5.2.9: Chapter Summary

Communities include all the different species living in a given area. Biotic interactions refer to the relationships among organisms. Competition occurs when organisms at the same trophic level use the the same resources, and one or both organisms is harmed. Many organisms have developed defenses against predation and herbivory, including mechanical defenses, warning coloration, and mimicry, as a result of evolution and the interaction with other members of the community. Two species cannot exist in the same habitat competing directly for the same resources. Species may form symbiotic relationships such as commensalism or mutualism. Community structure is described by its foundation and keystone species. Communities respond to environmental disturbances by succession (the predictable appearance of different types of plant species) until a stable community structure is established. The variety of these species is called species richness. Relative abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. In communities, some species play a bigger role than others. keystone species impact the biodiversity within an ecosystem by upholding an ecological community’s structure. Invasive species have an overall negative impact on the community. Community dynamics are the changes in community structure and composition over time. Succession describes the sequential appearance and disappearance of species in a community over time after a severe disturbance.

Ecosystems are communities that include the abiotic components of the environment as well. Energy is defined as the ability to do work, or to create some kind of change. Two of the physical laws that govern energy are the first and second law of thermodynamics. Energy flows from producers to consumers and recycled by detritivores and decomposers. Trophic interactions in a community can be represented by diagrams called food chains and food webs. Organisms in an ecosystem acquire energy in a variety of ways, which is transferred between trophic levels as the energy flows from the bottom to the top of the food web, with energy being lost at each transfer. Modeling of ecosystem energy is best done with ecological pyramids of energy, although other ecological pyramids provide other vital information about ecosystem structure.

After completing this chapter, you should be able to...

• Describe the differences between intraspecific and interspecific interactions in reference to competition
• Describe what is herbivory
• Give examples of defenses against predation and herbivory
• Describe what is considered a symbiotic relationships between species
• Compare and contrast between commensalism, mutualism, and parasitism
• Describe symbiosis as it relates to nitrogen fixation
• Describe how saprophytes, epiphytes, and carnivorous plants depend on other orgnanisms
• Describe how species richness and relative abundance play a role on biodiversity
• Describe the role of keystone species in a community
• Describe the role of invasive species in a community
• Describe community structure and succession
• Describe the difference between a community and an ecosystem
• Describe how organisms acquire energy in a food web and in associated food chains
• Explain how the efficiency of energy transfers between trophic levels affects ecosystem structure and dynamics
• Compare and contrast betrween a food chain and a food webs
• Describe energy transfer efficiency as it relates to trophic levels