Characteristics of Populations14.1

466 MHR • Unit 5 Population Dynamics

Demography is the study of populations —particularly the characteristics of populations thatcan be quantified, such as size, density, and growthrate. Demographers (those who study demography)are interested in what are sometimes called thevital statistics of a population. These include thecharacteristics mentioned above as well as others,such as the ratio of males to females and the ratioof old to young individuals in a population. Theseare among the factors that determine if, and by howmuch, the population is going to grow. Demographersgenerally start their study by defining thepopulation of interest.

Recall from the previous chapter that apopulation consists of a group of individuals, all of the same species, simultaneously occupying aparticular area. The boundaries of this area, whichpartly define the population, may be natural. Forexample, the shores of Lake Ontario define thelimits of the population of lake whitefish that livein the lake. Similarly, the shores of Isle Royale inLake Superior define that island’s moose (Alcesalces) population.

Many populations, however, have arbitraryboundaries set by a researcher or a governmentagency for management or study purposes (see the example in Figure 14.1). White-tailed deer(Odocoileus virginianus) in Algonquin Park oftentravel beyond the park’s boundaries, and there isnot much to distinguish between a maple treeinside the park and one just outside. Nonetheless,it is often useful to talk about the park’s deer ortree populations, with the understanding that the geographic limits of these populations aresomewhat arbitrary.

Regardless of how the boundaries of a populationare determined, all populations can be described in terms of two important characteristics: densityand dispersion. The density of a population is thenumber of individuals per unit of volume or area(for example, the number of oak trees per squarekilometre in Prince Edward County). Dispersionrelates to how the individuals in a population arespread out within its geographical boundaries. (Are

they spaced uniformly throughout the habitat, orare they clumped together in groups?) Both of thesefeatures can affect how a population grows and whatimpact the population has on the environment.

Figure 14.1 Although we might refer to Ontario’s blue jay(Cyanositta cristata) population, the birds themselves arenot restricted by provincial boundaries. Individuals fly in and out of the other provinces and across the border intothe United States daily.

Measuring Population DensityThe density of a population depends on its size —that is, how many individuals it contains. In rarecases, it is possible to count all the members of agiven population. Such a complete count isreferred to as a census. The governments of manycountries regularly conduct censuses of theirhuman populations.

Often, however, a complete census is impossibledue to time, energy, or financial constraints — apopulation may simply be too large to count.Therefore, ecologists must frequently estimate thesize or density of the population of interest. Thereare a variety of ways to do this. In most cases, thenumber of individuals in a number of samples(small portions or subsets of the entire population)are counted or estimated and then averaged. The

EXPECTAT IONS

Describe characteristics of a population, such as size and density.

Estimate the size and density of populations using various sampling methods.

467Chapter 14 Population Ecology • MHR

results are then extrapolated to the entire areaoccupied by the population, as shown in Figure 14.2.

Estimating Numbers by UsingTransects or QuadratsIn some cases, organisms are sampled along atransect, which is essentially a very long rectangle.In transect sampling, a starting point and directionare randomly chosen and a line of a certain length(for example, 100 m) is marked out. The occurrenceof any individual within a certain distance of thisline is recorded. This distance might be 1 m ifplants are being sampled, or perhaps 50 m if moremobile organisms (such as birds or mammals) arebeing studied. A sample transect is shown inFigure 14.3. Transects are particularly useful whenthe density of a species is low, or when organismsare very large (such as trees in a forest).

For plants and other types of organisms that tendto stay in one spot all their lives, ecologists generallyuse quadrats to sample a given population. First,several locations are randomly chosen within thearea, and at each site a quadrat of known size (forexample, 1 m2) is marked out. Next, the number of individuals of a species within the quadrat is

counted (as illustrated in Figure 14.4 on the nextpage), or the number of individuals of each ofseveral species are counted if more than onepopulation is being studied. The density of thepopulation is determined by calculating theaverage number of individuals per quadrat, anddividing by the size of the quadrat (for example,2.5 individuals per m2). The size of the populationcan then be estimated by extrapolating from thedensity figure. For example, if the geographicalarea occupied by a population is 1000 m2 and onaverage there are 2.5 individuals per m2, then thepopulation size is roughly 2500 individuals.

Figure 14.3 What is the area sampled along this transect?

100 m

5 m

Figure 14.2 Examine a random sample of 5 of these 25 quadrats. Based on yoursample, what is the average number of birds per quadrat? How many birds do youestimate there are in this flock?

468 MHR • Unit 5 Population Dynamics

Similar methods are used in studies of aquaticecosystems to estimate the size of variouspopulations. Often, samples of a known volume ofwater are collected. The water is then passed througha net or sieve, and the number of organisms in eachsample is counted. As with quadrats, the averagedensity (number of individuals per unit volume)can be used to estimate the size of the populationcontained in the entire body of water.

When using any sampling technique, it isimportant to take random samples — that is, samplesin which all individuals in the population have anequal chance of being represented. To have the bestchance of taking such a sample, it is important toknow something about how the individuals in thepopulation are distributed — in other words, tounderstand the population’s dispersion. In theMiniLab on the next page, you will practise onetechnique used for estimating populations ofmobile species.

Population DispersionImagine walking along a transect and making a noteon your data sheet each time you see a member ofthe population in which you are interested. Do youthink you would be counting individuals at regularintervals — perhaps one every 10 m? Or do you

think it more likely that you would come acrossseveral individuals at once, and then none, untilyou reach another group? This, of course, dependson how the members of the population are dispersed.

To simplify discussion, ecologists tend torecognize three general patterns of dispersion:uniform, random, and clumped (see Figure 14.5). It is important to realize, however, that there isactually a continuum of dispersion patterns. Manyspecies typically display patterns that fall betweentwo of these three types. Factors important indetermining dispersion include the distribution of

Ecologists are often more interested in knowing the relativedensities of two or more populations (that is, whether oneis more or less dense than another) rather than the actualor absolute density of each one. Relative densities can bedetermined by looking for signs of activity along transects.For example, the relative density of bears in an area is oftenestimated by looking for footprints, claw marks on trees,clumps of hair caught on bushes, and piles of droppings(usually called “scats” by biologists). Since it is difficult todistinguish between grizzly (Ursus horribilis) and blackbears (U. americanus) using some of these signs, samplesof hair or scats are brought back to the lab for DNA analysis.

B I O F A C T

Figure 14.4 When sampling the density of certain plants ina quadrat, it is often too difficult to count the number ofindividuals of a species. In this case, the proportion of the

quadrat covered or shaded by one or more plant species isrecorded. The area sampled is usually square or circular.

M I N I L A B

What Is the Population of Your School?The size of a population of mobile organisms is oftenestimated using one of a number of mark–recapturesampling techniques (sometimes referred to as capture–recapture sampling). Small, lightweight leg bands are oftenused to mark birds, fin or gill tags are used for fish, and eartags are sometimes used to mark captured mammals. It isimportant that the markers do not interfere with the abilityof the subject to forage and perform other activities relatedto survival or reproduction.

In this MiniLab, you will estimate the population of yourschool. To do so will require careful planning, your principal’spermission, and co-operation from other students. “Capture”a random sample of 25 or 50 students (for example, capturethe first 50 students passing you in the hallway of yourschool) and “mark” them in some way. You might use piecesof string or ribbon as armbands to mark your capturedindividuals. Release them and let them again becomerandomly mixed with the general population. You will have todecide in advance how long it might take them to becomere-mixed (an hour? a day?).

Take a second random sample of students (perhaps thesame size as your first sample) after re-mixing has occurred,and record how many of them are marked. The proportion

of individuals in the second sample that are marked can beused to estimate the size of the entire population by usingthe formula:

For example, if 100 students were caught initially, marked,and released, and 10 of these individuals were recapturedwhen a second random sample of 100 was taken, anestimate of the population size (N) could be obtained by re-arranging the formula given above:

100N

= 10100

therefore,

N = 100 × 10010

= 1000 students

Analyze1. Compare your estimate of the size of the school’s

population with the true size. How close were you?

2. (a) What problems do you think might affect theaccuracy of your estimate?

(b) Do you think any of these problems might also befaced by ecologists studying non-human animalpopulations?

3. How might you improve your sampling design?

number of individuals caughtand marked in the first sample

total population size=

number of markedindividuals recaptured

total numberin second sample

469Chapter 14 Population Ecology • MHR

resources, the types of interactions that typicallyoccur among members of a population, and thedistance offspring generally disperse from theirparents. In general, how the members of a populationare spaced throughout an area depends on a complexinterplay among these factors (each will be discussedin more detail below). As always, ecology andevolution are intertwined. The dispersion patterntypically seen in a population is produced by

behaviours and other features that increase theability of individuals to survive and reproduce.Dispersion patterns, which occur in ecologicaltime, are therefore the result of characteristics thathave been selected for over evolutionary time.

In some cases, it is primarily the distribution of food, water, or other needed resources thatdetermines the dispersion of individuals. If theseresources are not distributed evenly but rather are

uniform randomclumped

Figure 14.5 Think of examples of populations that fit each of these dispersion patterns.

470 MHR • Unit 5 Population Dynamics

clumped in specific places, then members of thepopulation also tend to be clumped (see Figure 14.6).For example, many types of plants tend to beclustered in locations where the moisture,temperature, and soil conditions are optimal for their growth and reproduction.

Figure 14.6 These animals show a clumped distributionbecause the lakes, rivers, and ponds they rely on are onlyfound in certain locations in the environment.

Clumping is also typical of species in whichindividuals gather into groups for protection frompredators or to increase hunting efficiency. Forexample, schools of fish and flocks of birds oftenform to protect individuals from predators.Humpback whales (Megaptera novaeangliae) gatherto feed so they can make “bubble nets” aroundtheir prey, and wolves hunt in packs to increase theprobability of catching food. Wolf packs, chimpanzeegroups, and lion prides are also important inensuring that young members of the population are cared for while some adults hunt or engage inother activities. In general, species in which theinteractions among individuals in a population canbe described as positive (that is, the interactionsdraw individuals together) tend to show clumpeddistribution patterns.

Finally, clumping is typical of species in whichoffspring grow in close proximity to their parents(Figure 14.7 shows one example). Some species ofplants, for example, tend to be found growing in

clumps even if there is suitable habitat elsewhere,simply because they produce seeds that cannottravel far from the plant that produced them.

Figure 14.7 Aspens reproduce asexually, with newindividuals sprouting from the roots of older ones. Theytherefore tend to grow in groves of genetically identicalindividuals.

By contrast, if the resources needed by membersof a population are evenly distributed across anarea and are in short supply, a uniform dispersionpattern is often seen. Many species of birds andmammals, for example, are said to be territorial.They defend a territory that contains the shelter orfood they need for survival, mating, or raisingyoung. Defending this territory involves keepingother individuals (or members of other families)out of the area. Since the amount of the resource(and thus the area) an individual or pair needs isrelatively constant within a species, the result isthat members of the population are more or lessevenly spread over the habitat.

Certain species of plants, for example the blackwalnut tree (Juglans nigra), achieve the same resultby producing chemicals that discourage the growthof other trees nearby. Secreted into the soil, thesechemicals ensure that an individual plant has itsown territory, containing the water or soil nutrientsit needs. Whether a territory is defended withbehaviours or chemicals, this type of interactionamong the individuals in a population can be

T H I N K I N G L A B

Sampling a Moose PopulationBackgroundKnowing about the dispersion pattern typical of a speciesand how this pattern can vary among populations can helpus understand the behaviour and ecology of the species. It is also valuable to know about dispersion before takingsamples to estimate population density, since the accuracyof your estimate can depend on the dispersion. In this lab,you will see how transects might be used to samplepopulations of a large mammal — the moose.

You Try It1. Study the three diagrams showing hypothetical moose

populations with two different dispersion patterns.Describe how the moose are dispersed in each diagram.

2. The shaded portions of the diagrams show thetransects that were used to sample each population.What proportion of the total study area (defined by thebox) was sampled in each of the three cases?

3. Count the number of moose encountered within thetransect areas for each case. (That is, do not count thetotal number of moose in any of the diagrams.) Fromthese counts, estimate the size of the three moosepopulations.

Dispersion pattern 2

4. The actual numbers of moose in the three populationsare 60, 133, and 133, respectively. How close wereyour estimates? What do you think affected theaccuracy of the estimates in each case?

5. How would you design a sampling experiment on a wildpopulation? Note that in real life, the time and expenseinvolved usually restricts the proportion sampled tobetween 10 and 20 percent of the total area of interest.

Dispersion pattern 1

Dispersion pattern 3

471Chapter 14 Population Ecology • MHR

described as negative (that is, the individualsrepulse or repel each other). Such interactions lead to uniform dispersion patterns.

For a population to display a random dispersionpattern, the resources needed by its members have

to be uniformly distributed. These resources alsomust be so abundant that there is no need forindividuals, pairs, or families to defend their share.In addition, randomness requires that there beneither positive nor negative interactions among

472 MHR • Unit 5 Population Dynamics

S E C T I O N R E V I E W

1. What is the biological definition of the term“population”?

2. Why is it important to be able to developaccurate estimates of natural populations of plants,animals, and other species of living organisms?

3. Why is it often necessary to estimate populationsize, rather than counting each member of apopulation individually?

4. Describe what is meant by the density of apopulation.

5. When is it useful to use transects to samplepopulations?

6. Develop a procedure in which transects are usedto sample the species within a region. Select a localecosystem for study.

7. Suppose that in a study of a particular species of rodent, a sample of 200 was trapped, marked,and released. Later, a second sample of 200 wasobtained and it was discovered that 20 percent ofthese animals were recaptured (that is, they werealready marked). Based on these data, estimate the size of the total population.

8. How might the abundance and distribution offood in a habitat influence how a species is dispersedin its environment?

9. Research the types of sampling techniquestypically used to estimate populations of migratingspecies, such as the caribou (Rangifer tarandus) innorthern Canada. How do researchers attempt toimprove the accuracy of their observations?

I

K/U

I

I

K/U

K/U

K/U

MC

K/U

Figure 14.8 Many species of bats tend to forage (find food)alone at night, but roost in colonies during the day.

Figure 14.9 Each homeowner in this development has hisor her own territory.

the members of a population, and that offspringdisperse more or less equally over all distances.Since these conditions are rarely met in nature,random dispersion patterns are unusual.

It is important to recognize that the dispersionpattern displayed by a population may vary betweenseasons, or even with the time of day (as with thebats shown in Figure 14.8). The scale at which apopulation is examined is also important. Forexample, the human population of Canada has aclumped dispersion. Looking at the country overall,

most of its human population is concentrated incities and towns. But if you focus on a smallerscale and look at the population of one city, youwill often find a more uniform distribution (asillustrated in Figure 14.9).

Whether members are clumped, randomlyscattered, or uniformly dispersed, many populationschange in size. In the next section, you will learnhow and why this happens and what factors affectthe rate at which such change occurs.