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Mutualistic Networks CHAPTER 11 Mutualistic networks Jordi Bascompte and Jens M. Olesen 11.1 A historical perspective on network At the same time, however, there are early pre- thinking in mutualism decessors of community studies, some of them resulting in the compilation of large datasets of Plant–animal mutualisms such as those between interactions later used in network research. One flowering plants and their pollinators or seed dis- classic subject of discussion in these community- persers have played a major role in the generation wide mutualistic studies was whether the major- of biodiversity on earth. Beyond early notes and ity of interactions are specialized or generalized. In some predecessors of ideas, it is fair to say that this context, specialist species are those interacting studies of mutualistic interactions among free- with only one or a few other species. An import- living species began with the work of J.G. Köl- ant contribution here is the classic paper by Waser reuter (1733–1806; Waser 2006), later followed by et al. (1996). This paper made the point that pre- C.K. Sprengel (1793), who become a direct inspi- vious studies had overemphasized the prevalence ration of Charles Darwin. of specific, one-on-one interactions, and that when Darwin himself is often cited as one of the found- viewed at a community level, most species interact ers of research on mutualisms. His book On the with a large set of other species. Various Contrivances by Which British and Foreign Or- In another significant contribution, Daniel chids are Fertilised by Insects, and on the Good Effects of Janzen wrote a highly influential paper that sig- Intercrossing (Darwin 1862) was very important in nificantly contrasted with the above dominant advancing the idea that one cannot understand the focus on the one-species-to-one-species approach evolution of floral morphology without taking into ( Janzen 1980). He introduced the idea that mutu- account the evolution of insect morphology. That is, alisms occur in species-rich communities. That is, the evolution of plants and animals are not inde- the interaction is not between one plant species pendent of each other. and one animal species, but between one set of With the development of the modern evolution- plants and one set of animals. The catch-all term ary synthesis in the 1930s, mutualisms began to be “diffuse coevolution” was coined to refer to this viewed from a population and evolutionary per- situation. Several papers subsequently elaborated spective. This involved formulation of systems of upon this concept, emphasizing the high variabil- trait “syndromes” and studies of pairwise inter- ity of mutualistic interactions (Herrera, 1982). One actions (Baker 1983). Indeed, most early studies of drawback of this approximation, however, is that plant–animal mutualisms focused on single pairs it seems to preclude any analytical tractability to of species. This set up a research agenda organized community-wide mutualism (Thompson 2005). As around highly specific interactions. Paradigmatic a consequence, the theoretical framework in mu- examples of these specific interactions are fig wasps tualistic studies seems to predict that these inter- and figs (Cook and Rasplus 2003) and yucca moths actions have to lead to either highly specific or and yuccas (Pellmyr 2003). highly diffuse assemblages. Mutualism. Edited by Judith L. Bronstein. © Oxford University Press 2015. Published 2015 by Oxford University Press. 204 MUTUALISM 120 3600 100 3000 80 s 2400 60 Papers 1800 Citation 40 1200 20 600 0 0 2002200320042005200620072008200920102011201220132014 2002200320042005200620072008200920102011201220132014 Year Year Figure 11.1 Temporal increase in the number of papers (left) and citations to papers (right) on mutualistic networks. Search performed on the Web of Science on 29 January 2015 (topic: mutualistic networks). Several exceptions that brought tractability to the species richness, and so on. The number of papers understanding of multispecific mutualism (in terms on mutualistic networks, and the citations these pa- of allowing the search of repeated patterns) are worth pers have gathered, have risen exponentially in the mentioning. For example, the geographic mosaic last few years (Figure 11.1). theory of coevolution (Thompson 2005) described The first studies on mutualistic networks focused predictable patterns in the geographic distribution on plant–pollinator and plant–seed disperser mu- of mutualistic interactions involving small groups of tualisms, due to the main interests of the initial core species. Thus, the sign and strength of a mutualistic of authors who introduced this approach. Thus, this interaction may vary across the landscape. In some reflects historical contingency rather than implying patches, the interaction is mutually beneficial, while that other types of mutualisms cannot be seen as it becomes antagonistic in other patches. Whether networks. Indeed, in subsequent years, network the net effect is positive or negative depends on the analysis has been applied to the study of plant– presence of other mutualists and antagonists. The ant mutualisms, cleaning mutualisms, and several geographic mosaic theory has represented a major other systems, including cooperative interactions advance in our understanding of how coevolution in socioeconomic systems (see Bascompte and Jor- proceeds in complex natural settings (see Chapter 7). dano 2014 for a review). Network theory has pro- The second major approach to community-wide vided a framework that can be used to characterize mutualism has been brought by network analysis. all mutualisms. Thorough this chapter, however, we The first papers introducing a network approach will mainly focus on pollination and seed dispersal to mutualism mainly used concepts from food-web mutualisms for illustrative purposes. theory and looked at global properties such as con- nectance ( Jordano 1987, Memmott 1999, Elberling and Olesen 1999). Several years later, there was a 11.2 Rationale for a network approach resurgence of interest in mutualistic networks (Bas- to mutualism compte and Jordano 2007, Bascompte and Jordano 2014). These foundational papers applied a new Despite several recent claims and the early predeces- generation of tools and models to large and detailed sors noted above, many coevolutionary researchers datasets, facilitating a search for invariant prin- still think in a pairwise way. The reason is rooted in ciples or regularities across mutualistic networks the reductionistic tradition of breaking down com- despite obvious differences in latitude, habitat, plex systems into their basic elements in the hope MUTualiSTIC NETWORKS 205 that their study can be scaled up to understand entire This relates to classic work on specialization versus communities. Indeed, we have lacked a conceptual generalization in mutualistic communities. We de- framework to address multispecies interactions. Net- vote the rest of this chapter to the study of a particu- work theory provides this conceptual framework. lar organization of mutualistic networks that allows This approach treats species as nodes, and mutual- linking the scale of the entire network to that of a istic interactions as links between two such nodes. small subgroup of species. Technically, the type of network that fits these sys- tems is called a bipartite network, that is, a network 11.2.1 The example of asymmetric formed by two sets of nodes (here, plants and ani- specialization mals) with interactions between, but not within, sets. Network studies can bring insight in three im- The network approach has introduced two advances portant ways. First, network theory can unveil in our understanding of the level of generalization of complex patterns; that is, it can help us to visualize mutualistic interactions. First, it describes the specific the structure of highly diverse mutualisms. Second, form of the continuum between the frequency of spe- network tools help us to analyze this complexity cialists and that of generalists within a community. and to test hypotheses about its origins. Third, net- Second, it has made the point that specialists do not work theory emphasizes the relationship between typically interact with specialists, as once believed. network structure and dynamics, which may help Regarding the first point, network theory has us evaluate the community-wide effects of species established the concept of connectivity distribu- extinctions and other drivers of global change. tions, which describe the cumulative probability of Network theory has rapidly become a popular a species interacting with one, two, . ., k other spe- approach in community ecology in general, and cies. Mutualistic networks have been found to be in the study of mutualism in particular. As with heterogeneous: while the majority of species have any new paradigm, there is the risk that it can be- one or a few interactions with other species, a few come a fancy new way of stating what we already species are super-generalists, acting like hubs in knew, or a quantitative exercise poorly rooted in technological networks. Technically, the connectiv- the ecological and evolutionary realities of the ity distribution in these networks is best described system being studied. Another potential problem by a broad scale distribution ( Jordano et al. 2003). with the application of network theory is that—as This naturally leads to a situation in which “special- with any other approach—there are trade-offs be- ists” or “generalists” are not discrete
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