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KEYWORDS FOR DATASET: Ecology, Desert Vegetation, Longevity, Plant Growth
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ACCOMPANYING DATA PROVIDED BY: Martin Cody, PhD
                               UCLA, Department of OBEE
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GENERAL EXPLANATION OF THE STUDY
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The accompanying data is from a study on the longevity and population dynamics
of perennial plants in the Mojave Desert. Anecdotal evidence and conventional 
wisdom have long supported the notion that perennial desert plants are slow 
growers, with relatively few births, and are capable of extreme longevity, 
with some plants living over 1000 years. One aim of this study was to test 
this conventional wisdom scientifically. While claims of maximum longevity
clearly cannot be tested directly, they can be tested with the aid of 
statistical inference.  The basic idea is simple: we assume that a plant
of a given species survives each year with some fixed probability, say p. 
Next, based on survivorship figures for some observable period of time, we 
estimate p, attaching a 95% confidence interval to it. Then we look at the 
endpoints of this interval; these represent the smallest and largest 
possible values of p. Using each of these values, we solve for the "longevity 
figure"--the age past which only the oldest 5% of all individuals live. 
(For example, if .8 was the lower bound for p, then the lower bound for the 
longevity figure would be the x such that (.8)^x=.05, which gives x=13.43.)
The two solutions for the longevity figure give an upper and lower bound for
the plant's longevity, and the interval between these bounds is a 95%
confidence interval for that longevity. 

Thus a species' longevity may be estimated purely by the survivorship 
rate over a fixed number of years. In this case a small plot of land was
censused once in 1981 and then again fifteen years later, in 1996. The 
number of births, deaths, and survivors was recorded for each species, and 
this provided the information necessary for longevity estimates. In addition
the precise position of each plant within the plot was recorded. This 
information was used to investigate the spatial distributions of the 
various species: Are the plants distributed uniformly across the plot? 
If the plot is sectioned by a grid, does the number of plants in each square
follow a Poisson distribution? The data on the plant's positions was also used 
to answer questions concerning plant growth patterns, such as whether plants
with more neighbors are more or less likely to survive than those with fewer
neighbors. Or whether some species help each other by growing closely 
together, while others hurt each other. This very simple data set, then, 
can be used not only to estimate longevity and thus confirm anecdotal 
evidence about the life spans of perennial desert plants, but also to 
answer many questions concerning the growth patterns and population dynamics
of those plants.

The study is in press with The Journal of Vegeatation Science (11/30/99).
 
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BRIEF DESCRIPTION OF THE DATA          
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The site from which these data were collected is a 20 meter x 18 meter 
square of land near the base of the Granite Mountains in the east-central 
Mojave Desert that was chosen for its topographical uniformity and lack of any
apparent disturbance. The first census of the site was taken in April 1981. 
A Cartesian coordinate system, with x-axis running along the 18 m. side, was 
imposed on the square to pinpoint the location of each plant. Then each plant
was identified by species, tagged, and its x- and y-coordinates were 
recorded. The same information was recorded for each plant fifteen years 
later when the site was re-censused in 1996. 

During the first census, the site contained a total of 312 plants and
22 plant species.

321 plants and 23 plant species were recorded during the second census. 

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HOW TO USE THE DATA FILES
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The data file is a comma delimited text file. Each row contains information 
on an individual plant for the following four variables: "x-coordinate", 
"y-coordinate", "species", and the plant's history ("Dead/Alive/New"). 

All coordinates are given in meters.

Plant species are abbreviated by the first letters of genus-species, and when 
there was more than one plant of a given species, the species abbreviation is 
followed by an arbitrary number. Thus "Ld2" represents the second observed 
individual of the species "Larrea divaricata." 

The plant's history variable, given in the column labeled "Dead/Alive/New",
gives information about the plant's history over the two census periods. 
A "D" indicates that the plant was alive during the first census but was 
dead at the time of the second. An "A"indicates that the plant was still
alive during the second census, and an "N" indicates a new plant, one that
was born between the two censuses. 

NOTE: For technical reasons there are a few repetitions in the species 
column. There may, for example, be two entries (rows) marked "Ld10", even 
though the rows refer to seperate individuals. Also, a few entries have 
not been included in the data set.    

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STATISTICAL TESTS AND ANALYSES USED IN THE STUDY
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1) Binomial Confidence Intervals
2) Linear Regression Analysis
3) Residuals Analysis
4) Chi-Square Test for Independence