=========================================================================
KEYWORDS FOR DATASET: Birds, Diversity, Vegetation Structure, Density
=========================================================================
===============================================================
ACCOMPANYING DATA PROVIDED BY: Martin Cody, PhD
UCLA, Department of OBEE
===============================================================
================================
GENERAL EXPLANATION OF THE STUDY
================================
The accompanying data set is from a study on the diversity patterns of
bird communities in North American oak woodlands and Australian eucalyptus
woodlands. Because the study's entire data set is very large, the only data
included here are from sites in Californian oak woodlands. Very roughly, the
data provide information about the location, vegetation structure, and
number of species of breeding bird pairs at various sites throughout
California's woodlands. The term "woodlands" refers to moderately thick
forests with uneven canopy cover, and the numerical indexes of vegetation
structure provided by the data can be thought of as a kind of scientific
description of the overall look of a forest--the height of its trees,
the density of its foliage, the variation from forest floor to forest
ceiling.
Although the study itself was primarily concerned with somewhat complex
comparisons between the bird diversity patterns in North America and Australia,
these data can be used to study ecological aspects of the bird diversity
problem--a subject emphasized in much of the earlier literature on species
diversity. These ecological aspects focus on predicting the number and density
of different bird species from information about the vegetation structure
and location of a site, particularly its latitude. Thus the data can be
used to answer the following kinds of questions: Are there more species of
birds in denser forests? Are birds concentrated more densely in areas
where vegetation is more dense? In general, is there more diversity in
locations at higher latitudes, or lower latitudes? And, perhaps most
importantly, which technical measure of vegetation structure is the best
predictor of the number of bird species in a forest. Many other, more
subtle questions can be explored using these data as well.
Explanations of the measures of vegetation structure and other variables
are given below. It should be noted, finally, that the natural logarithms
of variables, rather than the raw variables as given in the data, are
generally used when examining relationships or identifying good predictors.
Thus the correlation between the log of the "Profile Area" and the number
of bird species in a forest--to take a concrete example--is stronger than
the correlation between the "Profile Area" and the number of species.
This study has not yet been published (12/09/99).
=============================
BRIEF DESCRIPTION OF THE DATA
=============================
Data were gathered at 40 different California oak woodland sites. Most of
the data were collected in spring, during the bird breeding season, in
1994.
The sites were around 5 hectares in size--the equivalent of about 12
acres, or .019 square miles--and were situated in relatively homogeneous
habitat. At least three visits were made to each site, and the number of
breeding pairs of each bird species were counted by sight and by analyzing
sound recordings.
Measurements of vegetation structure were also made during the visits.
=========================
HOW TO USE THE DATA FILES
=========================
The data file is a comma delimited text file. Missing data are indicated by
periods ("."). The following list explains each of the seven variables. To
understand the measurements of vegetation structure, you'll need the concept of
a forest's vegetation density at a particular height. Roughly, this is just
a measure of how thick the foliage is at a given cross-sectional height.
To attach a number to this thickness, two researchers position themselves at
the height whose vegetation density they wish to measure and one holds a
flat wooden board and moves away from the other. The stationary researcher
then notes the distance to "semi-obscurity": how far away his partner is
when the view of the board is half obscured by foliage. Since this distance
will be smaller in thicker foliage, its reciprocal is taken as the measure
of vegetation density. Thus higher numbers will correspond to thicker
foliage.
ELEVATION..................The height of the site above sea level. Given in
meters.
PROFILE AREA...............Roughly, a measure of the total amount of
foliage at a site. Technically, this number is
obtained by plotting vegetation density versus height
above ground and taking the total area under
the graph. The units, however, are not
typical area units, as vegetation density is the
reciprocal of a distance; the units are
specially designed to measure profile area, and
are called f.p. units (foliage profile units).
HEIGHT.....................Roughly, the height of the top of the forest
canopy. Technically, the height above which
horizontal distance to semi-obscurity exceeds
30 meters. Given in meters.
HALF-HEIGHT................Roughly, the height that divides total foliage
in two: half the forest is above it, and half below.
Technically, the median of the graph of vegetation
density versus height. Given in meters.
LATITUDE...................The latitude of the site. Given in degrees.
LONGITUDE..................The longitude of the site. Given in degrees.
NO. SPECIES................The number of different species of breeding bird
pairs.
TOTAL DENSITY..............A measure of the bird population density. It is
simply the number of pairs of birds per hectare.
================================================
STATISTICAL TESTS AND ANALYSES USED IN THE STUDY
================================================
1) Two-sample t-test for difference in mean.
2) Jacknife estimation.
3) Linear regression.