Community and Biodiversity
Consequences of Drought
Tom Whitham
Northern Arizona University &
Merriam-Powell Center for Environmental
Research
Flagstaff, AZ USA
Pinyon mortality
North side of the San Francisco Peaks, AZ
Photo mosaic by Paul Heinrich taken December 2003
Key Questions
1. As the dominant plant suffer water stress
and die during a record drought, how will
the rest of the dependent community be
affected?
2. Will all vegetation types be equally affected
or is it just restricted to certain species like
pinyon and ponderosa pine?
3. What are the management implications for
preserving biodiversity?
Mean Arthropod Richness / Tree
On Trees Growing Under Low Stress
14
B
High Stress
Low Stress
B
12
10
8
A
A
B
6
4
A
2
Mean Arthropod Abundance / Tree
0 1B: Overall Mean Arthopod Abundance Is Highest
Figure
Predators
Parasites
OnHerbivores
Trees Growing
Under Low
Stress
High Stress
Low Stress
600
B
40
20
A
In an arthropod
community of
266 species,
species richness
is 2 X higher and
abundance is
12 X higher in
low stress sites
than high stress
sites.
B
A
A
B
Trotter et al. 2004
0
Herbivores Predators
Parasites
Figure 2: Canopy Arthropod Community Composition Differs
Significantly
Between Trees
Growing In High
and Low
Stress Changes
Arthropod
Communities
Stress Environments
0.6
High Stress
Sites
0.4
Y Axis
0.2
High Stress 1
High Stress 2
High Stress 3
Low Stress 1
Low Stress 2
Low Stress 3
0.0
-0.2
-0.4
-0.6
-0.6
Low Stress
Sites
-0.4
-0.2
0.0
X Axis
0.2
0.4
0.6
Trotter et al. 2004
Ectomycorrhizae on fine root of pinyon pine
Environmental Stress Shifts Ectomycorrhizal Community
O'Neill Crater RFLPs
R = 0.652
P < 0.00001
Low Mortality
Sites
High Mortality
Sites
Swaty et al. 2004 Ecology
Low Mortal
High Morta
Watering
experiment
conducted by
Crescent Scudder
to test for
community
release.
May 23, 2003
Mean Athropod Abundance on
Arthropod
Non-watered
vs Abundance
Watered Pinyons
40
p=.028
30
Watered Pinyons
Non-watered Pinyons
20
10
0
Watered Control
Mean Arthropod Richness on
Non-watered vs Watered Pinyons
20
Species Richness
p=.031
Supplemental
watering
increases
arthropod
abundance
and species
richness.
15
10
Scudder unpub. data
5
0
Watered Control
Low Stress
High Stress
Plant stress affects arthropod
and microbial communities differently.
Ring width expansion (mm)
2.0
a
a
Tree ring widths
1.5
Climatic stress negatively
affects pinyon growth.
b
b
1.0
c
c
d
d
0.5
d
d
0.0
% Ectomycorrhizal Colonization
100
Mycorrhizal abundance
80
c
60
40
b
a
a
Mycorrhizal mutualists
increase with intermediate
stress, but decline at high
stress levels.
a
20
0
1
Low
2
3
4
Tree Rank
Stress Level
5
High
Swaty et al. 2004 Ecology
Tree Rings Predict Arthropod Species Richness
30% Richness vs. Ave 97-01
40
Species
Richness
per Tree
Y Data
30
20
10
0
0
500
1000
1500
2000
2500
X Data
Average Tree Ring Width 1997-2001
Ave 97-01 vs 30% Richness
Plot 1 Regr
Adrian Stone unpub. data
3000
Moth Resistant Pinyon
These phenotypes
are genetically based
and have extended
phenotypes that
have community
consequences.
Susceptible Pinyon
Photo by Tom Whitham
Moth Resistant Tree 3X More Lilkely to Die During
2002 Drought Than Moth Susceptible Trees
80
% mortality
70
60
50
40
30
20
10
0
Moth
Resistant
Pinyons
Stulz et al. unpub. data
Moth
Susceptible
Pinyons
The extended phenotypes of moth resistant and susceptible
trees affect a diverse community of about 1000 species.
Whitham et al. 2003 Ecology
Drought impacts on
dominant plants will
negatively affect their
dependent communities.
Photos by Tom Whitham & Alicyn Gitlin
Trees like
cottonwoods should
be of special
concern due to low
coverage.
Gitlin et al. unpub. data
Landscape coverage (%) Population mortality (%)
Mortality of
dominant plants
at 20 randomly
selected sites for
each species
within a 80km
radius of Flagstaff.
50
P = <0.0001
40
30
20
10
20
0
15
10
5
0
Juni. Pond. Asp. Cotton. Manz. Pin.
Dominant plant species
Summary
1. Through its effects on community drivers
(i.e., dominant and keystone species),
drought negatively affects biodiversity.
2. Arthropod and mycorrhizal mutualist
communities are negatively affected by
extreme drought, but differ in their
community responses at low stress levels.
3. Unexpected outcomes are likely (e.g., insect
resistant trees are more likely to die).
4. Extreme drought is a bottleneck event that
is also an evolutionary event.
Mistletoe (Phoradendron juniperinum) on One-Seed Juniper
(Juniperus monosperma)
As a two-way
interaction,
a classic example
of a parasite-host
interaction
Juniper and Mistletoe Share a Common Avian Seed Dispersal Agent
Stands with mistletoe
support 2x more seed
dispersing birds
Stands with mistletoe
have 4x more
juniper seedlings
van Ommeren & Whitham 2002 Oecologia
Scales associated with reversals
•
•
•
•
Number of species
Time
Spatial scales
Any mix of above
Interactions Increase With the Addition of Factors
% of Significant Interaction Terms
100
Observed
Total Observed Significant Interactions
Observed
ObservedReversals
Reversals
80
60
40
20
0
0
1
2
3
4
Number of Statistical Factors (Species, Time, Space)
# of Statistical Factors
Bailey & Whitham 2003
Management & Research Issues
1. In the absence of long-term community-level studies,
fundamental errors in interpretation are likely due to the high
probability of reversals. Need to support such studies to
minimize these errors.
2. Need to minimize human impacts that exacerbate the effects of
drought.
3. Need special emphasis on rare habitat types that are especially
sensitive to drought (e.g., riparian habitat and springs).
4. Marginal or edge habitats that suffer chronic stress can be
barometers of change and may be crucial to preserve as
sources of extreme genotypes that may be best adapted to
changing environments.
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