Do small-scale exclosure/enclosure experiments predict the effects of large-scale extirpation of freshwater migratory fauna?

Abstract

A variety of theoretical and empirical studies indicate that the abilities of small-scale experiments to predict responses to large-scale perturbations vary. Small-scale experiments often do not predict the directions of large-scale responses, and relatively few empirical studies have examined whether small-scale experiments predict the magnitudes of large-scale responses. Here we present an empirical example of small-scale manipulations predicting not only the directions but also the magnitudes of the effects of whole-catchment, decades-long decimation of migratory freshwater shrimp populations. In streams of Puerto Rico (USA), we used arena sizes of <2 m2 in 1- to 4-week exclosure/enclosure experiments. Effects of small-scale experiments largely matched those of large-scale shrimp loss above dams for a variety of response variables (abiotic and biotic factors including epilithic fine sediments, algae and organic matter, and invertebrate grazers, detritivores, and predators). The results of our extrapolation contrast with studies of small- versus large-scale perturbations in the temperate zone. Our findings are likely explained by: a set of response variables that are more dominated by within-patch processes than exchange processes, an experimental manipulation that encompassed the characteristic scales of response variables, our use of open arenas lacking cage artifacts, and/or our combination of two distinct experimental approaches (exclosures and enclosures). Based on our study design, we suggest that extrapolation across experimental scales can be greatly enhanced by embedding open arenas within large-scale conditions that represent all treatment levels.

Toward Quantifying the Relative Importance of Invertebrate Consumption and Bioturbation in Puerto Rican Streams

“…After storm events, shrimps rapidly clear deposited fine material from benthic substrata (Pringle & Blake 1994, March et al 2002, Greathouse et al 2006a). In Puerto Rican streams with relatively high consumer biomass, the quantity of epilithon often returns to prestorm levels within hours to days (Pringle & Blake 1994; W. F. Cross pers.…”

“…Although bioturbation by consumers does not directly influence the quality of organic matter, sediment clearing and redistribution has large effects on the composition and nutrient content of bulk epilithic material. In Puerto Rico, both observational (Greathouse et al 2006b) and experimental (Pringle et al 1999, Greathouse et al 2006a) studies have shown that shrimps increase the nutrient content of epilithon by removing loosely attached low-nutrient particulate material and inorganic sediment. The remaining benthic material is often composed of a tightly adhering nutrient-rich biofilm with a significant algal component.…”

A life-history account ofMacrobrachium spinipes(Schenkel, 1902) (Cherabin) in a large tropical Australian River

“…Process subsidies have been documented through the subsequent ecosystem impacts following loss of these species caused by dam development (Benstead et al 1999, Greathouse et al 2006a. Material subsidies by migratory shrimp have not been evaluated in any detail.…”

Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

“…These generally showed a positive effect of nutrient concentration on periphyton (Elwood et al, 1981) or on grazers, via periphyton (Hershey et al, 1988(Hershey et al, , 1993Hinterleitner-Anderson, Hershey & Schuldt, 1992). Other studies explicitly addressed the top-down control of periphyton biomass (Yasuno et al, 1982;Kohler & Wiley, 1997;Taylor, Mcintosh & Peckarsky, 2002;Greathouse, Pringle & Mcdowell, 2006;Katano et al, 2007). However, only Peterson et al (1993) have documented top-down as well as bottom-up effects using an enrichment experiment.…”