CHARACTERISTICS OF A
POPULATION:
Definition: a
population is:
*a group of individuals of one species,
interbreeding
*in a given area (though the size and nature of
the area is arbitrarily defined for the purposes of the
study undertaken)
*whose structure is dynamic -
Why do we need to know the characteristics of populations?
read along.....Perch problems: sorting out a disturbing
trend
Lake Michigan's threatened perch population is the subject of an intensive study by Wisconsin Sea Grant fisheries' scientist Fred Binkowski. A few summers ago Binkowski and his colleagues spent a month off Milwaukee's shores catching more than 14,000 young yellow perch to determine why they are not surviving long enough to join Lake Michigan's adult population.
Initial results were alarming. Of the 14,000 fish captured only 46 of the fish were female while the majority of males were eight or nine years old.
The researchers theorized the seven or eight years of bad breeding may be the result of a commercial and sport fishery operating in Lake Michigan waters during that time. Since female fish grow at a greater rate than their male counterparts, they would have been more likely to have been caught by fishery nets.
Continuing their investigation, Binkowski and his group began lab-raising Lake Michigan Jude/Center for perch from three other locations where fish appear to be in good health. Through observing Aquatic early life stages of the perch through to reproductive maturity, Binkowski hoped to map perch development thereby pinpointing the time when fish switch from feeding on small organisms to larger food particles. This change in behavior may be the key to future fish survival.
Binkowski reported Lake Michigan fish take twice as long to proceed to the next feeding level.
"This could be connected to a slower growth rate," he said. "Although this apparent slower growth rate could be normal for the Lake Michigan perch, it could also be the result of something interrupting their normal development. The problem won't be solved quickly. We've allowed this thing to do on for seven years, and it's not going to be turned around in one year.
If you were doing this study, what
characteristics would you investigate? Remember you are on a limited
budget and have adequate support to only study 4 characteristics. Which
four of the listing below would you choose?
Characteristics: Spatial structure
1. Would it be important to study their Density ? (Number of
individuals per unit area):
Why is is density important?
Is there a maximum number?
a. competition for food
b. competition for territories
c. degeneration of environment. by fouling
d. attraction of predators including parasites
e. niches overlapping with other species' competitors
f. increased time in social interactions
g. coevolutionary dependents
Is a Minimum Number is required?
a. Enough mates to go around
b. Enough to deal with fluctuations in density
c. enough genetic variability
How would you measure
this characteristic?
2. Would it be important to study their Distribution? :(The
geographic and ecological range)
*Is a suitable habitat present?
*Can individuals of the species migrate there?
Starlings were introduced in the US
1890 and 91 just 160 were released and accounted for
60 years later millions exist over the whole country
How would you measure this characteristic?
3. Would it be important to study their Dispersion? (Spacing of
individuals with respect to one another)
*Clumped; what are the gains and losses associated with 'groups' or clumps of individuals?
*Random spaced - no set pattern
*Spaced (hyperdispersion), meaning they are regularly spaced or evenly distributed ...Why?
How would you measure this characteristic?
Characteristics: Spatial and Temporal Variations
Would it be important to study their Age Structure: relative
frequencies in each age class
Why is age structure important: It is a....
a. Reflection of environment. perturbations - disturbances open up
space etc., release resources only to shut down supplies later
b. Effects the breeding structure: for example in countries where there is a
very large pre-reproductive population, even if ZPG ( zero population growth
occurs, with couple only producing 2 offspring, the population will still expand
in the near future)
c. Effects the competition when size and age are correlated: in all organisms
in which growth is developmentally regulated ( i.e.. we stop growing length
wise after 20 though this is not true for many reptiles, perennial plants etc.)
a high load of older types may prevent the growth of younger individuals.
How would you characterize an age
structure?
2. Would it be
important to study their Sexual
Structure?
- according to Fishers Theorem the most
'efficient' sex ratio should be
a. 50:50 ratio both sexes have equal contributions with respect to
their genetic progeny to the next generation, i.e. too many males
- fewer females contribute more of their genes, etc.
But nature being what it is - we find a number of exceptions to
the expectation-
- a.* Fish which change sex with age and
Plants which change sex with age
b.* Ramets vs. genets; if a population contains both sexes ( i.e.. dieocious plants) and can reproduce asexually, then sex ratios may be skewed if the either sex is more successful in the given environment...
A human example now in the making is female infanticide 'prevalent' through China, India and some other eastern countries. Here, high population density has lead to fewer offspring. As male offspring are critical for parental survival in later years, parents are using scans to determine embryo sex and whether to abort their offspring early on. This has resulted in a reduction of young females in these countries, limited selection when mating/marriage occurs later in life. What other problems may occur over time?
How would you measure this characteristic?
3. Would it
be important to study their
Dispersal
?
movement of individuals between populations and their movement
between local areas within populations
Emigrate (leave)
A---------------->
<----------------
Immigrate (enter)
How important are 'outside; genetic contributions to the
population?
a. Large implications for genetic structure ........if a population is small in size, and no new individuals are recruited then it will most likely become Inbred. In a stable environment inbreeding may maximize fitness, but in an varying environment it may be disastrous Generally to keep genetic diversity high requires only a few individuals to immigrate per year..
b. Massive dispersal - If resources are limited then large influxes or losses may have impact on how resources are distributed and implications on survival and competition..
How would you measure this characteristic?
4.
Would it be
important to study their Genetic
Structure:
high or low genetic diversity?
implications on status dependent on whether the environment varies
or not
Stable environment. vs.. Varying environment- important to note here is, is
change predictable or not... how would this impact on the necessity of genetic
variability..
Supergenes- are cluster of genes which are related in terms of functioning...
highly adapted individuals when bred within a diverse population may 'lose'
their collective gene structure making their offspring less well adapted...
how can they maintain their integrity in such a setting?
How would you measure
this characteristic?
5. Would it
be important to study their Size
Structure??- that is the
distribution of sizes of individuals in the population
itself:
important in long lived organisms in terms of
who gets the resources Part A. If you were
doing this study, what characteristics would you go
after? Remember you are on a limited budget and have
adequate support to only study 4 characteristics. Which
four of the would you choose? Your answer is? Justify
it...
If all the same size, all may die out together since individuals can't
compete selectively- no dominants- we see this phenomenon in planted forests
or even aged forests ... without "selective cutting" the population as a whole
may suffer..There are many recorded instances where even age populations which
have arisen through major disturbances, die out as a whole in a short period
of time.
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