Abiotic & biotic factors
A coral reef and a temperate forest are both ecosystems. Identify ONE abiotic factor unique to the aquatic ecosystem and ONE unique to the terrestrial ecosystem, as covered in your study guide.
Aquatic unique: availability of oxygen (or salinity/currents). Terrestrial unique: wind or fire.
Explain why equatorial regions are warmer than polar regions. Your answer must reference the behavior of solar radiation, not Earth's distance from the sun.
Sunlight strikes equatorial regions more directly (perpendicular), concentrating energy over a smaller surface area. At higher latitudes, sunlight hits at a slant, spreading the same energy over a larger area, delivering less energy per unit area.
A songbird defends a 2-acre territory during a year of abundant food but expands to a 5-acre territory the following year when food is scarce. What principle from the study guide explains this flexible behavior, and what does it reveal about how animals balance the costs and benefits of territorial defense?
Territory size is adjusted according to the costs and benefits of maintaining it. When food is scarce, a larger territory is needed to secure enough resources — the benefit of more food outweighs the higher energy cost of defending a bigger area.
Two warbler species live in the same spruce tree. Species A forages at the treetop; species B forages in the middle branches. When species B is experimentally removed, species A begins foraging throughout the entire tree. What does this reveal about the difference between a species' fundamental niche and its realized niche?
The fundamental niche is the full range of conditions and resources a species could use without competitors. The realized niche is the narrower range actually used when competitors are present. Species A's realized niche was compressed to the treetop by interspecific competition with species B; removing B allowed species A to expand to its fundamental niche.
A grassland food chain reads: grasses → grasshoppers → frogs → snakes → hawks. A pesticide is applied at very low concentration to the grasses. At which trophic level would you expect the highest concentration of the pesticide, and what is the name of the process that explains this? Why are top-level consumers especially vulnerable?
Hawks (the top trophic level) would have the highest concentration — this is biomagnification. Each consumer eats many organisms from the level below, so the pesticide accumulates and becomes more concentrated at each successive trophic level. Top consumers are most vulnerable because they accumulate the toxin from every level below them.
Two neighboring forests are studied. Forest A has wolves, elk, willows, beavers, and a river. Forest B has only elk, willows, and a river — wolves were removed 20 years ago. Ecologists note that Forest B's willows are nearly gone and the river banks have eroded. Which ecological concept explains the disproportionate impact of removing ONE species?
Keystone species — removing a single species (the wolf) causes dramatic cascading changes to community structure.
A large region of tropical rainforest is cleared for cattle ranching. Using the study guide's explanation of the water cycle, predict TWO specific consequences for the regional environment.
(1) Water vapor in the air decreases because transpiration from trees is eliminated, reducing precipitation. (2) The water cycle is disrupted — less moisture returns to the atmosphere over land, potentially causing regional drying or altered precipitation patterns.
A bee colony is attacked by a hornet. A worker bee stings the hornet and dies in the process. The worker shares approximately 75% of its genes with its sisters in the hive. Explain using kin selection theory why this self-sacrificing behavior can be favored by natural selection.
Kin selection favors altruistic behaviors when they benefit closely related individuals who share genes. By sacrificing itself to protect the hive, the worker increases the survival and reproduction of sisters who carry most of the same genes — effectively passing those shared genes to the next generation indirectly.
A farmer plants 500 acres of a single corn variety in dense rows. A fungal pathogen enters the field and destroys 90% of the crop within two weeks. A neighboring farm with 12 different crop varieties loses only 15% of its yield to the same pathogen. Explain this outcome using TWO principles from the study guide.
(1) When many potential hosts of the same type live close together, it is easy for a pathogen to spread from one to another — the monoculture provides ideal conditions for rapid spread. (2) Higher species diversity reduces pathogen impact — in the diverse farm, most plants are not susceptible hosts, breaking the chain of transmission.
A terrestrial ecosystem is experimentally sealed so no rock weathering occurs and no new soil forms. Over centuries, plant growth declines dramatically even though carbon and nitrogen remain available. Which biogeochemical cycle is responsible for the decline, and why can't this nutrient be replenished from the atmosphere the way nitrogen can?
The phosphorus cycle. Phosphorus has no atmospheric component — it cannot enter the ecosystem as a gas. The only natural source for terrestrial ecosystems is the weathering of rocks. Without new rock input, phosphorus is gradually lost through soil erosion into aquatic systems and cannot be replaced.
A scientist wants to study how a single black bear responds to changes in food availability across seasons. A second scientist studies all black bears in Yellowstone. A third studies bears, elk, wolves, and all plants together. Name the ecological level each scientist is working at, in order.
Organism level; population level; community level.
Over the open ocean, evaporation exceeds precipitation. Over land, precipitation exceeds evaporation and transpiration. What must be true about the movement of water between these two regions for the global water cycle to remain balanced?
Water must move from the oceans to land via the atmosphere — evaporated ocean water is carried inland by winds, falls as precipitation over land, and eventually flows back to the ocean through rivers and runoff, completing the cycle.
A population of deer has 500 individuals at the start of the year. Over the year: 120 are born, 40 immigrate from neighboring areas, 90 die, and 25 emigrate. What is the population size at year's end, and what does a net positive change suggest about current conditions relative to carrying capacity?
500 + 120 + 40 − 90 − 25 = 545 individuals. A net positive change suggests the population is currently below carrying capacity — resources are sufficient to support growth.
Milkweed plants produce cardiac glycosides that are toxic to most insects. Monarch butterflies, however, can tolerate these toxins and sequester them in their own bodies, becoming toxic to birds. Explain how this represents coevolution AND mutualism-like dynamics, using specific roles of each organism.
Coevolution: milkweed evolved toxins in response to herbivory; monarchs evolved tolerance as a counter-adaptation — each species acts as a selective force on the other. The monarch also gains a secondary benefit by sequestering the toxins as its own chemical defense against bird predators, while the milkweed's toxins serve their original defensive function.
Atmospheric CO₂ has been rising for over a century. The study guide states that normally the return of CO₂ by respiration closely balances its removal by photosynthesis. Explain precisely what human activity breaks this balance and why photosynthesis alone cannot compensate.
Burning fossil fuels releases carbon that was locked underground for millions of years — carbon that was never part of the active photosynthesis-respiration cycle. This adds a massive new source of CO₂ that existing photosynthetic capacity cannot offset, because the rate of fossil fuel combustion far exceeds the rate at which plants can fix the extra carbon.
A species of fish thrives in cold, oxygen-rich mountain streams but dies when placed in warm, slow lowland rivers. The study guide states that populations are adapted to local conditions. Using BOTH a biotic and an abiotic factor, explain why this fish cannot simply expand its range into warm lowland rivers.
Abiotic: warm lowland water has lower dissolved oxygen, which the fish cannot tolerate. Biotic: lowland rivers contain different predators or competitors the fish is not adapted to.
A farmer in an arid region begins pumping massive amounts of groundwater to the surface to irrigate crops. Predict TWO negative environmental consequences according to the study guide.
(1) Groundwater supplies are depleted over time. (2) Increased evaporation occurs at the surface as the pumped water evaporates, altering local moisture and potentially the microclimate.
You survey a meadow and find that wildflowers are clustered in dense patches near a stream, while being absent elsewhere. In a nearby pine forest, trees are spaced uniformly about 10 meters apart. Name the dispersion pattern in each habitat and identify the mechanism driving each pattern.
Wildflowers: clumped dispersion — driven by unequally distributed resources (water near the stream). Pine trees: uniform dispersion — driven by territorial interactions or competition for light and soil nutrients.
A tapeworm living in a deer's intestine and a tick feeding on the deer's blood are both parasites, but they differ in a key classification. State the difference, then explain why pathogens like bacteria and viruses are considered MORE dangerous to community structure than macroparasites like tapeworms.
The tapeworm is an internal parasite; the tick is an external parasite. Pathogens are more dangerous to community structure because they spread rapidly among hosts — when many potential hosts live close together, microscopic pathogens transmit easily and can devastate entire populations, while macroparasites typically don't spread between hosts as explosively.
A mountaintop ecosystem has extremely high primary production but supports only two trophic levels — plants and herbivores. A lowland ecosystem has moderate primary production but supports five trophic levels including a top predator. Explain why trophic level count depends on energy availability, and why the top predator in the lowland requires such a large geographic territory.
Each trophic level transfer loses energy — only a fraction of energy is passed up. More total energy input (primary production) is needed to support more levels. The top predator is the furthest removed from photosynthetic production, so an enormous amount of vegetation is needed at the base to funnel enough energy up through all intermediate levels to sustain even a small population of top predators — requiring vast territory.
A pronghorn antelope can run at 55 mph — far faster than the wolf, its only current major predator, which tops out at 35 mph. Explain the evolutionary mechanism responsible for this mismatch, naming the extinct organism involved and the ecological concept that connects past selection to present traits.
The American cheetah (extinct ~12,000 years ago) exerted selection pressure on pronghorn, driving the evolution of extreme speed. The pronghorn's speed is an 'evolutionary ghost of predators past' — a trait shaped by a selective force that no longer exists.
The same biome type — for example, temperate grassland — appears on multiple continents that have been separated for millions of years. The organisms in each grassland look and behave similarly but are not closely related. Explain using TWO concepts from the study guide why this occurs.
(1) Similar climates (temperature and precipitation patterns) in geographically distant locations support the same biome type. (2) Convergent evolution — unrelated organisms independently evolve similar traits and forms in response to the same environmental pressures.
A scientist claims that all animal behavior is purely innate and genetically hardwired. Using the concept of imprinting and the study guide's broader statement about behavior, construct a specific argument against this claim.
Imprinting shows that learning can modify innate behavior — a gosling is innately primed to follow a parental figure, but WHICH figure it follows depends on environmental experience during a limited developmental window. The study guide states most behaviors are a mixture of innate and learned behavior, directly refuting the claim that behavior is purely genetic.
The dinoflagellates inside coral tissue use the coral's CO₂ and NH₃ waste as raw materials for photosynthesis, returning sugars that provide over half the coral's energy. Ocean warming causes corals to expel their dinoflagellates (bleaching). Using the mutualism framework from your study guide, predict the cascade of consequences for the coral after bleaching.
The mutualism breaks down — the coral loses its primary energy source (over half its energy from photosynthetic sugars). Without the dinoflagellates recycling CO₂ and NH₃, waste products accumulate. The coral is energy-deprived and metabolically stressed, leading to starvation, reduced calcification, vulnerability to pathogens, and ultimately death if the dinoflagellates do not return.
Climate change is described as a potential agent of natural selection. A population of snowshoe hares turns white in winter for camouflage against snow. As winters shorten and snow-free periods lengthen due to climate change, white hares become highly visible against brown ground for more of the year. Trace the complete chain of events from this abiotic change through natural selection to a predicted evolutionary outcome in the population.
Abiotic change (less snow cover) alters the selective environment → white hares are more visible to predators for longer periods → white hares suffer higher predation rates → they survive and reproduce less than any hares with less-white or brown-tinted coats → those coat variants have higher fitness → over generations, alleles for white winter coats become less common in the population → the population