Announcements September 8, 2006

Population Biology Lecture Objectives: 1.Learn the population characteristics that determine population growth rate 2.Understand why the number of individuals in a population may change over time. 3.Understand the different types of population growth curves 4.Understand the difference between K- selected and r-selected species

Population Characteristics Population – All organisms of the same kind found within a specific geographic region Populations can differ in: *Birthrate *Death rate *Sex ratio *Age distribution *Density *Population growth rate

Birthrate (Natality) Birthrate (b)—Number of individuals added through reproduction over a particular time period. - In many species, birthrate is influenced by the amount of food available - Sexually reproducing species must also find mates - In humans, expressed as # of babies/1000 individuals/year

Asexual Reproduction Females that reproduce asexually do not need to find a mate New Mexico Whiptail Daphnia

Mortality Survivorship curve – a graph showing the proportion of individuals likely to survive to each age Three main types:  Type I  Type II  Type III

Type I survivorship curve Survival is high until old age Examples: Dall sheep Humans in developed countries

Type II survivorship curve Mortality is spread evenly among all age groups Example: Many species of birds

Type III survivorship curve Very high mortality among the young Examples: Plants Animals that produce many offspring

Survivorship curves (continued) One major factor determining the shape of the survivorship curve is the degree of parental care How long do parents care for the offspring after they are born?

Population growth rate Population growth rate (r) – the rate at which the size of the population changes r = b – d Assumes no immigration or emigration— when this happens population growth rate is then more generally, (birth + immigration) – (death + emigration)

Population growth rate If r is positive, birthrate is higher than the death rate and the population size increases If r is zero, birthrate equals death rate and the population size stays the same If r is negative, birthrate is less than the death rate and the population size gets smaller

r > 0 (+) r = 0 r < 0 (-) Example for a population of algae

Dispersal: Movement of individuals Emigration: leaving a population, often from crowded areas or in response to environmental change Immigration: emigrating individuals become immigrants in a new population (joining a population) Other factors influencing population growth rate Dispersal

Other factors influencing population growth rate Sex Ratio Sex Ratio—Relative number of males and females in a population. In many sexually reproducing species, with separate sexes, sex ratio is 1:1 In other species (e.g., asexuals, social insects) can have far more females than males

Other factors influencing population growth rate Age Distribution (Age Structure) Age Distribution—Number of individuals of each age in the population. Greatly influences reproductive rate of a population.

Summary of factors influencing population growth rate Birthrate Death rate Sex Ratio Age distribution Immigration Emigration How many individuals are in a population at any given time? How many individuals will be in the population one generation later?

Which of these age distributions has a Type III survivorship curve? ABC

Population Density (N)—Number of individuals per unit area. High population density may lead to increased competition for resources, increased emigration, increased death rate Population Density Population 1 N = 4 flowers/m 2 N = 12 flowers/m 2 Population 2

Population Growth Curve Biotic Potential—Inherent reproductive capacity.  Generally, biotic potential is much above replacement level. Natural tendency for increase. When not limited, populations tend to grow exponentially How to combine number of individuals (N) and population growth rate (r)?

Exponential Growth Note: This equation is provided only to help. You do not need to memorize it. You will not be asked about the equation on a test. Change in number over time Population growth rate Number of individuals The number of individuals in each generation is a multiple of the previous generation

Example of exponential growth

Populations do not grow exponentially forever

Population Growth Curve

Typical Growth Curve Lag Phase—First portion of the curve; slow population growth. Exponential Growth Phase—More organisms reproducing causing accelerated growth; continues as long as birth rate exceeds death rate. Stable Equilibrium Phase—Death rate and birth rate equilibrate; population stops growing.

Carrying Capacity —Number of individuals of a species that can be indefinitely sustained in a given area.

Environmental Resistance Environmental Resistance—Any factor (limiting factor) in the environment influencing carrying capacity. Four main factors:  Raw material availability  Energy availability  Waste accumulation and disposal  Organism interaction

Logistic Equation Note: This equation is provided only to help. You do not need to memorize it. You will not be asked about the equation on a test.

Causes of population crashes Population size can sometimes overshoot carrying capacity Environmental resistance factors can change, lowering carry capacity When population > carrying capacity, death rates > birth rates Population crashes!!

Example 1: bacterial population crash due to toxic waste accumulation

Example 2: Population cycling due to organism interaction Hare population: birth rate limited by food availability death rate determined by predators, food Lynx population: birth and death rate determined by food availability (hares)

Example 2: Population cycling due to organism interaction Start: both have low population density Hares – high food, low predators = pop increase over next generations Lynx – as hares increase, more food = pop increase Hares – when high pop density, increased competition for food and increased predation = low birth rate & high death rate = sharp pop decrease Lynx – when high pop density and few hares, low food = low birth rate & high death rate = sharp pop decrease Back to start

Population Cycles

Remember: Several factors influence population growth rate: BirthrateDeath rate Sex RatioAge Distribution ImmigrationEmigration But, there are also additional things about the individuals that influence how the population grows

Life history traits – characteristics of an individual that influence survival and reproduction Age at maturity years 3-6 years Atlantic Salmon African elephant 2 months House Mouse

Life history traits – characteristics of an individual that influence survival and reproduction Atlantic Salmon African elephant House Mouse 1 calf every 3-8 years 1,500 to 8,000 eggs once 5-8 young every month Number of offspring produced

Life history traits – characteristics of an individual that influence survival and reproduction Atlantic Salmon African elephant House Mouse Number of reproductive events ~ ~6-12

Life history traits – characteristics of an individual that influence survival and reproduction Atlantic Salmon African elephant House Mouse Lifespan years3-6 years ~2 years

How do the following life history traits affect the rate of reproduction? Age at maturity # offspring # reprod. events life span 2 momanyone long 2 yrfewmany short 2 momanymanylong 2 yrmanymanylong

K-strategists: Remember that “K” is Carrying Capacity long lived produce few offspring parental care Reproductive strategies and population fluctuations

–controlled by density-dependent limiting factors factors more severe as population increases example: food K-strategists: stable populations at carrying capacity (K) examples: humans, deer, lions Reproductive strategies and population fluctuations

Gypsy moth caterpillars Remember that “r” is population growth rate r = b - d r-strategists: small short life many offspring Reproductive strategies and population fluctuations

Gypsy moth caterpillars r-strategists: rarely reaches K little or no parental care Reproductive strategies and population fluctuations

Gypsy moth caterpillars –good at using temporarily abundant resources r-strategists: controlled by density-independent limiting factors: weather, pond drying Reproductive strategies and population fluctuations

Points to know: Understand the three types of survivorship curves. What factors influence population growth rates? What are some life history traits? Know the difference between K- and r- strategists and what kind of limiting factors each has.