Decomposers are the microorganisms and fungi that break down dead organisms and waste. As they digest these materials, nitrogen that was locked in proteins and DNA is released into the soil mainly as ammonia, which becomes ammonium. This process, called ammonification, returns nitrogen to the soil so other organisms, like plants, can reuse it. While other creatures such as earthworms help by physically breaking down litter, the specific role of decomposers is to chemically decompose organic matter and recycle nitrogen back into usable forms. Examples include bacteria and fungi.

Genetic variation in sexual reproduction comes from mixing genetic material from two parents and from the reshuffling of genes during meiosis. During meiosis, homologous chromosomes exchange segments in a process called crossing over, creating chromosomes with new combinations of alleles. At the same time, the way chromosomes are independently assorted into gametes means each gamete carries a different set of parental alleles. When a sperm and egg meet in fertilization, their randomly chosen genetic material combines to form a zygote with a unique genetic makeup. This combination of two parents’ alleles plus recombination during meiosis is what drives the variety seen in offspring. While mutations can add new changes, and fertilization adds even more variety, the core mechanism described here is the mixing and rearranging of parental genes through crossing over and independent assortment.

Understanding what an organism does in its environment centers on its niche. The niche is the specific role the organism plays, including how it gets its energy (what it eats and at what level in the food web) and how it interacts with other living things (such as prey, predators, competitors, or mutualists). For example, a bee’s niche includes feeding on nectar and pollen and helping plants reproduce through pollination, while also interacting with flowers, predators, and other insects. This is different from the place where it lives (its habitat), which is simply the physical environment like a forest or meadow. It’s also not about how many individuals are in an area (population) or the entire system of living things and their surroundings (ecosystem). Niche best captures the organism’s functional role and its relationships in the environment.

Limiting factors are environmental features that prevent a population from growing without bound. When resources like food, water, space, or favorable conditions are limited, not all individuals can survive and reproduce, so the population levels off at a certain size. These factors can be biotic, such as predators, disease, or competition with others, or abiotic, such as drought, extreme temperatures, or habitat destruction. The statement that best describes a limiting factor is a factor that restricts population growth, because it directly limits how large the population can become. It's not about something that would increase growth, nor about something that never changes, nor something that only affects predators—the limiting factor is anything in the environment that reduces the potential for the population to keep increasing.