ANSWERS TO REVIEW QUESTIONS – CHAPTER 42
1.
The Conservation Commission of the Northern Territory of Australia is interested in
determining the number of saltwater crocodiles, Crocodylus porosus, in a 10 km stretch of
the Adelaide River. On the first day they catch and mark 40 crocodiles with a blue tag. On
the second day they catch and mark 50 crocodiles with a red tag, 20 of which were already
marked with a blue tag. What would be your estimate of the size of the population? (Box
42.1)
Using the formula presented in Box 42.1:
day 1 and day 2: 40 × 50/20 = 100
This method assumes the population is closed. Since a 10 km stretch of river is an arbitrary unit, it is
likely that some crocodiles move in and out of this zone during the sampling period. In addition,
animals may die or hatch during the period, although this is less likely in a long-lived animal over a
short period of time. The method also assumes all individuals are equally likely to be caught. There
may be behavioural or physical differences between the sexes or between individuals of differing ages
or sizes that affect their catchability. In addition, the experience of being captured may modify their
behaviour and affect their subsequent catchability. Since we are not told about the trapping method, we
cannot judge how far this assumption is likely to be violated. Finally, the method assumes that tags
remain on the animal and are legible from one sampling occasion to the next.
2.
Name four abiotic factors and four biotic factors that could limit the distribution of a
species. (pp. 1045–1046)
The chapter lists a number of abiotic factors of importance to plants: light, rainfall, temperature, soil.
Biotic factors listed include pollinating species, herbivores, pathogens and competitors.
3.
Soil moisture, temperature and nutrients are environmental gradients that are said to affect
plant distribution and abundance directly. Altitude, rainfall and soil type are said to affect
plant distribution and abundance indirectly. Discuss. (p. 1045)
Moisture, temperature and nutrients are factors that have a direct effect on a plant’s metabolism.
Experiments varying these factors in plants will produce changes in metabolic rate, and hence growth
rate, fecundity and survival. Altitude tends to be a determinant of microclimate, affecting temperature
and soil moisture; rainfall will also be a strong determinant of soil moisture; and finally soil type will
govern both soil nutrient availability and the ability of the soil to retain moisture. However, plants
grown under the same nutrient, temperature and moisture conditions, but at different altitudes, will
generally exhibit the same growth rates. Thus plants respond to the effect of altitude because changes
in altitude tend to change the direct factors rather than because altitude has an innate quality that affects
plant growth. Similarly, rainfall and soil type produce effects through direct factors.
4. What is habitat selection theory? (p. 1048)
All individuals of a species attempt to live in places which maximise their chances of survival and
reproduction.
5. What is a closed population and where might you find one? (p. 1049)
A closed population is one that has no emigration or immigration. We might find such populations on
islands (actual islands or ‘ecological’ islands such as ponds or rock outcrops) or in isolated reserves.
6.
A butterfly population in the mallee is believed to respond directly to summer rainfall.
When the summer rainfall is below 100 mm, each adult female produces, on average, 0.1
adult females; when between 100 mm and 200 mm the annual fertility is 1; when above 200
mm, annual fertility is 5. If the probability of summer rainfall being one of these three
rainfall classes is equal, will the population increase or decrease in the long term? Do you
think this is a permanent or ephemeral population? (pp. 1055–1056)
This question is best treated as a class exercise. A simple way to approach it is to use a spreadsheet
which has a random number function. The formula below will work in a Microsoft Excel spreadsheet.
Type the initial population size of 100 in cell A1, and then type the formula into cell A2. Drag down
from cell A2 to cell A11. The result in A11 is the population size after 10 generations. Retype 100 into
cell A1 and the spreadsheet will rerun the simulation. Run for at least 100 iterations and see how often
you get a result of zero in A11. After 1000 iterations I produced a probability of extinction of 0.44
using this formula, which suggests that the population would have a very low probability of surviving
in the long term.
=INT(IF(0.33333>RAND(),A1*0.1,IF(0.666666>RAND(),A1*1,A1*5))+0.5)
7.
You are not sure whether the size of a population of numbats, Myrmecobius fasciatus, is
food-limited or limited by fox predation. Design some experiments to test these alternative
hypotheses. (Refer to Chapter 42 for principles of experimental design.)
This can be tested by adding or removing individuals from a closed population and directly measuring
the effects of reduced predation and/or food manipulation. Refer to Figure 42.7. Students should pay
close attention to the principles of experimental design presented in the opening chapter.
8.
What kinds of species might be space-limited? Provide some examples. Could a species of
parrot be space limited? (pp. 1054–1055)
Sessile (non-motile) organisms such as barnacles may become space-limited, where the amount of
space in the niche environment runs out. Parrots are unlikely to be space-limited because they can
move from one habitat to another and have very large foraging areas.
9.
Name two groups of organisms where birth and death rates are affected by the sizes of the
individuals in a population. (p. 1051)
Insects and plants.
10. Consider a common species of barnacle that dominates part of the rocky intertidal zone. A
starfish predates on the barnacle, selectively choosing patches of about 10 m 2 and
consuming all individuals in the patch. Discuss the consequences of this predation on
patterns in the distribution of adult barnacles at different spatial scales. (pp. 1046–1048)
Barnacles, like most sessile organisms, have a widely dispersing juvenile stage, in the form of a
planktonic larva. The spatial packing of the adults inhibits the larvae from attaching and
metamorphosing in regions of high barnacle density. The starfish thus creates a turnover in the barnacle
population by clearing out patches which are then recolonised. The barnacle patch is thus going to be a
spatial mosaic of individuals of different ages, with similar age groups occupying each approximate 10
m2 patch. Since the larvae are planktonic, each patch will represent a genetic mix, with individuals
drawn from substantial distances apart in adjacent positions. How far the mosaic is complicated by
relatively young patches being partially overlaid with new predation will depend upon the density and
motility of the starfish.
Variation in density and age-structure of barnacles in the intertidal zone is thus going to depend on
abiotic factors, such as characteristics of the substrate and desiccation, and on biotic factors such as the
intraspecific competition between the barnacles, and the distribution, abundance and predation biology
of the starfish.