Abstract
Body size is frequently claimed to be a major determinant of animal trophic interactions, yet few studies have explored relationships between body size and trophic interactions in rivers, especially within the tropics. We examined relationships between body size and trophic position (TP) within fish assemblages in four lowland rivers of the Lower Mekong Basin in Cambodia. Stable isotope analysis (based on δ15N) was used to estimate TP of common fish species in each river, and species were classified according to occupation of benthic versus pelagic habitats and major feeding guilds. Regression analysis yielded strong correlations between body size and TP among fishes from the Sesan and Sreprok rivers, but not those from the Mekong and Sekong rivers. The Mekong fish assemblage had higher average TP compared with those of other rivers. The relationship between body size and TP was positive and significantly correlated for piscivores and omnivores, but not for detritivores and insectivores. The body size–TP relationship did not differ between pelagic and benthic fishes. Body size significantly predicted TP within the orders Siluriformes and Perciformes, but not for Cypriniformes, the most species-rich and ecologically diverse order in the Lower Mekong River. We conclude that for species-rich, tropical fish assemblages with many detritivores and invertivores, body size would not be an appropriate surrogate for TP in food web models and other ecological applications.
Highlights
Body size is recognized as an important determinant of community structure because it influences ecological processes that affect consumer–resource interactions, life-history traits, population dynamics and metabolic rates [1,2,3,4,5,6]
We conclude that for species-rich, tropical fish assemblages with many detritivores and invertivores, body size would not be an appropriate surrogate for trophic position (TP) in food web models and other ecological applications
75 million people living in Southeast Asia, those who live in the Lower Mekong Basin, depend on inland fisheries for food security
Summary
Body size is recognized as an important determinant of community structure because it influences ecological processes that affect consumer–resource interactions, life-history traits, population dynamics and metabolic rates [1,2,3,4,5,6]. Empirical studies in marine and freshwater ecosystems have supported the positive relationships between body size and TP. Similar findings are reported by Nakazawa et al [15], who investigated relationship between body size and δ15N (an index of relative TP) using long-term data (40 years) for freshwater fishes from Lake Biwa, Japan. There is ample evidence of positive relationships between body size and TP of fishes, other studies have shown no relationship between TP and size [16,17,18] or a negative relationship [19,20], which suggests that phylogeny and associated morphological and behavioural constraints influence how body size affects species interactions [20,21]. Even though the body size–TP relationship in fishes appears to be strongly influenced by gape limitation [13,22], other morphological traits could play roles [14]
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