Practice question

Succession models are interpreted from how resident species affect the arrival and replacement of later species. Facilitation occurs when an early colonist improves conditions for a successor; tolerance occurs when later species can invade regardless of earlier occupants and eventually win through traits such as shade tolerance; inhibition occurs when established organisms suppress newcomers until disturbance or death creates space. A statement that an initial species makes the environment more suitable for a later one therefore indicates facilitation. A species able to invade whether another species is present or absent, and then outcompete it, reflects tolerance rather than dependence on facilitation. The keyed response identifies the pairing “Model X: tolerance, Model Y: inhibition” as the incorrect statement, which can be coherent if Model X shows facilitation and Model Y shows tolerance. However, the defining succession diagrams are absent from the spreadsheet extract, so the model labels and key cannot be conclusively checked. The reliable method is to follow the direction and sign of each species interaction rather than infer the model from replacement alone.