Transcript Ch 25: Extinction + Conservation

Fall 2009 IB Workshop Series
sponsored by IB academic advisors
Preparing for
Graduate School
Thursday, Oct. 1
4:00-5:00pm 135 Burrill
Learn about the ingredients for deciding whether and when to go
to grad school. Also covered will be the timeline for the GRE and
application and admission process.
Ecological footprints of some
nations already exceed available
ecological capacity.
Our ecological ‘footprint’…
1) meet with CERC director
about energy usage
2)
Conservation Biology IB 451
- Every other year in the spring
- Next time it will be taught is spring 2011
Ch 26: Biodiversity, Extinction +
Conservation
Objectives
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Types of biodiversity
Values of conserving biodiversity
Causes of extinction
chance
deterministic factors
small population size
Conservation of single species
population bottleneck + genetic diversity
small populations + inbreeding depression
documentation of loss of alleles/heterozygosity
method of restoring allelic diversity +
increasing population size
Biological diversity is incompletely
catalogued: 1.5 of 10-30 million!
Components of Biodiversity
• Ecological diversity
• Genetic diversity
• Geographic diversity
Values of biodiversity
•
•
•
•
•
Moral
Aesthetic
Economics
Ecotourism
Indicate
environ.
quality
• Maintain
ecosystem
function
Extinction is forever…
• Background = natural rate (1 sp. / year)
• Mass extinction (up to 95% of all species)
• Anthropogenic (1 sp. / day!)
Deterministic causes of extinctions:
the ‘evil quintet’
1 habitat destruction and fragmentation
(67% of cases)
2 overkill
3 chains of extinction
4 introduced species
5 emerging diseases
Natural fragmentation: extinctions and
recolonizations related to distance to
mainland
1. Habitat reduction and fragmentation
lead to endangered species
Smaller fragments support fewer animals.
Habitat reduction and elimination
• Some habitats are eliminated altogether.
• Fragmentation causes other problems:
•
reduced total area
•
reduced habitat heterogeneity
•
reduced connectivity
•
greater inter-fragment distance
•
unable to migrate with changing climate
•
reduced interior/edge ratio
3. Overkill for non-food item.
3. Overexploitation +
4. chains of linked extinctions
• often changes species composition of a
community
4. Introduction of exotic species--->
• Eliminate native species and alter ecosystem
• Especially vulnerable are islands, aquatic systems
5. Emerging Diseases
Conservation planning: Approach 1
• Focus on ecological requirements and area
needed by individual, often ‘charismatic’ species.
Focus on rare, endangered species.
How is ‘rarity’ defined?
classic
rare sp.
Difference in vulnerability and
conservation plans:
• Small species:
•
small range size
• human population densities-->
• must protect threatened habitat
• Large species:
• intrinsic qualities (long development,
low reproduction, low pop size -->
• concentrate on increasing lx + mx
Small populations at > risk to
extinction via chance events, e.g.
• Demographic stochasticity
• Genetic stochasticity
• Environmental stochasticity and natural
catastrophes
Stochastic population processes produce a
probability distribution of population size.
Probability of stochastic extinction XXXXX over time
(t), but decreases as a function of XXXXXXXXX.
Criteria for long-term survival:
• Have Minimum Viable Population (MVP) =
smallest population that can sustain itself in
face of environmental variation---> avoid
stochastic extinction
• have wide distribution so that local
catastrophe doesn’t wipe out entire
population
• have some population subdivision to
prevent spread of disease
How small is small?
• 50/500 estimate
• 50 short-term: keep inbreeding low
• 500 long-term: allows evolution to occur
without genetic drift
• Effective population size = 11% of actual
population size
How big a preserve is necessary to ensure
MVP?
***What’s main ‘take-home’ message?
100
>100
<15
% pop.
persisting
0
10
Years
50
Population Viability Analysis (PVA):
Put demographic info into model with
stochasticity added -->
Predict probability of extinction within
100-1000 years
Useful only if well-studied species
***What’s main ‘take-home’ message?
100%
No = 60
50 km2
10%
Cumulative
extinction
probability
1%
.1%
.01%
2500 km2 No = 3000
0
Years
1000
Population Bottleneck: period of small pop. size.
…subject to genetic stochasticity
Populations undergoing a population
bottleneck experience founder events and
genetic drift,. Each causes a loss in genetic
variation.
+ genetic
drift
Allele
becomes
fixed = no
variation.
The drift-mutation balance preserves more
genetic variation in large than small populations.
Smaller populations have less minisatellite
variation; it has been lost by genetic drift.
Inbreeding decreases the frequency of
heterozygotes in a population. Allows
expression of deleterious recessive alleles.
Loss of genetic variability has
both qualitative & quantitative
aspects
Qualitatively, specific alleles will either
be lost or retained
Quantitatively, genetic variance (or
heterozygosity) will be lost
Extinction vortex of small populations due
to positive feedback loops.
Population started by 1 pair--> little population
growth. Then new male arrives --> explosion. WHY?
Pedigree shows
high level of
inbreeding in
small wolf pop.
Selfing reduces reproductive fitness.
Population bottleneck. Partial rescue by
immigration from source population.
Bottlenecks will usually have a
greater qualitative than
quantitative impact
i.e., the loss of alleles,
especially rare ones, is much
greater than the loss of
genetic variance (or
heterozygosity) per se
Loss of alleles:
Original number of alleles = 4
allele freq. = p1 = .70 p2 = p3 = p4 =.10
N = 2 (two individuals)
E = # alleles retained
2x#ind
(1- .70)
. = .0081 - little influence
E = 4 - (1- .10) = .6561 (1- .10) = .6561 - large influence
(1- .10) = .6561 E = 4 - (.0081 + .6561 + .6561 + .6561)
= 2.02 alleles left of original 4
Table 1
# INDIVIDUALS
IN SAMPLE (N)
1
2
6
10
50
>>50
AVERAGE # OF 4 ALLELES
RETAINED
P1=.70,
P1=.94,
P2=P3=P4=.10 P2=P3=P4=.02
1.48
2.20
3.15
3.63
3.99
4.00
1.12
1.23
1.64
2.00
3.60
4.00
Two conclusions:
1. More alleles are lost in populations
with small numbers of individuals.
2. Alleles with a low frequency in the
original population tend to be lost
much more easily in the small
population than alleles with high
frequencies.
In the short run, the loss of rare
alleles is probably not very
important, especially in
benign environments.
In the long run, though, such
alleles might be crucial; in an
evolutionary sense.
Table 2
#
founders
1
2
6
10
20
50
100
% original percentage
heterozygosity
lost
retained
50%
50
75
25
91.7
8.3
95
5
97.5
2.5
99.5
0.5
100
0
Changes following the
reduction in size
When numbers are low, a
population is, in effect, going
through a serious bottleneck
every generation, and the
effects are cumulative.
Table 3
% Genetic Variance
(heterozygosity)
Remaining after t generations
Pop
Size (N)
2
6
10
20
50
100
1
5
10
100
75
91.7
95
97.5
99
99.5
24
65
77
88
95
97.5
6
42
60
78
90
95
<<1
<<1
<1
8
36
60
Conclusions:
• Small populations of constant size always
lose heterozygosity through time.
• The smaller the population is, the more
rapidly heterozygosity is lost.
• The higher the number of generations a
population of small size is bred, the more
heterozygosity is lost.
The crucial issue is whether the
population remains small or grows to
a relatively large size.
It is perennial low numbers that erode
genetic variation.
Additional Problems Faced by
Populations of Small Size
• Demographic Stochasticity - wildly
fluctuating probabilities of survival and
reproduction
• Environmental Stochasticity - wipe out
small populations; particularly when there
is only one or few individuals
• Allee Effect - inability of the social
structure to function (e.g., finding mates)