4/9 real

nutrient cycles vs energy flow

nutrients are recycled within ecosystems while energy flows through and is lost as heat

gross primary productivity (GPP)

total energy captured by producers via photosynthesis

percent of solar energy captured as GPP

~1%

net primary productivity (NPP)

energy remaining after plant respiration; energy available to consumers

NPP formula

NPP = GPP − R

percent of GPP that becomes NPP

~40% (about 60% lost to respiration)

main factors controlling GPP

climate (temperature + precipitation) and leaf area index (LAI)

leaf area index (LAI)

number of leaf layers above a given ground point (vertical structure)

high LAI ecosystems

tropical rainforests

low LAI ecosystems

deserts

why lower leaves are often shed

reduced light → lower return on energy investment

harvest method for NPP

measure biomass accumulation by collecting and weighing plant material

limitation of harvest method

misses belowground biomass and herbivory losses

gas flux method

measures CO₂ exchange to estimate photosynthesis and respiration

eddy covariance

measures ecosystem-level CO₂ flux using towers

NDVI

satellite-based index using red and near-infrared reflectance to estimate vegetation productivity

secondary productivity

energy stored as biomass in consumers

consumed energy

energy eaten by an organism

egested energy

energy lost in feces/urine

assimilated energy

consumed − egested energy

net secondary productivity (NSP)

assimilated energy − respiration (growth + reproduction)

GPP vs consumer analogy

GPP corresponds to assimilated energy

NPP vs consumer analogy

NPP corresponds to net secondary productivity

relationship between NPP and NSP

generally positive (more plant energy → more consumer energy)

exception to NPP–NSP relationship

eutrophic systems can reduce NSP despite high NPP

eutrophication

excess nutrients (N, P) causing high productivity but poor energy transfer

why eutrophication reduces NSP

favors inedible producers or inefficient consumers

terrestrial NPP pattern

highest in tropics, decreases toward poles

NPP vs temperature

increases with temperature

NPP vs precipitation

increases then plateaus/declines at very high rainfall

why high rainfall can reduce NPP

nutrient leaching and runoff

main limiting nutrients in aquatic systems

nitrogen and phosphorus

additional marine limiting nutrients

iron and silica

iron limitation in oceans

open ocean lacks iron → low productivity “wet desert”

iron fertilization effect

causes temporary phytoplankton blooms

limitation of iron fertilization

carbon returns to atmosphere after decomposition

upwelling

movement of deep nutrient-rich water to surface, increasing productivity

high productivity aquatic systems

coral reefs, marshes, estuaries, upwelling zones

trophic levels

hierarchical feeding positions in a food web

primary producers

autotrophs (plants, algae)

primary consumers

herbivores

secondary consumers

carnivores that eat herbivores

tertiary consumers

higher-level carnivores

direction of arrows in food webs

direction of energy flow (resource → consumer)

trophic depth

number of trophic levels in a system

terrestrial trophic depth

typically 3–4 levels

aquatic trophic depth

can be 6–8+ levels

factors affecting trophic depth

ecosystem size, disturbance, productivity

detritivores

organisms that consume dead organic matter

role of detritivores

all energy eventually passes through them

omnivore

organism that feeds at multiple trophic levels

trophic position of omnivores

fractional (e.g., 2.5)

nitrogen isotope pattern

increases ~3–3.5‰ per trophic level

why nitrogen isotopes increase

preferential excretion of lighter ¹⁴N

carbon isotopes (C₃ plants)

~−28‰

carbon isotopes (C₄ plants)

~−12‰

why C₃ vs C₄ differ

different photosynthetic pathways fractionate carbon differently

what carbon isotopes indicate

type of primary producers in diet

example of C₄ foods

corn, sugarcane

what high C₄ signature in humans means

diet high in processed/corn-based foods

trophic position formula

TP = Σ(PDᵢ × Tₛᵢ) + 1

PDᵢ

proportion of diet from source i

Tₛᵢ

trophic level of source i

vegan trophic position

2

100% herbivore diet trophic position

3

mixed diet (50% plants, 50% herbivores)

2.5

chronosequence

using sites of different ages to study succession

example of chronosequence

Presque Isle sand deposits

early succession richness

low but increases rapidly

mid succession richness

slows and stabilizes

late succession richness

may decline due to competitive exclusion

intermediate disturbance pattern

highest diversity at intermediate stages

why early succession increases diversity

many open niches

why late succession decreases diversity

strong competition excludes species