4. Discussion
Pollen limitation is generally considered a common phenomenon and many
comparative studies report relatively high occurrence (62–73%) in
various habitats (Burd 1994, Ashman et al. 2004). However, we only
recorded significant pollen limitation for approximately 41% of species
(9 out of 22 surveyed) in our wet meadow community. Our findings are in
concordance with a similar unusually low occurrence of pollen limitation
in a temperate grassland community in western Norway (Hegland and
Totland 2008). This study focused on pollen limitation and its
relationship to plant species visitation rates and specialisation levels
and revealed only two out of eleven (~18%) studied
plant species to be significantly pollen limited. Moreover, Bennett et
al. (2018) even documented no pollen limitation in investigated study of
nine species in a Romanian meadow community. It might seem that the low
levels of pollen limitation revealed in the pollen supplementation
experiments are in agreement with the assumptions from the model by Haig
and Westoby (1988), which stipulates that seed set in flowering plants
should be equally limited by both pollen and resource availability. It
further suggests that pollen supplementation should not increase seed
set in populations at their evolutionary equilibrium, because resources
should be unavailable for maturation of their additional fertilized
ovules. However, Burd (2008) adjusted this model for stochastic
variation in both ovule fertilization and resource availability, which
made the model broadly in accordance with the recent meta-analysis (Burd
1994, Ashman et al. 2004), in which pollen limitation is found in most
surveyed species.
The reported inconsistencies in the magnitude of pollen limitation could
stem from several non-mutually exclusive reasons:
(1) Effect of sampling size and experimental design. Using power
tests (via simulation) for pollen supplementation experiments, Thomson
(2001) illustrated that moderate pollination deficits of up to 15% will
usually not be detected with sample sizes of 20 individuals, and even 40
are insufficient for minor deficits. But, unfortunately, lower sampling
effort (such as 20-30 individuals in our study) is an inevitable result
of various logistic constrains and trade-offs between the data quantity
and quality in most community studies (Hegland and Totland 2008,
Wolowski et al. 2013, Lázaro et al. 2015, Bennett et al. 2018).
(2) Publication bias . The community approach, where multiple
plant species are studied simultaneously, may lead to a better
understanding of patterns in pollen limitation. It is because
environmental characteristics, such as nutrient levels within a given
community, are relatively homogenous in such studies and the role of
plant traits in pollen limitation can, therefore, be better assessed.
Nevertheless, there have been few studies focused on the relationships
between plant traits and pollen limitation across whole communities
(Motten 1986, Hegland and Totland 2008, González and Pérez 2010,
Wolowski et al. 2013, Lázaro et al. 2015). All these studies recorded
lower levels of pollen limitation in natural systems compared to the
pollen limitation documented in comprehensive reviews that are mostly
based on single-species studies (Burd 1994, Ashman et al. 2004).
Therefore, the publication bias, favouring statistically significant
responses which then become available for further studies, together with
the omission of ‘grey literature’ and studies not written in English
(Auger 2017), complicates our understanding of pollen limitation (Knight
et al. 2006).
(3) Effect of pollinator abundance. Hegland and Totland (2008)
discussed their results of low pollen limitation in the context of a
possible higher pollinator abundance in the studied community, which
could substantially reduce the quantitative pollen limitation. A partial
cause of low pollen limitation in our study could be that the targeted
semi-natural locality is situated in a relatively well-preserved and
mosaic-like landscape with a limited influence of intensive agriculture.
Such semi-natural, diverse, and heterogeneous environments support
pollination services (Steffan‐Dewenter and Westphal 2008, Viana et al.
2012) and thus increase the plant reproductive success, as suggested by
Bennett et al. (2018) in their Romanian meadow community.
(4) Effect of plant community composition and study species
selection . In our investigated community, only a few plant species with
morphologically highly specialized flowers, which are expected to be
more prone to pollen limitation, were present. Therefore, this community
may have a lower pollen limitation than communities with a greater
proportion of specialized flowers.
(5) Choice of the pollen limitation measure . An important factor
determining the recorded magnitude of pollen limitation may also be the
choice of its measure. Knight et al. (2006) compared 263 studies working
with different measurements of the production component of reproduction
and revealed the largest effect for relative fruit set, and the lowest
effect for production of seeds/flower and seeds/fruit. However, because
the magnitude of pollen limitation was inter-correlated among these
response variables, Knight et al. (2006) assumed that pollen limitation
occurs simultaneously at different stages of the plant reproduction, but
with varying intensity. Also in our study the numbers of pollen-limited
species varied substantially between the two applied measures, seed
production and seed mass. Furthermore, Hegland and Totland (2008)
pointed out that the two main components of plant reproductive success,
seed production and seed mass, are often not included in the same
studies.
In our studied community, two species showed significant positive seed
weight response after pollen supplementation, though we expected the
negative relationship. Several studies demonstrated that seed mass
decreases with pollen availability due to seed size–number trade-off
(Ashman et al. 2004). For example, Ågren et al. (2006) recorded reduced
mean seed size in hand-supplemented Primula farinosa by about
12%, but a larger total mass of seeds than in naturally-pollinated
plants. The opposite effect, i.e. increased seed weight after pollen
supplementation could be explained by the increased pollen quality
(Hegland and Totland 2008). Aizen and Harder (2007) suggested that the
cross-pollen used for supplementation may have higher quality than the
mixture of self- and cross-pollen available under the natural
pollination. It seems evident that the magnitude of pollen limitation is
dependent on the treatment level, e.g. whether the experimental design
is applied only on a fraction of the plant’s flowers or on the whole
plant (Zimmerman and Pyke 1988). Unfortunately, because we treated only
flower pairs, we can only speculate on the proportion of resource
allocation in our study (Wesselingh 2007). The low differences in the
seed weight between the treatments could be caused by the ability of the
plant to compensate for any possible higher cost of an additional seed
production induced by the supplemental pollination in only one flower.
Despite analyses of several floral and life history traits connected to
plant reproduction, we only found the significant relationship of PL to
the number of pollinator functional groups. This finding is in
accordance with the meta-analysis of pollen limitation in different
world regions (Vamosi 2006, Wolowski et al. 2014, Rodger and Ellis
2016), where the more pollinator-specialized plant species were also
more pollen limited (but see Hegland and Totland 2008). However, Lázaro
et al. (2015) pointed out that this relationship is not entirely clear
and it is very important to distinguish between morphological (based on
floral shape) and ecological (based on realized interactions)
specialization. They found a strong negative relationship between pollen
limitation and ecological generalization, but only for species with the
morphologically specialized flowers. As a possible explanation they
suggested that the morphologically specialized flowers benefit more from
generalizing their pollination system in the lack of a primary
pollinator (Lázaro et al. 2015). The high ecological generalization may
however result in the stronger pollen limitation because of lower
flower-visitor diversity with abundant low-efficiency pollinators
transporting high loads of incompatible pollen (Gómez et al. 2010).
Accordingly, we recorded stronger pollen limitation in the species with
specialized, as well as highly generalized pollination systems. This
supports the prediction that many mutual relationships between plants
and visitors should be non-linear (Young 1988, Morris et al. 2010).