8.5 Community Ecology

1. AList the different plant species found in each meadow, without counting the individuals of each.
   A student who thinks diversity is just the number of species picks this. A species list gives the number of species but not their relative abundance, which a diversity index also needs.
2. BCount the total number of plants in each meadow, without recording which species each belongs to.
   A student who thinks diversity depends on the number of individuals picks this. A total count without species identities gives neither the number of species nor their relative abundance.
3. CCount the individuals of each species in several randomly placed quadrats of equal size in each meadow. Correct
   Species diversity depends on both the number of species and their relative abundance, so the individuals of each species must be counted (n and N for Simpson's Diversity Index). Several randomly placed quadrats of the same size in each meadow give a representative sample and a fair comparison.
4. DCount the individuals of each species in one quadrat placed in the middle of each meadow.
   A student who thinks one sample represents a whole meadow picks this. A single quadrat in one place may miss species and does not show how much counts vary; several randomly placed quadrats are needed.

CED 8.5.A.1 · Read this in Fix

1. ABox Y Correct
   A community is all the interacting populations of different species in an area. Box Y includes the populations of every kind of organism in the pond (fish, frogs, insects, algae, plants, fungi and microorganisms) but none of its nonliving parts.
2. BBox W
   A student who confuses a community with a population picks this. Box W contains only one species, the bluegill: it is a population. A community includes the populations of all the species living there.
3. CBox X
   A student who thinks a community is made up only of animals picks this. Box X leaves out the algae, plants, fungi and microorganisms, which are also populations in the pond and interact with the animals.
4. DBox Z
   A student who includes nonliving parts in the community picks this. Box Z adds the water, sediment and sunlight, so it represents the pond ecosystem; a community includes only the living populations.

CED 8.5.B.1 · Read this in Fix

1. AChemical energy in the phosphate and other minerals it takes up from the soil
   A student who thinks soil minerals are a source of energy picks this. Phosphate supplies matter, such as phosphorus atoms for nucleic acids and phospholipids, but not energy; the fungus's energy comes from the tree's sugars.
2. BSunlight absorbed by the tree, whose sugars carry that energy to it Correct
   Through this interaction the fungus gains access to energy it could not obtain alone: the tree captures light energy in photosynthesis and stores it in sugars, some of which pass to the fungus. In return, the fungus increases the tree's access to phosphate, a source of matter, from the soil.
3. CLight energy captured by the fungus itself in photosynthesis, as plants do
   A student who thinks fungi are plants picks this. Fungi do not carry out photosynthesis; this fungus obtains sugars, and the energy in them, from the tree.
4. DEnergy passed back and forth between the fungus and the tree, and so reused
   A student who thinks energy cycles between organisms picks this. Matter such as phosphate and carbon passes between the partners, but energy is not recycled: it enters as light captured by the tree and is eventually lost as heat.

CED 8.5.B.2 · Read this in Fix

1. AMutualism, as K and L live in close association
   A student who thinks every close association (symbiosis) is mutualism picks this. Mutualism requires + effects in both directions, as between L and M in the model; between K and L the effects are + and −.
2. BCommensalism, as K gains at the expense of L
   A student who confuses commensalism with parasitism picks this. In commensalism one species benefits and the other is neither helped nor harmed (+ and 0); when one species benefits at the other's expense, the relationship is parasitism or predation.
3. CParasitism, as K benefits and L is harmed Correct
   The model shows a + effect of L on K and a − effect of K on L, so K benefits and L is harmed. Because K lives on the body of L for most of its life and benefits at L's expense, the relationship is parasitism, a symbiosis with + and − effects.
4. DCompetition, as one of the two species is harmed
   A student who thinks any interaction in which a species is harmed is competition picks this. Competition harms both species (− and −), as between M and N in the model; here K benefits.

CED 8.5.B.3 · Read this in Fix

1. AIndividuals reproduce faster because the species needs to fill the space the competitor left.
   A student who thinks populations change to meet a need picks this. Individuals do not reproduce faster because the species needs to fill space; the increase comes from greater access to resources.
2. BFood and other resources are shared among fewer individuals, so births rise or deaths fall. Correct
   Competing species use some of the same limited resources, such as food or space. With the competitor gone, more of these resources are available to each individual of the remaining species, so its birth rate can rise or its death rate fall, and the population increases.
3. CThe competitor had been eating the remaining species, so fewer of its individuals now die.
   A student who confuses competition with predation picks this. Competitors harm each other by using the same limited resources, not by eating each other; the remaining species benefits because more resources are available.
4. DWith no competitor, the population is no longer limited and would go on growing without end.
   A student who thinks a population grows without limit once a competitor is removed picks this. Other factors, such as the total amount of food and space, predators and disease, still limit the population, so it levels off at a new size.

CED 8.5.B.4 · Read this in Fix

1. AQ is more diverse, as its index is 0.85 and P's is 0.25.
   A student who stops after calculating Σ(n/N)² picks this. Σ(n/N)² is the probability that two individuals picked at random belong to the same species, so a large value means low diversity; the index is 1 − Σ(n/N)², 0.75 for P and 0.15 for Q.
2. BQ is more diverse; it has 5 species, whereas P has only 4.
   A student who thinks diversity is just the number of species picks this. Q has more species, but 92 of its 100 individuals belong to one species; Simpson's index, which also reflects how evenly individuals are spread among species, is 0.75 for P and 0.15 for Q.
3. CP and Q are equally diverse; each has 100 organisms.
   A student who thinks diversity depends on the number of individuals picks this. Both communities have 100 individuals, but diversity depends on the number of species and how evenly individuals are spread among them: 0.75 for P and 0.15 for Q.
4. DP is more diverse; its index is 0.75, and Q's is 0.15. Correct
   N = 100 in each community. For P, Σ(n/N)² = 4 × (0.25)² = 0.25, so the index is 1 − 0.25 = 0.75. For Q, Σ(n/N)² = (0.92)² + 4 × (0.02)² = 0.848, so the index is 1 − 0.848 = 0.15. Q has one more species, but 92 of its 100 individuals belong to species A, so it is far less diverse.

Working N = 100 in each community. P: n/N = 0.25 for each of species A–D (and 0 for E), so Σ(n/N)² = 4 × 0.0625 = 0.25 and Diversity Index = 1 − 0.25 = 0.75. Q: (92/100)² + 4 × (2/100)² = 0.8464 + 0.0016 = 0.848, so Diversity Index = 1 − 0.848 = 0.152 ≈ 0.15. P has the higher index, so P is more diverse. Distractors: stopping at Σ(n/N)² gives 0.25 for P and 0.85 for Q, reversing the conclusion; counting species alone favors Q (5 species versus 4); counting individuals gives 100 in each.

CED 8.5.A.1 · Read this in Fix

1. AAlgae would increase, as more minnows would eat more of the zooplankton that feed on algae. Correct
   This is a trophic cascade. With few bass, fewer minnows are eaten and the minnow population grows; more minnows eat more zooplankton, so zooplankton decline; with fewer zooplankton grazing, the algae increase. A change at the top of a food chain affects populations several links below it.
2. BAlgae would not change, as bass do not feed on algae and so do not affect them.
   A student who thinks a change affects only the populations directly linked to it picks this. The bass affect the algae indirectly, through the minnows and the zooplankton.
3. CAlgae would decrease, as the loss of any species harms every other species in the lake.
   A student who thinks the loss of any species harms all others picks this. Removing a predator benefits its prey (the minnows), and the effects alternate down the chain, so the algae increase.
4. DAlgae would increase at first, but all populations would then return to their original sizes.
   A student who thinks communities always return to their original balance picks this. With the bass gone, the minnows' main predator is missing, so the populations do not return to their former sizes; the community changes.

CED 8.5.B.3 · Read this in Fix

1. AEach species moves aside to leave food for the other, so that it helps the other species survive.
   A student who thinks organisms act to help other species picks this. Each species shifts toward heights where it meets less competition, which benefits that species itself; nothing in the data suggests that either gives way for the other's sake.
2. BEach bird's beak changed during its life so that it could catch the insects found at a new height.
   A student who thinks individuals change their bodies to meet a need picks this. The data show a change in where the birds feed, a change in behavior; an individual's beak does not change shape during its life because of where it needs to feed.
3. CSpecies A is being driven out of the forest, as competing species cannot live together for long.
   A student who thinks competing species cannot coexist picks this. Both species are present and feeding where they live together; by partitioning the forest they reduce competition, which can allow them to coexist.
4. DFeeding at different heights reduces competition for the same insects, so they can coexist. Correct
   Where each species lives alone, both feed mostly at middle heights (50%), so where they live together they would compete for the same insects. Where they live together, A feeds mostly high (70%) and B mostly low (65%), so their use of the forest overlaps much less. This niche partitioning reduces competition, which can allow the two species to coexist.

CED 8.5.B.3 · Read this in Fix

1. ABoth Y and Z reduced the population of X, although Y had a larger effect on it than Z.
   A student who treats any difference between means as real picks this. The mean with Z (198 cells/mL) is lower than alone (210 cells/mL), but the error bars overlap widely, so this difference could be due to chance.
2. BY reduced the population of X, but an effect of Z on X is not supported by the data. Correct
   The mean density of X was 210 cells/mL alone and 120 cells/mL with Y; the ±2 SE bars (196–224 and 104–136) do not overlap, so Y likely reduced X's population, as expected if the two compete for the limited food. With Z the mean was 198 cells/mL, but its bar (180–216) overlaps that of X alone, so an effect of Z is not shown.
3. CZ had no effect on the population of X, as the error bars for those two groups overlap.
   A student who thinks overlapping error bars prove there is no effect picks this. Overlap means an effect of Z has not been shown, not that Z has no effect; a small effect could be missed with 8 cultures.
4. DX held back its own growth when Y was present, so as to leave enough food for Y.
   A student who thinks organisms act to help other species picks this. X's population is lower with Y because the two share a limited food supply, so each X cell obtains less food; X does not limit its growth for Y's benefit.

Working No test statistic is calculated; the decision rests on the ±2 SE error bars. X alone: 210 cells/mL (bar 196–224). X with Y: 120 cells/mL (bar 104–136); the bars do not overlap (136 < 196), so the reduction is likely real. X with Z: 198 cells/mL (bar 180–216); the bar overlaps that of X alone (196–216), so the 12 cells/mL difference may be due to chance and an effect of Z is not supported. Overlap does not prove that Z has no effect.

CED 8.5.B.4 · Read this in Fix

1. APrey density falls steadily over the 20 years, as more and more of the prey are eaten.
   A student who thinks predators drive their prey toward extinction picks this. Prey density rises and falls in cycles, between about 15 and about 80 per km²; it recovers whenever predators become scarce.
2. BPredator density is higher than prey density at every point in the 20 years.
   A student who thinks predators are more numerous than their prey picks this. The populations are plotted on different axes: predator density never exceeds about 12 per km², while prey density reaches about 80 per km².
3. CEach peak in predator density is reached two years after a peak in prey density. Correct
   Prey density peaks at years 4 and 14, and predator density at years 6 and 16, two years later each time. As prey become abundant, more predators survive and reproduce; as predators increase, more prey are eaten and the prey population falls, followed by the predator population.
4. DPrey density rises without limit in each period when predator density is low.
   A student who thinks a population grows without limit when its predators are scarce picks this. Each rise in prey density levels off at about 80 per km² and then falls, as predation increases.

CED 8.5.B.4 · Read this in Fix

1. AThey would grow as before, as their leaves can take in the nitrogen gas they need directly from the air.
   A student who thinks plants use nitrogen gas directly from the air picks this. Plants cannot use N₂; they take up nitrogen compounds, which here come from the bacteria.
2. BThey would soon begin to convert nitrogen gas themselves, as they need a new source of nitrogen.
   A student who thinks organisms acquire new abilities when they need them picks this. The plants do not gain the ability to convert N₂ because they need nitrogen; that ability belongs to the bacteria.
3. CThey would grow more, as they would no longer give up their sugars to the bacteria in their roots.
   A student who considers only the cost of an interaction picks this. The plants would keep the sugars they supplied, but they would lose their main source of nitrogen, which limits growth far more in nitrogen-poor soil.
4. DThey would grow less, as they could no longer obtain enough nitrogen to build proteins and nucleic acids. Correct
   Through their interaction with the bacteria, the legumes gain access to nitrogen they cannot obtain from nitrogen-poor soil, and the bacteria gain sugars. Without the bacteria, the plants lack the nitrogen needed to build proteins and nucleic acids, so they grow less.

CED 8.5.B.2 · Read this in Fix

1. ADeer grazing reduced the preferred species, freeing light and space for the species deer avoid. Correct
   Interactions between populations change communities over time. The deer eat the preferred species, reducing their populations; the species the deer avoid then face less competition for light, water and space, so their populations increase, and the species composition of the community changes.
2. BThe avoided species increased in order to replace the lost plants and keep the community in balance.
   A student who thinks communities act to stay in balance picks this. Populations do not increase in order to replace others; the avoided species increased because competition from the preferred species was reduced.
3. CThe rise of the avoided species is unrelated to the deer, which affect only the plants that they eat.
   A student who thinks a change affects only the populations directly linked to it picks this. The deer affect the avoided species indirectly, by reducing the populations of their competitors.
4. DPreferred plants developed bad tastes during their lives and passed these on to their offspring.
   A student who thinks traits acquired during an individual's life are inherited picks this. Offspring do not inherit changes that a plant develops during its life; the change in the community comes from changes in the sizes of different species' populations.

CED 8.5.B.1 · Read this in Fix