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Biodiversity refers to the variety of living organisms present in an ecosystem, including the range of species, the genetic variation within those species, and the variety of habitats and ecological processes they form. It is a measure of variety and difference rather than of total quantity or of a single population limit.
Producers such as plants, algae and some bacteria carry out photosynthesis, capturing light energy and storing it as chemical energy in organic molecules. This makes them the entry point for energy into most ecosystems, while other groups obtain their energy by consuming or breaking down organic matter already formed.
Trophic levels are counted from the base of the food chain, with producers forming the first trophic level. The grasshopper feeds directly on the grass, so it is the primary consumer and occupies the second trophic level, while the lizard and hawk occupy higher levels.
Most energy taken in by an organism is lost as heat through respiration, used in movement and maintenance, or remains in uneaten and undigested material. As a result, only roughly one tenth of the energy at one trophic level is typically passed on to the next, which limits the number of trophic levels a food chain can support.
Cellular respiration breaks down organic molecules to release energy and produces carbon dioxide as a by-product, which diffuses back into the atmosphere. Photosynthesis removes carbon dioxide from the atmosphere, while the other two processes concern nitrogen and water rather than the carbon cycle.
The combustion of fossil fuels such as coal, oil and gas for electricity, transport and industry releases large quantities of carbon dioxide that were stored underground for millions of years. This continuous addition of long-stored carbon is the dominant human contribution to rising atmospheric carbon dioxide concentrations.
When resources are abundant and limiting factors are largely absent, the population can grow at a rate proportional to its size, producing an exponential curve. Logistic growth only develops later, once crowding and resource limits begin to slow the rate of increase.
Abiotic components are the non-living physical and chemical features of an environment, such as temperature, light, water and soil minerals. Soil temperature is a physical condition, whereas fungi, earthworms and grass are living organisms and therefore biotic components.
Resource partitioning occurs when species that could compete divide a shared resource by using different parts, times or types of it. By feeding in different zones of the tree, the two warblers reduce direct competition and can coexist, whereas competitive exclusion would mean one species eliminates the other.
Mutualism is a symbiotic interaction in which each partner gains a benefit, such as food, shelter or protection, from the association. This distinguishes it from parasitism, where one partner is harmed, and from commensalism, where one partner is unaffected.
Primary succession begins on a surface with no existing soil or living community, such as new volcanic rock or a retreating glacier. Pioneer species such as lichens help break down rock and build soil, allowing later species to establish, whereas secondary succession starts where soil and seeds already remain.
Pioneer species must establish where conditions are harsh and soils are thin, so they typically tolerate exposure and poor nutrients and produce many easily dispersed propagules. Species requiring deep fertile soil or shade usually appear later in succession, once the pioneers have modified the environment.
Added nutrients promote rapid growth of algae, forming a bloom. When the algae die, decomposers break them down and consume dissolved oxygen, which can cause oxygen depletion and harm fish and other organisms.
Fat-soluble substances that persist are not easily excreted and tend to be stored in body fat, so each successive consumer ingests the accumulated burden of everything below it. This biomagnification produces the highest concentrations in long-lived top predators.
Energy enters ecosystems as sunlight captured by producers and is progressively lost as heat through respiration, so it must be continually resupplied. Matter such as carbon and nitrogen is not destroyed but is returned to the environment by decomposers and reused by producers.
A thick waxy cuticle limits evaporation from the leaf surface, and sunken stomata trap humid air that reduces the gradient driving water loss. Broad leaves with many stomata and large open flowers would increase water loss, so they are not water-conserving adaptations.
A keystone species has an effect on community structure that is much larger than its abundance would suggest, such as a predator controlling herbivore numbers. Its removal can trigger major changes throughout the community even though it may be relatively rare.
The Lincoln index estimates population size as the number marked multiplied by the total in the second sample, divided by the number of marked animals recaptured. Here that is 200 multiplied by 100 divided by 20, which gives an estimate of 1000 individuals.
Density-dependent factors have an effect that grows stronger as population density increases, because more individuals compete for the same limited resource. Competition for nesting sites intensifies as numbers rise, whereas droughts, eruptions and frosts act regardless of how many individuals are present.
Species richness is simply the count of different species recorded in an area, so comparing sites requires listing the species present at each. Biomass, tree height and photosynthesis rate measure abundance or ecosystem function rather than the number of distinct species.
Nitrogen-fixing bacteria, often living in nodules on the roots of legumes, reduce atmospheric nitrogen gas to ammonia that can be incorporated into organic compounds. This is the main route by which inert atmospheric nitrogen enters the biological cycle.
Removing the insects breaks the link between the grass and the small birds, so the birds lose a direct food source and may decline or switch prey. The grass may initially increase because grazing pressure falls, and the hawk is affected only indirectly through its prey.
Net primary productivity depends on the rate of photosynthesis minus respiration, so greater availability of the key limiting inputs, light and water, raises productivity during the growing season. Grazing removes plant material, and reduced canopy area lowers light capture.
The net rate of change is the difference between birth and death rates, which is 15 per 1000 per year. Applying this to a population of 5000 gives 5000 multiplied by 15 divided by 1000, an increase of 75 individuals, so the answer must be checked carefully against the arithmetic.
Island biogeography theory predicts that immigration rates are higher on islands near a source of colonists, while extinction rates depend mainly on island area. A closer island therefore tends to reach a higher equilibrium number of species than a distant one of similar size.
Photosynthesis is the only process listed that takes carbon dioxide from the atmosphere and converts it into organic carbon compounds in producers. Respiration, combustion and decomposition all release carbon dioxide back to the atmosphere. Therefore, photosynthesis is the key removal mechanism.
Producers, such as plants and algae, carry out photosynthesis to capture solar energy and store it as chemical energy in glucose and other organic compounds. Consumers and decomposers obtain energy by eating or breaking down organic matter; they do not convert solar energy directly. Thus, producers form the base of the energy flow.
Species richness is the count of different species in a given area. Genetic diversity refers to variation within a species, ecosystem diversity refers to the variety of habitats, and population density is the number of individuals per unit area. Therefore, species richness is the correct term for the variety of species.
Livestock, especially cattle and sheep, produce methane during digestion through enteric fermentation. Burning coal, deforestation and cement manufacturing primarily release carbon dioxide. Therefore, livestock digestion is a major methane source.
Decomposers, such as bacteria and fungi, break down dead organisms and waste, releasing nutrients like nitrogen and phosphorus back into the soil for reuse by producers. Nitrogen fixation is done by specific bacteria, photosynthesis by producers, and consumption by consumers. Thus, decomposers recycle nutrients.
Deserts receive very little rainfall, have sparse vegetation adapted to dry conditions, and often experience large day–night temperature swings. Tropical rainforests are wet and warm, temperate grasslands have moderate rainfall, and tundra is cold with permafrost. Therefore, desert is the correct biome.
Wind is continuously replenished by natural processes and is considered renewable. Coal, natural gas and petroleum are fossil fuels formed over millions of years and are finite. Therefore, wind is the renewable resource.
On average, only about 10% of the energy at one trophic level is transferred to the next level; the rest is lost as heat through respiration and in waste. This is known as the 10% rule. Therefore, 10% is the correct approximate value.
The net increase is births minus deaths: 50 - 30 = 20 individuals per year. Adding this to the initial population of 1000 gives 1020. Therefore, the population size after one year is 1020.
In fire-prone environments, some plants have seeds that remain dormant until a fire provides the heat cue to germinate, ensuring seedlings establish in the post-fire environment. Thin leaves, shallow roots and bright flowers are not specific adaptations to fire. Therefore, heat-triggered germination is the correct adaptation.
Resource partitioning occurs when species use different parts of a resource, such as different feeding heights, reducing competition and allowing coexistence. Competitive exclusion would result in one species eliminating the other, while mutualism and parasitism are different types of species interactions. Therefore, resource partitioning is the correct term.
Density-independent factors affect a population regardless of its size, such as natural disasters, extreme weather and drought. Competition, disease and predation are density-dependent because their impact increases with population density. Therefore, a severe drought is the density-independent factor.
Nitrification is the two-step process carried out by bacteria where ammonium is oxidised to nitrite and then to nitrate. Denitrification converts nitrate to nitrogen gas, nitrogen fixation converts atmospheric nitrogen to ammonia, and ammonification releases ammonium from organic matter. Therefore, nitrification is the correct process.
A point source is a single, identifiable discharge point, such as a pipe from a factory. Agricultural runoff, atmospheric deposition and leaching from lawns are non-point sources because they come from diffuse areas. Therefore, the pipe discharge is a point source.
The Lincoln-Petersen index estimates population size as (number marked in first sample × total caught in second sample) / number of marked recaptured. Here, (100 × 80) / 16 = 500. Therefore, the estimated population size is 500.
An ecological footprint measures the demand a person or population places on nature, expressed as the area of land and water required to produce the resources consumed and to absorb the wastes generated. Species richness, energy transfer and population growth rate are different concepts. Therefore, the first option is correct.
Biological control uses living organisms, such as predators, parasites or pathogens, to control pest populations. Applying pesticides is chemical control, monoculture is a cropping practice, and plastic mulch is a physical method. Therefore, introducing a natural predator is biological control.
The greenhouse effect is a natural process where certain gases in the atmosphere trap some of the heat radiated from Earth's surface, maintaining a habitable temperature. Human activities enhance it, but it is not caused solely by humans, nor does it trap all heat. Ozone depletion is a separate phenomenon. Therefore, the first statement is correct.
Without natural herbivores or other controls, an introduced plant may experience rapid population growth and spread, outcompeting native vegetation. It is unlikely to die out immediately, evolve quickly, or have no effect. Therefore, it may become invasive.
An ecosystem encompasses all living organisms in an area plus the abiotic factors (such as water, soil, and climate) with which they interact. A community consists only of the interacting populations of different species. Therefore, the first option correctly describes the distinction.
Low genetic diversity reduces the population's capacity to adapt to changing conditions, such as new diseases or climate shifts, increasing extinction risk. It does not confer increased disease resistance, higher reproduction, or greater tolerance. Therefore, reduced adaptability is the correct risk factor.
Ecosystem services are benefits humans receive from ecosystems. Forests regulate local climate by releasing water vapour through transpiration, which cools the air and influences rainfall. Synthetic fertilizers, plastics and fossil fuels are industrial products, not ecosystem services. Therefore, climate regulation is the correct example.
Frogs are prey for snakes. If frog numbers fall, snakes have less food and their population may decline. Grasshoppers would likely increase because frog predation decreases, hawks would not increase if snakes decline, and grass would likely decrease due to more grasshoppers. Therefore, the snake population may decline.
High nutrient concentrations in Lake A promote algal blooms, which increase turbidity and reduce water clarity. Lake B, being nutrient-poor, has fewer algae and clearer water. More suspended sediment or agricultural runoff would contradict the description of Lake B. Therefore, algal blooms explain the reduced clarity in Lake A.
The lag occurs because predators need time to reproduce and increase their numbers after prey become abundant; the energy gained from eating prey is not immediately translated into new predators. Prey typically reproduce faster than predators, not slower, and prey are often limited by food. Therefore, the time required for predators to reproduce explains the lag.