Energy Flow & Nutrient Cycle Big bugs have little bugs upon their backs to bite ‘em Little bugs have lesser ones an so ad infinitum.

Food Chains Artificial devices to illustrate energy flow from one trophic level to another Trophic Levels: groups of organisms that obtain their energy in a similar manner

Total number of levels in a food chain depends upon locality and number of species Highest trophic levels occupied by adult animals with no predators of their own Secondary Production: total amount of biomass produced in all higher trophic levels Food Chains

Nutrients Inorganic nutrients incorporated into cells during photosynthesis - e.g. N, P, C, S Cyclic flow in food chains Decomposers release inorganic forms that become available to autotrophs again

Energy Non-cyclic, unidirectional flow Losses at each transfer from one trophic level to another -Losses as heat from respiration -Inefficiencies in processing Total energy declines from one transfer to another -Limits number of trophic levels

Energy Flow

ProducerPrimary Consumer Secondary Consumer Tertiary Consumer Food Chain Nutrients Decomposer grass grasshopper snake hawk fungi Energy Flow through an Ecosystem heat

Transfer Efficiencies Efficiency of energy transfer called transfer efficiency Units are energy or biomass E t = P t P t-1 P t = annual production at level t P t-1 = annual production at t-1

Transfer Efficiency Example Net primary production = 150 g C/m 2 /yr Herbivorous copepod production = 25 g C/m 2 /yr Typical transfer efficiency ranges *Level 1-2 ~20% *Levels 2-3, …: ~10% E t = P t P t-1 = P copepods P phytoplankton = 25 =

Feces Growth Cellular Respiration

Food Webs Food chains don’t exist in real ecosystems Almost all organisms are eaten by more than one predator Food webs reflect these multiple and shifting interactions

Antarctic Food Web

Some Feeding Types Many species don’t fit into convenient categories Algal Grazers and Browsers Suspension Feeding Filter Feeding Deposit Feeding Benthic Animal Predators Plankton Pickers Corallivores Piscivores Omnivores Detritivores Scavengers Parasites Cannibals Ontogenetic dietary shifts

Food Webs… Competitive relationships in food webs can reduce productivity at top levels Phytoplankton (100 units) Phytoplankton (100 units) Herbivorous Zooplankton (20 units) Carnivorous Zooplankton A (2 units) Fish (0.2 units) Herbivorous Zooplankton (20 units) Carnivorous Zooplankton A (1 units) Fish (0.1 units) Carnivorous Zooplankton B (1 units)

Recycling: The Microbial Loop All organisms leak and excrete dissolved organic carbon (DOC) Bacteria can utilize DOC Bacteria abundant in the euphotic zone (~5 million/ml) Numbers controlled by grazing due to nanoplankton Increases food web efficiency

Solar Energy Microbial Loop CO 2 nutrients Phytoplankton Herbivores Planktivores Piscivores DOC Bacteria Nanoplankton (protozoans)

An Ecological Mystery

Kelp Forests Keystone Species

An Ecological Mystery Long-term study of sea otter populations along the Aleutians and Western Alaska 1970s: sea otter populations healthy and expanding 1990s: some populations of sea otters were declining Possibly due to migration rather than mortality 1993: 800km area in Aleutians surveyed -Sea otter population reduced by 50%

Vanishing Sea Otters 1997: surveys repeated Sea otter populations had declines by 90% : ~53,000 sea otters in survey area : ~6,000 sea otters Why? - Reproductive failure? - Starvation, pollution disease?

Cause of the Decline 1991: one researcher observed an orca eating a sea otter Sea lions and seals are normal prey for orcas Clam Lagoon inaccessible to orcas- no decline Decline in usual prey led to a switch to sea otters As few as 4 orcas feeding on otters could account on the impact - Single orca could consume 1,825 otters/year