Notes towards Conservation Biology Chapter 6
Introductory/Title Slide (1)
Hello. My name is Gwen Raitt. I will be presenting this chapter
Mechanisms of extinction act at the population/metapopulation level (Barbault &
Sastrapradja 1995).
Populations
A population is a group of individuals of a given species living in a specific geographic
area at one time (Miller 2002, Wikipedia Contributors 2006a). Population size and
survival depend on: the availability of resources such as food, clean water and clean air
(Barbault & Sastrapradja 1995, Miller 2002), the amount of suitable habitat available
(Barbault & Sastrapradja 1995), the amount of predation/parasitism (Barbault &
Sastrapradja 1995, Dobson 1996), the prevalence of disease and finally social interactions
(Barbault & Sastrapradja 1995). Clockwise from the top, the pictures show Cape Gannet
(Morus capensis) behaviour, the deer tick (Ixodes scapularis - left to right: larva, nymph,
male & female) and a cheetah (Acinonyx jubatus) with its kill.
Mechanisms of Extinction in Single Populations
Population extinction is certain if, in the long term, the mortality rate is higher than the
birth rate (Barbault & Sastrapradja 1995). Extinction mechanisms act by raising the
mortality rate, lowering the birth rate or doing both (Barbault & Sastrapradja 1995,
Dobson 1996). The mechanisms may be grouped into four categories for single
populations (Barbault & Sastrapradja 1995). Firstly, Allee effects - decreasing the
population density below a critical threshold results in decreased interaction between
individuals and hence a decreased birth rate (Barbault & Sastrapradja 1995, Dobson
1996). Secondly, chance (stochastic) variation occurs in birth and death rates, affecting
the population size. This is known as demographic uncertainty (or stochasticity). Very
small populations are vulnerable to extinction caused by demographic uncertainty.
Thirdly, environmental uncertainty reflects the effects of chance changes in the
environment in which the populations occurs (Groombridge 1992, Barbault &
Sastrapradja 1995, Dobson 1996). This includes such unpredictable events as ‘natural’
catastrophes (Groombridge 1992, Dobson 1996) which may be aggravated or caused by
human behaviour (Brown 2001, Pauchard et al. 2006). The picture is of a flood in
Mozambique – the houses conveniently show that the water is above its normal level.
Finally, loss of genetic diversity (biodiversity loss) affects the chances of population
extinction (Groombridge 1992, Barbault & Sastrapradja 1995). One impact of loss of
genetic diversity is the reduced fecundity and viability known as inbreeding depression,
which may occur in the offspring if closely related individuals mate. Another impact is
the reduction of genetic variability in small populations due to genetic drift (changes in
allele frequencies) (Barbault & Sastrapradja 1995).
Mechanisms may interact, compounding the effect on the population (Groombridge
1992). Population size is critical to survival (Barbault & Sastrapradja 1995).
Human impact populations by causing three of the categories to occur individually or in
combination: reducing population density (i.e. increase the likelihood of Allee effects),
changing the environmental conditions (e.g. increasing the likelihood of and power of
natural disasters (Brown 2001)) and forcing the loss of genetic diversity.
Metapopulations
A metapopulation is made up of a number of spatially separated, extinction-prone local
populations (or subpopulations) that are linked by migration (Groombridge 1992,
Barbault & Sastrapradja 1995, Wikipedia Contributors 2006b). Other than the classical
metapopulation, the following types are recognized: mainland-island metapopulations
have at least one large stable population that is not likely to become extinct which
provides immigrants to other habitat fragments that may be more extinction prone
(Barbault & Sastrapradja 1995), source-sink metapopulations occur if some populations
have a growth rate that exceeds the capacity of the habitat forcing emigration to other
populations (Groombridge 1992, Barbault & Sastrapradja 1995) and non-equilibrium
metapopulations are the result of recent habitat fragmentation and may not survive as no
equilibrium exists between colonizations and extinctions and the development of such an
equilibrium is not guaranteed (Barbault & Sastrapradja 1995).
Metapopulation survival depends on: local population survival, unoccupied suitable
habitat at suitable distances (i.e. within migration distance of occupied habitats) and
sufficient migration for colonization of unoccupied habitat to occur (Barbault &
Sastrapradja 1995). The picture shows a Jackass Penguin (Spheniscus demersus)
subpopulation (colony).
Mechanisms of Extinction in Metapopulations
Extinction of a metapopulation is certain if the extinction rate of local populations
exceeds the rate at which new populations are established. Local population extinction
mechanisms are those of single populations (Barbault & Sastrapradja 1995). The
mechanisms acting at the metapopulation level may be grouped into two categories.
Colonization-extinction uncertainty is analogous to demographic uncertainty for single
populations. This is a threat if the network containing the metapopulation only has a few
habitat patches and the local populations have a high risk of extinction (Barbault &
Sastrapradja 1995). Regional uncertainty is equivalent to environmental uncertainty for
single populations. The risk of extinction by regional uncertainty decreases as the
distance between subpopulations increases (Barbault & Sastrapradja 1995).
Humans impact metapopulations at the local population level and change regional
conditions.