EPHM7110 Chap.7 Population Growth and the Limits That Bind It
Population Growth and the Limits That Bind It
Two symbols, and everything else is commentary
Demography is affected by reproductive patterns, and the course reduces population growth to two quantities: the rate of population change, and the carrying capacity, which is the maximum population size the environment can sustain. Every curve in the chapter is a statement about how the first behaves as a population approaches the second.
Carrying capacity is set by critical resources in limited supply, and the three named are food, mates and water, so saying which of them is limiting at a particular site is what turns a curve into a management statement.
The two shapes are identical until they are not
Exponential growth is what happens under ideal conditions with unlimited resources, producing the J curve.
Logistic growth adds environmental resistance, the factors that reduce growth rate, producing an S curve that flattens against the ceiling. Both start in the same place and follow the same path while numbers are low, so an early growth record is consistent with either.
A curve becomes informative only after it has passed the point where the two shapes diverge, and until then the honest statement is about the observed rate rather than about the shape.
Life history traits move as a package
One end of the range carries fast growth, a great many young, next to no parental care, breeding started early, most offspring dying before maturity, small size and suitability to unstable conditions.
The other carries the opposite of each, with a low growth rate, few offspring, high parental care, late reproduction, most offspring reaching maturity, large size and suitability to stable conditions close to the ceiling. They move together because they are answers to the same question about how to spend a limited reproductive budget.
Body size is the trait most often used as a shortcut and the least reliable of the set, so check age at first reproduction and number of offspring before classifying anything.
Where the deaths happen decides where a measure should act
A survivorship curve shows the percentage of an original cohort surviving to a particular age.
One type has high survival through most ages and falls away among the old; a second has a constant rate at every age; a third has heavy early loss after which survivors live long. Put that beside a protection decision and it becomes practical. Protecting adults of the third type adds little, because adult survival was never the bottleneck, while protecting spawning and nursery habitat acts where the deaths are.
What this chapter covers
- 01
Rate of population change and carrying capacity as the two quantities
- 02
Exponential and logistic growth, and why an early record cannot separate them
- 03
Environmental resistance, and naming which resource is limiting
- 04
The seven life history traits that move together
- 05
Why body size is the least reliable trait to classify on
- 06
Three survivorship curve types and where each one loses individuals
- 07
Density dependent and density independent factors
- 08
Competition, parasitism and obligate against facultative mutualism
Separate a rate from a level in a visitor pressure dispute
- 3Name the confusion in the operator's argument.
- 2Say what each of the two further facts rules out.
- 2State the management response that follows.
Key terms
- Carrying Capacity
- The maximum population size an environment can sustain, set by critical resources in limited supply.
- Environmental Resistance
- The set of factors that reduce a population's growth rate as its numbers rise.
- Exponential Growth
- Growth under ideal conditions with unlimited resources, producing a J shaped curve.
- Logistic Growth
- Growth that flattens against the carrying capacity as environmental resistance increases.
- Survivorship Curve
- A plot of the percentage of an original cohort that survives to each age.
- Density Dependent Factor
- An influence on a population whose effect depends on how dense that population is, usually biotic.
- Obligate Mutualism
- A relationship in which two species benefit from each other and are entirely dependent on the partnership.
Population Growth and the Limits That Bind It FAQ
A population has grown steadily for five years. Can you tell which curve it is on?
Not yet. Both shapes trace the same path while numbers remain far below the ceiling, because the resources that eventually bite have not started to bite. The record separates them only after it reaches the region where one keeps accelerating and the other begins to flatten, so an honest report from an early period describes the measured rate and names the level at which a change of shape would be expected.
Why does protecting adults sometimes achieve nothing?
Because the stage where individuals are being lost may not be the adult stage. In species that produce enormous numbers of young with no care, almost all of the loss happens in the first days of life and the few that get through live long, so adult survival was never the constraint.
Effort spent guarding grown animals in such a species buys very little, while work on spawning and nursery conditions acts where the loss actually occurs.
Which of two threatened species should limited funding go to first?
The trait package is a legitimate part of that argument. Where reproduction is early and output is high, money converts quickly into recovered numbers, so a small intervention can be enough. Where maturity is late and output is low, the same money buys time rather than recovery, and a single bad decade can undo protection built over many years. Saying which of the two you are buying is what makes the ranking defensible.
Why do obligate partnerships change a conservation plan?
Because they make one species insufficient as a unit of protection. If neither partner can persist without the other, then securing only one of them secures nothing, and the plan has to cover both ranges, both sets of threats and both timetables. Recognising the dependency early is much cheaper than discovering it after a reserve has been drawn around the more visible of the two.
Exam move
Sketch the two growth curves and mark the point where they separate. Then take a real population you know something about and decide, from the data you have, whether you could tell which side of that point it is on. The answer is usually no, and knowing why is more useful than either curve.
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