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Data from: Latitudinal variation in the constitutive and inducible defenses of a canopy-forming rocky intertidal seaweed

Ryan, Sean1; Matassa, Catherine1

Published Oct 08, 2025 on Dryad. https://doi.org/10.5061/dryad.sn02v6xd0

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Abstract

A long-standing theory in biogeography is that stronger biotic interactions at lower latitudes select for better-defended phenotypes. However, greater environmental variability at temperate latitudes may also shape defensive strategies by increasing temporal or local spatial variation in consumer pressure and thus selecting for greater phenotypic plasticity, or inducible defenses. Distinguishing between inducible and constitutive defense strategies is therefore necessary to test for latitudinal defense gradients, but also for understanding how species interactions and community dynamics vary across ecological and evolutionary scales. 

We investigated latitudinal variation in antiherbivore defenses of a cosmopolitan rocky intertidal seaweed (Fucus vesiculosus) against a similarly common and abundant grazer (periwinkle snails, Littorina littorea). We used a multi-phase common garden experiment with seaweeds from three different regions along the US Atlantic coast and snails from the southern- and northern-most study sites. We manipulated and measured snail grazing on seaweeds in a series of no-choice and two-choice assays to identify regional differences in seaweed functional defenses (i.e., reduced grazing). 

Across all assays, grazing rates declined with seaweed latitude. Prior grazing reduced the palatability of southern, but not northern, seaweeds. Changes in seaweed nutritional content (C:N) and phlorotannins (a putative chemical defense) correlated with induced but not constitutive functional defenses.  

Our results indicate that the constitutive anti-herbivore traits of F. vesiculosus increase with latitude and negatively covary with defense plasticity. This result suggests that the selective pressure of herbivory is stronger for northern seaweed populations, while southern populations may face a different set of tradeoffs leading to defense plasticity, such as increasing environmental stress.  

The strength of trophic interactions plays an important role in community dynamics and food web stability. Our findings add to a growing literature highlighting the importance of ecological context in shaping trophic interactions and suggests that estimates and comparisons of interaction strength need to consider spatiotemporal variation in prey defenses. Defensive traits that vary with latitude or along environmental stress gradients may be particularly important for predicting the effects of climate change on trophic interactions and their consequences for community dynamics and ecosystem function.