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each habitat contains some unique specialist species while 2,500
multiple habitats often contain the same generalist spe- 2,500
cies. So, what happens when we start to lose habitats due 2,500
to human activities? Given that specialist species are only 2,000
found in a single habitat type while generalists are found 2,000
across multiple habitat types, the loss of a single habitat type Total plant phosphorus (mg/m 2 ) 2,000
1,500
generally leads to the loss of the specialist species, but not
the loss of the generalists since they can live in other hab- (a) Total plant phosphorus (mg/m 2 ) Total plant phosphorus (mg/m 2 ) 1,500
1,500
itat types. However, as habitat loss continues and multiple 1,000
habitats are destroyed, this too eventually leads to the loss of 1,000
the generalist species. Habitat loss can also lead to reduced 1,000
numbers of any species that require large, continuous hab- 500
itats, such as large migrating herds of buffalo or gazelles, or 500 0 2 4 6 8 10 12 14
large predators such as wolves and mountain lions that roam 500 0 2 4 6 8 10 12 14
over several kilometers of land in search of prey. (a) 0 2 4 6 8 10 12 14
(a) 160
Consequences of Species Diversity 160
160
As we discussed in the unit opening case study, scien- 140
tists have discovered that having a higher species diversity 140
results in ecosystems that are more productive and resil- Total root biomass (g/m 2 ) 140
120
ient. That is, they have greater primary productivity (as Total root biomass (g/m 2 ) 120
discussed in Module 6) and they are better able to either Total root biomass (g/m 2 ) 120
resist the impact of disturbances — such as drought, hurri- 100
canes, or fires — or they are better able to rapidly recover 100
from disturbances. To see the impacts of species diversity 100
on productivity, researchers have examined soil fungi that 80 0 2 4 6 8 10 12 14
live in close proximity to plant roots and act as mutualists (b) 80
with the plants. The fungi search for phosphorus in the 80 0 2 4 6 6 8 8 10 12 14
12
2
0
4
14
10
soil and provide some of this important nutrient to the (b)
(b)
plants. In exchange, the plant roots provide the fungi with 130
sugars produced through photosynthesis. When research- 130
ers conducted an experiment in which they manipulated 120
130
the number of species of fungi in the soil, they found 120
that soil with a higher diversity of fungi caused the plants 110
120
to increase the biomass of their roots and their shoots 110
100
(i.e., stems and leaves), as shown in FIGURE 8.4. Total shoot biomass (g/m 2 ) 110
Scientists have also examined how species diversity affects Total shoot biomass (g/m 2 ) 100
90
the stability of ecosystems. An excellent example of this Total shoot biomass (g/m 2 ) 100
90
comes from an experiment in which scientists manipulated 80
90
the number of grassland plants in Minnesota and then mon- 80
itored how much the abundance of herbivores, predators, 70
80
and parasites varied over 11 years as the ecosystem expe- 70 0 2 4 6 8 10 12 14
rienced environmental change including years of droughts (c) 70 0 Number of fungal species in the soil 14
12
2
4
8
6
10
2
6
4
10
12
8
and years of abundant rainfall. As you can see in FIGURE 8.5, (c) 0 Number of fungal species in the soil 14
the researchers found that increasing the number of plant (c) Number of fungal species in the soil
species resulted in greater stability in the number of her- FIGURE 8.4 The effects of fungal species diversity on plant
bivore species and the number of predator and parasite growth. When researchers manipulated the number of fungal
species. species in the soil, which function as mutualists with plants,
Environmental scientists often focus on species diver- they found that increased fungal diversity leads to increased
sity as a critical environmental indicator. The number of plant growth. The fungi take up phosphorus from the soil and
frog species, for example, is used as an indicator of regional pass it to the plant roots. As the number of soil fungi increases,
environmental health because frogs are exposed to both it causes (a) an increase in the amount of phosphorus found in
the water and the air in their ecosystems. A decrease in the the plants, (b) an increase in the biomass of the plant roots, and
number of frog species in a particular ecosystem may be an (c) an increase in the biomass of the plant shoots, which includes
indicator of environmental problems in that location. Spe- the stems and leaves. (Data from van der Heijden, A. G. A., et al. 1998. Mycorrhizal
cies losses in several ecosystems can indicate environmental fungal diversity determines plant biodiversity, ecosystem variability, and productivity. Nature
problems on a larger scale. 396: 69–72.)
ModUle 8 ■ Introduction to Biodiversity 101
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