Award Date

8-2011

Degree Type

Thesis

Degree Name

Master of Science in Environmental Science

Department

Environmental Science

First Committee Member

William J. Smith, Jr., Chair

Second Committee Member

Scott R. Abella

Third Committee Member

Craig J. Palmer

Graduate Faculty Representative

Thomas C. Piechota

Number of Pages

120

Abstract

Ecological studies have shown worldwide that vegetation is being affected by climate change. Species are shifting to new elevations and physiographic positions to adapt to changes in their environment. More specifically, paleoecology studies in the Mojave Desert have shown shifting vegetation patterns in response to past warming and precipitation changes. Recent studies have shown mortality among desert plants related to extended drought and warming. However, few studies have shown how the geographic distribution of Mojave Desert species has changed during this most recent period of warming. This study addresses this gap in the literature by focusing on several plant species in the Newberry Mountains, which are located at the southeastern boundary of the Mojave Desert in a transitional area to the Sonoran Desert. The study area is co-managed by the Bureau of Land Management and the National Park Service, and as a result, has been relatively undisturbed.

A 1979 dataset utilized for analysis includes 111 vegetation transects from the Newberry Mountains. 107 of these transects were re-surveyed over 2007-2008 to allow for direct comparison over a 30-year period. Using Parameter-elevation Relationships on Independent Slopes Model (PRISM) data, I found that average annual minimum temperature in the Newberry Mountains has increased 1.5° C over the last 30 years, with the greatest increases seen in the higher elevations. Precipitation has also declined across the study area by three centimeters on average, with the greatest decreases in the high elevations. Maxent uses species presence locations, along with environmental and climate variables, to predict the probability of species' occurrences across the study area. A past weakness of Maxent has been a lack of on-the-ground data. This 30-year dataset allows for "ground-truthing" the model. Maxent is used to predict species locations in 1979, project locations in 2008 given climatic change, and then results are compared to actual locations in 2008. Using ArcGIS, projected occurrence rasters for 2008 are subtracted from actual 2008 rasters to evaluate the effectiveness of projecting changes with Maxent. Additionally, the 1979 probability maps are subtracted from 2007-2008 maps to determine actual change over the last 30-years.

Findings are that Maxent does a poor job of projecting 2008 species distributions when using 1979 locations with 2008 climate data. However, when using locations and climate variables from the sample period, Maxent accurately represents species' realized niches. The 2008 projection models over-predict species habitat when compared to 2008 models using actual locations. Species found at higher elevations that are more reliant on precipitation as a predictor variable show decreasing suitable habitat within the Newberries. Species widely distributed across the study area show little to no change. Since the only model variables that changed are species presence locations, 1970's climate variables, and 2000's climate variables, I conclude that the species in the Newberry Mountains that are most reliant on higher precipitation levels are migrating to higher elevations in order to adapt to the current climate change.

Keywords

Climatic changes; Desert plants; Ecology; Nevada – Clark County; Nevada – Newberry Mountains; Phytogeography; United States – Mojave Desert

Disciplines

Climate | Desert Ecology | Environmental Indicators and Impact Assessment | Environmental Monitoring | Environmental Sciences | Plant Sciences | Population Biology

Language

English