Ecology: Populations

1. Ecology: Populations Unit 9, Part 1

2. Populations • A population is a group of organisms belonging to the same species and living within a certain area. • In a pond where there are 38 bass, 42 frogs, and 21 turtles, there are three different populations present: the frogs, the bass, and the turtles.

3. Population Size • The size of a population is always changing, as new members are born and old members die.

4. Population Growth • Under ideal conditions, like unlimited food, absence of disease, lack of predators, and favorable temperatures, the size of a population would increase indefinitely and without limit. • This is called ideal population growth and can be expressed by a population growth curve, like the one on the next slide.

5. Ideal Growth Curve

6. Biotic Potential • The highest rate of reproduction under ideal conditions is called a population’s biotic potential. • For example, if there were no limiting environmental factors, a single bacterium would be able to produce, via binary fission, enough offspring to cover the earth 30 cm deep in just 36 hours.

7. Population Growth Curves • In most populations, growth occurs slowly at first, then rapidly increases, then slows down again. • This produces an S-shaped curve. • Eventually, the population stops increasing altogether, and the population size remains stable after that. Normal Growth Curve

8. Population Growth Rate Curve • A graph can also be used to show not only how much a population grows, but the rate at which it grows. This is called a growth rate curve. • When the population growth levels off, the population growth rate approaches zero. • This is similar to the acceleration of a car. When it reaches a constant velocity, acceleration equals zero.

9. Carrying Capacity • When a population arrives at the point where its size is no longer increasing, it has reached the carrying capacity of the environment. • The carrying capacity is the greatest number of individuals in a certain population that a given environment can support with its resources. • The carrying capacity can vary with the time of year or as conditions change.

10. Birth and Death Rates • At the carrying capacity, the number of organisms born or produced in a given period of time, or birth rate, balances the number of organisms that die during that time, or the death rate. • At that point, the size of the population remains fairly stable.

11. Limiting Factors • If you were to make growth curves of yeast, deer, mice, snakes, different trees, shrubs, or other complex organisms, you would see an S-shaped curve. • In each case, something causes the population to stop increasing. This is called a limiting factor. • Example of limiting factors include space, food supply, and amount of predation. Limiting factors vary from population to population.

12. Density-Dependent Limiting Factors • Population density refers to the size of a population that occupies a given area at any given point in time. A small population of frogs living in a given pond has a low population density. A large population living in the same pond has a high population density. • Factors that act on the population, such as a shortage in the food supply, are tied to population density.

13. Density-Independent Limiting Factors • Factors such as temperature and oxygen supply are not related to population density and are said to be density independent. • These factors are often related to physical aspects of the environment.

14. Density-independent factors have the same effect on a population regardless of its size. • The effects of density-dependent factors vary with the size of the population. The larger a population, the more intense a density-dependent limiting factor will be.

15. Predation and Food • Predation is the feeding of one organism on another, and it can be a factor limiting the size of a population. • It is difficult to tell for sure if predation alone is a limiting factor, but it is certain that the size of most populations is at least partly kept in check by predators.

16. Predation: Density-Dependent Factor • The greater the number of an animal, such as rabbits, in a given area, the greater the chances that a predator, such as coyotes, will find and kill one. • As the population of rabbits (the prey) falls due to predation, the fewer rabbits are killed by the coyotes (the predator).

17. Effects of Predation • If predators were removed from an environment, the size of the prey population would increase at first. But, as population increased, supplies and resources would decrease. This would cause prey population to decrease again, eventually leveling off near the carrying capacity.

18. Disease and Populations • Population density is closely related to the spread of certain diseases. For example, malaria is caused by the protist Plasmodium, which is carried by mosquitoes and transferred to humans through bites from the mosquitoes. • The denser the human population, the greater the chance of the disease being passed to others. The spread of malaria is also dependent on the mosquito population, without which the disease could not be passed.

19. Other Biotic Relationships • Many relationships and interactions exist among organisms of different species. • Some of these relationships are necessary to the survival of both organisms. • Other relationships are of no consequence to either organism.

20. Parasitism • A predator does not always have to be a larger animal, such as a lion, hunting down a smaller animal, such as a rabbit. • Sometimes, a very small organism will attack a larger organism and kill it. These smaller organisms are called parasites, and include tapeworms, flukes, ticks, and fleas. • Parasites live on other organisms, called hosts, and steal nutrients from them. • Parasitism is a density-dependent factor; the more hosts there are to feed on, the better the parasites are able to thrive and reproduce.

21. Commensalism • In some relationships, one organism benefits from a host without aiding or harming the host. • This is called commensalism. • An organism that benefits is called a commensal. • An example is a remora (a fish that attaches itself to the belly of a larger animal) which feeds from the leftovers of a shark’s meals. The shark is not harmed or helped through this relationship.

22. Mutualism • When both organisms benefit from a relationship, it is called mutualism. • Cycling of nitrogen relies on a mutualistic relationship. Many forms of bacteria can convert nitrogen gas to a usable form (this process is called nitrogen fixation). Many of these bacteria live in the roots of certain plants, where they break down dead plant material for food. • Within the root cells, the bacteria convert the nitrogen to ammonia, which is used by the plants to make proteins.

23. Ecosystems: Regulation of Populations Unit 9, Part 2

24. Interspecific Competition • Competition for food, supplies, resources, space, etc., among different species is called interspecific competition. • For example, when several predators feed on the same prey, the food supply may become limited, making the competition a density-dependent limiting factor.

25. Interspecific Competition can lead to… • Extinction - as animals compete for resources, one species will win the fight and wipe out the other species in that environment • Movement (emigration) - to reduce competition, one species may move to another close-by environment • Adaptation - as conditions change, some variations may be favored and lead to adaptations over time

26. Intraspecific Competition • Often, competition occurs between members of the same species. This is called intraspecific competition. • This may become a density-dependent limiting factor when members of a population seek out resources such as food, water, space, or nesting sites.

27. Life Cycles and Life Spans • Excessive intraspecific competition is avoided in many ways. One is by having alternate life cycles within a species. • For example, adult frogs do not compete with tadpoles because the habitats and foods of frogs and tadpoles are different. • Life spans may also reduce competition. In many insects, adults die shortly after the young are produced so the young and old do not compete for the same resources.

28. Dominance • Social behavior can reduce intraspecific conflicts. Consider how levels of authority reduce conflicts and preserve order in human populations, such as school administration or police forces. • This type of pattern, based on dominance and chains of command, is called social hierarchy.

29. Role Separation • Competition is also reduced in societies where members have definite roles. • In insect societies such as bees and ants, roles are determined genetically and physiologically. Usually, there is a queen, workers, soldiers, and foragers. Spreading out the work reduces conflict and competition among members of the society.

30. Behavioral and Physiological Changes • Overcrowding can lead to changes among members of a society. If rats are overcrowded and cannot escape, they may become aggressive to the point where they kill other rats. They may not build nests and the hormonal stress may reduce litter size. • This affects both birth and death rates, which changes the population size of the animals.

31. Emigration • Another solution to overcrowding in some populations is emigration, or moving out of an area. • Some organisms will remain in the original environment to keep it going, while others will move to a new area and start a new society.

32. Territoriality • Territoriality, the behavior of occupying and defending specific territories, is common among insects and vertebrates. • Because males having a territory are often the only ones to mate, population growth is limited. Possession of territories may also spread members of a population over a large area, ensuring a better food supply and more resources for a greater number of its members.

33. Ecosystems: The Human Population Unit 9, Part 3

34. The Human Population • The growth curve of human population resembles the curve seen in ideal conditions. • This suggests that there are no limiting factors on human population growth because of the rapid increase.

35. Declining Growth Rate • The birth rate is the number of live births that occur each year for every 1000 people. • The death rate is the number of deaths that occur each year for every 1000 people. • When people live longer, death rate decreases. When they continue to have the same number of babies as before, the birth rate increases. When the birth rate and death rate don’t even out, population sizes change.

36. Uneven Growth Rates • Declines in growth rate, though widespread throughout the world, are seen most in developed countries. • Two factors account for this: first, birth rates are almost four times higher in underdeveloped countries, and second, a large part of the population in underdeveloped countries is of reproductive age, which means they will more than make up for the lower birth rates in developed countries.

37. Zero Population Growth • Zero population growth is a condition in which the birth rate equals the death rate and the rate of population growth equals zero. This would be a good thing to accomplish, but does not appear to be attainable any time soon.

38. Future of the Population Explosion • There are many factors that have influenced the boom in human population. • Agricultural, cultural, sanitation, medical, and technological factors have all played a role in the increase of population size. • Human population cannot expand indefinitely due to limiting factors such as food supply, medical care, space, and natural resources. • How can we limit human population size?

39. Limiting Human Population Size • Most natural checks on population growth cause an increase in the death rate. One alternative for controlling growth is to decrease the birth rate, using artificial ways of preventing pregnancy, family planning, and even government involvement.

40. Why Not Let The Population Grow? • As mentioned before, food supplies, space, and natural resources are all limiting factors that affect the growth of the human population. • Control of the population is important for ecological reasons as well. Pollution and depletion of fossil fuels are increasingly serious as the population expands. Slowing the rate of growth would reduce such problems.