Data from: Test of biotic and abiotic correlates of latitudinal variation in defences in the perennial herb Ruellia nudiflora

1. Geographic variation in abiotic factors and species interactions is
widespread and is hypothesized to generate concomitant patterns of species
trait variation. For example, higher rates of herbivory at lower latitudes
are thought to select for increased plant defences, although latitudinal
variation in defences may also be influenced directly by abiotic factors
and indirectly by predators and parasitoids reducing herbivore pressure.
2. We measured defences of the herb Ruellia nudiflora among 30 populations
spanning a latitudinal gradient from northern Yucatan to southern Belize
that vary substantially in leaf herbivory (four-fold), seed herbivory
(25-fold), and seed herbivore parasitism (14-fold). These surveyed
populations span one-third of the species’ latitudinal distribution (5° of
latitude), the entire precipitation gradient of its distribution, and
one-third of the temperature gradient of its distribution. Our prior work
showed that leaf herbivory decreased with latitude and that seed herbivory
increased with latitude. Here, we measured leaf trichome density and leaf
and seed phenolics and tested whether latitudinal variation in climate,
herbivory, and parasitism explained latitudinal variation in these
defensive traits. 3. Patterns of variation in leaf trichomes fully
supported predictions, with trichome density increasing with a parallel
increase in herbivory towards lower latitudes. While seed phenolics were
positively associated with herbivory, and seed herbivory tended to
increase with latitude, the predicted (positive) association between
latitude and defence was not detectable. There was no detectable
association between parasitoids and seed defences. In addition, the
association between leaf herbivory and phenolics was weak, and leaf
phenolics were not associated with latitude. Importantly, variation in the
abiotic environment was associated with plant defence, indicating that
abiotic factors can play a major role in shaping plant defences,
independently of herbivory. 4. Synthesis: Latitudinal variation in abiotic
factors may drive concomitant patterns of variation in plant defences,
independently of herbivory. Collectively, these findings highlight the
need for assessing geographic variation in plant defences from a
multi-factorial perspective, testing for the simultaneous influence of
biotic and abiotic factors.