chapter 10: how do organisms acquire energy? heterotrophs

heterotrophy

use organic sources of carbon synthesized by others to derive energy

• can be found across all organismal groups

herbivores

organisms that eat plants

carnivores

organisms that eat animals

detritivores

organisms that eat dead organisms that eat dead organic matter (it used to be alive but is no longer living)

functional groups

herbivores, carnivores, and detritivores

Food economics: Trade offs

heterotrophs need to balance the ease of obtaining food and its quality

• ease of obtaining food = quality of food

Food quality: ecological stoichiometry

the balance of these 5 elements in ecological interactions

• carbon

• oxygen

• hydrogen

• nitrogen

• phosphorous

carbon

provides structure to organisms

oxygen

parts of water molecules (organisms mostly made of water)

hydrogen

other part of water molecules

nitrogen

part of amino and nucleic acids

• limiting factor in all organisms

phosphorous

essential for cellular processes such as ATP energy transfer

relative abundance of C and N

c:n ration dictates what and how much each type of heterotroph needs to eat (C/N)

plants

High C:N ratio 

• lots of carbon to build up structure 

• ex. xylem, cellulose, lignin

• high carbon, low nitrogen

animals, fungi, bacteria

low C:N ratio

• structural components are less carbon-rich

• low carbon, high nitrogen

herbivory: feeding on plants

• + ease of obtaining food

• - food quality

• high carbon, low nitrogen

nutritional quality

• high C:N ratio means plants are difficult to ingest and digest (particularly those high in cellulose and lignin)

• adaptations to teeth

• compensate for low nutritional quality by eating a lot

• adaptations to digestive systems. Relationship with symbionts

nutritional quality examples

• insect mandibles and palps allow for slicing and manipulation for plant material

• rodents’ long continuously growing incisors to gnaw tough material

• ruminants’ complex digestive system and repeated chewing of cud allows for maximum nutrient extraction

plant defenses

• plants fight back with adaptations to deter herbivory

• physical (thorns, spines) and/or chemicals (alkaloids, cyanide, tannins)

plant defenses examples

• monarchs and other insects feed on milkweed plant

• tolerate cardiac glycosides (plants defense chemical) 

• giraffe’s long tongue allows it to maneuver around thorns on acacia trees

herbivory challenges

• nutritional quality

• plant defenses

• exposure to predators

exposure to predators

• eating a lot → more exposure to being preyed upon

• defense strategies

exposure to predators examples

• porcupines don’t have to rush to eat: defense strategies against predators (quills)

• use size and horns/antlers to actively fight predators

carnivory: feeding on animals

• little variation in the C:N ratio across animal species

• a predator can use multiple prey species and get the same nutrition

• few digestive adaptations

• strong selection to efficiently capture and consume prey

• + food quality

• - ease of obtaining food

carnivory: adaptations

• bring really fast

• sharp claws for grabbing prey

• teeth for tearing and efficiently consuming prey

detritivores: feeding on non-living organic matter

• + ease of obtaining food

• ± food quality

detritivores

must ingest and digest dead organic matter via internal processes

• most abundant food source on the planet is dead plant material

• rich in carbon and energy, low in nitrogen

• living plants → developed nitrogen use efficiency

• fresh ___may also still have plant chemical defenses (ex. leaves falling on the ground)

decomposers

break down dead organic matter externally and absorb nutrients externally and absorb nutrients directly through their cell structures

detritivores and decomposers

both are vital to decomposition and nutrient cycling

Nitrogen use efficiency (NUE)

reabsorb nitrogen before dropping leaves

Detritivores are limited to

• chemical composition of the detritus

• abiotic factors: soil moisture is very important for soil-dwelling detritivores and decomposers 

Mixotrophy and omnivores

some organisms exploit more than one carbon source

Omnivores

gain energy from both plant and animal matter

mixotrophs

can gain energy from photosynthesis (inorganic) and from consuming organic material

• includes number of algae, bacteria and protist species, hemi-parasitic plants, carnivorous plants

myco-heterotrophs

obtain food from fungal hyphae 

• symbiotic relationship

• no capacity to do photosynthesis

• ex. monotropa uniflora ghost pipe, corallorhiza maculata leafless orchid

types of mixotrophs

• hemi-parasites

• epiphytes

• insectivores plants

hemi-parasites

obtain food from a living plant host

• ex. mistletoe

• photosynthesis and obtain nutrients from host

epiphytes

grow on other plants but don’t parasitize on them

• ex. epiphytic fern and orchids

• autotroph

insectivorous plants

obtain additional nutrients from trapped insects

• ex. venus fly trap, sundews

measuring energy limits in plants

photosynthesis reaches a plateau above Isat

measuring energy limits in animals

food intake saturates at a certain level of food density

• the relationship between food density and feeding rate characterized by functional response (type I-III)

what influences an organism’s feeding rate

• can only physically shove so much food in their mouths

• takes time to digest food and make room for more food

• takes time to find food

• takes time to handle/process food

• consider safety while foraging; sometimes it’s safer to hide than eat

Functional response curve: type I

describe the food intake per unit time as function of prey density or the amount of food available

• feeding rate increases linearly 

• food are readily available (encounter rate drives intake; negligible searching time)

• quick food processing → no handling time

• intake is limited purely by exposure rate, not by processing time

• feeding rate abruptly levels off

• plateau reflects satiation rather than handling constraints

• ex. some zooplankton, filter feeders (suspension and sediment)

functional response curve: type II

at low food density, feeding rates increases linearly (most common) 

• food intake is limited by food searching

• at intermediate food density, feeding rate begins to slow down

• handling time limits further intake

• at high food density, food is widely available

• limited by handling and processing time (digestion of food)

• ex. moose, wolves, bears, parasitoid wasps

functional response curves: type III

• feeding rate is s-shaped

• at low food densities, feeding rate increases slowly

• inefficient due to prey refuge, lack of foraging experience, or switching behavior

• limited by age, exposure to prey, encounter rate

• at intermediate food densities, feeding rate increases rapidly

• efficiency(learning/foraging experience, prey become easier to find)

• at high prey density, feeding rates levels out

• intake limited by handling/processing time

• ex. juveniles learning to hunt, prey switching, searching for rare prey

optimal foraging theory

describes how organisms feed as an optimizing process (maximizes or minimizes some quantity, such as energy intake or predation risk)

• behavior of organisms (understanding behavior)

marginal value theorem

an organisms should spend time in a patch that maximizes their energy gained in the patch per unit time. Once energy gain levels off, the organisms should leave the patch.

marginal value theorem: orange curve

short travel time among patches

• shorter travel → leave sooner

marginal value theorem: blue curve

longer travel time among patches

• longer trave → stay longer

optimal foraging by plants

plant forage by growing and orienting structures that capture either energy or nutrients.

• roots grow down with gravity (positive gravitropism)

• branch and make root hairs to access soil with more nutrients

• shoots grow up to reach light

• thigmotaxis: response of an organism to touch

optimal foraging by plants: limiting factors

nutrients in soil (ex. N,P), water and sunlight

• plants adjust location depending on which resource is more limiting

in nutrient poor soils but high light

invest more in roots

in low light but nutrient rich

invest more in shoots and leaves