Introduction
Chapters 12, 13, and 14 emphasized interactions between species in which at least one member is harmed (predation, herbivory, parasitism, and competition). But life on Earth is also shaped by positive interactions, those in which one or both species benefit and neither is harmed.
Most vascular plants, for example, form beneficial associations with fungi that improve the growth and survival of both species. In fact, fossil evidence indicates that the earliest vascular plants formed similar associations with fungi more than 400 million years ago (Selosse and Le Tacon 1998). These early vascular plants lacked true roots, so their interactions with fungi may have increased their access to soil resources and aided their colonization of land.As this example suggests, positive interactions have influenced key events in the history of life as well as the growth and survival of organisms living today. As we'll see in this chapter, positive interactions can have wide-ranging effects on many species, thus shaping communities and influencing ecosystems. We will begin our study of positive interactions with definitions of some key terms and an overview of the scope of these interactions in ecological communities.