Stochastic colonization, transient and boom-bust dynamics shape invasions by native taxa

Boom-bust dynamics (BBD) is a population dynamic pattern described in invasion ecology, where a population suddenly increases (boom) before quickly declining (bust), eventually stabilizing at much lower levels. This initial outbreak is considered a transient phase preceding a long-term population equilibrium. If BBD is common in invasions, understanding it is crucial for invasion interpretations and management. Nevertheless, how frequently BBD occurs is little known due to a scarcity and quality of data, and its underlying drivers remain poorly understood, especially among native species recolonizing restored habitats. Restored ecosystems offer a rare opportunity to investigate BBD in the absence of non-native traits. Here, we analyzed BBD in a community of 47 breeding waterbirds across 53 wetland patches from 1984 to 2023, where restoration measures (e.g., regulated hunting, improved hydrology) triggered an invasion (recolonization) process. Strikingly, although the study spanned 3-8 generation times, most species and the whole community showed long transient dynamics far from equilibrium. BBD was observed in over 75% of cases, exhibiting single and recurring patterns, particularly in wetlands with stochastic hydrology and among generalist species. Patch features influenced the occurrence of BBD nonlinearly, likely through interactions with other features, especially with species ecological typology. Rare species showed particular dynamics, characterized by high colonization-extinction turnover. Our findings, although restricted to waterbirds, suggest that, at least in this dataset, BBD are prevalent during invasions. Moreover, only approximately 20% of colonizations achieved long-term persistence, while the majority resulted in local extinctions. This underscores that failed, often undetected, colonization attempts may be widespread. Importantly, our study also shows that BBD can emerge in native species through intrinsic ecological processes alone, without invoking exotic traits or the species' ecological novelty. This challenges the prevailing view that BBD are uniquely associated with alien species and calls for a reinterpretation of population dynamics after colonization.