Studies on latitudinal patterns in plant defence have
traditionally overlooked the potential effect that resource availability
may have in shaping plant defence. Likewise, latitudinal patterns of
tolerance traits have rarely been studied, yet they can be a critical
component of plant defence. Therefore, the aim of our study was to
examine latitudinal variation in the production of tolerance and
resistance traits against herbivory along a latitudinal range and a
natural gradient of resource availability from upwelling conditions.
We conducted experiments simulating macroherbivore (e.g., bird, fish) damage on the seagrass Z. marina
at 10 sites across the Eastern Pacific coast (Canada–Mexico) and Quebec
and analysed several traits related to resistance and tolerance
strategies against herbivory. In addition, we examined the effects of
potential seagrass changes in defence strategies by performing a series
of feeding experiments with mesoherbivores in a subset of sites.
We found that eelgrass resistance defences did not
follow a linear latitudinal pattern but rather followed a bell‐shaped
curve which correlated with bottom‐up control. In sites with higher
nutrient availability, plants allocated resources to tolerance
strategies and had lower resistance traits. Furthermore, seagrasses did
not respond linearly to increased herbivory pressure; while they
tolerated moderate levels of herbivory, they underwent a significant
reduction in tolerance and resistance under high herbivory levels, which
also made them more susceptible to consumers in feeding experiments.
Our results highlight the importance that nutrient
availability has in shaping latitudinal patterns of plant defence
against herbivory and show how these defences may not respond linearly
to increased herbivory pressure in seagrasses.
