Community Ecology Notes

Community Ecology

What is a Community?

A community in ecology refers to a group of species that interact with one another within a particular environment. These interactions can significantly affect the distribution and abundance of the species involved.

Species Interactions

Species interactions can fundamentally shape communities, influencing evolutionary processes and the survival of species. The principal types of species interactions include:

1. Consumption (+/-)

In consumption interactions, one organism consumes another, leading to beneficial outcomes for the consumer while negatively impacting the consumed organism. Examples include predation, herbivory, and parasitism.

2. Competition (-/-)

In competition, individuals or species vie for the same limited resources, such as food, water, or space, resulting in lower fitness for both competitors. This dynamic can lead to competitive exclusion, where one species outcompetes another.

3. Commensalism (+/0)

Commensalism is an interaction where one species benefits while the other is unaffected. An example is barnacles that attach to whales; barnacles gain mobility to food-rich waters, while the whale is neither helped nor harmed.

4. Mutualism (+/+)

Mutualism occurs when two species provide benefits to each other, enhancing each other's fitness. For instance, treehoppers extract sugar from the phloem of plants and secrete honeydew, which ants collect as food. This relationship not only supports the ants but also protects the treehoppers from predators.

Experimental Evidence for Mutualisms

Observational and experimental studies, such as those involving treehoppers and ants, document how these interactions benefit both parties. In mutualistic relationships, the dynamics can vary, sometimes leading to situations where one partner may benefit at the other's expense (cheating).

The Niche Concept of Ecology

The niche encompasses the range of resources a species utilizes and the environmental conditions it can tolerate. This concept is vital for understanding species interactions within a community.

Fundamental vs. Realized Niche

The fundamental niche defines the theoretical limits within which a species can survive and reproduce in the absence of competition or predation. In contrast, the realized niche is the actual conditions under which the species lives when affected by biotic interactions.

• Fundamental Niche: Theoretical range of conditions

• Realized Niche: Actual conditions occupied, influenced by competitive interactions.

The Competitive Exclusion Principle

This principle states that two species competing for identical resources cannot coexist indefinitely if one is a superior competitor. Over time, one species will outcompete the other, leading to its exclusion from the environment.

Asymmetric Competition

The model of asymmetric competition illustrates how different species exhibit varying levels of fitness based on competition. This situation can lead to distinct outcomes in population dynamics and resource usage.

Competitive Exclusion and Niche Overlap

Competitive exclusion is often a result of complete niche overlap, where two species vie for identical resources. When species are closely matched in competition, one may eventually dominate.

Experimentally Testing Niche Differences

Experiments have shown the differences between fundamental and realized niches. One significant study involves barnacle species:

• Chthamalus has a fundamental niche that extends beyond its realized niche, which is constrained by competition with Balanus. Research on these species' distribution at varying tidal levels illustrated how competitive interactions shape their ecological niches.

Competition Leading to Niche Differentiation

Competition can also drive species to occupy different niches, enhancing biodiversity. This is observable in the case of Galápagos finches, where natural selection has led to distinct beak depths among species in the presence of competition. Niche differentiation allows coexistence by reducing direct resource competition.

Learning Objectives

• Describe and distinguish among various species interactions: commensalism, competition, consumption, and mutualism.

• Explain the importance of the niche concept in ecology.

• Identify the criteria leading to competitive exclusion.