Biology 301 Disturbance and Experimental Analysis
1. Without release from resource competition, one often sees zonation patterns in habitats such as the rocky intertidal where within a given zone one species wins in competition and excludes other species
2. Release from resource competition can occur by any process that reduces the ability of the dominant competitor to occupy all of the resource. Such processes may be predation or a physical disturbance such as log damage. Disturbance is defined as any process that increases the availability of a limiting resource.
3. If the process that reduces the ability of the dominant competitor to occupy all of the resource acts in spatially distinct areas (e.g. not uniformly but rather in patches within the distribution zone of the dominant competitor), then organism distributions will be in patches within that zone. This is typical of many habitats including some zones of the rocky intertidal.
B. Predation/Disturbance and the number of species (species richness)
1. Predators which prey preferentially on the dominant competitor can affect the patterns of species abundance in an area.
2. In the absence of a process such as predation to prevent competitive exclusion and with a species capable of excluding all of its competitors, there will be only one species present, the dominant competitor.
3. If the predator eats the dominant competitor in sufficient numbers to prevent competitive exclusion of other species, the number of species present will increase, unless the intensity of predation is so great that very few species can exist. This effect is called the Intermediate Disturbance Hypothesis and has been demonstrated in a number of systems including the rocky intertidal and terrestrial plant communities.
C. Experimental analysis
1. Competition: changes in the resource base result in changes in population dynamics
a. Remove/add competitors thus altering resource indirectly
b. Remove/add resource directly
2. Predation: changes in predator access either temporally or spatially result in changes in prey abundance
a. Cycles of population abundance with the predator population abundance lagging behind that of the prey are common
b. Remove the predator and look for increase in competition or some other limiting factor
c. In some cases but not all, predation may prevent competitive exclusion from going to completion
3. Physical disturbance
a. To give release from competition, resource availability must be affected
b. Examples: minor landslides (soil not dramatically eroded), logs plus wave force in the rocky intertidal
4. Physical stress
a. Manipulative reduction should yield changes in measures of individual success
b. Examples: desiccation, manganese concentration