Chapter 20- Population Ecology

Question 1

Define population.

Answer 1

• A group of individuals of a single species that live in the same general area.

Members of a population:
• Rely on the same resources.
• Are influenced by similar environmental factors.
• Have a high likelihood of interacting and breeding with one another.

Question 2

Define population ecology

Answer 2

is the study of how and why the number
of individuals in a population changes over time.

• What both promotes and limits population growth.
• Changes in age structure, the proportions of males to females, and how they are dispersed geographically.
• Population ecology has been a vital field in biological
science for endangered species management and recovery strategies.

Question 3

Distinguish among population size, density, and dispersion

Answer 3

• Population size: number of individuals in the
population at a specified time.
• Population density: the number of individuals per unit
area or volume.
• Population dispersion: the pattern of spacing among
individuals within the population’s geographic
boundaries.

Question 4

Explain how ecologists may estimate the density of a species.

Answer 4

Ecologists usually estimate population numbers using a
variety of sampling techniques.
• For example:
• Count the number of individuals in randomly located plots and extrapolate to estimate the population size in the entire area.
• This method is most accurate when ecologists use many sample plots and the habitat is fairly homogenous (not clumped).
• Estimate population size from an indicator (e.g., number of nests, burrows, tracks, or fecal droppings).
• Fish - use catch per unit effort.

Question 5

Describe the mark-recapture method for estimating population size, and calculate an
estimate for population size given a hypothetical example.

Answer 5

Mark-recapture method.
• Individuals are trapped and captured, marked with a tag, recorded, and then released.
• After a period of time has elapsed, traps are set again, and individuals are captured and identified.
• This second capture yields both marked and unmarked individuals.
• From counts of captured + recaptured individuals, researchers estimate the total number of individuals in the population.
• The number of individuals recaptured divided by the total number of animals captured in the second sample should equal the number of individuals marked and released in the first sample, divided by the estimated population size.

Question 6

State the 4 processes that directly affect population size.

Answer 6

Birth, death, immigration, and emigration

Question 7

Clumped dispersion

Answer 7

1. Clumped dispersion
   • Most common.
   • Occurs when individuals aggregate in patches.
   • Examples:
   • Plants and fungi are often clumped where soil conditions favor germination and growth.
   • Animals may clump in favorable microenvironments or habitat patches, to facilitate mating interactions, to avoid predation, or to be a more effective predator

Question 8

Uniform dispersion

Answer 8

2. Uniform dispersion
   • Occurs when individuals are evenly spaced.
   • Often results from interactions between individuals within the population.
   • Examples:
   • Some plants secrete chemicals the inhibit the growth of nearby individuals, thus avoiding competition for resources.
   • Territoriality in animals - the defense of a bounded space against encroachment by others.

Question 9

Random dispersion

Answer 9

3. Random dispersion
   • Occurs when the position of each individual is independent of the others, and spacing is unpredictable.
   • Occurs in the absence of strong attraction or repulsion among individuals in a population, or when key physical or chemical factors are relatively homogeneous
   throughout the range.
   • Example: plants with windblown seeds may be randomly distributed in a uniform habitat.

Question 10

Define survivorship and fecundity.

Answer 10

survivorship: the proportion of offspring produced that survive, on average, to a particular age.

Fecundity: the average number of female offspring
produced per female in the population.

Question 11

Explain how a life table is constructed.

Answer 11

• Life table: an age-specific summary of the survival and
reproductive patterns of a population.
• The best way to construct a life table is to follow the fate of a cohort (a group of individuals of the same age) from birth throughout their lifetimes until all are dead.
• This involves marking a group of individuals as soon as they are born, and then following their fate.
• For long lived species, life tables are created by examining the age structure of a population at one point in time (“snapshot approach”).

Question 12

Understand the significance of a net reproductive rate

(R0) of 1, >1 and <1.

Answer 12

The sum of all the lxmxvalues (i.e. across all age classes) = the net reproductive rate (R0) = the growth rate of a population per generation.
• R0 = The growth rate of a population per generation = the average number of female offspring that each female produces over the course of her lifetime.
• A female’s average lifetime reproduction is thus a function of her survival and her reproductive output at each age class.
• What does a net reproductive rate of 1 mean?
• That the population size is remaining constant from generation to generation.
• The population is reproducing itself at a rate that is simply replacing lost individuals.
• i.e. On average each original female is producing 1 individual over the course of her lifetime, thus replacing herself.
• If R is >1? If R <1 ?

Question 13

Define per capita rate of increase (r) and understand the significance of an r = 0, r >0, and r<0.

Answer 13

The difference between the per capita birth and death rates (i.e., b – d) is the per capita rate of increase (r).

• The value of r indicates whether a population is growing
(i.e. per capita birth rate exceeds per capita death rate; r >
0) or declining (i.e. per capita death rate exceeds per capita birth rate; r < 0).
• If r = 0, then there is zero population growth. Births and
deaths still occur, but they balance exactly.
• Within a population, r varies through time and can be
positive, negative, or zero.

Question 14

Describe the characteristics of populations that exhibit Type I, Type II, and Type III survivorship curves, and provide example organisms for each .

Answer 14

• Type I: relatively flat at the start, reflecting a low death rate in early and middle life, and drops steeply as death rates increase among older age groups.
• Humans and many other large mammals.

• Type II: constant death rate over an organism’s life span.
• Many species of rodent, various invertebrates, and
some annual plants.

• Type III: drops sharply at the start, reflecting very high death rates 
early in 
life, but flattens out as death rates decline for the few individuals 
that survive to a critical age. 
• Organisms that produce large numbers of offspring, but provide little or 
no parental care - e.g., 
many fishes, long-lived plants, 
and marine invertebrates. 
• Many species will fall 
somewhere in between
these basic survivorship 
patterns.

Question 15

Define survivorship curve

Answer 15

Survivorship curve: a plot of the proportion or number of
individuals in a cohort that are still alive at each age.
• Typically, biologists will plot the logarithm of the number of survivors versus age.
• Curve represents the rate of survival for individuals over the species’ average life span.

Question 16

Define life history and explain how limited resources may affect life histories ( hint: think about fecundity and survivorship).

Answer 16

Life history: The lifetime patterns of growth, survival,
maturation, and reproduction that characterize different species (or populations).

• With the exception of humans, organisms do not consciously choose when to reproduce or how many offspring to have
• An organisms’ life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism.
–>Life history is shaped by natural selection in a way that maximizes the individual’s fitness in its given environment.

Allow us to pinpoint the stage in life when the species has particularly low survivorship.
• Biologists can then put into place measures that protect or enhance survival of the species during that time period

Question 17

Define and distinguish between semelparity and iteroparity. Explain what factors may favor the evolution of each life history strategy.

Answer 17

1. Semelparous species produce all offspring in a single
   reproductive event, with no resources set aside for future
   survival.
   –>Reproduction is inevitably followed quickly by death.
   • Also known as “Big-bang reproduction”
   • Examples:
   • Salmon return to freshwater streams to spawn and then die.

Example: • Agaves:
• Grow in arid climates with unpredictable rainfall
and poor soils.
• Grow for years, accumulating nutrients in their
tissues, until there is an unusually wet year –> send
up a large flowering stalk, produce seeds, and die

2. Iteroparous species breed repeatedly at intervals throughout life, devoting some of their resources during the breeding season to survival to further breeding
   episodes

• When adult survival is low relative to juvenile survival, evolution should favor semelparity (i.e. don’t bother investing in future reproduction when you’ll likely die before you get there!)
• When the environment is more dependable, and adult survival is more likely, and where resource competition may be high, iteroparityshould be favored.
• Adult more likely to survive to next reproductive event and few, well provisioned offspring should have a better chance at surviving in a competitive environment.

Question 18

Contrast the exponential model of population growth with the logistic model.

Answer 18

David Suzuki…
• Exponential population growth occurs when r does not change with population size or density
• Represents continuous growth in an unlimited environment.
• Accelerating increase in population size –> J-shaped curve.
• Although r is constant, the increase in population size
gets larger over time because N is larger (i.e. more individuals are reproducing).
• Population grows at an ever-increasing pace!

Question 19

Define intrinsic rate of increase, rmax

Answer 19

When conditions are optimal for a particular species, meaning that births per individual are as high as possible and deaths per individual are as low as possible, r reaches a maximum value called the intrinsic rate of increase, rmax.

Question 20

Differentiate between density-independent and density-dependent factors that affect population growth.

Answer 20

• Density independent factors alter birth and death rates irrespective of the number of individuals in the population.
• Include weather patterns, or catastrophic events such as cold snaps, hurricanes, drought.
• These types of abiotic factors would affect similar proportions of the population regardless of population size.
• Density dependent factors change in intensity as a function of population size.
• Include competition for resources, predation, buildup of toxic metabolic wastes, aggressive interactions, and disease transmission.
• Halt population growth by increasing death rates or decreasing birth
rates. —> What type of feedback is this? NEGATIVE FEEDBACK

Question 21

Explain how density-dependent factors work to control a population’s growth, with examples (hint: think negative feedback).

Answer 21

• Halt population growth by increasing death rates or decreasing birth
rates. —> What type of feedback is this? NEGATIVE FEEDBACK

Question 22

Define carrying capacity, list several factors that influence carrying capacity, and explain how and why an environment’s carrying capacity affects the per capita rate of increase of a population.

Answer 22

Carrying capacity (K) is the maximum number of 
individuals that an environment can support 
indefinitely.

It is a property of the environment – depends on a number of factors including (but not limited to) food, space, water availability, soil , availability of resting or nesting sites, and the intensity of disease, parasitism, predation, and competition.

Question 23

Explain the meaning of rmax

Answer 23

When conditions are optimal for a particular species, meaning that births per individual are as high as possible and deaths per individual are as low as possible, r reaches a maximum value called the intrinsic rate of increase, rmax.

Question 24

Explain the meaning of (K-N)/K

Answer 24

represents the proportion of “unused resources” remaining.Slows down population growth as population size approaches carrying capacity.

Question 25

Explain the meaning of rmax(K – N)/K

Answer 25

—> In this model, the realized per capita growth rate (r) depends on population size!

Question 26

Explain why carrying capacity varies between species and in space and time.

Answer 26

• Any given region may have very different carrying capacities for different species – why?
• Different species have different demands for space and resources.
• Also: For a given species, some habitats are better than others due to differences in food availability, predator abundance…etc. Carrying capacity thus varies in space.
• Also: Conditions in some years may be better than others. Carrying capacity thus varies in time.

Question 27

Describe population cycles, explaining the leading hypothesis of lynx/hare fluctuations.

Answer 27

• One example is population cycles.
• Occurs when population size (or density) regularly fluctuates between highs and lows.
• Exhibited by many insects, birds, and mammals.
• Small herbivorous animals, including voles and lemmings, tend to have 3-4 year cycles
• Birds, such as ruffled grouse, have 9 – 11 year cycles.
• Most of the hypotheses for population cycles suggest that some sort of density-dependent factor intensifies dramatically at high population density, causing the population numbers to crash.
• Could involve predation, disease, or food shortages.

• Snowshoe hares are the primary food source for lynx.
• It is not surprising that lynx populations cycle in response to snowshoe hare populations.
• The question is: Why do hare numbers rise and fall in 10-year cycles?
• Work by Charles Krebs at UBC suggests that it’s a
combined effect of food availability and predation.
• Leading hypothesis: when hares are at high densities individuals are weakened by nutritional stress and are more susceptible to predators.
• When populations of hares reach high densities, they use up all their food and become nutritionally stressed, especially in winter.
• In addition, lynx populations (along with other predators) reach high densities in response to increases in hare
densities —> higher predation.
• The result? At high densities the hares are
more susceptible to predation and the population declines!