Chapter 51 Ecosystems Ecosystems n n n Population

Chapter 51 Ecosystems

Ecosystems n n n Population: all the individuals of a certain species that live in a particular area Community: all the different species that interact together within a particular area Ecosystems consist of all the organisms that live in an area along with the nonbiological (abiotic) components.

Ecosystems n n n Many global environmental problems have emerged recently. Ecosystem ecology follows the flow of energy and nutrients through ecosystems Humans have artificially affected the flow of these components

Energy Flow within Ecosystems n Energy enters an ecosystem primarily though sunlight:

Energy Flow and Trophic Structure n Species within an ecosystems are classified into different trophic levels: • Primary producers: autotrophs, photosynthetic- plants, algae, some bacteria • Consumers • Primary consumers: herbivores that eat producers (plants)- deer, rabbits, etc. • Secondary consumers: carnivores that eat herbivores: wolf eating a deer • Tertiary consumers: carnivores that eat carnivores: a hawk eating a snake that ate a mouse • Decomposers: fungi, bacteria that break down organic material (dead plants and animals)

Different Trophic Levels in an Ecosystem Trophic level 4 Feeding strategy Grazing food chain Secondary carnivore Cooper’s hawk 3 Decomposer food chain Carnivore Robin Owl Shrew 2 Herbivore Cricket Earthworm 1 Autotroph Maple tree leaves Dead maple leaves

Energy Flow in an ecosystem External energy source PRIMARY PRODUCERS CONSUMERS DECOMPOSERS ABIOTIC ENVIRONMENT

Decomposers Predators of decomposers: Spider Salamander Centipede Puffball Mushroom Earthworm Primary decomposers: Bacteria and archaea 305 nm Millipede Nematodes 49. 4 µm Pillbugs

Energy Flow and Trophic Structure n Key points about energy flow through ecosystems. • Plants use only a tiny fraction of the total radiation that is available to them. • Most energy fixed during photosynthesis is used for respiration, not synthesis of new tissues. • Only a tiny fraction of fixed energy actually becomes available to consumers. • Most net primary production that is consumed enters the decomposer food web.

Ecological Efficiency: percent of energy transferred from one trophic level to the next Energy source: 1, 254, 000 kcal/m 2/year 0. 8% energy captured by photosynthesis. Of this. . . … 55% lost to respiration … 45% supports growth (Net primary production) … 11% enters grazing food web … 34% enters decomposer food web as dead material

Ecosystem Processes n Production: rate at which energy/nutrients are converted into growth • Includes Primary Production: growth by autotrophs • Includes Secondary Production - growth by heterotrophs n n Consumption - the intake and use of organic material by heterotrophs Decomposition - the chemical breakdown of organic material

Figure 51. 3 a Terrestrial productivity 0– 100– 200– 400– 600– 800 >800 Productivity ranges (g/m 2/yr)

Figure 51. 3 b Marine productivity <35 35– 55 55– 90 >90 Productivity ranges (g/m 2/yr)

Very little of the energy consumed by primary consumers are used for secondary production 80. 7% respiration Energy derived from plants 1. 6% growth and reproduction 17. 7% excretion

Pyramid of productivity 4 Secondary carnivore 3 Carnivore 2 Herbivore 1 Autotroph Productivity Example: 100 g of plant becomes 5 -20 g of grasshopper then 0. 25 -1 g of mouse

The Different Trophic levels in an ecosystem is often pictured as a Food chain Pisaster (a sea star) Thais (a snail) Bivalves (clams, mussels)

Energy Flow and Trophic Structure n Food chains and food webs • Food chains are typically embedded in more complex food webs. • Many organisms feed at more than one trophic level

Food web Pisaster Thais Chitons Limpets Bivalves Acorn barnacles Gooseneck barnacles

Energy Flow and Trophic Structure n Food chains and food webs • The maximum number of links in any food chain or web ranges from 1 to 6. • Hypotheses offered to explain this: § Energy transfer may limit food-chain length. § Long food chains may be more fragile. § Food-chain length may depend on environmental complexity.

Food chains tend to have few links. Average number of links = 3. 5 10 Streams Lakes Number of observations 8 Terrestrial 6 4 2 0 1 2 3 4 Number of links in food chain 5 6

Biogeochemical Cycles n n The path an element takes as it moves from abiotic systems through living organisms and back again is referred to as its biogeochemical cycle. Examples: nitrogen cycle, carbon cycle, phosphorus cycle

Figure 51. 8 Assimilation Plants Herbivore Feces or urine Death Uptake Detritus Soil nutrient pool Loss to erosion or leaching into groundwater Decomposer food web

Biogeochemical Cycles n n A key feature in all cycles is that nutrients are recycled and reused. The overall rate of nutrient movement is limited most by decomposition of detritus.

Boreal forest: nutrients are put back into the soil slowly, so organic material builds up

Tropical rain forest: decomposition is rapid so there is very little organic build up Result: if living material is removed from tropical rain forests, the soil is nutrient poor to support new growth

The rate of nutrient loss is a very important characteristic in any ecosystem. Devegetation experiment Choose two similar watersheds. Document nutrient levels in soil organic matter, plants, and streams.

Clearcut Control Devegetate one watershed and leave the other intact. Monitor the amount of dissolved substances in streams.

Nutrient export increases dramatically in devegetated plot Net dissolved substance (kg/ha) Nutrient runoff results 1000 Devegetated 800 600 400 Control 200 1965– 66 0 1966– 67 1967– 68 Year 1968– 69 1969– 70

Biogeochemical Cycles n Nutrient flow among ecosystems links local cycles into one massive global biogeochemical cycle. • The carbon cycle and the nitrogen cycle are examples of major, global biogeochemical cycles. • Humans are now disrupting almost all biogeochemical cycles. This can have very harmful effects.

Humans are adding significant amounts of carbon into the atmosphere THE GLOBAL CARBON CYCLE All values in gigatons of carbon per year Photosynthesis: 102 Physical and chemical processes: 92 2 Atmosphere: 750 (in 1990) +3. 5 per year Respiration: 50 Decomposition: 50 Physical and chemical processes: 90 Ocean: 40, 000 Aquatic ecosystems Rivers: 1 Deforestation: 1. 5 Fossil fuel use: 6. 0 Land, biota, soil, litter, peat: 2000 Terrestrial ecosystems Human–induced changes

Human-induced increases in CO 2 flux over time Annual flux of carbon (1015 g) 6 Fossil fuel use 5 4 3 Land use 2 1 0 1860 1880 1900 1920 Year 1940 1960 1980

Figure 51. 12 b Atmospheric CO 2 concentration (ppm) 360 350 340 330 320 310 1960 1970 1980 Year 1990

THE GLOBAL NITROGEN CYCLE Only nitrogen-fixing bacteria can use N 2 § make ammonia (NH 3) or nitrate (NO 3) § limiting nutrient (demand exceeds supply) for plants § All organisms require nitrogen to make protein § Animals get nitrogen from their diets, not the air § Atmospheric nitrogen (N 2) =78% Bacteria in mud use N-containing molecules as energy sources, excrete (N 2) Run–off Nitrogen fixing cyanobacteria Mud Protein and nucleic acid synthesis Lightning and rain Industrial fixation Decomposition of detritus into ammonia Nitrogen-fixing bacteria in roots and soil

Human activities now fix almost as much nitrogen each year as natural sources Sources of nitrogen fixation Amount of nitrogen (gigatons/year) 160 140 Lightning Fossil fuels 120 Nitrogenfixing crops 100 80 Biological fixation 60 Nitrogen fertilizer 40 20 0 Natural sources Human sources