restricted access 23. Conservation of Coevolved Insect Herbivores and Plants
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325 It goes without saying that one cannot consider the conservation of plants without thinking about their relationship with the creatures that eat them—and herbivorous insects have long been among the worst enemies of the global flora (Becerra 1997, 2003, 2005). And as anyone who has worked extensively with butterflies or other herbivorous insects can tell you, the distribution, abundance, and phenology of food plants are absolutely key to understanding the dynamics and conservation of their populations. It is no accident that the entire field of coevolution sprung from a study of the reciprocal evolutionary interaction of butterfly caterpillars and their photosynthesizing victims (Ehrlich and Raven 1964). The prominence and popularity with amateurs of butterflies makes them the best-known large group of insect herbivores and an important model system in population biology (Boggs et al. 2003). Similarly, many of the examples in what follows will be drawn from work on that system. Thus, conserving either herbivorous insects or plants automatically requires consideration of the other group, their coevolutionary partners. The dramatic effect that insect herbivores can have on plant populations was demonstrated years ago by the control of a vast Opuntia cactus invasion in Australia by the introduction of a coevolved predator, the moth Cactoblastis cactorum Berg (Mooney and Hobbs 2000). So was the powerful evolutionary impacts that even a small insect herbivore could have on relatively large plants, as when the tiny lycaenid, Glaucopsyche lygdamus Doubleday, was shown to dramatically reduce seed set of lupine plants (Breedlove and Ehrlich 1968). The other side of the coin is even more obvious—known to every butterfly collector who observes populations disappearing as larval food plants are extirpated by development (e.g., Ehrlich and Hanski 2004, p. 60). The scale of human activities is such that many (and potentially all) populations of plants and herbivorous insects are being influenced by those activities, and thus so are patterns of their coevolution. The drivers that endanger coevolved populations are, of course, the same ones that endanger organisms in general. Humanity probably influences interactions of all organisms by brute destruction of habitats, and insect herbivores– plant interactions are no exception. Invasive plants can disrupt coevolutionary complexes by arriving in a new location without their native predators, outcompeting the native host plants of specialist insect herbivores or creating dangerous fire-prone thatch (Powell and Parker 1993). In the process, they can disrupt ecosystem services. For instance, in places where Latin American asters of the genus Eupatorium invade, many problems are exacerbated when the plant is no longer imbedded in complexes with coevolved insect herbivores such as the tephritid gall fly Procecidochares utilis Stone (Groves 1989): the stock carrying capacity of grazing lands is often reduced, and succession in milpa agriculture is slowed, among other problems. Changes in disturbance regimes due either directly to human activities or indirectly to introductions as previously noted can threaten host plants, their coevolved herbivorous insects, and the evolutionary dynamic between them. Climate change demonstrably can affect herbivore-plant complexes , and again likely affect the evolutionary dynamic within such complexes. For example, overharvesting of plants has had devastating effects on some cactus populations (Hernandez 1995; Ortega-Baes and Godinez-Alvarez 2006), and this clearly alters their coevolution with specialist insect herbivores. It reduces the food resources available to the insect populations and puts a selective premium on individuals with a tendancy to oviposition “errors” and/or less specialized adaptation to host defenses. In contrast, no butterfly is likely to have become globally extinct from overharvesting (although populations of some lycaenid species such as Lycaena dispar Haworth and Plebejus sephirus kovacsi Szabo (Balint 1993a, 1993b) have been hurried toward extinction by butterfly collectors). TW E NTY-TH R E E Conservation of Coevolved Insect Herbivores and Plants CAROL L. BOGGS AND PAUL R. EHRLICH Not only do these threats affect the population dynamics of species involved in plant-herbivore interactions, they also affect the evolutionary dynamics underlying the coevolutionary interaction. Much less is known about the effects of anthropogenic threats on these dynamics, yet they are equally critical to conservation of biodiversity in the longer run. These dynamics shape species interactions at multiple scales, producing biogeographic mosaics of types, strengths, and identities of interacting species pairs at the landscape scale as a result of adaptation and coevolution at the regional to local scale (Thompson 2005). The resulting spatial patterns, which are readily disrupted by human activity, are an important component of global-scale...


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