Animal migration is multifaceted in nature, but the relative strength of different cues that trigger resulting patterns of migration is not well understood. Partially migratory populations offer an opportunity to test hypotheses about migration more broadly by comparing trait differences of migrants and residents. We quantitatively reviewed 45 studies that statistically modeled migration propensity, extracting132 effect sizes for internal and external proximate drivers across taxa. Our meta-analysis revealed that internal and external drivers had medium (Cohen’s d > 0.3) and large (Cohen’s d > 0.5) effect sizes on migration propensity respectively. Predator abundance and predation risk had a large effect, as did individual behaviour (e.g., personality). The abiotic environment and individual physiology had a medium effect on migration propensity. Of the studies that examined genetic divergence between migrants and residents, 64% found some genetic divergence between groups. These results clarify broad proximate drivers of migration and offer generalities across taxa.

The relative effect of top-down versus bottom-up forces in regulating and limiting ungulate populations is an important theme in ecology. Untangling these effects is important for basic understanding of trophic dynamics and is critical for effective management. We integrated two independent sources of observations to compare the drivers of moose abundance across 55 replicated populations. Across populations, moose declined by nearly 20% over 20 years. At high density, moose were regulated by intraspecific competition. Predation primarily limited population growth, except at low density, where it was regulating. Harvest was largely additive and potentially contributed to population decline. Our results provide strong evidence for density dependent predation, highlighting that population dynamics are context dependent and vary strongly across gradients in climate, forest type and predator abundance. Consequently, management of moose would be optimized by taking different strategies across populations in accordance with their population trajectories and abundance of predators.

The relative effect of top-down versus bottom-up forces in regulating and limiting wildlife populations is an important theme in ecology. Untangling these effects is critical for basic understanding of trophic dynamics and effective management. We examined the drivers of moose abundance by integrating two sets of observations to create one of the largest existing spatiotemporal datasets on ungulate population dynamics globally. We documented a 20% population decline. At high density, moose were regulated by intraspecific competition. Predation primarily limited population growth, except at low density, where it was regulating. Harvest was largely additive and contributed to population decline. Our results provide strong evidence for density dependent predation, highlighting that population dynamics are context dependent and vary strongly across gradients in climate, forest type and predator abundance. These results clarify longstanding questions in population ecology and highlight the complex relationships between natural and human-caused mortality in driving ungulate population dynamics.