The tri‐trophic niche concept and adaptive radiation of phytophagous insects

Abstract

A conceptual divide exists between ecological and evolutionary approaches to understanding adaptive radiation, although the phenomenon is inherently both ecological and evolutionary. This divide is evident in studies of phytophagous insects, a highly diverse group that has been frequently investigated with the implicit or explicit goal of understanding its diversity. Whereas ecological studies of phytophagous insects increasingly recognize the importance of tri-trophic interactions as determinants of niche dimensions such as host-plant associations, evolutionary studies typically neglect the third trophic level. Here we attempt to reconcile ecological and evolutionary approaches through the concept of the ecological niche. We specifically present a tri-trophic niche concept as a foil to the traditional bi-trophic niche concept for phytophagous insects. We argue that these niche concepts have different implications for understanding herbivore community structure, population divergence, and evolutionary diversification. To this end, we offer contrasting empirical predictions of bi-and tri-trophic niche concepts for patterns of community structure, the process of population divergence, and patterns of evolutionary diversification of phytophagous insects.

Divergence in behaviour between the European corn borer,Ostrinia nubilalis, and its sibling speciesOstrinia scapulalis: adaptation to human harvesting?

“…This weed is not harvested, not grazed by mammals and mugwort shoots are fragile, often breaking into fragments (D. Bourguet 2000, personal observation): this would make gravity-based placement unreliable. Divergence in geotactic behaviour is therefore a major facet of host-plant specialization in this system, adding to the argument that natural enemies should be included as a dimension of ecological niche differentiation (Singer & Stireman 2005). Although there is little doubt that human harvesting of maize has been the dominant selective agent on geotaxis for the last 100 years, its effect may have been weaker further in the past.…”

“…Nonetheless, there is a concern that studies of insect specialization may have overlooked important dimensions of the process by focusing on limited aspects of the life cycle (Scheirs et al 2005). In particular, most studies have been concerned with the insect -plant interaction, at the expense of third-party actors such as natural enemies (Singer & Stireman 2005). Novel host species can not only free insect populations from some natural enemies, facilitating the process of hostshift (the enemy-free space hypothesis; e.g.…”

“…Although the source of divergent selection is traditionally looked for at the level of insect -plant interactions (e.g. plant secondary metabolites, Ehrlich & Raven 1964;or phenology, Feder & Filchak 1999), it is increasingly recognized that natural enemies can play an important role in driving insect diversification on host species, stressing the need to consider 'tri-trophic' ecological niches (Singer & Stireman 2005). For instance, different host-plant species can select for contrasting anti-predator cryptic patterns (Nosil et al 2002) or differ in the nature and abundance of the parasitoid fauna (Brown et al 1995;Feder 1995).…”

Coexisting generalist herbivores occupy unique nutritional feeding niches

“…Ecologists search for assembly rules to explain community structure (22), usually as the combined outcome of top-down and bottom-up interactions (6,7,23). Although both are important in terrestrial herbivore communities, two observations argue for a critical importance of understanding bottom-up factors.…”

Detection of refuge from enemies through phenological mismatching in multitrophic interactions requires season-wide estimation of host abundance

“…As such, it has been invoked repeatedly as a selective factor influencing the host-plant range, and thereby diversification, of phytophagous insects (e.g. Bernays and Graham 1988;Ohsaki and Sato 1994;Singer and Stireman 2005). Many phytophagous insects are specialized in their use of host plants (Ehrlich and Murphy 1988;Stamp 2001).…”