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.