Extended Data Fig. 1: Conceptual illustration of the models used to predict alien spread.

Flowchart illustrating the four types of models used to predict alien spread: Random Dispersal Model, Environmental Resistance (ER) Model, Climate Resistance Model, and Climate Matching Model. Grids show hypothetical probability surfaces used to model spread. The red square indicates the cell where the alien population established. Darker colours indicate that a cell has a higher probability of being invaded. Under the random dispersal model, alien spread is purely stochastic and all cells have an equal probability of being invaded. The other three models use conditions in either the invading species’ alien (red) or native (blue) ranges to guide dispersal. The environmental resistance model uses an invasion probability based on the distributions of native species in the invaded region. The native species with ranges overlapping the cell of alien population establishment are identified, and these species’ ranges are used to calculate the environmental resistance (1 – biotic similarity) to all other cells in the landscape. Biotic similarity is here taken as the proportion of native bird species in the establishment cell that are present at each other cell (yellow box). The climate resistance model calculates the probability of invasion based on the similarity of each cell’s climatic conditions to the conditions within the cell of alien population establishment. Under the climate matching model, invasion probability is determined by a cell’s climatic similarity to the average conditions occupied in the invading species’ native range.