Energy through Ecosystems

A2 Chapter 12

By the end of this session I will:• (d) define the terms producer, consumer decomposer

and trophic level; • (e) describe how energy is transferred though

ecosystems; • (f) outline how energy transfers between trophic levels

can be measured;• (g) discuss the efficiency of energy transfers between

trophic levels; • (h) explain how human activities can manipulate the

flow of energy through ecosystems (HSW6b);

The first one is down to you guys 2 MINUTES

(d) define the terms producer, consumer decomposer and trophic level;

GIVE EXAMPLES

Basic processes to consider!

• A trophic, or feeding, level consists of all organisms feeding at the same energy level

• Food chain– Passage of

food energy through ecosystem trophic levels in a linear path

How can we calculate the amount of energy at each trophic level?

Producers (autotrophs)

Consumers(heterotrophs)

What limits the length of the food chain?

What limits length of food chain?• H1: Energetics • Availability of energy limits to 5-7 levels• Depends on: NPP energy needed by consumers average ecological efficiency

• H2: Dynamic stability Longer chains less stable because: Fluctuations at lower trophic levels magnified at higher levels ---> extinction of top predators.

• A food web is a branching food chain with complex trophic interactions

• Species may play a role at more than one trophic level

• Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community

• About one order of magnitude of available energy is lost from one trophic level to the next

How heterotrophs use food energy

Biomass available at the next trophic level

Energy loss in an ecosystem

Cayuga LakeIn NY

– Reason why food chains generally consist of only 3 or 4 steps

Primary productivity

• Gross Primary Productivity (GPP): – total amount of photosynthetic energy captured in a given

period of time.• Net Primary Productivity (NPP):

– the amount of plant biomass (energy) after cell respiration has occurred in plant tissues.

NPP = GPP – Plant respirationplant growth/ total photosynthesis/ unit area/ unit area/unit timeunit time

Photosynthesis:• Light energy captured by pigments • Used to build bonds forming various complex

molecules – anabolic processes• Carbon dioxide absorbed/oxygen waste

product• Autotrophs: ‘self feeders’ – algae, certain

bacteria, plants• Only certain wavelengths of light effective

– 1. Pigments capture energy from sunlight

– Water is split, O2 released– 2. Using energy to make

ATP and NADPH– 3. Using ATP and NADPH to

power the synthesis of carbohydrates from CO2

Light-dependent reactions

Light-independent reactions

The Calvin cycle

6 CO2

carbondioxide

+ 6 H2Owater

+ Light energy C6H12O6

glucose+ 6 O2

oxygen

Absorption spectra of chlorophylls and carotenoids

Global O2 from photosynthesis

• 80% comes from marine cyanobacteria.– Synechococcus– Synechocystis

• 20% comes from terrestrial systems.– 5% of this comes from tropical

rainforests.

Primary productivity – marine ecosystems

Global variation in estimated NPP

Figure 9

Secondary Productivity

• Secondary productivity – the rate at which consumers convert the chemical energy of the food they eat into their own new biomass

• Involves heterotrophs• Essentially reverse of photosynthesis - May occur

with or without oxygen– Aerobic – most efficient– Anaerobic fermentative pathways (in anoxic

environment)• ATP immediate cellular energy form

Challenges We Face

• To feed 9+ billion people in 2050, global food production must increase by 70% -- 80% to come from yield increases and 20% from area expansion

• Developing countries must almost double production – with little possibility of land expansion in many countries

• Increased demand for better variety, quality and safety of agricultural products

• Land area (per cap.) declining: from 4.3 ha in 1961 to 1.5 ha in 2050

• Yield growth rate for cereals declining: 3.2% in 1960 to 1% in 2050

• Climate change likely to add greater and unpredictable stresses