Population Density’s Effect on Population Growth

1. Population Density’s Effect on Population Growth

2. Our Focus: • What environmental factors keep populations from growing indefinitely? • Why are some populations stable, and others aren’t? • Why it’s Important: • We need to understand how environmental factors affect the breeding and eating habits of populations of organisms. If we understand such regulation, we can help control and monitor the stability of the organisms that share our planet.

3. Population Density • What is it? • In simple terms, it is how concentrated a certain area is with one/several species. • When immigration and emigration offset each other, population grows when birthrate exceeds death rate. It will thus decline when death rate exceeds birth rate

4. Density Independent Growth • Density Independent: The birth/death rate does not change with population density. It is unaffected by environmental factors that come with density. • Example: Study by Andrew Watkinson and John Harper (University of Wales) found that mortality of dune fescue grass is dependent on the physical factors of the environment, not necessarily on the density.

5. Density Dependent Growth • Density Dependent: The birth/death rates are dependent on the population density. These factors are typically inversely proportional. • Example, cont: Watson and Harper found that reproduction in the dune fescue declines as population density increases. This is because when the population density is high, water and nutrients are more scarce due to being used up by the high number of individuals

6. How are those results possible for the same study? • Watkinson and Harper’s conclusion regarding dune fescue is that their death rate is density independent because it is purely due to physical factors that are free of other organisms, while their birth rate is density dependent because availability of certain resources is imperative for reproduction, but not necessarily for sustaining the population

7. Density Dependent Population Regulation • Negative feedback is vital when controlling population growth. If there wasn’t negative feedback between population density and birth/death rates, then the population wouldn’t be able to temper growth. • Density dependent regulation provides the necessary feedback, then, to help reduce birth rates and increase death rates.

8. Examples of Density Dependent Population Regulation • Competition for Resources: In a high density population, competition for declining nutrients and resources increase, which results in a lower birth rate. This is population dependent because the availability of these resources depends on the density of the population.

9. Territoriality: When population density is high, there is less space for vertebrates and some invertebrates to have their own territory. Without animals having their own territory, the chances of being able to take in enough food to reproduce decreases, thus decreasing the population. This is density dependent. Science.

10. Disease: When the population density is high, diseases are spread easier, thus effecting the survival of organisms. The impact of disease is very dependent on how dense a population is. • Predation: If the population density is high, then there is more prey for predators of this population to feed on, thus leading to a higher consumption of prey (the population itself. This decreases the population for obvious reasons.

11. Toxic Wastes: When the population density is high, metabolic by-products accumulate. Thus poisoning the organisms and decreasing the population • Intrinsic Factors: Some factors are physiological rather than environmental. When a population of animals in an area is high, reproduction will increase, but at a certain point, there is a drop in reproduction due to aggressive interactions and hormones. • Stress in organisms among high density populations can cause hormonal changes and decrease birth rates

12. Population Dynamics • Population Dynamics: Complex interactions between biotic and abiotic factors that cause fluctuations in population density. • Things such as weather can cause these fluctuations. • “Nice” weather=lots of yummy food=more babies • “Bad” weather=low food=fewer babies

13. Some populations of animals have cycles of increases and decreases • Example: Lynx and snowshoe hares go through a 10 year cycle • Immigration/Emigration/Metapopulation • Metapopulation is a number of local population that are linked. When patches of population immigrate or emigrate, it will naturally affect the population density.