Elements on the move: How ungulate migration expands Alpine biogeochemical footprints

Résumé fourni par la source

Through depositing waste products, animals influence the spatial distribution of elements across landscapes. Yet the relationship between animal movement and element distribution remains poorly characterized. We developed a spatially explicit agent-based model to test how migratory versus resident red deer (Cervus elaphus) influence nitrogen redistribution across an alpine landscape in the Central-Eastern Italian Alps. Specifically, we asked how both local-scale and landscape-scale movement alter the spatial extent and magnitude of nitrogen deposition. We parameterized our model with GPS telemetry from 2021 to 2024 and remotely sensed vegetation data. We simulated four different scenarios which allowed us to disentangle the relative effects of large-scale (migration persisting) and fine-scale (resident behaviour) movement: (i) mixed migratory-resident (300 deer), (ii) fully resident (300 deer), (iii) reduced resident (150 deer) and (iv) reduced migratory (150 deer). The potential for nitrogen intake, assimilation, and excretion occurred hourly across a seasonally dynamic landscape. Across all scenarios, tree cover density and slope consistently emerged as positive predictors of nitrogen transport. Thus, regardless of resident or migratory status, red deer act as mediators of local element transport. Similarly, proximity to roads/trails reduced nitrogen inputs and created closed systems, indicating that barriers constrain both local and landscape-scale element transport. Migration substantially expanded the spatial extent of nitrogen redistribution and enabled the upward movement of elements, both locally upslope and into higher elevation habitats, effectively transporting elements against gravitational forces. Consequently, the loss of migration is likely to weaken these large-scale element linkages and reduce associated ecosystem functions. Our results demonstrate that different animal movement patterns play distinct and complementary roles in connecting element pools across landscapes. While both resident and migrant foraging redistribute elements locally, migratory movements link lowland and alpine habitats, expanding the spatial reach of element redistribution. Thus, loss of migration not only reduces the spatial extent of element distribution but also alters the topographic pathways through which elements are cycled. These findings highlight the broader ecosystem consequences of declining animal movement extent, and migration in particular, and underscore the importance of conserving behavioural diversity to maintain element heterogeneity and ecosystem functioning in mountain systems.

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