Introduction to Ecosystems
Help Questions
AP Environmental Science › Introduction to Ecosystems
A kelp forest has kelp, sea urchins, sea otters, and decomposers; sunlight and nutrients vary. Which is an abiotic factor?
Decomposer bacteria breaking down dead kelp, forming detritus-based food pathways for small invertebrates and fish.
Kelp competing with other algae for space and light, altering primary productivity and habitat structure in the coastal zone.
Sea otters preying on sea urchins, reducing grazing pressure on kelp and changing the community’s species composition over time.
Dissolved nitrate concentration in seawater, which can limit kelp growth and influence overall ecosystem productivity.
Explanation
Abiotic factors are non-living components of an ecosystem, such as temperature, light, and nutrient levels, which influence organism growth and distribution. In the kelp forest, dissolved nitrates are essential for kelp photosynthesis and growth, acting as a limiting factor for productivity. Biotic factors, like predation by sea otters on urchins, interact with these abiotic elements to shape community structure. Ecosystems rely on the balance between biotic interactions and abiotic conditions to maintain stability and function. This distinction helps in understanding how environmental changes can disrupt ecosystem health and biodiversity.
A population of insects shows a survivorship curve with high juvenile mortality and few individuals reaching adulthood. Which type is it?
Type III, because many young die early while a few survivors live much longer, common in insects and many marine species.
Type IV, because survival increases with age due to learning, producing an upward-sloping survivorship curve in most populations.
Type I, because most individuals survive to old age and mortality increases sharply late in life, typical of large mammals.
Type II, because individuals have a constant probability of dying at any age, producing a straight-line decline in survival.
Explanation
Survivorship curves describe age-specific mortality patterns in populations, fundamental to understanding life history strategies. Type III curves show high early mortality with few survivors to adulthood, typical for species like insects that produce many offspring with little parental care. This contrasts with Type I (low early mortality, like mammals) and Type II (constant mortality, like birds). Ecosystems with Type III species often have high reproductive output to compensate for losses. These patterns influence population dynamics and community structure. Studying them helps predict responses to environmental changes. In essence, they reflect evolutionary adaptations to mortality risks.
In a stream, mayfly larvae are sensitive to pollution; their absence suggests degraded water quality. What are mayflies an example of?
Keystone predators, because they control fish populations through top-down regulation, maintaining stream biodiversity and stability.
Ecosystem engineers, because they build dams and physically restructure stream channels, creating wetlands and new habitats.
Invasive species, because they spread rapidly into new habitats and outcompete native organisms, reducing species richness.
Indicator species, because their presence or absence provides information about environmental conditions such as dissolved oxygen and pollution.
Explanation
Indicator species signal environmental conditions, like mayflies indicating clean water due to pollution sensitivity. Their absence suggests degradation, useful for monitoring. Ecosystems use such species for quick assessments. They reflect broader community health. Fundamentals include bioindicators in conservation. This differs from keystones, which disproportionately affect structure. Understanding indicators aids water quality management.
A meadow includes rabbits, grasses, foxes, and soil microbes. Which statement best distinguishes a community from an ecosystem?
A community describes global climate patterns and dominant vegetation, while an ecosystem describes only local predator-prey interactions.
A community includes abiotic factors like soil moisture and temperature, while an ecosystem includes only living organisms in one area.
A community includes interacting populations of different species, while an ecosystem includes those organisms plus abiotic components.
A community is a single species in an area, while an ecosystem is multiple species competing for the same limiting resource.
Explanation
A community consists of all interacting populations of different species in a given area, focusing on biotic relationships like predation and competition. An ecosystem expands this to include abiotic factors such as soil, water, and climate, which influence nutrient cycling and energy flow. In the meadow, rabbits, grasses, foxes, and microbes form a community, but adding soil and other non-living elements makes it an ecosystem. This distinction is key to understanding how physical environments shape biological interactions. Ecosystems provide a holistic view of how life sustains itself through integrated biotic and abiotic processes.
A forest has high species richness but one tree species dominates most biomass. Which term describes the dominance pattern?
Ecotone, because a boundary between ecosystems always contains one dominant species that excludes all other tree species.
Species richness, because richness measures how evenly individuals are distributed among species rather than total number of species.
Net primary productivity, because dominance is determined only by how much solar energy is converted into biomass each year.
Species evenness, because low evenness occurs when one species is much more abundant than others despite many species present.
Explanation
Species diversity in ecosystems includes both richness (number of species) and evenness (relative abundance). High richness means many species are present, but low evenness occurs when one dominates, as in this forest where one tree species comprises most biomass. This pattern is common in ecosystems where competitive advantages allow dominance without excluding others entirely. Evenness affects ecosystem resilience, as more even distributions can buffer against disturbances. In contrast, richness alone doesn't capture abundance disparities. Fundamentals of ecology emphasize measuring both aspects for a complete diversity picture. This helps in assessing forest health and biodiversity conservation.
In a food web, removing wolves leads to more deer and fewer young trees. Which concept best describes this pattern?
Mutualism, because wolves and deer both benefit when wolves are removed, leading to increased biodiversity and tree recruitment.
Primary succession, because predator removal exposes bare rock and initiates soil formation followed by pioneer species colonization.
Biomagnification, because wolf removal increases toxin concentrations in deer tissues, reducing tree growth through chemical transfer.
Trophic cascade, because changes at a top predator level indirectly affect lower trophic levels and vegetation through altered herbivory.
Explanation
Trophic cascades describe indirect effects rippling through food webs, such as wolf removal increasing deer herbivory and reducing tree recruitment. This top-down regulation maintains ecosystem balance. Producers suffer from unchecked consumers, altering habitat. Ecosystems demonstrate interconnectedness through such cascades. Studying them reveals the far-reaching impacts of species loss.
A beaver builds a dam, creating wetlands that increase plant and animal diversity. What role is the beaver playing?
Ecosystem engineer, because it physically alters habitat structure, changing resource availability and community composition.
Invasive species, because it enters a new ecosystem and outcompetes native organisms, reducing biodiversity through exclusion.
Primary producer, because it converts sunlight into biomass, forming the energetic base of the wetland food web.
Decomposer, because it breaks down dead organic matter and returns inorganic nutrients to the soil and water.
Explanation
Ecosystem engineers modify habitats, creating new niches and enhancing biodiversity, as beavers do by damming streams to form wetlands. This alters water flow, nutrient distribution, and species composition. Producers and consumers benefit from increased resources. Ecosystems rely on such species for structural diversity. This role highlights how individual actions scale to community-level changes.
In a tundra, lichens grow slowly; caribou graze; wolves prey on caribou. Which is a density-dependent factor for caribou?
Seasonal daylight changes reducing photosynthesis, limiting lichen biomass similarly whether few or many caribou are present.
A volcanic eruption depositing ash, decreasing lichen growth and caribou survival independent of population size in the region.
A severe blizzard that kills caribou regardless of herd size, reducing survival through exposure and limited forage access.
Increased disease transmission within larger herds, raising mortality as contact rates increase with caribou population density.
Explanation
Density-dependent factors regulate populations based on their size, such as disease spread increasing with crowding, which limits growth through higher mortality. In tundra ecosystems, caribou herds face resource competition and predation, but density-independent factors like blizzards affect all individuals equally. Lichens as producers support grazers, with wolves as predators adding top-down control. Population dynamics in ecosystems balance these factors to prevent overpopulation. This concept illustrates how ecosystems maintain equilibrium through feedback mechanisms.
After a wildfire, grasses colonize first, then shrubs, then trees. What concept does this sequence illustrate?
Competitive exclusion, because grasses permanently prevent shrubs and trees from occupying the habitat after the fire.
Biomagnification, because chemicals increase in concentration from grasses to shrubs to trees across trophic levels.
Secondary succession, because soil remains after disturbance and communities reestablish through predictable changes in species composition.
Primary succession, because fire removes soil and requires new soil formation before any plants can establish on bare rock.
Explanation
Secondary succession follows disturbances that leave soil intact, allowing faster recovery through stages from grasses to shrubs to trees. In post-wildfire ecosystems, pioneer species facilitate soil nutrient recovery, supporting later colonizers. This process increases biodiversity and complexity over time. Primary succession, in contrast, starts on bare rock. Understanding succession reveals how ecosystems restore themselves and build resilience.
A river ecosystem receives fertilizer runoff, increasing algal growth and decreasing dissolved oxygen. What process causes oxygen decline?
Nitrification produces oxygen as a byproduct, so more fertilizer should raise dissolved oxygen and prevent fish kills.
Predation by fish increases, causing oxygen to be used up by predators rather than decomposers during algal blooms.
Increased photosynthesis consumes dissolved oxygen, so higher algal growth directly removes oxygen from the river water.
Decomposition of excess algae increases microbial respiration, which consumes dissolved oxygen and can create hypoxic conditions.
Explanation
Eutrophication occurs when excess nutrients stimulate algal blooms, leading to oxygen depletion as decomposers break down dead algae through respiration. This process creates hypoxic zones harmful to fish and other aquatic life in river ecosystems. Photosynthesis by algae temporarily increases oxygen, but decomposition dominates, consuming it. Nutrient cycling is disrupted, affecting overall ecosystem productivity. Understanding this highlights the impact of human activities on water quality and biodiversity.