Introduction to ecosystems: species interactions
Walk through a longleaf pine forest in southern Georgia at dawn.
A red-tailed hawk drops onto a cotton rat.
Lichens crust the bark of a pine, and fungi wrap the pine's roots underground.
Two kinds of woodpecker work the same trunk, one probing the bark and the other drilling into dead branches.
Every one of these organisms needs food, water, space or light, and none of those resources is unlimited.
How organisms share, take and fight over limited resources is what this first section is about, and it sets up everything in this unit: the biomes, the cycles of matter and the flow of energy all come back to who eats whom and who competes with whom.
An ecosystem is all the organisms living in an area together with the nonliving (abiotic) parts of their environment: water, soil, air, sunlight and temperature.
The living parts (biotic) interact with each other and with the abiotic parts.
You need to know three kinds of interaction between species: predation, symbiosis and competition.
For each one, ask the same question: does each species gain (+), lose (−) or stay unaffected (0)?
Predators and prey
You need to know that "In a predator-prey relationship, the predator is an organism that eats another organism (the prey)."
The predator gains (+) and the prey loses (−).
A gray fox eating a cottontail rabbit is predation; so is a bluegill eating a mosquito larva, and so is a deer eating an oak seedling, since herbivores are predators of plants in this broad sense.
Predator and prey populations are linked by food.
When prey are plentiful, more predators survive and raise young, so the predator population grows.
More predators eat more prey, so the prey population falls.
With less food, fewer predators survive, so the predator population falls too, and the prey can recover.
The model below shows the cycle that results.
Read the model carefully.
The predator curve peaks after the prey curve, because predators need time to turn extra food into extra young.
The predator curve is also lower, because each predator eats many prey over its life.
The classic real example is the snowshoe hare and the Canada lynx in northern Canada, where fur-trading records kept for more than a century show both populations rising and falling on a cycle of about ten years.
Hare numbers are also driven by their own food supply, so the real cycle is not caused by the lynx alone, but the lag between the two peaks is real.
The link to resources is direct.
The prey is the predator's resource.
When prey become scarce, predators must travel farther, switch to other prey, or starve, and a predator that eats only one kind of prey has no fallback.
That is why a question that tells you an animal "feeds only on" one food is pointing you at its vulnerability: lose the food and the predator declines.
Symbiosis
You need to know that "Symbiosis is a close and long-term interaction between two species in an ecosystem.
Types of symbiosis include mutualism, commensalism, and parasitism."
The words "close and long-term" separate symbiosis from a single meal: the two species live together or meet over and over for much of their lives.
Type
Species 1
Species 2
United States example
Mutualism
Yucca plants and yucca moths in the Southwest: the moth pollinates the yucca flowers, and its larvae eat some of the seeds
Mutualism
Mycorrhizal fungi and tree roots: the fungus takes in water and phosphorus for the tree, and the tree supplies the fungus with sugars
Commensalism
0
Spanish moss hanging from live oaks in the Southeast: the moss gets a place in the light and takes water from the air, and the oak is largely unaffected
Commensalism
0
Cattle egrets following grazing cattle in Florida: the birds catch insects the cattle stir up, and the cattle neither gain nor lose
Parasitism
−
Blacklegged ticks feeding on white-tailed deer: the tick gets a blood meal, and the deer loses blood and can be infected with disease
Parasitism
−
Brown-headed cowbirds laying eggs in other birds' nests: the host raises the cowbird chick, often at the cost of its own young
Each type has a logic you can explain, not just a sign to memorize.
- Mutualism works because each species supplies a resource the other cannot easily get for itself. Coral animals and the algae living in their tissues are a marine example you will meet again: the algae make sugars by photosynthesis for the coral, and the coral gives the algae shelter and the carbon dioxide and nutrients from its waste. Bacteria in the root nodules of beans and clover are another: they fix nitrogen for the plant, and the plant feeds them sugar. That partnership is the heart of the nitrogen cycle later in this unit.
- Commensalism is the hardest to prove, because "unaffected" is difficult to measure. On the exam, treat it as one species gaining while the other neither gains nor loses in any way the question shows.
- Parasitism differs from predation in that a parasite usually lives on or in its host for a long time and rarely kills it quickly. A parasite that killed every host fast would destroy its own resource.
Competition and resource partitioning
You need to know that "Competition can occur within or between species in an ecosystem where there are limited resources.
Resource partitioning—using the resources in different ways, places, or at different times—can reduce the negative impact of competition on survival."
Competition is a (−, −) interaction: both competitors lose, because each uses up some of a resource the other needs.
It happens only where a resource is limited.
When there is more food than every animal can eat, there is nothing to compete for.
- Intraspecific competition is competition within one species: two male elk fighting over a territory, or seedlings of one pine species crowding each other for light.
- Interspecific competition is competition between species: coyotes and red foxes hunting the same rodents, or sugar maples and American beeches shading each other's seedlings.
If two species compete for exactly the same limited resource in exactly the same way, one usually does better and the other declines locally.
Many species that look like competitors avoid this by resource partitioning, which the definition breaks into three kinds:
- Different ways: using different parts or sizes of a resource. Two finch species on one island eat seeds, but the one with the larger, deeper beak cracks the large seeds and the other eats the small ones.
- Different places: using the same resource in different parts of the habitat. In a classic study published in 1958, the ecologist Robert MacArthur watched several species of small warblers feeding on insects in the same spruce trees in Maine and Vermont. Each species spent most of its feeding time in a different zone of the tree, as the diagram shows.
- Different times: using the same resource at different times of day or year. Red-tailed hawks hunt rodents in daylight, and great horned owls hunt the same fields at night.
Partitioning reduces competition because each species now depends mainly on a share of the resource the other species takes less of.
Each gets more food, so more individuals of both species survive and reproduce, and the two species can coexist.
That causal chain (less overlap, so less competition, so better survival for both) is exactly what an Explain part about partitioning needs.
How the availability of resources shapes interactions
Put the three interactions together and one rule runs through them: the scarcer a resource, the stronger the interactions over it.
In a drought year in a Texas grassland, grass is scarce, so grazers compete harder, predators find thinner prey, and parasites weaken hosts that are already short of food.
In a wet year the same interactions relax.
Recent exams have asked students to describe how a predator would respond when its prey becomes scarcer: the predator population declines, or the predator switches to other prey, or it moves into new areas to hunt.
The answer needs the response and the link to the lost food.
You also need to read these interactions in data.
A population graph where one species rises as the other falls points to competition or predation; two species whose numbers rise and fall together may be partners in mutualism, or may both be responding to the same resource.
Ask what each species gains or loses, and give the direction.
- Every free-response part is worth 1 point, except a Calculate part, worth 2. There are no levels and no half points.
- Identify: a word or short phrase. Nothing more is needed, and extra text can contradict you.
- Describe: the relevant characteristic, saying more than a vocabulary word. A trend names the direction of both variables.
- Explain: cause and effect with a direction: what changes, which way, and why. The effect without the reason, or the reason without the effect, earns nothing.
- Propose: one realistic, specific action that fits the actor (an individual, a city, a state) and the limits in the question.
- Justify: usually an additional advantage of the solution, other than the one the question names, with how the solution produces it.
- Make a claim: a position the data support, citing the data.
- Calculate: one point for the setup (every number and every operation shown) and one for the answer, scored separately. Show units; give a percent change its direction.
- Constraints are scored: "other than ...", "while still allowing ...", "a city", "in a marine ecosystem". An answer that breaks one scores 0.
- Do not restate the question. Give one example when one is asked for, since only the first is scored. Write complete sentences for describe, explain and justify, with no text abbreviations.
- Parts stand alone: a wrong answer in one part never costs a later point unless the question links them.
- Predator-prey relationship: In a predator-prey relationship, the predator is an organism that eats another organism (the prey).
- Symbiosis: Symbiosis is a close and long-term interaction between two species in an ecosystem. Types of symbiosis include mutualism, commensalism, and parasitism.
- Mutualism: a symbiosis in which both species benefit (+, +).
- Commensalism: a symbiosis in which one species benefits and the other is neither helped nor harmed (+, 0).
- Parasitism: a symbiosis in which one species (the parasite) benefits and the other (the host) is harmed (+, −).
- Competition and resource partitioning: Competition can occur within or between species in an ecosystem where there are limited resources. Resource partitioning—using the resources in different ways, places, or at different times—can reduce the negative impact of competition on survival.
Great blue herons and green herons both hunt fish in the same marshes in South Carolina.
Great blue herons, with long legs, wade into water up to about 50 cm deep and take larger fish.
Green herons, with short legs, hunt from the bank and from branches over the water and take smaller fish.
Explain how the feeding behavior of the two heron species allows them to coexist in the same marsh.
Full-point answer: The two herons partition the fish resource by place and by prey size: great blue herons take larger fish in deeper water, while green herons take smaller fish near the bank.
Because each species mainly eats fish the other does not, competition between them is reduced, so enough individuals of both species find food to survive and the two species can coexist.
Point 01. Accept: they use the resource in different places (deep water against the bank), so competition for fish is reduced and both survive.
Does not earn: "they eat different things" with no link to reduced competition; "the great blue heron is bigger so it wins" (describes a difference, not coexistence).
Weaker answer: "The herons coexist because they use resource partitioning."
This scores 0.
It names the term but gives no cause (how they divide the resource) and no effect (less competition, so both survive).
Describe the relationship between a parasite and its host.
Full-point answer: A parasite lives on or in its host over a long period and takes nutrients from it, so the parasite benefits while the host is harmed, for example a tick feeding on a deer's blood.
Point 01. Accept: one species benefits and the other is harmed in a close, long-term interaction.
Does not earn: "the parasite uses the host" (says nothing about who gains or loses); "both are harmed" (that describes competition).
Weaker answer: "It is a symbiotic relationship."
This scores 0.
It is a vocabulary word that every type of symbiosis shares, so it describes no characteristic of parasitism.
In a forest in northern Minnesota, a disease reduces the number of snowshoe hares by more than half in one winter.
Lynx in the forest feed mainly on snowshoe hares.
Describe how the lynx population is likely to respond to the decline in snowshoe hares.
Full-point answer: With fewer hares to eat, fewer lynx find enough food to survive and raise kittens, so the lynx population will decrease over the following year or two.
Point 01. Accept: lynx will move to other areas to find hares, so the local population falls; lynx will switch to other prey such as grouse or squirrels.
Does not earn: "the lynx will be affected" (no direction); "the lynx population increases because there is less competition" (ignores the loss of food stated in the stem).
Weaker answer: "The lynx population will change because they eat hares."
This scores 0.
It restates the stem and gives no direction.
For an Explain part about competition or partitioning, write the full chain in one or two sentences: how the species divide the resource (way, place or time), so competition is reduced, so more of each species survive and they coexist.
Naming "resource partitioning" without the chain earns nothing.
- Calling any feeding relationship symbiosis. A hawk catching a rat once is predation; symbiosis needs a close, long-term interaction. The mistake is tempting because both involve two species living near each other.
- Mixing up commensalism and mutualism. Ask whether the second species gains anything. If it gains, it is mutualism; if it neither gains nor loses, it is commensalism.
- Writing that parasites kill their hosts. Most parasites harm their host without killing it quickly; an organism that kills and eats its food at once is a predator.
- Saying resource partitioning means species "share" resources. Partitioning means they divide them, each using a different way, place or time, which is why competition falls. "Share" suggests both use the same thing, which is the opposite.
- Describing a predator's response without a direction. "The predator will be affected" earns nothing; "the predator population will decrease because it has less food" earns.
Check your understanding
1. Bumblebees collect nectar from the flowers of wild blueberry plants in Maine and carry pollen from flower to flower. Which of the following best describes this interaction?
- (A) Commensalism, because the bumblebee benefits and the blueberry is unaffected
- (B) Competition, because both species use the energy in the nectar
- (C) Parasitism, because the bumblebee takes nectar the blueberry needs
- (D) Mutualism, because the bumblebee gets food and the blueberry is pollinated
Total [1]
2. Identify the type of species interaction in which both species are harmed.
Total [1]
3. Gray foxes and coyotes both hunt rabbits and rodents in the same canyon in southern California. Field cameras show that gray foxes hunt mostly in dense brush, while coyotes hunt mostly in open grassland.
Part A. Identify the type of resource partitioning shown by the two predators.
Part B. Explain how this partitioning increases the survival of both predators.
Total [2]
4. Describe one way, other than by feeding at different times of day, that two bird species eating insects in the same forest could partition their food resource.
Total [1]
5. Water hyacinth, a floating plant, has spread over the surface of a pond in Louisiana. Below its mats, native rooted plants receive much less light.
Describe how the spread of water hyacinth affects the native rooted plants, using the idea of competition.
Total [1]
6. In a desert in Arizona, two species of kangaroo rat eat seeds. In a wet year seeds are plentiful; in a drought year few seeds are produced.
Describe how the competition between the two kangaroo rat species is likely to differ between the wet year and the drought year.
Total [1]
Answers
1. (D). Both species gain: the bee gets nectar (food) and the blueberry's flowers are pollinated, so it can produce fruit and seeds. (A) misses the blueberry's gain from pollination. (C) treats the nectar as a loss, but the plant makes nectar to attract pollinators. (B) is wrong because the two species are not using a limited resource that both need; the plant makes the nectar.
2. Point 01. Competition. Accept: interspecific competition; intraspecific competition (both describe a (−, −) interaction). Does not earn: parasitism (only the host is harmed); predation.
3. Scoring:
- Point 01 (Part A): Partitioning by place (habitat): the gray foxes hunt in brush and the coyotes in grassland. Accept: spatial partitioning; using different places. Does not earn: "by time" (the cameras show different places, not times); "by prey" (both take rabbits and rodents).
- Point 02 (Part B): Because each predator hunts mainly in a different habitat, they catch fewer of the same individual prey, so competition between them is reduced and each finds more food, so more gray foxes and coyotes survive. Accept: less overlap means less competition for prey, so both populations can persist. Does not earn: "they don't compete" (no mechanism); "gray foxes are better hunters in brush" (no link to survival of both).
4. Point 01. The two species could feed in different places, for example one species feeding on insects in the treetops and the other on insects on the forest floor. Accept: eating different sizes or kinds of insect (different ways); one species gleaning insects from leaves while the other catches them in the air. Does not earn: "one feeds at night" (the excluded answer, feeding at different times); "they share the insects" (no partitioning).
5. Point 01. The water hyacinth outcompetes the native plants for light: its floating mats absorb the sunlight before it reaches the water below, so the native plants photosynthesize less and their growth and numbers decrease. Accept: the hyacinth takes the limited light (and nutrients), so the native plants decline. Does not earn: "the hyacinth kills the native plants" (no resource named); "the native plants are affected" (no direction).
6. Point 01. Competition is stronger in the drought year: seeds are a limited resource then, so each rat that eats a seed leaves less for the other species, while in the wet year there are more seeds than both species need. Accept: in the wet year there is little or no competition because the resource is not limited. Does not earn: "they compete more in the wet year because there are more rats" (reverses the effect of the resource); "competition changes" (no direction).