Energy Flow & Food Webs
A food web is a way to view the interconnected feeding relationships within the ecosystem. The Flathead Watershed ecosystem has complex terrestrial and aquatic food webs. Energy necessary for plant and animal life flows through the ecosystem in both predatory and detrius food chains. In the predatory food web, green plants, algae, or plankton are fed on by plant eaters (herbivores) which are fed on by flesh eaters (carnivores) and omnivores that eat both plant and flesh. In the detritus food web, plant and animal matter are fed on by bacteria and fungi (decomposers), detrial feeders (detrivores), and their predators (carnivores).
|Figure 2.31: Simplified food web. Source: Author - Cunningham, W., et. al. 2007; Publisher - McGraw-Hill, N.Y. with permission of The McGraw-Hill Companies.|
The processes of living (like hunting, feeding, and mating) are all fueled by energy originally captured by plants in photosynthesis. When respiration takes place, carbon bonds are broken and carbon combines with oxygen to form carbon dioxide. This process releases energy that is either used by an organism or lost as heat. Energy is then passed from organism to organism up the food chain. Ecological efficiency is the percentage of energy transferred from one trophic level (the position each organism fills in their respective food web) to the next. On average, 90% of an organism’s energy is used for its life processes and only 10% is passed on to its predator in the next trophic level. There are rarely more that 4 or 5 trophic levels in a food web.
|Figure 2.32: Energy flow diagram. Source: Lori Curtis|
Trophic Levels - Terrestrial
|Figure 2.33: Trophic levels. Source: Lori Curtis|
Plants are primary producers; they form the first trophic level of the terrestrial food web. Some common examples in the Flathead Watershed are grasses, sedges, fescues, forbs, beargrass, huckleberry, serviceberry, common snowberry, and cone-producing pine and fir trees. These autotrophs—organisms that use the energy from the sun and nutrients from the abiotic or physical environment (air, water, rock, and soil) to perform photosynthesis—turn inorganic compounds into organic compounds that feed primary consumers in the second trophic level. Although the rest of the ecosystem depends on the energy from these organisms, a portion of that energy is continually lost through plant respiration and death.
Heterotrophs—organisms that use organic carbon for growth—include herbivores (plant eaters), carnivores (meat eaters), and detrivores (decomposers). The primary consumers—the herbivores that eat the vegetation of the first trophic level—comprise the second trophic level. Some of the herbivores in the Flathead Watershed ecosystem include elk, white-tailed deer, mountain goats, bighorn sheep, Canada geese, and squirrels. As with primary producers, energy is lost from the system by herbivores through respiration (the exchange of carbon dioxide for fresh air) and organism death. The amount of nutritional energy retained by these herbivores depends largely on the quality of their diet and the amount of digestible materials in their food.
Some approximate percentages of assimilation efficiency are shown in various diets below:
- 15% of the energy from decomposing material
- 30-40% of the energy from grasses
- 60-70% of the energy in young plant materials
- up to 80% from seeds.
The third trophic level is comprised of carnivores who obtain their energy by eating flesh of the herbivores from the second trophic level. In parts of this ecosystem, the top carnivore is the gray wolf. Other carnivores include the Canada lynx, cougar, fox, wolverine, bald eagle, peregrine falcon, hawk, and owl.
As with primary producers and herbivores, energy is also lost by secondary consumers through respiration and organism death. The assimilation efficiencies of animal food by carnivores vary between 60% and 90%. And within those foods, vertebrate (species with spinal columns or backbones) prey are more efficiently digested than insect prey, as the indigestible insect exoskeletons make up a larger portion of the prey body than do scales, feathers, or hairs on vertebrates. Assimilation efficiencies of insectivores are between 70% and 80% and carnivores is 90%.
Omnivores are part of both the second and third trophic levels. Humans are omnivores because we are physiologically capable of eating animal flesh and vegetation. Some humans, however, choose not to eat animal protein. In the Flathead Watershed ecosystem, two of the largest animals—grizzly bears and black bears—are omnivorous. Other ecosystem omnivores include raccoons, chipmunks, martens, snowshoe hares, chickadees, nuthatches, frogs, salamanders, wild turkeys, turtles, grouse, jays, and woodpeckers.
The ultimate omnivores are the decomposers that feed on fallen vegetation, fecal matter, and dead animals. The temperate climate of the Flathead Watershed allows dead plant material (detritus) to accumulate, enabling this ecologically important detritus-based food chain. For example, earthworms feed on leaves and other plant matter that has not yet decayed. Nematodes eat algae, fungi, small animals, fecal matter, dead organisms, and living tissue. A similar set of trophic levels and processes occurs in the aquatic food web, though they tend to occur in a more complex web of connectivity. (See the Flathead Lake Food Web Watershed Perspective).
Exploitation efficiency varies in each predator based on how well they are able to hunt or gather and ingest their prey. Assimilation efficiency depends on how much of the energy is captured by the predator once it has eaten its prey. Some portion of this assimilated energy is used for growth and reproduction (the net production efficiency).The gross production efficiency is the product of the assimilation efficiency and the net production efficiency and represents the overall efficiency of biomass production within each trophic level. Active warm-blooded terrestrial animals have low net production efficiencies rarely exceeding 5%, while aquatic animals can exceed 30%.
Biomass in each trophic level declines up the food chain partially because a relatively high proportion of energy passed from one level to the next is lost in respiration. In agriculture, there is a much greater energy efficiency of producing low trophic level crops of soy and potatoes versus higher trophic level fish, chicken or beef for human consumption. This disparity in efficiency reflects the reduction in production from moving up trophic levels. It is therefore more energetically efficient to establish a diet from lower trophic level foods.
Each terrestrial and aquatic species has its own set of ecological efficiencies within its environment. As an example, the energy cost of maintaining body temperature for the white-tailed deer in the watershed is partially related to heat loss from exposure to cold and wind. The insulative properties of fat and body hair are important in preventing heat loss in the winter and enabling it in the summer. By curling up underneath dense vegetation, deer can minimize heat loss to the environment in the winter. By lying in a cool field shaded by trees, they can minimize exposure to heat.
Hair depth is a determinant in the thermoregulatory qualities of the coat of the white-tailed deer. The summer coat is thin, 0.04 to 0.18 inch (1 to 3.5 mil) deep, while the winter coat is thicker 0.2 to 1.1 inches (5 to 27 mil) deep. The fur is comprised of two layers: long guard hairs on the outside and short underfur. While the fur coats are critical to the conservation of energy for the deer, the growth of the two coats per year also costs the deer energy and protein. The metabolic cost of producing the hair is equal to the amount of protein and energy in the hair plus the energy overhead required for its production.
|Figure 2.34: Flathead white-tailed deer. Source: Lori Curtis|
Energy is transferred away from the deer through evaporation (the transfer of energy by the change from liquid water to vapor in the air), conduction (the transfer of energy by molecule-to-molecule contact), radiation (the movement of energy through a medium without influencing it), and convection (the transfer of energy by movement of the medium surrounding an object). The white-tailed deer is also an important energy source for many carnivores including mountain lions, wolves, wolverines, coyotes, foxes, and bald eagles. For most of these carnivores, deer is only a small part of their diet.
All of the biogeochemical cycles work together to maintain ecosystem functioning and to make the earth a livable planet for humans and all other species. Since the nineteenth century, human activity has altered over half the earth’s surface through urbanization, industrialization, forestry, and agriculture. The resulting changes in composition of soils, waterways, and the atmosphere all affect these essential cycles to varying degrees. It is necessary that we understand our impact on and relationship with these cycles in order to maintain a healthy, functioning ecosystem in the Flathead Watershed and beyond.