43
total diversity of a macro system such
as an ecosystem, blome or continent
(Suffling et al 1988). The diversity
literature is dominated primarily by
studies of alpha diversity (Peet 1974,
Pielou 1975) in part because it has
been easier to measure than beta or
gamma diversity.
Regardless of the level of diversity
being examined, two components of
diversity are recognized. The first is
richness, or the number of species or
number of habitats present, and the
second is evenness, or the
distribution of individuals or habitats
among species or landscapes (Peet
1974, Pielou 1975). Thus, estimates
of alpha diversity for a given region
requires two variables, the number of
species present (also called species
richness) and the relative
abundance of each species. Beta
diversity requires knowing the
number of habitats in a given
landscape (also called landscape
richness) and the relative
abundance of each habitat across a
given landscape.
METHODOLOGY
Sources of Data. Preprocessing, and
the Study Area
The data used to calculate diversity
are the number of pixels for each
spectral class in all or a portion of a
SPOT multispectral image. I have
personally visited more than half of
the 423 islands in the study and
conclude that the spectral classes of
the preprocessed data correlate well
to specific habitats or landscapes. I
assume therefore, that the diversity
of spectral classes for a given island
is a measure of habitat or landscape
diversity. Consequently, I use the
terms spectral diversity, landscape
diversity and habitat diversity
interchangeably.
Landscape diversity and richness
was measured directly from a full
SPOT multispectral scene centered
on Penobscot Bay, Maine (Figure 1.).
The image was acquired October 27,
1988 and totaled 64.25 km 2 of which
423 islands comprised17.27 km 2 .
The remaining 46.98 km 2 was
Fig. 1. SPOT Image scene of Penobscot Bay, Maine showing the larger islands. © [1988] ONES,
provided by SPOT Image Corporation, Reston, VA.
comprised of water and mainland.
Data on mammal species richness
on 18 islands was provided by
Crowell 1986.
Data were extracted from the image
using GAIA software. GAIA allows
the display, manipulation and
analysis of a variety of digital earth
images as well as the integration of
vector maps on any 8 bit Macintosh II
(Podolsky and Morehouse 1990 and
Podolsky, et. al. 1990). File
compatibility and data complexity
dictate that the raw SPOT images be
preprocessed. Preprocessing is a
two step procedure. Step one
converts SPOT data to Macintosh
format and step two reduces the 16.7
million reflectance values (256