus among experts is that pollinators
are in decline, and publication of “The For-
gotten Pollinators” (Buchmann and Nabhan
1996) raised awareness of the problem.
Bees, flies, and butterflies are considered
the best native pollinators, and the Unit-
ed States alone has approximately 4000
species of bees (Moisset and Buchmann
2011) and 575 species of butterflies (NABA
2016). Although evidence is growing that
many pollinators and their functions are
declining (Potts et al. 2010; Burkle et al.
2013), not enough information is available
to assess the conservation status of most
species (National Research Council 2007).
Nevertheless, the Xerces Society lists 31
species of bees (Xerces Society 2016a) and
58 species of butterflies (Xerces Society
2016b) in North America that are vulnera-
ble, imperiled, critically imperiled, or even
possibly extinct. Of the butterflies, 24 are
listed as federally endangered. Some evi-
dence indicates that while at least one of the
46 bumble bee species known to occur in
North America has gone extinct, half may
now be at risk (Grixti et al. 2009; Williams
et al. 2014). Other bee genera have received
less attention, despite accounting for >95%
of known species (Bartomeus et al. 2013)
and playing essential roles as pollinators
of most native tree and forb species in our
forests. A study using historical data sets
found a 50% reduction in bee species over a
120-year period, resulting in major changes
to the plant-pollinator network (Burkle et
al. 2013). This underscores the paucity of
information on the status of most native
bees in North America (Cane and Tepedino
2001). The many factors implicated in the
declines of bee and butterfly populations
include habitat fragmentation, nonnative
plants, pathogens, nonnative insects, bio-
control agents, overgrazing by white-tailed
deer, herbicides and insecticides, fire (too
frequent), shrub encroachment due to fire
suppression, right-of-way management,
harvesting of wild plants, logging of ma-
ture forests, and losses of open forests and
forest clearings (van Swaay et al. 2006;
Miller and Hammond 2007; Cameron et
al. 2011; Schweitzer et al. 2011; Szabo et
al. 2012; Fartmann et al., 2013).
Forests currently cover more than one
third of the land area in North America
(World Bank 2016) and provide important
resources for many pollinators. In addition
to supporting forest specialists (Winfree et
al. 2007), a large number of generalists are
known to move readily between forests,
agricultural fields, and other land-use
types (Blitzer et al. 2012; Monasterolo et
al. 2015). Some forest conditions favor
pollinators more than others and there is
a growing interest in optimizing manage-
ment practices for pollinator conservation.
Natural Areas Journal 36:427–439
2
Corresponding author: mulyshen@fs.fed.
us; 706-559-4296
Conserving
Pollinators in North
American Forests: A
Review
James L. Hanula
1
1
USDA Forest Service
Southern Research Station
320 Green Street
Athens, GA 30602
Michael D. Ulyshen
1,2
Scott Horn
1

428 Natural Areas Journal Volume 36 (4), 2016
Over the past century, forests of the United
States have undergone dramatic changes.
Forests were altered by extensive logging
and repeated wildfires in the late 19th and
early 20th century (Ahren 1929, 1933).
Bohart (1971) suggested that this forest
clearing, at least in eastern North America,
resulted in higher bee populations than
existed prior to European arrival. Support
for this idea comes from more recent work
by Winfree et al. (2007), who found a
negative correlation between bee numbers
and forest cover in the northeastern United
States. Reforestation and fire exclusion to
prevent wildfires were implemented over
large areas to restore ecosystems degraded
by earlier logging and agricultural practices
(Lilliard 1947; Clark 1984; Williams 1989;
Stanturf et al. 2002; Barnett 2014). These
practices, which promote high planting
densities, increased tree growth, and
continued fire exclusion, have resulted
in unnaturally dense stands with closed
canopies and dense shrub layers beneath
(Carroll et al. 2002; Schwilk et al. 2009).
As part of the “National Strategy to
Promote the Health of Honey Bees and
Other Pollinators,” the federal gov-
ernment released “Pollinator-Friendly
Best Management Practices for Federal
Lands” (USDA 2016). These management
practices were developed from the most
current scientific research; however, they
emphasize the need for updates as new
evidence becomes available. Thus, as the
practices are implemented and evaluated,
it is expected that they will be revised to
reflect new findings. The primary goal of
this article is to review what is currently
known about conserving pollinators in
forested regions of North America, with
a focus on how pollinators are impacted
by common forest management practices,
roadside and powerline corridors, and
nonnative plant species. The objectives
of pollinator conservation can range from
maintaining the greatest number of spe-
cies possible, maximizing an ecosystem
service, or sustaining viable populations
of endangered species. It is important to
recognize that no single approach can
be expected to benefit every species of
pollinator, given differences in host and
habitat requirements, nesting behavior,
and other life history characteristics. While
we seek to identify general patterns based
on existing evidence, we necessarily stop
short of making specific management rec-
ommendations. The optimal management
plan for a particular location will depend
on a variety of local factors, including
conservation priorities, forest type, land-
use history, etc. Using this article as a
starting point, managers are encouraged to
consult local experts or delve deeper into
the literature most relevant to their focal
organism(s) and system of interest. This
review is limited to pollinating insects,
with bees and butterflies dominating the
current literature.
Aside from sharing a need for floral re-
sources, the ecology of butterflies differs
from that of bees in some important ways.
For example, bees require nectar and pollen
throughout their life cycle, while butterflies
only utilize nectar as adults. Most larval
lepidopterans (butterflies and moths) are
leaf-feeders that do not require any parental
provisioning of floral resources. Bees, by
contrast, must collect sufficient pollen and
nectar to support their developing brood
as well as their own energy needs. While
most bee species develop in underground
nests or in other relatively protected plac-
es, butterfly caterpillars are exposed on
their host plants where they may be more
sensitive than most bees to management
tools like prescribed fire or mowing. Con-
servation efforts aimed at both butterflies
and bees should keep these differences in
mind (Alanen et al. 2011).
In this article, we consider a forest condi-
tion or management practice to generally
benefit pollinators when it results in a
measureable increase in the number of
species and/or abundance of bees and/or
butterflies. Despite our focus on community
responses, we recognize that conservation
goals will vary among study systems and
may sometimes be limited to particular
species of concern. Moreover, it should be
noted that abundance alone may not always
be the best metric with which to gauge
an impact. Reproductive performance,
for instance, can sometimes be more
meaningful (Palladini and Maron 2014).
Finally, although our focus is centered on
conserving pollinators in North American
forests (excluding Mexico), key references
from other Northern-hemisphere temperate
forests are also considered. This review is
organized into three main sections. First we
discuss the effects of forest management
on pollinators, with a focus on thinning/
gap creation and prescribed fire. Next, we
consider the value of roadside and power-
line corridors and how best to manage these
avenues of open habitat. Finally, we review
the variable effects of nonnative species
on bee and butterfly communities before
ending with some concluding thoughts.
FOREST MANAGEMENT
As a group, pollinators are generally more
abundant in open forests relative to closed
forests (Fye 1972), although information
on forest-obligate species remains limit-
ed (Winfree 2010). Temperature and the
amount of light within a habitat are the
most important abiotic factors affecting
foraging by bees (Herrera 1997; Polatto
et al. 2014), and soil-nesting bees seem
to benefit from patchy ground with ample
sun exposure (Vaughan et al. 2015). With
some important exceptions, butterflies are
generally more numerous in nonforested
habitats than in forests (Schmitt 2003;
Miller and Hammond 2007; Schweitzer
et al. 2011), and, like bees, benefit from
more open forest conditions. In a study of
successional stages following coppicing
(i.e., harvesting young stems sprouting
from the roots of previously-cut trees)
in France, for instance, higher butterfly
species richness and density occurred in
relatively open early to mid-successional
stages compared to more closed canopy
late-successional stages (Fartmann et al.
2013). This pattern held true for resident
and migratory species, as well as threatened
species, and was attributed to warmer con-
ditions and greater availability of nectar and
larval host plants in the more open forests.
Likewise, Benes et al. (2006) showed that
the transition from relatively open forests
to closed-canopy forests brought about by
abandonment of coppicing in the Czech
Republic negatively impacted butterflies.
More recently, Hanula et al. (2015) ex-
amined seven forest types typical of the
Piedmont region of the southern United
States and found that lower leaf area in-
dex (i.e., more light) resulting from lower

Volume 36 (4), 2016 Natural Areas Journal 429
stand densities (basal area) was associated
with greater numbers and species richness
of bees. The highest species richness and
bee abundance occurred in mature pine
forests with open canopies and little shrub
cover, which are created and maintained
by regular thinning and frequent prescribed
burning (Figure 1). High species richness
and bee abundance also occurred in re-
cently cleared forests. In contrast, mature
pine forests with similar canopy cover,
but a dense shrub layer, had fewer bees
and fewer species because of the shading
provided by the shrubs (Figure 1). Forests
that provided the best long-term pollinator
habitat had high herbaceous ground cover
and were being managed as foraging habitat
for red-cockaded woodpeckers (Leucono-
topicus borealis del Hoyo and Collar), an
endangered species.
Although traditionally used to achieve
different objectives, several widely used
forest management techniques, such as
thinning, or prescribed fire (see below),
result in more open forests and have the
potential to benefit pollinator communities.
Encouragingly, this suggests improving
forest conditions for pollinators may be
consistent with other management goals.
Thinning and Gap Creation
Forests have traditionally been thinned (i.e.,
the selective removal of trees to reduce
tree density) to improve tree vigor, which
results in increased growth rates and a lower
incidence of pest outbreaks. By making
forests more open, however, thinning may
also result in benefits to pollinators. Simi-
larly, certain management approaches (e.g.,
group-selection harvests) create gaps in the
forest canopy, resulting in open areas that
may also benefit pollinators. Indeed, both
thinning and gap creation have consistently
been shown to benefit pollinators in a vari-
ety of forest types across North America.
For example, Romey et al. (2007) examined
the effects of small scale (approximately
2 ha) tree removals from a northern hard-
wood forest in New York resulting in 30,
60, and 100% overstory tree removal and
found the greater the forest cover removed,
the higher the bee abundance and diver-
sity in the openings. All three treatments
increased bee community attributes over
untreated controls. In the pinyon-juniper
woodlands of the southwestern United
States, Kleintjes et al. (2004) studied the
effect of thinning the overstory canopy
by 70% followed by mulching of the
Figure 1. Upper left is upland hardwood forest with a dense shrub layer consisting of native species; to its right, the same forest after the shrub layer was
cut and subsequently burned (Photos by T. Waldrop) creating improved pollinator habitat for both bees and butterflies (Campbell et al. 2007). Lower left
is a mature loblolly pine stand with a dense midstory of shrubs and small trees that was poor habitat for bees; lower right is a similar stand that has been
frequently burned and provides good habitat for bees (Hanula et al. 2015).

430 Natural Areas Journal Volume 36 (4), 2016
logging residue and found the treatment
increased both forb and grass cover, and
butterfly species richness and abundance.
Also working in the southwestern United
States and finding a similar result, Waltz
and Covington (2004) reported increased
butterfly richness and abundance in thinned
and burned ponderosa pine forests com-
pared to untreated control stands. In that
study, the researchers saw few differences
in plant community composition between
the two treatments, suggesting that but-
terflies may be responding to the sunnier
conditions in the thinned forest treatment.
In Japan, Taki et al. (2010) found thinning
to result in higher numbers of both bees
and butterflies in Cryptomeria plantations.
Proctor et al. (2012) studied group selection
harvesting in northern hardwood forests
of Ontario, Canada, and found gaps had
more bees than intact forest and the two
habitats had different bee community
composition but similar numbers of bee
species. In the Czech Republic, Slamova
et al. (2013) found that a butterfly species
of conservation concern, Erebia aethiops
Esper, is threatened by canopy closure.
Interestingly, males were more numerous
in sparsely wooded areas, whereas female
densities were highest in grassland patches,
indicating the need for both habitat types.
In Germany, Hermann and Steiner (2000)
argued that the light-demanding forest
butterfly species Satyrium ilicism Esper is
facing extinction due to the abandonment
of practices such as coppicing that helped
maintain open forest conditions.
The interface between fields and forests is
often abrupt and there is great interest in
improving this transition zone to benefit
pollinators. Korpela et al. (2015) compared
pollinator communities in forest edges
that had been partially cleared of trees to
unmanipulated reference edges in an effort
to understand the effects of this field-forest
ecotone on pollinators in Finland. The
treatments involved clear-cutting 5 m into
the forest and thinning for an additional 20
m. Both treatments resulted in greater bum-
blebee abundance, total pollinator species
richness, and abundance of butterfly habitat
specialists relative to the reference. These
effects were most apparent at the clear-cut
edge than in the thinned areas, especially
for bumblebee abundance, presumably
due to improved microclimate and greater
floral resources in cleared areas. Floral
resource availability appeared to be less
crucial to butterflies than to bumblebees in
the study, suggesting these insects respond
more to open and warmer conditions. This
may reflect the fact that bumblebees must
collect enough nectar and pollen to support
their brood, whereas butterflies only need
enough nectar to fuel their own activities
(Korpela et al. 2015).
Prescribed Fire
Prescribed (i.e., controlled) fires are widely
used in forests to suppress the shrub-layer
and reduce fuel loads, thus stimulating
herbaceous vegetation and minimizing
the risk of wildfires. Studies from a wide
range of temperate forest types indicate that
prescribed fire can be a highly effective tool
in improving forest habitat for both bee and
butterfly communities. In the southwestern
United States, Nyoka (2010) compared
treatments designed to reduce wildfire
risk, which included thinning from below,
prescribed burning, and thinning followed
by prescribed burning. Only thinning plus
burning resulted in higher numbers of bees
as well as greater cover and species richness
of flowering plants compared to untreated
areas. Similar results were found for but-
terflies in ponderosa pine forests, where
Waltz and Covington (2004) reported sig-
nificantly greater numbers of species and
individuals from thinned and burned stands
compared to untreated controls. Huntzinger
(2003) reported many times more butter-
fly species in burned forests compared to
unburned forests at sites in Oregon and
California. These results were attributed
to higher total areas of sunlit patches in
burned forests. Working in southern Ap-
palachian hardwood forests, Campbell et
al. (2007a) found similar results when they
compared removal of a native understory
shrub, which was the dominant component
of the understory as a result of long-term
fire exclusion, to prescribed burning and
shrub removal followed by burning. Both
butterfly and bee communities responded
positively to the combined treatments, but
not to the treatments individually. Exam-
ining the underlying reasons showed that
the combined treatment resulted in hotter
prescribed fires that killed some of the
overstory trees, essentially thinning the
forest and reducing canopy cover. Wagner
et al. (2003) stressed the need for prescribed
fire, mechanical cutting, or a combination
of both for maintaining the open-canopy
shrubland barrens required by rare butter-
fly and moth species in northeastern US
forests. Strahan et al. (2015) examined the
plant community composition in ponderosa
pine forests following thinning, prescribed
burning, or both, and found that thinning
plus burning provided the greatest benefit
in restoring understory herbaceous plant
communities.
Campbell (2005) found that flower-visit-
ing Hymenoptera in general were more
abundant in recently burned longleaf
pine compared to undisturbed controls,
while bees in the family Halicitidae were
more abundant in all disturbed plots,
which included thin and burn, burn only,
thin only, and herbicide treatment of an
abundant native shrub layer followed by
burning. Breland (2015) found an increase
in bee richness the year immediately after
prescribed fire in longleaf pine savannas
compared to two years post burn. Con-
versely, Fultz (2005) examined the effects
of even and group shelterwood treatments
in lodgepole pine (Pinus contorta Dougl.
ex Loud.) in Montana on flower-visiting
insects and compared those to unlogged
controls and open meadows. In her study,
half of the group including even shelter-
wood treatment plots and all of the control
plots, were burned in 2002 and 2003 during
her second and third year of sampling.
Burning had no effect on bee abundance
or species richness, but open meadows
and the two shelterwood treatments had
higher numbers of individuals and species,
and the unlogged old growth controls had
the lowest, even though the controls had
prescribed burns during the study.
Despite the many reports of positive
effects of prescribed fire on pollinator
communities, it should be mentioned that
burning can also have negative effects,
depending on the intensity and frequency
of fire and the pollinator species involved.
This is even true for species that depend
on historically fire-maintained habitats,
especially for species with severely restrict-

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References
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Journal ArticleDOI
TL;DR: The nature and extent of reported declines, and the potential drivers of pollinator loss are described, including habitat loss and fragmentation, agrochemicals, pathogens, alien species, climate change and the interactions between them are reviewed.
Abstract: Pollinators are a key component of global biodiversity, providing vital ecosystem services to crops and wild plants. There is clear evidence of recent declines in both wild and domesticated pollinators, and parallel declines in the plants that rely upon them. Here we describe the nature and extent of reported declines, and review the potential drivers of pollinator loss, including habitat loss and fragmentation, agrochemicals, pathogens, alien species, climate change and the interactions between them. Pollinator declines can result in loss of pollination services which have important negative ecological and economic impacts that could significantly affect the maintenance of wild plant diversity, wider ecosystem stability, crop production, food security and human welfare.

4,608 citations


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01 Mar 2011-Oikos
TL;DR: The global number and proportion of animal pollinated angiosperms is estimated as 308 006, which is 87.5% of the estimated species-level diversity of fl owering plants.
Abstract: It is clear that the majority of fl owering plants are pollinated by insects and other animals, with a minority utilising abiotic pollen vectors, mainly wind. However there is no accurate published calculation of the proportion of the ca 352 000 species of angiosperms that interact with pollinators. Widely cited fi gures range from 67% to 96% but these have not been based on fi rm data. We estimated the number and proportion of fl owering plants that are pollinated by animals using published and unpublished community-level surveys of plant pollination systems that recorded whether each species present was pollinated by animals or wind. Th e proportion of animal-pollinated species rises from a mean of 78% in temperate-zone communities to 94% in tropical communities. By correcting for the latitudinal diversity trend in fl owering plants, we estimate the global number and proportion of animal pollinated angiosperms as 308 006, which is 87.5% of the estimated species-level diversity of fl owering plants. Given current concerns about the decline in pollinators and the possible resulting impacts on both natural communities and agricultural crops, such estimates are vital to both ecologists and policy makers. Further research is required to assess in detail the absolute dependency of these plants on their pollinators, and how this varies with latitude and community type, but there is no doubt that plant – pollinator interactions play a signifi cant role in maintaining the functional integrity of most terrestrial ecosystems. Plant – pollinator relationships may be one of the most ecologically important classes of animal – plant interaction: without pollinators, many plants could not set seed and reproduce; and without plants to provide pollen, nectar and other rewards, many animal populations would decline, with consequent knock-on eff ects for other species (Kearns et al.

2,448 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a global meta-analysis of 199 articles reporting 1041 field studies that in total describe the impacts of 135 alien plant taxa on resident species, communities and ecosystems.
Abstract: Biological invasions cause ecological and economic impacts across the globe. However, it is unclear whether there are strong patterns in terms of their major effects, how the vulnerability of different ecosystems varies and which ecosystem services are at greatest risk. We present a global meta-analysis of 199 articles reporting 1041 field studies that in total describe the impacts of 135 alien plant taxa on resident species, communities and ecosystems. Across studies, alien plants had a significant effect in 11 of 24 different types of impact assessed. The magnitude and direction of the impact varied both within and between different types of impact. On average, abundance and diversity of the resident species decreased in invaded sites, whereas primary production and several ecosystem processes were enhanced. While alien N-fixing species had greater impacts on N-cycling variables, they did not consistently affect other impact types. The magnitude of the impacts was not significantly different between island and mainland ecosystems. Overall, alien species impacts are heterogeneous and not unidirectional even within particular impact types. Our analysis also reveals that by the time changes in nutrient cycling are detected, major impacts on plant species and communities are likely to have already occurred.

2,293 citations

Journal ArticleDOI
29 Mar 2013-Science
TL;DR: Overall, wild insects pollinated crops more effectively; an increase in wild insect visitation enhanced fruit set by twice as much as an equivalent increase in honey bee visitation.
Abstract: The diversity and abundance of wild insect pollinators have declined in many agricultural landscapes. Whether such declines reduce crop yields, or are mitigated by managed pollinators such as honey bees, is unclear. We found universally positive associations of fruit set with flower visitation by wild insects in 41 crop systems worldwide. In contrast, fruit set increased significantly with flower visitation by honey bees in only 14% of the systems surveyed. Overall, wild insects pollinated crops more effectively; an increase in wild insect visitation enhanced fruit set by twice as much as an equivalent increase in honey bee visitation. Visitation by wild insects and honey bees promoted fruit set independently, so pollination by managed honey bees supplemented, rather than substituted for, pollination by wild insects. Our results suggest that new practices for integrated management of both honey bees and diverse wild insect assemblages will enhance global crop yields.

1,881 citations


"Conserving pollinators in North Ame..." refers background in this paper

  • ...Additionally, native pollinators from these natural areas contribute substantially to the pollination of adjacent crops, often without the need for managed honey bees (Garibaldi et al. 2013; Morandin and Kremen 2013)....

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Journal ArticleDOI
TL;DR: Recent declines in honeybee numbers in the United States and Europe bring home the importance of healthy pollination systems, and the need to further develop native bees and other animals as crop pollinators.
Abstract: ▪ Abstract The pollination of flowering plants by animals represents a critical ecosystem service of great value to humanity, both monetary and otherwise. However, the need for active conservation of pollination interactions is only now being appreciated. Pollination systems are under increasing threat from anthropogenic sources, including fragmentation of habitat, changes in land use, modern agricultural practices, use of chemicals such as pesticides and herbicides, and invasions of non-native plants and animals. Honeybees, which themselves are non-native pollinators on most continents, and which may harm native bees and other pollinators, are nonetheless critically important for crop pollination. Recent declines in honeybee numbers in the United States and Europe bring home the importance of healthy pollination systems, and the need to further develop native bees and other animals as crop pollinators. The “pollination crisis” that is evident in declines of honeybees and native bees, and in damage to web...

1,653 citations


"Conserving pollinators in North Ame..." refers background in this paper

  • ...Negative effects of grazing can include the elimination of food plants for caterpillars, reduction or elimination of nectar and pollen sources, destruction of ground nests of bees, and direct trampling of pollinators (Kearns et al. 1998)....

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