18.8 THIS IS HOW WE DO IT: Does it matter how much nectar a flower produces?

Most animal-pollinated plants use bribery: they produce nectar for consumption by visiting animals and get pollination in return. Would you expect this arrangement to lead to perpetual directional selection for plants producing more and more nectar in order to attract more pollinators? At first that might seem reasonable. Within a plant population, an individual’s fitness is closely linked to its relative success in reproducing.

But one of the most common themes in nature is the pervasiveness of trade-offs—which is important in the question of how much nectar to produce. Does producing more nectar always lead to higher fitness for the plant? Does nectar production have significant costs? These are important questions. How could we investigate them experimentally?

Researchers interested in these questions devised a powerful experiment. By crossing petunia plants from two closely related species that differed in their average nectar volume, they generated two distinct petunia populations. One population produced flowers with low amounts of nectar (low-nectar plants). Plants in the other population produced flowers with a high volume of nectar (high-nectar plants). The petunia populations resembled each other in all traits except nectar production. The low-nectar plants produced only 30% as much nectar as the high-nectar plants.

Armed with large numbers of flowers from each of the two populations, the researchers evaluated whether nectar volume influenced the behavior of the plants’ pollinator, a hawkmoth.

The researchers conducted 37 “choice” experiments in which they simultaneously presented one of each type of plant to an individual hawkmoth. They found that the hawkmoths showed no preference for either plant, approaching each with equal likelihood. Does this seem surprising?

This finding didn’t change when the researchers artificially increased the amount of nectar in the low-nectar flower (by first adding nectar with an eyedropper), creating plants with equal amounts of nectar. What additional observations would be helpful?

What the researchers did notice was that hawkmoths spent significantly less time probing the low-nectar flowers (6.9 seconds, on average) compared with the high-nectar flowers (11.1 seconds). This 47% reduction in probing time on the low-nectar flowers disappeared when the researchers supplemented the flowers with nectar to the same level as the high-nectar flowers.

From an evolutionary perspective, perhaps the most important observation in these experiments was this: The average number of seeds produced by each of the low-nectar plants was 328 (± 22), while the average number for the high-nectar plants was significantly higher, at 424 (± 25).

Based on these results, it appears that from an evolutionary perspective, it benefits a plant to produce flowers with a greater volume of nectar. But things are not quite so simple. Remember: benefits often come with costs.

In a clever twist, the researchers took samples of low- and high-nectar plants and hand-pollinated them, rather than having the moths probe and pollinate them. In this experiment, they discovered that the low-nectar plants were actually much more effective at producing seeds—producing an average of 480 (± 20) seeds to the high-nectar flowers’ 383 (± 20) seeds.

It seems that there’s a cost to producing more nectar, in that it reduces a plant’s ability to produce seeds, given equal amounts of pollination. The optimum nectar volume, rather than being “as much as a plant can produce,” more likely reflects a balance based on a tradeoff between the ability to produce seeds and the need to attract pollinators that spend time probing the flowers.