9: Community Structure and Dynamics
- Page ID
- 103364
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Figure \(\PageIndex{1}\): Until the late 1800s, Bison (Bison bison) numbered tens of millions in the Great Plains of the United States. By 1890, roughly 1000 Bison were left because the United States government campaign to eradicate the native people, their culture, and habitats they relied on. Slowly, a movement started to try and save Bison from extinction. It took until the early 2000’s for Bison numbers to reach a half a million. During this time, scientists were able to observe the Bison reintroduction back into the Great Plains. Bison were found to be the most critical species to restoring and maintaining the function and diversity in the Great Plains community. Both plants and animal populations in the community were strengthened from the return of the Bison. Understanding community dynamics is essential to conserving and restoring these systems and the species that define them. This is particularly critical for communities with one particular species that acts as a keystone to the health of the system. Image by NPS photos/Kim Acker (Public Domain)
Chapter Summary
Populations that interact within a given area form a community. Ecologists study the organization and function of communities from the scale at many spatial and temporal scales. Communities are complex systems that can be characterized by community structure (the number and size of populations and their interactions) and community dynamics (how the members and their interactions change over time). Scientists study ecology at the community level to understand how interactions between species and abiotic factors affect the way communities are structured, function, and change over time. Understanding community structure and dynamics allows us to minimize human impacts on ecosystems and manage ecological communities we benefit from.
After reading this chapter you should be able to:
- Define a community. Describe how Ecologist measure community structure through species richness and species diversity.
- List hypotheses for why biodiversity tends to be highest near the equator.
- Describe the role of foundational species, keystone species, and invasive species in community structure and stability.
- Explain the flow of energy through the trophic levels of a community.
- Describe the process of biomagnification.
- Explain how disturbances can increase biodiversity of a community,
- Compare primary and secondary ecological succession and be able to identify and describe examples of each.
- 9.1: Community Structure
- Community structure is the ecologist's term for indicating what organisms are present in a given environment, in what numbers, and how they relate to each other. Two important measures ecologists use to describe the composition of a community are species richness and relative abundance (or species evenness). In general, ecologists think that more diverse ecological communities are more stable (that is, more able to recover after a disturbance) than less diverse communities.
- 9.2: The Role of Species within Communities
- Communities are shaped by foundation species and keystone species, while invasive species disrupt the natural balance of an area.
- 9.3: Food Chains and Food Webs
- Feeding relationship among species can influence community structure. Grazing food webs have a producer at their base, which is either a plant for terrestrial ecosystems or a phytoplankton for aquatic ecosystems. The producers pass their energy to the various trophic levels of consumers. At the base of detrital food webs are the decomposers, which pass their energy to a variety of other consumers. Changes in the abundance of one species can affect the rest of the community.
- 9.4: Disturbance
- Ecological disturbance, is a temporary change in environmental conditions that causes a pronounced change in a community. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and abiotic elements. Disturbance plays a significant role in shaping the structure of individual populations and the biodiversity of whole ecosystems.
- 9.5: Succession
- Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state. Thus, disturbance can initiate successional change. Ecological succession is the process of more or less orderly and predictable changes in the species structure of an ecological community over time. It is a process by which an ecological community undergoes more or less orderly and predictable changes following a disturbance or the initial col