Ecotones and Ecological Gradients - Ecological Systems - page 150

boundary detection and gradient quantification methods have been made. Remote-
sensing tools, for example, can use data occurring over several orders of magnitude,
using satellite-derived data currently available at a resolution of 0.5 m to 100 km.
Further application of these tools at multiple spatial resolutions will provide a better
understanding of ecotones. Recent work has shown that remote sensing tools can be
effectively used to detect ecotones and to predict species richness and rarity (e.g.,
range size rarity) in ecotones, especially in mountains where latitudinal gradients
occur (e.g., [ 22 , 23 ]).

Patterns of Biodiversity in Ecotones

There have been multiple studies on patterns of biodiversity in ecotonal areas,
which have led to a range of results. Recent work is providing increasing evidence
that boundary regions between ecological communities can be highly diverse at
both the within-species and community levels. Ecotones have been shown to hold
especially high biological diversity over several spatial scales, at both the commu-
nity level (when examining species richness, i.e., the number of species in an area)
and at the within-species level (morphological and genetic diversity) as reviewed
by Kark and van Rensburg [ 17 ]. Other studies, however, have shown conflicting
results, making it difficult to generalize without carefully examining each case,
community, and region.
Early on, Odum [ 30 ] pointed at high species richness and abundance in ecotones
and suggested that the ecotonal community commonly contains many species that
are characteristic of, and sometimes restricted to, the ecotone. In a continental-scale
study of New World birds, Kark et al. [ 16 ] examined the relationship between
passerine richness and rarity of

2,300 passeriform species in 4,889 one-degree
New World grid cells and the distance of the cells to boundaries between adjacent
plant-based ecoregions. They found that areas nearer to transitions between
ecoregions had more bird species, and also scored more highly in terms of species
rarity. The findings of their work suggest that transitional environments harbor
many rare species, in addition to high richness. At the community level, there is also
some evidence for high species richness in ecotonal areas in marine systems. For
example, van Rensburg et al. [ 41 ] showed at the subcontinental scale in South
Africa that species richness and range size rarity at a spatial resolution of quarter
degree are generally negatively correlated with distance to areas of transition areas
between vegetation communities for both birds and frogs. Areas with more range-
restricted species were located significantly closer to areas of transition areas
between vegetation communities than expected by chance [ 41 ]. Similarly, in the
Gulf of Aden, Kemp and colleagues [ 18 ] found high-reef fish diversity in an
ecotone harboring a unique mixing of the three distinct faunas of Oman, the Red
Sea, and the Indian Ocean.