Energizing the Forest

Energizing the Forest

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester


Every living thing needs energy. Plants and some microorganisms use the process of photosynthesis to directly capture the energy of sunlight to meet this need. These organisms are called autotrophs, meaning “self-feeders.” Almost all other organisms rely on “eating” energy-containing organic matter produced by the autotrophs (see a previous Naturalist’s Note, “What’s the Yucky Stuff in the Creek” to read about one exception). These creatures are called heterotrophs, which roughly means “feeding on others.” Heterotrophs might eat autotrophs or other heterotrophs.


How does the energy move through the forest?

Ecologists have long been interested in studying how energy flows through the whole complex of autotrophs and heterotrophs in the forest. In “the old days” we were taught about food chains. In a food chain, trees captured the energy of the sun, and produced seeds, squirrels ate the seeds, and then hawks ate the squirrels. That’s correct as far as it goes, but is a bit too linear. Scientists now advocate a concept they call a “food web.” It’s pretty much the same idea, but recognizes that lots of heterotrophs have a varied diet and that a linear chain is too simplistic. For example, a tree produces seeds and a thimbleberry produces leaves. The seeds are eaten by squirrels. The leaves are eaten by insects. Birds eat the insects, and owls eat both the birds and the squirrels. For simplicity, in this posting I’ll stick with the food chain concept.

Each organism is the source of energy for the next organism up the food chain. However, there is one big factor to consider. Every organism uses up a certain amount of the energy it acquires to perform vital functions which might include staying warm or moving around. This “maintenance” energy is being used, or lost, at every level in the food chain. Thus, there is more energy available to the insects eating the plants than there is to birds eating the insects.


What effect does this maintenance energy loss have?

The most obvious effect of energy loss at each level in the food chain is that there is “more” of the organisms at the lower level of the food chain than at the upper level. Let’s look at a simple food chain at Tryon Creek State Natural Area (TCSNA). For this example I’ll use the chain of plant-worm-mole-owl.

At the very bottom, think of it as the “foundation,” are plants, which capture solar energy to make sugar. One example at TCSNA is the vine maple shown below.


Vine maple leaves harvesting solar energy


The next step is the earthworms, pictured below, which feed on dead plant matter.


Earthworm crossing Old Main Trail


The next step in our example is a mole, which eats the earthworms.


A mole crossing Big Fir Trail


The next link in the chain is an owl. This owl’s ready for a nice mole dinner!


Owl with mole – Photo by Scott Carpenter


To recap, our sample food chain is: plants capture energy from the sun, worms eat dead plant material, moles eat worms and owls eat moles. Owls are an example of what’s called an “apex predator,” meaning it is at the top of the food chain.

When you start thinking about food chains, you might develop a different perspective about the creatures you see in the forest. When you think of owls as eating moles you might view them differently. Take the great horned owl in the photo below:


Click on the owl’s picture to reveal a different perspective on owls.


Great Horned Owl, an apex predator

An owl is just earthworms which have been reconfigured to fly.

How does the “maintenance energy” loss effect the food chain?

As mentioned above, each link in the food chain uses up some of the energy it captures just to stay alive. So each link up the food chain contains less biomass (weight) than the link just below it. It is useful to think in terms of biomass per unit area of land. For this discussion, the area of land is TCSNA.

Based on recent TCSNA citizen science project owl numbers (thanks to Matthew Collins), and the estimates of weights of the different species, there are approximately 20 lbs. of owls living at TCSNA.

They are represented by this red square with an area of 1 square inch.

1 in red square


Biomass numbers for some of the forest organisms aren’t easy to come by. Apparently everyone figures they had something better to do than find out how many pounds of moles there are in an acre of forest. So instead of moles, I’ll use some biomass numbers for mice found in a study at the H. J. Andrews Experimental Forest near Corvallis. Owls eat mice, and other things, as well as moles.


Using the same scale as the red square for the owls, the weight of mice at TCSNA is estimated at 506 lbs. It is represented by the gray square below. One caution should be noted. The biomass numbers always represent just one point in time (“the standing crop” in ecology-speak). In reality, these mice are constantly having babies and some of the babies and adults are constantly being eaten. Therefore, over the course of the year, the heterotrophs of TCSNA probably have the opportunity to eat a lot more than 506 lbs. of mice.

grey square



Numbers for the standing crop of worms that I could find were so wildly variable, that I decided to skip those.




Extrapolating from plant biomass data collected at several Pacific Northwest forests, there are approximately 197 million lbs. of trees and shrubs at TCSNA. Using the same scale as for the owls and mice, this would be a square approximately the area of 6 or 7 average city lots. Think about cutting out the 1 inch red square representing owls, and laying it out on your front yard, and comparing the size of that red square to the sum total of the sizes of your lot, your neighbor’s lots on both sides, and the three lots across the street. When comparing the biomass of the plants to the biomass of the owls, owls almost don’t exist.


But owls are the top of the food chain, right?

As I said before, owls are considered to be an apex predator in our forest. In an area the size of TCSNA they would provide very little food for the next link up the chain. In order for a predator to survive by eating owls, it would have to cover a huge territory to get enough food. Think of a good size hawk hunting over a big chunk of the Willamette Valley!


But there’s another option!

We’ve probably all watched too many movies of lions killing gazelles on the plains of Africa. This has created a sense of “prey” and “predator” that has limited usefulness when we think about the flow of energy in an ecosystem. Rather than think about a large owl-eating predator that covers huge areas, let’s look at an alternative.

Since there are only enough owls to support a small weight of the next level up the food chain, what if the organisms at the next level of the food chain were really small? When scientists studied owls in northern Idaho, they found something very interesting!

Click on the blood drop below to find out what the scientists discovered.

red drop

One drop of owl’s blood

Owl’s blood containing red blood cells AND blood parasites (the horned demons)

That’s right, blood parasites were found in more than half of the northern saw-whet owls (a species we have at TCSNA). The parasites were getting energy from the owl’s blood. This isn’t the traditional view of “eating” something, but for energy flow purposes, it’s the same thing. The biomass of the blood parasites that are probably in owls at TCSNA is incredibly small, which fits into the pattern we have seen so far in the food chain. I am reluctant to declare that the blood parasites are the final link in the energy flow of our forest. However I will stop here, you get the idea!


All organisms in the forest need energy. As you can see, Mother Nature has been very creative in developing a variety of ways in which energy flows through our ecosystem. Enjoy the many manifestations of that energy as you, a child of the sun, hike through TCSNA.





Posted on August 24, 2015, in Plants & Wildlife, Tryon Creek and tagged , , , , , , , , . Bookmark the permalink. 1 Comment.

  1. I appreciate this new information.

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