Abstract
Previous field-based studies have evidenced patterns in gastrointestinal helminth (GIH) assemblages of rodent communities that are consistent with “enemy release” and “spill-back” hypotheses, suggesting a role of parasites in the ongoing invasion success of the exotic house mouse (Mus musculus domesticus) in Senegal (West Africa). However, these findings came from a single invasion route, thus preventing to ascertain that they did not result from stochastic and/or selective processes that could differ across invasion pathways. In the present study, we investigated the distribution of rodent communities and their GIH assemblages in three distinct zones of Northern Senegal, which corresponded to independent house mouse invasion fronts. Our findings first showed an unexpectedly rapid spread of the house mouse, which reached even remote areas where native species would have been expected to dominate the rodent communities. They also strengthened previous insights suggesting a role of helminths in the invasion success of the house mouse, such as: (i) low infestation rates of invading mice by the exotic nematode Aspiculuris tetraptera at invasion fronts—except in a single zone where the establishment of the house mouse could be older than initially thought, which was consistent with the “enemy release” hypothesis; and (ii) higher infection rates by the local cestode Mathevotaenia symmetrica in native rodents with long co-existence history with invasive mice, bringing support to the “spill-back” hypothesis. Therefore, “enemy release” and “spill-back” mechanisms should be seriously considered when explaining the invasion success of the house mouse—provided further experimental works demonstrate that involved GIHs affect rodent fitness or exert selective pressures. Next steps should also include evolutionary, immunological, and behavioral perspectives to fully capture the complexity, causes and consequences of GIH variations along these invasion routes.
Highlights
Parasitism has been considered a key factor explaining the successful range expansion of many introduced species [1,2,3]
We investigated the distribution of both rodent communities and their gastrointestinal helminth (GIH) assemblages along three distinct zones of Northern Senegal, including sites of the invasion front previously studied as well as newly sampled sites that were presumed to have been more recently invaded by the house mouse
We found that only 5% of the total variance of habitat characteristics was explained by the sampling zone (BWA: Monte-Carlo test, p = 0.001), indicating that habitats were similar with respect to the variables recorded in each zone (Supplementary Material 2)
Summary
Parasitism has been considered a key factor explaining the successful range expansion of many introduced species [1,2,3]. The aforementioned hypotheses have received ever-increasing supports from studies comparing invasive populations across one of their expansion routes [e.g., [16,17,18]] While this design requires substantial knowledge of the invasion history [19], it has the advantage of reflecting a spatial and temporal continuum in the invasion process [20]—which is fundamental to unravel the actual role of parasites in the successful spread of invaders. Comparisons between invasion routes taken by an invasive species appear essential to ensure consistent supports, and disentangle the consequences of stochastic and selective processes across invasion pathways. Such comparative studies— in the same expansion range—are still scarce as far as we are aware [but see [25]]
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