Download Population Ecology

Population Ecology
From Chapter 5.
I. Evolution can be defined as a genetic change in a population of organisms that occurs over time.
A. Charles Darwin proposed the theory of evolution by natural selection, which is based on
four premises.
1. Overproduction. Each species produces more offspring than will survive to
maturity.
2. The individuals in a population exhibit inheritable variation in their traits. The
discovery that genes consisting of DNA nucleotide base sequences serve as the
units of that inheritable variation represents one of the great triumphs of biology
in the latter half of the 20th century. We also now know that accidental,
random alterations in DNA such as base insertions, deletions, or alterations
provide the mutations that are the source of the variation which make
evolution possible. Three generalizations regarding mutations can be offered:
a. Many mutations are selectively neutral, conferring neither an advantage nor a
disadvantage to the organisms that carry them. For example, many
common alterations in DNA bases making up a gene do not result in a
change in the protein for which that gene codes. This is because the
four bases, abbreviated G, C, A, and T (or U in RNA) contained in
DNA are read in groups of three (called “codons”) in determining the
identity of each amino acid to include in a protein. Although there are
64 (i.e., 43) possible different three-letter combinations of these four
letters, they code for only 20 different amino acids. The code,
therefore, contains redundant codons, different triplets of bases that all
code for the same amino acid. For example, the amino acid serine will
be inserted into a growing protein whenever any of the following
codons are translated during protein synthesis: UCU, UCC, UCA,
UCG, AGU, AGC. A mutation that changed a codon from UCC to
UCG would be selectively “silent”.
b. Far more mutations are deleterious than are advantageous to the organisms
that possess them. This is a simple consequence of the fact that
random changes in a complex system are terribly likely to disrupt the
system, in much the same way that random changes under the hood of
a car are unlikely to improve engine performance.
c. However, advantageous mutations do occur and the lucky mutants that
possess them enjoy a selective advantage over other members of their
species. The occurrence of advantageous mutations is most easily
observed in cultures of bacteria maintained for many generations under
limitation by a single nutrient. One average, it takes about a month
before a mutant appears that has measurably superior ability to take up
the nutrient at low concentrations.
3. Limits to growth. Organisms compete with one another for the resources needed
to survive.
4. Differential reproductive success. Those individuals with the most favorable
combination of traits are most likely to survive and reproduce, passing those
genetic characters on to the next generation.
B. Evolution proceeds more rapidly when an organism has a mechanism (like sex) for genetic
recombination that allows an individual with two favorable mutations to be rapidly
produced from two individuals each with a single favorable mutation. Sex (involving
meiosis) is not the only means of genetic recombination that life forms have devised.
Many bacteria can import naked pieces of DNA and integrate them into their
chromosome. Many bacteria have little circles of DNA called plasmids that seem
designed to be copied and transferred to other bacteria.
C. A vast body of evidence supports evolution, including observations from the fossil record,
comparative anatomy, biogeography, and molecular biology. In addition, evolutionary
hypotheses are tested experimentally.
II. The Kingdoms of Life (p. 86) A natural, taxonomic level exists that is higher (older, or more
fundamental) than kingdoms such as green algae and plants, animals, or fungi. This level has
been called the Domain. So far, three domains have been discovered:
A. Eukarya (from a Greek roots meaning “true nuts” or “true kernels”)
1. This domain includes all life forms, like us, with most of their DNA packaged in linear
chromosomes contained within a membrane-bound nucleus.
2. Most, but not all, eukarya have organelles such as mitochondria or, if photosynthetic,
plastids (e.g., chloroplasts). Mitochondria and plastids contain additional DNA
arranged as a single closed circle.
3. Examples of “eukaryotes” include us, and our distant cousins like brewer’s yeast,
slime molds, sponges, nori (the red algae wrapped around sushi), and saguaro
cacti, as well as our close cousins like iguanas, Siamese fighting fish, canaries,
and gerbils. Many of the tests based on sequencing large information-rich
molecules (e.g., DNA and RNA) in cells that were used to detect the existence
of the different domains would be hard pressed to find any difference between
us and chimps.
4. Contains at least the following separate kingdoms: Animals, green algae and plants,
true fungi, the yellow and brown algae (Chromista), and a disputed number of
protozoan kingdoms including a few primitive ones that lack mitochondria.
B. Bacteria, called eubacteria on p. 86.
1. Lack nuclei and have most of their DNA packaged in a single, closed circle, which is
still called a (or “the”) chromosome.
2. Lack organelles.
3. Have lipids similar to those of Eukarya in their cell membranes.
4. Ribosomes are smaller (70 s) than those of most Eukarya (80 s)
5. Major kingdom-equivalent groups of bacteria include the cyanobacteria and
prochlorophytes (some of which became chloroplasts) which together invented
O2, the Gram-positive bacteria that include many successful soil-dwellers as
well as human pathogens (staph, strep, leprosy, tuberculosis), and the “purple
bacteria” (Few are actually purple) or proteobacteria including plenty of
pathogens (black plague, Salmonella), Escherichia coli (the bacteriologist’s
lab rat), and a number of isolates important in the great elemental
biogeochemical cycles of nitrogen, sulfur, and iron (and other metals).
C. Archaea, called archaebacteria on p. 86.
1. Lack nuclei and organelles as bacteria (Negative characters.)
2. 70 s ribosomes like bacteria
3. Cell membranes contain lipids utterly unlike those of other lifeforms.
4. One kingdom-equivalent grouping includes the bugs that produce swamp gas
(methane), and another includes sulfur-transforming chemoautotrophs that form
the basis of the food chain around deep-sea hydrothermal vents. A great many
of these organisms prefer to grow under conditions of high temperature (near
boiling) and strict absence of O2 strongly resembling those of the young
biosphere.
D. To detect the ancient relationships that unite archeans into domain separate from that of
bacteria taxonomists used “molecular clocks”. A molecular clock is a macromolecule
common to all the organisms to be compared that accumulates mutations at a
“moderately slow” rate relative to the time period over which the clock is to be used.
The are three “structural” pieces of RNA in the ribosome, a structure used by every
known life form to make proteins. The middle-sized or “small-subunit” (SSU) piece is
the most popular molecular clock for looking backwards over billions of years. To
determine the degree of relatedness of two organisms, their SSU rRNAs are sequenced
and compared. If the sequences are very similar, the organisms are more closely
related than if the sequences differ greatly. Naturally, many additional clocks have been
identified and sequenced to confirm and refine the evolutionary family trees identified
with SSU rRNA.
III. Predation is the consumption of one animal or microbial species (the prey) by another (the
predator). When the consumption is of a plant by an herbivore, terms like “grazing” or
“browsing” are used instead of predation. However, many protozoa “graze” on bacteria, even
though these protozoa are also called predators of bacteria.
A. During coevolution between predator and prey, the predator evolves more efficient ways
to catch prey, and the prey evolves better ways to escape the predator.
B. Two effective predator strategies are pursuit and ambush.
C. Adaptations that protect plants from being eaten include spines; thorns; tough, leathery
leaves; and protective chemicals, e.g., milkweed Fig. 5-3.
D. Strategies that help animals avoid being killed and eaten include flight, association in groups,
and camouflage, as well as a variety of mechanical and chemical defenses (e.g., Fig. 53 and arrow-poison frogs). Animals that possess chemical defenses often exhibit
warning coloration. Other animals, lacking chemical defenses sometimes mimic the
coloration of those that do, e.g., the Viceroy and the Monarch butterflies.
IV. Symbiosis (Gr. “living together”) is any intimate association between two or more species.
A. In a mutualism, both partners benefit. Three examples of mutualism are nitrogen-fixing
bacteria (rhizobia) and legumes, zooxanthellae and corals, and mycorrhizal fungi and
the roots of plants.
B. In commensalism, one organism benefits and the other is unaffected. Two examples of
commensalism are silverfish that are camp followers of no particular benefit to army
ants, and epiphytes (incl. many orchids and bromeliads) and larger plants.
C. In parasitism, one organism (the parasite) benefits while the other (the host) is harmed. One
example of parasitism is honeybees and mites. Some parasites are pathogens that cause
disease. One example of a pathogen is the bacterium (Agrobacterium tumefaciens,
one of the purple bacteria) that causes crown gall disease in many plants.
V. Competition occurs when two or more individuals attempt to use an essential common resource
such as food or mineral nutrients, water, shelter, living (or nesting/breeding) space, or sunlight,
i.e., space-based or resource-based competition.
A. Competition can occur among individuals within a population (intraspecific competition)
or between species (interspecific competition).
B. Interactions among species competing for the same resources may be subtle, indirect, and
difficult to assess. For example, direct observation of plant roots competing for
nitrogen is usually impossible.
C. Competition for mates, a strictly intraspecific form of competition, constitutes a potent
selective force for many eukaryotes.
VI. An organism's ecological niche includes its distinctive lifestyle and its role in the community. The
ecological niche takes into account all aspects of the organism's existence-that is, all of the
physical, chemical, and biological factors that the organism needs to survive, remain healthy,
and reproduce. Where can it live, and how does it earn its living?
A. Organisms are potentially able to exploit more resources and play a broader role in the life
of their community than they actually do. They could live in a broader range of
climates, elevations, and habitats than where they actually occur. Many animals could
eat a broader array of plants or animals than they do.
1. The potential ecological niche of an organism is its fundamental niche, whereas the
niche an organism actually occupies is its realized niche.
2. Interspecific competition is an important biological determinant of a species' realized
niche.(Fig. 5-10)
B. Many ecologists think that no two species can occupy the same niche in the same community
for an indefinite period of time. (Other ecologists suspect that the idea of niche is
defined in such a way as to guarantee that only one species can occupy one.)
1. Limiting factors (environmental resources such as the mineral content of soil,
temperature extremes, and amount of precipitation) tend to restrict an
organism's ecological niche.
2. In competitive exclusion, one species excludes another as a result of competition for
limited resources. Gause's experiment with two species of Paramecium (Fig.
5-11) is a classic study of competitive exclusion.
3. Some species reduce competition by resource partitioning, in which they evolve
differences in resource use. MacArthur's study of North American warblers is a
classic example of resource partitioning.
From Chapter 8.
VII. Population ecology is the branch of biology that deals with the number of individuals of one or
more selected species that are found in an area and how and why those numbers change over
time.
A. Populations have certain properties that individual organisms lack, such as population
density, birth and death rates, growth rates, and age structure.
B. Population density is the number of individuals of a species per unit of area (e.g., for plants)
or volume (e.g., cells/ml for bacteria) at a given time.
C. On a global scale the growth rate (r), the rate of change in a population's size, is due to two
factors, the birth rate (b) and the death rate (d).
1. On a global scale (when migration is not a factor),
r = b - d.
2. Populations increase in size as long as the number of births is greater than the
number of deaths.
D. In addition to birth and death rates, migration must be considered when examining changes
in local populations.
1. Emigration (e), the number of individuals exiting an area, and immigration (i), the
number of individuals coming into an area, affect a local population's size and
growth rate.
2. For a local population (where migration is a factor),
r = (b - d) + (i - e) = b + i - d - e.
VIII. Biotic potential is the maximum rate at which a species or population could increase in number
under ideal conditions.
A. Although certain populations, including ours, exhibit exponential population growth for
limited periods of time (the J curve), eventually the growth rate decreases to around
zero or becomes negative. Exponential growth simply cannot last forever.
B. Population size is modified by limits set by the environment, which are collectively called
environmental resistance. Environmental resistance includes the limited availability of
food, water, shelter/nesting sites, and other resources, as well as limits imposed by
disease and predation.
C. For any particular species, the carrying capacity (K) of the environment is the largest
population that can be maintained for an indefinite time by a particular environment.
D. The S curve (also called the logistic or sigmoidal growth curve) shows an initial lag
phase (when the population is small, and growth is absent or too small to notice),
followed by an exponential phase, followed by a leveling phase as the carrying capacity
of the environment is reached. The S curve is an over- simplification of how most
populations change over time. Although conditions in a laboratory experiment can be
devised to force bacteria to exhibit logistic growth, more commonly their populations
do not “coast into” the carrying capacity as gradually as a logistic curve does. Instead
growth looks exponential almost until the population reaches K and then the curve just
flattens out, as though it had hit a wall.
IX. Environmental factors limit population growth.
A. Density-dependent factors regulate population growth by affecting a larger proportion of
the population as population density rises. Predation, disease, and competition are
examples.
B. Density-independent factors limit population growth but are not influenced by changes in
population density. Hurricanes and fires are examples.
C. At one time, the competitive exclusion principle was interpreted to mean that the number of
species that can coexist in a stable equilibrium (i.e., in the absence of population cycles)
cannot exceed the number of density-dependent factors limiting their populations. This
assertion seems to hold true in specialized laboratory apparatus capable of housing two
or three microbial species at equilibrium, but does not seem terribly useful for real
ecosystem, where seasonality usually violates the “stable equilibrium” required by the
assertion. Experiments with habitats engineered to possess a single limiting factor can
maintain two (and possibly more) species in an oscillating equilibrium if the limiting
factor is supplied in a seasonally varying fashion.
X. Each species has its own life history strategy.
A. An r strategy (employed by an r-selected species) emphasizes a high growth rate. These
organisms often have small body sizes, high reproductive rates, and short life spans, and
they typically inhabit variable environments, such as river flood plains or frequently
burned areas. Little investment is made in each individual seed, egg, or larvae by rselected species which instead specialize in the production of vast numbers of offspring
or propagules. Species with an r strategy are rarely very effective at competing for
scarce resources.
1. When old fields are abandoned, r-selected trees and bushes (e.g., birches) are the
first to appear because they excel at rapid and wide dispersal of their seeds.
2. In lawns, r-selected trees are desirable, because they cast a dappled shade.
3. Many of the r-selected relatives of goldfish lay many hundreds or even a few
thousands of eggs in weeds during one spawning and them swim away and
leave them behind.
B. A K strategy (employed by K-selected species) results in maintenance of a population near
the carrying capacity of the environment. These organisms often have large body sizes,
low reproductive rates, and long life spans, and they typically inhabit stable
environments, in which they are superb competitors for resources. K-selected species
need not be terribly efficient at wide dispersal of jillions of propagules. Instead a small
number of propagules or progeny are produced but they are given every possible
competitive advantage by their parents.
1. Trees of mature forests that cast very black shadows (indicating that they are superb
competitors for light), such as beeches and hemlocks, are K-selected.
2. Virtually all of the cichlid fishes of the African great lakes (such as Victoria as
mentioned in the Chapter 5 intro) are powerfully K-selected. They may
produce only a dozen very large eggs per spawning. One or both parents
frequently incubate the eggs in the mouth until they have hatched, and continue
to protect they young fry for the first few, most vulnerable weeks of life.
3. We are about as K-selected a species as you can imagine. Fortunately! You think
we have population problems now? Imagine what the world would be like if
we were r-selected, and parents routinely had 500 kids that reached maturity at
age 5.
(See Table 8-1 for r- vs. K- strategy comparisons.)
C. Many species possess a combination of r-selected and K-selected traits, as well as traits
that are characteristic of neither r selection nor K selection. For example, many aphids
are:
1. Tiny (disposable) º r selected
2. Viviparous (live bearing) most of the year º K selected
3. Many bear 10-14 young per day º r selected!!
4. Wingless as adults, usually, º K selected
5. Mostly parthenogenic (Gk. “virgin birth” = self cloning) º r
6. These girls are clearly boom-and-bust specialists, but some aspects of their life
cycles surely look K selected.
D. There are three general types of survivorship curves. (See Fig. 8-7 BUT WATCH THE
AXES!)
1. Type I survivorship, in which death is greatest in old age, is typical of K-selected
species in animals. The text offers humans as a species which exhibits Type I
survivorship. That may even be a fair statement for humans with the good
fortune to be born in rich countries in modern times.
2. Type III survivorship, in which death is greatest among the young, is typical of
r-selected species in animals.
a. The text cites oysters as an example of species with Type III survivorship.
b. Humans in poor countries have a survivorship curve that looks pretty TypeIII-like for the first few years.
3. In Type II survivorship, death is spread evenly across all age groups.
a. Fig. 8-8 shows a rather Type II-like survivorship curve for herring gulls after
an initial drop reflecting a fair amount of chick mortality.
b. Go to any graveyard dating from the 1700s or 1800s in Massachusetts and
you will see evidence that there was a heck of a downward jerk in the
human survivorship curve produced by the high maternal mortality rates
of that time. Historically we may have looked a little more Type II than
we do now.
c. In Figs. 8-7 and 8-8 we see straight lines but in both cases the vertical axes
are logarithmic. If you plotted those straight lines on ordinary arithmetic
axes, you would see exponential decay curves. Organisms with Type
II survivorship curves essentially have half-lives, just like radioactive
elements do.