of S En r Tneer°s PS Effects °f a Normative, Invasive Lovegrass
on Agave palmeri Distribution, Abundance,
and Insect Pollinator Communities
by Denise L. Lindsay , 1 Pamela Bailey , 1 Richard F. Lance , 1 Michael J. Clifford , 2
Robert Delph , 2 and Neil S. Cobb 2
PURPOSE: This technical note is a product of the Department of Defense Legacy Resource
Management Program work unit titled “Effects of invasives on the distribution of keystone
desert plants on military lands.” The objective of the work unit is to provide a better under¬
standing of the impacts of invasive species on key components of ecosystems and pollinator
communities. The study documented herein emphasized the integration of invasive nonnative
plant invasion with other ecological processes through assessments of the spatial effects and fire
dynamics of Lehmann lovegrass (.Eragrostis lehmanniana) on the distribution and abundance of
Palmer’s agave (Agave palmeri ), investigations of changes in A. palmeri pollinator community
composition and diversity in the presence of high E. lehmanniana abundance, and implementa¬
tion of a focused network analysis of A. palmeri and the plants with which it directly interacts
with through shared pollinators. The purpose of this technical note is to provide information
(such as key insights into important ecological relationships that foster species persistence, bio¬
diversity, and community stability) that can be leveraged against ongoing work on pollinator
systems by Fort Huachuca, the state of Arizona, and the U.S. Army Engineer Research and
Development Center, to address management concerns for desert plant communities and their
associated threatened and endangered species.
INTRODUCTION: Invasive plants are considerable challenges for land managers in desert eco¬
systems, especially invasive grasses, which both benefit from and promote recurrences of fire,
often reducing the persistence of native species and converting native plant communities to
annual grasslands (Brooks and Pyke 2001). Invasive plants are capable of aggressively spreading
into new habitat and monopolizing essential resources such as nutrients, water, and light, conse¬
quently out-competing native species. Impacts of invasive species on natural environments have
contributed to the decline of 42 percent of federally threatened and endangered species nation¬
wide (U.S. Environmental Protection Agency (USEPA) 2001), and following direct loss of
habitat, invasive species are the next greatest threat to the survival of native species. Potential
negative impacts of invasive species include the disruption of ecosystem structure and function
via the alteration of community composition, the reduction of available resources, and
diminished reproductive efficiency.
The genus Agave is an important native taxa to assess the effects of invasive grasses because
agaves are keystone species (one whose impact on its ecosystem is disproportionately large
1 U.S. Army Engineer Research and Development Center, Vicksburg, MS.
2 Northern Arizona University, Flagstaff, AZ.
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Effects of a Nonnative, Invasive Lovegrass L Agave palmeriDistribution,
Abundance, and Insect Pollinator Communities
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U.S. Army Engineer Research and Development Center,Vicksburg,MS
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relative to its abundance) of semiarid and arid regions of the southwest with considerable eco¬
logical and economic value (Good-Avila et al. 2006). The Palmer’s agave (Agave palmeri ) is
state protected in Arizona and is currently threatened by the invasive African plant, Lehmann
lovegrass (Eragrostis lehmanniana ), as grasses strongly compete with agave seedlings (U.S. Fish
and Wildlife Service (USFWS) 1999). A. palmeri grows in sandy to gravelly places on limestone
in oak woodlands and grassy plains at elevations between 900-2,000 m in Arizona, New Mexico,
and Mexico (Flora of North America Editorial Committee 2002). Agave plays a key role in the
life history of the federally endangered lesser long-nosed bat (Leptonycteris curasoae) and state
listed Mexican long-tongued bat (Choeronycteris mexicana). It is an important nectar and pollen
resource for a large variety of pollinators, including bees, hummingbirds, orioles, butterflies, and
wasps (National Park Service (NPS) 2007). However, little is known regarding the effect of
agave on insect pollinators.
E. lehmanniana was introduced in southern Arizona in 1932 to control soil erosion and provide
forage for cattle and has since spread throughout the southwest (Crider 1945; Gori and Enquist
2003; Bock et al. 2007). It is now considered a major plant species on about 140,000 hectares
(ha), primarily located in southeastern Arizona (Halvorson and Guertin 2003), and has the
potential to spread to over 7,000,000 ha under predicted climate change scenarios (Huang and
Geiger 2008). The biomass of E. lehmanniana is typically two to four times greater than the
biomass produced by native grass vegetation (Anable et al. 1992); thus, it can indirectly impact
pollinators by crowding out native plants and reducing the availability of nectar sources and
nesting sites (e.g. woody stems and bare earth used by bees). Currently, E. lehmanniana grows at
elevations from 200-1,830 m (Flora of North America Editorial Committee 2007). The potential
for E. lehmanniana to dominate and influence ecosystems is likely increasing because under
changing climate conditions, its future distribution is predicted to be much greater than its cur¬
rent distribution (Schussman et al. 2006), with colonization spreading to areas higher in elevation
and much farther north than its present range.
Additionally, the lovegrass can indirectly affect plant interactions by altering fire dynamics
throughout the ecosystem. Prescribed fires to remove E. lehmanniana populations have been
unsuccessful in maintaining control, often resulting in regrowth during subsequent seasons
(Rogers 2004). Furthermore, the lovegrass has been shown to increase the frequency and inten¬
sity of natural fires (Kupfer and Miller 2005), which could alter A palmeri germination, growth,
abundance, and resource availability and foraging behavior of pollinators (Geiger 2006; Gucker
2006). Agave stalks are edible to wild herbivores such as deer, javelina, rodents, and rabbits
(USFWS 1999). Because agave stalks often remain available following fire when other food
resources are limited, herbivores may favor them, negatively impacting the availability of flow¬
ering stalks for pollinators, such as the nectar-feeding bats (USFWS 1999). In addition to the
direct mortality of agave, fire may alter the availability of blooms, as agaves physiologically
commit to bolt by early spring. If an agave survives a burn, bolting continues although the stalk
is then smaller and has fewer flowers (Howell 1996; USFWS 1999), and if an agave stalk bums
directly, the reproductive effort and the availability of nectar for that plant is completely lost
(USFWS 1999). Both of these situations result in reduced availability of nectar for pollinators.
This study assesses the impacts of invasive species on key components of ecosystems and polli¬
nator communities. To integrate aspects of invasive nonnative plant invasion with other ecologi¬
cal processes, the spatial effects of fire, soil type, and E. lehmanniana on the distribution and
density of A. palmeri are assessed, and changes in agave pollinator community composition and
diversity in the presence of high E. lehmanniana abundance are investigated. A network
approach was implemented (Jordano et al. 2006; Olesen et al. 2006) to describe and analyze how
agave interacts through shared pollinators with other plants, and to detect any differences in the
structures of these agave “ego networks” associated with low and high E. lehmanniana abun¬
dance. Network visualization and analysis of pollination communities can provide key insights
into important ecological relationships that foster species persistence, biodiversity, and commu¬
nity stability (Aizen et al. 2009; Carvalheiro et al. 2008; Fontaine et al. 2006; Memmott et al.
2004). The agave ego network was restricted to only those plants directly connected to
A. palmeri, making other common network measures, such as diameter or closeness centrality,
meaningless or redundant with other statistics. Information gained from this study will be lever¬
aged against ongoing work on pollinator systems by Fort Huachuca, the state of Arizona, and the
U.S. Army Engineer Research and Development Center, to address management concerns for
desert plant communities and associated threatened and endangered species.
Study Locale. The study was conducted on Fort Huachuca, located in Cochise County of
southeastern Arizona (Figure 1). Nearly 3,000 ha of Agave have been documented (Danzer and
Roberts 2003) on the 33,000-ha installation. Fort Huachuca has well characterized vegetative
communities, supports a number of rare plants and pollinators, and has a high diversity of polli¬
nators. The overall study area (21,200 ha) did not include the northeast section of the base, as
agave was not present and fire history data were not available. Study sites were selected using
prior data ’ to locate areas characterized by high (> 35 percent) and low (< 15 percent) abun¬
dance E. lehmanniana, and low, medium, and high densities of A. palmeri. These study sites
were chosen in paired locations for high and low abundance E. lehmanniana over relatively
homogenous terrain (elevation ranged from about 1450-1550 m) to minimize environmental
variance. Mean E. lehmanniana percentage was determined between two classes of study sites
(high = 52.83 percent ± 18.37 percent, low = 5.67 percent ± 4.59 percent) to be significantly dif¬
ferent (Fiji = 37.24, P = 0.0001) using PROC GLM (SAS Institute 2005). Surveys and sampling
were conducted during the summer of 2008.
Distribution of Eragrostis lehmanniana and Agave palmeri in Relation to Fire and
Soil. Geographic infonnation systems (GIS) data were obtained, including shapefiles of fire
history from 1975-2006, soil types (Soil Survey Geographic (SSURGO)), and agave distribution
and density (provided by D. Schlichting). High-resolution (1-m) 2007 color infrared imagery
(USDA National Agriculture Imagery Program) was obtained for determining canopy cover of
overstory tree species (e.g. Prosopis spp.). High-resolution imagery was classified using a super¬
vised classification to differentiate areas of tree canopy from grasslands. With this imagery, the
authors were able to remotely detect the presence of larger shrubs and trees (crown diameter
> 1 m), and quantify canopy cover.
1 Personal communication. 2009. University of Arizona, Tucson, AZ.
2 Personal communication. 2009. D. Schlicting, Range Training Lands Assessment Coordinator, Colorado State
University Center for Environmental Management of Military Lands, Ft. ITuachuca, AZ.
■fa Site Locations
I&&&1 Agave Distribution
Lehmann I xwcgrass Cover
Figure 1. Map of study area and sample site locations (stars) on Fort Huachuca, Arizona, including
Agave palmeri distribution (cross-hatched) and Eragrostis lehmanniana percent cover
(shaded). E. lehmanniana was masked to the extent of grasslands as determined by the
Southwest Regional Gap Analysis Program (SWReGAP) (Lowry et al. 2007).
Spatial analyses were performed by overlaying shapclilcs and rasters to determine the interac¬
tions of spatial distributions of fire, soil-type, E. lehmanniana, and A. palmeri. To increase sam¬
pling efficiency, 1,000 random points were generated for the high, medium, and low density
A. palmeri datasets (Beyer 2004). Trends in the distribution and density of A. palmeri were ana¬
lyzed relative to the distribution and density of E. lehmanniana by using an inverse distance
weighting interpolation from point data (all percent cover of E. lehmanniana ) collected in 2004
and 2006 1 and 2008 which occurred on the study area. The output of this interpolation was a
spatial dataset of percent cover of E. lehmanniana. The relationship of E. lehmanniana density to
fire occurrence was also assessed, to further investigate the effects of E. lehmanniana on
A. palmeri through potential impacts to A. palmeri survival, and thus A. palmeri pollinator
guilds. This was accomplished by creating 1,000 random points in the area of A. palmeri distri¬
bution (high, medium, and low) and masking the E. lehmanniana dataset to each fire occurring
from 1975-2006. At each random point within each fire year, the percent cover of
E. lehmanniana was compared to cover not burned during that year. An analysis of variance
(ANOVA) was used to compare A. palmeri density with both fire occurrence and percent cover¬
age of E. lehmanniana, and all significant values were accepted at the 0.05 probability level
using SPSS 16.0 (SPSS, Inc. 2007).
Interpolated percent cover of E. lehmanniana was masked to the extent of grasslands as deter¬
mined by the Southwest Regional Gap Analysis Program (SWReGAP; Lowry et al. 2007) and
the distribution of A. palmeri was excluded. Then 1000 random points were generated that over¬
laid the potential E. lehmanniana distribution (e.g., grasslands and non-agave) and extracted
E. lehmanniana percent cover on these points. Percent cover of E. lehmanniana was also deter¬
mined by generating 1,000 random points within the distribution of A. palmeri and compared to
percent E. lehmanniana in non-agave areas with an ANOVA. To determine which soil types are
preferred by E. lehmanniana, 1,000 random points were generated and soil type was extracted
from a SSURGO soils dataset, and percent cover was extracted from the interpolated E. lehman¬
niana dataset. An ANOVA was used to determine whether percent cover of E. lehmanniana dif¬
fered significantly between soil types.
Agave palmeri Surveys. The relative abundance and size class of A. palmeri were quantified
at each of 10 sites characterized by high (N = 5) and low (N = 5) E. lehmanniana abundance in
the grassland vegetation community. The total number of live and dead A. palmeri were quanti¬
fied per site, and size class was calculated by measuring the average diameter of each living
A. palmeri using a standard measuring tape. Diameter was calculated by averaging two perpen¬
dicular measurements across the top of the plant. Comparisons of the number of live and dead
A. palmeri between sites with high and low E. lehmanniana abundance were conducted with an
analysis of variance using PROC GLM (SAS Institute 2005). A Kolmogorov-Smirnov test was
performed to compare size class differences between high and low E. lehmanniana abundance
sites using PROC NPAR1WAY (SAS Institute 2005).
Pollinator Sampling. Two related pollinator studies were conducted. The first was a directed
assessment of insect pollinators visiting A. palmeri flowers. For this study, pollinators from
A. palmeri were sampled during the peak agave flowering season (July and August). Once per
month, pollinators were collected on blooms from 7-10 individual A. palmeri per site, at each of
12 sites characterized by high (N = 6) and low (N = 6) E. lehmanniana abundance. Individual
agaves were systematically sampled for 2 consecutive minutes with battery-powered handheld
vacuums modified for insect collection while perched on orchard ladders to reach flowering
stalks that range in height from 3-6 m. Insects were identified to the lowest possible taxonomic
1 Personal communication. 2009. D. Schlichting, Range Training Lands Assessment Coordinator, Colorado State
University Center for Environmental Management of Military Lands, Ft. Huachuca, AZ.
level. Efforts were made to observe pollination by hummingbirds on agave, but due to very few
interactions and difficulties with species identification, hummingbird data were not included in
the analyses. Likewise, data on nocturnal pollinators were not included.
A. palmeri pollinator community differences between high and low E. lehmanniana abundance
sites were assessed by comparing mean species richness and species abundance among sites with
a one-way analysis of variance using PROC ANOVA (SAS Institute 2005). Species indicator
analysis was conducted with a Monte Carlo test of significance to determine whether specific
pollinator taxa responded to high or low E. lehmanniana abundance. A multi-response permuta¬
tion procedure (MRPP) was used to determine pollinator community composition differences
between high and low E. lehmanniana abundance sites.
The second pollinator study examined plant-pollinator networks from a complementary ongoing
study of all pollinators associated with grasslands on Fort Huachuca to assess networks for
A. palmeri insect pollinators. For this study, 16 plots (100 m x 25 m) characterized by high
(N = 8) and low (N = 8) E. lehmanniana abundance were surveyed for pollinator-plant interac¬
tions monthly from April through September, with the exception of June. Each plot was divided
into five sampling lanes, four of which were randomly selected for sampling by a randomly
assigned field technician. Sampling was conducted using the same handheld vacuums used for
the agave-centric pollinator sampling and focused on capturing all insects found on flowers (any
species) along each of the four selected transects over a 20-minute period, with collection on
individual plants limited to 2 consecutive minutes. Plots were sampled once per month, with the
order of sampling both among and within plot pairs randomly assigned. Plants on each plot were
identified to species and collected insects were identified to the lowest possible taxonomic level.
As explained earlier, hummingbirds and nocturnal pollinator activity on plots was not recorded.
All flower-feeding Hymenoptera and Lepidoptera, and the plant species on which they were
collected, were used to create rectangular weighted adjacency matrices and corresponding bipar¬
tite (or 2-mode) networks: one set for native (combined data from all low E. lehmanniana
abundance sites and all months) and another set for invaded (combined data from all high
E. lehmanniana abundance sites and all months). In these matrices, plant species comprise the
row categories and insect pollinators the column categories, with the number of individuals of an
insect species captured on a plant represented in the corresponding matrix cell. For the bipartite
network, each plant and pollinator corresponds to a node, and the number of pollinators captured
on a plant provides a weighting for the edges (= links) between plant and pollinator nodes. To
visualize and analyze the structure of plant-plant interactions (= shared pollinators), the weighted
adjacency matrices were dichotomized and collapsed to create new, square weighted adjacency
matrices and corresponding unipartite (or 1-mode) networks. In the new weighted adjacency
matrices, the constituent plants comprise both column and row categories and the matrix cells
correspond to the number of pollinator species shared by pairs of different plant species. For the
unipartite network, each species of plant corresponds to a node, and the number of pollinator
species shared by two plant species corresponds to a weighted edge between nodes. In order to
focus analyses on A. palmeri, reduced matrices corresponding to the unipartite (1-mode) “ego
networks” of agave (all plants linked directly to A. palmeri through shared pollinators) were
extracted from the broader data sets. 1 Because many of the available analyses can only be used
1 Treatment of the full pollination network data will be presented in a separate publication.
to analyze unweighted (binary) networks, the weighted agave ego network matrices were
dichotomized to create unweighted adjacency matrices and corresponding unweighted unipartite
networks. The significance of differences in standard network measures, described earlier, for
unipartite networks from areas with high and low E. lehmanniana abundance were determined
following bootstrap procedures described by Snijders and Borgatti (1999). All matrix processing
and network analyses were executed using UCINET 6.0 (Borgatti et al. 1999), and network
creation and visualization were executed with Pajek 1.02 (Batagelj and Mrvar 1998). Network
parameters of interest (reviewed in Borner et al. 2007) included those related to topology, such
as: number of nodes or size of the network (AO, number of edges or links ( E ), density of the net¬
D = -
and several measures of network connectedness, including mean number
of edges per node |/i) or mean degree centrality ( C D ), mean betweenness centrality ( C H j; the
proportion of shortest network paths between other nodes that incorporate a node), mean eigen¬
vector centrality ( C E ; a measure of the degree to which a node is a component of overall con¬
nectedness in the network), and mean Bonacich power ( C p ; when the attenuation factor, [1, is
positive, power is a positive function of being connected to well-connected nodes). Network
creation and visualization were executed with Netdraw 2.085 (Borgatti 2002), with random posi¬
tioning of nodes and strength of weighted edges (number of shared pollinators) represented by
scaled line thicknesses (stronger edge = thicker line).
Agave palmeri and Eragrostis lehmanniana distributions. A. palmeri was present in
1837 ha across the study area (21,200 ha; Figure 1), with an estimated 249 ha being designated
as high density, 993 ha designated as medium density, and 595 ha designated as low density. The
overall agave distribution grew with a mean of 25.7 percent E. lehmanniana cover, while mean
E. lehmanniana cover in the study area was 7.3 percent ± 0.3 percent. Although the presence of
high E. lehmanniana abundance did not significantly alter the number of live (F\c, = 0.71,
P = 0.4231) or dead (F\$ = 2.38, P = 0.1615) A. palmeri among the sample sites, areas of low
density agave had significantly higher percent coverage of E. lehmanniana than either medium-
or high-density areas of agave across the study area (F = 42.50, P < 0.0001; Figure 2). Addition¬
ally, a significantly higher ratio of smaller agave plants (< 0.4 m radius) to larger agave plants
(> 0.4 m radius) was found in sites corresponding to high E. lehmanniana abundance
( KSa= 1.9578, P = 0.0009). Overall, A. palmeri plants ranged in size from 0.03-2.64 m in
There were several fundamental differences among levels of agave density. Percent canopy
cover of overstory trees was highest (1.9 percent) in high-density agave areas and lowest
(1.0 percent) in low-density agave areas. Agave density also varied by the relative abundance of
soil type. High density agave was largely (76.0 percent) found on the Terrarossa-Blacktail-Pyeatt
Complex, while low-density agave was found equally on the Terrarossa Complex and White
House Complex (Table 1). Soil type also influenced percent cover of E. lehmanniana on Fort
Huachuca. Aside from the Ubik Complex, the three most common soil types where agave was
found (Terrarossa-Blacktail-Pyeatt Complex, Terrarossa Complex, and White House Complex)
had the most E. lehmanniana cover of any soil type (13-19 percent; Table 1). Percent cover of
E. lehmanniana was significantly higher (F = 398.33, P < 0.001) within the distribution of agave
than in non-agave areas (Figure 3).
Figure 2. Low-density Agave palmeri corresponds to significantly higher percent
coverage of Eragrostis lehmanniana than either medium- or high-density
agave (F = 42.50, P < 0.0001).
Table 1. Influence of soil type on Agave palmeri distribution and Eragrostis
lehmanniana abundance. 1
Mean ± SD
Percent Soil Abundance of
Agave palmeri Distribution
Comprised of by
White House Complex
3.2 ± 0.4
Carbine Very Gravelly Loam
7.8 ± 1.3
1 High densities of agave were preferentially found on Terrorossa-Blacktail-Pyeatt Complex, as was E. lehmanniana. Percent
cover of E. lehmanniana was higher in soil types preferred by agave, but was also abundant on other soil types (e.g., Ubik
Figure 3. Eragrostis lehmanniana exhibits significantly higher percent cover in
areas where Agave palmeri occurs compared to areas with no agave
(F = 398.33, P < 0.001).
Interactions of natural and prescribed bum history with E. lehmanniana distribution and density
showed no discernible pattern. Areas with high E. lehmanniana abundance did not burn more
frequently than areas of low E. lehmanniana abundance. For A. palmeri, however, areas of high
and medium density were significantly associated with more frequent burning (F = 3.26,
P < 0.05; Figure 4).
Pollinator Community Analysis. There was no significant difference in pollinator species
richness (F 23 = 0.14, P = 0.7076) or species abundance (Fi^ = 0.50, P = 0.4868) between sites
with high and low E. lehmanniana abundance (Table 2). Pollinator community composition
analysis revealed no significant differences between high and low E. lehmanniana abundance
sites (r = -0.015, P = 0.726). Of the 70 taxa identified (Appendix A), only one species was an
indicator of either high or low E. lehmanniana abundance; with a mean observed indicator value
of 20.8 ± 3.67, Agapostemon angelicus was found to be an indicator species of A. palmeri
located in high E. lehmanniana abundance sites (P = 0.0472).
Figure 4. Agave palmeri density significantly decreases with increasing
percentage of area burned per year (F = 3.26, P < 0.05).
Table 2. Pollinator species richness and species abundance for eight high (>35 percent)
and eight low (<15 percent) Eragrostiis lehmanniana abundance sites.
Eragrostis lehmanniana Abundance
Pollinator Network Analyses. In each agave ego network (native vs. invaded), agave was
directly linked (shared > 1 pollinators) with 11 other plants (Figure 5). In addition to A. palmeri,
however, the two networks only have three plant species in common, including Acacia augustis-
sima, Calliandra eriophylla, and Prosopis velutina. The native network included 30 different
insect pollinators, while the invaded network contained 14 different insect pollinators. Nine spe¬
cies of pollinators, Apis melifera, Dialictus microlepoides, Hemiargus isola, Microclepi spp.,
Bruchophagus spp., Myrmecosystus spp., Crematogaster spp., Lydella radieus, and Trupanea
spp. were found within the agave ego networks in both native and invaded plots. In both of the
larger community networks, the agave ego network played an important role, comprising
40 percent of the plant species. However, there were notable differences between the native and
invaded agave ego networks, including a higher degree of pollinator sharing among plants in the
native network relative to the invaded network ( E = 92 vs. 72; D = 63.64 percent vs.
43.64 percent). Significant differences (one-tailed t-tests, 10,000 bootstraps) between the
unweighted unipartite networks (native vs. invaded, p-value) included mean node degree
centrality (c d = 7.667 vs. 5.833, p = 0.04l) and mean node power
(c p = 1160.109 vs. 662.263, p > 0.001 j. In regards to the weighted unipartite networks, the same
trends hold true with C D (14.500 vs. 8.500,/; = 0.010), C B (22.792 vs. 2.583 , p = 0.034), and
mean cluster coefficient (c = 1.821 vs. 1.213, p = 0.002 j. There were no significant differences in
betweenness centrality (c b = 1.667 vs. 2.583, p = 0.653) or eigenvector centrality
[ C E = 0.280 vs. 0.276, p = 0.910 j for the unweighted networks, nor in eigenvector centrality for
the weighted networks (C E = 0.268 vs. 0.246, p = 0.697). Agave also appears to play a more cen¬
tral role in the native ego network as indicated by a higher two-step reach (the percentage of
other nodes within two-links of agave; 93.10 percent versus 86.21 percent).
DISCUSSION: High abundance of E. lehmanniana significantly altered the distribution of
A. palmeri size classes, resulting in a higher ratio of small to large plants. Areas with a higher
ratio of small to large agave plants are a management concern on Fort Huachuca because stands
of small plants are considered to be important future nectar-feeding centers, and should thus be
protected. 1 However, areas of low-density agave had significantly higher percent coverage of
E. lehmanniana than either medium- or high-density areas of agave, indicating that high
E. lehmanniana abundance tends to exclude A. palmeri. Although the small/young agave plants
are important, the more dense stands of agave also need to be protected, as total amount of nectar
produced is the main conservation concern for the endangered lesser long-nosed bat.
Although Kupfer and Miller (2005) found that the presence of E. lehmanniana increases both the
frequency and intensity of natural fires, Geiger (2006) determined that the proportion of E. leh¬
manniana does not increase following burns. Similarly, it was determined that areas with high
E. lehmanniana abundance did not bum more frequently than areas of low E. lehmanniana abun¬
dance. However, fires did occur significantly more in areas of high-density agave than in areas of
low-density agave. Because fire has the potential to reduce or eliminate agave bloom production
by damaging or destroying plants, this could have a negative effect on overall nectar availability.
Agave density also varied by soil type, with high-density agave being associated with the
Terrarossa-Blacktail-Pyeatt Complex, also following an observation of Geiger (2006) that sur¬
vival of agave varies with soil type. Because E. lehmanniana was preferentially found on the
same three soil types where A. palmeri most commonly occurred, and percent cover of
E. lehmanniana was significantly higher within agave areas, the two species will likely be in
close association for the foreseeable future. This suggests that areas of agave may be more prone
to invasion by E. lehmanniana than areas without agave.
1 Personal communication. 2009. D. Schlichting, Range Training Lands Assessment Coordinator, Colorado State
University Center for Environmental Management of Military Lands, Ft. Huachuca, AZ.
Figure 5. Unipartite networks of plants linked through shared pollinator species from a) native grass
dominated plots, and b) invaded plots dominated by the nonnative grass Eragrostis
lehmanianna. Line thickness reflects edge weighting (number of pollinator species shared).
There was no significant difference in A. palmeri pollinator species richness, species abundance,
or community composition between sites with high and low E. lehmanniana abundance, sug¬
gesting that E. lehmanniana does not have a negative influence on the agave pollinator guild.
Agave flowering stalks often tower over the maximum height of E. lehmanniana, thus allowing
pollinators to access agave blooms with ease. High E. lehmanniana abundance is concomitant
with low densities of A. palmeri, which suggests that pollinator activity should also follow this
pattern. However, sites with high E. lehmanniana abundance could be outcompeting other native
flora, thus increasing the amount of pollinator activity on the limited numbers of A. palmeri in
high E. lehmanniana abundance sites. Agapostemon angelicus, a native, pollen-feeding sweat
bee, was the only pollinator observed to be an indicator species of A. palmeri in high E. lehman¬
niana abundance sites. The sweat bees are considered generalist species, pollinating a wide vari¬
ety of flower species. Though A. angelicus could be utilizing A. palmeri as a major pollen
source, the association may be due to E. lehmanniana providing cover or nesting material for
A. A. angelicus; however, that may be unlikely considering that Agapostemon spp. nest in ground
burrows (Michener 2000). In general, bees are the most common pollinator, a trend also
observed in this study. A. palmeri pollinators collected in this study included 30 species of
Hymenoptera (bees), 21 species of Diptera (flies), 9 species of Coleoptera (beetles), 4 species of
Lepidoptera (butterflies), and 2 species of Hemiptera (aphids, leafhoppers, and cicadas). Two
species of Coleoptera and two species of Araneae (spiders) that were herbivores or predators
were also collected.
Network descriptions of the interconnectedness and co-reliance among plants that share pollina¬
tors provide potentially important insights into the combined community’s robustness and resil¬
ience to changes in composition, such as loss of species (Aizen et al. 2009; Fontaine et al. 2006;
Memmott et al. 2004). Network approaches also provide important insights into the role of a
particular species, plant or pollinator, in supporting community structure, as well as that species’
susceptibility to extinction within the community (Carvalheiro et al. 2008). In the case of agave
in the Sonoran desert grassland community that was studied, it appears that A. palmeri and its
one-step ego network (the plants to which it is directly linked through shared pollinators) are
major components of the overall community pollinator network and likely lend a large degree of
stability to the community pollination dynamics. It also appears that A. palmeri and the plants in
its ego network are well established and supported by multiple pollinator linkages, but appear to
be significantly more linked within the native network. One possible reason for the apparent
greater connectedness of the native agave ego network may be a rarefaction bias in the sampling
results arising from the higher density of agave in native habitat and concomitant higher prob¬
ability of detecting more pollinator species. Percent cover of agave was significantly higher
within native sites (F = 4.88, P = 0.0444). In the sampling component of this study for the
directed assessment of A. palmeri insect pollinators, where numbers of agave sampled in native
and invaded habitat were equivalent, significant differences in pollinator diversity did not exist.
An additional factor could be different portions of plants in the low and high E. lehmanniana
abundance sites that are pollinator generalists, or pollinators that are flower generalists.
CONCLUSIONS: The nonnative grass E. lehmanniana has negatively impacted the native plant
A. palmeri, which is an important resource for many pollinators in the desert communities of the
Southwestern United States. E. lehmanniana may exclude A. palmeri, as areas of high E. leh¬
manniana abundance were associated with significantly lower densities of A. palmeri, greater
numbers of small/young A. palmeri plants, and lower pollinator network connectedness.
Although E. lehmanniana abundance had no significant effect on fire frequency, medium- and
high-density A. palmeri areas were associated with increased fire frequency, which can decrease
overall nectar production through direct or indirect means. Due to similar soil preferences,
E. lehmanniana and A. palmeri are likely to continue be found in close association; therefore,
continued study and monitoring of the invasion and impacts of E. lehmanniana on these desert
communities and their associated threatened and endangered species would benefit future man¬
POINTS OF CONTACT: For additional information, contact Denise Lindsay (601-634-2362,
firstname.lastname@example.org), Pamela Bailey (601-634-2380, email@example.com.
mil), or Dr. Richard Lance (601-634-3791, firstname.lastname@example.org). This technical note
should be cited as follows:
Lindsay, D. L., P. Bailey, R. F. Lance, M. J. Clifford, R. Delph, and N. S. Cobb.
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MS: U.S. Army Engineer Research and Development Center.
ACKNOWLEDGEMENTS: The authors thank Sheridan Stone, Dean Schlichting, Pamela
Landin, Susan Langley, Gwynne Pollard, and John-Paul Flodnett for their assistance with field
efforts and data accumulation. Funding for this project was provided by the Department of
Defense Legacy Resource Management Program (MIPR: W31RYO80857494). The study
described and the resulting data presented herein, unless otherwise noted, were partly obtained
from research conducted under the U.S. Army Environmental Quality Technology Program by
the U.S. Army Engineer Research and Development Center. Permission was granted by the
Chief of Engineers to publish this information. The views expressed in this manuscript are those
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NOTE: The contents of this technical note are not to be used for advertising, publication, or promotional purposes.
Citation of trade names does not constitute an official endorsement or approval of the use of such products.
Appendix A. Agave palmeri pollinator species list grouped by feeding
Genus and Species
Dialictus spp. 1
Dialictus spp. 2
Cotinus texana arizonica
Genus and Species
Bruchophagus spp. 1
Bruchophagus spp. 2