Aquatic food webs in anthropogenic landscapes

Abstract

Anthropogenic influences are reshaping all ecosystems, requiring renewed understanding of determinants of food-web structure in anthropogenic landscapes. Currently, there is a lack of understanding of how the presence of human settlements in catchments affect food-web properties in streams. In addition, there are few quantitative studies of the trophic basis of secondary production in shallow wetlands of human-dominated landscapes, particularly in tropical east Asia. In this thesis, I first examined how human settlements in headwater stream catchments affect food-web properties. A global analysis of 27 food webs revealed that streams in catchments with a higher Settlement Index (a measure of population density, built-up area, and proximity to urban centres) exhibited greater linkage density, trophic vulnerability (number of consumers per species), and trophic generality (number of resources per species), indicating a reduction in specialisation (i.e., more ‘generalist’ food webs). Other food-web properties (species richness, connectance, and mean food-chain length) were unrelated to the Settlement Index, nor were any properties affected by farmland coverage. To test whether the ‘generalist’ food webs in catchments with higher settlement were a result of broader diets, I compared the dietary breadths of a set of five predators in four streams (two with uninhabited catchments; two with settlements) during both dry and wet seasons. Gut-content analysis revealed that, while catchment condition and seasonality explained some variation in the prey eaten, predator diet breadth was mainly determined by species identity, and the diet breadth of the overall predator guild was unaffected by seasonality or catchment condition. The trophic basis of secondary production in three marshlands during both dry and wet seasons was investigated using stable-isotope analysis (SIA) and fatty acid (FA) analysis. Seasonal changes in all food webs were minor. Vascular plants, detritus, and microalgae had similar isotopic signatures and were the main carbon sources, but filamentous algae and an unsampled food source appeared to support secondary production in some instances. The FA profiles of consumers suggested consumption of detritus, but there was evidence of contribution of other foods such as filamentous algae, periphyton, and cyanobacteria. The trophic basis of production in three tidal shrimp ponds in Mai Po Nature Reserve were investigated using SIA during both dry and wet seasons over two years. Community-wide isotopic signatures showed only minor differences between years or among sites, but δ13C values were more depleted during the wet seasons. Mixing models revealed that algae were a major food source for most consumers, while mangrove detritus or particulate organic matter were also significant for many taxa; reed was generally unimportant. Overall, stream food-web properties are affected by settlement of catchments, but the impact was not always obvious when population densities were low, and future investigations should include a wider range of sites. Secondary production in marshes and tidal shrimp ponds probably depended on both the algal and vascular plant/detrital pathways. However, different food resources in both habitats had overlapping bulk isotopic signatures that confounded further resolution; future studies could benefit from the application of compound-specific SIA.