1. Population Growth and Interactions
Chapter 20
Population Growth and Interactions

2. Chapter Outcomes
Describe and apply models that represent population density and distribution of individuals within populations
Describe the four main processes that result in population change and explain how these processes are related
Analyze population data to determine growth rate and per capita growth rate

3. Chapter Outcomes
Describe how a population’s biotic potential and the carrying capacity of its habitat determine its pattern of growth
Compare r-selected and K-selected reproductive strategies in terms of life cycles and patterns of population growth

4. Chapter Outcomes
Describe the interactions among population members and among members of different populations within a community
Explain how producer-consumer interactions affect population growth
Describe defense mechanisms that have evolved within populations

5. Chapter Outcomes
Understand that symbiosis includes mutual, commensal, and parasitic relationships
Distinguish between primary and secondary succession

6. 20.1 – Population Growth
Quantitative measurements of populations are like snapshots of moments in time
Ecologists often rely on a number of measurements over a long period of time to make inferences about population growth
Both the distribution and growth of a population can be significant when studying populations and communities

7. Population Distributions
habitat can play a role in how populations are distributed
population distributions can follow one of three general patterns:

8. Patterns of Distribution
Clumped

9. Patterns of Distribution
Random

10. Patterns of Distribution
Uniform

11. Population Size and Density
population size simply describes the number of organisms in an area
it is often more useful to compare populations by describing population density (the number of individuals in a given area or volume – for aquatic species)

12. Population Density Formula
D = N / A where:
D = density of organisms (organisms/unit)
N = # of organisms
A = size of area in units

13. Example:
Ex: There are 200 lemmings in a 25 ha area. Determine the population density of the lemmings:

14. Population Change 4 factors determine population size: Natality –
Mortality –
Immigration –
Emigration –

15. population change can be calculated in the formula:
if all the factors remain the same except for an increase in the birth rate, population increases
population change can be calculated in the formula:
∆N = (births + immigration) – (deaths + emigration)
We can also calculate a per capita population growth rate
cgr = (births + immigration) – (deaths + emigration)
Initial number of organisms

16. Population Growth Example
Ex: In a Banks Island Breeding site, 40 cranes were born, and there were 55 deaths. There was no immigration or emigration of cranes. The original population was Calculate the population growth.

17. Biotic Potential biotic potential depends on a number of factors:
Offspring – the maximum number of offspring per birth
Capacity for Survival – the chances of offspring reaching reproductive age
Procreation – the number of times per year an organism reproduces
Maturity – the age at which reproduction begins

18. Carrying Capacity
Generally, growth in small populations begins slowly and then the rate of growth increases
However, the growth must eventually slow because there is a maximum number of organisms that an ecosystem can support

19. Growth Phases
Lag
Log (or Exponential Growth)
Stationary
Death

20. S-Curves Many populations exhibit an S-shaped (sigmoidal) growth curve
This is also known as a logistic growth pattern
The population number increases until it reaches the carrying capacity of the ecosystem
At this point, the population fluctuates near the carrying capacity

21. J-Curves
J – shaped curves are representative of quick growth and then a sharp decline in the population
this occurs when a population quickly outgrows the carrying capacity of an ecosystem
as a result, there is a crash in the population, which is followed by a relatively stable stationary phase
these J-curves are most often associated with organisms that can reproduce very quickly (insects, bacteria, etc.)

22. Comparison of J & S-Curves

23. Limiting Factors in Populations
if there are a number of substances required for growth, then the one with the least concentration will be a limiting factor for growth (law of the minimum)
as well, in some cases, too much of a substance can harm an organism
therefore, the greater an organisms’ range of tolerance for high and low concentrations of nutrients, the greater its survival ability (Shelford’s law of tolerance)

24. the overall optimum ranges for abiotic factors for each species is different because each species reacts to each factor differently
any abiotic factors that are not affected by population density are density independent
such factors include temperature & climate
factors that are dependent on the population density are density dependent
these are factors such as limits to food supply, disease, and predation
often problems involving density-dependent factors are alleviated when a population density returns to lower levels

25. r and K Selected Populations
K-selected populations are:

26. r-selected populations are:

27. 20.2 – Interactions in Ecological Communities
An ecological community is a collection of interacting populations within an area
In any community, individuals must compete for limited resources
The competition between populations is the driving force behind population dynamics

28. Intraspecies and Interspecies Competition
Gause’s Principle states that if two populations occupy the same niche, one of them will be eliminated
this principle would represent a worst-case scenario in interspecies competition (the competition between two different species)
there also exists intraspecies competition, where members of the same species compete for resources such as food, space and mates

29. Predation
predator and prey populations are often closely tied to one another (for instance, if a prey population declines, it is likely that the predator species will as well)
however, predators are important in ecosystems as they reduce the number of primary consumers that are feeding on producers

30. Populations of Lynx & Hares

31. Predator-Prey Cycles
Predator-prey cycles often depend on a number of factors:

32. in some cases, animals develop camouflage to escape detection (either by predators or prey)
other organisms produce physiological adaptations (such as plant toxins)
some animals will engage in mimicry, where they will develop markings similar to those on a poisonous or dangerous animal
often predators and prey coevolve in an attempt to gain an upper hand

33. Symbiosis There are 3 types of symbiosis: Commensalism Parasitism
Mutualism

34. Succession
succession is the slow, orderly replacement of one community by another through the development of vegetation
climax communities are eventually formed through this process
a climax community is a stable, mature community
primary succession occurs where there previously was no community (on places such as barren volcanic islands)
secondary succession occurs after the partial or complete destruction of a community

35. Steps in Primary Succession
Bare land is formed.
Pioneer species, such as mosses and grasses that are relatively hardy move in and decrease soil temperature and evaporation, while increasing soil fertility.
Small shrubs that tolerate full sunlight move in, stabilizing and enriching the soil.
Small, fast-growing trees replace the shrubs and deplete the soil of nutrients and sunlight.
A climax community forms, produced from shade-tolerant trees which have a high sapling survival rate.

36. Generalizations Regarding Succession
Species composition changes more rapidly during the earlier stages of succession.
The total number of species increases dramatically during the early stages of succession, levels off during the intermediary phases, and declines as the climax community is established.
Food webs become more complex and the relationships more clearly defined as succession proceeds.
Both total biomass and nonliving organic matter increase during succession and begin to level off during the establishment of the climax community.