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Effects of conspecific and heterospecific floral density on the pollination of two related rewarding orchids

Article (PDF Available) inPlant Ecology 212(8):1397-1406 · August 2011with141 Reads
DOI: 10.1007/s11258-011-9915-1
Karl Duffy at University of Naples Federico II
  • 24.24
  • University of Naples Federico II
Jane Stout at Trinity College Dublin
  • 37.36
  • Trinity College Dublin
Abstract
Variation in within-population floral density can affect interactions between plants and pollinators, resulting in variable pollen export for plants. We investigated the effects of conspecific and heterospecific floral densities on pollination success both of two related, self-compatible, nectar-rewarding orchid species in Ireland, Spiranthes romanzoffiana (rare and listed as endangered) and its congener, S. spiralis (more abundant and not of conservation concern). Floral densities, insect visitation rates, and orchid pollen transport were recorded in multiple quadrats in four populations of both orchid species over their flowering season. We found that conspecific and heterospecific co-flowering plant density affected pollination in both orchid species. For S. romanzoffiana, higher heterospecific density increased pollen removal. For S. spiralis, higher conspecific visitation increased pollen removal and increased heterospecific density decreased pollen deposition. In addition, increased conspecific density increased pollen deposition in both species. This study shows that plants may interact to facilitate or compete for different components of the pollination process, namely; pollinator attraction, pollen removal and deposition. Such interactions have immediate consequences for endangered plant species, as increases in both conspecific and heterospecific coflowering density may ameliorate the negative effects of rarity on pollination, hence overall reproductive success. Keywords Bombus –Competition–Facilitation–Pollen export– Spiranthes
Figures
Fig. 2 The relationship between a density of heterospecific co-flowering species and proportion of flowers with pollinia removed and b conspecific density and the proportion of flowers with massulae deposited in S. romanzoffiana  
The relationship between a density of heterospecific co-flowering species and proportion of flowers with pollinia removed and b conspecific density and the proportion of flowers with massulae deposited in S. romanzoffiana
Effects of conspecific and heterospecific floral density
on the pollination of two related rewarding orchids
Karl J. Duffy Jane C. Stout
Received: 21 October 2010 / Accepted: 18 March 2011
ÓSpringer Science+Business Media B.V. 2011
Abstract Variation in within-population floral den-
sity can affect interactions between plants and poll-
inators, resulting in variable pollen export for plants.
We investigated the effects of conspecific and heter-
ospecific floral densities on pollination success both of
two related, self-compatible, nectar-rewarding orchid
species in Ireland, Spiranthes romanzoffiana (rare and
listed as endangered) and its congener, S.spiralis
(more abundant and not of conservation concern).
Floral densities, insect visitation rates, and orchid
pollen transport were recorded in multiple quadrats in
four populations of both orchid species over their
flowering season. We found that conspecific and
heterospecific co-flowering plant density affected
pollination in both orchid species. For S. romanzof-
fiana, higher heterospecific density increased pollen
removal. For S. spiralis, higher conspecific visitation
increased pollen removal and increased heterospe-
cific density decreased pollen deposition. In addi-
tion, increased conspecific density increased pollen
deposition in both species. This study shows that
plants may interact to facilitate or compete for
different components of the pollination process,
namely; pollinator attraction, pollen removal and
deposition. Such interactions have immediate conse-
quences for endangered plant species, as increases in
both conspecific and heterospecific coflowering den-
sity may ameliorate the negative effects of rarity on
pollination, hence overall reproductive success.
Keywords Bombus Competition Facilitation
Pollen export Spiranthes
Introduction
Among animal-pollinated plants, it is estimated that
less than 1% of pollen is exported to conspecific
flowers (Harder and Thomson 1989; Harder and
Johnson 2008). Such low pollen export has implica-
tions not only for the evolution of both pollen and plant
mating systems (e.g., Harder and Barrett 2006; Harder
and Johnson 2008), but also for the conservation of
animal-pollinated plants (Kearns et al. 1998). Many
ecological factors have been proposed to explain such
low pollen transfer efficiency (PTE), for example:
limited pollen pick-up by the pollinator (Sahli and
Conner 2007), grooming by the pollinator (Harder
1990), and pollen discounting (Harder and Routley
2006). However, one major factor determining the fate
K. J. Duffy (&)J. C. Stout
Department of Botany, School of Natural Sciences,
Trinity College Dublin, Dublin 2, Ireland
e-mail: duffyk@tcd.ie
Present Address:
K. J. Duffy
School of Biological and Conservation Sciences,
University of KwaZulu-Natal, Private Bag X01,
Scottsville, Pietermaritzburg 3209, South Africa
123
Plant Ecol
DOI 10.1007/s11258-011-9915-1
of pollen is conspecific density (the number of
flowering individuals per unit area) as this can
influence pollination such that pollen limitation is
reduced in plants which occur in higher density
patches (Sih and Baltus 1987; Knight 2003). This is
driven by pollinator foraging behavior: dense patches
are preferentially selected because travel times
between flowers are reduced, thus enhancing pollina-
tor foraging efficiency (Heinrich 1979; Harder 1990;
Mitchell et al.2004). Increases in density are also
likely to increase the probability that pollen will be
transferred between conspecific individuals (Karron
et al.1995). However, increasing density can have
potential negative effects on the pollination of indi-
vidual plants via conspecific competition for pollina-
tors (Ratchke 1983; Duffy and Stout 2008).
As plants often flower in communities with
individuals of many species, heterospecific co-flow-
ering plants may also affect the behavior of pollin-
ators towards rare or isolated plants through
interspecific competition and facilitation (Campbell
and Motten 1985; Moeller 2004). A plant that occurs
with low conspecific density within a patch along-
side high density heterospecific co-flowering plants
may suffer from interspecific competition for polli-
nator attention (Waser 1978; Ratchke 1983; Bell
et al.2005). Such individuals may also be prone to
pollination limitation, resulting from not just reduced
visitation rates, but also improper pollen transfer
(IPT) (the transfer of pollen between co-flowering
heterospecific flowers) and/or pollen wastage (Ratch-
ke 1983; Stout et al.1998; Wilcock and Neiland
2002). Alternatively, facilitation can occur whereby
co-flowering conspecific or heterospecific plants
form an increased floral display and all individuals
receive increased visitation (Ratchke 1983; Moeller
2004). Such facilitation by co-flowering species has
been recently demonstrated in both experimental
(Ghazoul 2006) and natural populations (Johnson
et al. 2003).
Orchids are useful model organisms for the study
of the effects of floral density on pollination, as they
share population characteristics with many angio-
sperms, such as occurring in patchy distributions in
mixed floral communities (Neiland and Wilcock
1998; Duffy and Stout 2008) and can suffer pollen
limitation within a flowering season (Johnson and
Bond 1997). Since their pollen is aggregated into
pollinia, it can be tracked directly in the field and
allows direct assessment of pollination success. In
addition, competition may be particularly detrimental
for orchids as IPT may result in the loss of the entire
male component of an individual flower (Dressler
1981). The negative effects of IPT on female function
may also be low due to a more precise pollen transfer
in orchids. However, orchids that have sectile pollinia
may have their pollen transferred among many
flowers (e.g., Johnson et al.2005). Competition and
facilitation with heterospecific co-flowering plants
may affect PTE in orchids strongly (Neiland and
Wilcock 1999; Johnson et al. 2003), although the
extent of competition may depend on their density
and whether they provide a reward (Internicola et al.
2006).
Most studies of pollination of orchids have focused
on nectarless species, with very few focused on
nectar-rewarding species (e.g., Smithson 2002;
Johnson et al. 2003; Internicola et al.2006). The
pattern of both intra- and inter-specific floral density
on pollination are not clear and may be markedly
different in rewarding species than in rewardless
species. Because rewardless species depend on poll-
inators to feed on rewarding heterospecifics, inter-
specific facilitation of pollination may be more
predominant in natural populations of these species.
For example, Johnson et al. (2003) found that co-
flowering species can increase pollen removal and
receipt in natural populations of the nectarless
Anacamptis morio. Other studies showed increased
fruit set when co-flowering with morphologically
similar rewarding species (e.g., Juillet et al.2007)
and that the addition of an artificial nectar reward
increases pollination success, regardless of local
density of the orchid (Jersa
´kova
´et al.2008).
However, field studies are lacking on the effects of
conspecific and heterospecific density effects on the
pollination success of rewarding orchid species.
Here, we investigate the pollination ecology of two
nectar-rewarding orchid species to test whether con-
specific and heterospecific density influence pollen
export and import in natural populations. We expect
increased pollen removal and deposition with
increased density in both S. romanzoffiana and
S. spiralis. We expect the pollination of S. romanzof-
fiana to be more affected by inter-specific effects as it
flowers during peak flowering of many other plants,
and occurs in fewer, smaller flowering populations.
Heterospecific co-flowering plants may have a positive
Plant Ecol
123
or neutral effect on the pollination of S. spiralis
because it occurs in larger conspecific populations and
flowers later in the year than many of the other plants in
the habitats in which it occurs, hence competition with
heterospecifics is less likely to impact on pollination.
Specifically, we test the hypothesis that pollen
removal and deposition in both S. romanzoffiana and
S. spiralis are affected by:
a) increased density of conspecific orchid flowers
within populations (number of flowers per unit
area)
b) increased density of heterospecifics co-flowering
within populations
c) insect visitation rate to both the orchid and
heterospecific co-flowerers.
Methods
Study species and populations
We investigated two members of the orchid genus
Spiranthes,S. romanzoffiana, and S. spiralis. Both
species are non-autogamous and share similar traits
such as; self-compatibility, bee pollination, and offer
a nectar reward, but differ in their flowering time and
habitats in which they occur (Catling 1983; Willems
and Lahtinen 1997). These species do not co-occur in
Ireland. The pollen of both species is aggregated into
a pair of sectile pollinia held together by a viscidium.
The pollination mechanisms of these species have
been described by Catling (1983)(S. romanzoffiana)
and Darwin (1862)(S. spiralis). Both species are
protandrous which may effectively reduce self pol-
lination and insects tend to forage acropetally (bot-
tom to top) on inflorescences (Catling 1983).
Medium- to long-tongued bees are the documented
pollinators of both of these species (Catling 1983;
Duffy and Stout 2008; Jacquemyn and Hutchings
2010). When bees probe the flowers, almost always
both pollinia are removed together, and pollen is
deposited as pollen sheets (massulae) on the stigma,
with multiple flowers potentially receiving pollen
from a single pollinium.
Spiranthes romanzoffiana (Cham.) is perennial
orchid that is widespread in North America, yet
confined to the fringes of Ireland and Britain in
Europe (Lupton 2008). The typical habitat is damp
peaty meadows, pastures and lakeshores (Summer-
hayes 1968; Lupton 2008). In Ireland, S. romanzof-
fiana begins flowering in mid- to late-July and
continues through to the end of August. The height
of the inflorescence varies between 5 and 35 cm and
can bear up to 30 tubular white flowers in a three-
ranked arrangement. Fruit set is variable; it is 0% in
European populations (Duffy and Stout 2008) while
can be up to 75% in North American populations
(Larson and Larson 1987). The reason for the lack of
fruit set in European populations is not yet fully
understood, but is not due to pollen limitation
(Forrest et al.2004; Duffy and Stout 2008). However,
minute quantities of seeds are contained in the
unripened fruits (Lupton 2008; KJ Duffy pers.
obs.), which may maintain current populations.
Hence, S. romanzoffiana is considered endangered
in the Republic of Ireland and is protected by the
Wildlife Act (1976) and the Wildlife Amendment Act
(2000), under the Flora Protection Order (1999).
Spiranthes spiralis (L.) Chevall. is a small, long-
lived perennial orchid with a distribution that ranges
throughout Europe from North Africa as far north as
Denmark to Russia in the east and Ireland in the west
(Tutin et al. 1980). It is endangered in other parts of
Europe, for example, the Netherlands (Jacquemyn
et al. 2007) and France (Machon et al. 2003). It grows
in relatively dry, nutrient poor meadows, or calcar-
eous grassland. It occupies open grazed areas with a
constant land-use. In Ireland, S. spiralis begins
flowering in late August and continues through to
mid-September. The height of the inflorescence
varies between 5 and 25 cm and can bear up to 25
small, white flowers that are arranged as a spiral on
the upper half of the flowering stalk. Fruit set can be
variable; between 0 and 78% (mean: 35%) in the
Netherlands (Willems and Lahtinen 1997). Seeds are
wind-dispersed, but most fall in the vicinity of
maternal plants (Machon et al. 2003).
This study was conducted in the west of Ireland in
2006. We selected four un-managed populations of
varying size (based on the number of flowering
orchids) of each of S. romanzoffiana and S. spiralis
(Fig. 1). Populations of S. romanzoffiana were mon-
itored from 25 July to 26 August; populations of
S. spiralis were monitored from 2 September to 16
September, representing the flowering period for each
species. Populations of S. romanzoffiana contained 14
(Carraig a moiltı
´n), 21 (Knockmore), 71 (Loch
Plant Ecol
123
Allen), and 102 (Loch Cullen) flowering individuals.
Populations of S. spiralis contained 71 (Yellow
Beach), 81 (Ballyconnell), approx. 250 (Strandhill),
and approx. 500 (Mullachmore) flowering individu-
als. These populations were selected on the basis that
they represent the range of population sizes of both
species in the country (KJ Duffy pers. obs.). Other
heterospecific co-flowering vegetation at S. romanz-
offiana populations included: Leontodon hispidus,
Lythrum salicaria,Mentha aquatica,Potentilla
erecta, and Prunella vulgaris. Other heterospecific
co-flowering vegetation at S. spiralis populations
included: Campanula rotundifolia,Leontodon autum-
nalis,Leucanthemum vulgare,Mentha aquatica,
Senecio jacobea, and Trifolium repens.AsP. erecta,
L. vulgare, and S. jacobea are species not visited by
bees (Dramstad and Fry 1995) and were visited very
infrequently in our study, we excluded these species
from our calculations of heterospecific effects on
pollination.
Visitation observations, pollinia removal
and deposition
Observations were made in dry weather between
8:30 am and 6 pm over 10 days and 3–8 days in S.
romanzoffiana and S. spiralis population, respec-
tively. The floral densities of both orchids and each of
three co-flowering heterospecific species were
recorded in randomly selected 2 m 92 m patches
(10 replicates in each S. romanzoffiana population,
3–8 replicates in S. spiralis populations; Ballycon-
nell =8, Mullachmore =8; Strandhill =7, Yellow
Beach =3), measured using a quadrat and measuring
tape, of: a) flowering orchids and b) each of three
neighboring co-flowering heterospecific species, on
each of 10 days in all S. romanzoffiana populations
and 3–8 days in S. spiralis populations in dry
weather. This particular size of quadrat was selected
because this size accounted for the variation in
density in the field of both orchid species, bees often
Fig. 1 Location of study populations of Spiranthes romanzoffiana and S. spiralis in the Republic of Ireland; open symbols indicate
S. romanzoffiana populations; closed symbols indicate S. spiralis populations
Plant Ecol
123
forage within a few meters within patches, and
allowed multiple independent observations in each
population throughout the flowering period. Each
patch was observed for 15 min and was only
observed once during the season to avoid pseudore-
plication. All insects entering patches, visiting flow-
ers and probing for nectar and pollen, and the number
of flowering units (individual flowers for Spiranthes
spp.; individual flowers, inflorescences or capitula for
co-flowering heterospecifics, depending on the spe-
cies) visited by each individual were recorded. We
identified bee visitors to species level (except Bombus
lucorum/terrestris complex, whose workers are indis-
tinguishable in the field; Edwards and Jenner 2005),
and grouped other visitors as lepidopterans, syrphids
or other dipterans. As observations of direct insect
visitation to orchids can be infrequent (Tremblay
et al. 2005) we also measured pollinia removal and
deposition (presence/absence of massulae on the
stigma) in S. romanzoffiana and S. spiralis using a
109hand lens or a 129head lens to examine all
flowers within each patch at the end of each
observation period. We calculated visitation rate per
flowering unit per hour as the number of visits to
flowering units in 15 min/total number of flowering
units in the patch 94. In S. romanzoffiana popula-
tions a range of taxa were observed visiting all
flowers: Bombus pascuorum (36.4%), B. lucorum/
terrestris (27.4%), Apis mellifera (16.3%), Syrphids
(8.2%), B. hortorum (5.2%), Dipterans (5.9%), and
Lepidopterans (0.6%). Similarly, we observed a
range of taxa visiting all flowers in S. spiralis
populations: B. pascuorum (33.8%), B. lucorum/
terrestris (32.2%), A. mellifera (22.5%), Syrphids
(8.1%), Dipterans (2.7%), and Lepidopterans (0.8%).
We included only bees in our calculation of both
conspecific and heterospecific visitation rates, as
these were the only insects observed carrying Spir-
anthes pollen.
Data analysis
We tested for the relationship between: (i) density of
conspecifics (ii) bee visitation rate to conspecifics, (iii)
density of heterospecifics, and (iv) bee visitation rate to
heterospecifics, on both the proportion of pollinia
removed and proportion of pollinia deposited, sepa-
rately for S. romanzoffiana and S. spiralis. The
composition of heterospecific co-flowering species
did not vary much between populations of either orchid
species; therefore we pooled the data and analyzed the
visitation to all heterospecifics together. For both
orchid species, we tested for collinearity between the
explanatory variables by following the approach
outlined by Zuur et al. (2009). We calculated variance
inflation factors (VIF) for each fixed factor in all
models and we omitted variables that showed signif-
icant correlation and high VIF values. We used mixed-
effect models and included population as a random
effect in each model. This was because we studied four
populations for both orchids and our observation
replicates were nested within-population. In addition,
the response variables are binary (i.e., presence/
absence of pollinia/massulae) and are likely to reach
asymptote, therefore a binomial link function was
used. Laplace approximation was used to generate the
models. We found a significant correlation between
heterospecific floral density and heterospecific visita-
tion rate in S. spiralis, therefore we only included
heterospecific floral density in the model of S. spiralis
pollination. Fixed factors included in final analysis had
VIF values \1.5, which is below the recommended
threshold VIF value of 3 for evidence of collinearity
between fixed factors (Zuur et al. 2009).To perform the
mixed-model analyses, we used the lme4 package
(Bates et al. 2009) in R 2.11.1 (R Development Core
Team 2009).
Results
A total of 52 visits were observed to S. romanzoffiana
flowers and 48 visits were observed to S. spiralis
flowers during a total of 16.5 h of daytime observa-
tions. Bees (Bombus spp. and Apis mellifera) were the
only visitors observed for both species. A total of
1,393 visits to co-flowering heterospecifics were
recorded during a total of 30 h of daytime observa-
tions in S. romanzoffiana populations representing a
range of insect taxa. A total of 323 visits to
co-flowering heterospecifics were recorded during a
total of 19.5 h of daytime observations in S. spiralis
populations.
Pollinia removal and deposition occurred in all
populations of both species. Among populations,
S. romanzoffiana had a mean 0.31 flowers with
pollinia removed per patch (range: 0.19–0.42) and a
Plant Ecol
123
mean 0.04 flowers with massulae deposited per patch
(range: 0.02–0.06). Among populations, S. spiralis
had a mean of 0.48 flowers with pollinia removed
per patch (range: 0.44–0.54) and a mean of 0.12
flowers with massulae deposited per patch (range:
0.05–0.14). With S. romanzoffiana, there were
positive relationships between pollinia removal and
heterospecific density and pollinia deposition and
conspecific density (Table 1; Fig. 2a, b). With S. spi-
ralis, there was a positive relationship between the
proportion of flowers with pollinia removed and the
conspecific visitation rate (Table 2; Fig. 3a) and
between pollinia deposition and conspecific density
(Table 2; Fig. 3b). However, heterospecific density
was negatively related to pollinia deposition
(Table 2; Fig. 3c).
Discussion
In accordance with our expectation, there was
increased pollen deposition with increased conspecific
density in both S. romanzoffiana and S. spiralis. Also,
there was increased pollen removal in S. romanzof-
fiana with increased heterospecific floral density,
indicating facilitation of pollen removal. Increased
visitation rate to flowers of S. spiralis increased pollen
removal, while increases in heterospecific density
reduced pollen deposition. Consequently, the results
of this study show the variable nature of pollen export
and import in hermaphrodite plants. As highlighted by
Thomson (1982) there is no reliable way of predicting
whether the interaction between any two plant species
will be characterized by competition or facilitation for
Table 1 Mixed-effect models for factors influencing proportion of flowers with pollinia removed and proportion of flowers with
massulae deposited in patches of S. romanzoffiana
Proportion pollinia removed Proportion massulae deposited
Random Effect Standard deviation Standard deviation
Population \0.001 0.594
Fixed Effects Estimate S.E. Estimate zPEstimate S.E. Estimate zP
Intercept -1.457 0.195 -7.462 \0.001 -3.714 0.588 -6.311 \0.001
Conspecific density
a
0.024 0.015 1.558 0.119 0.072 0.036 2.025 0.043
Conspecific visitation rate 0.089 0.094 0.946 0.344 0.052 0.179 0.292 0.770
Heterospecific density 0.015 0.007 2.061 0.039 0.006 0.017 0.360 0.719
Heterospecific visitation rate -0.011 0.016 -0.660 0.509 0.037 0.035 1.084 0.278
a
Conspecific density was centred
0 10203040
0.0 0.4 0.8
Mean heterospecific density
Proportion pollinia removed
0 5 15 25
0.0 0.1 0.2 0.3 0.4
Conspecific density
Proportion massulae deposited
(b)(a)
Fig. 2 The relationship between adensity of heterospecific
co-flowering species and proportion of flowers with pollinia
removed and bconspecific density and the proportion of
flowers with massulae deposited in S. romanzoffiana
populations (number of flowering plants in parenthesis): open
square Carraig a moiltı
´n(N=14), open circle Knockmore
(N=21), open triangle Loch Allen (N=71), inverted open
triangle Loch Cullen (N=102)
Plant Ecol
123
pollination and this may differ between years (Dudash
and Fenster 1997) and populations (La
´zaro and
Totland 2010). For example, a previous study on
S. romanzoffiana revealed a negative relationship
between pollinator visitation and conspecific inflores-
cence density (Duffy and Stout 2008). In this study,
we found a positive effect of conspecific density
on pollen deposition in both S. romanzoffiana and S.
spiralis populations. This suggests intra-specific facil-
itation of pollination, and supports a previous study
which showed that aggregated inflorescences of
S. spiralis have greater fruit set that sparsely distrib-
uted ones (Willems and Lahtinen 1997), and probably
reflects the optimization of foraging patterns by
flower-visiting insects (Andersson 1988).
Pollen removal in S. romanzoffiana was higher
when heterospecific density increased, but heterospe-
cific density had no effect on pollen deposition.
Visitors move from S. romanzoffiana to other
co-flowering species, hence causing pollen wastage
and probably lose pollen while travelling between
flowers (possibly via grooming or breakage of the
sectile Spiranthes pollinia). Indeed, Bombus pascuo-
rum and B. lucorum/terrestris were often observed
visiting heterospecifics immediately after visiting
S. romanzoffiana (KJ Duffy pers. obs.). When
heterospecific co-flowering plants are more abundant,
foragers may find them more profitable, causing
foragers to focus on them (as majors) while occa-
sionally including S. romanzoffiana in their foraging
regime (as minors) (Heinrich 1979). It is possible that
bees focus on S. romanzoffiana and S. spiralis for
nectar and collect pollen from co-flowerers; nectar in
bagged flowers was higher than in open flowers in
S. romanzoffiana and S. spiralis (KJ Duffy unpub.
data), suggesting depletion by insect foragers.
With S. spiralis, we found intraspecific facilitative
effects, as there was an increase in pollen removal
with increased conspecific visitation, and increased
pollen deposition with increased conspecific density.
Table 2 Mixed-effect models for factors influencing proportion of flowers with pollinia removed and proportion of flowers with
massulae deposited in patches of S. spiralis
Proportion pollinia removed Proportion massulae deposited
Random effect Standard deviation Standard deviation
Population 0.029 \0.001
Fixed Effects Estimate S.E. estimate zPEstimate S.E. estimate zP
Intercept -0.798 0.214 -3.739 \0.001 2.532 0.348 -7.268 \0.001
Conspecific density -0.001 0.003 -0.336 0.736 0.022 0.005 4.001 \0.001
Conspecific visitation rate 0.536 0.215 2.491 0.013 0.172 0.350 -0.491 0.623
Heterospecific density -0.002 0.017 0.090 0.928 -0.061 0.029 -2.060 0.039
0.0 0.4 0.8 1.2
0.0 0.4 0.8
Conspecific visitation rate
Proportion pollinia removed
(a)
0 20406080
0.0 0.2 0.4
0.0 0.2 0.4
Conspecific density
Proportion massulae deposited
(b)
02468 12
Mean heterospecific density
Proportion massulae deposited
(c)
Fig. 3 The relationship between aconspecific density and
proportion of flowers with pollinia removed, bconspecific
density on the proportion of flowers with massulae deposited,
and cheterospecific density on the proportion of flowers with
massulae deposited in S. spiralis (number of flowering plants
parenthesis): Filled diamond Yellow Beach (N=71), filled
square Ballyconnell (N=81), filled triangle Strandhill
(N=[250), filled circle Mullachmore (N=[500)
Plant Ecol
123
However, there was a decrease in pollen deposition
when heterospecific density increased; hence increas-
ing heterospecific density can negatively affect
overall reproductive success in S. spiralis. In the
Netherlands, Willems and Lahtinen (1997) found that
the removal of co-flowering heterospecifics increased
fruit production in S. spiralis, suggesting there was
interspecific competition for pollinators.Although
S. spiralis was rarely surrounded by dense flowering
vegetation in our study populations, we detected a
similar effect of heterospecific competition for pollen
deposition. Also, it could be that S. spiralis flowers
later in the season when fewer pollinators are
available, thereby increasing competition for limited
pollinator attention. We found a very low proportion
of flowers had pollen deposited per patch in
S. romanzoffiana populations (less than one in twenty
on average), compared with S. spiralis populations
(more than one in nine flowers on average). This is
could be due to fewer co-flowering heterospecifics in
S. spiralis populations compared with S. romanzof-
fiana or low numbers of conspecifics in S. romanz-
offiana populations.
Fruit maturation failed to occur in S. romanzoffiana
despite pollination, which makes it impossible to
compare overall reproductive success with density.
However, low numbers of embryo-containing seeds
(*100–200 seeds) were found in the withered fruits,
which makes it possible that S. romanzoffiana repro-
duces sexually (KJ Duffy pers. obs.). Whether these
seeds are viable to establish new individuals in
suitable habitats requires further long-term investiga-
tion. It may be that fruit failure is the result of genetic
inbreeding. Indeed, Forrest et al. (2004) found low
genetic diversity in Irish populations of S. romanzof-
fiana with microsatellite markers, which suggest
autogamous reproduction or an extreme genetic
bottleneck. However, Lupton (2008) used AFLP
markers and microsatellite markers and showed that
there is genetic variation and differentiation within
and between Irish populations; hence examination of
patterns of genetic variation in S. romanzoffiana
requires further work. As S. romanzoffiana is endan-
gered in Ireland and is known from very few
populations, current populations therefore require
monitoring and protection. Other co-flowering spe-
cies may maintain pollinator species around S.
romanzoffiana populations by providing other nectar
or pollen rewards. Also, although S.spiralis is not
considered to be in decline in Ireland, it is rare in
other European countries (e.g., the Netherlands;
Jacquemyn et al. 2007), and therefore populations
should be monitored to ensure severe population
declines do not occur.
Although conspecific density has been shown to
affect pollination in rewarding orchids (e.g., Brys
et al. 2008; Duffy and Stout 2008; Johnson et al.
2009); this study shows that heterospecific co-flow-
ering species may also play an important role in their
pollination. Orchids are not known to provide a
pollen reward for pollinators, therefore require
co-flowering heterospecifics to provide such rewards
(e.g., for pollen-collecting bees; Duffy and Stout
2008). Rewarding orchids may compete more effec-
tively for pollinator attention compared with nectar-
less orchids, regardless of density, as they have a
greater opportunity to attract and maintain pollinator
fidelity, although they may suffer greater geitonog-
amous pollen transfer. In rewardless species, an
increase in heterospecific co-flowering density may
increase pollination of the orchid (i.e., facilitation) by
attracting greater numbers of pollinators to patches
within populations (e.g., Johnson et al. (2003)).
Further research on the effects of density on
pollination in rewarding orchids in natural popula-
tions should focus on manipulative density experi-
ments and the staining of pollinia with histochemical
stains (Peakall 1989). Staining pollinia can allow for
estimation of the contribution of geitonogamous
versus xenogamous pollen movement to reproductive
success. For example, Johnson et al. (2009) used
histochemical stains to show that pollen transfer does
not vary with population size in the orchid Satyrium
longicauda; however, they found more there was
self-pollination in small populations. Neither density
manipulations or pollen staining were performed in
our study as it would be extremely difficult to
precisely count massulae on the stigmas of these
Spiranthes species to get a reliable estimate of the
contributions of geitonogamous pollen movement.
Spiranthes romanzoffiana is endangered and trans-
planting individuals to manipulate density may
negatively impact on population survival. Also,
S. spiralis does not suffer a reduction in fruit set
when self-pollinated by hand (Willems and Lahtinen
1997; Jacquemyn and Hutchings 2010). Although the
quality of seed produced may be affected by self-
pollination, recording stigmas with pollen deposited
Plant Ecol
123
was enough to quantify pollination success in these
species.
In conclusion, even though plant species may be
endangered and occur in few flowering populations,
they may not necessarily suffer an overall reduction
in pollination. The negative effects of low numbers
and size of populations on pollination can be
ameliorated by increases in within-population density
and the presence of co-flowering heterospecific
species. In addition, more abundant species may also
suffer similar conspecific and heterospecific density
effects on pollen transport, to endangered species.
Increased knowledge in this area is important for
developing our understanding of how insect-polli-
nated plant species respond to accelerating anthro-
pogenic-mediated habitat reduction and modification.
Acknowledgments We are grateful to Tony Arthur, Don
Cotton, Tommy Earley, Steve Johnson, Naomi Kingston,
Darach Lupton, Magali Proffit, and Brendan Sayers for
advice and practical support. We thank an anonymous
reviewer for helpful comments that improved the manuscript.
We thank the EU-funded ALARM project for including us in
their discussions on plant pollination. This work was funded by
a National Parks and Wildlife Service research grant (D/C/81),
a University of Dublin, Trinity College Start-Up Grant (both
awarded to JCS) and University of Dublin, Trinity College
Postgraduate Award (awarded to KJD). Work was carried out
on S. romanzoffiana under Department of the Environment,
Heritage and Local Government license number 8/2004.
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  • ... Whereas hundreds of studies demonstrate competition for pollinator visits (reviewed in: Mitchell et al. 2009), relatively few have found positive interactions between pollinator-shar- ing plant species (Moeller 2004;Duffy & Stout 2011;Liao et al. 2011). Mutualism, i.e. positive interactions in both directions, has been detected in even fewer cases (Waser & Real 1979;Thomson 1981). ...
  • ... linator attraction as already demonstrated in rewarding (Duffy & Stout, 2011;Sun, Gross, & Schiestl, 2014;Sun, Schlüter, Gross, & Schiestl, 2015) and food-deceptive species (Galizia et al., 2005;Jersáková, Jürgens, Šmilauer, & Johnson, 2012;Sletvold & Ågren, 2011). In the second case, our in situ manipulation experiments emphasize the significance of the open perianth, such as callus, as strong visual signals for pollinator attraction and reproductive success in the sexually deceptive species (Peakall et al., 2010;Rakosy et al., 2012;Spaethe, Moser, & Paulus, 2007). ...
  • ... The relative importance of pollination quality, however, remains an aspect deserving attention, since interspecific pollen transfer, rather than visitation rate, is the component that is directly affected by hybridization. The possibility for quantitative facilitation in coflowering (but non-hybridizing) species has been investigated theoretically (Feldman et al. 2004;Hanoteaux et al. 2013) and empirical data supporting such a facilitation mechanism have been reported from several experimental and natural plant communities (Ghazoul 2006; Duffy andStout 2011;Sieber et al. 2011;Seifan et al. 2014). We are unaware of any study exploring the possibility for a qualitative facilitation that can result from a neutral hybridization with no changes in the fitness (in the sense of equal competitive ability and reproductive ouput of the colonizing species, the common species, and/or hybrid offspring). ...
  • ... Increased female fecundity in dense conspecific neighbourhoods has been reported in other flowering plant species (e.g. Knight, 2003; Duffy and Stout, 2011; Waal et al., 2014). In the specific context of tropical tree species, positive density-dependent reproduction is expected to be especially relevant because most tropical plant species generally occur in low population densities and rely on animals for cross-pollination. ...
  • ... S. spiralis is a relatively small, white-flowered orchid that is selfcompatible but requires pollinators for seed set (Jacquemyn & Hutchings 2010 ). Its flowers provide nectar reward to pollinators , which in northern Europe are mainly bumblebees and honeybees (Claessens & Kleynen 2011; Duffy & Stout 2011; Willems & Lahtinen 1997). In Greece, S. spiralis grows in a variety of habitats including olive groves, shrublands and forest openings, and occasionally in urban environments (Kantsa 2007; Krigas & Kokkini 2004; Tsiftsis, Tsiripidis, Karagiannakidou, & Alifragis 2008). ...
    ... S. spiralis is thought to have originated in the Mediterranean (Jacquemyn & Hutchings 2010) and its distribution is classified as European–Caucasian (Pignatti, Menegoni, & Pietrosanti 2005 ). It is common in the Mediterranean, particularly in the eastern part (Jacquemyn & Hutchings 2010), whereas its populations are in decline or endangered in northern Europe (Duffy & Stout 2011; Jacquemyn & Hutchings 2010; Willems & Lahtinen 1997). This study was conducted on the island of Lesvos (1630 km 2 ; NE Aegean, Greece), in the centre of the geographical distribution of S. spiralis, during the autumn and winter 2008. ...
    ... Insects, particularly bumblebees, are vital for S. spiralis pollination (Duffy & Stout 2011; Willems & Lahtinen 1997 ). Because bumblebees are rare in Mediterranean habitats (Petanidou & Ellis 1993) we decided to investigate closer the dependence of the plant on its flower visitors and probable pollinators. ...
  • ... Such interactions have immediate consequences for http://www.aloki.hu ● ISSN 1589 1623 (Print) ● ISSN 1785 0037 (Online) DOI: 10.15666/aeer/1301_181192  2015, ALÖKI Kft., Budapest, Hungary endangered Spiranthes species, as increases in both conspecific and heterospecific coflowering density may ameliorate the negative effects of rarity on pollination, hence overall reproductive success (Duffy & Stout, 2011). Lack of suitable insects to pollinate the flowers of Orchis militaris has been characteristic of British populations since the beginning of 19th century according to herbaria data (Farrell 1985). ...
  • ... For example, aggregations of con-and heterospecific co-flowering plants may facilitate increased pollinator visitation to all individuals by increasing the floral display (Rathcke 1983;Moeller 2004). Facilitation may also occur within species, where pollen removal and deposition increase with density of neighbouring conspecifics (e.g.Duffy & Stout 2011). However, the latter mechanism may also act independently of pollinator visitation rate, that is increased mate availability (conspecific density) may result in higher seed set because of a higher probability of conspecific pollen transfer, even if visitation rates or pollinator abundance do not increase (Kunin 1993;Moeller 2004). ...
    ... Similarly, aggregated (clumped) dispersion patterns reduce the extent of heterospecific pollen movement (e.g.Campbell 1986;Feinsinger et al. 1986) while retaining the benefits of joint attraction of pollinators (Moeller 2004). While several studies have demonstrated that density and dispersion affect pollinator visitation rates and/or fecundity of co-flowering individuals (e.g.Duffy & Stout 2011) as well as the intensity of interspecific competition for pollinators (Hanoteaux, Tielb€ orger & Seifan 2013), few have attempted to tease apart the confounding mechanisms (i.e. pollinator visitation, mate availability and heterospecific pollen transfer) which underlie these effects (but see Feinsinger,Tiebout & Young 1991;Kunin 1993;Rodger, van Kleunen & Johnson 2013). ...
    ... Facilitation may also occur within species, where pollen removal and deposition increase with density of neighbouring conspecifics (e.g. Duffy & Stout 2011). However, the latter mechanism may also act independently of pollinator visitation rate, that is increased mate availability (conspecific density) may result in higher seed set because of a higher probability of conspecific pollen transfer, even if visitation rates or pollinator abundance do not increase ( Kunin 1993;Moeller 2004). ...
  • ... In particular, fecundity of generalist plants can be enhanced by abundance of other species that attract pollinators (Johnson et al. 2003; Ghazoul 2006; but see Feldman 2008). Such increases in fecundity may be related to increased visitation rates and pollen transfer between conspecifics in the presence of increased heterospecific abundance (Duffy & Stout 2011). Although these 'facilitation' effects are not strictly Allee effects, which involve intraspecific as opposed to interspecific facilitation, they may shed light on whether the underlying basis for an Allee effect is mate or pollinator limitation. ...
  • ... Pollinators see communities of flowering plants as "biological markets" that offer a wide variety of flowers from which they can choose those with the best rewards ( Chittka and Raine, 2006). The distribution of visitors among flowers is strongly affected by competition between plants, mechanisms of facilitation for the attraction of pollinators ( Ghazoul, 2006;Duffy and Stout, 2011), and competition between pollinators for the exploitation of floral resources ( Pleasants, 1981). Plants need to attract and compete for the attention of pollinators to receive their services. ...
  • ... In general, self-incompatible species are more limited due to the quantities of pollen that are received compared with self-compatible species (Burd, 1994; Knight et al., 2005; Larson and Barrett, 2000). Predation on floral peduncles, which decreases the availability of opened flowers, may enhance pollen limitation phenomena given that patches with lower density of flowers should decrease pollinator attraction (Brys et al., 2008; Duffy and Stout, 2011; Williams et al., 2001). Moreover, given that autogamy rarely occurs in R. weyleri, the pollen limitation could be related to pollen discounting processes as well (Lloyd, 1992). ...
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