Population and abundance PART 2

Population demographics:
study of the size and age structure of the population of interest and the changes in the population over time.


interested in rates of change (r)
births (b) and deaths (d): r = b - d (rate of change)

Types of population growth models:


Density independent:
1. simplest models
2. assume change in pop. size is not constrained by any environmental factor


Density dependent:
1. incorporate constraints to population growth

Factors limiting populations:


Environment:
1. physical characteristics of the environment
2. Availability of suitable habitat


Biological interactions:
1. food supply
2. abundance and distribution of predators

Fundamental Niche:
defined by the range of abiotic factors within which a species can live and reproduce


Realised Niche:
the actual environmental range in which a species lives and reproduces.

Body size and population density:
Log (population density) = a + b x Log [body wt]


e.g. 85g quenda = 1.3 - 0.66 x log(0.085) = 1.3 + 0.707 = 2.007
10^2.007 = 102 individuals per km2

Limitation by predators (kangaroos):

  1. water - regular source has been available for livestock
  2. food - trees have been cleared and area of grasslands/pastures has been increased
  3. predators - dingos have been shot and poisoned, removing a major predator

Limiting factors:
1. populations are limited by a variety of different factors that vary between species
2. human activity has modified limiting factors (e.g. water, habitats, etc.)


Knowledge of limiting factors can be used to:
1. increase the population of rare and endangered species
2. reduce the abundance of pest species
3. manage sustainably the species we harvest

Demography:
study of the size and structure of populations and changes in populations
1. many of the techniques originate from the insurance industry
2. life tables in insurance


Age structure of the population is important
1. determines the rates of change in populations

Survivorship curves:
1. Type 1- good survival from birth to dependency, high survival till towards the end of life, low survival at the end (mammals)

  1. Type 2- linear decrease in survival through life (birds)
  2. Type 3- low survival early in life, good survival after juveniles (fish)

Demographic traits reproduction:
1. r selected (short-lived species e.g. anchovies)
-greater reproductive output
-low egg and larval survival
-juvenile survival predictable
-low adult survival
-short-lived and early maturity


2. k selected (long-lived species e.g. whales, dolphins)
-lesser reproduction output
-better survival
-juvenile survival variable
-long-lived and late maturity

Demographic traits survival/mortality:
1. mortality is a key driver of population change
-varies greatly with stage of development
2. life tables a way of summarising age specific mortality in a population
3. produce a life table by:
-take a cross-section of the population at one point in time
-follow the age structure of the population over time

Life history tables:
1. help us understand the current status of the population

  1. the potential response of the population to change in the environment or disturbance
  2. strategies for releasing rare and endangered species back into the wild

Significance of age data:
1. mortality/survivorship recorded directly for a cohort followed through time
2. age at death recorded
3. age structure recorded directly

Growth in an unlimited environment (density - independent growth):
1. an animal/plant in an environment with unlimited resources and no competitors or predators will keep increasing
2. size of population depends on - size at beginning, birth and death rates an Nt = rNt-1
3. exponential growth equation

Assumptions of the exponential growth model:
1. population is closed - no immigration or emigration
2. populations grow without any constraints or limits to their growth
3. the rates of births and deaths are constant

Logistic models:
1. logistic growth models account for an upper limit to population growth
2. they area modification to the exponential model
3. logistic models must include - population size at any instant in time and reproductive rate is relative to the population size


carrying capacity (K) = the max no. of individuals that can be supported in the current environment

logistic equation:


dN/dt = rN( K - N / K)
dN/dt = rate or population growth at an instant in time
N = population size
K = carrying capacity
r = instantaneous rate of increase in population

Assumptions:
1. mortality entirely dependent on density
2. birth and mortality are continuous functions
3. the environment is constant
4. individuals in the population react in the same way to changes in density

Carrying capacity for earth:
carrying capacity = (ha land) x (yield per ha) x (kj per crop unit) / no. kj needed per person per year