Figure 2 : Three exotic E populations (A, B, C) experience
different levels of enemy pressure relative to native species (N),
depending on the product (enemy pressure P = D*I) of proportional
differences in both enemy diversity (D) and enemy impact (I), where D =
DE/DN, and I =
IE/IN. P > 1 indicate
enemies have a stronger negative effect on exotic species, P <
1 indicate enemies have a stronger negative effect on natives. Dashed
grey lines from each population link to exterior squares that show how
those populations would look if only a single process was studied
(diversity, a and b; or impact of a few select enemies, c and d). Dashed
blue lines indicate equal enemy diversity (richness, abundance or their
combination) between natives and exotics (vertical), and equal
per-capita impact of those enemies (horizontal). The solid blue line
indicates where enemies affect the performance of native and exotic
species equally, with support for the ERH below this line. Axes are
logged to facilitate symmetrical interpretation (i.e. on the x-axis, 0.1
corresponds to natives having ten times as many enemies as exotics; 10
corresponds to aliens having ten times as many enemies as natives).