Chapter 17 – Biodiversity and Ecosystem
Function of Grazing Ecosystems
S.J. McNaughton
Amy Obraske & Aaron Hefty
17.1 Introduction
I. Biodiversity of communities is due to niche diversification of the co-occurring species
and such diversification will lead to greater community productivity due to more
effective resource exploitation
II. This chapter examines evidence that biodiversity affects both the productivity and
stability of ecosystems where the producers are principally herbaceous perennials
17.1.1 Intellectual Origins
I. Mac Arthur suggested that the more alternative pathways for energy flow in an
ecosystem there were, the less likely that pathway destruction or disruption would
unsettle the system
II. Elton 1958 - Maintained that “the balance of simple communities is more easily upset
than that of richer ones; that is more subject to destructive oscillations in populations and
more vulnerable to invasions
A. He presented six lines of evidence for support (pg. 362)
B. Believed cause and effect were operating: greater species diversity led to
greater community stability
III. Ecosystem processes are les likely to be disrupted as the number of pathways
involved in those processes increases, also that population fluctuations are less likely in
more diverse ecosystems
Question: Which one of these concepts is better or more important, or are
they both the same?
17.1.2 Conceptual Development
I. Margalef’s principle tenets
A. ecosystems have a structure; they are composed of different elements arranged
in a definite pattern
B. greater number of species allows for a higher number of specific relations in
food webs
C. structure becomes more complex as time passes
D. conditions become more predictable, the average life span of individuals is
longer, the number of produced offspring lower and internal organization of the
ecosystem urns random disturbances into quasiregular rhythms
II. The relative amount of energy necessary for maintaining an ecosystem is related to the
degree of structure of organization
17.1.3 An Individual Remark
I. Found no evidence that the greater diversity of the Jasper Ridge Grasslands was
accompanied by greater stability, but found a negative correlation with productivity and
species richness
A. inconsistent with Darwin’s observations between diversity and productivity
17.2 Theory and Empiricism
17.2.1 Conceptual Definitions
I. higher community diversity is accompanied by greater species packing in the
environmental space or greater functional redundancy
A. Alternatively stated, diversity can act as a stabilizer of ecosystem function if:
1. Higher diversity is due to greater niches packing
2. Time- or space-varying environmental factors are the same as, or
associated with, those responsible for niche diversification and overlap
17.3 How to Test
I. Two fundamental ways to test the hypothesis that ecosystem functional properties are
related to biodiversity (Figure 17.1)
A. The variation of ecosystem functional properties in communities of different
diversities can be examined through space for a spatially heterogeneous
environment or time for a temporally heterogeneous environment
B. ecosystem properties of systems of different diversities can be experimentally
displaced from the control state and the responses followed through time
Question: Are there other hypotheses or approaches that would also address this
question?
17.4 Tests
117.4.1 Diversity and Productivity
17.4.1.1 New York Old Fields
I. Primary productivity measured in two ways
A. Average productivity was calculated from the slopes per day of biomass above
and below ground over the growing season.
B. Diversity was measured as species richness divided by the log of number of
individuals to correct differences in density based on the familiar richness/number
of individuals relationship.
II. Results
A. Average net Productivity was negatively related to species diversity
B. Measured as total productivity, in contrast, there was no relationship between
diversity and net productivity.
1. This result was due to differences in seasonal distributions of species
growth patterns. This can be seen directly in figure 17.1
A. In older more diverse fields species growth periods were spread
throughout the growing season, reaching their species biomass at
different times.
B. In younger and less diverse fields, the species growth was more
synchronized, tending to reach peak biomass simultaneously.
2. Suggests that specialist herbivores which depend on one or a limited
number of plant species should be more abundant in low diversity
grasslands
3. Generalist herbivores, in contrast, capable of shifting from host to host
as the season progresses, should be more common in more diverse
grasslands.
17.4.1.2 Serengeti Grasslands
I. Spatial heterogeneity of species composition, pattern diversity, was a major contributor
to species diversity at all spatial scales
A. variety of factors effected species diversity
1. Climate
2. Edaphic factors
3. Herbivores
Question: How do these affect diversity? Are there any more factors that might
affect species diversity?
17.4.2 Diversity and Stability
17.4.2.1 New York Old Fields: Producers
I. Tested to see if the stability of ecosystems functional properties is related to diversity
A. Stability was measured as the resistance of producer, herbivore, and carnivore
community productivities to nutrient supplementation at the producer level
1. Diversity
-There was a significant difference due to age, but not to treatment
2. Primary producers
-96% stimulation of energy flow in the less diverse community and
-63% stimulation in the more diverse community
3. Combined above and below ground productivity
4. Dominant Species
-Dominant species on the more diverse community was totally
unresponsive to the fertilization
B. Conclusions
1. Provided convincing evidence that the energy flow of the more diverse
community was much more stable in response to a nutrient supplement
than in less diverse communities
2. Further suggests that biological control of the ecosystem processes
through species interactions accompanying greater diversity, rather than a
simple, physical environmental difference between the fields.
17.4.2.2 New York Old Fields: Consumers
I. Less straightforward results than producers
II. Consumers were characterized by a spring and summer biomass peaks during the
growing season
Question: What might create two biomass peaks?
III. Results
A. Statistically there were no differences in species richness at either trophic level
for the spring peak, but was higher at both trophic levels in the older field during
the summer productivity peak
B. Diversity of herbivores increased with fertilization in both the younger and
older fields during the spring peak and in only the younger field during the
summer peak
C. At the carnivore trophic level, fertilization increased the diversity of the spring
peak in both fields, but had no effect upon the summer peak
D. Consistent tendency for consumer diversity also to be higher in the older
successional fields
IV. Conclusions
A. the more diverse community of herbivores on the older field responded more
to the experimental fertilization.
B. For carnivores, the only significant response was a greater spring productivity
on the older field.
17.4.2.3 New York Old Field: Conclusions
I. The older, more diverse plant community was much less sensitive to nutrient
enrichment, at both the entire community level and the level of the dominant species,
than the younger, less diverse field
17.4.2.4 Serengeti Grazing Ecosystem
I. Ten tests to check the hypothesis that community diversity is positively associated with
functional stability
A. Test 1
1. Stability was measured as resilience
2. The more diverse community had recovered to 89% of control values,
while the less diverse community had recovered only 31% of control
B. Test 2
1. Stability was measured as resistance of a functional property to
environmental fluctuation
2. Stability increased with diversity
C. Test 3
1. Green biomass stability was related to the rainfall variability, being less
in more variable locations
2. Resistance of the green biomass to rainfall fluctuations was greater in
more diverse grasslands
D. Test 4
1. Let rainfall and grazing influence green biomass
2. Stability, again, was directly related to the degree of variation in
precipitation
E. Test 5
1. Looked at relationship between grassland diversity and the resistance to
grazing
2. Grazing resistance was positively related to vegetation diversity (Figure
17.5)
3. Gazelles had the least pronounced effect on grassland biomass as they
grazed, whereas wildebeest had a major effect
Question: Why do you think gazelles had a lower impact on grazing
ecosystems?
F. Test 6
1. For multiple species of grazers, there was no evidence that the grassland
biodiversity influenced grazing resistance during the dry season
G. Test 7
1. During the wet season, no evident difference between the species, and,
pooled data indicated that grazing resistance during that season was also
positively related to grassland diversity (Figure 17.6)
H. Test 8
1. Indicated that grazing resistance due to vegetation diversity was more
effective in more mature grasslands than in those maintained in am
immature state by heavy grazing
I. Test 9
1. Measured resilience versus species diversity
2. Found that resilience was positively related to diversity (Figure 17.7)
J. Test10
1. Measured resistance of grassland productivity to species extinction
2. Found that the vegetation biodiversity was not related to the stability of
productivity to extinction of a species in the grasslands
17.4.2.5 Resistance to Grazing: A Partial Mechanism
I. the results indicated why more diverse vegetation was more resistant to grazing: the
selectivity was higher in more diverse grasslands and that was principally due to
avoidance
17.5 Stability of Species Composition to Drought and Grazing: Yellowstone Grazing
Ecosystem
I. Equation for the resistance of community species composition to change (pg. 378)
II. There was a significant positive relationship between the resistance of the species
community composition to changes in abundance and community biodiversity
17.6 Conclusions: biodiversity and Ecosystem Function
17.6.1 Biodiversity, Productivity, and Stability
I. To not ask does biodiversity effect ecosystems? But rather how and why does
biodiversity affect ecosystem function?
II. Indicated that greater stability at a given trophic level is not necessarily coupled to
stability at other trophic levels
III. Should search for the specific properties of natural ecosystems that may contribute to
their ability
17.6.2 Biodiversity, System perpetuation, and Global Change
I. Urged that a rigorously experimental approach to the relationships between biodiversity
and ecosystem function is likely to lead to the most rapid advances in our understanding
of those ways in which biodiversity affects function, and those in which it doe not.
II. Further experimentation will contribute to a greater understanding of the role of
biodiversity in ecosystem function and the consequences of that role for the effects of
global change on the Earth’s biota