WK7 - INTERACTIONS: Competition: Part 3: Interspecific Competition

Interspecific competition is competition between different species.
It relates to topics in community dynamics and explains control of species richness.
Tom Shona developed mechanisms of interspecific competition.

Consumptive/Exploitative: Using up resources, such as food, making it unavailable to others.
Preemptive: Dominating space and preventing others from using it; example: barnacles dominating space in the intertidal zone.
Overgrowth: Blocking other species' access to light or resources; example: large trees outshading smaller shrubs.

Chemical Competition: Plants using allelopathy to release chemicals that inhibit other plant species.
Territorial Competition: Defending territories to ward off other individuals.
Encounter Interactions: Short-term interactions for space, incidental fights to occupy a location.
Consumptive competition is the most dominant type of interspecific competition.

The species richness is lower locally than regionally.
The question is: why aren't all regional species found locally?
Each local patch is theoretically habitable by more species than are actually present.
The central question is whether interspecific competition limits local species numbers.
If the number of species in a local area is just a function of area, then as area increases, the number of species increases by the same amount.

Competition theory suggests limiting similarity: species that are too similar cannot coexist.
Checkerboard patterns: Exclusive presence of one species on one island, and another related species on another, suggesting interspecific exclusion.
Depressed population density: Species only found in parts of a region might be vulnerable to disturbance or inbreeding depression.

If two competing species coexist in a stable environment, it is due to niche differentiation.
In unstable environments, species may respond differently to environmental shifts, allowing coexistence.
Niche: Environmental factors influencing growth, survival, and reproduction.
Fundamental vs. Realized Niche: If there's no differentiation, one species will exclude the other.
Two species with identical niches cannot coexist indefinitely.

Circumstantial Evidence: Observations without environmental control (least confident).
Laboratory Experiments & Mathematical Models: Controlled conditions.
Field Experiments: Manipulations in natural settings (most confident but difficult to control).

Example: McKinley's and bar-tailed cuckoo-doves on islands near Papua New Guinea.
One or the other species or neither is found on each little island. So we never find both species in the same place. And this is circumstantial evidence that they do not coexist together.
Only one species or neither is present on each island, suggesting competitive exclusion.
Alternate explanations are possible, such as microclimatic differences or specific habitat needs.

Example: Darwin's finches in the Galapagos Islands.
Beak size differs when species are found alone versus together, which prevents competition for resources with each other.
Geospiza fortis increases while Geospiza fuligenosa reduces
Beak size influences feeding habits; differentiation prevents competition for resources.
Character displacement is a shift in traits to reduce competition.
$character<em>{speciesA} \neq character</em>{speciesB}$ when species are separate
$character<em>{speciesA} \rightarrow$ with distinction $\leftarrow character</em>{speciesB}$ when species are together
Competitive relationships depend on context (abiotic/biotic factors).

Controlled manipulation of factors.
Example: Beetle species competition changes with temperature.
Calandre species outcompetes the rhizopurpha at $29^{\circ}C$ .
Rhizopurtha outcompetes the calandre at a slightly higher temperature.
Example: Yeast growth over time, grown alone versus together.
When species are grown alone, these yeast grow to a certain size and number, and they reach their carrying capacity and they flatten out.
Saccharomyces and Schizosaccharomyces reach different carrying capacities alone versus together, indicating competition. In a mixed population, the Schizosaccharomyces reaches a much lower carrying capacity than it does alone, and that's due to being outcompeted by the saccharomyces species.

Rodent species in the Arizona desert.
Species include kangaroo rat (largest, eats seeds), pocket mouse (eats seeds and vegetation), and grasshopper mouse (insectivore).
Experiment: Removal of kangaroo rats and observation of other species.
Small gramophores (seed eaters) increased when kangaroo rats were removed.
Insectivores were unaffected, reflecting a lack of niche overlap.
Permanent research plots allowed baseline data collection before removals.
$Kangaroo Rat \downarrow \rightarrow Small : Granivores \uparrow$
$Kangaroo Rat \downarrow \rightarrow Insectivores : No : Change$

Competition reduces local species richness compared to regional richness through competitive exclusion.
Next lecture: Exploitative interactions in the form of predation.