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The energy of the entire system (Ch. 37) Population • A population is a group of individuals of a single species that occupy the same general area • Density and dispersion patterns are important population variables. Population density is the number of individuals of a species per unit area or volume. • Environmental and social factors influence the spacing of individuals in various dispersion patterns • Clumped, Uniform, Random Yum, plenty of lunch in this random population How does a population grow A population is defined by how it grows. Multiple factors may limit population growth. • • • • • Population growth is density dependent (how many individuals exist in the population). Birth rates decline and death rates rise in response to increasing population density. Organisms compete for limited resources Density affects health of organisms Abiotic factors may limit population growth before limiting factors become important Some populations have "boom-andbust" cycles: Some populations fluctuate in density with regularity. Example: snowshoe hare and lynx cycles Structural features of communities A community includes all the organisms inhabiting a particular area • • Biological community: an assemblage of all the populations of organisms living close enough together for potential interaction Key characteristics of a community Species diversity: variety of different kinds of organisms Species richness Relative abundance Dominant species Response to disturbances Trophic structure: feeding relationships among species • Trophic structure is a key factor in community dynamics. • Trophic structure: a pattern of feeding relationships consisting of several different levels • Food chain: sequence of food transfer up the trophic levels. Moves chemical nutrients and energy • • Producers • Autotrophs that support all other trophic levels • Plants on land • In water, mainly photosynthetic protists and cyanobacteria Primary consumers • Herbivores that eat plants, algae, or phytoplankton • Secondary, tertiary, and quaternary consumers • • Eat consumers from the level below them Detritivores (decomposers) • • • Animal scavengers, fungi, and prokaryotes Derive energy from detritus produced at all trophic levels Decomposition is essential for recycling nutrients in ecosystems Trophic level Quaternary consumers Hawk Killer whale Tertiary consumers Snake Tuna Secondary consumers Herring Mouse Primary consumers Zooplankton Grasshopper Producers Plant A terrestrial food chain Phytoplankton An aquatic food chain Food chains interconnect, forming food webs. A food web is a more realistic view of trophic structure: • Consumers usually eat more than one type of food • Each food type is consumed by more than one type of consumer Quaternary, tertiary, and secondary consumers Tertiary and secondary consumers Secondary and primary consumers Primary consumers Producers (plants) Ecosystem structure and dynamics Ecosystem ecology emphasizes energy flow and chemical cycling • • An ecosystem consists of all the organisms in a community and the abiotic factors with which they interact Ecosystem dynamics involve two processes • • Energy flow through the components of the ecosystem Chemical cycling within the ecosystem LE 37-11 Chemical cycling Energy flow Chemical energy Light energy Heat energy Chemical elements Primary production sets the energy budget for ecosystems • Primary production: amount of solar energy converted by producers to chemical energy in biomass. • • • Biomass: amount of organic material in an ecosystem Net primary production: amount of biomass produced minus amount used by producers in cellular respiration Varies greatly among ecosystems Open ocean Estuary Algal beds and coral reefs Desert and semidesert scrub Tundra Temperate grassland Cultivated land Boreal forest (taiga) Savanna Temperate deciduous forest Tropical rain forest 0 500 1,000 1,500 2,000 Average net primary productivity (g/m2/yr) 2,500 Energy supply limits the length of food chains • Only about 10% of the energy stored at each trophic level is available to the next level • Pyramid of production shows loss of energy from producers to higher trophic levels. • Amount of energy available to top-level consumers is relatively small. • Most food chains have only three to five levels. Tertiary consumers Secondary consumers Primary consumers 10 kcal 100 kcal 1,000 kcal Producers 10,000 kcal 1,000,000 kcal of sunlight CONNECTION A production pyramid explains why meat is a luxury for humans • • • Human meat or fish eaters are tertiary or quaternary consumers Humans eating grain have ten times more energy available than when they process the same amount of grain through meat Using land to raise animals consumes more resources than using the land to cultivate crops Trophic level Human meat-eaters Secondary consumers Primary consumers Human vegetarians Corn Producers Cattle Corn