The Sonoran Desert encompasses a large portion of Arizona as well as the western side of the state of Sonora in Mexico and most of the Baja penninsula. I am fortunate to live in this unique ecosystem and I hope to share some of its beauty and fascinating creatures with you.
“In every walk with nature one receives far more than he seeks.”
– John Muir
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
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.
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,
a legume tree, for nutrients. The host tree is harmed over time and
with heavy infestation of the mistletoe by the depletion of nutrients.
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.
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.
- 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.
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
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.
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.