One fundamental part of ecology is an ecosystem. An ecosystem refers to all the living, or biotic, things within an environment, like plants and fungi; it also considers all the nonliving, or abiotic things within the environment, like water and soil. In an ecosystem, energy flows as a system from prey to predator. This unidirectional transfer of energy through a predator’s consumption of prey is a food chain. Food chains are simple with only one organism eating another. However, in ecosystems, the connections between the multiple predators and their food sources are complex. Often, a predator has many prey. Thus, food webs are constructed to represent the interlocking nature of food chains.
As previously stated, energy flows in one direction. In a food web, energy from the sun is converted in primary producers. Grass, crops, and flowers are all primary producers that photosynthesize to convert sun energy to chemical energy. Then, primary consumers eat the plants. These first-level consumers are herbivores like rabbits. Secondary consumers eat the primary consumers. For example, foxes sometimes eat rabbits. At the top of the food web are tertiary consumers, who eat the secondary consumers. These consumers are also called top predators, and they are typically wolves, snakes, or birds of prey. Birds of prey are hawks, eagles, owls, and other raptors.
In this lab, students studied one particular bird of prey: barn owls (Tyto alba). Barn owls, like other raptors, have a unique digestive system that presses all the undigested bones, feathers, and such from the owl’s prey into a pellet which they regurgitate. These pellets can be dissected for bones and scored to see what organisms make up the owl’s diet. In a temperate area, one would expect barn owls to eat small mammals like moles. In a dry area, barn owls would likely eat lizards or insects. Students sorted into groups of two and dissected pellets. When students score the bones, they will likely find the owl eats more small mammals, suggesting they are from a temperate zone. This will be expected if the owls are from the area because Chattanooga is in a temperate zone.
In groups of two, students obtained a pellet from a barn owl as well as two wooden probes, forceps, a magnifying glass, and a bone chart. Gently, students broke the pellet with their fingers or forceps, careful not to break any fragile bones within the sample. Any bones or skulls found were extracted and compared to the bone chart. Students then guessed which organism the bone matched. The bone types and the prey were scored. They repeated this until the pellet had been completely dissected. In the end, the groups combined their data.
Students discovered that all bones found belonged to small mammals of rodents, shrews, moles, and birds. No insects, amphibians, or reptiles were found in the pellets. In all, students saw 73 rodents, 53 shrews, 39 moles, and 14 birds. This data is represented in Figure 1. Additionally, they found 203 ribs, 104 vertebrates, 73 hind limbs, 63 fore limbs, 45 jaws, 45 pelvic bones, 26 scapula bones, and 25 skulls, as shown in Figure 2.
Figure 1. Proportion of prey types found within the owl pellets from class data set
Figure 2. Proportion of bone types found within the owl pellets from the class data set
A single student group found eleven rodents, fifteen shrew, and eleven moles, as shown in Figure 3.
Figure 3. Proportions of prey found in a single owl pellet
From the experiment, students found that the proposed hypothesis was accepted. As shown in Figure 1, the owls had eaten only small mammals. No insects, reptiles, or amphibians had shown in the pellet, suggesting that the owl is from a temperate zone. However, it is important to note that the data may be somewhat incomplete. Many bones found were rib bones and vertebrates (as shown in Figure 2), which were very small and hard to assign to the corresponding organism. To confirm, ecologists would likely want to track the specific owl to its habitat and see what prey are available.
One group specifically found 11 rodents, 15 shrews, and 11 moles, as shown in Figure 3. If owls produced one pellet a day, an owl’s entire 30-day diet would likely consist of 330 rodents, 450 shrews, and 330 moles. Farmers would likely want to keep a barn owl around because a single owl could take care of a lot of rats and mice that might interfere with their work. If a single owl ate 330 rodents a month, that would keep a farm relatively pest-free.
Lastly, ecologists must consider the entirety of the barn owl diet. In a temperate zone, there are often still insects for an owl to feed on, but no group found any insects. Likely, small mammals were either easier to find and catch for the owls, or it was a season in which the barn owl’s preferred insect prey was not available.
Exploring Ecology 