Question

the type of environment in which an organism lives is called its habitat. a habitat is designated by describing such characteristics as temperature, altitude, amount of light and geographical features. the relationship of an organism to the rest of its habitat-particularly in regard to its food and enemies-determines its niche, or place within the habitat. although many different kinds of living things can share the same habitat, a niche is exclusive to each type of organism. many times, the ecological niche is very particular to a species, lessening competition between two species.
much ecological work is devoted to the study of communities. a community is the group of organisms that share a particular habitat. a community may be as small as the life in a pond or as large as the living elements of a forest. many communities are well-defined. some communities have blurred boundaries and cannot be distinguished from other communities easily.
certain large communities are called biomes. a biome is distinguished by its types of species and by the temperature and humidity of the area in which it occurs. biomes include the tropical rainforest, the temperate deciduous forest, the boreal forest, the freshwater marsh, the prairie, the tundra, and the desert. the ocean is sometimes considered as one biome, subdivided into the intertidal, littoral, and deep-sea zones (habitats of bottom-living organisms) and the pelagic zone (habitat of free-floating and free-swimming organisms).
summary sentence: (the author is saying that....)

Explanation:

The text defines habitat, niche, community, and biome. It explains that a habitat is an organism's living environment. A niche is an organism's role in its habitat. A community is a group of organisms sharing a habitat. Biomes are large communities distinguished by species, temperature, and humidity. Examples of biomes are given.

Answer:

The author is saying that a habitat is an organism's living environment (described by factors like temperature, altitude, etc.), a niche is an organism's role in its habitat (related to food and enemies, often species - specific to reduce competition), a community is a group of organisms sharing a habitat (ranging in size and boundary clarity), and biomes are large communities (distinguished by species, temperature, humidity; examples include tropical rainforest, etc.).