Interspecific Competition

Interspecific Competition

  • Interspecific competition is competition between different species, influencing community dynamics and species richness.

    • dominant theme in ecology theory for many years

    • community dynamics

    • explanation of control of ‘local’ richness

      • What limits number of species in community?

      • Why aren’t there as many species locally as at a larger scale?

Mechanisms of Interspecific Competition (Shoener's Theory)

  • Tom Shoener developed mechanisms to explain how species compete.

  • These mechanisms can be categorised into:

    • Resource/scramble competition

    • Interference/contest competition

  • Consumptive competition is considered the most dominant type of interspecific competition.

Resource/scramble Competition

  • Consumptive/Exploitative Competition

    • Using up resources like food, limiting access for others.

  • Pre-emptive Competition

    • Dominating space and preventing others from using it in anticipation of limitations

    • Example: Barnacles dominating intertidal zones.

  • Overgrowth

    • Blocking access to light or resources from othger species.

    • Example: Trees outshading shrubs and prevent them from coming out — further prevents competition in the future to ensure the understory remains clear for their seedlings in the next gen

Interference Competition

  • Chemical Competition

    • (Allelopathy)

      • Plants releasing chemicals into the soil they themselves can tolerate but other species can not - that inhibit other species.

      • Effectively removing competition from space

  • Territorial Competition

    • Defending territories through fights and displays.

    • ward off other individuals encroaching on their space

  • Encounter Interactions

    • Short-term interactions for space

    • Often incidental interactions - organisms bump into each other and fight to occupy that location in the short term

Local vs. Regional Species Richness - The important questions

  • The question is, why are there fewer species locally compared to a broader regional scale?

  • Consider a local habitat patch with a species richness of four bird species versus a regional area with a greater species richness of seventeen bird species.

  • Each local patch is theoretically habitable by all species in the region.

  • This raises two important questions:

  1. Is the number of species in the ‘local’ area a fixed fraction of the species that could occur there (regional pool)?

  2. So, is the interspecific competition limiting the number of species in local communities?

Area and Species Scaling

  • If species number scaled linearly with area, a region four times larger than a local area with five species would have 20 species.

  • In reality, species numbers level off as the region gets larger.

Competition Theory

  • Limiting similarity: Species that are too similar cannot coexist for long due to direct competition for resources/space

    • One species will eventually outcompete the other

    • Gause’s competitive exclusion principle

  • Chequerboard patterns: Exclusive presence of species in archipelagos suggests out-competition.

    • one species has exclusive space on one island while another species is exclusively found on another island —> they are never found together

    • suggests they are outcompeting each other on their respective islands

  • Character divergence and displacement: need to have traits that are diverging from each other —> otherwise displacement happens

  • Depressed population density: Species found only in small areas of a whole region may lack resilience and perhaps suffer from inbreeding depression as well.

Competitive Exclusion Principle (Gause's Principle) and limiting similarity

  • Two competing species can coexist in a stable environment if they have niche differentiation.

  • In an unstable environment, species may coexist if they respond differently to environmental shifts

    • e.g., temperature changes - one species does better in the cold and one does better in warm conditions

    • allows them to co-exist due to this fluctuation

  • Niche: Environmental factors influencing growth, survival, and reproduction of a species OR all the factors necessary for its existence.

  • No differentiation or habitat constraints lead to the exclusion of one species, one species will inevitably be eliminated.

  • Two species with identical niches cannot coexist indefinitely.

Evidence from least to most strength in inference (confidence)

  • Circumstantial evidence: Observations without environmental control which are inconclusive.

  • Laboratory experiments and math models: Controlled conditions and mathematical models.

  • Field experiments: Manipulations in natural environments are the most reliable but are difficult to control.

Circumstantial evidence: Chequerboard Patterns: Cuckoo Doves

  • The McKinley's cuckoo dove and bar tailed cuckoo dove demonstrate a chequerboard pattern in Papua New Guinea islands.

  • Only one species or neither is found on each island, suggesting competitive exclusion.

    • Both species are never found in the same place

  • Suggests that if there was a dispersal from one island to another, one species would outcompete the other (unable to mix)

  • This is circumstantial evidence, therefore there may be alternative explanations that we could not identify from observations alone, such as microclimatic factors.

    • If you continue to see this pattern over and over again across different species, you may feel more confident in this circumstantial evidence

Character Displacement: Darwin's Finches

  • In the Galapagos Islands, finch beak sizes vary depending on whether species are found alone or together.

  • Example: Geospiza fortis and Geospiza fuligenosa.

  • In isolation beak size is different that when species are found together:

    • G. fortis beak depth ranges from 9 to 12.

    • G. fuligenosa beak depth ranges from 7 to 10.

  • When together on Santa Cruz, beak depths diverge:

    • The beak depth of G fortis increases.

    • The beak depth of G fulugenosa reduces in the other direction.

  • Beak size and shape influences diet, and differentiation in beak depth reduces competition for resources as they are no longer in direct competition because they now eat different sized food.

    • If birds kept the same beak size (overlapping), when present together there would be competition amongst birds in each species

  • EVIDENCE that interspecific competition is occurring between different species —> hence character displacement involves traits diverging to reduce competition.

Context-Dependent Competition

  • Different abiotic or biotic backgrounds can switch competitive advantage

  • Which species wins may be changed by subtle differences in:

    • Climatic conditions

    • Availability of refuges (habitat heterogeneity)

    • Community composition

Competition Experiments in the Lab

  • Controlled lab experiments can manipulate factors easily —> hence you can be more sure of the results

  • Example: Beetle species competition under different temperatures.

    • Researchers controlled for humidity/moisture

    • Measured which beetle species did better under each temperature

    • Calandra species outcompetes Rhizoperpha at 29 degrees.

    • Rhizopurtha outcompetes Calandra at slightly higher temperature of 32.3 degrees.

    • A small change in temp can have a big impact of competition between species and competitive ability

  • Yeast growth experiments demonstrate differences when species are grown alone versus together.

    • Species 1: Saccharomyces

    • Species 2: Schizosaccharomyces

  •  When grown alone, they grow until their carrying capacity and then flatten out

  • A mixed population results in a lower carrying capacity for each species compared to when grown alone due to being outcompeted by the other species.

Field Study: Rodents in Arizona

  • Species include:

    • Kangaroo rat (largest, eats seeds - large granivore).

    • Pocket mouse (eats seeds and vegetation - small granivors).

    • Grasshopper mouse (insectivore).

  • Researchers removed kangaroo rats and observed the response of other species.

  • Removal plots with exclusion barriers were used and compared to control plots.

  • Results showed that when kangaroo rats were removed, small granivorous (seed eaters) increased, indicating kangaroo rats were competing with the small granivorous rodents for food. —> overlap in niche between large and small granivores

  • Insectivorous rodents showed no change, as they do not compete for the same resources. -—> no overlap in their niche and the niche of small granivores

  • Repeated experiments in permanent research plots confirmed these effects, showing similar species abundance in control and removal plots before removal, but significant differences after removal.

(9=3)(\sqrt{9} = 3)

Conclusion

  • Competition between species is an important factor determining why local species richness is lower than regional richness.

  • Competitive exclusion reduces the number of species in a local environment.