Simple and Complex Interactions

While the role of abiotic factors in governing species interactions is dealt with in various chapters of this book (e.g. Chap. 7 by Terlizzi and Schiel, Chap. 9 by Benedetti-Cecchi and Chap. 13 by Gili and Petraitis), we will focus here on the biotic factors that affect species interactions. Due to the large number of examples on biotic interactions, we can not and do not attempt to give a complete overview on this topic. Rather, we will present selected examples, mainly from competitive and trophic interactions among macroscopic individuals, describing the principal mechanisms that turn simple into complex interactions. One gradient of complexity concerns the number of interacting species. In this regard, we define the simplest level of species interactions as (1) among conspecific individuals and populations (intraspecific level), followed by interactions (2) between species (interspecific level), and how this reflects on (3) larger sets of species (community level), as the highest level of complexity. Orthogonal to this cline of complexity based on the number of participating species, a number of non-mutually exclusive factors further affect and complicate species interactions, including (1) context specificity, (2) variability, (3) modulation and (4) simultaneous action of several interactions (Fig. 16.1 ).