Invaders’ trophic position and their direct and indirect relationship influence on resident food webs

ABSTRACT Many ecosystems are undergoing simultaneous colonization and spread of multiple alien species. Invaders often negatively affect communities by reducing population sizes of resident species or even decreasing community diversity through extinctions. Direct and indirect interactions between them can amplify or mitigate their impacts on native communities. In this study, we compare the effects of two invaders on the resident model food webs under two scenarios: separate versus simultaneous invasion. We examined the resident food webs’ response from two perspectives: the number of extinct species as a measure of diversity loss, and the net change in total biomass of the food web. Using dynamic simulations based on the Allometric Bioenergetic Model , we tracked these changes in silico and compared the results of the two scenarios. We examined how the invaders’ trophic positions relative to each other and their direct/indirect interactions influence the additive or non-additive nature of the outcomes of their co-invasion. Our results have corroborated previous field and experimental observations, showing that when co-occurring invaders occupy different trophic levels, their combined effects may be dampened if one invader preys on the other. Further, we have shown that the probability of synergistic effect increases when invaders form a trophic cascade or share a common predator. However, we have also shown that all of these relationships are influenced by i) the metric used to track changes in the resident community, ii) the trophic level at which the invasion occurs, and iii) whether the invader has a predator (either resident or another invader). Highlights The impact of a joint invasion on the resident community depends on the used metric Additivity of the co-invasions’ impact is influenced by the invaders’ trophic position The effects of the invaders’ direct/indirect interactions depend on the trophic level Invaders in top position have stronger effects than those of intermediate position