Resource Supply and Disturbance as Controls
Over Present and Future Plant Diversity
S.E. Hobbie, D.B. Jensen, and F.S. Chapin, III
Nick Hall and Shannon Murphy
I.
Introduction
a. Increasing human use of the earth’s surface has already reduced genetic and
species diversity through the extirpation of populations
b. The authors argue that the impact of global change on plant diversity can be
adequately predicted only by considering the interactions among climate,
resource availability, and disturbance regime.
II.
Future Resource and Disturbance Regimes.
a. The most obvious effects of global change on resources are those related
directly to changing atmospheric chemistry and climate
b. Increased CO2 can have limited effect in stimulating an ecosystem if the C/N
ratio is too high from increased plant litter. The litter has a negative effect on
the nutrient availability and thus creates a problem in ecosystem stimulation.
c. Temperature increase in cold climates can improve nutrient cycling because
the microbial activity would increase creating greater availability of resources
in the soil that were once considered limiting.
d. Fire and gap formation are two natural disturbances commonly responsive to
global change. Fires are caused generally by decreasing moisture in the
environment. Fire cycles nutrients by burning dead plant material and
creating new plant growth eventually releasing greater amounts of moisture
into the air and re-stabilizing the ecosystem.
i. Question 1: Can you think of any other nutrient cycling processes
that can bring equilibrium to an otherwise unbalanced system?
III.
Plant Genetic Diversity
a. Genetic diversity contributes to both the short term success of individuals and
populations by enhancing fitness.
b. It also enhances long term capacity by allowing capacity for adaptation to
environmental changes.
c. Although the ecological variation in the genetic makeup in figure 18.1 is
significant (47% -35%) the levels of variation at the species level is much
lower (24%).
i. Question 2: Why would the variation be much greater across the 8
traits ecologically than at the species level?
IV.
Plant Species Diversity
a. Regional Patterns
i. In resource poor environments few species can survive and thus low
species diversity is present.
ii. As resources increase the diversity increases to a point where light
then becomes a limiting resource.
iii. This implies that the highest species diversity is present with only
intermediate resource availability. (Somewhat like a bell curve)
1. Question 3: Given the intermediate resources greatest
variation hypothesis, which ecosystem do you think provides
the greatest variation on earth and why?
b. Latitudinal Patterns
i. Many hypotheses have been created to explain the large number of
species found in the tropics. One being that with so much food
available plants may specialize in certain areas of consumption and
food items.
ii. Other hypothesizes include (1) the creation of more niches in the
environment and (2) That tropical regions are predictable in available
resources and thus the species can adapt to resource availability.
1. Question 4: Which of these hypothesizes do you find the most
plausible? Should they be separate or linked?
V.
Diversity of plant Functional Groups
a. General considerations
i. They define a functional group as “Those species with ‘similar’
impacts on ecosystem processes such that substitution among species
within a functional group has ‘minor’ impacts on ecosystem processes’
ii. Functional groups may be specific or broad (Maybe explain the
example that it could be as specific as a keystone species or as broad
as all deciduous trees in a dry tropical forest
iii. We do not need a separate functional group for each ecosystem issue
because many ecosystem processes often depend on plants traits
iv. Correlations occur for several of reasons including cause effect
relationship, allocation trade-offs and evolutionary constraints
b. Control by Resource and Disturbance
i. If plant functional groups are defined by size then they argue that light
is the primary determinant of a functional group (Question: Why
would this be? Can you think of examples?)
ii. Plant size strongly influences vegetations effect on the atmosphere,
water and energy flux, gross photosynthesis and annual nitrogen
throughput of an ecosystem
iii. Reduced functional group diversity often occurs in high resource
environments
c. Types of functional groups
i. The distribution of functional groups are predictable along resource
gradients
ii. High resource environments support a high relative growth rate
through high capacities for photosynthesis and nutrient uptake
iii. Low resource environments have inadequate resources, this limits
the plants growth size and tissue nutrients are retained for a longer
life (question here) Can’ t think of one right now )
d. Climatic Predictors
i. Climate allows for a basis of prediction but we still must consider
resources, disturbance and competitive interaction
ii. Climate may be a good predictor for distribution because the niche at
which they occupy can be grouped by climate
1. If however we define the functional group as its effect on the
ecosystem then climate should only be considered for resource
availability and the physiognomic types that grow there
iii. Historical factors such as migration rate can also effect the functional
group within an area
e. Future Diversity
i. Global Change will have major direct and indirect effects on resource
availability
ii. High latitude plants should be reduced diversity of functional groups
iii. At mid latitude an increase in functional groups would be observe
VI.
Landscape Diversity
a. In places that have not been extensively altered by humans landscape is
influenced by resource availability and disturbance
b. By contrast, places of low resource availability have a high functional group
diversity because small changes in resource supply have a large effect on
abundance of different groups
c. The frequency, severity and size of disturbances are all important factors in
influencing landscape.
i. Question 5: Can you think of examples of where this occurred?
d. Human land use also alters the landscape
i. Human land uses have a tendency to increase the landscape
homogeneity
VII.
Consequences of Changing Biodiversity
a. Ethically wrong for humans to consume all resources and not be concerned
with biodiversity
b. Humans depend on other species and their diversity
VIII.
Conclusion
a. The impact of global change on plant diversity can be adequately predicted
only by considering the interactions among climate, resource availability and
disturbance regime
b. Authors expect genetic diversity to continue declining with global change due
to many factors. Decline in resource availability will have the same effect.
i. Question 6: What are some of the most important ways humans
depend on biodiversity? Why should we be concerned with
decreasing biodiversity?