Ecotones and Ecological Gradients - Ecological Systems - page 149

biology [ 11 , 13 ]. Studies on ecotones in the 1980s often focused on material flow
(e.g., water and nutrients) across communities and on ecosystem processes in these
boundary regions [ 12 ]. Much of the work focused on wetlands and on riparian
zones, where land-water interfaces occur (reviewed in [ 45 ]). Later work in the
1990s more directly examined the effect of ecotones on biological diversity, and
especially on the relationship between ecotones and processes leading to morpho-
logical divergence, patterns of genetic and phenotypic diversity, species richness,
rarity, and their conservation implications (reviewed in [ 17 ]).

Approaches for Measuring Ecotones

Due to the fact that ecotones can be rarely delimited by a fine line, their measure-
ment and mapping is not simple. A wide range of research approaches and tools
have been used to detect and quantify ecotones. These include, among others,
simulation modeling, geographic information systems (GIS), remote sensing, and
statistical tools that enable quantification and analysis of ecotones of different types
and over several spatial scales. Diverse approaches for the quantification of the
steepness of gradients exist [ 17 ]. Methods for measuring and characterizing
ecotones depend on the data available (e.g., quantitative or qualitative, grid- or
transect-based data), with one of the simplest approaches, proposed by Womble in
1951, being the quantification of the magnitude of the first and second derivatives
(rates of change in a given variable or several variables along a spatial gradient)
[ 44 ]. These approaches often examine the values of variables in an area (e.g., a 1

1 km grid square) relative to its neighboring regions. The basic idea is to detect
areas of sharp environmental transition by finding the areas with the highest rate of
change in the value of a given variable or several variables between adjacent
squares (pixels). Specific software for the detection of boundary regions and
analysis is now available (e.g., BoundarySeer: http://www.terraseer.com/
products_ boundaryseer.php ) , enabling more widespread use of advanced statisti-
cal tools for the study of areas of transition [ 5 , 8 ]. These tools can also be applied to
the study of ecotonal regions.
In recent years, new approaches to quantify changes in diversity across gradients
and boundary regions have been developed and are being applied. Among these is
a range of new beta-diversity estimates of species turnover in space [ 20 , 26 ]. These
have been developed in the past decades, since Wilson and Shmida's [ 43 ] review on
beta-diversity estimates. Beta-diversity and species turnover are often used when
studying gradients, and although they do not focus necessarily on ecotonal areas,
they can be applied to the study of ecotones.
One of the most promising directions in ecotone and boundary measurement is
the use of tools developed in other areas of science. These include fields such as
physics, remote sensing, and image analysis, where substantial advancements in