APES Test 3 Populations Midterm Review

Question 1

Terms used to describe populations

Answer 1

A population is defined as a group of organisms of the same species that inhabits a defined geographic area at the same time
Individuals in a population generally breed with one another, rely on the same resources to live, and are influenced by the same factors in their environment
Two important characteristics of populations are the density of the population and how the population is dispersed.

Question 2

Population density

Answer 2

Refers to the number of individuals of a population that inhabit a certain unit of land or water area
Ex. number of squirrels that inhabit a particular forest

Question 3

Population dispersion

Answer 3

Refers to how individuals of a population are spaced within a region, there are three main ways in which populations of species can be dispersed, and you should know all of them
Uniform dispersion: uniformly dispersed within region, seen in forests, in which trees are uniformly distributed so that each receives adequate light and water, uniform dispersion is often result of competition for resources in an ecosystem
Random dispersion: randomly dispersed, randomly spaced,position of each individual is not determined or influenced by positions of other members of the population, ex. Is plants that are interspersed in field or forests – the location of their growth is random and relative to other species, not their population
Clumped dispersion: clumped into groups, most common dispersion pattern for populations, individuals “flock together,” this makes sense for many species — many species of plants tend to grow together in a location or habitat that is near their parents and suits their requirements for life; fish swim in schools to avoid predation; and birds and many other animals migrate in groups

Question 4

Biotic potential

Answer 4

Biotic potential of a population is the amount that the population would grow if there were unlimited resources in its environment
Not a practical model for population growth simply because in reality the amount of resources in the environments of populations is limited

Question 5

Carrying capacity

Answer 5

Carrying capacity of a particular species in a particular environment is defined as the maximum population size for the species that can sustainably be supported by the available resources in that environment
Given geographic region will have different carrying capacities for populations of different species – because different species have different requirements for life
Ex. within certain area, one would expect a population of bacteria to be quite a bit larger than a population zebras, because individual bacteria are much smaller than individual zebras; thus, each bacterium requires fewer resources to live than each zebra
These differences in population size may be driven not just by the different sizes of individual organisms of each species, but by each species’ resource requirements and the particular array of resources available in the area

Question 6

Competition

Answer 6

Every ecosystem, members of a population compete for space, light air, water, and food

Question 7

Population Growth Graphs

Answer 7

If we looked at growth of a population of bacteria in a petri dish with plenty of food, the curve produced by plotting the increase in their number over time would be in the shape of a J, because the bacteria would grow exponentially (exponential, unrestricted growth)
This exponential population growth rate is seen where resources are unlimited, but in nature such ideal conditions are rare and fleeting
In reality, resource availability and the total resource base are limited and finite on any timescale
In more realistic model for population growth, after initial burst in population, growth rate generally drops and the curve ultimately resembles a flattened S

The slowdown of population growth is the result of increased mortality rather than decreased fecundity, the reality tends to be a little messier than the word slowdown implies: it will probably involve overshoot, which occurs when a population exceeds its carrying capacity. There are environmental impacts of population overshoot, includign resource depletion
If resource depletion is severe enough, carrying capacity of environment may be lowered
The severity of these effects varies, but resource depletion generally leads to dieback of the population, which can be severe to catastrophic, because the lack of available resources leads to famine, disease, and/or conflict
Once the dieback occurs, the population once again falls below carrying capacity; if the events were not too catastrophic, the environment can recover and the reduction in carrying capacity may not be permanent

Question 8

Logistic growth model

Answer 8

much better model for reality,
Logistic growth model basically says that when populations are well below the size dictated by the carrying capacity of the region they live in, they will grow exponentially
But as they approach the carrying capacity, the resource base of the population shrinks relative to the population itself
This leads to increased potential for unequal distribution of resources, which will ultimately result in increased mortality, decreased fecundity, or both
Result is that population growth declines to, or below, carrying capacity, and the size of the population will eventually become stable

Question 9

Generalist species

Answer 9

live in a variety of environments, eat a variety of foods, broad ecological tolerance for environmental conditions, have an advantage when the environment changes

Question 10

specialist species

Answer 10

require a specific habitat
limited diet
narrow ecological tolerance for environmental conditions
have an advantage in constant habitats

Question 11

K selected species

Answer 11

Populations whose growth is limited by the carrying capacity of the environment; live in stable environments where competition for resources is relatively high
tend to be large, have longer lifespans
have fewer offspring per reproduction event
expend significant energy for each offspring
It is important to preserve as many members of the offspring as possible because they produce so few; parents have tremendous investment in offspring
mature after many years of parental care; have long life spans
reproduce more than once in their life
live in stable environments
Ex. humans, lions, cows

Question 12

r selected species

Answer 12

Populations below the carrying capacity of their environment, means that population growth is constrained only by species’ own biological limits
Competition for resources in r-selected species’ habitats is usually relatively low
tend to be small
have many offspring
expend or invest minimal energy for each offspring
mature early; have short life spans
may reproduce only once in their lifetime
are able to thrive in disturbed environments
High capacity for reproductive growth
Ex. bacteria, algae, protozoa

Question 13

r and k species, general rules

Answer 13

r = generalist, k = specialist
Specialists typically live in areas where there is less competition and it makes sense to put extra energy into offspring; but this is a rule of thumb at best
Invasive species tend to be r-selected, whereas most adversely affect by invasive species tend to be K-selected
Many are in between, or lean towards one rather than being alll the way to one side
Some can change strategies in different conditions or at different times
But groups are useful for broad comparisons

Question 14

Carrying Capacity

Answer 14

Defined broadly as the number of organisms that an ecosystem can support of any one particular species
Eg. the theoretical maximum population size

Question 15

Biotic Potential

Answer 15

The reproductive potential of a population- eg. how quickly can a population grow, when left unchecked?

Question 16

Overshoot

Answer 16

Population exceeds carrying capacity- usually followed by correction (collapse)- decrease in births, increase in deaths

Question 17

Reaching Carrying Capacity- K (Population Equilibrium)

Answer 17

Population Equilibrium defined as Birth Rate= Death Rate
No population growth or decline
In a functional ecosystem, population equilibrium is reached at carrying capacity

Question 18

Limiting/Regulating Factors (Biological Resistance Factors)
Density Dependent

Answer 18

Typically (but not exclusively) biotic
Food
Water
Shelter
Nest Sites
Interspecific Relationships
Nutrients

Question 19

Limiting/Regulating Factors (Biological Resistance Factors)
Density Independent

Answer 19

Typically abiotic or human caused
Volcanoes or other natural disasters
Pollution
Extreme Weather Events
Climatic Shifts

Question 20

Carrying capacity can change

Answer 20

environment can change, degrading carrying capacity
ex. carrying capacity of moose was decreased because of increased presence of ticks because of warmer temperatures

Question 21

Survivorship Curves

Answer 21

Depict death in a population as a function of age
Represent the number of individuals in a population born at a given time (called a cohort) that remains alive as time goes on
Different species have different survivorship curves depending on their life circles and life strategies, including r and K selection
Survivorship curve differ for k and r selected species
Lifespan is more complex than a mean/median, as many organisms experience steep die offs early in life with several long lived individuals
Most actual populations exhibit some combination of Type I, Type II, and Type III patterns

Question 22

Survivorship Curves: Type I

Answer 22

Most individuals survive to advanced age, with sharp increase in mortality as teh population approaches the species’ maximum age
Typically K species

Question 23

Survivorship Curves: Type III

Answer 23

Significant early die off, with a few individuals making it to advanced (and reproductive) age
Species with this curve produce high numbers of offspring that encounter bottlenecks to survival that wipe out most young, and parents provide little or no nurture to their young
Typically r selected species

Question 24

Population Cycles

Answer 24

When we observe populations in natural habitats, there are two patterns that are more specific and involve more factors than just overshoot and dieback: the boom-and-bust cycle and the predator-prey cycle
These two patterns aren’t explicitly tested, but relate to patterns that are

Question 25

Boom-and-Bust Cycle

Answer 25

Boom-and-bust cycle is very common among r-strategists
In this type of cycle, there is a rapid increase in the population and then an equally rapid drop-off
These rapid changes may be linked to predictable cycles in the environment (temp or nutrient availability, for example)
These cycles may reflect regular changes in rainfall, temp, or nutrient availability over the course of the year
Or they may reflect longer and less regular cycles
When the conditions are good for growth, the population increases rapidly
When the conditions for that population worsen, its numbers rapidly decline
You might say that their strategy is “get it while the getting’s good”

Question 26

Predator-Prey Cycle

Answer 26

Ex. rabbits and coyotes
We discussed how in a year of relatively high rainfall, rabbits have plenty of food, which enables them to reproduce very successfully
In turn, because the coyote is a predator of the rabbit, coyotes would also have plenty of food, and their populations would also rise rapidly
However, if rainfall is below average a few years later, then there would be less grass, the populations of rabbits would decline, and the coyote population would decline in turn. The graph of the predator-prey relationship looks like the following
Important to notice that coyote population doesn’t change at exactly the same time as the rabbit population, coyote population actually rises after the rabbit population does
That is because the rabbit population has to have time to build up to fairly high levels before the coyotes can find enough to eat
When there is enough good, the coyote mothers have enough energy to give birth to and feed their pups, only then can the coyote population increase
This cycle plays a large role in understanding why many endangered species are large carnivores
Large predator populations can suffer directly if humans alter their natural habitats, but they can also suffer indirectly if humans kill of their prey
If prey population falls so low that the predator cannot find food, then the predator populations will decline, sometimes to the point of extinction

Question 27

There are population limiting factors that are purely the result of size of the population itself

Answer 27

Ex. in many populations of species in nature, birth and death rates are influenced by the density of the population
Other density-dependent factors that influence population size are increased predation (which occurs because there are more members of the population to attract predators); competition for food or living space; disease (which can spread more rapidly in overcrowded populations); and the buildup of toxic materials