Ecosystem Ecology Ecosystem ecology emphasizes energy flow and chemical cycling. An ecosystem consists of all of the organisms in a community as well as the abiotic components they interact with.

ECOSYSTEM

Energy from the sun fuels global ecosystems The Earth is not a closed system. Energy comes in from outside (i.e., from the sun), moves through ecosystems and ultimately is dissipated into space as heat.

Primary Efficiency How much solar radiation is converted into biomass? Tropical Rain Forests : 1-3% Temperate deciduous forest : 0.5-1% Hot deserts : 0.01-0.2%

Food Chains Show 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

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

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

Transfer Efficiency Example Net primary production = 150 g C/m2/yr Herbivorous copepod production = 25 g C/m2/yr = Pcopepods Et = Pt Pt-1 = 25 = 0.17 Pphytoplankton 150 Typical transfer efficiency ranges *Level 1-2 ~20% *Levels 2-3, …: ~10%

Net Productivity of species n Assimilation of species n Production Efficiency = Net Productivity of species n Assimilation of species n Group Production Efficiency Respiration Mammals and birds 1 – 3% 97 – 99% Insects 10 – 41% 59 – 90% Why the difference?  Homeothermy (it takes a lot of energy to keep warm)

Why Is Energy Transfer Not 100% Efficient? When energy is changed from one form to another, some is degraded in the form of heat. This law of thermodynamics is true for all physical-chemical systems, including living ones. One ecologist summarized this fact by the statement: “There is no such thing as a free lunch.” All physical transactions come at a cost of energy.

Secondary Efficiency Worldwide : mean = 10% per level

Energy and Biomass Pyramids Kaneohe Bay 10 J Tertiary consumers 100 J Secondary consumers 1000 J Primary consumers 10,000 J Primary producers 1,000,000 J of sunlight

Biomass Why does the ocean have such a low biomass of primary producers?

Inverted Pyramids In lakes & open sea algae are the main primary producers. Algae multiple fast but also consumed at a higher rate therefore the pyramid appears inverted.

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

Nutrients Unlike energy, nutrients are constantly recycled from one form to another and pass through multiple trophic levels to decomposers to abiotic forms and back to living organisms again.

(decomposers and detritus feeders) Heat First Trophic Level Second Trophic Third Trophic Fourth Trophic Solar energy Producers (plants) Primary consumers (herbivores) Tertiary (top carnivores) Secondary (carnivores) Detritvores (decomposers and detritus feeders)

Detritivores Main decomposers are fungi and bacteria. These break down organic matter and release chemical elements into the soil, water and air where producers can recycle them into organic compounds. Without the action of decomposers life would cease because essential nutrients would remain locked up in detritus and unavailable to organisms.

Detritivores Detritivores (species that feed on the dead remains of organisms) are also known as decomposers. These include bacteria, fungi and invertebrate animals of numerous species. 2O & HIGHER LEVEL CONSUMERS DECOMPOSERS 1O CONSUMERS PRODUCERS

Fungal decomposition of a tree stump

Primary Production About 1% of the visible light that strikes earth is converted by photosynthesis into chemical energy. This is enough energy to create about 170 billion tons of organic matter annually.

Gross and net primary productivity In an ecosystem gross primary productivity (GPP) is the amount of light energy converted into chemical energy per unit time. Net primary productivity (NPP) is gross primary productivity minus the energy used by the primary producers for respiration.

Net primary productivity Net primary productivity indicates how much energy is available for use by other trophic levels. It is measured as biomass of vegetation added to the ecosystem per unit area per unit time (g/m2/yr).

Net primary productivity NPP is influenced by light and nutrient availability and differs among ecosystems. Tropical rain forests have high NPP as do estuaries and coral reefs. Lakes, tundra and the open ocean have relatively low NPP.

Global net primary productivity Tropical rainforests contribute about 22% of the Earth’s total NPP and open ocean about 24%. The open ocean has a much lower rate of NPP, but covers a far larger area. Various temperate forests and grasslands and the continental shelf (shallow continental waters) contribute most of the rest.

Global net primary productivity Overall, terrestrial ecosystems contribute about 66% of NPP and marine ecosystems the remainder.

NET PRODUCTION IN SOME MAJOR BIOMES Net Production / Unit Area ( grams / meter 2 / year) BIOME RANGE AVERAGE Desert & Semi-desert 0-250 40 Artic & Alpine Tundra 10-400 140 Coniferous Forest 400-2,000 800 Deciduous Forest 600-2,500 1,250 Grassland 200-1,500 600 Tropical Forest 1,000-3,500 2,000

Limits on primary productivity In marine and freshwater ecosystems both light and nutrients are important in controlling NPP. The inability of light to penetrate the water limits photosynthesis to the upper layers. More than 50% of solar radiation is absorbed in the first meter. Even in clear water, only about 5-10% of radiation reaches a depth of 20m.

Regional annual net primary production for Earth

Limits on primary productivity Nutrient limitation is a major factor affecting NPP in aquatic biomes. In marine environments the nutrients limiting primary productivity are usually nitrogen and phosphorus, which are scarce in the photic zone.

Decomposition and nutrient cycling rates The rates at which nutrients cycle is strongly affected by the rates at which decomposers work. In the tropics, warmer temperatures and abundant moisture cause organic material to decompose 2-3 times faster than it does in temperate regions.

Decomposition and nutrient cycling rates High rate of decomposition means little organic material accumulates as leaf litter. In tropical forest about 75% of nutrients are in woody trunks of trees and only about 10% in soil. In temperate forest about 50% of nutrients are in the soil because decomposition is slower.

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?

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

Carrying Capacity – the number or weight of animals of a single or mixed population that can be supported permanently on a given area Ecological Carrying Capacity – maximum density of animals that can be sustained without inducing negative effects on ecosystem Economic Carrying Capacity – density of animals that enables maximal sustained harvesting and is always lower than the ecological carrying capacity