Chapter 51 Ecosystems

Chapter 51

Ecosystems n Many global environmental problems have emerged recently. n Ecosystems consist of all the organisms that live in an area along with the nonbiological components. n Energy and nutrient flows link the biotic and abiotic environments.

Energy Flow and Trophic Structure n All ecosystems consist of four components that are linked by the flow of energy: Primary producers Consumers Decomposers Abiotic environment (Fig. 51.1)

External energy source PRIMARY PRODUCERS CONSUMERSDECOMPOSERS ABIOTIC ENVIRONMENT Figure 51.1

External energy source PRIMARY PRODUCERS CONSUMERSDECOMPOSERS ABIOTIC ENVIRONMENT Figure 51.1

Energy Flow and Trophic Structure n Key points about energy flow through ecosystems. Energy enters ecosystems in the form of sunlight that is used in photosynthesis by producers. Plants use only a tiny fraction of the total radiation that is available to them. Only a tiny fraction of fixed energy actually becomes available to consumers.

Energy Flow and Trophic Structure n Key points about energy flow through ecosystems. Most net primary production that is consumed enters the decomposer food web. From there, only a small fraction is used for secondary production by herbivores and carnivores. Most energy fixed during photosynthesis is used for respiration, not synthesis of new tissues. (Fig. 51.2)

Energy source: 1,254,000 kcal/m 2 /year 0.8% energy captured by photosynthesis. Of this... …45% supports growth (Net primary production) …11% enters grazing food web …34% enters decomposer food web as dead material …55% lost to respiration Figure 51.2

Energy source: 1,254,000 kcal/m 2 /year …11% enters grazing food web …34% enters decomposer food web as dead material 0.8% energy captured by photosynthesis. Of this... …45% supports growth (Net primary production) …55% lost to respiration Figure 51.2

0– – – – –800 >800 Productivity ranges (g/m 2 /yr) Figure 51.3a Terrestrial productivity

<35 35–55 55–90 >90 Productivity ranges (g/m 2 /yr) Figure 51.3b Marine productivity

80.7% respiration 17.7% excretion 1.6% growth and reproduction Energy derived from plants Figure 51.4

Predators of decomposers: Spider Centipede Mushroom Earthworm Primary decomposers: Bacteria and archaea Millipede Nematodes Pillbugs Salamander 305 nm49.4 µm Figure 51.5 Puffball

Energy Flow and Trophic Structure n Trophic structure Organisms that obtain their energy from the same type of source occupy the same trophic level. Each feeding level within an ecosystem represents a trophic level.

Energy Flow and Trophic Structure n Trophic structure Organisms at the top trophic level are not eaten by any other organisms. Productivity is highest at the lowest trophic level. (Fig. 51.6a,b)

Trophic level Feeding strategy Secondary carnivore Carnivore Herbivore Autotroph Grazing food chain Decomposer food chain Cricket Maple tree leaves Owl Shrew Earthworm Dead maple leaves Cooper’s hawk Robin Figure 51.6a Trophic levels

4 Secondary carnivore 3 Carnivore 2 Herbivore 1 Autotroph Productivity Figure 51.6b Pyramid of productivity

Energy Flow and Trophic Structure n Food chains and food webs Food chains are typically embedded in more complex food webs. (Fig. 51.7a,b)

Pisaster (a sea star) Thais (a snail) Bivalves (clams, mussels) Figure 51.7a Food chain

Pisaster Thais Chitons Limpets Bivalves Acorn barnacles Gooseneck barnacles Figure 51.7b Food web

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. (Fig. 51.7c) 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.

Number of observations Number of links in food chain Streams Lakes Terrestrial Figure 51.7c Food chains tend to have few links. Average number of links = 3.5

Biogeochemical Cycles 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. (Fig. 51.8)

Assimilation Loss to erosion or leaching into groundwater Soil nutrient pool Decomposer food web Detritus Death Herbivore Uptake Plants Feces or urine Figure 51.8

Boreal forest Figure 51.9 upper

Tropical rain forest Figure 51.9 lower

Biogeochemical Cycles n A key feature in all cycles is that nutrients are recycled and reused. n The overall rate of nutrient movement is limited most by decomposition of detritus. n The rate of nutrient loss is a very important characteristic in any ecosystem. (Fig a,b)

Devegetation experiment Choose two similar watersheds. Document nutrient levels in soil organic matter, plants, and streams. Figure 51.10a upper

Figure 51.10a lower Clearcut Control Devegetate one watershed and leave the other intact. Monitor the amount of dissolved substances in streams.

Devegetated Net dissolved substance (kg/ha) 1965–661966–671967–681968–691969–70 Control Year Figure 51.10b Nutrient runoff results

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. (Fig , 51.13a) Humans are now disrupting almost all biogeochemical cycles. This can have very harmful effects. (Fig a,b; 51.13b)

THE GLOBAL CARBON CYCLE All values in gigatons of carbon per year Physical and chemical processes: 92 2 Ocean: 40,000 Rivers: 1 Land, biota, soil, litter, peat: 2000 Decomposition: 50 Respiration: 50 Photosynthesis: 102 Physical and chemical processes: 90 Deforestation: 1.5 Fossil fuel use: 6.0 Atmosphere: 750 (in 1990) +3.5 per year Aquatic ecosystemsTerrestrial ecosystems Human–induced changes Figure 51.11

THE GLOBAL NITROGEN CYCLE Nitrogen fixing cyanobacteria Mud Decomposition of detritus into ammonia Nitrogen-fixing bacteria in roots and soil Industrial fixation Protein and nucleic acid synthesis Atmospheric nitrogen (N 2 ) Bacteria in mud use N-containing molecules as energy sources, excrete (N 2 ) Run–off Lightning and rain Figure 51.13a

Land use Fossil fuel use Year Annual flux of carbon (10 15 g) Figure 51.12a Human-induced increases in CO 2 flux over time

Year CO 2 concentration (ppm) Figure 51.12b Atmospheric CO 2

Natural sourcesHuman sources Amount of nitrogen (gigatons/year) Sources of nitrogen fixation Lightning Biological fixation Fossil fuels Nitrogen fertilizer Nitrogen- fixing crops Figure 51.13b