Effects of primary productivity and ecosystem size on food-chain length in Raohe River, China

Abstract

Food chains illustrate the fundamental relationship among different producers and consumers in the natural world. The length of a food chain describes the relationship from primary producers to top predators, and has long been a central concept in community ecology. Many different drivers, including productivity and ecosystem size, explain natural variations in the lengths of food chains. A significant amount of knowledge remains to be obtained about the roles of these various drivers in determining the lengths of food chains and the mechanisms by which they operate in river ecosystems. Raohe River in China is one of main rivers that flow into Lake Poyang. We examined the main channel and tributaries of Raohe River and collected the most common species in them. We found a total of 336 species belonging to 12 orders, 26 families, and 43 genera. We also collected 10 water samples. We conducted stable nitrogen isotope analysis of the consumers and used calculated nitrogen ratios (δ15N) to determine the lengths of the food chains at various sites in Raohe River. Subsequently, we examined the effects of primary productivity and ecosystem size on food-chain length. We used chlorophyll-a concentration obtained by analyzing filtered water samples to calculate the primary productivity of the river. The lengths of food chains in Raohe River ranged from 1.86 to 3.71. The longest food chains were at the Lake Poyang and Lianhu sample sites, and the shortest food chain (<2.0) was at Xiagang sample site. Results indicated that food chains in these rivers lengthened from the upper reaches to the lower reaches, reflecting an increased number of consumer species in the lower reaches. Significant positive correlations were observed between the lengths of the food chains in Raohe River and primary productivity (y = 0.41x + 2.13, R2 = 0.58, p < 0.01) as well as ecosystem size (river width: y = 0.61x + 1.28, R2 = 0.43, p < 0.05; catchment area: y = 0.32x + 1.48, R2 = 0.42, p < 0.05). Together, these analyses tested the reliability of productive-space hypothesis, which is often applied to river ecosystems. Our data suggest that larger ecosystems result in longer food chains in rivers by integrating catchment area and river width through river networks, thereby enhancing environmental stability in river ecosystems.