Island biogeography of mutualistic interaction networks

Abstract

[Aim] The seminal theory of island biogeography, based on changing rates of immigration and extinction, should be seen in a geological context, as an island's maturity influences the richness of its biota. Here, we develop an island biogeography of biotic interactions, recognizing that, besides species richness, biodiversity also encompasses the multitude of interactions among species. By sampling interactions between plants and pollinators across the Canarian archipelago, we illustrate how the local richness, specialization and endemism of biotic interactions vary with island age and area.

[Location] Canary Islands (27.62° N–29.42° N and 13.33° W–18.17° W).

[Methods] On five islands, covering the full age range of the archipelago, plant–pollinator interactions were catalogued and their strength estimated. Network parameters (e.g. interaction richness and specialization) and the number of single‐island interactions (equivalent to single‐island endemics) were estimated from interaction matrices and related to island area and age.

[Results] Plant species richness, interaction richness and average degree of specialization of pollinator species showed hump‐shaped relationships with island age. Pollinator richness varied with island area and plant richness. Plant specialization increased with island age, and the proportion of single‐island interactions (pSII) exhibited a U‐shaped relationship with age.

[Main conclusions] The previously reported hump‐shaped relationship between species richness and island age, both on the scale of islands and of habitats, was confirmed for plant species in local networks. Both plants and pollinators were more generalized on the youngest island, which may be due to a predominance of generalist colonists. Pollinator specialization peaked on mid‐aged islands, whereas plants showed the highest specialization on old islands, potentially reflecting their different life histories. The U‐shaped relationship between the proportion of single‐island interactions and island age might be explained by (1) young islands having a high proportion of unique interactions, due to interactions between generalists, and (2) old islands having unique interactions due to an accumulation of unique pairwise interactions that have evolved through time. Thus, island age – which not only captures time per se, but also the geomorphological changes of islands – may act as a regional driver of local network structure, and so the contemporary networks we observed across the Canarian archipelago illustrate the development of a network through geological time.