Ecosystem Interactions
A severe drought occurs in the Serengeti. Identify one abiotic factor that changes and predict one biotic factor that could change as a result.
Rainfall/water availability decreases → plant growth decreases, which could decrease herbivore populations.
resources that affect a population size
limiting factors
A wildebeest population increases while the amount of available grass stays constant.
What will most likely happen to competition per individual?
increase
Why doesn't carrying capacity represent the largest population ever observed in an ecosystem?
Carrying capacity represents the population the environment can sustain over time, not a temporary maximum.
Why is protecting an ecosystem often more effective than protecting only one species?
Protecting habitat preserves resources and interactions needed by many species, helping maintain biodiversity and ecosystem function.
Wildebeest eat grass. Lions eat wildebeests/
If the wildebeest population decreases significantly, predict two possible effects on this system.
Grass could increase because fewer wildebeest are eating it, while lions could decrease because less prey is available.
the maximum amount of organisms that can survive in one area based on available resources
carrying capacity
Explain why the same resource might be a limiting factor during one year but not another.
Resource availability and population size can change. For example, water might be abundant during a rainy year but become limiting during a drought.
A population fluctuates:
920 → 1,080 → 940 → 1,060 → 930 → 1,070
Estimate its carrying capacity and explain.
Approximately 1,000, because the population repeatedly fluctuates above and below that value
A protected area contains many wildebeest but does not include part of their migration route. Explain why the population could still be threatened.
Wildebeest depend on migration to reach resources. Protecting only part of their habitat may prevent access to important food and water.
Two ecosystems receive exactly the same amount of rainfall but contain different plant and animal species. Explain why the ecosystems may respond differently to a drought.
Ecosystems contain different components and interactions. Species may have different resource requirements or abilities to survive reduced water availability. The rain may affect plants differently too.
What evidence suggests this population is approaching carrying capacity?
Year l Population
1 503
2 652
3 791
4 900
5 949
6 951
7 948
Population growth is slowing and staying around 950, suggesting limiting factors are preventing continued rapid growth.
A grassland can support 20,000 herbivores. It contains:
If the zebra population increases substantially, explain how wildebeest could be affected even if the wildebeest population initially stays the same.
Zebras and wildebeest compete for grass. More zebras would increase competition and decrease the amount of food available per wildebeest.
A wildebeest population temporarily rises above carrying capacity.
Predict what will happen to:
Competition: ______
Food per individual: ______
Population growth: ______
Competition: increase
Food per individual: decrease
Population growth: slow/decrease
A conservation area increases in size, but an invasive plant spreads throughout it.
Explain why simply increasing the amount of protected land may not guarantee biodiversity increases.
Habitat quality and species interactions also matter. An invasive species could compete with native species and disrupt ecosystem interactions even inside protected land.
A student says:
“Abiotic factors only affect organisms directly.”
Use the Serengeti ecosystem to provide evidence that this statement is incorrect.
Abiotic factors can have indirect effects. For example, decreased rainfall can reduce grass growth. Less grass means less food for wildebeest, which can decrease wildebeest populations.
A population decreases sharply for two years and then begins increasing again. Give two different ecological explanations that could account for this pattern.
Examples include a temporary drought followed by increased rainfall, disease followed by recovery, temporary food shortage, changes in predation, or migration.
A drought causes a wildebeest population to decrease.
Classify each as direct or indirect:
Drought → less water available to drink
Drought → less grass → less food
Less drinking water = direct effect
Less grass/food = indirect effect
A student argues:
“Migration increases the carrying capacity because fewer animals remain in the ecosystem.”
What is wrong with this reasoning?
Migration can change the number of organisms in an area, but carrying capacity depends on the resources and environmental conditions available. A decrease in population size does not automatically mean carrying capacity changed.
Two conservation plans are proposed:
Plan A: Protect a small area containing many wildebeest.
Plan B: Protect several connected areas along the wildebeest migration route.
Using what you know about wildebeest ecology, explain a potential advantage of Plan B.
Connected protected areas could preserve access to resources throughout migration rather than protecting the animals only while they are in one location
A disease kills 30% of the lions in one area of the Serengeti. Predict an indirect effect this could have on grass populations in this ecosystem.
If lions die, there will be more wildebeest. If there are more wildebeest, there will be less grass
Scientists notice that a wildebeest population decreases during a drought. Why does this observation not prove that ONLY the drought caused the population decrease?
The pattern shows a relationship, but other factors—such as disease, predation, migration, or competition—could also affect population size. Scientists would need additional evidence.
Why do limiting factors often become stronger as a population approaches carrying capacity?
As population density increases, more individuals must share finite resources such as food, water, and space. Competition increases, slowing population growth.
During a drought, wildebeest migrate to an area receiving more rainfall. Explain the relationship among:
rainfall → grass → migration → survival
Greater rainfall increases grass growth. Wildebeest migrate toward areas with more available food and water, increasing their chances of obtaining the resources needed for survival.
Explain how conservation could affect an ecosystem's carrying capacity without directly adding any organisms to the ecosystem.
Conservation can protect habitat and resources such as vegetation, water, and space. Maintaining or increasing available resources can help maintain or increase carrying capacity.