5.2- LImits to Growth

Vocabulary:
Limiting factor- a factor that controls the growth of population
Density-dependent limiting factor- operate strongly when population reaches a certain level
Density-independent limiting factors- affect all populations in similar ways , regardless of population size and density


Key Points:
1. Acting separately or together, limiting factors determine the carrying capacity of an environment for a species
2.Density-dependent limiting factors include competition, predation, herbivory, parasitism, disease, and stress from overcrowding.
3. Unusual weather such as huricanes, droughts, or floods, and natural disasters such as wildfires, can act as density-independent limiting factors.


Limiting Factors
-The productivity of an ecosystem can be controlled by a limiting nutrient.
-Limiting factors keep most natural populations between extinction and overrunning the planet.


Density-Dependent Limiting Factors
-Density-dependent limiting factors do not affect small, scattered populations as much

Competition
-Competition is an example of a density-dependent limiting factor, because the higher the population density, the sooner the resources are used up.
-Competition for food can lower birthrates, raise death rates, or both.

Predation
-Another example of a density-dependent limiting factor is predation
-Population of predators and prey cycle up and down over time.
-Herbivory can contribute to population changes
-Herbivores are predators to plants, so the population of herbivores and plants raise and lower, as well.
-Sometimes, human activity limits populations
Parasitism and Disease
-Parasites and disease-causing organisms feed at the expense of their hosts
-This is another example of density-dependent limiting factors

Stress From Overcrowding
-If populations are too crowded, species can sometimes fight amongst themselves


Density-Independent Limiting Factors
-Populations can "crash", or drop

True Density Independence
-It is difficult to say that a limiting factor acts only in a density-independent way
-Human activities can also place ecological communities under stress