Morphological adaptation. Fur thickness in North American mammals has a major impact on the degree of insulation the fur provides.
Polar bears are able to live in zoos in warm climates. How thick would you expect the hair of a polar bear to be in a zoo in Miami, Florida? Answer: It depends on what factors regulate hair growth. If hair growth responds to temperature, then we would expect hair length to be much lower in the Arctic because it is much warmer in Miami. If hair length is triggered by shorter day lengths, a sign of impending winter, then we would expect hair length to increase as winter approaches; however, even in the heart of winter, day length is much longer in Florida than in the Arctic, so hair length might also be shorter in Miami for that reason.
Behavioral adaptation. The Puerto Rican lizard Anolis cristatellus maintains a relatively constant temperature by seeking out and basking in patches of sunlight; in shaded forests, this behavior is not possible, and body temperature conforms to the surroundings.
When given the opportunity, lizards regulate their body temperature to maintain a temperature optimal for physiological functioning. Would lizards in open habitats exhibit different escape behaviors than lizards in shaded forest? Answer: Very possibly. How fast a lizard runs is a function of its body temperature. Researchers have shown that lizards in shaded habitats have lower temperatures and thus lower maximal running speeds. In such circumstances, lizards often adopt alternative escape tactics that rely less on rapidly running away from potential predators.
The relationship between body size and generation time. In general, larger animals have longer generation times, although there are exceptions.
If resources became more abundant, would you expect smaller or larger species to increase in population size more quickly? Answer: Because of their shorter generation times, smaller species tend to reproduce more quickly, and thus would be able to respond more quickly to increased resources in the environment.
Survivorship curve for a cohort of the meadow grass Poa annua. After several months of age, mortality increases at a constant rate through time.
Suppose you wanted to keep meadow grass in your room as a houseplant. Suppose, too, that you wanted to buy a plant that was likely to live as long as possible. What age plant would you buy? Answer: As young a plant as possible, because survival rates decline with age.
Reproduction has a price. Increased fecundity in birds correlates with higher mortality in several population of birds, ranging from the albatross (lowest) to the sparrow (highest). Birds that raise more offspring per year have a higher probability of dying during that year.
Do you think that species in this graph are ordered by body size? Answer: Yes, larger species tend to have lower birth and death rates. Thus, body size will tend to decrease along the x-axis.
Reproductive events per lifetime. Adding eggs to nests of collared flycatchers (which increases the reproductive efforts of the female rearing the young) decreases clutch size the following year; removing eggs from the nest increases the next year's clutch size. This experiment demonstrates the trade-off between current reproductive effort and future reproductive success.
Why does this relationship exist? Answer: Probably because a relationship exists between the number of offspring raised and the amount of energy expended. Thus, birds that raise fewer offspring probably retain larger energy stores, which makes them better able to raise more offspring the following year.
The relationship between clutch size and offspring size. In great tits, the size of the nestlings is inversely related to the number of eggs laid. The more mouths they have to feed, the less the parents can provide to any one nestling.
Would natural selection favor producing many small young or a few large ones? Answer: It depends on the situation. If only large individuals are likely to reproduce (as is the case in some territorial species, in which only large males can hold a territory), then a few large offspring would be favored; alternatively, if body size does not affect survival or reproduction, then producing as many offspring as possible would maximize the representation of an individual's genes in subsequent generations. In many cases, intermediate values are favored by natural selection.
Relationship between population growth rate and population size. Populations far from the carrying capacity (K) will have large growth rates-positive if the population is below K, and negative if it is above K. As the population approaches K, growth rates approach zero.
Why does the growth rate converge on zero? Answer: Because when the population is below carrying capacity, the population increases in size. As it approaches the carrying capacity, growth rate slows down either from increased death rates, decreased birthrates, or both, becoming zero as the population hits the carrying capacity. Similarly, populations well above the carrying capacity will experience large decreases in growth rate, resulting either from low birthrates or high death rates, that also approach zero as the population hits the carrying capacity.
Many populations exhibit logistic growth. (a) A fur seal (Callorhinus ursinus) population on St. Paul Island, Alaska. (b) Laboratory populations of two populations of the cladoceran Bosmina longirsotris. Note that the populations first exceeded the carrying capacity, before decreasing to a size that was then maintained.
Why is there a hump in the population growth curve in (b), followed by a decline in the population? Answer: There are a number of possible explanations. One is that the population overshot the carrying capacity and then declined back to it. Another is that the carrying capacity decreased, perhaps due to environmental change.
Density-dependent population regulation. Density-dependent factors can affect birthrates, death rates, or both.
Why might birthrates be density-dependent? Answer: There are many possible reasons. Perhaps resources become limited, so that females are not able to produce as many offspring. Another possibility is that space is limited so that, at higher populations, individuals spend more time in interactions with other individuals and squander energy that otherwise could be invested in producing and raising more young.
Density dependence in the song sparrow (Melospiza melodia) on Mandarte Island. Reproductive success decreases and mortality rates increase as population size increases.
What would happen if researchers supplemented the food available to the birds? Answer: The answer depends on whether food is the factor regulating population size. If it is, then the number of young produced at a given population size would increase and the juvenile mortality rate would decrease. However, if other factors, such as the availability of water or predators, regulated population size, then food supplementation might have no effect.
Fluctuations in the number of pupae of four moth species in Germany. The population fluctuations suggest that density-independent factors are regulating population size. The concordance in trends through time of the species suggests that the same factors are regulating population size in all species.
What might those factors be? Answer: Most likely some aspect of the environment, such as winter severity, drought occurrence, or predator population size.
Linked population cycles of the snowshoe hare and the northern lynx. These data are based on records of fur returns from trappers in the Hudson Bay region of Canada. The lynx population carefully tracks that of the snowshoe hare, but lags behind it slightly.
Suppose experimenters artificially kept the hare population at a high and constant level; what do you think would happen to the lynx population? Conversely, if experimenters artificially kept the lynx population at a high and constant level, what would happen to the hare population? Answer: If hare population levels were kept high, then we would expect lynx populations to stay high as well because lynx populations respond to food availability. If lynx populations were maintained at a high level, we would expect hare populations to remain low because increased reproduction of hares would lead to increased food for the lynxes.
History of human population size. Temporary increases in death rate, even severe ones such as that occurring during the Black Death of the 1300s, have little lasting impact. Explosive growth began with the industrial revolution in the 1800s, which produced a significant long-term lowering of the death rate. The current world population is 6.3 billion, and at the present rate, it will double in 53 years.
Based on what we have learned about population growth, what would you predict will happen to human population size? Answer: If human populations are regulated by density-dependent factors, then as the population approaches the carrying capacity, either birthrates will decrease or death rates will increase, or both. If populations are regulated by density-independent factors, then if environmental conditions change, then either both rates will decline, death rates will increase, or both.
Why the population of Mexico is growing. The death rate (red line) in Mexico fell steadily throughout the last century, while the birthrate (blue line) remained fairly steady until 1970. The difference between birth and death rates has fueled a high growth rate. Efforts begun in 1970 to reduce the birthrate have been quite successful, but the growth rate remains high.
Is population growth rate increasing? Answer: No, population growth rate, the difference between birthrate and death rate, is decreasing. Nonetheless, it is still positive, which means that population size is increasing.
Population pyramids from 2000. Population pyramids are graphed according to a population's age distribution. Kenya's pyramid has a broad base because of the great number of individuals below childbearing age. When the young people begin to bear children, the population will experience rapid growth. The Swedish pyramid exhibits a slight bulge among middle-aged Swedes, the result of the "baby boom" that occurred in the middle of the twentieth century.
What will the population distributions look like in 20 years? Answer: The answer depends on whether age-specific birth and death rates stay unchanged. If they do, then the Swedish distribution would remain about the same. By contrast, because birthrates are far outstripping death rates, the Kenyan distribution will become increasingly unbalanced as the bulge of young individuals enter their reproductive years and start producing even more offspring.
Distribution of population growth. Most of the worldwide increase in population since 1950 has occurred in developing countries. The age structures of developing countries indicate that this trend will increase in the near future. World population in 2050 likely will be between 7.3 and 10.7 billion, according to a recent United Nations study. Depending on fertility rates, the population at that time will either be increasing rapidly or slightly, or possibly, in the best case, declining slightly.
Is this an example of density-dependent population regulation? If so, what factors are regulating population size? Answer: Yes. Birthrates around the world are declining. Nonetheless, in some places, death rates are increasing as population increases; thus, both birth and death rates appear to be density-dependent.
Ecological footprints of individuals in different countries. An ecological footprint calculates how much land is required to support a person through his or her life, including the acreage used for production of food, forest products, and housing, in addition to the forest required to absorb the carbon dioxide produced by the combustion of fossil fuels.
Which is a more important cause of resource depletion, overpopulation or overconsumption? Answer: Both are important causes and the relative importance of the two depends on which resource we are discussing. One thing is clear: The world cannot support its current population size if everyone lived at the level of resource consumption of people in the United States.
1). The term homeostasis refers to
a). the maintenance of a consistent internal environment.
b). the ability to conform internal temperature to environmental temperature.
c). an organism's biotic potential.
d). the carrying capacity of a population.
2). Which of the following is not considered a population?
a). the ginkgo trees (Ginkgo biloba) in New York City
b). the birds in your hometown
c). the human inhabitants of Pennsylvania
d). the grizzly bears (Ursus arctos) of Alaska
3). A clumped population may be due to
a). weak interactions between the members of a population.
b). intense competition for uniformly distributed resources.
c). uneven distribution of resources in the environment.
d). intense territoriality.
4). The tidewater goby (Eucyclogobius newberryi) is an endangered species of fish that occurs as metapopulations in isolated coastal wetlands of California. Large wetlands serve as sources of individuals for populations in small wetlands, which function as sinks due to inferior habitat quality. What effect would most likely be seen on the population of gobies if a barrier to migration between wetlands developed?
a). The populations in the small and large wetlands would evolve independently of each other.
b). The populations in the large wetlands would most likely go extinct.
c). The populations in the small wetlands would most likely go extinct.
d). The populations in the small and large wetlands would most likely go extinct.
5). Which of the following factors does not determine the growth rate of a population?
a). the population's sex ratio.
b). the species' generation time.
c). the age structure of the population.
d). the optimal temperature at which an organism can reproduce
6). A population with a larger proportion of older individuals than younger individuals will likely
a). grow larger and then decline rapidly.
b). continue to grow larger indefinitely.
c). grow smaller and may stabilize at a smaller population size.
d). not experience a change in population size.
7). Humans are an example of an organism with a type I survivorship curve. This means
a). mortality rates are highest for younger individuals.
b). mortality rates are highest for older individuals.
c). mortality rates are constant over the life span of individuals.
d). the population growth rate is high.
8). According to the Population Reference Bureau (2002), the worldwide intrinsic rate of human population growth (r) is currently 1.3%. In the United States, r = 0.6%. How will the U.S. population change relative to the world population?
a). The world population will grow, while the population of the United States will decline.
b). The world population will grow, while the population of the United States will remain the same.
c). Both the world and the U.S. populations will grow, but the world population will grow more rapidly.
d). The world population will decline, while the U.S. population will increase.
9). The logistic population growth model, dN/dt = rN[(K – N)/K], describes a population's growth when an upper limit to growth is assumed. This upper limit to growth is known as the population's ____________, and as N gets larger, dN/dt _______________.
a). biotic potential/increases
b). biotic potential/decreases
c). carrying capacity/increases
d). carrying capacity/decreases
10). Which of the following is not an example of a density-dependent effect on population growth?
a). an extremely cold winter
b). competition for food resources
c). stress-related illness associated with overcrowding
d). competition for nesting sites
Test Your Visual Understanding
1). The song sparrow Melospiza melodia exhibits density-dependent population growth on Mandarte Island. List three reasons that the number of young per female may be sensitive to population size.
Answer: Competition for food results in decreased nutritional status for females, competition for food results in decreased nutritional status for offspring, reduced food availability requires parents to be away from the nest for longer periods of time which leaves the young vulnerable to predation, competition for nesting sites, and stress-related responses to increased population size.
Apply Your Knowledge
1). Throughout most of North America, large carnivores have either been extirpated (driven to local extinction) or their populations are at extremely low levels. Explain how the loss of carnivore populations may have changed the vegetation of North America.
Answer: Decreases in carnivore populations often lead to an increase in herbivore populations. As herbivore populations increase they consume more vegetation. This can lead to large-scale changes in vegetation abundance and distribution. This phenomenon can be observed on the small scale by looking at a pasture full of cows. All of the vegetation that is palatable to the cows will be grazed. Those plants that the cows do not eat (such as thistle) will be ungrazed.
2). Do humans show more r-selected life-history traits or K-selected traits? How does this correlate with current global human population growth?
Answer: In general, humans show more K-selected life history traits. For example, humans have a late age at first reproduction; have a long lifespan, a low mortality rate, and extensive parental care. Currently, human population growth is showing an exponential rate of increase. This is due to technological advances, which have made human populations less density-dependent.
3). Give your opinion: What constitutes the greatest threat to the future of the planet, the rapidly growing population in developing parts of the world or high resource consumption in the developed world?