Fiia Haavisto and Veijo Jormalainen
Large perennial seaweeds, such as the brown alga Fucus vesiculosus L., are important foundation species in the rocky shores where they often form dense monospecific stands. They typically confront intense herbivory. Consequently, they have evolved an ability to sense water-borne cues released by grazing of their neighbours and to use them to trigger their own defences before becoming grazed. While such water-borne resistance is found in several seaweeds, knowledge of this phenomenon is based on small-scale laboratory studies and we lack evidence of water-borne resistance in nature. In natural populations, chemically mediated interactions may be disturbed by several abiotic factors like currents and water turbulence. Also, biotic factors such as the amount of grazing damage on the cue-emitting individual or the sensitivity of the cue-receiving individual may limit the spread of resistance among individuals in the sea.
We experimentally tested whether F. vesiculosus shows water-borne resistance induction in field conditions. We exposed algal individuals to water-borne cues at distances of 0.5 and 2 m from their grazed neighbours and measured their resistance responses. We found that grazing induced resistance in the directly grazed algae, and that the induction spread to the undamaged neighbours within 2 m. However, the spreading of induction was spatially variable, likely reflecting natural variability in water currents in the sea. Furthermore, investment of resources into defences may be limited by resource allocation to other functions which can be realised as trade-offs between resistance and growth. We found such a cost of overall resistance, including induced and constitutive resistance, but no costs of water-borne induced resistance, suggesting that water-borne resistance may represent an economical priming process to prepare for anticipated herbivory. Our field experiment demonstrates, for the first time, that water-borne spread of herbivore defence is not restricted to laboratory conditions but occurs also in nature. Defence induction may, thus, gradually spread over large seaweed stands enabling them to cope with the intense herbivory found in these habitats.