Abstract |
- The impact of herbivores on primary producers in differing oceanographic
regimes is a matter of intense ecological interest due to ongoing changes in their abundance,
that of their predators, and anthropomorphic alteration of nutrient cycles and climatic
patterns. Interactions between productivity and herbivory in marine habitats have been
studied on temperate rocky shores, coral reefs, mangroves, and salt marshes, but less so at
tropical latitudes. To determine how herbivore–alga dynamics varied with oceanographic
regime, we used the comparative-experimental approach in rocky intertidal communities on
the Galápagos Islands from January 2006 to January 2009. This setting was selected because
strongly contrasting oceanographic conditions occurred within a range of ~181 km, with
significant differences in temperature, nutrients, phytoplankton productivity, and intertidal
communities, and in abundance of macro-herbivores, including marine iguanas. Experiments
and measurements were conducted at two sites in each of three oceanographic regimes
characterized by low, intermediate, and high bottom-up inputs. At sites of low inputs, macro-herbivores
(fish, crabs, iguanas) had a consistent top-down effect, reducing algal abundance,
and leaving a few grazer-resistant varieties. At sites of intermediate and high inputs, consumer
impacts were stronger during La Niña (cool phase) than during El Niño (warm phase). At sites
of high inputs, algal biomass was naturally relatively high and was dominated by the edible
algae Ulva spp. Macro-grazers reduced algal biomass, but their primary effect was indirect, as
articulated corallines displaced other species of algae in their absence. Prior results from the
tropics had revealed dominant effects of top-down interactions and recruitment in structuring
intertidal communities. Our results suggest that, when a broader oceanographic scenario is
taken into account, the relative importance of top-down and bottom-up forces are context
dependent, varying with oceanographic regime and climatic variability.
- Keywords: Top-down, Productivity, Oceanographic conditions, Galápagos, Marine iguanas, ENSO, Bottom-up, Climate, Herbivory, Rocky shores, Algae, Tropics
- Keywords: Top-down, Productivity, Oceanographic conditions, Galápagos, Marine iguanas, ENSO, Bottom-up, Climate, Herbivory, Rocky shores, Algae, Tropics
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