Species Diversity Within and Among Ecosystems - Ecological Systems - page 257

geography and taxonomic groupings, with a disproportionately large number of
species in the Class Insecta. It is very difficult to estimate this variability, as data are
limited for marine and fresh water ecosystems and for non-vertebrate species, but
there are around 10 plant species and at least 100 invertebrate species for each
vertebrate species. Despite their low numbers compared to invertebrates, consider-
able datasets have been compiled for vertebrates, allowing numerous patterns of
variations to be explored [ 10 ], although the extent to which these patterns can be
generalized remains unknown [ 27 ].
One of the most obvious patterns is that for virtually all taxonomic groups,
species richness tends to decrease from the tropics to the poles [ 28 ]; the tropics hold
much higher species richness than do temperate, boreal, or polar regions. This is
more so than would be expected on the basis of area alone [ 10 ], although it is
complicated by physiographic and climatic factors such as mountain ranges or
rainfall patterns. There is also a broadly similar spatial distribution of diversity
between taxa, where differences seem to be driven by particular biological traits
[ 10 , 29 ]. In general, these differences will increase with increasing evolutionary
distance between taxa [ 30 ]. Patterns of marine and freshwater species richness are
less understood, but studies [ 31 , 32 ] have demonstrated a latitudinal gradient in the
shallow water benthos, with decreasing richness toward the poles.
There are also patterns of endemism and evolutionary distinctiveness that are
important aspects of species diversity, as species with long and independent
evolutionary histories and few surviving relatives contain irreplaceable genetic
diversity. Most species have small range sizes [ 33 ], which tend to co-occur in
centers of endemism [ 10 , 31 ]. Centers of endemism are also concentrated in the
tropics, with overlap, at certain scales, across birds, mammals, and amphibians [ 34 ]
and a similar pattern is expected for plants [ 35 ]. However, this pattern does not
appear to represent the situation for invertebrates or microorganisms [ 10 ]. There is
enormous variation between species in terms of the evolutionary age [ 36 ]. Evolu-
tionary distinctiveness among species can be explored using taxonomic
relationships and as with species richness, the available data indicates that tropical
rainforests are regions with the greatest number of taxa with the longest indepen-
dent evolutionary history [ 37 ].
Vertebrate Species Status, Trends, and Threats
Conservation assessments have been primarily carried out for vertebrates as the
conservation community tends to use these species as flagships or indicators of
ecosystem health and global biodiversity. Therefore, this section will focus on
vertebrate species, as sufficient data exists on the status and trends of the world's
mammals, birds, amphibians, and cartilaginous fishes, and weighted approaches can
be used to gain an understanding for reptiles and bony fishes [ 38 ]. Other taxonomic
groups will not be discussed as there is not currently sufficient data for a global
assessment. However, vertebrates represent only 5% of animal species and a fraction