Chapter 6
Population Ecology
• They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback.
Core Case Study: Southern Sea Otters: Are They Back from the
Brink of Extinction?
Figure 8-1Figure 8-1
Population Trends from 1990 - 2012
17,000 in 1800’s Less than 100 in 1914
Case Study: CA Condor
• Large pre-historic population; ice age
• In 1987, only 22 condors were left.
• Captured all wild birds; captive breeding program
• Decline due to:– Pesticides (DDT)– Poisoning (lead)– Starvation– Accidents
Levels of Complexity
Population• Population – composed of all individuals that
belong to the same species and live in a particular area at a given time.– Unit of evolution– Cause of decrease and increase in population– Boundaries can be arbitrary– Dynamic (always changing)
Changes in Population Size: Inputs & Outputs
• Populations increase through births and immigration
• Populations decrease through deaths and emigration
Communities & Ecosystems
• Community - incorporates all of the populations of organisms within a given area.– Boundaries can also be arbitrary– Scientists interested in interaction between species– Ex: Maple-Beech-hemlock community– Communities that experience similar temperature & rainfall are
grouped into biomes• However, species of trees varies from community to community. Ex:
Eucalyptus vs. oak (Aust. vs N. America/Europe)
• Ecosystems – consists of all abiotic & biotic components in a particular location.– Focused on the flow of energy & matter; cycling of nutrients
through a system
Population Ecology
births + immigration > deaths + emigration = population growth
births + immigration < deaths + emigration = population decrease
Population Ecology is the study of the factors that cause populations to change.
– Knowing factors that affect population are useful for controlling it– CA Condor (extinction) or Argentine ants (invasive)
Population Characteristics
• Population Size (N) – total number of individuals within a defined area @ a given time.
• Population Density – is the number of individuals per unit area (or volume in case of aquatic organisms).– Help estimate if species is rare, abundant, or
overpopulated– Help aid in wildlife management
Population Characteristic
• Population Distribution – description of how individuals are distributed with respect to one another.– How a population occupies space– Three types:
• Random – Trees in a forest• Uniform – territorial nesting birds, tree farm• Clumped – schooling fish, flocking birds, & herding
animals
Population Distribution
• Most populations live in clumps although other patterns occur based on resource distribution.
Figure 8-2Figure 8-2
Population Characteristics
• Population Sex Ratio – the ratio of males to females– Usually close to 50-50; fig wasps (20:1 females/males)– Knowing ratio helps scientists estimate number of
offspring in next generation
• Population Age Structure – description of how many individuals fit into a particular age category.– Helps predict how rapidly a population can grow– Large proportion too young or too old to reproduce will
help understand the growth or decline of a population
Age Structure: Young Populations
Can Grow Fast• How fast a population grows or declines
depends on its age structure.– Prereproductive age: not mature enough to
reproduce.– Reproductive age: those capable of
reproduction.– Postreproductive age: those too old to
reproduce.
Age Structure Diagram
Factors That Influence Population Size
• Density Dependent Factors: influence an individual’s probability of survival & reproduction that depends upon the size of population.– Amount of available food (greater affect on large
populations); predation, disease, competition, parasitism
– Gause Experiments – shows how food supply controls population growth
• Rapid growth early, then population plateaued later • Double the amount of food, populations also doubled• Confirmed limiting resource = food• Limit to how many individuals the food supply can sustain is
Carrying Capacity (K) without destroying the habitat.– Predict how much individuals an environment can sustain
• Density-Independent Factors: have same effect on an individual’s survival & reproduction at ANY population size.– Examples: tornado, hurricanes, floods, fires, volcanic
eruptions, and other climatic events– Mortality (death) increases in such an event
regardless of population size– Bird populations are regulated by these factors
Factors That Influence Population Size
Gause’s Experiments
Population Growth Models
• Important tools in explaining population fluctuations.
• Used to predict population size at any moment in time.
• Growth Rate – number of offspring an individual can produce in a given time period, minus the deaths.
• Maximum potential for growth is called the intrinsic growth rate (r). Under ideal conditions.
Population Growth Models
• Exponential Growth Model:– N0 = starting population
– r = intrinsic growth rate– N = future size of population– t = time elapsed @ which population grows– r = 0 if population is constant; r > 0 if
population is increasing; r < 0 if population is decreasing
Exponential Growth Model
• Populations not limited by resources, growth is very rapid.
• Rate is fixed• More births occur with
each step in time.• This model produces a J-
Shaped curve .• Exponential growth is
density independent.• Gause’s experiment
showed that no population can grow indefinitely
The Logistic Growth Model
• Modified model to incorporate environmental limits on exponential growth.
• Logistic growth model – a population whose growth is initially exponential but slows as population reaches carrying capacity (K).
• Produces an S-shaped curve
Variations on the Logistic Growth Model
• Overshoot – producing more offspring than environment can hold, leading to a die-off– Seasonal: mate in winter but
population is larger than spring carrying capacity
– Reindeer are a good example (see graph)
– Overshoot & die-off can be a recurring cycle as populations will oscillate around the carrying capacity.
Types of Population Change Curves in Nature
• Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.– Stable: fluctuates slightly above and below carrying
capacity.– Irruptive: populations explode and then crash to a
more stable level.– Cyclic: populations fluctuate and regular cyclic or
boom-and-bust cycles.– Irregular: erratic changes possibly due to chaos or
drastic change.
Natural Population Curves
Fig. 9-7 p. 194
Population grew exponentially then crashed to only 8 individuals . . . . Most likely due to lack of food
The Role of Predation in Controlling Population Size
The Role of Predation in Controlling Population Size
Top-down control: predator controls the cyclic fluctuations of the prey
Bottom-up control: herbivore-plant interaction influences the cyclic fluctuations of the predator
Reproductive Strategies & Survivorship Curves:
r-Selected• High intrinsic growth rate• Reproduce often w/ lots of
offspring• Large fluctuation around
carrying capacity• Ex: insects, rodents, small fishK-Selected• Low intrinsic growth rate• Abundance of species is
determined by carrying capacity.• Small fluctuations• Ex: Large mammals, most birds
Survivorship Curves: Short to Long Lives
The way to represent the age structure of a population is with a survivorship curve.
Late loss population (Type I) high survival rate; live to an old age.
• Elephants, rhinoceros, humans, whales, large reptiles
Constant loss population (Type II) constant; die at all ages.
• Corals, Jellyfish, songbirds, rodents
Early loss population (Type III) low survival rate; die at young ages.
• Annual plants• Insects