ORIGINAL
ARTICLE
Changes in rain forest tree diversity,
dominance and rarity across a seasonality
gradient in the Western Ghats, India
Priya Davidar1,2*, Jean Philippe Puyravaud3,4 and Egbert G. Leigh Jr1
INTRODUCTION
Rainfall and seasonality are important predictors of alpha
diversity in tropical rain forests. Why this relationship exists,
however, is not clearly understood (Phillips et al., 1994;
Clinebell et al., 1995; Leigh, 1999; Pitman et al., 2002; ter
Steege et al., 2003). Do wet aseasonal forests support more tree
species because they have higher stem density (ter Steege et al.,
2003), because trees on wetter soils are shorter-lived (Phillips
et al., 1994), because understorey plants live longer where
there is no drought (Huston, 1994; Pitman et al., 2002),
because light gaps are more frequent (Phillips et al., 1994) or
because a greater abundance of species-specific pests and
pathogens allow more species to coexist than in drier forests
(Janzen, 1970; Connell, 1971; Givnish, 1999; Wright, 2002)?
The Janzen?Connell hypothesis (Janzen, 1970; Connell, 1971)
suggests that tropical rain forests are diverse because species-
specific pests cause mutual repulsion among conspecifics
1Salim Ali School of Ecology and
Environmental Sciences, Pondicherry
University, Kalapet, Pondicherry, India,
2Smithsonian Tropical Research Institute,
Balboa, Republic of Panama, 3Centre ValBio,
PB 33 Ranomafana, Ifanadiana, Fianarantsoa,
Madagascar and 4Department of Ecology and
Evolution, State University of New York at
Stony Brook, Stony Brook, NY, USA
*Correspondence: Priya Davidar, Smithsonian
Tropical Research Institute, Box 2072, Balboa,
Republic of Panama.
E-mail: davidarp@tivoli.si.edu
ABSTRACT
Aim We assessed the effects of latitude, altitude and climate on the alpha diversity
of rain forest trees in the Western Ghats (WG) of India. We tested whether stem
densities, dominance, the prevalence of rarity, and the proportion of understorey
trees are significantly correlated with alpha diversity.
Location The WG is a chain of mountains c. 1600 km in length, running parallel
to the western coast of the Indian peninsula from above 8
 N to almost 21
 N
latitude. Wet forests occur as a narrow strip in regions with heavy rainfall.
Methods To assess tree diversity we used data from 40 small plots, < 1 ha in
area, where all trees ? 3.18 cm d.b.h. had been inventoried. These plots were
distributed across 7 latitudinal degrees and at elevations between 200 and 1550 m.
Fisher?s alpha was used as a measure of diversity. For each plot, the proportion of
trees belonging to the understorey, the proportion of trees belonging to the most
abundant species in the plot, as a measure of dominance, and the proportionate
representation of singletons, as a measure of rarity, were estimated, and correlated
with Fisher?s alpha, elevation, rainfall and seasonality.
Results Annual rainfall and seasonality increased towards the north, but were
not significantly correlated. Tree diversity increased significantly with decreasing
seasonality. Tree diversity was not significantly correlated with stem density or
with the proportion of understorey tree species, but was significantly correlated
with tree dominance and rarity. Dominance increased and rarity significantly
decreased with increasing seasonality.
Main conclusions This study demonstrates that seasonality influences rain
forest tree diversity in the WG of India. The relationship between alpha diversity,
dominance and rarity lends correlative support for the Janzen?Connell pest
pressure hypothesis.
Keywords
Alpha diversity, India, Janzen?Connell hypothesis, pest pressure, seasonality
gradient, tropical rain forest, Western Ghats.
Journal of Biogeography (J. Biogeogr.) (2005) 32, 493?501
? 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi doi:10.1111/j.1365-2699.2005.01165.x 493
(Janzen, 1970; Connell, 1971), largely by diminishing recruit-
ment near conspecific adults (Wills & Condit, 1999; Peters,
2003).
The rain forests of the Western Ghats (WG) of India provide
a good model system for testing the relative effects of rainfall
and seasonality on tree diversity, because the monsoon regime
causes dry season lengths to be longer where annual rainfall is
higher (Pascal, 1988), contrary to the climatic pattern experi-
enced by Neotropical forests. The independent gradients in
rainfall and seasonality allow correlative tests of hypothesis
relating to alpha diversity. In this study we ask how alpha
diversity of trees in the rain forests of the WG is related to
climatic variables such as annual rainfall and seasonality, by
using data from tree inventories ranging across 7 latitudinal
degrees. We then test four specific hypotheses: is a plot?s tree
diversity correlated with (1) the density of stems on the plot;
(2) the proportion of species on the plot that are represented
by understorey species, whose maximum height is less than
15 m; (3) the proportion of trees on the plot belonging to its
most common species; or (4) the proportion of species on the
plot that is ?rare?? We evaluate the association between the
above variables and the climatic gradient prevalent in the WG.
Rain forests occur as a narrow strip along the WG from the
southern tip of India to above 16
 N latitude. These rain
forests (or wet evergreen forests) have lower species richness
(species ha)1) than the extensive forests of the Amazon or
Southeast Asia (Pascal & Pelissier, 1996; Parthasarathy &
Karthikeyan, 1997). Because they have been well described
floristically and many aspects of their biology are well known,
they can be used to test several hypotheses on diversity that can
be addressed by a correlative approach. Without long-term
data on species biology, we cannot address processes in detail,
but some simple observations may help us to discard certain
hypotheses and allow future research to focus on processes that
appear more likely to be important.
STUDY AREA
The WG, a global biodiversity hotspot (Myers, 1990), is a chain
of mountains c. 1600 km in length, running parallel to the
western coast of the Indian peninsula from above 8
 N to
almost 21
 N latitude (Fig. 1).
The rain forests of the WG have been influenced consid-
erably by continental movements and associated climate
changes. As India drifted northwards in the Paleocene, its
flora was enriched by colonization from Africa (Morley, 2000).
The rain forest has undergone large-scale contraction due to
increasing aridity over the subcontinent and now exists as a
narrow belt along the WG. It recolonizes the grasslands when
human disturbance is low (Puyravaud et al., 1994, 2003). The
flora is characterized by many relictual species and a high
proportion of species endemic to the WG (Ramesh et al.,
1997).
Many inventories have been conducted to describe the
floristics and assess stem densities, species diversity and basal
area of relatively pristine forests (Pascal, 1988; Pascal &
Pelissier, 1996; Parthasarathy & Karthikeyan, 1997; Ayyappan
& Parthasarathy, 1999; Parthasarathy, 1999, 2001). A typology
of the forests has been conducted using bioclimatic informa-
tion (Pascal, 1988), but this is the first large-scale analysis of
alpha diversity for rain forest trees in the WG.
The length of the major dry season, which falls between the
South West and the North East monsoons, is widely supposed
to be the most important influence on floristic types in the
Ghats (Pascal, 1988). The length of the dry season increases
from the south to the north, because the South West monsoon
lasts longer towards the south. The North East monsoon only
affects the southern region, where it further decreases the
length of dry seasons there. In addition, orographic effects
create sharp west?east gradients in rainfall (Ramachandran &
Banerjee, 1983; Gunnell, 1997).
MATERIALS AND METHODS
Plot characteristics
Tree inventories from 40 small plots, located at sites from
c. 8
24? to 14
51? N latitude, were used to assess patterns of
tree diversity (full details can be found in Appendix S1 in the
Supplementary Material). These plots were chosen from a data
base of 135 plots, as follows. To be chosen, a plot must have 75
or more stems, and its site must have over 2000 mm of rain
per year and high basal area for that climatic zone (charac-
teristic of historically unlogged forests: Elouard et al., 1997),
and it must have suffered low human disturbance (no record
of logging, as judged from Forest Department working plans
and low current human impact). Total area sampled was
4.65 ha. These plots, although laid by different investigators,
SRI LANKA
INDIA
Western
Ghats
70 75 80 85
Longitude (decimal degrees)
5
10
15
20
25
L a
tit
u
de
(de
c i
m
a
l d
e
g r
e
e
s )
Figure 1 Map of the study area with location of the plots (solid
circles).
P. Davidar et al.
494 Journal of Biogeography 32, 493?501, ? 2005 Blackwell Publishing Ltd
followed the same methodology. All stems > 3.18 cm diameter
at breast height (d.b.h.) were identified and recorded. Species
identifications in all plots were by experienced botanists and
unidentified species indicated as such.
Plot sizes ranged from 0.09 to 0.2 ha. All stems ? 3.18 cm
(10 cm girth at breast height) were inventoried, and height
recorded. Eight of the plots were from published sources and
32 from unpublished data (full details can be found in
Appendix S1). Twenty-six of the plots were south, and 14
north of the Palghat Gap, a major break in the WG at around
10
50? N. Twenty-five plots, 30 ? 30 m in dimension
(0.09 ha), were laid in randomly selected sites (using geo-
graphical coordinates) in the southern WG (SWG), for the
purpose of biodiversity characterization (Davidar et al.,
unpubl. data). Seven 0.16-ha plots from the Kogar region
were originally used as controls for assessing the effect of
human disturbance on plant diversity (Garrigues, 1999;
Puyravaud & Garrigues, 2002). The rest of the plots were
from a published data base (Pascal, 1988), the number of
individuals per species was estimated from information given
in the publication. The geographical coordinates of the plots
were transformed into the equivalent degree decimal units for
analyses.
We used Fisher?s alpha (Fisher et al., 1943), which is
relatively independent of plot size (Condit et al., 1996; Leigh,
1999) to measure alpha diversity. The distribution of trees over
species in 1-ha plots appears to follow a log series distribution
(Leigh, 1999). In the forests studied by Condit et al. (1996), for
any given number of contiguous stems, the number of species
is roughly the same regardless of the lower diameter limit of
stems sampled. Therefore, Fisher?s alpha allows a relatively
unbiased comparison of diversity on plots where different sizes
and numbers of stems are sampled (Leigh, 1999).
Climatic parameters
Rainfall data were available between the years 1990 and 2002
for many sites. For 28 plots, the mean annual rainfall and dry
season length were obtained for periods from 1 to 13 years,
from meteorological stations located close to the site. Some of
these plots were clustered within 5 km and shared a station.
For CWG plots there were monthly rainfall data for 1 year
for eight sites, and 7?10 year data for another six sites. In the
SWG there was monthly rainfall data for 13 years at six sites,
and for 6 years at three sites in the Agastyamalai and
Mahendragiri ranges. For 12 sites, which were far away from
settlements, the average annual rainfall and dry season length
for each site was obtained from bioclimatic maps (Pascal,
1982), and cross-checked with the location of the plots in
topographic sheets to assess the intensity of the monsoon
winds. For example, plots on the crest or the west-facing slope
of a mountain might have a different rainfall profile from plots
on the eastern slopes. Rainfall and seasonality from reference
sites were used for sites with similar topographic profiles
within a region. The subset of the plots with accurate climatic
information was used for preliminary analyses, and as these
results did not differ from that of the full data set, all the plots
were used for the analyses.
For sites where climatic data were available, the mean length
of the dry season was estimated as the number of consecutive
months where the monthly rainfall is < 100 mm. This was a
biologically meaningful assessment of seasonality as it is related
to the water holding capacity of the soil. In the Kodagu region
(c. 12
30? N), where the soil is fairly deep, the available
water capacity (AWC) was estimated as 81 mm m)1 of soil
(Peterschmitt, 1993), which is sufficient to avoid physiological
drought in regions with long dry seasons and sufficient soil
depth (Elouard et al., 1997). Mean seasonality was calculated
as the average length of the dry season for the given number of
years. In sites with two monsoons and a major and minor dry
season, the length of the longer dry season was used. The
relationship between mean annual rainfall and mean season-
ality were analysed for selected CWG and SWG sites for which
> 6-year data were available.
The elevation for the unpublished inventory plots was
measured using a pocket altimeter, and crosschecked with
topographic maps.
Floristics
Species in the unpublished plots were identified through
voucher specimens collected from each tree and deposited at
the herbaria of the Salim Ali School of Ecology and Environ-
mental Sciences, Pondicherry University and the French
Institute of Pondicherry. Specimens were identified to species
with the help of floras using vegetative and reproductive
characters (Gamble & Fischer, 1915?1936; Saldanha &
Nicholson, 1976; Saldanha, 1984; Pascal & Ramesh, 1987;
Matthew, 1999). We also used herbaria of the French Institute
of Pondicherry, Botanical Survey of India, Coimbatore and St
Xavier?s College in Palayamkottai. Few species remained
unidentified.
Species?abundance relationships and stem density
To look at the species?abundance patterns, the density (stems
ha)1) was calculated for each species in each plot and averaged
for all the plots. The densities were then log transformed and
fitted to a normal curve. Normality was tested using the
Kolmogorov?Smirnov (K?S) test.
Log stem densities were correlated with altitude, annual
rainfall, mean seasonality and Fisher?s alpha. Variables with
significant correlations were used in multiple regression
analyses.
Alpha diversity, altitude and climate
South of 9
 N latitude, the transitional limit between low
and medium elevation forests is near 800 m. As most of the
medium elevation plots but two fell south of 9
 N, we
considered this altitudinal limit as suitable for our data
set.
Tree diversity in the Western Ghats, India
Journal of Biogeography 32, 493?501, ? 2005 Blackwell Publishing Ltd 495
The effects of latitude on mean annual rainfall and mean
seasonality, and the relationship between climatic variables and
Fisher?s alpha were analysed using correlations and linear
regressions. Except for latitude and longitude, the data
followed a normal distribution allowing for regression analy-
ses. Separate analyses were conducted for the central WG
(CWG) plots which lie north of 11
 N, and the SWG plots,
south of 11
 N.
Effect of understorey species
Using the maximum heights of each species (Davidar, unpubl.
data; Puyravaud, unpubl. data) and the forest stratum to which
they belonged within the forest (Pascal, 1988), the species were
assigned to the canopy (25?35 m), the subcanopy (15?25 m),
the understorey (5?15 m) and the emergent layer (> 35 m).
The proportion of species in each plot that belonged to the
forest understorey was estimated and correlated with Fisher?s
alpha and climatic variables.
Dominance and rarity
The proportion of stems on a plot belonging to its most
abundant species was used as an index of dominance. This was
regressed against Fisher?s alpha and mean seasonality.
We derived an index of rarity by totalling the number of
species in a plot represented by just one individual, and taking
the proportionate representation of these species among the
plot?s stems. Another index of rarity was derived by arranging
trees in increasing order of abundance, counting the number
of species required to account for one-tenth of the plot?s stems,
and dividing this by the total number of tree species on the
plot. Both indices were used for the analyses and the one that
gave a better fit was selected.
Statistical analyses
Correlations were performed between all the variables. Regres-
sion analyses were conducted between selected variables with
significant correlations and sometimes non-significant corre-
lations. All tests were conducted using Systat (version 10; SPSS
Inc., 2000).
RESULTS
Patterns of rainfall and seasonality
Annual rainfall and seasonality increased significantly from
south to north (Fig. 2), but were not significantly correlated
with each other (Fig. 2, Tables 1 & 2). However, regional
patterns differed and rainfall and seasonality were signifi-
cantly correlated for the SWG sites, but not for the CWG
sites (Fig. 3). The SWG sites had a greater variation in
seasonality (range of CVs: 0.156?0.565) than the CWG
sites (range of CVs: 0.078?0.216). These results suggest that
inter-year variation in climatic parameters between sites is a
result of the varying effect of the monsoon regime over the
Ghats.
Stem densities, species richness and alpha diversity
The log density of species differed significantly from a normal
distribution (K?S, d ? 0.11, P < 0.01). Log stem density
(stems ha)1) was marginally related to latitude and rainfall
but did not influence any other variable including Fisher?s
alpha (Tables 1 & 2).
Fisher?s alpha values ranged from 2.72 in Bhagvati in the
CWG to 28.7 in Vanamutti in the SWG. The mean value for
Fisher?s alpha for all 40 plots was 12.92 ? 0.87 (SE).
Fisher?s alpha was not significantly correlated with either
annual rainfall or altitude, but decreased significantly with
increased seasonality in all plots (Tables 1 & 2). Fisher?s
alpha was lower for the medium elevation plots than for low
elevation plots (Table 2; Fig. 4). When analysing regional
trends, Fisher?s alpha was not related to annual rainfall
either in the CWG or the SWG (CWG: n ? 14,
y ? 13.63 ) 0.001x, R2 ? 0.03, n.s.; SWG: n ? 26,
y ? 9.86 + 0.001x, R2 ? 0.05, n.s.), and was significantly
related to seasonality for the SWG sites (y ? 21.23 ) 2.36x,
n ? 26, R2 ? 0.20, P ? 0.03), but not for the CWG sites
(n ? 14, y ? 25.8 ) 2.9x, R2 ? 0.12, n.s.). This is probably
because seasonality does not vary much between the CWG
sites.
8 9 10 11 12 13 14 15
Latitude
M
ea
n 
an
nu
al
 ra
in
fa
ll (
mm
)
(degree decimals)
1000
2000
3000
4000
5000
6000
8 9 10 11 12 13 14 15
Latitude (degree decimals)
0
1
2
3
4
5
6
7
M
ea
n 
se
as
o
n
a
lit
y 
(m
on
ths
)
Figure 2 Latitudinal trends in annual rain-
fall and seasonality in the Western Ghats.
P. Davidar et al.
496 Journal of Biogeography 32, 493?501, ? 2005 Blackwell Publishing Ltd
Dominance, the prevalence of rarity, and the
proportion of species belonging to the understorey
We measured rarity by the proportion of stems in a plot
contributed by species with only one tree apiece, because it
provided stronger correlations than the alternative metric. The
alternative measure seemed arbitrary where the species with
one tree apiece accounted for more than 10% of the trees on
the plot. Fisher?s alpha decreased significantly with increasing
tree dominance in plots (Tables 1 & 2; Fig. 5), and increased
significantly with the prevalence of rarity (Tables 1 & 2;
Fig. 5). Dominance and rarity were significantly negatively
correlated (Table 1). Dominance increased significantly with
increasing seasonality (Tables 1 & 2; Fig. 5), whereas rarity
declined significantly with increasing seasonality (Tables 1 & 2;
Fig. 5).
Table 1 Correlation coefficients of variables used in the study for 40 plots
Latitude
(decimal degrees)
Altitude
(m)
Rainfall
(mm) Seasonality Log N
Fisher?s
alpha
Understorey
species Dominance
Latitude 1.00
Altitude )0.66*** 1.000
Rainfall 0.34 0.05 1.000
Seasonality 0.82*** )0.77*** )0.04 1.00
Log N 0.52* )0.33 0.45** 0.36 1.00
Fisher?s alpha )0.31 0.22 0.00 )0.46** )0.06 1.00
Understorey species )0.32 0.23 )0.06 )0.15 )0.05 )0.1 1.00
Dominance 0.14 )0.22 )0.11 0.41* )0.02 )0.78*** 0.16 1.00
Rarity )0.68*** 0.51* )0.21 )0.68*** )0.24 0.69*** 0.24 )0.45**
*P < 0.05, **P < 0.001, ***P < 0.0001.
Table 2 Linear regressions between selected dependent and independent variables
Dependent variable Independent variable/s Equation N R2 P
Mean annual rainfall (mm) Latitude (decimal degrees) y ? 1568 + 2.4x 50 0.19 0.001
Mean seasonality (months) Latitude (decimal degrees) y ? 2.24 ) 0.005x 50 0.77 0.0001
Log stem density Annual rainfall y ? 4.33 ) 0.0001x 40 0.10 0.05
Fisher?s alpha Mean seasonality y ? 18.82 ) 1.68x 40 0.21 0.003
Fisher?s alpha Dominance index y ? 28.53 ) 31.78x 40 0.61 < 0.0001
Fisher?s alpha Rarity index y ? 5.98 + 80.61x 40 0.48 < 0.0001
Dominance index Mean seasonality y ? 0.38 + 0.04x 40 0.17 < 0.01
Rarity index Mean seasonality y ? 0.16 ) 0.02x 40 0.45 < 0.0001
1000 3500 6000 8500
Mean annual rainfall (mm)
1.5
2.6
3.7
4.8
5.9
7.0
M
ea
n 
se
a
so
n
a
lit
y 
(m
on
ths
)
Figure 3 Relationship between annual rainfall and seasonality in
seven CWG sites (circle) and nine SWG sites (triangle). CWG:
y ? 4.52 + 0.0001x, n ? 7, R2 ? 0.3, n.s.; SWG:
y ? 4.87 ) 0.001x, n ? 9, R2 ? 0.69, P ? 0.006.
0 1 2 3 4 5 6 7
Mean seasonality (months)
0
10
20
30
40
Fi
s h
e
r's
a
lp
ha
Figure 4 The relationship between seasonality and Fisher?s alpha
in low (circle) and medium (triangle) elevation plots. Alti-
tude < 800 m: y ? 6.64 ) 0.15x, n ? 18, R2 ? 0.49, P < 0.001;
altitude > 800 m: y ? 3.53 ) 0.08x, n ? 22, R2 ? 0.27, P ? 0.01.
Tree diversity in the Western Ghats, India
Journal of Biogeography 32, 493?501, ? 2005 Blackwell Publishing Ltd 497
Of the 32 species that dominated in the 40 plots, 21 (65%)
were distributed across the seasonality gradient from regions
with short, to regions with long dry seasons, and the rest were
confined to sites with either short or long dry seasons.
DISCUSSION
The WG provides an arena for independently testing the effects
of rainfall and seasonality on tree alpha diversity. This is
because rainfall and seasonality are not correlated over the
WG. The South West monsoon strikes the southern part of the
WG first, and then advances northwards. It then retreats in
reverse, creating a longer rainy period and shorter dry season
length towards the south. Rainfall and seasonality were tightly
correlated for sites in the SWG, whereas they were not in the
CWG.
Our study indicates that seasonality, and not annual rainfall,
is the variable that drives tropical rain forest tree diversity.
Alpha diversity is related to seasonality both for the WG as a
whole and within the SWG where there is a greater variation in
dry season lengths. In Neotropical rain forests, species richness
and diversity are related to annual rainfall and seasonality, as
both are negatively correlated (Gentry, 1988; Clinebell et al.,
1995; Leigh, 1999; ter Steege et al., 2003).
Alpha diversity for the WG sites decreased with altitude
independently of dry season lengths. Species richness decreases
with altitude for reasons that are not clearly understood
(Gentry, 1988; Lieberman et al., 1996; Leigh, 1999). In
Neotropical sites there was an initial increase of diversity with
altitude probably due to the decrease in dry season lengths, but
diversity declined at higher altitudes (Leigh, 1999). In our
study Fisher?s alpha was not significantly correlated with
altitude, but diversity was lower for medium elevation sites
than for the low elevation sites.
Do stem densities influence alpha diversity?
In the Amazon basin, Fisher?s alpha tends to be higher where
density of trees ? 10 cm d.b.h. is higher, perhaps because
tree density is higher at wetter and less seasonal sites (ter
Steege et al., 2003), whereas in the WG, stem densities do
not influence the alpha diversity of trees. Stem densities of
trees in the WG tend to increase significantly with rainfall, as
in the Neotropics (Wright, 1992), but were not related to
seasonality.
Effect of understorey species
Higher alpha diversity in aseasonal forests has been attributed
to an increase in the number of smaller tree species (Givnish,
1999; Pitman et al., 2002; ter Steege et al., 2003) and
functional guilds in the understorey (Huston, 1994). In the
WG, the proportion of species confined to the understorey
does not influence alpha diversity.
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0
10
20
30
Fi
sh
e
r's
a
lp
ha
0.1 0.2 0.3 0.4 0.5
Rarity index
0
10
20
30
0 1 2 3 4 5 6 7
Mean seasonality (months)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
D
o
m
in
a
n
ce
in
d e
x
Dominance index
0 1 2 3 4 5 6 7
Mean seasonality (months)
0.1
0.2
0.3
0.4
0.5
R
ar
ity
in
de
x
(a)
(b)
Figure 5 (a) The relationship between alpha
diversity, tree dominance and rarity, (b) the
relationship between dominance, rarity and
mean seasonality.
P. Davidar et al.
498 Journal of Biogeography 32, 493?501, ? 2005 Blackwell Publishing Ltd
Dominance, rarity and alpha diversity
Pitman et al. (2002) have suggested that higher species richness
at Yasuni than at Manu in the Amazon basin could be due to
the relative increase in the proportion of rare species. There
were more species represented by one, two or three individuals
at Yasuni than at Manu. In this study we find that Fisher?s
alpha increased significantly with rarity, and decreased signi-
ficantly with dominance. Here, both rarity and dominance
influence Fisher?s alpha, as one would expect. Higher pest
pressure in less seasonal sites would increase diversity by
reducing the abundance of common species. If pest pressure is
more intense in less seasonal forests, then rarity should
decrease and dominance should increase with seasonality.
These predictions are supported by our results. In aseasonal
forests, insect activity is fairly even through the year (Wolda,
1983, 1988; Wright, 1992), and herbivores consume young
leaves more rapidly than in drier forests (Coley & Barone,
1996). Higher pest pressure in aseasonal sites might maintain
tree species at low densities whereas in more seasonal sites the
common species will constitute a higher proportion of stems
(Janzen, 1970). If relative abundance is governed primarily by
pest pressure, then the most common species, which are the
most successful at escaping pests, should occupy the widest
range of habitats. However, if dominant species were restricted
to long or short dry seasons, then the pest pressure hypothesis
will not be supported. The majority (65%) of the 32 dominant
species documented in these plots were distributed across the
seasonality gradient.
Many studies have demonstrated the importance of density-
dependent mortality in influencing tree distribution patterns
in tropical rain forests (Condit et al., 1992; Okuda et al., 1997;
Wills & Condit, 1999; Harms et al., 2000; Peters, 2003), and
seeds and seedlings do suffer higher mortality under the parent
tree as predicted by the Janzen?Connell model (Augspurger,
1983, 1984; Howe et al., 1985; Sinha, 1990; Wong et al., 1990;
Gilbert et al., 1994). Givnish (1999) had suggested that
density-dependent mortality should increase at wetter sites.
Our study shows that dominance, the ability of one species to
occupy a plot, and the increase of rare species at more
aseasonal sites, are consistent with the predictions of the pest
pressure hypothesis, but these results need to be tested more
explicitly across seasonality gradients. Hille Ris Lambers et al.
(2002) have shown that the proportion of tree species
experiencing density-dependent mortality is similar in tem-
perate and tropical forests although the intensity might differ.
Similarly the intensity of pest pressure could vary across the
seasonality gradient.
In conclusion, this study has demonstrated that seasonality
is an important predictor of tree alpha diversity in the WG of
India. It is likely that higher species diversity in the more
aseasonal wet forests is due to pest pressure that maintains
competitively superior tree species at lower densities. Our
study suggests that similar processes could regulate alpha
diversity in large, species-rich forests as well as in smaller,
species-poor rain forests such as those in the WG.
ACKNOWLEDGEMENTS
This study was undertaken during PD?s tenure as a Senior
Fellow at the Smithsonian Tropical Research Institute. The
fieldwork was funded by a DOS-DBT grant to PD and a Piren
grant to JPP. We thank the following institutions for providing
rainfall data: Department of Meteorology and Statistics,
Karnataka; Bombay Burmah Trading Corporation Manjolai;
Tamilnadu Electricity Board and UPASI. We are grateful to
Richard Condit and S. J. Wright for generously sharing their
ideas and for helping with analyses. We thank J. P. Garrigues,
S. Aravajy, M. Ramalingam, B. Rajagopal, M. Arjunan and
D. Mohandass for field support and plant identifications.
Critical comments by Annette Aiello, Nigel Pitman and
anonymous reviewers greatly improved the manuscript.
SUPPLEMENTARY MATERIAL
The following material is available from http://www.blackwell
publishing.com/products/journals/suppmat/JBI/JBI1165/
JBI1165sm.htm
Appendix S1 Location and other information pertaining to
40 inventory plots used for the analyses.
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BIOSKETCHES
Priya Davidar is a Professor at Pondicherry University, India,
and Research Associate at the Smithsonian Tropical Research
Institute, Panama. She is interested in tropical forest ecology,
particularly tree diversity and the ecology of mutualisms. She
conducts biodiversity assessments in the Western Ghats and
the Andaman islands of India.
Jean-Philippe Puyravaud is the Director of Centre Valbio
in Ranomafana, Madagascar. He has conducted research on
the impact of human disturbance on the rain forest of the
Western Ghats of India. He is interested in the conservation
and restoration of tropical rain forests.
Egbert Giles Leigh, Jr is interested in the conditions
favouring the evolution of mutualism, the role of mutualism
in evolution, and what can be learned from the analogy
between economies and ecosystems. He is a biologist on the
scientific staff at the Smithsonian Tropical Research Institute,
Panama.
Editor: Philip Stott
Tree diversity in the Western Ghats, India
Journal of Biogeography 32, 493?501, ? 2005 Blackwell Publishing Ltd 501