Author
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Swope, Sarah |
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Submitted to: Oecologia
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/19/2013 Publication Date: N/A Citation: N/A Interpretive Summary: Plants often make less seed than they are capable of. One of the reasons for this is that they do not receive enough pollinator visits to pollinate all the ovules they produce, a phenomenon known as pollen limitation. Pollen limitation is thought to be important in regulating population size and it has been well-studied in rare plants. But pollen limitation has been largely ignored by invasion biologists, despite the fact that more than half of the invaders in North America are dependent on pollinators and we are principally concerned about the size of their populations. The ecological literature shows that herbivore damage can deter pollinator visitation and lead to pollen limitation. This is relevant to the management of invasive plants with biocontrol because it suggests that agents whose damage indirectly deters pollinators may have an enhanced impact on their weeds. I conducted a field experiment to determine whether (a) pollen-limitation occurs in the invasive weed Centaurea solstitialis, (b) whether infection by a biocontrol pathogen affected the degree of pollen-limitation, and (c) whether this varied across a soil moisture gradient. Plants growing on north-facing slopes, where soil moisture was higher, experienced mild pollen limitation in the absence of the pathogen and more pronounced pollen limitation when they were infected. Plants on south-facing slopes appear to suffer from resource limitation rather than pollen limitation. The pathogen directly reduced seed set in the weed by 70 – 78% but on the north-facing slopes where plants suffered from pollen-limitation, it indirectly reduced seed set by another 42% by exacerbating pollen limitation. The mechanism underlying this outcome appears to be the size of the floral display: infected plants made fewer inflorescences, which attracted fewer pollinator visits, which led to greater pollen-limitation. Exploiting a weed’s dependence on pollinators by selecting agents that deter pollinator visitation may improve our ability to control widespread weeds, although this outcome is dependent on abiotic factors that vary over very small spatial scales (slope aspect, in this case). Technical Abstract: Pollen-limited seed set is rarely studied in invasive plants despite the fact that >50% of the most problematic invaders in North America are pollinator-dependent. Further, pollinators are rarely considered in the use biocontrol agents to manage invaders. The ecological literature shows that herbivore damage can deter pollinator visitation, suggesting that agents whose damage also indirectly deters pollinators may have an enhanced impact on their weeds. I conducted a field experiment in which I manipulated infection by a biocontrol pathogen and pollen supplementation to test for (a) pollen-limitation in the invasive weed Centaurea solstitialis, (b) whether infection affected the degree of pollen-limitation, and (c) whether this varied across a soil moisture gradient. Plants growing on north-facing slopes, where soil moisture was higher, experienced mild pollen limitation in the absence of the pathogen and more pronounced pollen limitation when they were infected. Plants on south-facing slopes appear to suffer from resource limitation and the addition of the pathogen exacerbated that. Pathogen infection directly reduced seed set in the weed by 70 – 78%. On the north-facing slopes where plants suffered from pollen-limitation, it indirectly reduced seed set by another 42% by reducing visitation. The trait that mediates this indirect pathogen-pollinator interaction is the size of the floral display: infected plants made fewer inflorescences which led to greater pollen-limitation. Exploiting a weed’s dependence on pollinators by selecting agents that deter visitation may improve our ability to control widespread weeds, although this outcome is dependent on abiotic factors that vary over very small spatial scales. |
