Module 8 Lesson 8 - 5
Lesson 8 — Populations and Communities Changing over Time
Lesson Summary
Communities change over time in the process of succession. Ecological disturbances (both human-caused and natural) can destroy communities and return them to rock or to soil. Primary succession begins from bare rock that has little life and may be the result of avalanche or glacial melting. Secondary succession occurs after fire has destroyed a community or when human settlements or agricultural fields are abandoned, but some soil has life existing as roots, seeds, and other organisms. In either case, a predictable pattern of invasion and replacement of species occurs, ending in a stable climax community.
The first species to invade is the pioneer species, and the last species is the climax species by which the community is known. Animal species undergo succession as well. As producer species change with the succession, so does the basis for new consumer food chains and food webs.
The analysis of complex ecosystems to identify the relationships between community members is difficult. Each organism belongs to a given trophic level and has a role within the community. However, organisms within the same trophic level compete with each other (the previous trophic level) in interspecific competition for food, water, and other scarce resources. Competition between members of the same species (intraspecific competition) happens the same way. Those with alleles that allow the individual to compete better live to reproduce and improve the population gene pool within that environment.
Predator-prey or producer-consumer interactions describe a community's food chain. Predators act as selecting agents, removing those with less successful alleles and leaving those more genetically fit. Predators and prey cycle together through time as the populations of one controls the numbers of the other. Prey species and producers have evolved many chemical, structural, and behavioural defenses that protect them from consumption. The more effective the defense, the more likely the organism is to survive to pass its genes to the next generation.
As an organism fills its niche (does its job) in the food chain, it may have positive or negative effects on those with whom it lives. Mutualistic relationships benefit both parties, commensal relationships benefit one and do not affect the other, and parasitic relationships benefit one organism at the expense of the other.
Succession refers to the changes that occur in communities as a geographical area ages. Whether primary (from rock) or secondary (from soil), pioneer species invade a barren habitat, changing the environment so they can no longer survive there, creating
an environment for a more suitable organism to replace it, terminating in a climax species that remains indefinitely until the succession starts over. As producer species change through succession, the consumer species that rely on them change as
well. The changing cast of characters in successions is part of a natural process and does not indicate a problem.
Assignment