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Plant-Pollinator Relationships

May 7, 2012

Photo credit: Aussiegall, Flickr Creative Commons. CC-BY 2.0.

Pollination is arguably among the most elegant reproduction systems on earth: it allows plants, which spend most of their lives immobile, to extend their genetic presence beyond their immediate surroundings, and to react to uncontrollable factors that tirelessly affect them wherever they take root. The truly spectacular diversity of plants that we know today wouldn’t be possible without these complex interactions between plants, pollinators, and their environment. Our most familiar pollinator, the European honeybee, is only the tip of the iceberg: an entire spectrum of animal pollination strategies can be found, all of which are a snapshot of these organisms’ evolutionary histories and their constant struggle to persist. Yet pollination isn’t necessarily an animal-mediated practice, nor does it always need to occur between two or more plants. In this first of three posts on pollination ecology, we will explore the factors that drive the development and persistence of pollination strategies.

Biotic vs. Abiotic Pollination

The most commonly recognized plant reproductive strategy is undoubtedly biotic pollination, which is facilitated by other living things (including bees, birds and bats). Plants can also spread their pollen through abiotic, or non-living, methods, like wind or water. Many plants can reproduce using more than one pollination strategy (Aguiar et al. 2012). The tendency of a particular species of plant towards a pollination strategy generally depends on the availability and types of pollinators within a community of plants and animals (Benadi et al. 2012).

The European honey bee, Apis mellifera, extracts nectar from a flower in exchange for pollination services. Photo credit: Severnjc, Wikimedia Commons (Public Domain).

Generalist Pollination Dynamics

When species are capable of multiple pollination methods, they are often more responsive to changing community conditions. In fact, the ability to thrive in diverse environments is a staple of many invasive species. An excellent example of this flexibility is shown by Aguiar, et al. (2012) in a recent study of the orchid species Oeceoclades maculata, which reproduces through both biotic and abiotic pollination. The authors found that within its natural range in Brazil, the plant is primarily self-pollinated by rainfall, with flowers that bloom in conjunction with the rainy season. However, some pollination also occurs from the visits of a few species of native butterflies, allowing sexual reproduction and promoting genetic diversity.

The researchers compared these native characteristics to an invasive population of the same species on the Caribbean island of Puerto Rico. Their flowers still bloomed at the height of the rainy season (which occurred at a different time of year), but no animals provided pollination services even though comparable nectar was offered. This represents a notable change in reproductive capability: the invasive orchids were now completely dependent on rain-assisted self-pollination to reproduce. As a result, orchids’ response to environmental changes could potentially be limited relative to its native populations. However, it was their original flexibility that allowed them to colonize a new habitat at all, and thriving there could eventually lead the new population to develop into a distinct species. Research supports this idea, finding that geographic isolation drives more speciation than other factors such coevolution (Althoff et al. 2012). The authors of the orchid study also suggest that the unpredictability of pollinators could have a been a significant hurdle to early flowering plants (Aguiar et al. 2012) and a strong selective pressure to have a reproductive ‘back-up plan.’

The variable pollination strategies of the monk orchid (Oeceoclades maculata) have helped it become a widespread invasive species. Photo credit: Americo Docha Neto. CC-BY-SA-3.0,2.5,2.0,1.0 GFDL. http://www.sagebud.com/monk-orchid-oeceoclades/

Explaining Specialized Pollination Strategies

While it is intuitive that plants will fare better in changing conditions when they possess genetic variability and alternative reproductive strategies, how can we account for current observable plant diversity? If the most broadly successful colonizers are generalists that can readily adjust to new environments, why don’t we find only a handful of the most successful plant species established across many ecosystems? These questions may seem simple to those familiar with ecological concepts, but the ability to answer them explicitly is a central problem to the field of ecology. Theoretical research attempting to model specific mechanisms of plant-pollinator dynamics has found that most competitive pollination interactions should lead to a loss of biodiversity (Kunin and Iwasa 1996). In other words, less common plants should become more rare as they lose access to pollinators and/or space. Yet this is not the only result we observe. More recent models were able to show that such positive feedback loops can be circumvented by utilizing a classic form of competition avoidance: niche specialization (Benadi et al 2012). When specialization was high enough in pollinator competition models, relative ecosystem stability was possible (however there are other, non-pollination-related stabilizing mechanisms currently known, but not modeled in the cited study). Such specialists sacrifice the ability to respond to changing conditions for a unique place in a relatively stable ecosystem, and would not be very successful if disturbances or changing climate disrupted their pollinators and habitats.

Butterfly pollination. Photo credit: Aussiegall, Flickr Creative Commons. CC-BY 2.0.

Indeed, we are currently witnessing disturbance-driven changes in ecosystems that are affecting diverse and specialized organisms most drastically (Franzen and Oeckinger 2012). Many of these changes are caused directly or indirectly by humans, including habitat fragmentation and rapid climate change, and represent serious threats to pollination dynamics and thus biodiversity (Phillips et al. 2010). These topics will be discussed in upcoming posts on pollination ecology.

This is the first of three posts on pollination dynamics. View others:
2. Habitat Fragmentation and Pollination Dynamics
3. Pollination Dynamics in a Changing Climate

References

1. Aguiar, J. M., Pansarin, L. M., Ackerman, J. D., Pansarin, E. R. 2012. Biotic versus abiotic pollination in Oeceoclades maculata (lindl.) lindl. (orchidaceae). Plant Species Biology 27(1):86-95.

2. Althoff D. M., Segraves, K., Smith, C., Leebens-Mack, J., Pellmyr, O. 2012. Geographic isolation trumps coevolution as a driver of yucca and yucca moth diversification. Molecular Phylogenetic Evolution 62(3):898-906.

3. Benadi, G. G., Blüthgen, N., Hovestadt, T., Poethke, H. 2012. Population dynamics of plant and pollinator communities: Stability reconsidered. American Naturalist 179(2):157-68.

4. Franzen M and Oeckinger E. 2012. Climate-driven changes in pollinator assemblages during the last 60 years in an arctic mountain region in northern scandinavia. J Insect Conservation 16(2):227-38.

5. Kunin, W., and Iwasa, Y. 1996. Pollinator foraging strategies in mixed floral arrays: Density effects and floral constancy. Theoretical Population Biology 49(2):232-63.

6. Phillips, R. D., Dixon, K. W., Hopper, S. D. 2010. Pollination ecology and the possible impacts of environmental change in the Southwest Australian biodiversity hotspot. Philosophical Transactions of the Royal Society B: Biological Sciences. 365(1539).

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2 Comments leave one →
  1. May 11, 2012 9:33 am

    Well-researched, well-written, and just plain cool stuff. I like the question about why we don’t see a few very successful species everywhere, but I would also argue that if we expand that to “genus” and “family,” then we actually do see them everywhere. Take the genus Prunus (cherry trees), for example. Looking forward to the next one!

  2. timlundy permalink
    May 21, 2012 2:41 pm

    Ivan – You are absolutely right in that we do see many families/genera that have found widespread success across many habitats, and could be support for an evolutionary/cultivation history that promoted some serious generalization adaptations. My argument, though, is that even in the case you mentioned (cherries), we don’t see them dominating in every habitat they appear. I may not have been entirely clear in this point, but in a world where generalism was the only way to make a living (and if Prunus was the best of said generalists) we would expect to find cherry forests spreading across basically any habitat it could colonize. Instead we see most dominance based on adaptations to relatively specific climates and habitats.

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