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Population Ecology
Mr. Nichols
PHHS
Population Ecology
• A population is a group of individuals of the same
species that live in a particular area and have the
potential to interbreed.
Flock of Starlings at Dusk – U.K.
Population ecologists are
primarily interested in
a) understanding how biotic and abiotic factors influence the
density, distribution, size, and age structure of populations.
b) the overall vitality of a population of organisms.
c) how humans affect the size of wild populations of organisms.
d) studying interactions among populations of organisms that
inhabit the same area.
e) how populations evolve as natural selection acts on heritable
variations among individuals and changes in gene frequency.
Life History Characteristics:
• Growth
• Change of form
• Dispersal
• Timing of reproduction
• Size at birth or germination
• Number and size of offspring
• Age at death
Life History - Growth
• Growth – for at least part of their life history,
all organisms grow by assimilating energy and
nutrients – final body size species-specific.
Life History – Change of Form
• Change of form - many organisms have
dramatically different forms or stages in their
life cycle.
Life History - Dispersal
• At some time in their lives, most organisms go
through dispersal – enhances reproductive success.
Belding’s Ground Squirrel
Spiders
Milkweed
Life History Characteristics
• Growth
• Change of form
• Dispersal
• Timing of reproduction
• Size at birth or germination
• Number and size of offspring
• Age at death
LIFE HISTORY STRATEGIES (LHSs):
Patterns of lifespan and reproduction
that characterize a species.
LHSs are a result of natural selection,
which acts on individuals, NOT species
Individuals that have a life history that
maximizes fitness will be favored by
natural selection…
…thus, particular patterns of survival
and reproduction will eventually be
shared by all members of a population.
Three Main Life History Strategies:
1) Survivorship
2) Maturity
3) Reproductive Output
3) Reproductive Output
a) Parity
# reproductive episodes in lifetime
Mayfly
Salmon
Agave
Semelparous species
Iteroparous Species
“The fundamental idea that evolution accounts for the
diversity of life is manifest in a broad range of life histories
found in nature.” Based on what you know about
evolution by natural selection, you can predict that species
that have evolved semelparity have done so because:
A) semelparous parents produce more offspring if they invest all of
their resources in reproduction than they would if they saved
enough resources to survive until they can reproduce again.
B) semelparous parents produce offspring that are more likely to
survive than offspring produced by iteroparous parents.
C) iteroparous parents are less likely to provide parental care than
semelparous parents.
D) semelparous parents and iteroparous parents are equally likely to
produce offspring; semelparity evolved for other reasons.
E) iteroparous parents are more likely to die before they can
reproduce than are semelparous parents.
Two factors influence evolution
of semelparity vs iteroparity:
• Survival probability of offspring
• Probability that adults will survive to
reproduce again
Both probabilities are low in harsh or
unpredictable environments, so semelparity
will be favored.
3) Reproductive Output
a) Parity
b) Fecundity
# offspring per reproductive episode
elephants
rodents
spiders
3) Reproductive Output
a) Parity
b) Fecundity
c) Parental Investment
Energetic effort put into offspring:
i) Size of offspring
• Some plants produce a large number of small seeds, ensuring that
at least some of them will grow and eventually reproduce.
•
Other types of plants produce fewer large seeds that provide a large
store of energy that will help seedlings become established.
General Relationship between Offspring Size
and Number of Offspring
Many
Number
of
Offspring
Few
Small
Large
Offspring Size
3) Reproductive Output
a) Parity
b) Fecundity
c) Parental Investment
Energetic effort put into offspring:
i) Size of offspring
ii) Parental care
LHS of a hypothetical “super-organism”?
Real LHSs are compromises in the
allocation of energy!
Reproductive “Trade-offs”:
a) Reproduction vs Future Survival
Reproduction vs Survival (Mortality)
Parents surviving the following winter (%)
How does caring for offspring
affect parental survival in kestrels?
100
Male
Female
80
60
40
20
0
Reduced
brood size
Normal
brood size
Enlarged
brood size
Fig. 53-13
In some bird species, the male provides no care. If
this were true for the European Kestrel, how would
the experimental results differ?
A) Females in all three groups likely would have
the same survival values as in the graph.
B) Males in all three groups likely would have
higher survival % than females.
C) Patterns for both males and females likely
would remain the same.
D) Only females with reduced brood sizes likely
would show a reduced survival.
Reproductive Trade-offs:
a) Reproduction vs Future Survival
a) Reproduction vs Future Growth
b) Current vs Future Reproduction
Annual Meadowgrass
Reproduction vs Future
Growth
Current vs Future
Reproduction
Particular combinations of LHSs often
favored in particular body sizes…
…but there are always exceptions to the rule!
Big Brown Bat (Eptesicus fuscus)
Baby bat
Longer lifespan (14 yrs) and lower fecundity (1-2)
than expected for a mammal of that size (small)
A few, large offspring.
Parental care in carrion beetles;
very unusual for an insect.