Hemiparasitism: a way station to holoparasitism or an evolutionary stable strategy

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Hemiparasitisma way station to
holoparasitismor an evolutionary stable
strategy?

• Prof. Joseph E. Armstrong
• Illinois State University
• Behavior, Evolution, Ecology, and Systematics
  Section
• Department of Biological Sciences
• Normal, IL 61790-4120 USA
• ltjearmstr_at_ilstu.edugt

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Is this the ultimate destination of
hemiparasitism?

• Destination Holoparasitism

You are here Hemiparasitism
Way back the other way Autotrophy
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Orobanchaceae
Holoparasitism has evolved from hemiparasitism
7-10 times in this clade.
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Why be a hemiparasite?

• Q. Why should an obligate parasite retain its
  photosynthetic ability?
• A. It must benefit by doing so.
• Q. When causes hemiparasitism to begin the
  journey to holoparasitism?
• A. When the cost of maintaining its PS ability is
  greater than the benefit received from PS.

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Under what circumstances does PS benefit a
hemiparasite?

• A1. In a fluctuating environment PS and
  parasitism may be functional tradeoffs such that
  hemiparasites enhance their gains via one or the
  other at different times.
• A2. Hemiparasites might alter their environment
  in such a way that they enhance their gain via PS
  and parasitism at the same time.

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Pedicularis canadensis

• Prairie grasslands eastern deciduous forest
• Obligate hemiparasite
• Wide range of host species
• Fully photosynthetic
• Root parasite water, nitrogen, some carbs
  seasonally.
• Short stature, but not a spring ephemeral

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A nutrient-limited grassland

• Pedicularis canadensis has a profound impact on
  the prairie community.
• It significantly reduces the height of the
  prairie canopy,
• which enhances light reaching the prairie under
  story.
• It differentially affects host species thereby
  altering the community structure.

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Presence of Pedicularis canadensis (Pc) reduces
prairie canopy height by 36.
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Amount of light reaching Pedicularis canadensis
doubles as prairie canopy height decreases
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Presence of Pedicularis canadensis (Pc) alters
the relative composition of the prairie community.
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Rich soil prairie community

• Canopy height and community structure not altered
  by presence of hemiparasite.
• At height of 20 cm, hemiparasite is in dense
  shade higher humidity.
• Pedicularis canadensis barely persists on tiny
  islands of poor soil (pioneer cemetery-prairie
  remnant).

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No evidence of functional tradeoffs between PS
and parasitism.
Nutrient/water limited
Not so limited
PS Parasitism Enhanced
PS Parasitism Reduced
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Nutrient limited
Not (as) limited

• Increased light in under story enhances PS.
• Open canopy enhances transpiration.
• Thus increasing gains via both PS and parasitism.
• In this environment hemiparasitism is an ESS.

• Light levels remain low in under story
• Transpiration is reduced in shade and higher
  humidity
• Thus reducing gains via both PS and parasitism.
• Hemiparasite does not persist.

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What circumstances would favor a shift toward
parasitism?

• Loss of nutrients to parasite must not overly
  affect host.
• Hemiparasite must be able to obtain adequate
  resources without significantly altering
  community structure.
• Benefit received from parasitism must be greater
  than benefit from PS.

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In a deciduous forest community

• Pedicularis canadensis also occurs in eastern
  deciduous forest communities where trees are
  among its primary hosts.
• Much greater size difference between host and
  parasite means little impact on trees.
• Presence of parasite does not alter forest canopy
  or under story light regime. PS only significant
  in spring, but Pedicularis is not a spring
  ephemeral.

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Prediction

• In forest communities, Pedicularis canadensis no
  longer can alter its environment to enhance PS,
  so PS capacity should decline.
• Much larger host size means parasite can take
  more without unduly harming host, and woody
  hosts may provide more carbohydrates in xylem
  stream, especially in the spring, reducing need
  for PS.
• Forest environment shifts the balance toward
  holoparasitism. (Where did other holoparasites
  evolve?)

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• Test by determining if Pedicularis canadensis has
  reduced photosynthetic capacity in eastern
  deciduous forest communities.

• Measurements in the field.
• Glass house experimental conditions.

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Acknowledgements

• My colleague Dr. Victoria Borowicz
• My students J. Sunley, D. White, A. Hedberg.
• McLean County Parks (study sites)
• Reference Hedberg, A., V. Borowicz, and J.
  Armstrong. 2005. Interactions between a
  hemiparasitic plant, Pedicularis canadensis L.
  (Orobanchaceae) and members of a tall grass
  prairie community. J. Torrey Bot. Soc. 132 in
  press.