Food Web Structure in Tropical Highland Stream Ecosystem

Abstract

The structure and properties of tropical stream food webs were investigated in nine second and third order streams, three in continuous sub-montane forest, three in tea plantations and three in maize fields. All were located on the Mambilla Plateau, north-east Nigeria. Components of all trophic levels (e.g., fine particulate organic matter [FPOM], coarse particulate organic matter [CPOM], algae, benthic invertebrates and fish) were sampled, and analysed by examination of gut contents and using stable isotopes techniques. Gut contents of key invertebrate species were categorised into eight types of food items: filamentous algae, diatoms, fungi, CPOM ≥ 1 mm (usually leaf litter and wood fragments), FPOM 1 mm (often amorphous detritus) and animal parts. Several food web properties were calculated and a basic food web was constructed using density data. Invertebrate species richness and density were higher in forest than in tea plantations and maize fields. Community and dietary analyses revealed that streams in the tea plantation and maize fields had simpler food webs incorporating fewer species and shorter food chains than those in continuous forest. However, potamonautid crabs (macroconsumers) and fish were present in all streams. Significant differences occured in three food web parameters calculated for streams in catchments differing in land uses: web size (ANOVA F2, 6 = 15.509, P = 0.004), predator prey ratio (ANOVA F2, 6 = 17.337, P = 0.003), standing biomass of invertebrates (AFDM); CPOM mass (ANOVA F2, 6 = 8.727, P = 0.016) and algal (ANOVA F2, 6 = 37.599, P = 0.001). Stable isotope analysis revealed that primary consumers assimilated a mixture of autochthonous and allochthonous carbon resources (CPOM and algae) in all streams.