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 att1, 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.