Lecture 10 - heterotrophs and energy acquistion

heterotroph

an organism that obtains both energy and carbon from consuming organic matter produced by other organisms 

• plants, animals, microbes

main heterotrophic groups

• herbivores - consume plants

• carnivores - consume animals

• detritivores - decomposers of dead organic matter

• omnivores - mixed diet

what limits the quality of herbivore diets

plant tissue is C-rich but N-poor (C:N ~40:1) and contains structural (carbohydrate, lignin) and chemical (defensive secondary compounds) barriers

how do herbivores overcome plant defenses

• specialized enzymes

• gut symbionts (cellulose-digesting bacteria)

• detoxification of secondary metabolites

• selective feeding

contrast detritivores and decomposers

detritivores = ingest dead material

• earthworms, isopods

decomposers = chemically break it down externally 

• bacteria, fungi

how does food quality change from plants → animals → detritus

increasing N content and energy density

plants < detritus < animals (high protein, low fiber)

what is the “optimal foraging theory”

predicts organisms maximize energy gain per unit time (E/t) by balancing benefits (energy content) and costs (search, handling, risk)

what are the 3 components of foraging cost

1. search time (Ts)

2. handling time (Th)

3. energy spent capturing or processing prey 

when should a predator broaden its diet

when preferred prey become rare (low E/t for specialist) or handling times are short for alternative prey

functional response

relationship between food density and individual feeding rate

describe the 3 functional response types

1. Type 1: linear filter feeding (eg. sponges)

2. Type II: hyperbolic plateau due to handling time (eg. wolves)

3. Type III: sigmoid learning / switching curve (eg. birds prey switching

assimilation efficiency (AE)

fraction of ingested energy absorbed

AE = A / I x 100%

typical: herbivores ~20-50%, carnivores ~80% → plants harder to digest 

production efficiency (PE)

fraction of assimilated energy converted to growth and reproduction

PE = P / A x 100%

ectotherms > endotherms (less respiration cost)

combine AE and PE to define trophic transfer efficiency

TE = AE x PE ~10 → only 10% of energy transferred to next trophic level (10% rule)

what determines food-chain length in ecosystems

energy availability (transfer losses) and ecosystem productivity (limit higher trophic levels)

differentiate endothermic and ectothermic energy budgets

endotherms = spend most on metabolism (heat maintenance) and less on growth

ectotherms = allocate more to growth → higher PE

what is the general animal energy budget equation

C = R + U + F + P → consumption = respiration + urine + feces + production (growth/reproduction)

how do predators maximize energy intake

by selecting prey with the highest energy/handling time ratio and adjusting search effort as prey density changes

why are food chains short

energy losses (~90%) at each transfer limit energy available to top predators

ecological stoichiometry

study of the balance of C:N:P elements in consumer vs resource and how that affects growth and nutrient cycling