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A Stage Structured Model of the Impact of Buffelgrass on Saguaro Cacti and their Nurse Trees

The saguaro cactus (Carnegiea gigantea), a keystone species in the Sonoran Desert, has
faced population decline in recent years. The immediate threat to the saguaro cactus is
the increase in wildfires fueled by the invasive species buffelgrass (Pennisetum ciliare). The
increasing rate of wildfires could result in the collapse of the Sonoran Desert ecosystem. A
stage structured model is developed and analyzed to capture interactions between saguaro
cacti, their nurse trees, and buffelgrass. In order to model the impact of buffelgrass, the
interactions between the saguaro growth stages, juvenile and adult, with the nurse trees,
called the foothills palo verde (Parkinsonia microphylla), are studied. Then the presence of
buffelgrass among the populations is analyzed to determine the influence of buffelgrass on nurse
tree and cacti populations. This model consists of a system of non–linear ordinary differential
equations which considers commensalism between juvenile saguaros and their nurse trees and
their eventual competition as juveniles mature to adulthood. The analysis of this system
includes qualitative analysis of the equilibria of the system as well as a numerical analysis
of the sensitivity of key parameters. From this research, it was concluded that buffelgrass
has led to an increase in wildfire frequency through the propagation of unnatural wildfires,
which can lead to a decrease, or possible extinction, of the saguaro population. Therefore, in
the interest of preserving the saguaro cactus population, the buffelgrass population must be
limited through the use of herbicide and harvesting.


Article Number: MTBI-14-03M

Year: 2017


Lucero Rodríguez-Rodríguez - University of Texas Rio Grande Valley

Erin Stafford - Tulane University

Anna Williams - University of Texas at Austin

Brian Wright - University of Redlands

Christopher Kribs - University of Texas at Arlington

Karen Ríos-Soto - University of Puerto Rico at Mayaguez

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