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

The niches of herbivorous insects are best understood in a tri-trophic perspective, as top-down effects of predators and parasitoids can determine the host plant range of herbivores. 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 test hypotheses surrounding reconfiguration of tritrophic networks in anthropized environments: what is the role of bottom-up and top-down forces in this host range expansion? and does habitat breadth limit this novel trophic relationship? Field and laboratory monitoring of larval survival and performance on a native (Solanum acerifolium) and exotic host in the Mindo valley (Ecuador), combined with measurement of plant physical defenses, shows 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. These findings support the theory that herbivores experience little top-down regulation on exotic hosts, and suggest that, in this case, bottom-up pressure from plant defenses is stronger in the absence of a co-evolved relationship. In this system, top-down forces tend to expand rather than restrict host plant range, contrary to many previous studies. 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 ecotone plants, likely linked to the stronger defenses of sun-grown leaves. Environmental conditions thus modulate the rewiring of novel trophic networks in heavily impacted landscapes, and limit a native herbivore`s control on an invasive plant.

食草昆虫的生态位最佳地从三营养级视角进行理解，因为捕食者和寄生者的顶向下效应能够决定食草动物的宿主植物分布范围。近期在热带农业环境中引入一种杂草型开阔地植物（Solanum myriacanthum）及其随后的普通森林边缘蝴蝶（Mechanitis menapis）对该植物的宿主范围扩展，为检验人类活动化环境中三营养网络重构的假设提供了契机：底部和顶部的力量在此宿主范围扩展中扮演何种角色？以及生境宽度是否限制了这种新型营养关系的建立？在厄瓜多尔的Mindio山谷（Ecuador）对本土植物（Solanum acerifolium）和外来宿主在野外和实验室中对幼虫的存活率和表现进行监测，并结合植物物理防御力的测量，发现幼虫在Solanum acerifolium上的死亡率主要受顶向下效应影响，与寄生有关，而在S. myriacanthum上则主要受底部效应影响，可能与观察到的植物防御力增强有关。这些发现支持了食草动物在外来宿主上受到的顶向下调节较少的理论，并暗示在此案例中，由于缺乏共同进化的关系，植物防御力的底部压力更为显著。在此系统中，顶向下力量倾向于扩展而非限制宿主植物范围，这与许多先前的研究结果相反。S. myriacanthum在开阔牧场上不如在半遮蔽生境（森林边缘、灌丛）中被广泛占据：发现的卵较少，表明成年蝴蝶在恶劣的开阔环境中的扩散受限，而一龄幼虫的存活率低于生态过渡植物，可能与其叶面较强的防御力有关。因此，环境条件调节了在严重受影响的景观中新型营养网络的重组，并限制了本地食草动物对入侵植物的控制。