Testing mechanisms of the intermediate disturbance hypothesis using long-term
A.
Pastore,
C.
Prather,
E.
Gornish,
R.
Ellis,
and
T.
Miller
data of saxicolous lichens
Department of Biological Science, Florida State University, Tallahassee, FL 32306
For more information on this and other thrilling projects, please contact Abigail Pastore at apastore@bio.fsu.edu
Results
Introduction
The Intermediate Disturbance Hypothesis (IDH) has been frequently invoked over the last 30
years for its simple prediction that diversity can be explained as a result of a tradeoff between
colonization and competition at different levels of disturbance. Despite many documented
patterns of species diversity that appear consistent with the IDH, few studies actually test the
presumed underlying tradeoff.
A preliminary analysis in 1978 of rock dwelling
lichen communities revealed a unimodal
relationship between rates of disturbance
(shearing of the rock face) and diversity of
lichen species (Fig 1). At that time, permanent
plots were established that were followed until
present to test the prediction of a trade off
between colonization ability and competitive
ability as well as other dynamics that could
affect lichen species diversity.
1980
1985
1988
1990
1992
1997
2004
2007
2009
Figure 2. A) Rates of colonization for the six most abundant species on the disturbed plots.
B) Competitive abilities of the six most abundant lichen species measured on control plots.
C) Radial growth rate of the six most abundant species measured on disturbed plots.
A
Kruskal-Wallis: chi-squared = 3.117,
df = 5, p-value = 0.682
B
Figure Legend
asp = Aspicilia spp.
rd = Rhizocarpon disporum
xl = Xanthoparmelia lineola
lm = Lecanora muralis
pl = Unidentified lichen
ls = Lecidella stigmataea
Figure 4.
Changes in lichen
cover through
time on a single
plot.
Kruskal-Wallis: chi-squared = 11.010,
df = 5, p-value = 0.0512
Figure 1. Each point represents a 1 m2 plot in the Gila
National Forest, NM censused for lichen species in 1978.
Methods
Ten paired plots were established on horizontal rock faces near the Ben Lily Memorial in the
Gila National Forest NM in 1978. All 10 cm x 10 cm plots were similar in slope, elevation
(2100 m), and light exposure. One of each pair of plots was randomly selected, and the entire
surface of the rock face removed with a chisel to simulate natural disturbance. Beginning in
1978, all control and disturbed plots were photographed approximately every two years,
resulting in 18 photographs of each of the 20 plots over a 32-year period, allowing us to
document lichen recolonization and competition. The colonization rate for each lichen species
was determined by counting the number of new colonies on the disturbed plots over the 32
years. Competitive abilities were determined by placing a 100 point grid on photographs of the
control plots and recording the lichen species at each point for every year photographed. We
quantified competitive ability, Ci, of a lichen species i as
n
Ci =
åW
C
Kruskal-Wallis: chi-squared =14.995,
df = 5, p-value = 0.0104
Conclusions
Figure 3. Percent cover of the six most abundant species over all control plots averaged
over years.
ij
j=1
n
n
åW + å L
ij
j=1
ij
j=1
where Wij is the number of instances a grid point transitioned from species j to species i, and Lij
is the number of instances a grid point transitioned from species i to species j, for the total
number of species, n.
To further explore the community dynamics we also measured percent cover and average
growth rate for each species. The lichen species at each grid point in the control plots was
recorded, summed over all plots, and then averaged over each year as a measure for percent
cover. Growth rates were determined by picking a random individual of each species on each
disturbed plot and quantifying radial growth over 10 ±2 years.
Kruskal Wallis tests were performed to assess significant differences among species for the
response variables measured (colonization rate, competitive ability, growth rate, and
abundance). Spearman’s rank correlation tests were performed across plots and across species
to assess relationships between response variables.
Kruskal-Wallis: chi-squared = 80.498,
df = 5, p-value = <0.0001
Table 1. Results of Spearman Rho tests for correlations between colonization,
competition, growth, and abundance among the lichen species. Table gives Spearman
Rho values – none are significant (P > 0.05).
We did not find evidence for a simple trade-off between competitive ability and colonization ability.
Therefore, our results do not support the mechanisms theorized to contribute to the patterns
described by the IDH. However, if we consider a broader suite of traits, we can explain the majority
of the patterns of relative abundance. For example, the most abundant species overall are Aspicilia
spp. and Xanthoparmelia lineola. Aspicilia has the highest colonization rate and competitive ability,
while Xanthoparmelia also has a high competitive ability and the highest growth rate. Most of the
other species have either low colonization rates or very slow growth rates, which may explain their
low percent cover. This is not always the case, however (see Lecanora muralis).
A final consideration is the unique natural history of lichens. We expected, for example, that X.
lineola might be the best competitor as its foliose growth form would overgrow and shade crustose
species such as Aspicilia. Instead, we observed that Aspicilia often persisted underneath the X.
lineola, re-appearing when X. lineola flaked off in later years. Results of our study suggest that we
need to reconsider how species are successful, beyond simple colonization and competition traits.
.
Acknowledgements
This work was initiated by TEM in 1978 with the encouragement of Diane Davidson and Jim Brown. Patricia, Rick,
and Bill Miller have helped with data collection.