Ecosystem Ecology

What is an ecosystem?

An ecosystem = a biotic community + its abiotic environment.

The two universal “currencies” in all ecosystems are:

Energy & Chemicals (matter)

How Autotrophs Acquire Energy

“Self‑feeding”

Photosynthetic organisms (plants, algae, some bacteria)

Convert sunlight → chemical energy (glucose)

How Heterotrophs Acquire Energy

“Other‑feeding”

Consume autotrophs or other heterotrophs to obtain energy

What are the Trophic Levels?

Primary producers (autotrophs)

Primary consumers (herbivores)

Secondary consumers (carnivores)

Tertiary consumers (top predators)

Detritivores & decomposers (critical at all levels)

Food chains

Linear pathways of energy flow.

Food Webs

Complex networks of feeding interactions.

Detritivores

Consume dead organic matter

Examples: worms, dung beetles, vultures

Decomposers

Break down organic matter chemically

Examples: fungi, bacteria

“The real heavy lifters” of nutrient recycling

Pyramid of Biomass

Shows total mass of organisms at each trophic level.

Pyramid of Energy

Shows energy flow; always upright.

Pyramid of Numbers

Shows number of individuals at each level.

Explains why “big, fierce animals are rare.”

The 10% Rule

Only ~10% of energy at one trophic level is passed to the next.

Why only 10%?

Not all food is eaten

Not all eaten food is digested

Energy lost as heat during respiration

• This limits food chain length.

Energy Flow

One‑way flow

Sun → autotrophs → heterotrophs → lost as heat

chemical cycling

Matter cycles through ecosystems

Governed by the Law of Conservation of Mass

Involves biological + geological + chemical processes

Pools:

Storage locations for nutrients

Have sizes

Examples: atmosphere, soil, biomass, ocean

Fluxes:

Movements of nutrients between pools

Have rates

Examples: photosynthesis, respiration, decomposition, weathering

Key points of the carbon cycle are

Major reservoirs: living organisms, fossil fuels, rocks, atmosphere

Photosynthesis moves carbon from atmosphere → biomass

Respiration, burning, weathering return carbon to atmosphere

Fossil fuel use is increasing atmospheric CO₂

CO₂ rise correlates with global temperature increase

Key points of the nitrogen cycle are:

Atmospheric N₂ is abundant but not usable by most organisms

Two critical steps make nitrogen accessible:

• Nitrogen fixation → ammonia (by nitrogen‑fixing bacteria)

• Nitrification → nitrites & nitrates (by nitrifying bacteria)

Denitrifying bacteria return N₂ to atmosphere

Nitrogen often limits productivity in terrestrial & marine ecosystems

Why Chemical Cycles Matter

Support plant growth

Regulate climate

Maintain soil fertility

Influence global temperature

Essential for life processes

Climate Change Context

NOAA & NASA data show rising CO₂ and rising global temperatures

Fossil fuel combustion is the primary driver