Data from: Neighbourhood effects on plant reproduction: an experimental-analytical framework and its application to the invasive Senecio inaequidens

1. Density-dependence is of fundamental importance for population and
range dynamics. Density-dependent reproduction of plants arises from
competitive and facilitative plant-plant interactions that can be
pollination-independent or pollination-mediated. In small and sparse
populations, conspecific density-dependence often turns from negative to
positive and causes Allee effects. Reproduction may also increase with
heterospecific density (community-level Allee effect), but the underlying
mechanisms are poorly understood and the consequences for community
dynamics can be complex. Allee effects have crucial consequences for the
conservation of declining species, but also the dynamics of range edge
populations. In invasive species, Allee effects may slow or stop range
expansion. 2. Observational studies in natural plant communities cannot
distinguish whether reproduction is limited by pollination-mediated
interactions among plants or by other neighbourhood effects (e.g.
competition for abiotic resources). Even experimental pollen supply cannot
distinguish whether variation in reproduction is caused by direct density
effects or by plant traits correlated with density. Finally, it is unknown
over which spatial scales pollination-mediated interactions occur. 3. To
circumvent these problems, we introduce a comprehensive experimental and
analytical framework which simultaneously (1) manipulates pollen
availability and quality by hand pollination and pollinator exclusion, (2)
manipulates neighbourhoods by transplanting target plants, and (3)
analyses the effects of con- and heterospecific neighbourhoods on
reproduction with spatially-explicit trait-based neighbourhood models. 4.
Applying this framework to Senecio inaequidens, one of Europe’s fastest
plant invaders, we found that the seed set was strongly pollen-limited.
Reproduction had increased by pollinator-mediated facilitation by both
con- and heterospecific neighbours which may lead to (community-level)
Allee effects. Pollination-independent interactions, such as amelioration
of abiotic conditions through neighbours, contributed to additional
positive neighbour effects. However, these pollination-independent
interactions were weaker than the pollination-mediated interactions and
they occurred over smaller spatial scales. Finally, the strength and
direction of neighbourhood effects depended on neighbour traits and thus
changed with the trait composition of the neighbourhood. 5. Synthesis. By
manipulating both pollen availability and target plant locations within
neighbourhoods, we can comprehensively analyse spatially-explicit
density-dependence of plant reproduction. This experimental approach
enhances our ability to understand the dynamics of sparse populations and
of species’ geographical ranges.