Ekologi Chapter 5

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The petersen method can be used to analyze

trends in populations that have been

threatened and are now recovering.

Ex. The humpback whale (), whose numbers

were greatly reduced by the whiling





In one study off the central coast of Ecuador,

79 humpback whales were photographed and

cataloged in 1996. The following year, 72

whales were photographed in the same area,

of which 3 were identified from the 1996

photographs; total estimate population is

1.896 whales.





Other Methods for Comparing

Population Sizes

Ecologists use a variety of sampling methods

that

enable them to compare the relative sizes of populations

without providing an estimate of population

size.



T raps. T raps include

mousetraps spread

across a field, light traps

for night-flying insects,pitfall traps in the

ground for beetles,

suction traps for flying

insects and planktonnets (Figure 5.4).





3. V ocalization frequency. The number of calls

heard per 10 minutes in the early morning can

be used to gauge the size of bird populations.

The same method is used for frogs, crickets

and cicadas.



4. Number of artifacts. This count is useful for

animals that leave evidence of their activities,

such as bird tracks in sand plots, mud

chimneys made by burrowing crayfish, tree

squirrel nests and pupal cases from emerged

insects.



5. Questionnaires. Questionnaires filled out by hunters, fishers and trappers provide estimates of population changes in economically importantanimals.

6. Cover. The percentage of t he ground surfacecovered by individuals is an especially importantmethod for comparing the relative sizes of manyplant populations. This method is also used byecologists studying invertebrates in the rockyintertidal zone.



7. Roadside counts. The number of birds or

ot her highly

visible organisms observed while one drives a standard distance has been used as an index

of

abundance.



Two points important to remember

Detailed, accurate information is easily

obtained for many plant species, but we have

only rough estimates of abundance





implications

1. conservation programs: can succed only if

we know what comprises a good habitat for

the protected species.

2. pest control programs require us to know

what makes habitats poor for undesirable

species.



Why are some habitats good and others poor for a

particular species?

two approaches:

± look for general attributes of species that arerelated to abundance.

For many animals, the most important of theseattributes is body size.

± describe the variation in abundance from site to

site for particular species and correlate thisvariation with environmental parameters, such astemperature, rainfall and soil type.







the graph predicts a density of 1 individual/km2 for a1-kg bird

and 20 individuals/km2 for a 1-kg mammal. Secondly,

larger animal species have lower population densities.

Thus, we would expect a density of about 60individuals/

km2 for 200 g squirrels but only about 0.5 individual/

km2 for 250 kg elk.



This inverse relationship between population

density and body size also applies to plants

and invertebrate animals.

The greater abundance of small plants and

animals affects the structure of ecological

communities and ecosystems,



Abundance in Relation to Limiting

Environmental

Factors



There is no general theory that allows us to

predict exactly what resource is limiting for a

particular species.





Limitation by Physical Factors

The abundance of many plants can be directly

linked to physical factors such as temperature,

rainfall and soil nutrients.

For example, the scribbly gum, a species of

Australian eucalypt, grows only in areas where

the mean annual temperature is between 9°C

and 14°C (Figure 5.7).









Clearly, some factor other than the availability

of cavitiesperhaps food supplymust limit

flying squirrel numbers in this part of South

Carolina.



Limitation by Predators

The abundance of the large kangaroos in

Australia has increased since the British

colonized the continent.

The paradoxical increase in kangaroo

populations has occurred in spite of intensive

shooting programskangaroos are considered

pests by ranchers and are harvested for meatand hides.



The reason seems to be that ranchers have

improved the habitat for large kangaroos in

three ways.

1. water has been made available for sheep

and cattle, removing the impact of water

shortage for kangaroos in arid environments.



2. kangaroos feed on grass, and ranchers have

cleared timber and produced grasslands for

livestock.

Thus, both the water supply and the food

supply have increased.



3. predation by the dingo (Canis familiaris

dingo) on kangaroos has been removed. T o

protect their sheep, ranchers shot and

poisoned dingoes in south-eastern Australia,

and the Australian government constructed a

5,400-km-long fence to keep dingoes from

recolonizing that part of the country (Figure5.10a).







Limitation by Food Supply





Limitation by Disturbances

Forest fires may cause habitat alterations that

favor some species and harm others.





Limitation by Habitat Structure

Changing the structure of a habitat can affect

the number of animals that use the habitat,

even if the food supply is unchanged.



5.3 POPULATIONS ARE NOT

CONTINUOUSLY DISTRIBUTED IN SPACE

if a species lives in a patchy habitat its spatial

distribution will also be patchy.

Patches of habitat may occur naturally or asthe result of human actions.

Ex. forests have been largely cleared for

agriculture, open fields separate small patches

of forest.



Populations and Metapopulations

When a species is divided into small

populations because of a patchy habitat, and

occasional migration between patches is

possible, the small populations collectively are

called a metapopulation.



Metapopulations are often distributed over

islands of habitat that are surrounded by a

sea of unsuitable habitat.

Metapopulations form a link between the

factors that limit distributions and those that

limit abundance.

















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Ekologi Chapter 5

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