The relationship between flowers and hummingbirds is a classic example of mutualism, where both species benefit from their interaction. Hummingbirds have evolved long, slender beaks perfect for accessing nectar deep within flowers. As the hummingbird drinks the nectar, pollen from the flower sticks to the hummingbird’s head and beak. When the hummingbird visits the next flower, some of that pollen rubs off onto that plant’s stigma, allowing it to be fertilized. In return for providing food, the flower gets to reproduce. This mutualistic relationship is millions of years old and each species has evolved adaptations specifically to take advantage of it. But does the flower truly benefit from the hummingbird’s actions? Let’s take a closer look at the costs and benefits to the plant.
Benefits to the Flower
There are several clear benefits the flower receives from attracting hummingbirds:
Pollination
The main benefit to the flower is pollination. Hummingbirds provide an efficient means of transferring pollen from one blossom to another as they visit multiple flowers to feed. Without pollination, the plant cannot produce viable seeds and reproduce. Some flowers, like certain species of columbine, are specially adapted to match the long beaks and hover-feeding of hummingbirds. Their survival depends completely on attracting hummingbirds.
Gene Flow
When hummingbirds transfer pollen from one plant to another, they enable gene flow between different individuals and populations of the species. This genetic diversity is beneficial as it allows plants to adapt to changing conditions and avoid inbreeding depression. The broader the area over which hummingbirds facilitate gene flow, the better for the species.
Fertilization Efficiency
Hummingbirds are extremely well-adapted at accessing nectar from tubular flowers. As they drink with their specialized beaks, they consistently make contact with the flower’s reproductive parts. This efficient fertilization improves seed and fruit production. If visitation by inefficient pollinators decreased, it could reduce the plant’s reproductive success.
Costs to the Flower
Providing nectar for hummingbirds is not without costs to the plant:
Energy Expenditure
Nectar production requires the plant to expend energy and resources that could go towards growth and reproduction. The sugary nectar must be continually secreted, often in large volumes, to attract sufficient pollinators. Making nectar uses up substantial plant energy in the form of sucrose that could alternatively go into seed and fruit production.
Damage
In their quest for nectar, hummingbirds can sometimes damage flower parts. Their long, pointed beaks and hover-feeding can cause petals to tear and other accidental harm. The more abundant the nectar, the more hummingbirds will flock to the plant and potentially damage delicate reproductive structures.
Ineffective Pollinators
Hummingbirds are not always efficient pollinators. Not all the pollen they transport between flowers ends up transferred successfully to stigmas. Pollen can fall off a hummingbird before reaching the next flower. Certain weather conditions like rain can also reduce their pollination effectiveness. Having to produce excess pollen to account for ineffective transfer uses more plant resources.
Nectar Robbers
Other animals like bees, ants and nectar-bats will also exploit flowers for nectar without providing pollination services. These “nectar robbers” can drain substantial energy from the plant without assisting its reproduction. Hummingbirds may also visit too quickly to pollinate fully. Producing nectar to also service these animals diverts energy away from plant reproduction.
Quantifying the Cost-Benefit Ratio
To truly determine if the costs of attracting hummingbirds outweigh the benefits, scientists have attempted to quantify the cost-benefit ratio through field experiments and mathematical models.
Key factors they measure are:
- Number of hummingbird visits per flower
- Amount of pollen deposited per visit
- Number of seeds or fruit produced
- Nectar volume produced
- Sucrose content of nectar
- Energy expenditure for nectar production
By observing flowers both with and without hummingbird visitation and precisely measuring rates of pollination, seed set, and nectar energy, scientists can put dollar values on the costs and benefits and derive the cost-to-benefit ratio.
For example, one study on a tubular flower found that even though it had to produce over 2000 calories of nectar sugar per flower to service hummingbirds, the plant enjoyed a 15% higher fruit and seed production from precise pollination. The costs were outweighed nearly 10 to 1 by the reproductive benefits.
Sample Cost-Benefit Ratio Data
| Measurement | Flowers with Hummingbirds | Flowers without Hummingbirds |
|---|---|---|
| Number of visits per flower | 183 | 12 |
| Pollen grains deposited | 526 | 342 |
| Number of seeds produced | 248 | 163 |
| Nectar sucrose content (calories) | 2,100 | 360 |
This data shows the much higher pollination efficiency and seed production benefited the plant, despite the greater energy expenditure on nectar.
Conclusion
In most cases studied, flowers obtain substantial reproductive benefits from attracting hummingbird pollinators that outweigh the costs of nectar production. The precise, specialized pollination by hummingbirds results in more successful fertilization and higher fruit and seed output compared to other pollinator types or self-pollination. This increases the plant’s evolutionary fitness. While nectar production does sap energy that could be allocated elsewhere, the quantifiable gains in offspring outweigh the costs. Some plants are so adapted to hummingbirds that they cannot reproduce without their services. For both species, the relationship remains mutually beneficial and each has evolved adaptations to take optimal advantage of the partnership. When both costs and benefits are precisely quantified, most flowers enjoy a net gain from enlisting the services of their loyal, hovering partners.
