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 are:

• Competition: Both species are negatively affected as they compete for the same limited resource.

• Mutualism: Both species benefit from the interaction.

• Predation: One species (predator) benefits by killing and eating the other (prey).

• Herbivory: An animal consumes plant parts or algae, benefiting the herbivore and harming the plant.

• Parasitism and Pathogens: The parasite or pathogen benefits at the expense of the host.

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 short supply, reducing the carrying capacity for both populations.

Mutualism

In mutualistic relationships, both partners benefit. For example, reef-building corals and photosynthetic dinoflagellates: the dinoflagellates gain shelter and nutrients, while corals receive sugars produced by photosynthesis.

Predation and Prey Adaptations

Predation drives the evolution of diverse adaptations in prey species to avoid being eaten. These adaptations include:

• Camouflage: Blending in with the environment to avoid detection.

• Mechanical defenses: Physical structures like spines or shells.

• Chemical defenses: Production of toxins or distasteful substances.

Herbivory and Plant Defenses

Plants have evolved various defenses against herbivores, such as spines, thorns, and chemical toxins. Some toxins are distasteful or harmful, and some plants produce chemicals that disrupt the development of herbivores. Coevolution can occur, where plants and herbivores evolve in response to each other’s adaptations.

Parasitism and Pathogens

Parasites live on or in a host, obtaining nourishment and often harming the host. Pathogens are disease-causing microscopic parasites, including bacteria, viruses, fungi, or protists. Both can significantly affect community composition, especially when non-native pathogens are introduced.

Trophic Structure and Food Webs

Trophic Levels and Food Chains

Every community has a trophic structure, which is the pattern of feeding relationships among organisms. The sequence of food transfer is called a food chain:

• Producers (autotrophs): Support all other trophic levels.

• Primary consumers (herbivores): Eat producers.

• Secondary consumers: Eat primary consumers.

• Tertiary consumers: Eat secondary consumers.

• Quaternary consumers: Eat tertiary consumers.

• Detritivores and decomposers: Break down dead material, recycling nutrients.

Food Webs

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

Species Diversity and Keystone Species

Species Diversity

Species diversity has two components:

• Species richness: The number of different species in a community.

• Relative abundance: The proportional representation of each species.

Keystone Species

A keystone species has a disproportionately large impact on its community relative to its abundance. Removal of a keystone species can lead to a dramatic decrease in species diversity, as demonstrated by experiments with sea stars in intertidal zones.

Community Change and Succession

Disturbance and Succession

Disturbances such as storms, fires, and human activities can alter communities by removing organisms or changing resource availability. Ecological succession is the process of community change following a disturbance:

• Primary succession: Colonization of barren rock, where soil is initially absent.

• Secondary succession: Occurs after a disturbance removes a community but leaves the soil intact.

Ecosystem Structure and Dynamics

Energy Flow and Chemical Cycling

An ecosystem includes all organisms in a community and their abiotic environment. Energy flow moves through the ecosystem, while chemical cycling transfers matter within the ecosystem.

Primary Production

Primary production is the conversion of solar energy to chemical energy by photosynthesis. The total amount is gross primary production, while net primary production is the energy available to consumers after producers use some for their own respiration.

Energy Pyramids and Food Chain Length

A pyramid of production shows the flow of energy through trophic levels. Only about 10% of the energy at each level is available 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

Chemical Cycling

Biogeochemical cycles describe the movement of elements like carbon, nitrogen, and phosphorus between living (biotic) and nonliving (abiotic) components of ecosystems. These cycles can be local or global.

The Carbon Cycle

Carbon cycles globally between the atmosphere, living organisms, fossil fuels, soils, and oceans. Photosynthesis and respiration are key processes in this cycle.

The Phosphorus Cycle

Phosphorus is required for nucleic acids, phospholipids, ATP, and bones. The phosphorus cycle does not have an atmospheric component and depends on the weathering of rocks.

The Nitrogen Cycle

Nitrogen is essential for proteins and nucleic acids. The nitrogen cycle relies on bacteria to convert atmospheric nitrogen into forms usable by plants, such as ammonium and nitrate.

Human Impact and Ecosystem Services

Eutrophication

Excess nutrients from fertilizers can cause eutrophication in aquatic ecosystems, leading to algal blooms, decreased species diversity, and oxygen depletion.

Ecosystem Services and Sustainability

Natural ecosystems provide essential services such as water purification, nutrient recycling, waste decomposition, and climate regulation. Sustainability involves managing resources to meet current needs without compromising future generations.