A tritrophic model of the bush lupine, the ghost moth caterpillar and its nematode parasite

Author

Amber Buntin

Graduation Date

2010

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Environmental Systems: Mathematical Modeling, 2010

Committee Chair Name

Christopher Dugaw

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

Ghost moth caterpillar, Trophic cascades, Bush lupine, Humboldt State University -- Theses -- Mathematical Modeling, Markov chain, Entomopathogenic nematode

Abstract

Systems of three species sometimes exhibit trophic interactions or go through a process called a "trophic cascade". One type of trophic cascade is when a predator inhibits the growth of an insect host that is, in turn, an herbivore of a plant species. Therefore the predator indirectly aids the survival of that trophic level. Specifically, entomopathogenic nematodes, ghost moth caterpillars, and bush lupine form a dynamic system with this trophic cascade effect. Trophic cascades have been studied in above ground systems, but rarely in below ground, terrestrial systems. We modeled this system with a discrete-time, discrete-state stochastic process, or Markov chain in which states of each of the individual species (absent or present) comprise the state of the entire system. We completed a systematic parameter study to see which parameter has the greatest affect on each species as well as a seasonal analysis. The seasonal analysis consisted of using three seasons: winter, El Nino summer (wet), and non-El Nino summer (dry); one year consisting of one winter and one either wet or dry summer. Separately, we alternated wet and dry years, added consecutive dry years and used data to control season and observed the fraction of sites occupied by each species. The transition probability with the strongest effect on the plants and the herbivores was the colonization of plants. This makes sense in nature since herbivores do not colonize unless a plant is present. The nematode species was most affected by the colonization of nematodes parameter indicating further field studies of this transition probability are necessary. Due to the sensitivity and variability of data collection, field studies involving nematodes have been difficult, and estimation of such parameters is nearly impossible. We also observed in the seasonal analysis that the model quickly approaches a stable seasonal cycle when using only all El Nino years or all non-El Nino years. When alternating and modifying wet and dry seasons, the fraction of sites occupied by each species rapidly approaches a stable (not seasonal) cycle. The model provides insight into trophic cascades in general and indicates the plants have bottom-up control of the system.

https://scholarworks.calstate.edu/concern/theses/ww72bd70k

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