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The chocolate tree, Theobroma cacao, known simply as cacao, is native to tropical America. Cacao grows where the environment is consistently warm and humid. Physical environment alone, however, does not determine where cacao can live. Cacao reproduction, and ultimately distribution, is limited by the requirement that tiny flies called midges pollinate them (Fig. 47.1). The midges’ eggs and young need deep shade and a thick layer of moist rotting vegetation, whether from the natural fall of leaves onto the forest floor or from banana plants left to decay in cacao plantations. The midges’ needs mean that cacao cannot reproduce in areas with full sun or on clean ground that otherwise would support their growth—at least not without human help. The natural distribution of cacao plants, therefore, depends on interactions with both the physical environment and other species.

The reliance of chocolate trees on midges is not an unusual arrangement. Every species interacts with other species in ways that affect growth and reproduction. These other species may be the insects that help a plant pollinate its flowers or ones that eat its leaves. They may be a fungus that helps a tree gain nutrients or another that attacks it. They may be other species that compete for resources, predators and pathogens that reduce population size, prey species that provide a specialized food resource, or microbes that provide or take nutrients while living inside their hosts (Case 5: The Human Microbiome). Associations among species, whether positive, negative, or neutral, form a complex web of interactions in which each species influences the population size and distribution of other species, whether they are partners, enemies, or food.

In this chapter, we explore where and how species live, how they interact with one another in a community, and how different species and the physical environment form ecosystems seen around the world.