Plant-pollinator mutualisms and biodiversity

A plant-pollinator mutualism is an association between a plant and a pollinator wherein each partner benefits from the other. Typically, the plant is cross-pollinated with other plants of the same species—a system which mixes the genetic material and creates strong and vigorous seeds. The pollinator gets pollen and nectar—or both—which it uses to nourish itself and the next generation.

Neither the plant nor the pollinator behaves altruistically. That is, neither plant nor pollinator says to itself, “Self, I’m going to help that poor plant (or pollinator) get the things it needs.”

Instead, those plants that have more of what the pollinator needs are more likely to get pollinated. And once they are pollinated, they pass those same genetic traits to the next generation. Likewise, the pollinators that are best equipped to gather nectar or pollen are most likely to survive and pass their genes to the next generation. As time goes on, the plants and the pollinators become more and more suited for each other.

In her article “Mighty Mutualisms: the Nature of Plant-pollinator Interactions” Carol Landry estimates that plant-pollinator mutualisms involve about 170,000 plant species and 200,000 animal species.

Mother nature can be an extremist sometimes. An extreme form of mutualism, called an obligate mutualism, occurs when the interdependence between a plant and a pollinator is so specific that no other organism can take its place. In other words, one specific pollinator is required to pollinate one specific plant and that pollinator needs that specific plant as well. This is also the most precarious kind of mutualism because if one partner becomes extinct, the other goes as well.

A good example of an obligate mutualism is the yucca plant and the yucca moth. The yucca plant is absolutely dependent on the yucca moth to pollinate its seeds. The yucca moth larvae cannot survive without yucca seeds to eat. The system works because the larvae eat only some—not all—of the seeds.

Plant-pollinator mutualisms are believed to be at least partly responsible for the large diversity of flowering plant species that showed up 90-130 million years ago. A good example of what may seem to be “unnecessary” diversification occurs within the fig family. Approximately 750 species of fig tree are pollinated by approximately 750 species of fig wasps. Most of the wasps pollinate only one or two species of fig, so the mutualisms are very specific. How does this happen?

Think of it this way. Say you have a mountain called Big Rock Mountain. To the east you have a population of flowers called Awesome Red Daisies that are pollinated by bees called Stinging Mothers.

Awesome Red Daisies have 6 really important genes: a, b, c, d, e, and f. All these genes are pretty common except for c. There are only a few c genes in the population—it is a gene that makes the flower yellow.

Stinging Mothers have 8 really important genes: 1, 2, 3, 4, 5, 6, 7, 8. All are pretty common except 2. Gene 2 gives the bees a long tongue. As a rule Stinging Mothers have short tongues, although a few have long tongues.

Life is fine on the east side of Big Rock Mountain until one day a tremendous storm (the biggest storm in un-recorded history) blows some Awesome Red Daisy seeds and a few of the Stinging Mothers to the west side of the mountain.

Oddly enough, most of the seeds that landed in the west were the type that carried the gene for yellow flowers. So when the flowers bloomed in the spring, many were yellow. This was okay with the Stinging Mothers since they could see red or yellow. But the problem was this: the flowers were longer than the ones in the east. It just so happened that there was a genetic trait linked to yellow color that caused the flowers to be long and deep.

As a result of the oddly-shaped flowers, only those bees that had a long tongue were able to collect nectar. All the other Stinging Mothers died from starvation so their genes did not get passed to the next generation. But oddly enough, the gene for long tongue was associated with stinglessness.

After many generations, all the Awesome Red Daisies on the west side of Big Rock Mountain were yellow and all the Stinging Mothers had long tongues and couldn’t sting. After many more generations the flowers and bees on the west side of Big Rock Mountain were so different from those on the east side that they could no longer interbreed. They had become separate species: Awesome Yellow Daisies and Stingless Mothers.

So there you have it. The same type of accidental and random occurrences in nature can cause one species of fig and its pollinator to become 750 species of figs and their pollinators. The changes happen slowly over millions of years, but they have given us the vast biodiversity we are now trying so hard to destroy.

Rusty