Abstract
Understanding the factors affecting the dynamics of spatially‐structured populations (SSP) is a central topic of conservation and landscape ecology. Invasive alien species are increasingly important drivers of the dynamics of native species. However, the impacts of invasives are often assessed at the patch scale, while their effects on SSP dynamics are rarely considered. We used long‐term abundance data to test whether the impact of invasive crayfish on subpopulations can also affect the whole SSP dynamics, through their influence on source populations. From 2010 to 2018, we surveyed a network of 58 ponds and recorded the abundance of Italian agile frog clutches, the occurrence of an invasive crayfish, and environmental features. Using Bayesian hierarchical models, we assessed relationhips between frog abundance in ponds and a) environmental features; b) connectivity within the SSP; c) occurrence of invasive species at both the patch‐ and the SSP‐levels. If spatial relationships between ponds were overlooked, we did not detect effects of crayfish presence on frog abundance or trends. When we jointly considered habitat, subpopulation and SSP features, processes acting at all these levels affected frog abundance. At the subpopulation scale, frog abundance in a year was related to habitat features, but was unrelated to crayfish occurrence at that site during the previous year. However, when we considered the SSP level, we found a strong negative relationship between frog abundance in a given site and crayfish frequency in surrounding wetlands during the previous year. Hence, SSP‐level analyses can identify effects that would remain unnoticed when focussing on single patches. Invasive species can affect population dynamics even in not invaded patches, through the degradation of subpopulation networks. Patch‐scale assessments of the impact of invasive species can thus be insufficient: predicting the long‐term interplay between invasive and native populations requires landscape‐level approaches accounting for the complexity of spatial interactions.
Highlights
IntroductionThe variation of patch-scale characteristics has significant effects on local demographic rates (Hanski 1998), still the dynamics of subpopulations living in discrete habitats are determined by processes acting at the landscape level, such as connectivity and dispersal (Friedenberg 2003, Semlitsch 2008, De Roissart et al 2015)
In a second step (SSP-level analysis), we considered the same variables of the patch-level analysis, plus two variables representing the role of processes acting at the spatially-structured populations (SSP) level: the number of clutches laid in nearby wetlands, and the occurrence of P. clarkii in surrounding wetlands
Long-term analyses of population networks are a powerful approach to understand the consequences of invasive alien species (IAS), as they allow to tease apart processes occurring at the patch and at the whole SSP scales
Summary
The variation of patch-scale characteristics has significant effects on local demographic rates (Hanski 1998), still the dynamics of subpopulations living in discrete habitats are determined by processes acting at the landscape level, such as connectivity and dispersal (Friedenberg 2003, Semlitsch 2008, De Roissart et al 2015) This can lead to source-sink dynamics, where the viability of a subpopulation often depends by the immigration/emigration from nearby populations, and some patches cannot sustain local populations without immigration from connected patches (Hale et al 2015, Seward et al 2019). Sites close to populations with high net reproductive rate (which act as sources) can have more positive population dynamics than expected solely on the basis of their in-site recruitment (Hanski 1998, Figuerola and Green 2002, Hanski and Gaggiotti 2004)
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