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Distribution, Abundance, and Niche Differentiation
in Neotropical Euglossine Bees
Brady W. Boyer and Tomás A. Carlo
Department of Biology, The Pennsylvania State University, University Park, 16802
Background
Methods
Orchid bees (Euglossini) are a diverse insect group (>200 spp.) native to neotropical
forests. Males have the curious behavior of collecting fragrances which they store in a
specialized spongy pouch on their hind tibia. Hundreds of orchid species depend
exclusively on orchid bees as they have evolved to exploit their fragrance-collecting
behavior (Figure 1). The bees also pollinate many trees and plants and are considered
keystone pollinators. Their solitary habits make them extremely difficult to study, and
thus their foraging ecology remains unknown. Here we used a stable-isotope approach
to infer the foraging niches of orchid bees on a tropical landscape in Peru. We tested the
prediction – based on limiting similarity theory – that bees of similar sizes will show
different isotopic signatures. Isotopes allow inferring trophic niches of species: animals
that differ in 13C and/or 15N levels obtain food from different sources (Newsome et al.
2007). Our interest is to determine whether these bees are competing with each other
for similar resources or if they each have their own specialized resource niches. We also
compared the structure of orchid bee communities found on three habitat types:
primary forest, agricultural landscape(agroscape), and urban areas of Puerto Bermúdez,
Peru.
We collected bees using chemical baiting methods, taking advantage that male
orchid bees are attracted to some synthetic chemicals. In order to attract the biggest
array of bee species we used the two best general attractants: Methyl salicylate (the
fragrance of wintergreen), and Cineole (eucalyptus oil) (Roubick & Hanson 2004). To
make a bait, we added five mL of both substances to a sponge. Sponges were placed in
a branch 1 to 1.5 m off the ground (Figure 2a). Each bait was exposed for five hours. All
bees that approached the bait were captured (or nearly so) with a net. We placed four
replicate sampling sponges, spaced 70-300 m from each other, in each of the three
habitat types (urban, agricultural, and primary forest locations, Figure 2b). Captured
bees were pinned, photographed, and identified.
The species Euglossa despecta, E. mixta, E. ignita, E. imperialis, Eulaema meriana,
and Eulaema bombiformis were analyzed with mass spectrometry to examine the 13C
and 15N isotopic signatures. We performed Continuous-flow mass spectrometry on 1-3
mg of abdomen tissue (total samples = 67).
a
b
Analyses:
To compare isotopic signatures among similarly-sized bees we used three pairs of
species: Euglossa despecta and Euglossa mixta were small, Euglossa ignita and
Euglossa imperialis were medium-sized, and Eulaema meriana and Eulaema
bombiformis were large species (Fig. 3). Signatures were compared using GLMs with
normal error distributions.
Species diversity was compared among habitats using Shannon’s diversity index. The
frequency of bees carrying orchid pollinaria was compared using Generalized linear
models (GLMs) with binomial errors.
b
a
(Photo by H. Nijssen)
Figure 1: (a) Euglossa imperialis with pollinaria from an orchid
attached to the dorsal side of its thorax. (b) A male orchid bee
visiting an orchid
a
b
H=-1.831(±)0.11
(Google Earth)
Primary Forest
Figure 2: (a) One of the chemical baits used in the agricultural area. (b)
A view of the sampling locations at the three different habitats.
H=-1.982(±)0.10
δ13C
Agroscape
H=-1.50(±)0.07
(Photo by Dr. Jim Marden)
Figure 4. (a) We captured a total of 130 bees. Primary forest had the largest number of
species (11) , followed by the agroscape (10), and urban (5). Diversity values (Shannon’s H
Index) were similar for both primary forest and agroscape, while diversity was drastically
reduced in the urban areas. Species composition was also very similar between primary
forest and agroscape. (b) However bees carrying pollinaria were only found in primary
forest, with one bee in each sampling station of the forest (GLM X2 = 5.123, DF = 2, P =
0.077).
δ 15N
Urban
NNs
Conclusions
Differences in 15N and 13C values of similar sized species (Fig. 3) reveal differences in the plant
sources the species rely upon. It can be inferred from this data that the small bees and the large bees
are in fact getting food from different species of plants. This in turn supports the hypothesis that
similar species partition resources in order to coexist. In the case of the medium sized bees, the lack
of isotopic difference does not necessarily imply that they are eating similar foods as different plant
species may have identical isotopic signatures.
The urban landscape had very low bee species richness, suggesting that orchid bees have trouble
coping with certain types of human development . On the other hand, there was no detectable
difference in bee diversity between agricultural habitat and primary forest habitats, suggesting that
orchid bees can use such landscapes. Still, the lack of pollinaria in agroscapes and urban areas
suggests that pollination services are negatively effected there. More research is needed to
understand the ecology of orchid bees and to understand anthropogenic impacts on their
populations.
Figure 3 . We found evidence for niche partitioning (or trophic differentiation) in the levels
of 13C between Euglossa despecta and Euglossa mixta, as well as in the levels of 15N
between Eulaema meriana and Eulaema bombiformis. However, the isotopic signatures of
Euglossa ignita and Euglossa imperialis show no significant difference.
Acknowledgments
National Science Foundation REU
Eberly College of Science Undergraduate Research Award
Museo de Historia Natural de Peru, Universidad Nacional Mayor San Marcos
César Arana, Diego Ardiles, Diana Silva