Population Growth, p. 1
POPULATION GROWTH
POPULATION GROWTH MODELS AND METHODS TO MEASURE
Biggest environmental problems is than of human population growth
What results in a population change in numbers:
births - natality
deaths - mortality
immigration - persons moving in (migrating)
emigration - individuals moving out
MATHEMATICAL MODEL FOR POPULATION GROWTH (pg. 118-120)
Under optimum conditions populations increase with time
one might think this is a gradual linear growth
In reality it's a geometric growth
to 2 to 4 to 8 to 16 to 32 etc.
with time the increase in number is quicker
at some point the model suggests growth is infinite
POPULATION GROWTH MODEL IS BASED ON TWO BASIC FACTORS (p. 118)
Biotic Potential - biological potential for reproduction
* age at reproduction
* number of offspring per reproduction
* portion of life in reproduction
Each species is different in their potential to reproduce
* cockroaches can lay 1000 eggs at a time
* elephants typically have only one at a time
Under ideal conditions will get exponential growth
variation between species as to the actual time (cockroaches vs elephants)
is the growth in hours, days, years or decades?
Population Growth, p. 2
In reality environmental conditions will limit population growth
* food, water, shelter become limiting
* we say a species has reached their Carrying Capacity
* may overshoot, but will drop back to limit
Also, environmental conditions may change with time
The above information can be put into a graph (fig 6.4, p. 119)
FACTORS INFLUENCING POPULATION GROWTH (pp. 120-126)
Density Independent Factors (regardless of pop density)
* weather and other abiotic environmental conditions
* biotic potential of the species
* a species living in a non-optimum environment
Density Dependent Factors (depends on pop density)
* predation & parasitism (other species)
* competition (influence of own species)
* food, water, shelter
THEORETICAL METHODS TO MEASURE POPULATIONS
a) Population Growth (already mentioned above)
* measure population at various times to count
* good if can do under controlled conditions
- get ideal growth
- can then compare to real world
* problems if:
- hard to count numbers
- slow population growth
b) Survivorship (p. 122-123 + fig 6.6)
* count all born in a short period of time
- follow through life till last dies
* typical to have 4 types
(a) few deaths at young age, many live to old age (ex. humans)
Population Growth, p. 3
(b) equal chance of death at any age (ex. birds)
(c) some deaths at young age, many live to old age (ex rabbits)
(d) most die at young age, the few that survive live long time (ex. oak trees, insects)
* good for study of populations which you can follow deaths
* problem if long life span
c) Age Structure distribution (p. 142 + fig 7.11)
* age all individuals in an area
* great if only a short period of time
* problem if hard to age individuals
APPLICATIONS TO CONSERVATION BIOLOGY (pp. 126-128)
POPULATION GROWTH ON ISLANDS
islands have long been known to have fewer species than mainlands ... why?
studies have been conducted in several places:
1) islands off California coast - with birds
2) islands in Caribbean - mammals and amphibians
3) islands off of keys in Florida - insects
Realized two major factors influence population size on islands:
1) size of island - larger means more spaces for individuals to live
a) diversity of habitats (number)
b) size of habitats (area)
2) distance from mainland - greater distance means harder to get there
The combination is important
a) large island near a mainland have more species
b) small island far from mainland has fewest species
Implications to numerous other ideas
1) typical island populations small that a mainland - colonized by few species
these individuals are typically not the average of the mainland species (size, color, etc)
the result is the small population colonizing an island
may result in genetic differences from the mainland = founder effect
(note fig 6.12)
Population Growth, p. 4
2) islands don't have to be oceanic - they can be islands of habitat
a) woodlots surrounded to fields (island to a squirrel)
b) mountain tops in South America over 10,000 ft
(some species can only survive in area above 10,000 ft)
c) high oxygen streams in mountains connected by low oxygen rivers nearer coast
(trout, dace require the oxygen)
The differences must be taken into account as we look at rare species management
a) Cheetahs in Africa lack genetic variation as if all were cousins (p. 127)
b) In the USA we have developed mechanisms to connect the various islands through
corridors (see p. 128)