In order to understand how hunting and other decimating factors influence populations, we must consider the factors that make wildlife populations increase and decrease.
Population Growth
Fundamental aspect of population biology or population ecology.
-Wildlife biologists study populations to determine whether they are increasing or decreasing
-what factors are responsible for causing increases and decreases?
-what regulates population size?
There are 4 factors that cause populations to change in size.
Birth- new individuals born in the population.
Death- animals that die
Immigration- permanent movement of individuals into the population (through dispersal)
Emigration- permanent movement of individuals out of the population " "
How can we determine the change in population size from one time period to the next?
Nt+1 = Nt + B - D + I - E
Nt - number of animals in the population at some time t.
Nt+1- number of animals in the population at the next time step.
B- Birth, D- death, I- immigration, E- emigration
With some effort, we can estimate birth and death rates in a population
but immigration and emigration rates are very difficult to measure.
How do real populations grow?
Start with a pair of quail in uninhabited area with unlimited resources.
1) cs=14
2) sex ratio =50:50
3) all indiv can breed at 1 yr
4) all breed
5) one clutch per year
6) one half of population dies over the winter
|
Time |
Number
breeding adults |
Number
offspring per adult |
Number
offspring |
Number
entering winter |
Number that
die |
Number
at end of winter |
| 0 | 2 | 7 | 14 | 16 | 8 | 8 |
| 1 | 8 | 7 | 56 | 64 | 32 | 32 |
| 2 | 32 | 7 | 224 | 256 | 128 | 128 |
| 3 | 128 | 7 | 896 | 1024 | 512 | 512 |
| 4 | 512 | 7 | 3584 | 4096 | 2048 | 2048 |
| 5 | 2048 | 7 | 14336 | 16384 | 8192 | 8192 |
| 6 | 8192 | 7 | 57344 | 65536 | 32768 | 32768 |
When plotted over time this gives an exponential or J-shaped curve.
How do we describe this mathematically?
Terminology
N Population size (usually measured at beginning of breeding season)
t Time
R The finite rate of growth of the population or the number of individuals per adult that are added in one year. It is defined as R=B-D. (Note difference between R and r).
In this case, B=7, D=4 so R=b-d=7-4=3.
N change in N from one time step to the next
t change in time
Growth of a population in an unrestricted environment.
N/t = RN
Since t=1 for 1 year intervals and Nt+1 = Nt + N
Nt+1 = RNt + Nt or
Nt+1 = Nt (R+1)
ex) N=2, R=3, N/t=2x3=6 and Nt+1 = 2(3+1)=8
ex) N=8, R=3, N/t=8x3=24 and Nt+1 = 8(3+1)=32
Note that the rate of increase per individual is assumed to remain constant with this equation.
THIS IS VERY UNLIKELY!
It is much more likely that as the number of animals increase, their rate of increase will decline. How do we describe this? With the logistic equation.
N/t=RN(1-N/K)
where K is the carrying capacity or the maximum number of animals that a habitat can support without the population declining.
The right hand term (1-N/K) is sometimes called the environmental resistance factor.
As N approaches K this term approaches 0 and the change in population size approaches 0.
Gives the familiar sigmoid curve- draw it!
Show overhead of sheep in Tasmania (from pops)
It is generally accepted that most (all?) populations grow according to the logistic growth model.
This means that as the population increases, the number of animals born per individual declines or the number of deaths per individual increases.
SHOW GRAPH FROM BOOK
Reasons for decline at high numbers:
1. Lack of food- Moose on Isle Royale
2. Lack of nest/roost sites- Seabirds
3. Increased disease transmission
4. Territoriality (could be a result of shortage of other factors)
Annual cycle- draw it
On an annual cycle this generally means that there are more produced in the breeding season than can be maintained over the winter.
=> it is futile to keep adding animals to a population- the excess will simply die.
=> game farms can only produce a temporary excess and therefore are
not a viable population management method and are not funded by P-R
Compensatory mortality
Those animals above the carrying capacity that are not removed by hunting will die of some other cause.
ex) Ruffed Grouse in Michigan
Protected at Gladwin
Hunted at Rifle River
Result- similar densities at two sites
=> mortality is compensatory
BUT- there is a limit. If hunting mortality exceeds natural mortality then the population will decline.
This is known as additive mortality because hunting mortality adds to natural mortality.
Waterfowl- many studies suggest compensatory mortality
Most species are limited by habitat- not by hunting mortality. Increase habitat and the populations will increase. This has lead to an emphasis on habitat acquisition and management for many game species.
Population regulation
At any one time, there generally is one factor that regulates a population.
Regulating factor- factor that affects the population in a density dependent manner.
i.e. as density increases, survivorship or fecundity decreases.
What causes this? Regulating factors
1. Food
2. Predation- only a few cases where predators regulate prey
ex) parasitoid wasps and their prey
ex) wolves and moose- depensatory
3. Nesting sites- some secondary cavity nesters
4. Territoriality
Most species are limited by habitat- not by hunting mortality. Increase habitat and the populations will increase. This has lead to an emphasis on habitat acquisition and management for many game species.
Population management
Need to first determine the goals. Two general goals.
Sport hunting- maximize hunter satisfaction
Market hunting- maximize yield
Maximum sustained Yield (MSY)- pop size at which maximum number of recruits is produced.
If population is growing according to logistic, this occurs at K/2- graph it.
=> maximize harvest by maintaining population size at half of K.
Problem- if natural variation causes population to decline, and if you maintain harvest at same rate, the population will crash.
ex) Anchovies off Peru
ex) Cod off New England
Another problem is the discount rate. Discount rate is the amount that you must discount future value to account for inflation and interest rates. i.e. $1000 right now is worth more than $100 per year for 10 years.
=> best strategy for long-term use of natural resource is not the same as economic strategy
ex) PACIFIC LUMBER
Agencies
USF&WS- lead agency for management of migratory birds.
- also responsible for T&E species designation and recovery
- manages some marine mammals in AK. Polar bear, Walrus others?
NFS does the rest
Flyway councils- people from fed and state agencies jointly make decisions regarding management of waterfowl.
-reps from each state AK, CA, OR, WA, NV, AZ (Pacific Flyway)
-collectively make recommendations.
-set bag limits that are universal across states
6 ducks/day, 12 in possession only one female
1 canvasback, 2 pintail, 2 redhead, etc.
-set total season that is universal across states (93 days)
-states decide exact dates of season
Current director- ??
Cal F&G- protecting and managing, and enhancing F&W and native plant resources
- set state game laws for game sp. other than waterfowl
-collect license fees
Fish and Game Commission
5 members appointed by governor
Will. conservation Board
acquires property
$ from bonds and horse racing
Law enforcement
USF&WS
1. Special agents- detectives (Terry Gross)
2. Wild. Inspectors- at borders and airports
3. Refuge officers- Kevin Foerster enforce fed. and state game laws on refuges
Cal. F&G
Game wardens- enforce state and fed. game laws
Human Population Growth
Humans are subject to the same ecological principles as other species with 2 crucial differences:
1. Humans can manipulate K
2. Humans have the ability to manipulate their environment to reduce death rates.
What is the outcome of this ability?
Unprecedented population growth rates over the last 200 years.
Not linear, not exponential, but faster than exponential!
Calculations of human population growth
Fremlin 1964 - assumed doubling every 37 yrs (growth rate in 1960's)
-predicted 60 million billion people in 2854
-at this point population would be limited by inability to dissipate heat generated by that many people
-density 120 people/m2
-could stack them in buildings 2000 stories high to give everyone 7.5m2
-not too bad 10 million Shakespears alive at one time
Von Foerster 1960-1966
-used an empirical approach to estimation of pop. size
-fitted a curvilinear regression line to 30 pop estimates over time
-found that growth rate has steadily increased over that time (not logistic, not exponential, but greater than exponential
N=(1.9 x 10^11)/(2026.87-t)^-99
=> pop growth will reach infinity on Friday Nov 13 2026
-what has population done since 1960?
-has actually increased faster than his predictions in 70's and 80's
- 5 May 1994 dropped below Von Foerster's predictions
OVERHEAD FIG 8.18
Population growth has only started to slow over the last couple of decades- mostly in developing countries
Presently 5.8 billion
increasing at 101 million/yr = 192/min = 10,000/hr
Where will it stabilize?
-hard to predict
-depends on changes in human behavior
-UN estimate 10.4 million in 2089 (this assumes # children/woman =2 in 2040)
-If population increases at current rates pop size will be 32 billion in 2089
Demographics- analysis of birth and death statistics
- great variation in demographics between countries
Developing countries
ex) US, Japan, Europe, etc.
-low growth rates (low fert, low death, low infant mort)
-high per capita income
-highly industrialized
Undeveloped countries
Moderately developed
ex) Mexico, Turkey, South Amer.
-high growth rates (high birth and death rates)
-moderate incomes
Less developed countries
ex) Ethiopia, Nigeria, Kenya, Bangladesh
-very high growth rates (very high birth and death rates)
-low per capita income
Why are birth rates so high?
Sub Sahara Africa- religion and custom, agricultural society
Demographic stages
Demographic transition- Predictable change in demographics that has been observed in most industrialized countries.
Stages in demographic transition
OVERHEAD FIG 8.10
1. Preindustrial- birth and death rates high => slow growth or stable
2. Transitional- improved health, reliable food
- lower death rates, high birth => high growth
ex) Latin America, Asia, Africa
3. Industrial- low birth => slower growth
4. Postindustrial- lowest birth, low death => little or no growth
ex) US, Canada, Europe
Most human population projections are based on the assumption that developing countries will follow this pattern over the next 20 to 30 years. Will this occur, it is hard to say.
Age structure of populations- affects growth and indicates potential for growth
OVERHEAD 8.12A,B,C
Age structure diagram
length of bar indicates numbers in each age class
2 sides- males, females
Types of age structure
Pyramid- more in prereproductive than reproductive => growing population
-will continue to grow even when fert =2
Why? younger indiv become reproductive
Stable- similar numbers in pre and reproductive stages
-will remain stable
Decreasing- lower numbers in pre reproductive stages
-will decline over time
Must beware of making predictions from age structure alone. Must assume no change in demographic parameters.
Overpopulation and overconsumption
Hunger, environmental degradation, underdevelopment, and poverty are aggravated by overpopulation. Many of the environmental problem we face can be traced to overpopulation and overconsumption.
ex) high pop densities in sub Saharan Africa have lead to degradation of land from grazing and crop production on marginal lands
- consequences- 20.2 million hectares of land/year are becoming desert in this region
=> short-term exploitation has long-term consequences
Types of resources
Nonrenewable- not replenished by natural processes (or rate of replenishment is orders of magnitude lower than rates of consumption)
ex) coal, oil, minerals, natural gas
Renewable- replaced by natural processes at rates that are comparable to rates of consumption
ex) trees, fresh water, productive soil, fish, game
** as long as these resources are not over exploited they will provide returns indefinitely
Sustainable use (or development)- development that meets the needs of present without compromising the abilities of future generations to meet their own needs.
-Difficult to agree on what we mean by present and future needs.
ex) Cutting of old-growth
-If define need simply in board feet of timber, it makes sense to cut down all of the old-growth and replace them with young growing trees. Then cut on short rotation that maximizes wood production. (Assuming there is no long term loss in productivity with this approach)
-If define need as trees and the species that they support, then it is necessary to preserve sufficient old-growth to maintain viable populations of those species that rely on this kind of habitat.
-humans presently consume about 40% of net primary productivity on the planet. This cannot be increased without consequences on the environment and other species.
How can we measure enviro. impact of resource consumption?
Resource consumption (by country)- population size x consumption per individual
-resource consumption can be reduced by reducing one or the other
-in less devel countries pop size is high while consumption is low (child in US consumes 12x the resources of a child in a devel country) therefore focus is on reducing pop growth
-in devel countries pop size and growth is low but resource consumption is high (focus on reducing consumption)
Environmental impact- resource consumption and extraction method
Extraction method refers to the way that resources are removed from the environment. Some cause less harm than others.
ex) coal mining- strip mining vs deep shaft mines
ex) clearcutting vs selective cut
ex) organic vs conventional agriculture
Overpopulation- more people than resource base can support.
Problem in developing countries, economic growth < resource growth
How do you reduce growth rates?
Five factors to consider 1. culture 2. marriage age 3. education 4. family planning 5. govt policies
1. Culture- values and norms of society
High birth rates are norm in many societies. Why?
a. high infant mort
b. societal roles of children
-provide labor- Africa
-provide security in old age
-child labor less important in industrial soc. cause kids in school and industry needs skilled labor
2. Marriage age-
-strong correl betw marriage age and fert rate
3. Education-
-women with more educ marry later and have fewer children
-more educ more knowledge of birth control
-educ and career gives women status apart from children
4. Family planning
-40% of drop in fert rate is caused by contraceptive use
-in male domin societies, women may be denied birth control
5. Govt policies
Indirect
-minimum age at marriage
-compulsory education until a certain age
-tax incentives
Direct
China
-medical care, educ, cash, housing for one child families
-penalties for 2+ families
How do we tackle problem of over population?
1. Reduce growth rate in devel countries
2. reduce consumption rates in devel countries
3. Encourage extraction methods that reduce impacts on environ.
4. Include environmental costs into items
All comes down to what our vision is for the future = values.