Box 2: Context changes how we observe biotic interactions
The component factors of enemy release interact with context. If this is not accounted for, this interaction produces inconsistencies in evidence for the ERH. To demonstrate this we discuss Fig. 4 in detail. Fig. 4 shows how different levels of resource availability affect enemy diversity through time. Three exotic populations (A, B, C) may be studied at the same time point (t1 ort2 ) in their invasion. If the populations come from different ecological contexts, their relative enemy diversities may differ. Alternatively, two populations could follow different trajectories because of their ecological contexts but appear to be on the same trajectory if they are studied at different points in time (enemy diversity on population A at t1 is identical to population B at t2 ). In this case, the equivalence is misleading; the ERH appears to be equally influential in both cases, but population B will saturate at a lower total enemy diversity. Finally, populations with the same trajectory can appear to be very different if they are studied at different points in their invasion history (e.g. population A at t1 vs.t2 ).
Depending on when populations A-C are studied, trends could range from ‘complete’ enemy release (all three populations shortly aftert0 ), to enemy reduction (all three populations att, populations B and C att2 ) to biotic resistance (population A in later invasion). As individual case studies, they would all tell different stories but can all be explained by the interaction of factors with context.