Competition in Ecology: Resources, Niches, and Interactions
Introduction to Competition
This chapter will not involve calculations, focusing instead on conceptual understanding of competition.
Defining a Resource
A resource is anything that is consumed or used by an organism.
Its availability directly impacts population growth:
Increased availability of a resource leads to an increased growth rate of the population.
Decreased availability (scarcity) leads to a decreased population growth rate.
Example: Sunlight is a resource for plants; they consume it, cause shade, and compete for it.
Non-Example: Temperature is generally not considered a resource because it cannot be consumed or monopolized, although it is crucial for many industries and organisms.
Interspecific Competition
Interspecific competition refers to competition that occurs between different species.
Competition always involves a struggle for resources.
The specific mechanisms of competition can vary significantly, sometimes being amplified or mitigated by other factors.
Experimental Design for Competition Studies
A common approach to study competition involves:
Observing populations of species grown together in a contained area with a certain amount of resources.
Observing populations of each species grown individually in separate contained areas with similar resource availability.
Comparing the growth and development of populations under these different conditions to understand their interactions.
Case Study: Bedstraw Species and Soil Preference
Species: Heath Bedstraw and White Bedstraw.
Observation: These plants were observed in different plots, with plant counts recorded:
In an acidic plot: Heath Bedstraw had 6 plants, while White Bedstraw had only 3 plants.
In an alkaline plot: White Bedstraw had 7 plants, while Heath Bedstraw had 3 plants.
Initial Conclusion: The species exhibit different preferences: Heath Bedstraw prefers acidic soil, and White Bedstraw prefers alkaline soil.
Competition Scenario: When grown together, the species compete, and the outcome depends on the soil conditions. This demonstrates that their preferences lead to different competitive advantages.
Case Study: Rodents and Ants (Seed Competition)
Context: A study conducted in the wild on interacting populations of rodents and ants.
Resource: Both rodents and ants consume seeds, making seeds a shared resource.
Experimental Design: Field plots were manipulated to either remove rodents or remove ants.
Results:
In plots where rodents were removed: The number of ant colonies increased significantly, and there were many more seeds left at the end of the experiment compared to control plots.
In plots where ants were removed: The individual mice (rodents) had a significantly larger average mass compared to control plots.
Conclusion: Both observations are consistent with brutal competition, indicating that each species has a reciprocal negative effect on the other by competing for the shared seed resource.
Case Study: Barnacle Competition in the Intertidal Zone
Organisms: Two sessile (non-moving) barnacle species, referred to as 'B' a species (Balanus) and 'C' a species (Chthamalus). These are crustaceans that settle and glue themselves to surfaces after a free-swimming larval stage, filtering particles from water.
Habitat: They live in the intertidal zone, the area between the highest and lowest tides, which experiences varying levels of water exposure and desiccation (drying out).
Upper intertidal: Organisms here are exposed to air more often and for longer durations.
Lower intertidal: Organisms here are covered by water more often.
Distribution: 'C' species (Chthamalus) is typically found in the upper intertidal, while 'B' species (Balanus) is typically found in the lower intertidal.
Experiment 1 (Toleration Test):
Researchers moved 'B' species (Balanus) to the upper intertidal zone (where 'C' species usually lives).
Result: 'B' species could not establish itself in this environment.
Conclusion: 'B' species cannot withstand the high levels of desiccation (drying out) in the upper intertidal area. 'C' species, however, tolerates more desiccation.
Experiment 2 (Competition Test):
Researchers facilitated '{C}' species colonizing the lower intertidal zone (where 'B' species usually lives) by removing 'B' species.
Result: While 'C' species could colonize the bottom if '{B}' species was absent, it was outcompeted when '{B}' species was present.
Conclusion: 'B' species is a superior competitor in the lower intertidal zone.
Fundamental vs. Realized Niche
This barnacle study is a classic example illustrating the difference between fundamental and realized niches.
Fundamental Niche: Describes the full range of environmental conditions (e.g., temperature, salinity, desiccation tolerance) and resources a species could potentially inhabit and use in the absence of biotic interactions (like competition or predation).
For the 'C' species (Chthamalus), its fundamental niche includes both the upper and lower intertidal zones (as it can survive in the lower zone if 'B' species is removed).
For the 'B' species (Balanus), its fundamental niche only includes the lower intertidal zone, as it physically cannot survive the desiccation in the upper zone.
Realized Niche: Describes the actual limited set of environmental conditions and resources a species occupies due to biotic interactions, primarily competition with other species for resources or presence of predators.
For the 'C' species (Chthamalus), its realized niche is restricted to the upper intertidal zone because it is outcompeted by the 'B' species (Balanus) in the lower intertidal zone.
Other Factors Influencing Competition Outcomes
Predation/Herbivory: Interactions with consumers can alter competitive dynamics.
Tadpole Example: In ponds, the survivorship of different tadpole species to adult stage can be affected by the presence of predatory newts.
Without predators, tadpole species (e.g., Spadefoot toads) can have high survivorship (e.g., above 90\%).
With predators (e.g., 2, 4, or 8 newts), survivorship generally decreases and competitive interactions among tadpoles change. Spring keeper tadpoles, for example, might do relatively better in the presence of predators, perhaps due to different defense mechanisms or predator preferences.
Goldenrod Example: Goldenrod plants are strong competitors. However, if herbivorous beetles consume goldenrod, it reduces goldenrod's competitive edge, allowing other plant species in the field to thrive better than they would otherwise.
Disturbance: Events like fire can significantly influence plant community structure and competitive outcomes.
Longleaf Pine Forests: These forests in the Southeastern US historically relied on frequent, low-intensity fires (often with grassy understories).
Human Impact: Fire suppression during the 20^{th} century allowed other plant species, not adapted to fire, to increase, outcompeting the fire-adapted Longleaf pine and its associated understory plants.
Apparent Competition
Definition: Apparent competition is an indirect interaction between two species that appears to be competition but is, in fact, mediated by a shared predator, parasite, or disease, rather than direct competition for a shared resource.
Key Distinction: It is not true competition.
Example: If an increase in the giraffe population leads to a greater asymptomatic spread of a virus that causes a decline in the zebra population, it might appear that giraffes are