Are high densities of fishes and shrimp associated with top‐down control of tropical benthic communities? A test in three Hong Kong streams

Abstract

Summary Top‐down control of benthic communities by fishes in temperate and Neotropical streams has received considerable attention, yet relatively little is known about the occurrence or strength of such top‐down control in tropical Asian streams. We undertook pool‐scale removals of an entire macroconsumer assemblage of fishes and predatory shrimp, as well as its component guilds (algivores versus predators), in three Hong Kong streams during the dry season and monitored the responses of periphyton, benthic macroinvertebrates (predominantly insects) and leaf‐litter breakdown. Pools supported high densities and biomass of predatory macroconsumers. Algivorous balitorid fishes had a mean density of 30.1 indiv m−2 (range: 1.5–58.2 indiv m−2) and a wet‐weight biomass of 20.9 g m−2 (range: 1.6–34.2 g m−2); values for predatory fishes (mainly a nemacheilid and a goby) were 9.4 indiv m−2 (range: 7.3–11.1 indiv m−2) and 29.7 g m−2 (range: 24.1–38.0 g m−2). Densities of predatory palaemonid shrimp were likewise high (mean: 14.0 indiv m−2; range: 8.9–19.0 indiv m−2). Despite high macroconsumer standing stocks, there was little evidence of top‐down control of benthic communities. The removal of algivorous fishes had no effects on algal biomass, periphyton accumulation, silt accrual or abundance of benthic insects. The removal of all predators (fishes and shrimp), as well as the removal of all macroconsumers, was associated with a small increase in the density of insect scrapers (especially baetid mayflies), although no such effect was evident when predatory fishes (but not shrimp) were removed. Apparently, shrimp substituted for the role of predatory fishes by cropping any excess prey. However, the removal of the entire predatory guild or all macroconsumers produced no additional changes in macroinvertebrate assemblages, nor any influence on periphyton or leaf‐litter breakdown. The pool‐scale approach used in this investigation, combined with the inclusion of three study streams, should have provided a realistic picture of the significance of top‐down control of benthic communities by macroconsumers. Statistical power may have been limited by logistical constraints on the number of replicate pools. Localised nutrient limitation caused by the removal of macroconsumers might have contributed to the apparent weakness of top‐down control of benthic communities. However, given the high standing stocks of fishes and shrimps, it is remarkable that experimental manipulations resulted in generally small or negligible effect sizes.

Benthic community structure and ecosystem functions in above- and below-waterfall pools in Borneo

“…One explanation is that the macroinvertebrates consistently evaded fish and shrimp predators; this has been shown in studies that artificially doubled the natural densities of fish and failed to observe a decrease in some invertebrate taxa (Gilliam et al, 1989;Dudgeon, 1991). Our study also did not reveal a consistent pattern in macroinvertebrate diversity of above-and below-waterfall pools, suggesting that communities of shrimp and fish do not influence these biotic patterns; however, it is possible that fish and shrimp predators are substitutable and thus functionally redundant (sensu Ho & Dudgeon, 2016). In Northern Australia, Garcia et al (2015) found that top-down effects on macroinvertebrates were context dependent with respect to factors such as benthic habitat, stream velocity and community structure, each of which influenced invertebrate diversity.…”

“…Moulton et al (2010) found that predacious fish inhibit shrimp and baetid mayflies from grazing in two Neotropical streams, thus increasing both abundance and quality of periphyton in pools containing fish. However, other experiments have found that top-down effects of fish on algae have less impact relative to bottom-up effects such as increases in nutrient levels (Garcia et al, 2015;Ho & Dudgeon, 2016). Interestingly, Ho & Dudgeon (2016) found no impact of high numbers of fish and shrimp on algal biomass or periphyton accumulation in three Hong Kong streams.…”

“…However, other experiments have found that top-down effects of fish on algae have less impact relative to bottom-up effects such as increases in nutrient levels (Garcia et al, 2015;Ho & Dudgeon, 2016). Interestingly, Ho & Dudgeon (2016) found no impact of high numbers of fish and shrimp on algal biomass or periphyton accumulation in three Hong Kong streams. This was suggested to be caused by the removal of macroconsumers, which may have reduced nutrient levels and therefore contributed to the apparently weak top-down effects on fish and shrimp (Ho & Dudgeon, 2016).…”

“…The different community structures of fish and shrimp above and below waterfalls can influence a range of ecosystem functions including organic matter decomposition and periphyton growth (Greathouse & Pringle, 2006;Moulton et al, 2010;Ho & Dudgeon, 2016). Below waterfalls, high numbers of fish can play an important role in controlling periphyton growth by clearing rock surfaces after frequent tropical flood events (Pringle & Hamazaki, 1998).…”

“…The separation of communities and its impact on ecosystem structure and function have been researched to some extent, with mixed results, in the Neotropics (Pringle & Hamazaki, 1998;March et al, 2002;Ramirez & Hernandez-Cruz, 2004;Covich et al, 2009). However, there have been few similar studies conducted in tropical Asia despite the similarity of the region's biotic and abiotic conditions to those of the Neotropics (Mantel et al, 2004;Ho & Dudgeon, 2016).…”