Chapter 35 - Population Dynamics

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Intro - European Starlings - worldwide range -

A. Once restricted to Europe and Asia

1. Now found in N. America, Eastern Australia, S. Africa, New Zealand

a. Omnivorous, aggressive and tenacious

b. displace cavity nesters such as woodpeckers, bluebirds and swallows from nesting sites

c. may pull other nestlings out of nests to make room for their own

2. introduction of exotic species was unregulated and popular in late 1800 and early 1900s

a. ring necked pheasant introduced for hunting purposes

b. house sparrows introduced for "aesthetic reasons" and pest control

c. a citizen’s group was trying to bring all the birds in Shakespeare’s works to New World

d. 120 starlings were released in Central Park in 1890 (Show map)

e. spread of starlings across the continent, Mexico to Canada

f. currently there are well over 100 million starlings in US

3. Population Ecology is concerned with changes in population size and the factors that regulate

populations over time

B. Populations are defined in several ways

1. Populations are generally confined geographically

a. for instance, a population of coyotes in the S. Lake Tahoe Basin.

2. Ecologists usually define a population as an interacting group of individuals of a species that use

common resources and are regulated by the same natural phenomena,

a. such as temperature,

b. food and water supply,

c. and predation.

3. Humans construct "population boundaries", when a population is continuously distributed

4. 2 important characteristics of any population are

a. density and

b. distribution or dispersal patterns.

I. Population structure and dynamics:

A. Density and dispersion patterns are important population variables

1. Density - number of individs of a particular species per unit area or volume - eg.

a. number of oak trees per km2, or

b. number of earthworms/m3 in soil.

2. Usually, you can’t count every individual so sampling allows estimates;

a. f or example, instead of counting all the individuals of alligators in the Everglades, plots.

b. the more plots you have, the more closely you approximate true numbers.

c. sometimes you don’t count actual individuals, but indirect methods-

d. scat or tracks, or bird songs, for example.

e. nests, or burrows.

3. mark and recapture - N =marked # in first catch x total catch 2nd time

recaptured marked individuals

a. you might use this method for voles in a meadow

b. catch particular number in first attempt, then wait 2 weeks and try again

c. eg 50 voles caught and marked in 1st attempt, 100 second time, 5 recaptured

1. 50x100 = 5000; 5000/5 = 1000 voles estimated to be in meadow

d. this method assumes marked individuals have the same chance of being caught as unmarked.

e. Problems with this assumption? animal may be more interested in food or wary of trap

5. Dispersion pattern - spacing of individuals in an area.

a. uniform - even pattern of dispersion - nesting birds

b. random - spaced in patternless, unpredictable way - clams in a mudflat when resources are

plentiful, or when conflicting needs for diff’t resources creates chaotic pattern

c. clumped - individuals aggregated in patches - most common dispersal pattern,

often results from uneven distribution of resources

B. Growth Models help us understand population growth

1. idealized models help us understand population growth

a. Exponential growth - unrestricted growth

1) G = rN

a) G = growth rate

b) N = size of population

c) r = intrinsic rate of increase; organisms inherent capacity to reproduce

(roughly birth - deaths for the period being discussed

d) J curve

b. Logistic model - limits on growth

1) G = rN(K - N)
                K

 

a) K is Carrying Capacity

b) overshooting carrying capacity may cause population crash, drop in K

c) K doesn't influence the equation very much until N starts getting large

relative to K.

d) when K is reached, (K-N)/K = 0 and growth rate is zero

e) growth rate is small when population is very small and very large;

f) growth rate is largest when N is intermediate level of K

C. Multiple factors may limit population growth

1. logistic model - as pop density increases, population growth slows

a. decrease in birth rate

b. increase in death rate

c. both of the above

2. Density Dependent factors - decrease growth rate or increase death rate

a. food - Fig. 35.4A - song sparrows clutch size decrease as pop increases

b. shelter/space - thin seedlings to achieve best productivity of plants

c. build up of toxins

d. disease -

1. high pops of mice, woodchucks other rodents decrease growth rate even when more food, shelter supplied -

a) stress hormones increase

b) shrinks repro organs,

c) delays sexual maturation,

d) depresses immune system

e. predator interest in large pops

3. Density independent factors

a. climate

b. weather (Fig. 35.4B) - some pops, especially insects, boom until change in conditions, then

bust dramatically, with no leveling off.

c. natural catastrophes

4. Most pops have regular fluctuation in numbers (Fig. 35.4c) -

a. song sparrow pops respond to many interactions of density dependent and independent

factors

D. Some pops have boom bust cycles -

1. predator prey relationships - Canadian lynx and snowshoe hare every 10 yrs. (Fig. 35.5)

2. food supply of prey affects predator pops, but why does hare pop boom and bust?

3. food supply of prey, over exploitation by predators, both predation and food supply

4. hormones and stress also may affect prey pops, coincidentally keeping predator pops from

getting too large

5. time lag in response to prey increase by predator populations - reproduce more slowly

II. Life Histories and their Evolution

A. Life tables- track mortality and survivorship in populations.

1. Life tables allow us to track mortality over time, insurance companies produce these and set premium rates using these numbers. Info used to make survivorship curves of difft species

2. survivorship curves - fig. 35.6

a. type I - whales, elephants and humans - have a few young, care for them, increase survival to maturity

b. type II - some rodents and hydras - constant death rate-

c. type III - oysters, lots of marine animals - high death rates for the very young, then low

death rates for those few individuals that survive to adulthood

B. Evolution shapes life history-

1. Life history includes events from birth thru reproduction to death in a species

2. include age at first reproduction

a. frequency of reproduction

b. number of offspring

c. amount of parental care given

d. energy cost of reproduction

3. Natural selection will favor combination of life history traits that maximizes and individuals output of

viable offspring, just as it shapes physical features

a. opportunistic life history - small animals, reproduce when young, many offspring

b. equilibrial life history - larger animals, reproduce later, few offspring

4. cichlids vs. killifish as guppie predators

a. guppies in cichlid pools tend to be smaller, mature earlier, have more offspring

b. guppies from both pools raised in lab w/ no predators kept their differences

5. r - selection - life history traits that maximize success in uncrowded, unpredictable surroundings

a. mature early

b. large numbers of offspring

c. weeds, insects, many invertebrates

6. K - selection - organisms w/ pops close to K of environment

a. maturity and reproduction at later age

b. few, large, well cared for offspring

c. polar bears, giraffes, humans

III. Human Population

A. continuous growth for over 1000 yrs. (Fig 35.8A and B)

1. at ~ 8000 bc - hunter gatherer to ag

2. plague

3. industrialization at 1700

4. falling death rates largely causing pop growth (Fig 35. 8B and C)

5. technological vs. biological changes

6. fluctuations around carrying capacity - exceed by a lot?

a. human population no longer fits comfortably on earth

b. in order for us to accommodate all the people expected on earth by 2025 and improve diet, we have to double food production

c. all arable land is currently being farmed

d. oceans currently being overfished

e. 2/3 of all the available water on earth will have to be in use

7. ecological footprints - amt of land in ha (hectare - 2.47 ac) per person, current demand on resources

a. US uses 8.4 ha/person; Bangladesh, India - .8 & .5

b. black means you are using more land than your country has, blue means you are within your country’s limits

c. Earth isn’t big enough to support whole human pop at US standards

B. Age structure and birth and death rates affect how a pop will grow in the future

C. Principles of Population ecology have practical applications

1. management of natural resources can mitigate human effects on natural systems

2. Principles of population ecology can be used in this management

3. Wildlife and fisheries managers as well as foresters try to practice renewable resource management

- harvesting crops w/out damaging resource

4. maximum sustained yield - consistent yield that can be obtained continuously, w/out harming the

original population

a. intermediate growth rates the highest - so harvest pops that are far below K, but not too far

b. frequently we don’t have all the info we need (Fig. 35.10A)

1. estimates were too high

2. discarding smaller cod caused higher than predicted mortality rate

5. spp in decline or facing extinction, we try to increase populations - Endangered spp program

a. red cockaded woodpecker - success story - brought back from near extinction by producing and preserving pockets of older pines and decreasing understory w/ fire mgt. - habitat requirements of that species

6. Integrated Pest Management (IPM) used to decrease pesticides and control pests - control excessive populations

ch 35 Study Questions

1. Describe the rise of the Starling population in the US. What is the worldwide distribution of starlings?

2. Explain how density and dispersion describe populations and how each of these variables is measured, both directly and indirectly.

3. Explain the mark and recapture sampling method.

4. Distinguish between exponential and logistic models of population growth, explaining the effects of existing population size and carrying capacity on growth rate.

5. Be able to draw and explain J and S curves and their axes, and relevant important points about each.

6. Differentiate between density dependent and density independent factors that limit population growth.

7. Describe the interrelationships among predator, prey and prey food that cause boom-bust cycles.

8. Name the attributes of life histories. Describe how natural selection affects life histories.

9. Draw survivorship curves I, II, and III; describe life history characteristics of each and give examples of organisms.

10. Distinguish between r and K selected species.

11. Discuss life tables and age structure diagrams, and examine them for human populations. Compare an age structure diagram of a stable population with an explosively growing population and a declining population.

12. What issues face a growing population? A declining population?

13. Outline the history of human growth of the population, including factors that affect growth.

14. Describe demographic transition. Do you think government regulation of population is ethical? Support your answer in 2 or 3 sentences.

15. How has the field of population ecology been used to manage populations we are trying to regulate?