Populations

What is a population?

-a group of actively interacting and interbreeding individuals in

space and time.

populations are characterized by:

-density, -distribution in space, -age structure, -sex ratio,-birth rate, -death rate, -emigration rate, -immigration rate,-genetic structure.

- Selection operates on individuals, populations evolve

Density Dynamics

Density change is a function of:

Birth rate, immigration rate

and

Death rate, emigration rate

Population Size

• Theory of population growth

Population Growth (Density Independence)

Nt+1 = RNt

where Nt is the population size at time tand R is the reproductive value-if R=2 then populations increase as 1,2,4,8,16

etc. =2t

rewritten as Nt = Rt

- If R=1 ? If R<1 ?

Life Tables

Life tables

Cohort vs Staticx - agelx – survivorshipmx –age specific fecundityR= net reproductive rate = Σ lx * mx G= generation = Σ x*lx * mx / RAssumes stable age distribution

LifetableSurvivorship Fecundity

x lx mx lxmx x(lxmx)0 1 0 0 0

1 0.7 0 0 0

2 0.3 1 0.3 0.6

3 0.2 2 0.4 1.2

4 0.1 3 0.3 1.2

5 0 0 0 0

sum(lxmx) = 3

Ro= 1

Generation time (G) = 3.00 rm = 0.00

X age classlx proportion surviving to start of age intervalmx fecundity (average number of young per femaleRo reproductive valueG generation time

rm = innate capacity for increase

Important concepts

• Habitat• movement (dispersal)

Habitat• habitat – physical condition where species

live – defined specifically for individual species.

• source/sink

Habitat Reduction and Fragmentation

Movement

Movement (Dispersal) is central to population health!

- Density

- Genetics

What factors affect movement?

Corridors

Connectivity

Spatially structured populations

Spatially structured populations

High dispersal rate = homogeneous populations, low extinction

Spatially structured populations

Low dispersal rate = heterogeneous populations, high extinction

Metapopulation

• Dispersal rate intermediate where individual populations go temporarily extinct.

Concepts

• Minimum Viable Population (MVP)

• Population Viability Analysis (PVA)

PVA for Spotted Owl

Estimating Abundance

Population Size

• Estimating population size– Indices– Density

Relative Abundance

Indices for Relative Abundance• An index of abundance is a measure that varies

directly with the abundance of the population of interest.

• Based on:• Scats, pellets, pellet groups• Point or transect counts (Bird Songs)• Tracks• Signs • Etc.

Index of Deer abundance in Nova Scotia

• Based on counting pellet groups –Pellet Group Index PGI

• Pellet groups are counted along a 1 km

transect that are within 1 m on either side of the transect.

• Counting takes place in the spring

2 m

1 km long

PGI = 7

This index can be turned into a density estimate

Deer Density

• Index can be converted to a density estimate

• Need to know how long a period the pellet groups have accumulated over

• Need to know how often deer defecate per day.

Birds

• Point count• Line transect count• Strip count• Variable distance line transect

Direct Density Estimates

Catch-effort Methods

Assumptions:1 – The population is closed2 – Probability of each individual being

caught in a trap is constant throughout the experiment

3 – All individuals have the same probability of being caught in sample i

Cat

ch p

er u

nit e

ffor

t

Accumulated catch

Number of

individuals

Leslie plot of catch-effort data

Capture-Mark-Recapture Techniques

• For Closed Populations:

• Single marking, single recapture – Petersen Method

• Multiple markings and recaptures – Schnabel Method

• For Open populations:

• Multiple capture and recapture - Jolly-Seber method

• How do we know that a population is decreasing and in need of protection?

Long-term monitoring! Increase or decrease?

Survey Data