Pollinator relationships

Flowering plants

Cacti and succulents are flowering plants (angiosperms), which use pollen to reproduce with one another. If plants are not pollinated by the wind, then they need an animal to help move their pollen from one flower to another.

Usually, the animals are attracted by the promise of food – the sweet, sugary nectar that the flowers produce. Cacti and succulents are pollinated by a range of different animals and millions of years of evolution have made some of these relationships between plant and animal very specialised and interesting.

Bees

Bees are famous for their role in pollinating flowers, including cacti and succulents. There are somewhere around 20,000 different kinds (species) of bees all over the world.

Most bees are generalist feeders. That means they will visit all sorts of different flowers, depending on what is available. Many bumblebees, like the ones you might see in the UK, are generalist feeders.

Other bees are specialist feeders and feed from limited kinds, or sometimes just one kind of flower. Some solitary bees from the Americas (Diadasia australis and Diadasia rinconis) are specialist feeders. They tend to feed on the flowers of cacti.

Things to think about

• What is the evolutionary advantage of having a specialist feeder visit a plant?There is a higher probability of them bringing compatible pollen from the same species of plant.
• Many cacti are seriously endangered. What would happen to the Diadasia bees if the cacti they feed on went extinct?This is the downside of being a specialist feeder – the bees would likely go extinct too.

Birds

Birds can also serve as a pollinator by transferring pollen that rubs onto their heads and beaks when they feed. Here are some examples of bird-pollinated flowers. As you can see, they look quite similar, even though these plants are not closely related to each other. This is called convergent evolution, when different evolutionary pathways end up at a similar end result because of similarities in the selective pressures affecting them.

Bird-pollinated plants often have red, pink and orange flowers. This is an adaptation that reduces the number of visits they get from bees. Bees see the world differently from humans (and birds). They pick up low wavelength (blues) down to in the ultraviolet (UV) part of the spectrum, which we cannot. They are less able to pick up colours of a high wavelength (reds to infrared). Because red flowers are less likely to be visited by bees, they are more reliably visited by birds, which can see them.

Flowers visited by birds also tend to be long, tube-shaped, and often hang down like bells. Birds will perch on the plant and stick their beaks and tongues into the flowers to reach the nectar at the base. Many succulents like Aloes, including the well-known Aloe vera, are visited by birds.

Things to think about

• What evolutionary adaptations would you expect to see on a bird that has evolved to feed from flowers?A long narrow beak that can fit into the throat of a flower.
• Can you think of a small and very active bird from the Americas that might feed on succulent flowers?Hummingbirds love to feed on the sugary nectar!

Moths

Many cacti and succulents flower at night and rely upon nocturnal animals for their pollination. They tend to look similar: a limited colour range of white/cream to pale yellow/green. Making the pigments for colourful flowers is a waste of energy for a plant that blooms at night, as nothing will see it! This is another example of convergent evolution: many night-blooming flowers that aren’t related to cacti, like orchids, are the same colour scheme.

Night-pollinated flowers are usually highly fragrant. The scent is very attractive to moths, which can tell flowers from different species apart from the chemical signals carried by the fragrance (pheromones). Highly fragrant flowers only tend to stay open for one night.

There are some cacti and succulents that only be pollinated by a single kind of moth. The senita cactus (Lophocereus schottii) is one of these plants. It relies entirely on the senita moth (Upiga virescens), and likewise, the moth relies entirely upon it. This is called obligate mutualism.

Things to think about

• Are there any other advantages to putting out dull-coloured flowers at night instead of brightly-coloured flowers during the day?Flowers can make a plant more visible to herbivores as well as pollinators.
• Why do you think the flowers of night-blooming cacti usually wilt after a single night?Fragrance takes energy to produce and there is no point continuing it into the following day if your pollinators aren’t active.

Flies

Some plants have evolved specifically to attract flies. What do flies like? Rotting meat, poo and other smelly things they can eat and lay their eggs on! Some plants in the genus Stapelia have developed flowers that mimic both the appearance and smell of dead animals, getting the common name “carrion flowers”.

Some are so convincing that flies will even lay their eggs on to the flowers, thinking that it will provide lots of rotting meat for their maggots to eat.