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Specialization and Rarity Predict Nonrandom Loss of Interactions from Mutualist Networks

Untangling the Web Interspecific interactions link species within complex trophic and nontrophic webs (see the Perspective by Lewinsohn and Cagnolo). Theoretical work has suggested that certain characteristics of species, or even interactions, may predispose them to extinction from a network. Aizen et al. (p. 1486) provide empirical evidence that plant-pollinator interactions are lost nonrandomly following habitat reduction in isolated hills in the Argentine pampas. Some types of interaction were more vulnerable to disruption than others, particularly when the specialization of the interacting was high and when the interactions were infrequent. Stouffer et al. (p. 1489) applied network theory to predict the dynamical importance of species across different food webs. Characteristic three-node motifs were identified, and species were characterized according to the relative frequencies with which they occupied unique positions within the motifs. These relative frequencies and the dynamic importance of the motifs were then used to identify a species-level importance within a food web. In the sierras of Argentina, specialized mutualistic plant-pollinator relationships increase vulnerability to habitat loss. The loss of interactions from mutualistic networks could foreshadow both plant and animal species extinctions. Yet, the characteristics of interactions that predispose them to disruption are largely unknown. We analyzed 12 pollination webs from isolated hills (“sierras”), in Argentina, ranging from tens to thousands of hectares. We found evidence of nonrandom loss of interactions with decreasing sierra size. Low interaction frequency and high specialization between interacting partners contributed additively to increase the vulnerability of interactions to disruption. Interactions between generalists in the largest sierras were ubiquitous across sierras, but many of them lost their central structural role in the smallest sierras. Thus, particular configurations of interaction networks, along with unique ecological relations and evolutionary pathways, could be lost forever after habitat reduction.