2.6.1-.4, 2.1.7 Population Dynamics

2.6 Ecosystem Changes

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 controlthrough predation by lynx and other predatorsand 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)

2Population: a group the same species that live in the same place at the same time Resources: food, water, shelter, space and matesIn theory populations can grow to an infinite size, but they are limited by resourcesThis causes individuals to compete for resources (remember intraspecific and interspecific competition!).

Population Growth

There are 4 main factors controlling population sizes:Natality - births increase the populationMortality - deaths decrease the populationImmigration - movement of individuals into an area increases the populationEmigration - movement of individuals out of an area decreases the population

Factors that Control Populations

Carrying capacity: maximum number of individuals of a species that can be sustained indefinitely in a given spaceNo population can grow indefinitely! Resources = Limited!!

Carrying Capacity

Limiting Factors of PopulationsFactors (biotic or abiotic) which prevent population numbers from growing too large and overrunning an ecosystem.Example: Disease/ parasitesdisastershunting & predationcompetition for resources (food, oxygen, nutrients)

J population growth curveExponential growth = starts out slow and then proceeds faster and fasterOccurs when a population has few resource limitationsSudden collapse ('diebacks')

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 populationPatterns of Population Growth

J Curve vs. S Curve

Population DynamicsA look at the factors that tend to increase or decrease the size of a populationThe population size is determined by the interplay of biotic potential and environmental resistance. Biotic potential- growth rate with unlimited resourcesEnvironmental resistance - all the factors acting jointly to limit population growth

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

Density Dependent Factors Density dependent factorsDepend on the size of the populationEffects of the factors increase as the population growsAct as negative feedbackTend to be bioticTwo categories:Internal factors = Within a single specieslimited resourcesreduced fertility ratesExternal factors = between speciespopulations of predators or preydiseases spread more easily in densely-populated areas

Density Independent FactorsDo NOT depend on the size of the populationTend to be abioticEffect the population regardless of its sizeExamples:1. Weather2. Earthquakes3. Floods 4. FiresR-strategists are most affected by these factors

Reproductive Strategies r Unstable environment K Stable environment

Small sizeMany offspring are producedEarly maturity Short life spanEach individual reproduces onceType III survivorship curveAffected by density Independent Large sizeFew offspring producedLate maturity (long parental care)Long life spanIndividuals reproduce more than once Mostly type I survivorship curveAffected by density dependent

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).

Survivorship Curves

Pop QuizWhat 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 Wisconsin? (Only at certain times in the year)What do you think the survivorship curve would be for a white-tailed deer population?

Ecological SuccessionHow has this ecosystem changed?Ecosystem #1

BEFOREAFTER

BEFOREAFTERHow has this ecosystem changed?Ecosystem #2

#1#2#3#4Ecological Succession: Change over TimeTwo Types of Succession Primary succession - An ecosystem starts from bare rockSecondary succession Ecosystem is built from a previous ecosystem Starts with soil

23Important TermsSere: A set of stages of changes in an ecosystem. A snapshot of ecosystemPioneer organisms: First species that begin to populate a sere, typically r-strategists.Ex. Weeds, lichensClimax community: Populations of organisms living together in a sere where all species are in balance. Ex. A mature forest, many K-strategists

Pioneer Species

Primary Succession No soil in a terrestrial systemNo bottom sediment in an aquatic systemTakes hundreds to thousands of yearsNeed to build up soils/sediments to provide necessary nutrients

26Primary succession will occur after a volcanic eruption

Primary succession occurs after a glacier retreats

Image source: http://www.callipygia600.com/Primary succession occurs after a glacier retreats

Primary succession occurs after a glacier retreats

Glacier Bay, Alaska

TimeBalsam fir,paper birch, and white spruce forest communityJack pine,black spruce,and aspenHeath matSmall herbsand shrubsLichens andmossesExposedrocksPrimary Ecological SuccessionSecondary SuccessionBegins with soil from previous ecosystemEcosystem has beenDisturbed, Removed, or DestroyedAbandoned farmsBurned forestsDeforestationA huge storm

33

AnnualweedsMature oak and hickory forestYoung pine forestwith developingunderstory of oakand hickory treesTimeShrubs andsmall pineseedlingsPerennialweeds andgrassesSecondary SuccessionYellowstone 1998 Forest Fire

Secondary Succession in Yellowstone Following the 1998 Fire

Figure 5.21: These young lodgepole pines growing around standing dead trees after a 1998 forest fire in Yellowstone National Park are an example of secondary ecological succession.

Conditions during succession Early Stages (Pioneer Communities) Gross Productivity = LowFew producersNet productivity = HighNo consumers doing respirationNo taxes to take away!!Nutrients in soilLater Stages (Climax Community )Nutrients in soil decrease Gross Productivity is HighMany producersNet productivity = lowMany consumers all doing respiration = a very large amount of taxes

37