Competition: Chapter 13

Objectives

Can design a lab or field experiment to evaluate effects of competition.

Can describe how intra-specific competition regulates population size.

Can describe conditions under which competing species co-exist.

Can describe how competition acts as an natural selection agent.

Definitions

Competition: mutually detrimental interaction among organisms when they share a common resource base that is insufficient to meet the needs of both.

Intra-specific: Competing organisms of same species

Inter-specific: Competing organisms of different species

Types of Competition

Exploitation: One organism limits other organisms access to limited resource by consuming the resource.

Interference: One organism limits other organisms access to limited resource by aggressive interactions.

Studies of Intra-Specific Competition

Experimental Design

Treatment Variable: Organisms cultured at different densities.

Response Variables: Organism survival, growth rate, body size, reproduction.

Random assignment of organisms to different densities.

Replicated groups of organisms at each density.

Control all other environmental factors to be the same for all density treatments.

Compare response variable(s) among different densites.

Fig 13.3 Population density, soil nitrogen, and grass size

Fig 13.5 intraspecific competition in alfalfa

Intraspecific competition among planthoppers: Denno and Roderick (1992)

Experiment:

Lab experiments of intraspecific competition:

Response Variable: Population size
Treatment Variable: Presence / Absence of competing species
Random assignment of cultures to treatments.
Replication of cultures within each treatment.
Control to keep all other factors the same.
Compare population size of species between cultures with / without competing species.

Gause (1934):Demonstrated resource limitation with Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of Bacillus pyocyaneus

Gause’s Paramecium Experiments
(Population Growth Without Competing Species)

Paramecium Populations With Competing Species

Why the Difference In Outcomes ?

Both P. aurelia and P. caudatum forage for yeast cells suspended in the water.

Competitive Exclusion

P. bursaria forages for yeast cells on the bottom of the culture dish.

Co-existence by Resource Partitioning

Fig 13.16: Competition Between Tribolium confusum and T. castaneum:

Fig 13.17:Outcome of Competition b/t Tribolium confusum and T. castaneum depends on Environmental Conditions

Field Experiments for Inter-specific Competition

Experimental Design

Response: Population size of species.
Treatment: Remove one or the other of two species from plots where both occur. Have “control” plots where two species remain.
Random assignment of treatment to plots.
Replication: Multiple plots for each treatment.
Control: Select plots so that all other factors are as similar as possible.
Compare population size of species between plots with / without competing species.

Competition Between Seed-eating Rodents and Ants: Table

Removal experiment with granivorous rodents: Fig 13.24

Connell barnacle experiments: Fig 13.20

Factors That Influence the Distribution of Barnacles: Fig. 13.21

Niche

The combination of environmental conditions, essential resources, and biotic interactions (e.g., competition, predation) that define where, when, and how a species makes its living.

Fundamental niche: All combinations of environment and resources where a species could live in the absence of other species.

Realized niche: All combinations of environment and resources where a species actually does live, as constrained by interactions with other species.

How Do Competing Species Co-Exist?

Intra-specific competition limits population size of each species, leaving resources to sustain the population of the other species.

Environmental variation allows each species to be superior competitor in some parts of the shared habitat.

How Do Competing Species Co-Exist?

Predators preferentially attack whichever species is most abundant (usually the competitively superior species).

Niche Differentiation: Competing species evolve to minimize overlap in their resource utilization.

Character displacement (morphological): Fig 13.8

Intraspecific variation in resource utilization: Fig 13.9

Fig 13.10 Variation in Resource (Seed) Availability

Selection for Larger Body/Beak Size During Drought: Fig 13.11

Character Displacement: Fig 13.26
Darwin’s Finches

Natural Selection for Character Displacement

Resource Partitioning by Warblers

In the Absence of Competing Species Resource Utilization is Expanded

Summary: Competition is Everywhere

Intra-specific competition regulates population size of many species.

Inter-specific competition plays a major role in determining where species live and what resources they consume.

Co-Evolution of competing species tends to minimize competition, allowing for co-existence.