Abstract
An important hypothesis for how plants respond to introduction to new
ranges is the evolution of increased competitive ability (EICA). EICA
predicts that biogeographical release from natural enemies initiates a
tradeoff in which exotic species in non-native ranges become larger and
more competitive, but invest less in consumer defenses, relative to
populations in native ranges. This tradeoff is exceptionally complex
because detecting concomitant biogeographical shifts in competitive
ability and consumer defense depend upon which traits are targeted, how
competition is measured, the defense chemicals quantified, whether
defense chemicals do more than defend, whether “herbivory” is
artificial or natural, and where consumers fall on the
generalist-specialist spectrum. Previous meta-analyses have successfully
identified patterns but have yet to fully disentangle this complexity.
We used meta-analysis to reevaluate traditional metrics used to test
EICA theory and then expanded on these metrics by partitioning
competitive effect and competitive tolerance measures and testing Leaf
Specific Mass in detail as a response trait. Unlike previous syntheses,
our meta-analyses detected evidence consistent with the classic tradeoff
inherent to EICA. Plants from non-native ranges imposed greater
competitive effects than plants from native ranges and were less
quantitatively defended than plants from native ranges. Our results for
defense were not based on complex leaf chemistry, but instead were
estimated from tannins, toughness traits, and primarily Leaf Specific
Mass. Species specificity occurred but did not influence the general
patterns. As for all evidence for EICA-like tradeoffs, we do not know if
the biogeographical differences we found were caused by tradeoffs per
se, but they are consistent with predictions derived from the
overarching hypothesis. Underestimating physical leaf structure may have
contributed to two decades of tepid perspectives on the tradeoffs
fundamental to EICA.