Data on the time spent in the study plots and the number of visits by animal species interacting with Prunus cerasifera fruits exposed in plots under three treatments (invasion, control, and double-control) in the Białowieża Primeval Forest foreland. These data were used in the publication: „Invasional meltdown shapes the invasibility of a primeval temperate forest”.

1. Multiple biological invasions lead to the emergence of novel associations between non-native species in their secondary range. When they result in mutual facilitation for both species, according to the invasional meltdown concept, they can positively affect the further spread of non-natives, e.g., by enhancing resource acquisition or dispersal. Most of the evidence supporting invasional meltdown, which promotes the spread of non-native plants, comes from the Mediterranean region and highly human-transformed ecosystems. 2. We assessed whether invasional meltdown can occur in primeval temperate forest and how it can affect the spread of invasive tree by invasive mammal. We used an example of raccoon dog (Nyctereutes procyonoides) and cherry plum (Prunus cerasifera) in the Białowieża Primeval Forest (NE Poland). Using camera traps, we investigated animals feeding on cherry plum fruits offered in 15 study plots. 3. The mean time raccoon dogs spent in plots was approximately 13-fold greater than that recorded for native mammal species, particularly in invasion and double-control plots. The probability of raccoon dog presence increased over time in control but remained stable in invaded and double-control plots. Fruit removal increased with raccoon dog visitation time and was relatively similarly high across all three treatments. 4. Synthesis: Even in species-rich, well-preserved landscapes, two invasive species can quickly develop novel associations, in <60 years of coexistence. Mutual interactions are two-sided: seed dispersal and provisioning valuable food resources. A consequence of invasional meltdown is a more effective spread of cherry plum, which recently entered the primeval forest. Our study is the first evidence that invasional meltdowns can also develop in low human impact ecosystems, believed to be resistant to biological invasions. Therefore, the next introductions of new, non-native species, can result in unpredictable effects, accelerating each other spread and increasing their impacts on native ecosystems’ functioning.