1. 2.6 Ecosystem Changes

2. Snowshoe Hare and Canada Lynx
Figure 5.18: This graph represents the population cycles for the snowshoe hare and the Canadian lynx. At one time, scientists believed these curves provided evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—through predation by lynx and other predators—and bottom-up population control, in which changes in the availability of the food supply for hares help to determine their population size, which in turn helps to determine the lynx population size. (Data from D. A. MacLulich)

3. Population Growth
Population: a group the same species that live in the same place at the same time
Resources: food, water, shelter, space and mates
In theory populations can grow to an infinite size, but they are limited by resources
This causes individuals to compete for resources (remember intraspecific and interspecific competition!).

4. Factors that Control Populations
There are 4 main factors controlling population sizes:
Natality - births increase the population
Mortality - deaths decrease the population
Immigration - movement of individuals into an area increases the population
Emigration - movement of individuals out of an area decreases the population

5. Carrying Capacity
Carrying capacity: maximum number of individuals of a species that can be sustained indefinitely in a given space
No population can grow indefinitely! Resources = Limited!!

6. Limiting Factors of Populations
Factors (biotic or abiotic) which prevent population numbers from growing too large and overrunning an ecosystem.
Example:
Disease/ parasites
disasters
hunting & predation
competition for resources
(food, oxygen, nutrients)

7. Patterns of Population Growth
“J” population growth curve
Exponential growth = starts out slow and then proceeds faster and faster
Occurs when a population has few resource limitations
Sudden collapse
('diebacks')

8. Patterns of Population Growth
“S” growth curve
(S for sigmoid)
Initially shows exponential growth then levels off at the carrying capacity.
Occurs when a population has limited resources.
Results in stable population

9. J Curve vs. S Curve

10. Population Dynamics
A look at the factors that tend to increase or decrease the size of a population
The population size is determined by the interplay of biotic potential and environmental resistance.
Biotic potential- growth rate with unlimited resources
Environmental resistance - all the factors acting jointly to limit population growth

11. Biotic Potential vs. Environmental Resistance
(Growth Factors)
Environmental resistance
(Decrease Factors)
Favorable light, temperature
High reproductive rate
Adequate food supply
Ability to migrate habitats
Ability to adapt to environmental change
Too much or too little light and temperature
Low reproductive rate
Inadequate food supply
Inability to migrate habitats
Inability to adapt to environmental change

12. Biotic Potential vs. Environmental Resistance

13. Density Dependent Factors
Depend on the size of the population
Effects of the factors increase as the population grows
Act as negative feedback
Tend to be biotic
Two categories:
Internal factors = Within a single species
limited resources
reduced fertility rates
External factors = between species
populations of predators or prey
diseases spread more easily in densely-populated areas

14. Density Independent Factors
Do NOT depend on the size of the population
Tend to be abiotic
Effect the population regardless of its size
Examples:
1. Weather
2. Earthquakes
3. Floods
4. Fires
R-strategists are most
affected by these factors

15. Reproductive StrategiesK
Stable environment r
Unstable environment
Small size
Many offspring are produced
Early maturity
Short life span
Each individual reproduces once
Type III survivorship curve
Affected by density Independent
Large size
Few offspring produced
Late maturity (long parental care)
Long life span
Individuals reproduce more than once
Mostly type I survivorship curve
Affected by density dependent

16. Survivorship Curves
Type I - high survival rate of the young, live most of their expected life span and die in old age. (ex. Humans)
Type II - relatively constant death rate, could be due to hunting or diseases. (ex. coral, squirrels, honey bees and many reptiles)
Type III - have many young, most of which die very early in their life. (ex. plants, oysters and sea urchins).

17. Survivorship Curves

18. Pop Quiz
What does parental care have to do with the shape of these curves?
What density dependent factor will always affect the population of white tailed deer in NYS? (Only at certain times in the year)
What do you think the survivorship curve would be for a white-tailed deer population?