Community Ecology Notes

Community Ecology

What is a Community?

• A community is a group of populations of different species that interact within a specific area. This interaction can be direct or indirect, influencing the survival and reproduction of the species involved.

• Community structure is influenced by the number, composition, and relative abundance of species present. Community structure includes species richness, evenness, and the types of species (e.g., dominant species, keystone species).

• Community ecology focuses on the factors that affect the structure of ecological communities. These factors include biotic interactions, abiotic conditions, and historical events.

Factors Influencing Community Structure

Community ecologists study:

1. Interspecific ecological interactions: Interactions between different species, such as competition, predation, mutualism, and commensalism.

2. Diversity and trophic structure: The variety of species in a community and the feeding relationships between them.

3. The influence of disturbance on community composition: How events like fires, floods, and human activities alter the types and numbers of species present.

4. Biogeographic factors affecting community diversity: How geographic location, climate, and barriers to dispersal influence species richness and composition.

Interspecific Interactions

• Interspecific interactions are relationships between different species that can be positive (+), negative (–), or neutral (0) in their effects on survival and reproduction. These interactions shape community structure and dynamics.

• Three main categories:

  • Competition (–/–): Both species are negatively affected.

  • Exploitation (+/–): One species benefits, and the other is harmed. Includes predation, herbivory, and parasitism.

  • Positive interactions (+/+ or +/0): At least one species benefits, and neither is harmed. Includes mutualism and commensalism.

Competition

• Occurs when different species use the same limited resource, negatively affecting both. Resources can include food, water, space, and nutrients.

• Example: Hawks and owls competing for the same prey. Both hawks and owls require the same food source to survive, leading to reduced access for both.

Competitive Exclusion

• One species may be more effective at obtaining a limited resource, leading to the local elimination of the inferior competitor. This principle suggests that strong competition can lead to the exclusion of one species.

Resource Partitioning

• An organism's niche includes the biotic and abiotic resources it uses. The niche defines the role and position of a species in its environment.

• Competitive exclusion principle: Two species cannot coexist in the same community if their niches are identical. This is because one will eventually outcompete the other.

• Ecologically similar species can coexist if there are significant differences in their niches. These differences allow them to avoid direct competition.

• Resource partitioning is the differentiation of niches, allowing similar species to coexist. Species evolve to use resources in slightly different ways.

• Example: Hawks hunt during the day, while owls hunt at night, reducing direct competition. This temporal partitioning allows both species to thrive.

Fundamental vs. Realized Niche

• Fundamental niche: The niche a species could potentially occupy. This is the full range of conditions and resources a species can use without competition.

• Realized niche: The portion of the fundamental niche that a species actually occupies, often limited by competition. This is the actual niche occupied in the presence of other species.

• Competition can cause a species' realized niche to be smaller than its fundamental niche. The presence of competitors restricts the range of resources and conditions a species can use.

Exploitation

• +/– interaction where one species benefits by feeding on another. This category includes predation, herbivory, and parasitism.

• Includes predation, herbivory, and parasitism.

Predation

• Predators kill and consume prey.

• Predators often have well-developed nervous systems and adaptations for catching prey. These adaptations can include sharp teeth, claws, speed, and camouflage.

• Prey species have evolved defenses:

  • Behavioral defenses: hiding, fleeing, forming herds or schools. These behaviors reduce the risk of predation.

  • Morphological and physiological defenses: mechanical or chemical defenses. Examples include spines, toxins, and camouflage.

  • Aposematic coloration: Bright coloration in prey with chemical defenses warns predators. This coloration signals that the prey is toxic or unpalatable.

  • Cryptic coloration: Camouflage that allows prey to blend into their environment. This makes it difficult for predators to detect them.

  • Batesian mimicry: A harmless species mimics a harmful one for protection. This deceives predators into avoiding the harmless species.

  • Müllerian mimicry: Two unpalatable species mimic each other. This reinforces the warning signal to predators.

• Mimicry can also be used by predators to approach prey more easily. Aggressive mimicry involves the predator resembling a harmless species to lure prey.

• Example: The mimic octopus can imitate various marine animals. This allows it to avoid predators and capture prey.

Herbivory

• An herbivore eats parts of a plant or alga, harming but usually not killing it. Herbivores consume plant tissues, affecting plant growth and reproduction.

• Herbivores have specialized adaptations like chemoreceptors. These adaptations help them locate and consume plants.

• Plants defend themselves with:

  • Mechanical defenses: spines/thorns. These physical structures deter herbivores.

  • Chemical defenses: toxins. These chemicals can be poisonous or distasteful to herbivores.

Parasitism

• A parasite derives nourishment from a host, harming it in the process. Parasites live on or in a host organism.

• Parasites usually don't kill their hosts directly but can weaken them. This can make the hosts more vulnerable to other threats.

• Parasites have specialized adaptations and complex life cycles. These adaptations help them infect and exploit their hosts.

Positive Interactions

• Interactions where at least one species benefits and neither is harmed. These interactions promote the survival and growth of the species involved.

Mutualism (+/+)

• Both species benefit.

• Can be obligate (required for survival) or facultative (optional). Obligate mutualism is essential for the survival of both species, while facultative mutualism is beneficial but not essential.

Commensalism (+/0)

• One species benefits, and the other is neither harmed nor helped. This interaction has no significant effect on one of the species involved.

• Can sometimes evolve into mutualism. Over time, the neutral species may evolve to benefit from the interaction.

Diversity and Trophic Structure

Species Diversity

• Richness: The number of different species in a community. A higher number indicates greater richness.

• Evenness: The relative abundance of each species. Evenness measures how equally abundant the species are.

• Diversity: Combines richness and evenness. Diversity indices, such as the Shannon diversity index, quantify both richness and evenness.

• Diverse communities:

  • Are more productive. They can utilize resources more efficiently.

  • Are more stable. They are better able to resist changes in environmental conditions.

  • Are better at withstanding and recovering from environmental stresses. They can recover more quickly from disturbances.

  • Are often more resistant to introduced species. Native species are better able to compete with new arrivals.

Trophic Structure

• Feeding relationships between organisms. Trophic structure describes how energy and nutrients move through a community.

• Trophic level: Position in the food chain. Each level represents a different feeding group.

• Food chain: Sequence of energy transfer between trophic levels. This linear sequence shows who eats whom.

• Energy transfer is inefficient; only about 10% of energy is converted to organic matter at the next level. This is due to energy loss as heat and metabolic processes.

• Food webs are more complex than simple food chains, with interconnected trophic levels. Food webs show the multiple feeding relationships in a community.

• Species can play multiple roles in a food web. Some species are both predators and prey.

• Decomposers obtain energy from dead organic matter, connecting grazer and decomposer food chains. Decomposers recycle nutrients back into the ecosystem.

Species with a Large Impact

• Impact may be due to size, abundance, or pivotal role in community dynamics. These species have a disproportionate effect on the community.

• Foundation species: create or define habitats. Examples include trees in a forest or corals in a reef.

• Keystone species: have a disproportionately large impact on the ecosystem. Their removal can lead to significant changes in community structure.

• Environmental engineers: physically alter the environment. Examples include beavers building dams or elephants creating clearings.

Keystone Species

• Species that other species in an ecosystem largely depend on. The removal of a keystone species can cause a trophic cascade.

• Removal leads to drastic ecosystem changes. This can result in the loss of biodiversity and ecosystem function.

• Types of keystone species: predator, modifier, prey, mutualist, host. Each type plays a critical role in maintaining community structure.

• Example: Wolves in Yellowstone National Park affect food web interactions and ecosystem engineering. Their presence regulates elk populations, which in turn affects vegetation and other species.

Influence of Disturbance

• Disturbance: An event that changes a community by removing organisms or altering resource availability. Disturbances can be natural or human-caused.

• Types, frequency, and severity of disturbances vary. Different disturbances have different effects on communities.

• Historically, communities were thought to be at equilibrium unless disturbed by humans. This view emphasized stability and predictability.

• This view emphasized competition and a single climax community controlled by climate. The climax community was seen as the end-point of succession.

• More recently, ecologists recognize multiple stable communities and the importance of disturbance. Disturbance is now seen as a natural and essential process.

• Disturbance can prevent communities from reaching equilibrium. It can reset succession and create opportunities for new species.

Intermediate Disturbance Hypothesis

• Moderate levels of disturbance promote greater species diversity. This hypothesis suggests that both high and low levels of disturbance can reduce diversity.

• High disturbance: stresses exceed tolerances; slow-growing species are excluded. Frequent or intense disturbances can eliminate species that are not adapted to them.

• Low disturbance: competitively dominant species exclude others. Without disturbance, strong competitors can outcompete other species.

Natural Succession

• Process of biotic community formation on a lifeless or relatively lifeless area. Succession describes the changes in community composition over time.

• Primary succession: starts from bare rock. This occurs in areas where no soil exists.

• Secondary succession: occurs after an ecosystem is disturbed but soil remains. This is faster than primary succession because soil and nutrients are already present.

Primary Succession

• Begins on bare rock with no life.

• Pioneer species, such as lichens, break down rock and create soil. Lichens secrete acids that weather the rock.

• Mosses grow, followed by weeds and grasses. These plants stabilize the soil and add organic matter.

• Shrubs and small trees grow as soil thickens. These plants provide shade and alter the microclimate.

• Pines are often the first trees, followed by deciduous trees. Deciduous trees add more nutrients to the soil.

• Climax community is a mature, stable, and sustainable ecosystem. This community is relatively stable and resistant to change.

• Examples: volcanic activity forming new land, glacier retreat. These events create new, lifeless areas for primary succession to begin.

Secondary Succession

• Occurs after a disturbance where soil is present.

• More rapid than primary succession.

• Pioneer communities are often dominated by r-selected species. These species are fast-growing and reproduce quickly.

• Examples: succession after a fire, volcanic eruption, or on abandoned farmland. These events leave the soil intact, allowing for faster recovery.

• Changing conditions during succession:

  • Biotic and abiotic conditions change. The environment becomes more favorable for a wider range of species.

  • Pioneer and intermediate communities alter conditions, promoting new communities. They modify the soil, light, and moisture levels.

  • Species diversity increases. As succession progresses, more species are able to colonize the area.

  • Ecosystem stability increases. The community becomes more resilient to disturbances.

Biogeographic Factors

Latitudinal Gradients

• Species richness is greatest in the tropics and declines toward the poles. This pattern is observed across many taxa.

• Key factors:

  • Evolutionary history: Tropical communities are older and have had more time for speciation. The tropics have been relatively stable over long periods.

  • Climate: Unequal solar radiation distribution leads to variations in precipitation. The tropics receive more solar energy and rainfall.

  • High temperature and precipitation in the tropics lead to high evapotranspiration. This supports high levels of primary productivity.

• Species richness correlates with evapotranspiration.

Area Effects

Species-