Natural environments are spatially and temporally heterogeneous. Many organisms respond to such heterogeneity by moving or migrating among habitats to enhance their growth, reproduction, and survival. Such movement of organisms can link spatially distant communities (Lundberg & Moberg 2003). In stream and riparian communities, fluxes of adult aquatic insects from rivers are known to be an important food source for riparian predators (Nakano & Murakami 2001; Sabo & Power 2002). In this dissertation, I show that adult aquatic insects, which emerge from productive rivers, can trophically support not only riparian predators of the river, but also predators in adjacent tributaries by their migrations from productive mainstems to less productive tributaries.
In my first summer at Angelo Coast Range Reserve in 2010, before embarking on Ph.D. research, I encountered massive numbers of dead adult aquatic insects in a small shaded tributary, Fox Creek. The numbers of adult aquatic insects I observed in the creek far exceeded the expected production of insects from such a small tributary. The observation made me suspect that the adult mayflies originated in other parts of the river. Careful inspection of this mysterious eruption of adult mayflies in the small creek revealed that they were all one species, Ephemerella maculata (Ephemerellidae), and that their nymphs only occur in the adjacent productive mainstem river, indicating a migratory life cycle.
In the Angelo Coast Range Reserve, there had been a number of novel previous studies on resource movements in the landscape; Sabo and Power (2002) had shown the importance of emerging adult aquatic insects from mainstem river to riparian lizard predators. Atlas et al., (2013) had shown the importance of aerial inputs in the stream food webs in small tributaries. The observation of the E.maculata migration from the productive mainstem to adjacent tributaries and associated movement of resources spatially linked those previous studies. It became apparent to me that the migration of adult E. maculata transported a trophic resource from the mainstem river to both the peripheral riparian food webs, and the food webs in adjacent tributaries.
Therefore, in my dissertation, I identified E. maculata as a key species of the ecosystem, and studied details of its life cycle, with special attention to its spatial and temporal distributions, and how their movements connect food webs in space and time. In the first chapter, I describe the life history of E. maculate in detail, with a focus on their migratory life cycle and its fitness advantages. In the second chapter, I document ecological consequence of the E.maculata migration, and experimentally demonstrated that trophic subsidy from the productive mainstem to less productive tributaries triples the growth of juvenile steelhead trout, which rear in cool but food-limited tributaries. In third chapter, I examined temporal aspects of the E.maculata life cycle, and showed how E.maculata is adapted to the thermally varying seasonal environment. In fourth chapter, I studied ecological consequences of the E.maculata life cycle’s distribution in space and time, and showed that spatial heterogeneity within the source habitat of their migration, the mainstem river, leads to asynchronous emergence of E.maculata and thereby prolongs the resource subsidy to tributaries, altering consumer responses.
Organisms move in space and shift their life cycle timing in response to the spatial and temporal heterogeneity in the environment. Such a life cycle can link food webs and communities in space and time. This dissertation presents a novel example of this phenomenon in an aquatic ecosystem.
Uno, H. (2016): Spatial and temporal linkage of stream-riparian food webs by seasonal migration of mayfly Ephemerella maculata. University of California, Berkeley.
This Paper/Book acknowledges NSF CZO grant support.