Abstract
Terrestrial invertebrates are important subsidies to fish diets, though their seasonal dynamics and importance to tropical stream consumers are particularly understudied. In this year-round study of terrestrial invertebrate input to two Trinidadian headwater streams with different forest canopy densities, we sought to (a) measure the mass and composition of terrestrial inputs with fall-in traps to evaluate the influences of seasonality, canopy cover, and rainfall intensity, and; (b) compare terrestrial and benthic prey importance to Anablepsoides hartii (Hart’s Rivulus), the dominant invertivorous fish in these streams, by concurrently measuring benthic and drifting invertebrate standing stocks and the volume and composition of invertebrates in Rivulus guts throughout the year. The biomass of terrestrial invertebrate fall-in was 53% higher in the wet versus dry season; in particular, ant input was 320% higher. Ant biomass fall-in also increased with the density of canopy cover among sampling locations within both streams. Greater precipitation correlated with increased ant inputs to the more open-canopied stream and increased inputs of winged insects in the more closed canopy stream. Concurrently, the biomass of benthic invertebrates was reduced by more than half in the wet season in both streams. We detected no differences in the total volume of terrestrial prey in Rivulus diets between seasons, though ants were a greater proportion of their diet in the wet season. In contrast, benthic prey were nearly absent from Rivulus diets in the wet season in both streams. We conclude that terrestrial invertebrates are a substantial year-round prey subsidy for invertivores in tropical stream ecosystems like those we studied, which may contrast to most temperate streams where such terrestrial inputs are significantly reduced in the cold season. Interestingly, the strongest seasonal pattern in these tropical streams was observed in benthic invertebrate biomass which was greatly reduced and almost absent from Rivulus diets during the wet season. This pattern is essentially the inverse of the pattern observed in many temperate streams and highlights the need for additional studies in tropical ecosystems to better understand how spatial and temporal variation in terrestrial subsidies and benthic prey populations combine to influence consumer diets and the structure of tropical stream food webs.
Highlights
The movement of organisms, energy, and nutrients across habitat boundaries is often an important contributor to recipient ecosystem processes, populations, and communities (Polis et al, 1997; Reiners and Driese, 2001; Gounand et al, 2018)
The majority of this research has identified strong seasonality in terrestrial invertebrate input to streams and their contribution to stream food webs; the bulk of the work has been geographically biased toward northern temperate ecosystems, and the effect that wet-dry season dynamics have on the magnitude and importance of terrestrial subsidies to tropical stream consumers is not well understood
We report on the effect of seasonality, precipitation, and canopy density on falling invertebrate biomass
Summary
The movement of organisms, energy, and nutrients across habitat boundaries is often an important contributor to recipient ecosystem processes, populations, and communities (Polis et al, 1997; Reiners and Driese, 2001; Gounand et al, 2018). Experimental reduction of terrestrial invertebrate input to this stream in summer using a greenhouse cover reduced fish growth and population biomass (Nakano et al, 1999b; Kawaguchi et al, 2003; Baxter et al, 2007), and induced a trophic cascade via a shift in fish diets from terrestrial to local aquatic prey (Nakano et al, 1999b; Baxter et al, 2004), which, in turn, decreased aquatic insect emergence and reduced riparian predators, such as spiders (Baxter et al, 2004) These examples serve to demonstrate that, in at least some temperate regions, seasonal inputs of terrestrial invertebrates subsidize populations of stream fishes with important consequences for community dynamics and ecosystem processes. Despite observations pointing to cross-biome differences in phenology and relative contribution of this subsidy to fish diets (e.g., Rundio and Lindley, 2008, 2012; Zhang and Richardson, 2011; Behn and Baxter, 2019), there have been few studies of this phenomenon conducted outside temperate latitudes
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