Urban development modifies lake food webs in the Pacific Northwest

Abstract

Residential shoreline and watershed development by humans are leading drivers of biodiversity loss in lake ecosystems that reduce abundances and diversity littoral invertebrates. Invertebrate biological and life history traits provide good indicators of environmental quality and ecosystem functioning, yet surprisingly few studies have utilized trait-based approaches to assess impacts of human development to lake littoral communities. My thesis addresses how human development modifies lake food webs by restructuring littoral macroinvertebrate communities and altering the flow of energy to lake consumers. In Ch. III, I used a traits-based approach to assess the impacts of development to littoral macroinvertebrate community structure, and I discuss environmental mechanisms and implications to ecosystem functioning. Multiple linear regressions revealed that functional diversity declined with increasing watershed development, concentrations of total phosphorus, and littoral macrophyte cover. Results from multivariate constrained ordination and fourth corner analysis indicated that high phosphorus concentrations and macrophyte cover filtered taxa with semivoltine life cycles and filter feeders from lake communities, and that both regional and local characteristics of watershed development were important determinants of invertebrate community structure. Urban development had particularly pronounced effects on invertebrate communities in deep littoral zones, for which overall community abundances declined as a result of removals of woody debris and increased phosphorus concentrations. My study indicates that lake shoreline development and nutrient loading favor assemblages of short-lived organisms and herbivores and act as environmental filters of other functional feeding groups. These changes to invertebrate community structure may have important implications for energy flow between terrestrial, littoral, and pelagic food webs. In Ch. II, I examined whether a non-native species provides a prey resource to consumers in lakes across gradients of urban development and native prey availability. I used stable isotopes of carbon, nitrogen, and hydrogen to assess resource use by consumers in undeveloped and developed lakes and determine whether non-native Chinese Mystery snail maintains the integration of benthic resources in food webs of developed lakes by providing an abundant prey resource. I found that consumers in undeveloped lakes were supported primarily by benthic resources, and lakeshore development dramatically reduced consumer reliance on these resources. This was at least partly due to a reduction in the availability of native snails, a high quality prey item, to the dominant littoral consumer, molluscivorous pumpkinseed sunfish (Lepomis gibbosus). In developed lakes with non-native Bellamya, generalist yellow perch (Perca flavescens) and piscivorous largemouth bass (Micropterus salmoides) consumed benthic resources in proportions similar to undeveloped lakes, and pumpkinseed sunfish consumed Bellamya in higher proportions than in undeveloped lakes.