Landscape management - Ecological Applications: Toward a Sustainable World - page 277

Environmental
constraints

Total
species pool

Dispersal
constraints

Habitat
species pool

Ecological
species pool
Internal
dynamics

Geographical
species pool

Community

Fig. 10.2 Relationships among fi ve types of species pools - the total pool of species in a
region, the geographical pool (species able to arrive at a site), the habitat pool (species able
to persist under the abiotic conditions of the site), the ecological pool (the overlapping set
of species that can both arrive and persist) and the community (the pool that remains in
the face of biotic interactions). (Adapted from Belyea & Lancaster, 1999, and Booth &
Swanton, 2002.)

To be par t of the community, in other words, a species has to be able to get there, must be able to
persist in the face of abiotic conditions and fi nd appropriate food resources (i.e. circumstances
match its niche requirements - Chapter 2), and must win out in any biotic interactions with com-
petitors, predators and parasites.
Let's explore what determines species richness by means of a conceptual model. For simplicity,
assume that the resources available can be depicted as a one-dimensional continuum, R units long
(Figure 10.3). The various species use the portion of this resource continuum corresponding to
their niche breadths ( n ), and the average niche breadth is ¯ . Some of these niches will overlap,
with an overlap o between adjacent species and an average niche overlap of ¯
Now you can see why some communities might be richer in species than others. First, for any
given values for niche breadth and overlap, a community will contain more species the greater the
range of resources present (i.e. the larger the value of R - Figure 10.3a). Second, for a given range
of resources R , more species can be fi tted in to the community if ¯ is smaller - in other words, if
the species are more specialized in their use of resources (Figure 10.3b). Alternatively, more
species can coexist if their niches overlap to a greater extent (greater ¯ - Figure 10.3c). Finally, a
community will contain more species the more fully saturated it is; conversely, it will contain fewer
species when more of the resource continuum is unexploited (Figure 10.3d).
Some communities, most notably tropical forests, contain remarkably high species richness. A
combination of circumstances seems responsible - very high productivity, a considerable range
of resources, and a long evolutionary history producing saturated communities with large numbers
of specialists. Such biodiversity hotspots around the world provide a particular focus for conserva-
tion action.

The partitioning of species richness at local and landscape scales
The total species richness of a region, known as gamma richness, is made up of two components,
alpha and beta, that relate respectively to our small-scale, homogeneous view of communities and
the larger-scale, landscape view of all local communities combined. Alpha richness is the number
of species present within a particular habitat area, whereas beta richness refers to the extra species
added when other habitat areas are included - thus, beta richness is the between-habitat compo-
nent of overall regional richness. If every habitat area had identical species inventories, beta rich-
ness would be zero, and regional (gamma) richness would simply equal the within-habitat alpha
diversity. But whenever there is heterogeneity in the distribution of species across habitats in a
region, beta richness will make an important contribution to gamma richness.