Habitat augmentation drives secondary invasion: an experimental approach to determine the mechanism of invasion success

Abstract

The entry of secondary invaders into, or their expansion within, native communities is contingent on the changes wrought by other (primary) invaders. When primary invaders have altered more than one property of the recipient community, standard descriptive and modeling approaches only provide a best guess of the mechanism permitting the secondary invasion. In rainforest on Christmas Island, we conducted a manipulative field experiment to determine the mechanism of invasion success for a community of land snails dominated by non-native species. The invasion of rainforest by the yellow crazy ant (Anoplolepis gracilipes) has facilitated these land snails, either by creating enemy-free space and/or increased habitat and resources (in the form of leaf litter) through the removal of the native omnivorous-detritivorous red land crab (Gecarcoidea natalis). We manipulated predator densities (high and low) and leaf litter (high and low) in replicated blocks of four treatment combinations at two sites. Over the course of one wet season (five months), we found that plots with high leaf litter biomass contained significantly more snails than those with low biomass, regardless of whether those plots had high or low predation pressure, at both the site where land crabs have always been abundant, and at the site where they have been absent for many years prior to the experiment. Each site was dominated by small snail species (<2 mm length), and through handling size and predation experiments we demonstrated that red crabs tend not to handle and eat snails of that size. These results suggest that secondary invasion by this community of non-native land snails is facilitated most strongly by habitat and resource augmentation, an indirect consequence of red land crab removal, and that the creation of enemy-free space is not important. By using a full-factorial experimental approach, we have confidently determined-rather than inferred-the mechanism by which primary invaders indirectly facilitate a community of secondary invaders.

Secondary invasion: When invasion success is contingent on other invaders altering the properties of recipient ecosystems

“…Despite being the strongest examples of secondary invasion, only Green et al. () specifically used that term with Grosholz () instead described his findings as an “accelerated invasion.” More examples of secondary invasion as we describe it have been published in recent years, including research on exotic land snails (O'Loughlin & Green, , ), soil seed banks (Gioria, Dieterich, & Osborne, ; Gioria, Jarošík, & Pyšek, ; Gioria & Pyšek, ; Gioria, Pyšek, & Moravcova, ), and invasive plants (Flory & Bauer, ; French, ; Stotz, Gianoli, Patchell, & Cahill, ); however, these are only the studies that have explicitly used the term “secondary invasion” (see Figure ). It is likely that other examples of invader‐facilitated invasion have been published in that time, but our ability to identify them is constrained by the absence of consistent and clear terminology for the phenomenon.…”

“…Codium fragile could only establish following the creation of a gap in the kelp forest, mediated by another invasive species growing epiphytically on the kelp (Levin et al., ). Potential invaders may also be inhibited by multiple properties of recipient ecosystems (Cross, ; Morales & Aizen, ; O'Loughlin & Green, ). For example, Morales and Aizen () demonstrated that exotic plants were facilitated directly by increased disturbance and indirectly by increased presence of exotic insect pollinators in disturbed areas.…”

“…;Morales & Aizen, 2002;O'Loughlin & Green, 2016). For example,Morales and Aizen (2002) demonstrated that exotic plants were facilitated directly by increased disturbance and indirectly by increased presence of exotic insect pollinators in disturbed areas.…”

Conchological and molecular analysis of the “non-scaly” Bornean Georissa with descriptions of three new species (Gastropoda, Neritimorpha, Hydrocenidae)

“…Since we have studied and examined all shell materials from BORN, MZU, ZMA, RMNH, MFN, NHMUK, and JJV, we find that it is useful to highlight some issues related to the “non-scaly” group that could be beneficial for future understanding. Firstly, the name G.williamsi was mentioned in several publications to refer to a species with distinct spiral ribs (Thompson and Dance 1983; Clements et al 2008; Nurinsiyah et al 2016; Maassen 2003; O’Loughlin and Green 2016; Vermeulen and Whitten 1998). After examination of the holotype of G.williamsi in the NHMUK, we find that this species name has often been misapplied.…”

Secondary invasion un‐undefined: The importance of consistent and clear terminology for classifying unique invasion phenomena

“…We appreciate that for researchers focused on the impacts of invasive plants and their management on native plant communities (e.g., Pearson, Ortega, Runyon, & Butler, ), “secondary invasion” is widely understood to mean where invasive species establish following management intervention of a different invader (what we referred to as “management‐mediated invasion”). However, this is quite different from how others have used the term or even described that same process outside of invasive plant management (e.g., Albaina et al., ; Baldwin, Carpenter, Rury, & Woodward, ; Flory & Bauer, ; Green et al., ; O'Loughlin & Green, ). We maintain that there is no intrinsic common feature that would justify the continued undefined use of “secondary invasion” to simultaneously label multiple and quite dissimilar invasion phenomena (O'Loughlin & Green, ).…”