Reciprocal subsidies: Dynamic interdependence between terrestrial and aquatic food webs

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Reciprocal subsidies Dynamic interdependence
between terrestrial and aquatic food webs

• Shigeru Nakano and Masashi Murakami

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Why study reciprocal trophic exchange?

• Across-habitat transfer of energy can shape
  communities
• Energy flows from high to low
• However, 2-way flow based on seasonality has not
  been studied
• (Polis, Anderson, Holt 1997 Cloc Garman
  1995)

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The facts

• Energy of stream and forest tightly coupled to
  temperature and light
• Strong edge effects create energetic interaction
  between the two zones
• Terrestrial plant productivity peaks in summer
  and decreases as temperature drops in fall
• Stream productivity highest when trees defoliate,
  and decreases as trees block light in summer
• Hypothesis This difference in peaks creates
  reciprocal energy exchange at different times of
  year

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Study Site

• 10 Ha plot along Horonai Stream in Tomakomai
  Experimental Forest, Hokkaido, Japan
• Riparian zone dominated by Ash, Cherry, Oak
• 2-5m stream 97 shaded in summer
• 14Km long, gradient lt1
• Air temp between -15 and 25C
• Water temp always 6C

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The Consumers

• Birds 4 all-year residents, 5 summer
  (May-August) residents, 1 winter (Oct-March)
  resident

• Fish 4 water column fish (2 Oncorhynchus spp., 2
  Salvelinus spp.) and 1 benthic fish (Cottus)

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Methods Prey abundance and flux

• Terrestrial
• Abundance determined by leaf beating and
  catching falling bugs in 80x80 cm tray once a
  month from 8o trees
• Flux measured from catches in 15 pan traps (1x1 m
  x 15cm deep) held in stream current (traps set
  for a week once each month)
• Biomass dried and weighed

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Methods Prey abundance and flux

• Aquatic
• Abundance determined via 225-µm mesh Surber net
  sampler (25x25 cm, 1 m deep) from 12 locations
  once a month
• Flux measured from numbers in emergent traps
  (1x1X1m, .5mm mesh) placed in 18 locations, 4
  days twice a month
• Biomass dried and weighed

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Methods Consumer diet

• Birds examined in 20m² areas for 5 hours
  immediately after dawn every day.
• Any bird entering an area was followed for as
  long as possible within 40m of the area.
• Assessments were made for 53.6 of prey caught,
  and classified into terrestrial or aquatic
• Bird abundance then determined by mapping method

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Methods Consumer diet

• Fish caught for 5 hours after dawn every day
  using a backpack electro-fishing unit
• Stomachs were pumped and fish were returned to
  stream. Contents preserved and classified in lab.
• Fish abundance then determined by three-pass
  removal method in four areas (50m long, 3.5m wide)

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Results

• Terrestrial biomass greatest in leafing period
  (June-August), and terrestrial flux into stream
  proportional to biomass
• Terrestrial flux comprised 60-100 of aquatic
  energy budget during leafing period (except in
  Cottus)

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Results

• Aquatic biomass was highest in spring, just
  before leafing began. Aquatic flux onto land was
  not correlated with biomass, but did peak at
  approximately the same time.
• Aquatic flux accounted for 50-90 of bird energy
  budget during the defoliation period.

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Significance?

• The authors indeed found a seasonal asynchrony in
  prey flux
• Such differences greatly augment the energy
  supply of linked habitats
• Therefore, this has serious consequences on
  habitat loss that may not have been previously
  considered.

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Good luck on your finals!