PDS AP Environmental Science 8th Period 2010-11

On our chapter 5 test, question 35 talks about the sustained increase in the primary productivity of the environment and how the sheep would respond. I did not think that the population would increase and stay at a significantly larger number of sheep because I thought the environment would not be able to hold an extra 1,000 sheep when it was leveled out at around 1,500. Why was the environment able to just take in 1,000 more sheep? Wasn’t it already at its carrying capacity?

Another question: how does extinction determine the world’s current biodiversity? This was a question on the test and I don’t understand. The question was “The two processes that determine the world’s current biodiversity are ______.” I said “allopatric and sympatric speciation” because this process plays a part in new species being developed considering geographical and behavioral separation between organisms. The answer he wanted was “extinction and speciation.” I can understand the speciation aspect, but how does extinction contribute to a greater biodiversity? Doesn’t extinction take away from biodiversity? For example, the K-T event wiped out a bunch of species at one time (I think it was like 75-90%…maybe that’s the Permian). Regardless, wouldn’t the result be one a very small number of species? So wouldn’t this actually DECREASE biodiversity? I’m confused…

Hey everybody, I was little confused on what the difference in directional, stabilizing, and disruptive forms of natural selection are, and what kind of scenario would lead a population to each type of scenario?

In the paradox of enrichment, why does this only happen when the growing season is longer (or does it)? When we ran the short growing season, the population sizes dropped significantly but did not go extinct. Why? If there was a decrease in the food supplies, then wouldn’t there be less food for both prey and predator, leading to both populations going extinct? Thanks for all the help!

In the case of a paradox of enrichment, is it possible that an r-related species of primary consumers might be able to cancel out the paradox due to so great reproduction, and allow them eventually to live past their predators. Like cockroaches being able to stand having such a big food supply and not die out while possessing several predators?

Hey guys-

so the book website defined a logistic growth curve as most likely to happen when there’s a small population with unlimited resources. Doesn’t it seem like unlimited resources would lead to an exponential growth curve because there are no limiting factors? I’m confused on the difference and exactly which types of populations lead to which curves. thanks!

The Isle Royale simulation in class points out the significance of free population growth, the presence of predators, and the availability of food in an ecosystem. The three simulations that we did focus on each of these and their effects on the populations of moose and wolves.

Exercise 1

This exercise focused on the population of moose when there are no predators present. We ran the simulation for about fifty years. The resulting graph looks similar to an S curve with a slight rise and fall before it reaches carrying capacity. This is an example of a population overshooting its carrying capacity, which then results in a dieback, also called correction, in the population until the carrying capacity is achieved.

A couple important things to note.

1 – A population grows fastest when it is medium sized.

2 – No population can sustain a J curve indefinitely. There is always a carrying capacity.

3 – The moose overshoot their carrying capacity because they have no natural predators. The dieback is then caused by the resulting food shortage when the moose are placed in direct competition with each other.

4 – The point of greatest population growth is called the inflection point.

Exercise 2

In this exercise, wolves were introduced to the island.

Population biologists/ecologists rarely have the luxury of counting every organism in an ecosystem or biome, but they still have to try in order to monitor relative biodiversity or determine if a population is threatened or endangered.  Today we simulated a popular sampling technique known as mark and recapture.  While we used pretzel goldfish as tagged fish, in reality a typical fish tag looks more like this:

See the blue tag (click to enlarge)?

Image Source: http://www.greenalgaecontrol.com/GAC%20Private%20Lakes.html

Of course, we make some assumptions when we use this method.  I think the professor that created this page does a nice job explaining the method and limitations of using it: Estimating the Size of Animal Populations (might be a useful read before finishing your lab write-up).

Finally, here is our class data for today.  Please analyze it before reaching and writing your conclusion:

Class Data (click to enlarge)

Of course, I’m available by email if you have any questions. -W

When thinking about population ecology, it is important to remember that populations evolve, not individuals. We’ve learned that populations do not operate independently of each other, but are connected and intertwined within communities. Therefore, we say that populations evolve together; they coevolve. In population ecology, there are 5 major relationships:

1. predation
2. mutualism
3. commensalism
4. competition
5. parasitism

Most of that is review from previous units, though. The main new stuff we learned involved characteristics of populations.

5 CHARACTERISTICS OF POPULATIONS:

1. Size: The size of a population can either increase, decrease, or stay the same. It can also move in patterns. It can be expressed as an equation:  (births+immigrations) – (deaths+emigrations) = size.

This is an example of a population size graph. It illustrates how populations can change size in patterns or cycles. It also demonstrates the idea that populations affect each other and do not operate independently… a key idea in population ecology.

2.  Density: Density is defined as the number of individuals per unit of area. A high density of individuals can have both pros and cons depending on the situations:

+ safety in numbers                                                                     – more intraspecific competition

+easily find mates                                                                         - infectious disease is more easily spread

- high density could actually attract predators (buffet)

<– high density of organisms in a population

3.  Distribution:  There are 3 types of distribution:

clumped- (picture on the left) all the birds are distributed together in a cluster/bunch

uniform- (picture in the center) the penguins are evenly spaced out

random- (picture on the right) the different species of trees are mixed in randomly

4.  Age Structure/Sex Ratios:  these are based off of age and sex “cohorts”; they are illustrated by histograms

5.  Growth Rates:  growth rates are expressed in percentages. There are 2 common situations: exponential growth and logistic growth. Exponential growth (constant growth rate) is sometimes called a J Curve. Logistic growth (unregulated growth rate) is sometimes called an S Curve. Their graphs may look something like this:

The graph on the left shows exponential growth, while the graph on the right shows logistic growth. Notice that in logistic growth, the curve levels off at a certain point “K”. This point is called the carrying capacity. These graphs might look simple, but growth rates are rarely this simple in the real world, they just tend to follow these two basic trends/stereotypes.

That’s basically it.

P.S. Always be on the lookout for the NAP ZONE:

Nap Zone: The Movie

In class on Monday we talked about Speciation, or the creation of new species. What helps drive the process of speciation is natural selection. We learned 4 different aspects of natural selection:

1. Genetic Variation (mutation)
2. Over Production of Offspring
3. Struggle for Existence (food limitation)
4. Differential Survival & Reproduction (heredity)

A source of genetic variation is mutation, which is considered “the fuel” for evolution. Remember… populations evolve, not individuals. As species reproduce each generation of offspring is different because the parents’ genes mix and the result can be a variety of things. Mutation is random, the behavior, shape or appearance that an organism inherits will not necessarily always be beneficial, although natural selection favors organisms that possess features that give them an advantage in survival. Thus organisms possessing unfavorable traits will die off and their trait will die with them (decreasing their presence within a population).

The 3 types of Natural Selection we learned about are:

1. Stabilizing: when the “average” individuals in the population are favored of the extremes of a trait
2. Disruptive: when individuals at both of the extremes of a trait are favored over the average individuals
3. Directional: when individuals possessing an extreme trait at one end of the curve are favored

*Disruptive Selection plays an important role in speciation because as variance within the population increases, dividing the population into 2 distinct groups, speciation becomes possible.

Lastly, we covered 2 types of Speciation:

1. Allopatric: speciation by geographic isolation. This type of speciation is much like a cheesy movie where the main character goes off having adventures that changes him so drastically that when he returns nobody in his hometown recognizes him. Organisms that were once part of the same population (same specie) and were separated geographical underwent mutation creating new traits only shared in that isolated population, eventually resulting in a new specie.        
2. Sympatric: speciation in which new species evolve from a single ancestral species while inhibiting the SAME GEOGRAPHICAL REGION. For example, there is small volcanic crater lake in Nicaragua that is completely isolated, so there is no physical way it could be connected to any other body of water. A study was done on the lake and this is what was concluded, “We find, first, that crater Lake Apoyo was seeded only once by the ancestral high-bodied benthic species Amphilophus citrinellus, the most common cichlid species in the area; second, that a new elongated limnetic species (Amphilophus zaliosus) evolved in Lake Apoyo from the ancestral species (A. citrinellus) within less than ~10,000 yr; third, that the two species in Lake Apoyo are reproductively isolated; and fourth, that the two species are eco-morphologically distinct.” (http://www.nature.com/nature/journal/v439/n7077/full/nature04325.html)

*From Mr. W, the brief video example of allopatric speciation shown in class: