Community Structure and Dynamics

Biological Communities and Community Ecology

A biological community consists of all the populations of different species that inhabit a particular area and interact with one another. Community ecology studies the factors that influence the composition and dynamics of these communities, including the interactions among species and their environment.

Interspecific Interactions

Interspecific interactions are relationships between individuals of different species within a community. These interactions significantly affect population structure and dynamics. The main types of interspecific interactions include competition, mutualism, predation, herbivory, and parasitism/pathogens.

Ecological Niche and Competition

An ecological niche is the sum of an organism’s use of biotic and abiotic resources in its environment. Interspecific competition occurs when the niches of two populations overlap and both require a resource in limited supply, reducing the carrying capacity for both populations.

Mutualism

Mutualism is an interaction where both species benefit. For example, reef-building corals and photosynthetic dinoflagellates: the dinoflagellates gain shelter and nutrients, while corals receive energy from sugars produced by photosynthesis.

Predation and Prey Adaptations

Predation benefits the predator but harms the prey. Prey species have evolved various adaptations to avoid predation, such as camouflage, mechanical defenses (spines, shells), and chemical defenses (toxins).

Herbivory and Plant Defenses

Herbivory is the consumption of plant parts or algae by animals. Plants have evolved defenses such as spines, thorns, and chemical toxins (some of which are used medicinally by humans). Some plant-herbivore interactions show coevolution, where both species reciprocally affect each other's evolution.

Parasitism and Pathogens

Parasites live on or in a host, obtaining nourishment at the host’s expense. Pathogens are disease-causing microscopic parasites (bacteria, viruses, fungi, protists). Both can significantly affect community composition, especially when non-native pathogens are introduced.

Ecosystem Structure and Dynamics

Trophic Structure and Food Chains

Every community has a trophic structure, the pattern of feeding relationships among organisms. A food chain is a sequence of food transfer from producers through various levels of consumers. Producers (autotrophs) support all other trophic levels, while consumers (heterotrophs) are classified as primary, secondary, tertiary, or quaternary consumers based on their position in the chain. Detritivores and decomposers recycle nutrients by breaking down dead organic matter.

Food Webs

A food web is a network of interconnecting food chains, providing a more realistic representation of trophic relationships in a community. Organisms may occupy multiple trophic levels within a food web.

Species Diversity

Species diversity includes two components: species richness (number of different species) and relative abundance (proportion of each species). High diversity often contributes to ecosystem stability.

Keystone Species

A keystone species has a disproportionately large effect on its community relative to its abundance. Removal of a keystone species can lead to a significant loss of species diversity and community structure.

Disturbance and Succession

Disturbances (storms, fires, floods, human activities) alter communities by removing organisms or changing resource availability. Ecological succession is the process of species composition change following disturbance. Primary succession occurs on barren surfaces, while secondary succession follows disturbances that leave soil intact.

Ecosystem Energy Flow and Chemical Cycling

Energy Flow and Chemical Cycling

An ecosystem includes all organisms in a community and their abiotic environment. Energy flow moves through the ecosystem (from sunlight to producers to consumers), while chemical cycling involves the transfer of matter within the ecosystem.

Primary Production

Primary production is the conversion of solar energy to chemical energy by photosynthesis. Gross primary production is the total energy captured, while net primary production is the energy available to consumers after producers' respiration.

Energy Pyramids and Food Chain Length

An energy pyramid shows the flow of energy through trophic levels. Only about 10% of energy is transferred from one level to the next, limiting the length of food chains and explaining why eating lower on the food chain (e.g., plants) is more energy-efficient.

Biogeochemical Cycles

Biogeochemical cycles describe the movement of chemicals between living (biotic) and nonliving (abiotic) components of ecosystems. Major cycles include the carbon, phosphorus, and nitrogen cycles.

• Carbon cycle: Involves photosynthesis, respiration, and decomposition; carbon moves between the atmosphere, organisms, and fossil fuels.

• Phosphorus cycle: Involves weathering of rocks, uptake by organisms, and return to soil and water; lacks an atmospheric component.

• Nitrogen cycle: Involves nitrogen fixation by bacteria, uptake by plants, and recycling through decomposition.

Eutrophication and Human Impact

Excess nutrients (nitrogen, phosphorus) from fertilizers can cause eutrophication in aquatic systems, leading to algal blooms, oxygen depletion, and loss of species diversity. Human activities threaten ecosystem stability and the essential services ecosystems provide, such as water purification, nutrient cycling, and climate regulation.

Sustainability

Sustainability is the responsible management of Earth's resources to meet current needs without compromising future generations' ability to meet theirs.