Biological Populations Miller Chapter 9 Populations Population Density

Biological Populations Miller Chapter 9

Populations • Population Density – Number/unit • Dispersion Clumped, random or uniform • Often clumped • Always question scale

How to measure density • Count all individuals • Random sampling – Habitat should be homogenous – Issue of scale!! • Mark-Recapture

Mark-Recapture N = # marked first x total # caught in 2 nd # recaptures in second Assumes same probability of getting caught twice

Measuring Populations • Other signs – Tracks, fecal matter, burrows, etc

Demography • Study of the statistics that affect population size

Survivorship Curves -Type I - low infant mortality-parenting K selected -Type II – steady population loss -Type III – many young - high infant mortality R selected

Population Growth Model = making predictions R = ∆N/ ∆t = average B - average D N = population size B= births during that time D = deaths during that time

Population Rate! b = per capita birth rate (# of offspring produced per unit time by an average member of the pop) *example 34 births in a pop of 1000 = 34/1000 = 0. 034 or 3. 4%

Zero Population Growth rate r = b-d If r is positive = population is growing If r is negative = population is shrinking If r is 0 = Zero Population Growth

Fertility Rate • Numbers of babies per female • Also called TFR (Total Fertility Rate) • For ZPG need TFR = 2. 1 – TFR < 2 means shrinking population – TFR > 2. 1 means growing population – Examples • Europe TFR = 1. 6 • World TFR = 3. 5

Exponential Growth Math • No Need to use exponential calculations • Doubling Rate = 70 / (growth rate%) or • Doubling Rate =. 70 / (growth rate dec. ) • Based on natural log 2 = 0. 693 approx =. 7

Exponential Growth Examples • Species rebounding from catastrophe • ***ALIEN species!!! Zebra, quagga mussles • Pests in monoculture agriculture

Logistic Growth • S shaped curve vs J shaped curve • Upper limit is the CARRYING CAPACITY

Logistic Growth • Starts with Exponential Growth • But slows down as it approaches the limit of the system called Carrying Capacity K

Carrying Capacity • When can K change? • What about populations in different areas? • Changes in shelter, soil nutrients, predators, water, can affect carrying capacity.

Logistic Growth Rate K - N = how many more individuals a population can hold K - N/ K = fraction of K still available for growth d. N/dt = r. N (K-N/K)

Logistic Models Population growth rate slows, the larger the population • Smooth S if pop adjusts instantaneously • Overshoot often happens

Population Limiting Factors • Why do pops eventually stop increasing? K • Why are some habitats better? • What can we do to stop the pop growth of introduced species? Need to understand exponential possiblities

Density Independent Factors • • Drought Floods Hurricanes Fire

Density Dependent Factors • Resource limitations – Examples – predation, food competition, disease (Intraspecific competition – same species and Interspecific – different species) – Death rate rises as density rises causing Negative feedback

Logistic Growth • Modified Exponential Growth Model - the growth rate slows as N approaches K - S shaped curve - Can overshoot and oscillate