February 2009

PLANT - ANIMAL MUTUALISMS:
Pollination and Seed Dispersal


Interdependency of plants and animals

 

MUTUALISM

Mutualism is a kind of biological interaction in which both partners benefit.  I start this month with how all plants and animals depend on each other.  Then I will discuss the first of two cases where animals help plants.  This month I will discuss pollination and next month I will discuss how animals help disperse plant seeds.  We will see that animal-assisted pollination and animal-assisted seed dispersal are more efficient than wind or water pollination and seed dispersal. 

 

PLANT PHOTOSYNTHESIS / ANIMAL RESPIRATION

The most universal mutualism on earth is the interdependence of plants and animals. Before plants evolved no oxygen was present and so no oxygen- breathing animals could exist on earth. During photosynthesis plants "breathe" in carbon dioxide (CO2) and "breathe" out oxygen (O2). During respiration animals breathe in oxygen (O2) and breathe out carbon dioxide. The diagram here shows that, even though the complicated biochemistry is completely different, end products of respiration and photosynthesis are roughly the opposite of one another. The other part of this mutualism is that plants produce sugars during photosynthesis and animals consume sugars during respiration.

POLLINATION

lily with 1 stigma (f) and 6 anthers (m)

Lily: stigma and anthers

For sexual reproduction to take place, the male sex cell (pollen) must get to and fertilize the female sex cell (egg). The first step is to get the pollen from the male organs, the anthers, to the female receptive surface, the stigma. This is pollination.

We know it is Spring at the Refuge because many plants are putting out new leaves and being pollinated by insects and the wind.  As Douglas Florian writes in his poem FRESH SPRING:

 

          Fresh flowers. 

          Fresh leaves.

          Fresh blossoms.

          Fresh breeze.

          Fresh beetle.

          Fresh bee.

          Hey Spring, get fresh with me!

Abiotic agents, like wind or water, and biotic agents, like birds and bees can be agents of pollination. Abiotic agents are much less reliable than biotic agents. If wind or water are the pollinating agents, plants have thousands of times more pollen-producing structures, anthers, than pollen- receiving structures, stigmas. When animals are the pollinating agents plants have about five male to one female part.

Many more plants are wind pollinated plants than water pollinated. Many non-flowering plants, like pines and cypress, and some flowering plants, like oaks and willows, are pollinated by wind in spring before the leaves are fully expanded.  At the Refuge during this month the cypress will exchange pollen as well as the willows. The male organs, especially of cypress, are more conspicuous than the female. If you live near large concentrations of pine trees, you can tell when the pine disperse pollen because everything gets covered with a fine yellow powder.
 

Remember how we use the scientific method?  Here's a verse of a song to the tune of This Land is Your Land by Pete Seeger.

 

If a flower's not scented,

Not brightly colored,

And many flowers are smaller

In clusters tighter

With stamens longer

The signs are stronger
         That this plant spreads pollen on the wind

POLLINATION BY THE BIRDS AND THE BEES

Both plants and animals benefit from pollination mutualisms. Plants benefit because the pollinators take pollen to other plants which ensures cross-pollination. The plant helps itself when it flowers in synchrony and has many flowers on one plant or dense local populations. The pollinators benefit from pollen and/or nectar. For many bees the combination of pollen and nectar is a complete food. Pollen is high in fat and protein and nectar is high in carbohydrates, particularly sugars. Worker bees get their quick energy from sugars in nectar. At the hive they evaporate the water to concentrate the sugar which makes honey. Other bees in the hive feed both honey and pollen to their growing larvae.

Bee on simple flower
with lots of anthers

 

 

Naturalized honeybees and native bumble bees are especially reliable pollinators because they learn to specialize on one species of plant when its flowers are abundant. Forager bees find a local abundance of flowers with lots of pollen or nectar. They communicate this information to others in their hive by a complicated combination of scent, taste, and dance to indicate the direction and distance to the food source. Bee keepers take advantage of this and market honey from a particular wild plant, like tupelo, or take their honey bees to pollinate a particular crop, like squash or almonds.  

 

CO-EVOLUTION OF SPECIALIZED POLLINATORS

The structure of flowers ranges from simple flowers with generalized pollinators to complex flowers with specialized pollinators. Specialized pollinators are more reliable. Some species of flowers have co-evolved to be highly dependent on specialized pollinators.

beetle covered with pollen in simple flower

 

 

Simple flowers are symmetrical with lots of easily accessible nectar or pollen. Examples are asters, magnolia, white water lily, and elderberry. These are pollinated by generalist beetles, flies, and small bees that do not require lots of food and do not have specialized mouthparts.

 

 

 

Some flowers have nectar guides that point the way for pollinators to go to the rewards. Often these are fluorescent,  visible only in ultraviolet light that insects can see. For example, here is silverweed in daylight (l.) and in UV light (r.), showing location of nectar.

 

Some plant species have structures and timing that attract only particular species of pollinator. This is the result of co-evolution of plant and pollinator so that flower structure, amount of pollen or nectar reward, arrangement of flowers on the plant, and time of blooming come to match a pollinators structure, food needs, sensory capabilities, and behavior. In other parts of the world there are plants that are specialized to be pollinated by bats, small marsupial mice, dung beetles, and carrion flies. You can study this in internet searches.

A hawk moth uncurls its tongue as it approaches night blooming flower...

 

In south Florida we have a plant that is co-evolved so that only hawk moths can pollinate it and another plant is co-evolved so that only hummingbirds can pollinate it. Each plant species has a flower with nectar at the end of a long tube and the pollinator gets the pollen while it accesses the nectar using long, thin mouthparts.

 

 

...and inserts its long tongue into the tube of the moonflower

Moonflowers are pollinated by long-tongued hawk moths and coral beans are pollinated by long-beaked hummingbirds. Both hawk moths and hummingbirds have high metabolic rates with body temperatures of ~108°F and both moon flower and coral bean have lots of high sugar nectar that their pollinators need. The moonflower is white with markings visible in the ultraviolet and has a sweet smell. This matches the hawk moth's sensory capabilities. It opens only at night to match the hawk moth's nocturnal activity.

 

A coral bean has no smell and is red. This matches the hummingbird's sensory capabilities. The coral bean produces nectar during the day to match the hummingbird's diurnal activity.

 

 

 

bumblebee 'figuring out' complicated flowers

 

 

Bumblebees are not co-evolved to pollinate particular species of plants but they are specialized to pollinate complicated flowers. Complicated flowers are asymmetrical and have hidden pollen and nectar. Examples are milkweed, penstemon, bromeliads, and pickerelweed. Bumblebees learn to push their way into these flowers in just the right way or step on the right part of a landing platform to open the flower. As relatives of honeybees they need nectar to make honey and they need pollen. Both pollen and honey are fed to their larvae.

 

 

Infrared photo of hot bumblebee on flower

 

Like hawk moths and hummingbirds, bumblebees are hot-blooded and have an insulating body cover. Thus bumblebees can pollinate flowers in the cool early morning in winter in Florida and even during the cold days of summer in the arctic. An image taken with sensitive infrared camera shows the small amount of heat that emanates from an insulated bumblebee as it pollinates a flower.

 

 

 

 

 

FIGS AND WASPS: THE ULTIMATE POLLINATION MUTUALISM

Figs and fig wasps are the most specialized plant and pollinator mutualism known. A different species of wasp is specific to each species of fig. The fig wasps are tiny and operate inside the fig which is a fruit with many tiny flowers inside. The reward to the wasp is the developing seed instead of nectar or pollen. And the whole life history of the wasp occurs inside and near the fig. When you eat a sweet-tasting fig you certainly get some protein and fat from the fig wasp larvae!

The diagram below explains the details of this fascinating co-evolved system. These incredible adaptations are an excellent example of the adage that "truth is stranger than fiction."

 

 

REVIEW QUESTIONS

a. No animals could exist on earth before plants evolved to breathe in carbon dioxide and breathe out oxygen.

b. Pollination and seed dispersal by wind is much more efficient than pollination and seed dispersal by animals.

c. In the mid-1800s in Madagascar, Darwin found a white, night-blooming flower with an 18 inch tube filled with copious nectar and predicted that a moth pollinator with an 18 inch long tongue would someday be found. Was he correct?

d. In a 5th grade biology quiz, one question was "write the definition of pistil."   One answer was that "the pistol is the flower's defense against Insects."