Symbiosis

Last week, fellow docents and I were having a conversation, prompted by a question to one of them from a visitor, regarding the relationship between a palo verde and a saguaro. It was obvious from our discussion that we needed a review of symbiotic relationships!

Symbiosis is defined as a relationship between two or more organisms that live closely together. There are three main types of symbiosis: commensalism, mutualism and parasitism. Commensalism is a relationship in which one organism benefits and the other is unaffected. Mutualism is a relationship in which both organisms benefit. Parasitism is when one organism benefits and the other one is harmed. To be successful, a symbiotic relationship requires a great deal of balance. Even parasitism, where one partner is harmed, is balanced so that the host lives long enough to allow the parasite to spread and reproduce.

These delicate relationships are the product of long years of co-evolution. Bacteria were the first living things on the planet, and all of Earth's other creatures have been living and evolving with them for hundreds of millions of years. Today, microbes are essential for many organisms' basic functions, including nourishment, reproduction, and protection.

So, back to the saguaro/palo verde relationship. The visitor wanted to know, if the nurse tree was benefitting the saguaro, what the payoff was for the palo verde? How would you classify that? The saguaro benefits from the protection provided by the palo verde. But, how about the palo verde? Most of the time, their relationship is probably commensal - benefiting the saguaro and causing no harm to the palo verde. However, in times of drought the shallower root system of the saguaro may intercept most of the rainfall.  These two species would then be said to enter a type of symbiosis known as interspecific competition. The saguaro could eventually hasten its nurse tree's death.  

There are thousands (millions?) of symbiotic relationships in the Sonoran Desert. Just take the saguaro, for example. Think of all the animals that depend on the saguaro for food, or shelter. The saguaro, in turn, depends on certain species to pollinate it, and to disperse its seeds. Bees, bats and white-winged doves get a food reward, while doing the important work of helping the saguaro to reproduce.

Parasitism - The desert mistletoe is an example of a parasitic symbiont that depends on its host,

usually a legume tree, for nutrients. The host tree is harmed over time and with heavy infestation of the mistletoe by the depletion of nutrients. 

Other common parasitic relationships include the coyote and the flea, or the mange-causing mite. Female mites can burrow into the skin. Coyotes with mange can lose their hair, which can make it difficult for them to control their body temperatures. Mange must be extremely severe before it disables a coyote. Most coyotes can survive with the disease for a long time. The brown-headed cowbird parasitizes the nests of other birds, called brood parasitism. The cowbird benefits by having another bird raise its young, which are usually larger than the bird’s own young and can out-compete them for food and space.

Animals are parasitized by viruses, bacteria, fungi, protozoans, flatworms (tapeworms and flukes), nematodes, insects (fleas, lice), and arachnids (mites). Plants are parasitized by viruses, bacteria, fungi, nematodes, and a few other plants.

Mutualism - Examples of mutualism abound. Just think about flowers and their pollinators. That’s a

win-win for both parties. Not all symbiotic relationships are visible by humans. Take the bacteria that live in the guts of herbivores, helping them to digest plant material, which is more difficult to digest than animal prey. This gut flora is made up of cellulose-digesting protozoans or bacteria living in the herbivores' intestines. Another example is the bacteria that allow legumes to fix nitrogen in the soil, living on nematodes on the plant roots.

Commensalism - Commensal relationships may involve one organism using another for transportation or for housing or it may also involve one organism using something another created, left after its death (metabiosis). Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies. Birds nesting in trees is a commensal relationship. Vines use trees to reach up to the light, causing no harm to the tree (most of the time!).  Consider the packrat home and all the many critters that seek shelter and food there.

Symbiosis can either be obligate or facultative (optional). In obligate relationships, one organism cannot live without the other. Perhaps one of the most well-known obligate mutualistic relationships is that of the yucca moth and its host plant.

The yucca moth, also commonly called the pronuba moth, is a small white moth that lives in the semi-arid habitats where yucca plants grow. The yucca moth is well-known for its co-dependent relationship with the yucca plant. The yucca moth’s larvae rely exclusively on the seeds of the yucca plant as a primary food source, and the plant relies exclusively on the yucca moth for pollination. One cannot exist without the other, creating an obligate mutualism between the moth and the plant. 

We talk about Keystone Species - a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. These are species on which so many other species are dependent. Think about the saguaro, and prairie dog, and the many different species for which they are so important. Another good example is the cottonwood tree and the many creatures that depend on it. I’m sure you can think of many more relationships in biology, for I haven’t begun to scratch the surface.  I have discussed only the 3 main types of symbiosis.  There are many more, and perhaps those are topics for another article.

We do well to remember that in each of these types of symbiosis, few situations are absolute.  In most cases, there is a continuum of types of interactions between species, rather than an exclusive category.