Density-dependent effects of ants on selection for bumble bee pollination in Polemonium viscosum

Area of Science:

• Ecology
• Evolutionary Biology
• Plant-Pollinator Interactions

Background:

• Mutualisms are often exploited by 'cheater' species that benefit without reciprocal contribution.
• Previous studies on mutualistic selection often neglect the impact of third-party species, such as cheaters.
• Understanding cheater effects is crucial for comprehending the evolutionary dynamics of mutualisms.

Purpose of the Study:

• To investigate how the abundance of nectar-thieving ants (cheaters) influences selection in a bumble bee-alpine skypilot pollination mutualism.
• To determine the direct and indirect effects of ant abundance on plant reproductive success (seed set).
• To assess the impact of cheaters on the evolution of plant traits, specifically flower size, in relation to pollinator attraction.

Main Methods:

• Field experiments were conducted in a high tundra population of *Polemonium viscosum* (alpine skypilot).
• The abundance of nectar-thieving ants was manipulated in experimental patches.
• Measurements included seed set, flower damage by ants, and bumble bee pollination services (pollen delivery and export).

Main Results:

• Ants significantly reduced seed set per flower by 20% primarily through direct flower damage.
• Increased ant density led to higher flower damage but did not significantly affect bumble bee pollen delivery or export, indicating negligible indirect effects on pollinator services.
• Selection for bumble bee pollination (measured by seed set) declined with increasing ant abundance, demonstrating density-dependent fitness returns.
• Despite reduced fitness returns, significant positive selection for larger flower size persisted across a wide range of ant abundance.

Conclusions:

• Cheating ants negatively impact plant fitness in this pollination mutualism, primarily through direct damage.
• The negative fitness consequences of cheaters are density-dependent, reducing the selective advantage of mutualist partner services.
• Even with high cheater abundance, selection for traits that attract pollinators (like large flowers) is not eliminated, suggesting a complex evolutionary balance.