Latitudinal variation in seagrass herbivory: Global patterns and explanatory mechanisms

Aim The aim was to quantify latitudinal patterns in seagrass-herbivore interactions in the context of a warming climate. Location: We carried out a global meta-analysis combined with a field experiment across 1,700km and 12 degrees of latitude in Western Australia. Time period: 1984-2014. Major taxa studied: Seagrasses. MethodsResultsWe first synthesized the global literature on herbivore exclusion experiments in seagrasses to test whether differences in herbivore impacts are related to latitude and sea surface temperature. We then quantified leaf production and consumption rates in the field at nine meadows of the seagrass Amphibolis antarctica across 1,700km, from tropical to temperate latitudes. Seagrass biomass and nutritional traits (nitrogen, C:N, phenolics) were also measured. Our meta-analysis showed that herbivores had a similar net impact on seagrasses across 37 degrees of absolute latitude, and there was little variation in herbivore exclusion effects at different temperatures. In the field, rates of both production and consumption of seagrass were greatest in the tropics and decreased with latitude. Seagrass nutritional quality was lowest in the tropics, where fish removed c.30% of primary production. Consumption of the more nutritious temperate seagrasses was lower overall but also highly variable and dominated by invertebrates. Main conclusionsIn tropical latitudes, faster growth rates compensated for greater consumption of A.antarctica by herbivores. This resulted in similar net impacts of herbivores across latitudes, because higher latitude plants grew more slowly but also suffered less herbivory. This match between consumption and production rates might explain the global patterns derived from the literature, which show little latitudinal variation in the effects of consumers on seagrasses. As ocean temperatures continue to rise and overall herbivory levels are expected to increase in temperate regions, the survival of seagrass meadows in higher latitudes will depend on the ability of plants to increase growth at compensatory rates.