Communities and Ecosystems
|43.1 A Community Includes All Life in an Area
1. Biotic communities are made up of coexisting species that are specialized in where, when, and how they live. Communities are most diverse near the equator. An ecosystem consists of biotic communities and the abiotic environment.
2. Each species has characteristic conditions where it lives (habitat) and resources necessary for its life activities (niche). Slight differences in niche allow different species to share surroundings. Because of interspecific competition, a species’ realized niche is often smaller than its fundamental niche.
3. Symbiotic relationships and predation influence community structure. In coevolution, the interaction between species is so strong that genetic changes in one population select for genetic changes in the other. Keystone species are members of communities that disproportionately affect community composition.|
43.2 Communities Change Over Time
4. As species interact with each other and their physical habitats, they change the community, a process called ecological succession. Primary succession occurs in a previously unoccupied area; secondary succession occurs after a disturbance. Succession leads toward a stable climax community, but complete stability is rare—most communities continue to change.
43.3 An Ecosystem Is a Community and Its Physical Environment
5. An ecosystem can range from a very small area to the entire biosphere. Ecosystems interact and change. Major terrestrial ecosystem types are called biomes.
6. A food chain begins when primary producers harness energy from the sun or inorganic chemicals, forming the first trophic level. The total amount of energy converted to chemical energy is gross primary production. The energy remaining after producers’ metabolism is net primary production.
7. Consumers comprise the next trophic levels. Primary consumers (herbivores) eat the primary producers. A secondary consumer may eat the primary consumer, and a tertiary consumer may eat the secondary consumer. Decomposers break down nonliving organic material (detritus) into inorganic nutrients.
8. Food chains rarely extend beyond four trophic levels because only a small percentage of the energy in one trophic level transfers to the next level. Ecological pyramids measure energy, numbers of organisms, or biomass in a food chain. Food chains interact, forming food webs.
43.4 How Chemicals Cycle Within Ecosystems—Biogeochemical Cycles
9. Biogeochemical cycles are geological and chemical processes that recycle chemicals essential to life.
10. Water cycles from the atmosphere as precipitation over land or water, then into organisms that release water in transpiration, evaporation, or excretion.
11. Autotrophs use atmospheric carbon in CO2 to manufacture carbohydrates. Cellular respiration and burning fossil fuels release CO2. Decomposers release carbon from once-living material.
12. Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia, which plants can incorporate into their tissues. Decomposers convert the nitrogen in dead organisms back to ammonia. Nitrifying bacteria convert the ammonia to nitrites and nitrates. Denitrifying bacteria convert nitrates and nitrites to nitrogen gas.
13. As rain falls over land, rocks release phosphorus as useable phosphates. Decomposers return phosphorus to the soil.
14. Bioaccumulation concentrates chemicals in cells relative to their surroundings. Biomagnification concentrates chemicals to a greater degree at successive trophic levels as the chemical passes to the next consumer rather than being metabolized or excreted.
43.5 A Sampling of Ecosystems
15. The top half-inch of the ocean contains abundant life.
16. Vernal pools exist for only a few months, but in that time house thriving communities.
17. Hardy trees and microorganisms live on cliff faces.
18. Fjords include layers of fresh water and seawater.