Abstract
Biological invasions are a threat to the maintenance of ecological processes, including pollination. Plant-flower visitor networks are traditionally used as a surrogated for pollination at the community level, despite they do not represent the pollination process, which takes place at the stigma of plants where pollen grains are deposited. Here we investigated whether the invasion of the alien plant Impatiens glandulifera (Balsaminaceae) affects pollen transfer at the community level. We asked whether more alien pollen is deposited on the stigmas of plants on invaded sites, whether deposition is affected by stigma type (dry, semidry and wet) and whether the invasion of I. glandulifera changes the structure of the resulting pollen transfer networks. We sampled stigmas of plants on 10 sites invaded by I. glandulifera (hereafter, balsam) and 10 non-invaded control sites. All 20 networks had interactions with balsam pollen, although significantly more balsam pollen was found on plants with dry stigmas in invaded areas. Balsam pollen deposition was restricted to a small subset of plant species, which is surprising because pollinators are known to carry high loads of balsam pollen. Balsam invasion did not affect the loading of native pollen, nor did it affect pollen transfer network properties; networks were modular and poorly nested, both of which are likely to be related to the specificity of pollen transfer interactions. Our results indicate that pollination networks become more specialized when moving from the flower visitation to the level of pollen transfer networks. Therefore, caution is needed when inferring pollination from patterns of insect visitation or insect pollen loads as the relationship between these and pollen deposition is not straightforward.
Highlights
Human activities have significantly increased the introduction and movement of alien species around the planet since the 18th Century [1]
A novel and exciting approach which lead to the publication of the first pollen transfer network, this study was from a single field plot and data were collected over just two consecutive days
Our objectives are twofold: 1) to quantify the amount of balsam, conspecific and heterospecific pollen deposition on stigmas of native plants, and to test whether this is affected by stigma type, and 2) to test whether the presence of a highly invasive alien plant changes the structure of the pollen transfer networks
Summary
Human activities have significantly increased the introduction and movement of alien species around the planet since the 18th Century [1]. Pollination of native species, for example, can be affected by plant invasion due to reduced visits of pollinator insects [6], which can lead to decreased seed set [7,8,9,10], the latter occurring by either reduced transfer of conspecific pollen or excessive heterospecific pollen deposition from the alien plant [11,12]. Alien pollen deposition on native stigmas is considered one of the main causes of pollination disruption worldwide [5,13] and while there have been recent attempts to understand pollen transfer at the community level [14,15,16,17], there are still large gaps in our understanding of the impact of alien plants on pollination. We adapt and expand their approach to investigate effects of a highly invasive plant species, Impatiens glandulifera, on the structure of bipartite pollen transfer networks. In contrast to Fang and Huang (2013) [15] where links were established between plant species that share the same pollen type deposition, we constructed bipartite networks where links are established between plant species and the pollen types deposited in its stigmas, analysed at the community level
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