Energy II. Heterotrophy

Learning goals:
* Be able to predict the outcome of food selection by heterotrophs associated with food quality
* Describe the basis and predictions of optimal foraging theory, focusing on the investment of energy in finding, capturing, and consuming food, as well as the benefits obtained


Heterotrophs have evolved mechanisms to acquire and assimilate energy efficiently from a variety of organic sources
The first organisms on Earth were probably heterotrophs that consumed amino acids and sugars that formed spontaneously in the early atmosphere.  Since that time, heterotrophs have evolved a wide range of methods for energy acquisition.

Heterotrophs consume energy-rich organic compounds (food) from their environment and convert them into usable chemical energy (ATP), by glycolysis.
Most compounds need to be converted to simpler forms during digestion (e.g. Proteins, carbohydrates, and fats are broken down into their component amino acids, simple sugars, and fatty acids)
The energy gain depends on the chemistry of the food, and the effort expended into obtaining it- some heterotrophs bathed in low quality food, while others expend lots of energy to seek high quality food.

Influence of food chemistry on heterotroph benefit is related to the type of organism the food came from
Plant, fungal, and bacterial cells have more structural components, such as cell walls, that are not easily digested; Animal cells are generally more energy-rich
Fats have more energy than carbohydrates per unit mass, and carbohydrates have more energy than amino acids, but amino acids also provide nitrogen

Heterotrophs range in size from archaea and bacteria (0.5 μm) to blue whales (up to 25 m); Feeding methods and the complexity of food absorption are accordingly very diverse among heterotrophs
Microbes (archaea, bacteria, and fungi) excrete enzymes into the environment to break down organic matter; they digest their food outside their bodies.  Hetertrophic microbes have adapted to a wide variety of organic energy sources (including nasty things like PCBs), and produce a wide variety of enzymes to break them down.  bioremediation - toxic wastes such as fuels, pesticides, and sewage are cleaned up using microorganisms that can break down the chemicals to less harmful substances.

Multicellular organisms have evolved specialized tissues and organs for absorption, digestion, transport, and excretion- increases efficiency of energy absorption
Animals have tremendous diversity in morphological and physiological feeding adaptations, which are adaptations to the types of foods they consume

Food for most animals is patchy in space and time
Animals should forage (seek out food) most efficiently- in a heterogeneous landscape the highest-quality food possible, which is the shortest distance away should be obtained

Optimal foraging theory proposes that animals will maximize the amount of energy gained per unit time, energy, and risk involved in finding food.
Theory assumes that evolution acts on the behavior of animals to maximize their energy gain
If food requires energy to find and handle it (e.g. kill, extract), then different sizes of food should provide different amounts of benefit- i.e. some size of prey should provide optimal amount of net energy gain (total energy – energy invested into finding and extracting prey).

Habitats are often heterogeneous, having patches with different amounts of food.  To optimize energy gain, an animal should remain in a patch with highest food density, until food density becomes equal to nearby patches.  Marginal value theorem: As a forager depletes the food supply, its energy gain decreases. When energy gain is equal to the average rate for the habitat, the forager should move to another patch = giving up time