Transcript Energy Flow
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 Food Chains • 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 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 Energy Flow through an Ecosystem Food Chain Producer grass Primary Consumer grasshopper Tertiary Consumer Secondary Consumer snake hawk heat heat heat Nutrients fungi Decomposer Transfer Efficiencies • Efficiency of energy transfer called transfer efficiency • Units are energy or biomass Et = Pt Pt-1 Pt = annual production at level t Pt-1 = annual production at t-1 Transfer Efficiency Example • Net primary production = 150 g C/m2/yr • Herbivorous copepod production = 25 g C/m2/yr Et = Pt = Pcopepods = 25 = 0.17 Pt-1 P 150 phytoplankton • Typical transfer efficiency ranges *Level 1-2 ~20% *Levels 2-3, …: ~10% Cellular Respiration Feces Growth 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) Herbivorous Zooplankton (20 units) Carnivorous Zooplankton A (2 units) Fish (0.2 units) Carnivorous Zooplankton A (1 units) Carnivorous Zooplankton B (1 units) Fish (0.1 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 Microbial Loop Solar Energy Phytoplankton Herbivores CO2 nutrients Planktivores DOC Piscivores Bacteria Nanoplankton (protozoans) An Ecological Mystery Keystone Species Kelp Forests 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% - 1970: ~53,000 sea otters in survey area - 1997: ~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