Community Interactions: Competition (-/-): Videos & Practice Problems

Competition occurs when organisms vie for limited resources, impacting their fitness negatively. This interaction can be symmetric, with no clear winner, or asymmetric, where one species outcompetes the other. The competitive exclusion principle states that two species with identical niches cannot coexist permanently; one will eventually dominate, leading to local extinction of the weaker competitor. In contrast, resource partitioning allows species with partial niche overlap to coexist by utilizing different resources, which can lead to character displacement, where species evolve distinct traits to minimize competition.

Community Interactions: Competition (-/-)

Competition is a fundamental ecological interaction where two or more organisms vie for the same limited vital resources, which can occur either directly or indirectly. The effects of competition can be categorized into two types: symmetric and asymmetric. In symmetric competition, there is no clear winner or loser, resulting in a balanced interaction. Conversely, asymmetric competition features a distinct winner and loser, leading to an imbalanced outcome.

Regardless of the type of competition, it is crucial to understand that competing for resources generally reduces the fitness of all involved organisms. This interaction is often described as a "minus-minus" scenario, meaning that both competitors experience a decline in their overall fitness compared to a situation where resources are accessed without competition. The loser of the competition loses access to the vital resource, while the winner expends energy and time to secure it, which also detracts from their fitness.

For example, consider a scenario with a squirrel trying to obtain an acorn. In the absence of competition, the squirrel can easily access the acorn, maintaining a full energy reserve. However, when competition arises, the losers struggle to obtain the resource, negatively impacting their fitness. Meanwhile, the winner, despite securing the acorn, depletes their energy reserves in the process, which is detrimental compared to the scenario without competition.

This overview highlights the essential nature of competition in ecological interactions and sets the stage for a deeper exploration of its various forms in future discussions.

Community Interactions: Competition (-/-) Example 1

In competitive situations, understanding the distinctions between symmetric and asymmetric competition, as well as direct and indirect competition, is crucial for analyzing interactions among species. Symmetric competition occurs when there is no clear winner or loser, indicating a balanced competition. In contrast, asymmetric competition features an obvious winner and loser, reflecting an imbalance in the competition.

Direct competition involves active interference between competitors, such as a physical confrontation over resources. For example, when a large bear chases away a fox from a deer carcass, it exemplifies direct competition. The bear not only confronts the fox but also ultimately consumes the deer, establishing itself as the clear winner. This scenario is categorized as direct asymmetric competition due to the evident victor.

On the other hand, indirect competition occurs when competitors do not directly confront each other but still affect one another's success, often through the depletion of shared resources. An example of this is a nocturnal mouse that consumes a food source, leaving little for a diurnal squirrel. Here, the mouse indirectly competes with the squirrel, resulting in indirect asymmetric competition since the mouse benefits at the squirrel's expense.

In situations where direct competition occurs without a clear winner, such as two lions injured in a territorial battle with no advantage gained by either, this is classified as direct symmetric competition. Both lions are engaged in a physical confrontation, yet neither emerges victorious, reflecting a balanced competition.

Lastly, when two species of birds feed on a declining insect population without direct interference, they are engaged in indirect symmetric competition. Both species experience a reduction in food availability, indicating that there is no clear winner or loser in this scenario.

In summary, recognizing the nuances of competition—whether direct or indirect, symmetric or asymmetric—provides valuable insights into ecological interactions and resource dynamics among species.

Which of the following scenarios is an example of direct, asymmetric competition?

• A. Cheetahs struggling to get enough food because a pride of lions are eating all the zebra in the territory.
• B. Two gorillas fighting for dominance, but both ending up injured and neither emerging as a clear winner.
• C. Sea turtles & crabs both eat algae in a coral reef, leaving less food available for both species.
• D. A plant secretes poison into the soil, inhibiting the growth of other plants in the same area.

Answer: (D)

Which of the following is always true regarding competition between organisms?

• A. There's a clear "winner" and a clear "loser".
• B. It decreases the fitness of all organisms involved.
• C. It can increase the fitness of one species if they "win" the competition.
• D. None of the above.

Answer: (B)

Competitive Exclusion Principle (Complete Niche Overlap)

The competitive exclusion principle is a fundamental concept in ecology that describes the outcomes when two species have completely overlapping niches. A niche encompasses the ecological role of a species, including its resource utilization, interactions with other organisms, and the environmental conditions it can tolerate. When a weaker competitor's niche overlaps entirely with that of a stronger competitor, competitive exclusion occurs, meaning that the two species cannot coexist indefinitely in the same environment.

In this scenario, the stronger competitor will outcompete the weaker one over time, leading to the local exclusion or extinction of the weaker species from that specific area or community. It is crucial to understand that this local exclusion does not imply global extinction; the weaker species may still exist in other regions where it does not face the same competitive pressures.

To illustrate this principle, consider two species of paramecium: Paramecium aurelia, the stronger competitor, and Paramecium caudatum, the weaker competitor. Both species utilize the same resources, resulting in competitive exclusion of P. caudatum. When these species are grown in isolation, they exhibit typical logistic growth curves. However, when grown together, competition negatively impacts both species, but the stronger competitor maintains a growth advantage. Over time, P. caudatum is unable to sustain its population and is driven to local extinction in the presence of P. aurelia.

This dynamic highlights the importance of niche differentiation and resource partitioning in ecological communities. Understanding the competitive exclusion principle helps explain species distribution and community structure, as it emphasizes the consequences of competition on biodiversity.

Resource Partitioning (Partial Niche Overlap)

Resource partitioning is a crucial ecological concept that describes how competing organisms can coexist by differentiating their niches. When a weaker competitor's niche partially overlaps with that of a stronger competitor, resource partitioning allows the weaker species to adapt and avoid competitive exclusion. This adaptation involves a shift from the fundamental niche, which represents the full range of conditions and resources a species could theoretically occupy without competition, to the realized niche, which is the actual space the species occupies due to the presence of competition.

For instance, consider two species of paramecium: Paramecium aurelia, the stronger competitor, and Paramecium bursaria, the weaker one. In scenarios where their niches only partially overlap, P. bursaria can adjust its niche to reduce competition. This adjustment is visually represented in graphs where the fundamental niche is depicted as a broader curve, while the realized niche is shown as a smaller, darker portion of that curve. The shift to a realized niche allows P. bursaria to coexist with P. aurelia without being driven to local extinction.

When both species are grown together, they exhibit logistic growth curves, indicating that competition negatively impacts both species. This interaction is characterized as a minus-minus relationship, where both organisms experience reduced growth. However, due to the partial niche overlap and subsequent resource partitioning, P. bursaria can thrive alongside P. aurelia, demonstrating the importance of niche differentiation in ecological communities.

Understanding these dynamics is essential for grasping how species coexist and the role of competition in shaping community structures. As we explore these concepts further, we will apply them to various ecological scenarios and problems.

Character Displacement

Character displacement is a significant evolutionary process that occurs between competing species, leading to the development of new traits that help reduce competition. This phenomenon is particularly important because competition is a negative interaction for both species involved, and minimizing it can enhance their survival and reproductive success.

A classic illustration of character displacement can be observed in the beak depth of two species of ground finches found in the Galapagos Islands: the small ground finch (Geospiza fuliginosa) and the medium ground finch (Geospiza fortis). The beak depth, which refers to the vertical height of the beak, directly influences the size of seeds that these birds can consume.

Data from three different islands reveals distinct patterns in beak depth among these finches. On the first island, only the small ground finch is present, with an average beak depth of approximately 10 millimeters. The second island hosts only the medium ground finch, averaging around 11 millimeters in beak depth. In contrast, the third island supports both species, leading to overlapping niches and subsequent competition.

To alleviate this competition, character displacement occurs on the third island. Here, the small ground finch evolves to have a smaller average beak depth of about 7 millimeters, while the medium ground finch develops a larger average beak depth of approximately 14 millimeters. This shift creates a gap in the beak depth data, effectively reducing competition for resources between the two species. The evolution of these distinct traits is a clear example of how character displacement can benefit both species by allowing them to coexist more harmoniously.

Understanding character displacement not only highlights the adaptive strategies of species in competitive environments but also underscores the importance of resource partitioning in ecological dynamics.

Community Interactions: Competition (-/-) Example 2

In this exercise, we explore key ecological concepts by matching terms with their definitions. Understanding these terms is essential for grasping the dynamics of species interactions and survival strategies in ecosystems.

The first term, fitness, refers to the ability of a species to survive and pass its genetic material onto offspring. This concept emphasizes the importance of reproductive success in the survival of species.

Next, we encounter the competitive exclusion principle, which states that two or more organisms with the same niche cannot permanently coexist. This principle highlights the competitive nature of species sharing identical resources, leading to the exclusion of one species over time.

The term realized niche describes the range of resources utilized by an organism as a result of competition. It represents the actual conditions under which a species exists, as opposed to its potential or fundamental niche.

Moving on, character displacement refers to the evolution of new traits in species to minimize competition. This adaptation often occurs as a response to resource partitioning, where species evolve distinct traits to exploit different resources.

Finally, resource partitioning is the process by which species' niches diverge from one another to avoid competition and enhance fitness. This strategy allows multiple species to coexist by utilizing different resources or habitats within the same environment.

By understanding these concepts, students can better appreciate the complexities of ecological interactions and the evolutionary strategies that species employ to thrive in their environments.

In which scenario is competitive exclusion most likely to occur?

• A. When there are multiple predators in a community.
• B. When there are 2 species with partially overlapping niches in a community.
• C. When there are 2 species with completely overlapping niches in a community.
• D. Both b & c.

Answer: (C)

The total range of resources that an organism can use & the conditions it can tolerate is its _____ niche, while the actual range of resources it uses & conditions it lives in is its _____ niche.

• A. Fundamental; realized.
• B. Realized; fundamental.
• C. Competitive; realized.
• D. Fundamental; competitive.

Answer: (A)

What could an ecologist do to find out if a species is occupying its fundamental or its realized niche?

• A. Remove a species it consumes & measure the effect.
• B. Observe how its use of resources changes upon the introduction of a competitive species.
• C. Take half the population out the community and observe the species' use of resources.
• D. Observe changes in the species' range of resources used after removal of a competitor.

Answer: (D)

Two barnacle species, Chthalamus stellatus & Balanus balanoides, can be found in communities on rocks along the coast. C. stellatus live higher up on the rocks than B. balanoides, who live closer to the surface of the water. In an experiment, all the B. balanoides are removed from the rock and over time the C. stellatus colonized the entire surface of the rock, including the area where only B. balanoides previously lived. In another area, all the C. stellatus were removed but the habitat of B. balanoides did not change. What does this suggest about the relationship between the two species?

• A. C. stellatus is a stronger competitor and forces B. balanoides into a realized niche.
• B. B. balanoides is a stronger competitor and forces C. stellatus into a realized niche.
• C. Both species have undergone resource partitioning to avoid competition.
• D. Both species have undergone character displacement to avoid competition.

Answer: (B)

In which of the following scenarios is character displacement most likely to occur?

• A. Two allopatric species that have complete niche overlap.
• B. Two allopatric species that have partial niche overlap.
• C. Two sympatric species that have partial niche overlap.
• D. Two sympatric species that have complete niche overlap.
• E. None of the above.

Answer: (C)

Here’s what students ask on this topic:

What is the competitive exclusion principle in ecology?

The competitive exclusion principle in ecology states that two species competing for the same limited resources cannot coexist at constant population values. When two species have completely overlapping niches, one will outcompete the other, leading to the local extinction or exclusion of the weaker competitor. This principle highlights that identical niches cannot be permanently shared by two species in the same habitat, as one will always have a competitive advantage, eventually driving the other to local extinction.

How does resource partitioning help species coexist?

Resource partitioning helps species coexist by allowing them to utilize different resources or occupy different niches within the same habitat. When species have only partial niche overlap, they can avoid direct competition by differentiating their resource use. This shift from their fundamental niche to a realized niche reduces competition and allows both species to survive and coexist. Over time, this can lead to character displacement, where species evolve distinct traits to further minimize competition.

What is the difference between symmetric and asymmetric competition?

Symmetric competition occurs when competing organisms are equally affected by the competition, with no clear winner or loser. Both species experience a similar reduction in fitness. In contrast, asymmetric competition happens when one species has a competitive advantage over the other, leading to a clear winner and a clear loser. The dominant species outcompetes the weaker one, which may result in the local exclusion or extinction of the weaker competitor.

What is character displacement and how does it relate to competition?

Character displacement is the evolutionary process where competing species develop distinct traits to reduce competition. This occurs as a long-term outcome of resource partitioning, where species with partially overlapping niches evolve differences in their characteristics, such as beak size in finches, to minimize competition for resources. By evolving these distinct traits, species can coexist more effectively, as the reduced competition is beneficial for both species involved.

What happens when there is complete niche overlap between two species?

When there is complete niche overlap between two species, they compete for the exact same set of resources. According to the competitive exclusion principle, this situation cannot persist indefinitely. One species will eventually outcompete the other, leading to the local extinction or exclusion of the weaker competitor. This means that the weaker species will be driven out of the area or community where the competition occurs, as it cannot coexist with the stronger competitor.