Chapter 52 Population Ecology

Population Ecology Study of Populations in relation to their environment. Environmental influences on Pop. Density Distribution Age structure size

Population: Individuals of a single species that occupy the same general area.

Important Characteristics Density 2. Dispersion

Density Number of individuals per unit area or volume. Ex: Diatoms - 5 million/m3 Trees - 5,000/km2 Deer - 4/km2

Dispersion The pattern of spacing among individuals within the boundary of the population.

How do you count a population? 1. actual count 2. Random plots, then extrapolate 3. Indirect indicators: number of nests; burrows; tracks; droppings 4. MARK & RECAPTURE METHOD

Mark and Recapture Method N= Population estimate Trap, capture, mark, release. Later-recapture, determine proportion of those recaptured that were marked. N= # marked in first catch X Total # in 2nd catch Number of marked recaptures Assumptions: 1. the proportion of marked animals in the 2nd trapping is equivalent to the proportion of marked animals in the total population 2. Those marked mix uniformly within the pop.

Population densities not static Birth Death Immigration Emmigration

Dispersal patterns 3 general patterns Know these!!

1. Clumped Pattern Most common pattern Results form patchy environmental conditions-food, nutrient availability. Ex: Mushrooms under rotting log. Groups enhance predation Safety. May increase chances for survival. Ex: Schooling behavior Flocks of birds humans

2. Uniform Dispersion Individuals are evenly spaced Results from direct antagonistic interactions Plants secrete chemicals-inhibit nearby germination & competition (alleopathy) territoriality

3. Random Dispersion Spacing varies unpredictably Absence of strong attractions or repulsions between individuals. RARE pattern in nature Example: windblown seeds land randomly & germinate.

Demography Study of vital statistics of a population & how they change over time. Add Lose 2 most important features Age structure-relative numbers of individuals of each age group in a population Sex ratios

1. Life Tables How long, on average, are individuals of a given age expected to live. Age-specific summary of survival patterns. Follow the fate of a cohort (indiv. Of same age) from birth to death.

Do males or females have a higher death rate? Who lives longer?

2. Survivorship Curves Graphic representation of data in Life Tables. Plot of the proportion or numbers in a cohort still alive at each age. 3 general Curve Types: ( be able to recognize and describe traits; members exhibiting each; example organism) Type I—humans, elephants Type II-squirrel Type III-oysters, clam

Draw and label in your notes

Type I Low early deaths. Steep decline in death rates among those older Produce few offspring, but provide good care. Ex: Humans Other large mammals

Constant death rate over life span. Type II Intermediate Constant death rate over life span. Ex: Annual plants small mammals ( grey squirrel), lizards

Type III High death rates for the young. Few live to adulthood ( sharp dip in curve initially) Few live to adulthood Associated with: Produce large numbers of offspring-but provide little or no care. Ex: Oysters

Variations Curve type may change between young and adults. Ex: Nestlings - Type III Adult Birds- Type II Stair-step curve Invertebrates—crabs; high mortality during molt, followed by low mortality

Life-History Traits Life History: Timing of reproduction and death. Highly diverse, but do show patterns Determines how populations grow. Results from Natural Selection. Darwinian “fitness”

Darwinian “fitness” Survive AND Reproduce

Reproduction and survival—Life History Traits 1. # of reproductive episodes # offspring per reproductive episode Age at first reproduction

2 Common Reproductive Patterns 1. Semelparity: “big-bang” reproduction 1 reproduction event Salmon, agave ( desert plants) , annual plants 2. Iteroparity Fewer offspring at a time Over many reproductive seasons

Population Studies & Reproductive Rates Focusing on females and female offspring. 2 Kinds of Reproductive Strategies identified R K

Life History Selection 1. "r" Selected species 2. "k" Selected species

“r" – Selected Species (density independent selection) Increase fitness by producing as many offspring as possible. Do this by one of these strategies: Early maturation Many reproductive events Many offspring in one reproductive event “Big bang” reproduction (semelparity) (pink salmon, agaves) is one time reproduction.

r-Selected Result Maximize reproduction so that at least a few offspring survive to the next generation. Most offspring die (Type III curve).

“K" – Selected Species (density dependent selection) Increase fitness by having most offspring survive. Maintain populations at or near “K” Do this by: High parental care Late maturation Few reproduction events Few offspring.

K- Selected –Results.. Maximize survivorship of each offspring . Few offspring, but most survive (Type I curve).

What is the strategy For a weed? For Garden Pests?

POULATION GOWTH Zero population growth: Per capita birth + Per capita death=0

Exponential Growth Produces a “J-shaped” growth curve. Ideal conditions unlimited resources. Example: Introduce into new or unfilled environment Rebounding population

Logistic Growth-

Logistic Growth-S curve K = carrying capacity The rate of population growth slows as the population reaches K. of the environment. Limiting factor influence S-shaped (sigmoid) curve

Population growth related to Life History Traits S-shaped” growth curve. Characteristic of “k" species. Common when resources are limited.

Populations are limited by space, food Populations are limited by space, food. That limit is called the CARRYING CAPACITY The graph shows a logistic population curve. At what level do the deer reach their CARRYING CAPACITY?

What Limits Population Size? Density-dependent factors: limited resources- space, food, water, air related to population size Density-independent factors: random occurrences that can limit population - earthquake, bad weather. not affected by population size Is disease density dependent, or density independent?

Additional Comments Populations often overshoot “K” ( go beyond) , then drop back to or below “K”.

Regular Population Cycles Cyclic changes in N over time. Often seen in predator/prey cycles. Ex: “Boom & Bust Cycle” of Snowshoe Hare -& Lynx

Why predators rarely kill and eat all the prey: Predators kill and consume other organisms. Carnivores prey on animals, herbivores consume plants. Predators usually limit the prey population, although in extreme cases they can drive the prey to extinction. Why predators rarely kill and eat all the prey: 1.Prey species often evolve protective mechanisms such as camouflage, poisons, spines, or large size to deter predation. 2.Prey species often have refuges where the predators cannot reach them. 3.Switch its prey as the prey species becomes lower in abundance:

Know how to read a graph for population growth, where K is; where the overshoot “K” is

Age Structure Diagrams Show the percent of a population in different age categories . Method to get data similar to a Life Table, but at one point in time.

HUMAN POPULATION GROWTH Grown since history 1A.D. 300 million Since 1650 ( 500 million), growing exponentionally. Over 6 billion today

Exponential Growth Produces age structures that are a triangle or pyramid shape.

Logistic Growth Produces age structures that have even sizes between most age categories.

Declining Populations Produce age structures with a narrow base and wider middles.

Reveal current and future growth trends.

Ecological Footprint Model to predict “K” Relative to The Available Ecologic capacity Land usage requirements to meet our varied human needs.

Summary Know density and dispersion patterns. Know Life Tables and survivorship curves ( and representative organism for the 3 types). Know what “K” is Be able to contrast and compare “r” and “k” strategies ( Suggest making a chart-include Logistic Curve, Exponential growth curve, characteristics of populations). When/how density independent, and dependent growth factors affect pop size. Know how (match) Age-structure pyramids relate to either Rapid, Slow, or Zero Pop. Growth; their shapes & what the shape means.

Summary Know exponential and logistic growth curves.Relate them to PGR