The Birds and the Bees

The Flowers and the Trees

This final example of mutualism should be familiar to most of you. I’ve even already shared a story related to it, though it was not the main focus. If you couldn’t guess from the title, I’m going to talk about pollinators, the animals that move pollen between flowers.

The story of pollination goes back a long way if we care to take it that far. Plants have developed a variety of new forms in order to colonize land more efficiently. This includes the growth of vascular tissues (specialized systems for transporting water and nutrients) and the evolution of seeds. For now, let’s start with the evolution of the flower itself.

Modern flowering plants, known as angiosperms, are not present in the fossil record until approximately 160 million years ago, a time known as the Jurassic Period (more widely known as part of the age of the dinosaurs). These plants continued to diversify and spread and by the end of the Cretaceous (66 million years ago) had supplanted cone-bearing conifers as the dominant type of trees. Today flowers dominate the diversity of the plant kingdom, with nearly 300,000 different species worldwide (compared to just over 1,000 for gymnosperms, the seed plants with cones).

One of the reasons why flowering plants have been so successful has been the symbiotic relationships they have fostered with a variety of different animals. Earlier groups of plants relied on water or wind to carry their reproductive material. This is not particularly efficient, as the majority of reproductive material those plants manufactured would not result in fertilization. By involving animals in the fertilization process, flowers ensured that their reproductive material would be carried directly where it needed to go.

Now, some of you may be wondering, “can’t flowers self-pollinate? Why do they need pollinators?”. Indeed, the pollen (the male reproductive structures) of a flower can touch the female parts of the same flower, but that does not help the plant much. If the fertilized embryo does develop, it will have the same genetic makeup as its parents. The reason why sexual reproduction evolved, despite its increased energy costs over asexual budding of single cells, is because it increases the genetic diversity of the species. Combining genes from different parents creates new combinations never seen before. The greater diversity also protects species populations from potential catastrophe if conditions change suddenly.

Since most plants are literally rooted in place once they begin to grow, they need assistants to help them move pollen between flowers. They mainly attract these animal pollinators with the promise of nectar, a sugary food source high in energy, and advertise with bright and colorful petals surrounding their reproductive organs. This has led to a wide variety of different animal pollinators.

This includes many different types of insects like bees, beetles, moths, and butterflies like the beautiful blue morpho, Morpho peleides, pictured above. There are also a variety of vertebrate animals that act as flower pollinators. This includes hummingbirds like the one filmed in my earlier story, as well as bats, sunbirds, monkeys, possums, rodents, and even lizards.

One of the things that fascinates me about the mutualistic relationships between flowering plants and their pollinators is that they are so varied. While the majority of both flowers and pollinators are not selective and will work with multiple species, there are some examples where the requirements are much more stringent and only a particular animal species will be able to pollinate a flower. This is quite common in the orchid family, whose flowers often take on remarkably unique shapes.

There are trade-offs for both of these strategies. If any type of pollinator will do, you will be less likely to be adversely affected by the loss of one particular pollinator species. However, each pollinator that visits your flower will also visit many other flower species. If instead you have a single pollinator suited only for you, that pollinator will be guaranteed to take the pollen to a flower of the same species. However, if anything were to happen to either species in the specialist relationship, the other connected species would be adversely affected as well.

Sadly, that last downside is becoming more and more common in recent years. In fact, the loss of pollinators globally has become such a problem that even generalist flowers are impacted. This matters because much of the produce that humans consume comes from flowering plants and those plants require pollinators in order to continue to survive and reproduce. Mutualistic relationships can be powerful evolutionary adaptations, but they can also be a double-edged sword. Losing one half of the equation can be just as detrimental as the relationship was beneficial. As humans continue to move forward into the future, we must remember that everything in nature is connected and severing those connections can have unintended consequences.

That’s all for this season on mutualism. I hope you all found the examples interesting. Next week we continue or symbiotic journeys into the world of commensalism.