Energy and Ecosystems Lecture 03

Food chain Passage of food energy through ecosystem trophic levels in a linear path A trophic, or feeding, level consists of all organisms feeding at the same energy level

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

Sea nettle Juvenile striped bass Fish larvae Fish eggs Zooplankton Figure 54.13 Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast Fish larvae Fish eggs Zooplankton

Ecological Pyramids A plant fixes about 1% of the sun’s energy that falls on its green parts Successive members of a food chain incorporate ~ 10% of energy available in organisms they consume Therefore, there are far more individuals at the lower tropic levels The biomass of the lower trophic levels also tends to be greater These relationships appear in a diagram as pyramids

Limits on Food Chain Length Each food chain in a food web is usually only a few links long Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis The energetic hypothesis suggests that length is limited by inefficient energy transfer The dynamic stability hypothesis proposes that long food chains are less stable than short ones Most data support the energetic hypothesis

Biomass available at the next trophic level About one order of magnitude of available energy is lost from one trophic level to the next Biomass available at the next trophic level How heterotrophs use food energy Reason why food chains generally consist of only 3 or 4 steps Energy loss in an ecosystem Cayuga Lake In NY

Energy Loss at Trophic Levels Lindeman efficiency Provides measure of energy transfer amongst trophic levels ratio of assimilation between trophic levels = energy (growth + respiration) of predator energy (growth + respiration) of food species

Laws of Thermodynamics and energy: Kinetic energy Light, heat Potential energy – energy in chemical bonds ATP (adenosine triphosphate) carbohydrates, lipids, proteins First Law: energy neither created nor destroyed … Second Law: energy transformations lead to progressively more dispersed (less funtional) forms of energy - entropy Light  heat  motion

Photosynthesis: (see ch. 6 for details) 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

Light-dependent reactions 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 + 12 H2O water + Light energy + 6 H2O water + 6 O2 oxygen 6 CO2 carbon dioxide C6H12O6 glucose

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 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 respiration plant growth/ total photosynthesis/ unit area/ unit area/unit time unit time

Primary productivity – marine ecosystems

Impacting Primary Productivity: Light intensity Nutrient availability N + Fe tends to be limiting in marine systems P limiting in fresh water system  eutrophication Lake ecosystem

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

Glycolysis With Oxygen aerobic Fermentation- anaerobic Electron Transport most ATP generated here

Two food chains: Grazing food chain Detritus food chain Herbivore  carnivore Detritus food chain Dead matter and waste from grazing food chain and primary production Provides input to grazing food chain

Energy Transfer – Loss between trophic levels: trophic efficiency = energy (growth + respiration) of predator energy (growth + respiration) of food species

Found in larger numbers, but still contain 90% less energy Ecological pyramids Fairly large animals Found in larger numbers, but still contain 90% less energy Inverted pyramid

How does energy efficiency of endotherms vs. ectotherms differ – why? What is meant by: Primary productivity? Secondary ? What processes occur during photosynthesis/ cellular respiration – how are they essentially complementary processes? How do the first and second laws of thermodynamics apply to energy transfer in ecosystems? How does energy efficiency of endotherms vs. ectotherms differ – why? What is the role of detritivores in energy transfer in ecosystems?