The grouping or assemblage of plants, animals and microbes we observe when we study a natural forest, a grassland, a pond, a coral reef or some other undisturbed area, is referred to as the area’s biota or biotic community. The plant portion of the biotic community includes all vegetation, from large trees down through to microscopic algae. Likewise, the animal portion includes everything from large mammals, birds, reptiles and amphibians through to earthworms, tiny insects and mites. Microbes encompass a large array of microscopic bacteria, fungi and protozoans. Thus, the biotic community comprises a plant community, an animal community and a microbial community. The particular kind of biotic community found in a given area is, in large part, determined by abiotic factors such as the amount of water or moisture present, the temperature, the salinity, or the type of soil in the area. These abiotic factors both support and limit the particular community. For example, a relative lack of available moisture prevents the growth of most species of plants, but supports certain species, such as cacti; these kinds of areas are deserts. Land with plenty of available moisture and a suitable temperature supports forests. The presence of water is the major factor that sustains aquatic communities. The first step in investigating a biotic community may be simply to catalogue all the species present. Species are the different kinds of plants, animals and microbes in the community. A given species includes all those individuals which have a strong similarity in appearance to one another and which are distinct in appearance from other such groups. Each species in a biotic community is represented by a certain population — that is, by a certain number of individuals that make up the interbreeding, reproducing group. Which of the following is not an abiotic factor?