Positive interactions between exotic species may increase ecosystem‐level impacts and potentially facilitate the entry and spread of other exotic species. Invader‐facilitated invasion success—”secondary invasion”—is a key conceptual aspect of the well‐known invasional meltdown hypothesis, but remains poorly defined and empirically underexplored. Drawing from heuristic models and published empirical studies, we explore this form of “secondary invasion” and discuss the phenomenon within the recognized conceptual framework of the determinants of invasion success. The term “secondary invasion” has been used haphazardly in the literature to refer to multiple invasion phenomena, most of which have other more accepted titles. Our usage of the term secondary invasion is akin to “invader‐facilitated invasion,” which we define as the phenomenon in which the invasion success of one exotic species is contingent on the presence, influence, and impacts of one or more other exotic species. We present case studies of secondary invasion whereby primary invaders facilitate the entry or establishment of exotic species into communities where they were previously excluded from becoming invasive. Our synthesis, discussion, and conceptual framework of this type of secondary invasion provides a useful reference to better explain how invasive species can alter key properties of recipient ecosystems that can ultimately determine the invasion success of other species. This study increases our appreciation for complex interactions following invasion and highlights the impacts of invasive species themselves as possible determinants of invasion success. We anticipate that highlighting “secondary invasion” in this way will enable studies reporting similar phenomena to be identified and linked through consistent terminology.

No abstract

Global biodiversity loss is the cumulative result of local species declines. To combat 26 biodiversity loss, detailed information on the temporal trends of at-risk species at local scales 27 is needed. Here we report the results of a 13-year study of temporal change in bird occupancy 28 in one of the most heavily modified biomes worldwide; the temperate woodlands of south-29 eastern Australia. We sought to determine if temporal changes in bird species were different 30 between three broad native vegetation types (old-growth woodland, regrowth woodland and 31 restoration plantings) and between species traits (body size, migratory status, rarity, 32 woodland dependency, or diet). We found evidence of decline for over a quarter of all bird 33 species for which we had sufficient data for detailed analysis (30 out of 108 species). In 34 contrast, only 14 species increased significantly. Temporal change of birds was linked to life-35 history attributes, with patterns often being habitat-dependent. Nectarivores and large-bodied birds declined across all vegetation types, whereas small-bodied species increased, particularly in restoration plantings. Contrasting with patterns documented elsewhere, resident but not migratory species declined, with this trend strongest in restoration plantings. Finally, our analyses showed that, as a group, common birds tended to decline whereas rare birds tended to increase, with effects for both most pronounced in restoration plantings. Our results highlight the benefit of targeted restoration planting for some species, but also demonstrate that many common species that have long-persisted in human-dominated landscapes are experiencing severe declines.

Decision triggers are defined thresholds in the status of monitored variables that indicate when to undertake management, and avoid undesirable ecosystem change. Decision triggers are frequently recommended to conservation practitioners as a tool to facilitate evidence-based management practices, but there has been limited attention paid to how practitioners are integrating decision triggers into existing monitoring programs. We sought to understand whether conservation practitioners' use of decision triggers was influenced by the type of variables in their monitoring programs. We investigated this question using a practitioner-focused workshop involving a structured discussion and review of eight monitoring programs. Among our case studies, direct measures of biodiversity (e.g. native species) were more commonly monitored, but less likely to be linked to decision triggers (10% with triggers) than measures being used as surrogates (54% with triggers) for program objectives. This was because decision triggers were associated with management of threatening processes, which were often monitored as a surrogate for a biodiversity asset of interest. By contrast, direct measures of biodiversity were more commonly associated with informal decision processes that led to activities such as management reviews or external consultation. Workshop participants were in favor of including more formalized decision triggers in their programs, but were limited by incomplete ecological knowledge, lack of skilled staff, funding constraints, and/or uncertainty regarding intervention effectiveness. We recommend that practitioners consider including decision triggers for discussion activities (such as external consultation) in their programs as more than just early warning points for future interventions, particularly for direct measures. Decision triggers for discussions should be recognized as a critical feature of monitoring programs where information and operational limitations inhibit the use of decision triggers for interventions.

Context Competition for space and resources within a fragmented landscape may change interspecific interactions within the remaining available habitat. These changes may inhibit the persistence of one species but facilitate the success of another. The yellow-throated miner (Manorina flavigula) is an example of a successful species, reportedly more common in the landscape as a result of fragmentation yet the consequences of its success are still relatively unknown. Aims To investigate whether the yellow-throated miner had negative impacts on bird community assemblages, particularly small insectivorous species, and whether its presence resulted in higher psyllid abundances and lower tree health, similar to impacts noted for other miner species. Methods We undertook this study near Walpeup in Victoria’s Mallee region, a highly fragmented, agriculture-dominated, semiarid landscape. Yellow-throated miner colonies and control sites free of miners were identified and surveyed for bird species present, psyllid abundance and measures of tree health. Conclusions The presence of the yellow-throated miner was associated with a significant reduction in bird species richness, lower abundance of small birds and a dissimilar community composition. Psyllid abundance was higher in miner colonies and tree health was significantly lower. Small insectivorous birds compete directly with miners for resources and, as such, are likely targeted by interspecific aggressive behaviour. The absence of small species from miner colonies most likely caused a trend in increased psyllid abundance and subsequently reduced tree health. Implications Our findings suggest that management of these miners is likely required to prevent further loss of biodiversity in this fragmented landscape. The loss of bird species and reduced tree health due to the influence of the yellow-throated miner presents one of the greatest threats to these communities nationally and a challenging conservation problem.