1. Lesson Overview
5.2 Limits to Growth

2. THINK ABOUT IT
What determines the carrying capacity of an environment for a particular species?
In its native Asia, populations of hydrilla increase in size until they reach _____________________, and then population growth stops. But here in the United States, hydrilla grows out of control.
Why does a species that is “well-behaved” in one environment grow out of control in another?

3. _____________________
A _______________is a factor that controls the growth of a population.
There are several kinds of limiting factors.
Some—such as competition, predation, parasitism, and disease—_____________________ ____________________.
Others—including natural disasters and unusual __________________ _____________________________.

4. _________________________________
What limiting factors depend on population density?
Density-dependent limiting factors include
____________________

5. Density-Dependent Limiting Factors
________________________________operate strongly only when population density—the number of organisms per unit area—reaches a certain level. These factors do not affect small, scattered populations as much.
Density-dependent limiting factors include competition, predation, herbivory, parasitism, disease, and stress from overcrowding.

6. ___________________
When populations become crowded, individuals ____________ _________________________________, and other essentials.
Some individuals obtain enough to _______________________.
Others may obtain just enough to live but not enough to enable them to raise offspring.
Still others may _______________________from lack of shelter.
Competition can lower ________________________________ ______________.

7. Competition
Competition is a ____________________________. The more individuals living in an area, the sooner they use up the available resources.
Often, ________________are related to one another. Many grazing animals compete for territories in which to breed and raise offspring. Individuals that do not succeed in establishing a territory find no mates and cannot breed.
For example, male wolves may fight each other for territory or access to mates.

8. This type of competition is a major force behind _________________.
Competition can also occur between members of _________ _____________that attempt to use similar or overlapping resources.
This type of competition is a major force behind _________________.

9. ________________________
The effects of _____________ on prey and the effects of ________ on plants are two very important density-dependent population controls.

10. Predator-Prey Relationships
This graph shows the fluctuations in __________ populations on Isle Royale over the years.
Sometimes, the moose population on Isle Royale grows large enough that moose become easy prey for wolves. When wolves have plenty to eat, their population grows.

11. Predator-Prey Relationships
As wolf populations grow, they begin to kill more moose than are born. This causes the moose ____________to rise higher than its ______________, so the moose population falls.

12. Predator-Prey Relationships
As the moose population drops, wolves begin to starve. _____________raises wolves’ death rate and lowers their birthrate, so the wolf population also falls.
When only a few predators are left, the moose death rate __________, and the __________________.

13. Herbivore Effects
__________can also contribute to changes in population numbers. From a plant’s perspective, herbivores are ___________.
On parts of Isle Royale, large, dense ___________ populations can eat so much ____________that the population of these favorite food plants drops. When this happens, moose may suffer from lack of food.

14. Humans as Predators
In some situations, ____________________ limits populations.
For example, _____________, by catching more and more fish every year, have raised ______ death rates so high that birthrates cannot keep up. As a result, cod populations have been dropping.
These populations can _____________ if we scale back fishing to lower the death rate sufficiently.
Biologists are studying __________and the ______________ of the cod population to determine how many fish can be taken without threatening the survival of this population.

15. Parasitism and Disease
_____________________________________feed at the expense of their hosts, weakening them and often causing disease or death.
For example, __________feeding on the blood of a hedgehog can transmit __________________ that cause disease.
Parasitism and disease are ___________________ __________________, because the denser the host population, the more easily parasites can spread from one host to another.

16. Parasitism and Disease
This graph shows a sudden and dramatic drop in the wolf population of Isle Royale around At this time, a _____________________ of wolves, canine parvovirus (CPV), was accidentally introduced to the island.
This virus killed all but ___ wolves on the island—and only three of the survivors were females.

17. Parasitism and Disease
The removal of wolves caused the moose population to skyrocket to ________.
The densely packed moose then became infested with _____________that caused hair loss and weakness.

18. ____________________________
Some species fight amongst themselves if _______________________.
Too much fighting can cause high levels of ________, which can weaken the body’s ability to resist disease.
In some species, stress from overcrowding can cause females to ___________________their own offspring.
Stress from overcrowding can __________________ ________________________________, and can also increase rates of emigration.

19. ________________________________
What limiting factors do not typically depend on population density?
Unusual weather such as
_________________
______________________________

20. Density-Independent Limiting Factors
_____________________________affect all populations in similar ways, regardless of population size and density.
______________such as hurricanes, droughts, or floods, and ________ _____________such as wildfires, can act as density-independent limiting factors.

21. Density-Independent Limiting Factors
A________________, for example, can kill off great numbers of fish in a river.
In response to such factors, a population may “_________.” After the crash, the population may build up again quickly, or it may stay low for some time.

22. True Density Independence?
Sometimes the effects of so-called density-independent factors can actually ___________ with population density.
It is sometimes difficult to say that a limiting factor acts _______ in a density-independent way.

23. True Density Independence?
On Isle Royale, for example, the moose population grew exponentially for a time after the wolf population crashed. Then, a bitterly ____________ with very __________________ covered the plants that moose feed on, making it difficult for moose to move around to find food.

24. True Density Independence?
Because this was an island population, _____________was not possible. Moose weakened and many died.

25. True Density Independence?
In this case, the effects of bad weather on the large, dense population were _________than they would have been on a small population. In a smaller population, the moose would have had more food available because there would have been______________________.

26. Controlling Introduced Species
In hydrilla’s natural environment, ______________ __________________________keep it under control.
Perhaps _________________________ devour it, or perhaps pests or diseases weaken it. Those limiting factors are not found in the United States, and the result is runaway population growth!
Efforts at artificial density-independent control measures—such as _____________________ _________________________—offer only temporary solutions and are expensive.

27. Controlling Introduced Species
Researchers have spent decades looking for natural predators and pests of hydrilla.
The best means of control so far seems to be an ______________called _________, which views hydrilla as an especially tasty treat.
Grass carp are __________________to the United States. Only ________________ grass carp can be used to control hydrilla. Can you understand why?