Page 299 - Environment: The Science Behind the Stories

Low
Few bird species
100
60 at high latitudes
100 110
70
90
130 30
60° 50
40 Niche dimension 2
140
60
170 110
70 Niche dimension 1
50° 80
Numbers 120 Temperate and polar latitudes:
represent 160 150 Variable climate favors fewer
numbers of 180 190 200 180 species, and species that are
breeding 200 170 widespread generalists.
Latitude 40° bird species 190 160 Number of bird species
210
170
160
180 140 150
130
140
210
120 110
180
30° 160 200 220 100 Niche dimension 2
140 150 90
120 160
100 280
200 240
220 Niche dimension 1
260 200
20° 300 340 360 280 240 Tropical latitudes: Greater solar
320 440 energy, heat, and humidity
420 promote more plant growth to
340
460 480 440 480 400 support more organisms. Stable
climate favors specialist species.
Many bird species 500 Together these encourage greater
diversity of species.
520
at low latitudes 660 600 560
High
(a) Latitudinal gradient in species richness for birds in North America (b) One hypothesis to explain
the latitudinal gradient
Figure 11.5 Species richness tends to increase toward the equator. Among birds (a), regions of arctic
Canada and Alaska are home to just 30–100 breeding species, whereas areas of Costa Rica and Panama host
over 600. A leading hypothesis (b) for this pattern explains how climate affects the degree to which organisms
specialize. Adapted (a) from Cook, R.E. et al., 1969. Variation in species density in North American birds. Systematic Biology 18:
63–84. (Originally published as Systematic Zoology). By permission of Oxford University Press.

The latitudinal gradient influences the species diversity (transition zones where habitats intermix; p. 131) often sup-
of Earth’s biomes (pp. 111–117). Tropical dry forests and port high biodiversity. Human disturbance can sometimes cre-
rainforests support far more species than tundra and boreal ate ecotones or patchwork combinations of habitats. Because
forests, for instance. Tropical biomes typically show more this increases habitat diversity locally, species diversity may
evenness as well, whereas in high-latitude biomes with low often rise in disturbed areas. However, this is true only at
species richness, particular species greatly outnumber others. local scales. At larger scales, human disturbance decreases
For example, Canada’s boreal forest is dominated by immense diversity because it replaces regionally unique habitats with
expanses of black spruce, whereas Panama’s tropical forest homogenized disturbed habitats, causing specialist species
contains hundreds of tree species, none of which greatly out- to disappear. Moreover, species that rely on large expanses
number others. of habitat disappear when habitats are fragmented by human
At smaller scales, diversity varies with habitat type. disturbance.
Structurally diverse habitats tend to allow for more ecological Understanding patterns of biodiversity is vital for land-
niches (pp. 79, 95) and support greater species richness and scape ecology (pp. 131–133), regional planning (p. 361),
evenness. For instance, forests generally support more diver- and forest management (pp. 332–341). These patterns also
sity than grasslands. inform wildlife conservation, as we shall see in this chap-
For any given area, species diversity tends to increase ter (and in Chapter 12; pp. 341–350), as we explore solu-
with diversity of habitats, because each habitat supports tions to the ongoing loss of biodiversity that our planet is
298 a somewhat different set of organisms. Thus, ecotones experiencing.

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