Download Ecology Question Set 1

Ecology Question Set 1
Due Friday, October 3, 2003 [10 pt]
Questions and Problems, Ch. 1, #1.9.
Fertility rate as a function of GNP
8.0
7.0
6.0
5.0
Fertility rate
1.
4.0
fertility rate
3.0
2.0
1.0
0.0
50000
45000
40000
35000
30000
25000
20000
15000
10000
5000
0
GNP per person
The relationship between the two variables is not tight; there is quite a range of fertility
rates among countries with per capita GNP less than 5000.
The general relationship is probably caused by several factors. [In terms of grading,
students may not come up with all these factors, or perhaps will think of others, but a fullcredit answer will include three good ideas.] Key is the $$ themselves do not directly
cause the differences in fertility!
 Infant mortality rate is high in these countries, and thus women may have more
children to insure that a few survive.
 Access to birth control is limited or non-existent.
 Perhaps these countries are more likely to have religious practices that prohibit
the use of birth control. However, Italy has the lowest fertility rates while being
primarily a Catholic country, so perhaps this factor doesn’t matter so much.
 Lower GNP countries tend to rely on subsistence living, so that having more
children means that the family will be more productive and be able to live better.
 GNP is often related to level of education, and educated women tend in
“developed” countries tend to delay childbirth, with the result that they have
fewer children.
Variability in the data is probably due to variability in some of the above factors. That is,
a country such as China with a low GNP has policies that reward families that have only a
single child, thus overriding some of the benefits. Also, there may be higher levels of
education and health care in some of the lower GNP countries. Again, China is an
example. Also, GNP of Russia, for example, reflects recent economic issues. In the past,
Russia was (I think?) more prosperous per capita, and their fertility rate may be reflective
of this. Certainly, the $$ per say are not the direct cause of fertility differences.
2.
Questions and Problems, Ch. 2, #2.11.
In general, many ecological processes occur more quickly than evolutionary processes, i.e.
they occur within the lifetime of individuals rather than through several generations. For
example, predation and competition can exert their effects on populations in a matter of
minutes, hours or days, often much shorter than the lifespan of the individuals involved in
the interaction. The additive effects of many ecological processes will result in
evolutionary change.
In some cases, the time scale of ecological and evolutionary processes may be similar.
For example, it takes monarch butterflies several generations to complete a single
migratory route. Depending upon the ecological conditions encountered along each step
and the severity of their effects, evolutionary changes could occur for the monarchs after
completing a single (or perhaps just a few) yearly migrations.
Secular dispersal is another example whereby, by definition, dispersal takes place at the
same rate as evolution.
3.
Questions and Problems, Ch. 2, #2.12.
Evolution is defined as the change in the characteristics of a population over generations.
SO, evolution has occurred if we make the assumption that color is indeed genetically
based, and thus the shift in frequency of color morphs persists into the next generation.
However, there has not been natural selection for the brown color morph, because the
differential survival had nothing to do with the color morph. It was a random act that just
happened to wipe out the one pond and not the other [unless being brown was linked with
the ability to more carefully choose a pond that wouldn’t dry out…]
Note that full credit answers will have some explanation of why you answered the
question the way you did, not just “yes” for the first part and “no” for the second…
4.
Questions and Problems, Ch. 3, #3.3 [See Gaston paper (handout)]
Range size might be considered as “numbers of degrees of latitude and longitude” covered
(Raeka, 1980 as quoted in Gaston, 1991), or similar description of area an area noting the
boundaries of a species (i.e. the state of California). In this case, range appears as a “solid
block of occupancy” (Gaston, 1991) even if the distribution is patchy within that overall
area. Alternately, range size could be considered as the number of occupied quadrats (of a
given size, such as 5º x 5º) or the total area actually occupied within a given range of
latitude and longitude, so that the range size would be equivalent to the sum of the
occupied areas. Note that the first way of measuring this would lead to a larger range size
than the second.
The patchiness of individuals complicates the issue. The degree of patchiness depends on
scale; at a fine enough scale, virtually all organisms are patchily distributed. How you
estimate range size by looking occupied areas will depend upon the size of the patch (or
quadrat) you use to judge emptiness vs. occupancy. The most conservative way to do this
(for an animal) is to look at individual home ranges and then sum those, taking into
account regions of overlap so they are not double (or triple or quadruple) counted. While
this may seem a good measure, the argument against this is that it equates more to
abundance. Critics of this method argue that abundance and range should be assessed
independently.
The measure you decide to use ought to be based on your purpose of making the
measurement. For example, range extent, rather than area occupied, might be most
important when “addressing questions of dispersal, invasion and the dynamics of
geographical range size” (Elton, 1958 as quoted in Gaston paper). Areas of occupancy, on
the other hand, might be useful if you are trying to figure out what aspect of the habitat is
important to that particular species, such as temperature, rainfall, substrate, etc…
Not discussed in the paper, but also important, is how difficult it is to decide where the
edges of the ranges are, since borderline areas can be sparsely populated. Exactly
when/where is “zero” reached?
5.
In Essay 2.1 (p. 26) the author states, “…the host species on the other hand should
evolve the ability to discriminate cowbird eggs from its own eggs.” Comment on this
statement based on the mechanism of evolution by natural selection. Think about
what is necessary for evolution to occur, and what factors (or lack thereof) would
keep evolution from occurring in the direction stipulated.
In order for this occur, there must be some variability in the population with respect to the
ability to discriminate one’s own eggs from cowbird eggs, and this variability must be
genetically based. If none of the individuals in the population have this ability, then the
entire population will be vulnerable. Key: Natural selection can only act on existing,
genetically-based variability. If there was genetically-based variability, then yes, the
discriminating individuals would have more surviving offspring that reach adulthood
(because the parents didn’t waste their efforts and energy on the cowbird young), and their
offspring in turn would be discriminating and thus more successful.
If at some point in time an appropriate mutation happened to occur that allowed for
discrimination, and then presumably, through natural selection, that gene would come to
dominate the population. BUT mere need for the gene isn’t going to magically create it.
6.
Questions and Problems, Ch. 4, #4.1
Distance moved = Dn ln R 0 = 100  50  ln 1000 = 13,141 m
If D(average distance moved) is doubled, the distance moved for the same number of
generations and same reproductive rate is also doubled, to 26,282 m.
If n (the number of generations) is doubled, the result is the same, 26,282 m.
If R0 is doubled, the effect is negligible: 100  50  ln 2000  13,785 m. Even changing
R0 tenfold has little difference. So basically, how many individuals you have is less
important than the average distance they are able to move in a generation and the number
of generations they have undergone that movement.
**Of course, using the average value of D to make the calculation is probably not
appropriate. See Reid’s paradox!
7.
Questions and Problems, Ch. 4, #4.5 [You are not expected to go to the paper listed
in the question (Case 1996). Instead, think creatively and come up with your own
possibilities!]
The limited data provided in Table 4.1 does suggest that invasions are more successful on
islands. For example, according to Table 4.1 the Continental U.S.A. has a rather low
percentage of success (13%) compared to the rest of the examples. On the other hand,
Australia, a continent, has a 33% rate, similar to Great Britain (a large island) and New
Zealand (also a relatively large island). There is certainly a lot of variability in success
among different islands. Unfortunately, sizes weren’t provided in the table, but the
general pattern seems to suggest that area as well as whether a place is an island makes a
difference.
What makes islands different, and thus potentially more susceptible to invasion? First,
while islands vary in size, they are smaller than mainland areas. Just because of this
smaller area, there are probably fewer niches and thus fewer native species. Also, they are
also some distance away from highly inhabited areas, and fewer species make it out to
islands by rafting and other means than can disperse across a contiguous area. (Water is a
key barrier to dispersal.) They also can be somewhat younger than mainland areas, such
as is the case with volcanically-formed islands. These factors, combined, mean that it is
quite likely that an island has fewer species than a similarly-sized chunk of mainland, so
chances are that potential predators, competitors and/or parasites that of an introduced
species are fewer in number.
The variability among different islands may have to do with their size and distance from
the mainland (see above paragraph) as well as the number of different niches available. A
place like Hawaii with its extensive topography is an ideal habitat for a variety of
organisms, while Tahiti is not quite so diverse.