Posted by: oikosasa | October 15, 2012

Double trouble in paradise

In the study “Density- and trait-mediated top–down effects modify bottom–up control of a highly endemic tropical aquatic food web” Christopher Dalton and co-workers have looked at bottom-up and top-down effects in anchialine ponds on Hawaii. Here’s Chrsitopher’s own story about the study:

For centuries, early Hawaiian residents divided land on the island into expansive parcels known as ahupuaʻa, with each ahupuaʻa containing all of the resources the residents would need to survive (water, food, shelter). Ahupuaʻa typically ran in radial lines extending from the top of the nearest volcano (Mauka; near the mountains) to the shore of the ocean (Makai; near the ocean). The expression Mauka to Makai lives on as an important model for island conservation today, emphasizing that the spectacular and unique biodiversity of these islands cannot be preserved without understanding the links between ecosystems.

Perhaps no single Hawaiian ecosystem better reflects the necessity of landscape-scale conservation as the anchialine ponds of coastal Hawaiʻi, connected through porous lava substrate to both tidal seawater and fresh groundwater. One of the most striking organism in the ponds is the endemic atyid shrimp, Halocaridina rubra (locally known as ōpaeʻula, or red shrimp).  This locally abundant invertebrate shows diel migration between daytime refuges in subterranean environments and nighttime foraging in productive surface habitats. In anchialine pools, the grazing of ōpaeʻula is often attributed a keystone function for maintaining a diverse and tuft-like epilithic crust of algae, cyanobacteria and heterotrophic bacteria.

We assess the roles of nutrients and invertebrate consumers in anchialine pond food webs by taking advantage of long-term, whole-ecosystem anthropogenic modification of bottom-up (nutrient enrichment) and top-down (grazing) controls. This study provides insight at the ecosystem scale for the interactions between top-down and bottom-up control in a system of dire need of information to direct management practices against threats from development and invasive species.

We collected quantitative samples of the epilithon quantity and composition, the abundance (day and night) of ōpaeʻula, the presence of fish and the dissolved nutrient concentrations. Sometimes this meant getting wet in the deepest pools, and sometimes it meant wading through algal and detrital mats up to half a meter deep. In the end, we captured a snapshot of the relatively simple food web structure and nutrient availability of twenty pools.

This study provides whole-ecosystem scale insight into the relative influence of bottom-up and both trait and density mediated top-down effects in pool food webs, and it also provides evidence to help guide management decisions. Our research suggests the ecological benefits of mitigating nutrient pollution are comparable to those of removing predatory, invasive fish, and monitoring these two factors can prevent the dramatic ecological change we observed in high nutrient ponds with fish.

Ultimately, preservation of Hawaiian anchialine ponds requires a perspective beyond the edges of these ecosystems. The nutrient enrichment that we observed in anchialine ponds was associated with land use (resorts, hotels and golf courses) immediately surrounding those ponds.

To truly understand and protect these hotbeds of endemism into the future, however, research must look from Mauka to Makai and assess the role of landscape context in driving change in these unique ecosystems.

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