Chapter 5 Population Ecology

Counting individuals What constitutes an individual organism?

Variations on the individual Unitary organisms - one zygote per embryo produces one organism

Variations on the individual Modular organisms - one zygote per embryo produces one module, which eventually produces more modules like itself Branching or shoot development in plants, budding in Hydra, sponges, fungi

How to count individuals in a population Some organisms - possible to easily count all individuals Others must be subsampled and estimated Plants and some animals - quadrat Soil, water dwellers - volume Animals - mark and recapture

Mark and recapture Random sample Release Try to recapture Theory - marked individuals will remix within population; proportion marked in next sample represents proportion in entire population

Mark and recapture example Population size (N) #marked on Day 1 = Total catch on Day 2 # of recaptures N = (# marked on Day 1) x (Total catch on Day 2) # of recaptures

Gilmore Creek Brown Trout 200 m stream reach=840 m 2 area 4.2 m average stream width Day 1: 169 trout captured, marked, released Day 2: 178 trout captured 80 marked (recaptures), 98 unmarked N = (# marked ) x (total catch Day 2) # of recaptures N = 169 x 178 = 377 trout 377 trout/840 m 2 = trout m 2

Life cycles Patterns of birth, death, growth are dictated by an organism’s life cycle 5 main types of life cycles

Life cycle types Annual Overlapping iteroparity Overlapping semelparity Continuous semelparity Continuous iteroparity

Semelparous Individual organism has single reproductive event during its life, then dies Invests large amount of energy in reproduction

Iteroparous Individual may have many reproductive events during season or life Invests lesser proportion of resources in reproduction

Annuals 12 months or less to complete life cycle Discreet, non-overlapping generations May or may not overwinter as non-seed/egg May be either semelparous or iteroparous Annual may be a misnomer for some plants with seeds that do not always germinate the year after being produced Seeds may lie dormant in seed bank for several years before germinating

Overlapping iteroparity Overlapping generations (yearlings, 2-year- olds, etc.), iteroparous Distinct breeding season Examples: temperate-zone trees, long- lived, seasonally breeding vertebrates (deer, most fish, snakes, birds)

Overlapping semelparity Overlapping generations (several age classes present [at least biennial]), semelparous New offspring in population every year (distinct breeding season) Require 2 or more years to mature and reproduce, then die Most common in plants, also in some species of octopus, salmon

Continuous semelparity No distinct breeding season because of favorable environmental conditions Many overlapping ages continually growing, reproducing, dying Example: some animals in tropical oceans

Continuous iteroparity No distinct breeding season Many overlapping ages Example: humans

Life tables Used to follow changes in births, deaths, growth of population through time Of differing complexity and usefulness depending on life cycle of organism being examined Easiest for annuals, more difficult for other types

Life table variables xlife stage or age class a x total number of individuals observed at each stage or class l x proportion of original number of individuals surviving to the next stage or class; survivorship d x proportion of original number of individuals dying during each stage or class; mortality q x mortality rate for each stage or class k x "killing power;" F x total fecundity, or reproductive output of entire population, for each stage or class m x individual fecundity, or mean reproductive output, for each stage or class l x m x number of offspring produced per original individual during each stage or class; product of survival and reproduction R 0 basic reproductive rate

Cohort life table Group of individuals “born” within same short time interval is followed from birth through death of last survivor

Grasshoppers - a cohort life table

Phlox - a cohort life table

Static life tables Life tables more difficult to construct for longer-lived organisms, and those with many overlapping generations Difficult to follow single cohort throughout entire life (many years) Static life table is produced - snapshot in time

Static life tables Need information on total population size and its age structure at some point in time Can get messy if older age classes have more individuals than younger age classes Different mortality, recruitment May need to smooth data to get things to work

Red deer - a static life table

Survivorship Curves Depict what proportion of population remains alive at various points in life 3 basic patterns displayed by living things

Survivorship Curves Type I - little mortality throughout early life Mortality concentrated in older age groups Example: humans

Survivorship Curves Type II - constant rate of mortality throughout life Constant proportion die age time/age interval Example: corals, squirrels

Survivorship Curves Type III - high early mortality Survivors have little mortality until old age Example: plants, fishes

Population Dynamics and Growth

Exponential Growth Time (t) Population size (N) -ideal habitat -maximum reproduction -unlimited resources Increase often followed by crash

2,000 1,500 Number of reindeer Year 1, Reindeer on an Alaskan island

5,000 4,000 3,000 2,000 1, Number of moose Year Number of wolves Moose population Wolf population Moose and wolves on Isle Royale

Logistic Growth Time (t) Population size (N) K -accelerating, decelerating Carrying capacity -growth slows as population size approaches carrying capacity -number that environment can support indefinitely Carrying capacity set by limiting factor

Number of sheep (millions) Year Sheep in Tasmania

Human population growth -exponential or logistic?

-appears exponential -history may suggest logistic -periods of rapid growth followed by stability

Human population growth -exponential or logistic? Cultural evolution -tool-making revolution -agricultural revolution -industrial (technological) revolution

Carrying capacity for humans Set by: -famine -disease -warfare Will these become more common as population approaches carrying capacity?

Population Demographics  What affects human population size and growth rate?  What affects human population size and growth rate? 1) Birth rate and death rate 2) Migration rate 3) Fertility rate 4) Age structure 5) Average marriage age 1) Birth rate and death rate 2) Migration rate 3) Fertility rate 4) Age structure 5) Average marriage age

Factors Affecting Human Population Size  Population change equation  Zero population growth (ZPG)  Birth rate (number/1000 people/year)  Death rate (number/1000 people/year) Population Change Population Change = = (Births + Immigration) – (Deaths + Emigration)

Birth and death rates  U.S and 9 (7 or 0.7%)  Rwanda - 52 and 18 (34 or 3.4%) Births per thousand population Year Demographic transition Depression End of World War II Baby boomBaby bustEcho baby boom  World - 26 and 9 (17 or 1.7%)

Infant deaths per 1,000 live births <10 <10-35 <36-70 < <100+ Data not available Factors Affecting Death Rate  Life expectancy  Infant mortality rate (IMR)

Rate of Natural Increase Developed Countries Rate of natural increase Crude birth rate Crude death rate Rate of natural increase = crude birth rate–crude death rate Developing Countries Rate per 1,000 people Crude birth rate Rate of natural increase Crude death rate Year © 2004 Brooks/Cole – Thomson Learning

Natural Rate of Increase <1% 1-1.9% 2-2.9% 3+% Data not available Annual world population growth 1% - triple in 100 years 2% - 7X in 100 years

Migration Rates  Affect regional populations  e.g., United States  Net gain of 4/1000 people/year  Add to 7 from BR - DR = 11 (1.1%)

Fertility Rates  Average number of children born to a woman during her childbearing years (ages 15-44)  Average number of children born to a woman during her childbearing years (ages 15-44)  Replacement level fertility rates for ZPG  Replacement level fertility rates for ZPG  Total fertility rates

Fertility Rates  Replacement level fertility rates for ZPG - developed countries - 2.1/woman - developing countries total world  Replacement level fertility rates for ZPG - developed countries - 2.1/woman - developing countries total world

Fertility Rates  Total fertility rates - developed countries (U.S. 1.8) - developing countries (Rwanda-8.5, Kenya-8.0) - total world  Total fertility rates - developed countries (U.S. 1.8) - developing countries (Rwanda-8.5, Kenya-8.0) - total world - 3.4

Births per woman < No Data Fertility Rates

 Time lag to ZPG - about 3 generations (~70 years) required to achieve ZPG once replacement level fertility rates are reached  Time lag to ZPG - about 3 generations (~70 years) required to achieve ZPG once replacement level fertility rates are reached

Ages 0-14 Ages Ages Rapid Growth Guatemala Nigeria Saudi Arabia Rapid Growth Guatemala Nigeria Saudi Arabia Slow Growth United States Australia Canada Slow Growth United States Australia Canada Male Female Zero Growth Spain Austria Greece Zero Growth Spain Austria Greece Negative Growth Germany Bulgaria Sweden Negative Growth Germany Bulgaria Sweden Population Age Structure

Average Marriage Age  or age at birth of first child  Higher marriage age leads to reduced reproductive period, which leads to lower fertility rates  Higher marriage age leads to reduced reproductive period, which leads to lower fertility rates

Average Marriage Age  Current U.S. marriage age - 24 (F)  Reduces 30-year reproductive period (15-44) to 21-year reproductive period (24-44) - 30% reduction  Reduces 30-year reproductive period (15-44) to 21-year reproductive period (24-44) - 30% reduction  Reduces 15-year prime reproductive period (15-29) to a 6-year prime reproductive period (24-29) - 60% reduction  Reduces 15-year prime reproductive period (15-29) to a 6-year prime reproductive period (24-29) - 60% reduction  Expectation: >25 needed to affect fertility rate

Current Needs for Large Families  Increased income  High infant mortality  Support for elderly  Few opportunities for women outside the home  Few opportunities for women outside the home  Family planning unavailable

r strategist Unstable environment, density independent K strategist Stable environment, density dependent Small size of organismLarge size of organism Low energy for reproductionHigh energy for reproduction Many offspring producedFew offspring produced Early maturityLate maturity (often after parental care) Short life expectancyLong life expectancy Reproduces onceReproduces more than once Type III survivorship curveType I or II survivorship curve Life History Strategies