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The recent introduction in a tropical agricultural environment of a weedy open-habitat plant ( Solanum myriacanthum ) and subsequent host range expansion of a common forest-edge butterfly ( Mechanitis menapis ) onto that plant provides an opportunity to examine reconfiguration of tritrophic networks in human-impacted landscapes. The objectives of this study were (1) determine if the caterpillars on the exotic host are more or less limited by plant defenses (bottom-up forces) and if they experience enemy release (decrease of top-down pressure) and (2) define how anthropic open pasture habitat influences the herbivore’s tritrophic niche. Field and laboratory monitoring of larval survival and performance on a native ( Solanum acerifolium ) host plant and the exotic ( S. myriacanthum ) host plant were conducted in the Mindo Valley, Ecuador. Plant physical defenses were also measured. Results showed that larval mortality was mostly top-down on  S. acerifolium , linked to parasitism, but mostly bottom-up on  S. myriacanthum , possibly linked to observed increased plant defenses. Thus, in the absence of co-evolved relationships, herbivores on the exotic host experienced little top-down regulation, but stronger bottom-up pressures from plant defenses. These findings provide a rare empirical example of enemy-free space as a mechanism underlying host-range expansion.  S. myriacanthum  was less colonized in open pastures than in semi-shaded habitats (forest edges, thickets): fewer eggs were found, suggesting limited dispersal of adult butterflies into the harsh open environments, and the survival rate of first instar larvae was lower than on semi-shaded plants, likely linked to the stronger defenses of sun-grown leaves. These findings show how environmental conditions modulate the rewiring of trophic networks in heavily impacted landscapes, and limit a biocontrol by a native herbivore on an invasive plant in open habitats.

Top-down and bottom-up controls on an herbivore on a native and introduced plant in a tropical agricultural landscape