Signs of Summer #12: Green Apples, Symbioses and the Gene Pool

Photo by D. Sillman

Photo by D. Sillman

Deborah and I were sitting out on our deck a few nights ago watching the flurry of activities that always precedes sunset. The main event of the evening involved two squirrels playing tag and chase up in one of the apple trees down in the orchard. They were bumping into each other, jumping over each other, and just having a great time. They shook the branches of the apple tree so hard with their acrobatics that a steady rain of tiny, green apples poured onto the ground. Standing under the apple tree were two fawns, and they were happily eating the sour, green apples as quickly as they fell.

Squirrels and deer interacting via the apple tree, who would have predicted that?

Ecology is the scientific study of the interactions going on in an ecosystem. These interactions can be between organisms and also between an organism and the non-living parts of its environment. A word that has grown in meaning over recent decades is “symbiosis.” At one time symbiosis meant two species that interacted and benefited each other, but now it defines all types of interactions between species. Symbiosis, then, is the essence of what is studied in ecology! Symbiosis is what was going on in and under our apple tree!

Looking out at my yard and field I can see many examples of symbioses. There are the young oak trees shading out the older spruce trees and grape vines wrapping up and over the crowns of the arbor vitae (both are examples of “perfect competition:” one species will “win” and one species will “lose”).

Photo by D. Sillman

Photo by D. Sillman

There are lichens growing on the rocks and also on a number of the tree trunks around the yard. Lichens are fungi that have algae living inside of their cells. The fungus relies on the algae to make sugars via photosynthesis, and the algae rely on the fungus to give it a place to live. Both organisms benefit from this interaction. It is a type of symbiosis called “mutualism.”

There is a cardinal’s nest up in the branches of one of the blue spruce trees. This interaction is a great benefit for the cardinal but has no real impact on the spruce. When one species is benefited in an interaction and the other is unaffected this sets up a symbiosis called “commensalism.”

On the ground beneath the spruce tree a robin pulls an earthworm out of my leaf pile. This is a symbiosis called “predation.” The robin benefits but the worm is harmed.

In any given time frame in an ecosystem a whole array of symbioses will occur. Out front in the yard a sharp-shinned hawk flies from her perch near the top of the spruce tree and dives down toward the birdfeeder. A blue jay perched in the lilac bush near the feeder sees the hawk coming and screeches an alarm. The feeder birds have an extra second or two to fly into the protection of the arbor vitae or scattered out across the street away from the hawk’s dive path. The hawk does not get a kill, and flies off chased out of the area by several blue jays including the one that raised the initial alarm.

The hawk gets negatively affected in all of these interactions (no supper and a blue jay chase squad ruining any chances for another attack!). The feeder birds are benefited in their interaction with the blue jay, but how does the hawk or the feeder birds in turn affect the blue jay? I would say that they didn’t affect him at all. Now, if there had been blue jay fledglings or some flock members at the feeder, then there would have been a self-benefit in sounding the alarm, but there weren’t. So, the interaction between the blue jay and the hawk generates harm to the hawk and no effect on the blue jay (a symbiosis called “amensalism”).

And, what about the observation that started this whole flow of ideas: the squirrels and fawns around the old apple tree? What kind of symbiosis is this? The fawns are definitely benefiting by the fresh abundance of apples, and the squirrels are neither harmed nor helped. So, it’s another example of commensalism! And what does the apple tree get out of this? Its fruit have seeds that are spread around the yard, the field, and the whole neighborhood in the feces of the deer (there is a lot of deer feces everywhere!). The apple tree, then, gets the chance to generate new apple trees far from the parental tree. The deer are benefited, the apple tree is benefited, so it’s another example of mutualism!

In an ecosystem there is a complex matrix of interactions that often influence each other in unexpected ways. The impact of predators on their prey species is a great example of this. Take our sharp-shinned hawk and the bird feeder community of small bird species. Sharp shinned hawks swooping in on a feeder are most likely to take the slowest, the least experienced, or the individuals that are physically impaired as their prey. The individuals most likely to survive the attack are the healthiest or the “most fit” individuals. The impact of a predator that is in balance with its prey populations, then, is improvement of the quality of the prey’s population! This has been observed in birds around suburban bird feeders, it has also been observed in the elk in Yellowstone after wolves were introduced, and in wildebeests in Tanzania in areas where a healthy population of lions are active!

I am glad that there are no lions lurking in my back yard, but it is good that there are small predators working the flocks and clusters of the various prey species out there. I like to think that my yard will have a very healthy, nimble, and fit chipmunk population because of the predation pressure exerted by my cat, Mazie, and that maybe my dog Izzy’s impact by chasing rabbits around the field each morning will be to make the rabbits more alert and somehow wilder. Izzy has never come close to catching any of the rabbits, of course, and probably wouldn’t know what to do with them if she ever did! But, if ever one of the rabbits was slow (or stupid) enough to be caught by a scatterbrain terrier who runs with her eyes closed, then the removal of that individual from the population would do nothing but improve the gene pool!

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One Response to Signs of Summer #12: Green Apples, Symbioses and the Gene Pool

  1. mary mcnavage says:

    A very interesting article and a great photo by Deborah of the fawns.

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