Positive effects of ecosystem engineers on stream communities and processes

Abstract

Ecosystem engineering is a process by which organisms modify habitat characteristics and influence community structure and ecosystem function. These engineer-mediated habitat modifications often have positive effects on community members by improving or creating novel habitats that ameliorate harsh conditions. Despite the far-reaching consequences of such positive interactions, most of what we know about ecosystem engineering is limited to marine or terrestrial habitats and focused on sessile, long-lived foundation species. Less recognition has been given to mobile, smaller bodied, and shorter-lived insect engineers within freshwater ecosystems. This knowledge gap is significant as freshwaters are one of the most threatened habitats globally, and freshwater insects are experiencing alarming rates of decline. My dissertation seeks to uncover how organism interactions modify physical and resource environments in ways that can affect community structure and ecosystem function. My objectives were to: 1) synthesize literature to develop a conceptual framework aimed at describing how two distinct mechanisms of positive interactions scale over time and space; 2) measure how net-spinning caddisfly (Hydropsychidae) engineers and their abandoned engineering structures differentially facilitate communities; 3) quantify the importance of beneficiary functional traits and environmental gradients in determining the strength of facilitation between caddisflies and invertebrate communities; and (4) test how caddisflies can generate hotspots of community assembly and ecosystem function. I found that caddisfly ecosystem engineers and their abandoned structures increased invertebrate colonization; however, occupied structures supported greater colonization of Chironomidae compared to abandoned structures. Additionally, I found that the strength of caddisfly facilitation increased with increasing elevation and was dependent on small-bodied beneficiaries. Furthermore, I found that caddisfly engineers generated ecological heterogeneity by aggregating both resources and consumers, with consequences for elemental cycling. Overall, my dissertation emphasizes the role that biology can play in modifying environments and how these alterations can positively influence biological communities with consequences for ecosystem function.