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Testing for Janzen–Connell Effects in a West African Montane Forest

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Testing for Janzen–Connell Effects in a West African Montane Forest

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The Janzen–Connell hypothesis proposes that density dependent seed and seedling mortality, combined with increasing seed and seedling survival away from the parent tree, together promote regular spacing of species and thus α diversity. This hypothesis has rarely been tested in tropical Africa, and rarely in montane forests anywhere. We tested this hypothesis using a combination of field experiments and observations in the most floristically diverse dry submontane forest in Nigeria. We investigated distance effects on seedling herbivory, seedling survival and seedling height growth. We found a significant decrease in herbivory with distance from conspecific adult trees for all three species of experimentally planted seedlings (Entandrophragma angolense, Deinbollia pinnata and Sterculia setigera), and also for naturally occurring seedlings of Pouteria altissima but not of Newtonia buchananii or Isolona pleurocarpa. The relative density of large seedlings/saplings of P. altissima, N. buchananii and I. pleurocarpa increased significantly at greater distance from conspecific adult trees; however, we found no significant distance effect on survival or height growth over 3 mo for all three experimentally planted species. Taken together, our results are some of the first to show that Janzen–Connell effects occur on the African continent and in particular montane tropical forest and suggest that such effects may be pantropical.
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Testing for Janzen–Connell Effects in a West African Montane Forest
Arne Matthesius
2
, Hazel Chapman
1
,and Dave Kelly
School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
ABSTRACT
The Janzen–Connell hypothesis proposes that density dependent seed and seedling mortality, combined with increasing seed and seedling survival away from the
parent tree, together promote regular spacing of species and thus adiversity. This hypothesis has rarely been tested in tropical Africa, and rarely in montane forests
anywhere. We tested this hypothesis using a combination of field experiments and observations in the most floristically diverse dry submontane forest in Nigeria. We
investigated distance effects on seedling herbivory, seedling survival and seedling height growth. We found a significant decrease in herbivory with distance from
conspecific adult trees for all three species of experimentally planted seedlings (Entandrophragma angolense,Deinbollia pinnata and Sterculia setigera), and also for
naturally occurring seedlings of Pouteria altissima but not of Newtonia buchananii or Isolona pleurocarpa. The relative density of large seedlings/saplings of P. altissima,
N. buchananii and I. pleurocarpa increased significantly at greater distance from conspecific adult trees; however, we found no significant distance effect on survival or
height growth over 3 mo for all three experimentally planted species. Taken together, our results are some of the first to show that Janzen–Connell effects occur on the
African continent and in particular montane tropical forest and suggest that such effects may be pantropical.
Key words: Afromontane forest; density dependence; insect herbivory; species diversity.
ONE OF THE MOST FASCINATING QUESTIONS IN ECOLOGY has been what
drives the high species diversity of tropical forests. The processes
responsible remain largely unresolved but one of the main con-
tenders is a hypothesis based on negative density dependence (Jan-
zen 1970, Connell 1971, Freckleton & Lewis 2006, Comita &
Hubbell 2009). The most tested and compelling explanations focus
on the role of host-specific, natural plant enemies which, by de-
stroying seeds and seedlings, create space for the establishment of
other species and so contribute to the diversity of the forest. First
articulated by Janzen (1970) and Connell (1971), this model
(called the Janzen–Connell hypothesis) proposes that density de-
pendent seed and seedling mortality, combined with increasing
seed and seedling survival away from the parent tree, together pro-
mote regular spacing of species. Host-specific seed and seedling
predation (Schupp 1992), herbivory (Basset 1992, Novotny &
Basset 2005) and/or pathogen attack (Freckleton & Lewis 2006,
Augspurger & Wilkinson 2007) drive these mechanisms.
The complex set of assumptions, predictions and hypotheses
generated by the Janzen–Connell hypothesis has been tested more
than 50 times with variable results (Carson et al. 2008). There is,
however, compelling support for the density and distance predic-
tions of the hypothesis from both lowland tropical forests (Clark &
Clark 1984, Burkey 1994, Hammond & Brown 1998, Romo et al.
2004) and temperate forests (Packer & Clay 2000, Lambers et al.
2002). Other studies, mainly from Central and South America, do
not support the hypothesis (e.g., Hubbell 1979, 1980; Schupp
1992).
Givnish (1999) stressed the need to evaluate whether Jan-
zen–Connell effects are important in different forest types and Ba-
rot and Gignoux (2004) suggested that Janzen–Connell effects may
vary among different communities. In a review of tests of the Jan-
zen–Connell hypothesis, Carson et al. (2008) found only 1 (Wenny
2000) of 53 studies had been carried out in a tropical montane for-
est and the vast majority of studies had been conducted in the neo-
tropics within just a small number of forest types and locations.
Here, for the first time, we test for the presence of Janzen–Connell
effects in a dry, relatively species-rich, and relatively undisturbed
African submontane forest. We focused on the role of insect herb-
ivory because there is good evidence for host-specific invertebrate
herbivores in tropical forests (Janzen 1980, Augspurger 1984, Clark
& Clark 1987, Howe 1990, Gilbert et al. 1994, Norghauer et al.
2006).
Specifically, we sought to test two predictions of the Jan-
zen–Connell hypothesis using six relatively common forest tree
species, with a mixture of manipulative and observational methods:
(1) seedling survival increases with distance from conspecific adult
trees and (2) seedling leaf herbivory decreases with distance from
conspecific adult trees.
METHODS
STUDY SITE AND SPECIES.—Ngel Nyaki Forest Reserve includes ca
7200 ha of dry, submontane forest along the escarpment of the
Mambilla Plateau, located on the Nigerian/Cameroon border in
Taraba State, Nigeria. The forest lies on a steep southwest facing
slope at 1400–1600 m asl. The mean annual rainfall of ca 1800 mm
occurs mainly between mid-April and mid-October (Nigerian
Montane Forest Project Rainfall data). Mean monthly maximum
and minimum temperature for the wet and dry season are 261C and
131C, and 231C and 161C, respectively (Upper Benue River Basin
records).
Sixty-nine tree species have been recorded from Ngel Nyaki
forest, which is the most floristically diverse forest of its type in
Received 20 September 2009; revision accepted 8 April 2010.
1
Corresponding author; e-mail: hazel.chapman@canterbury.ac.nz
2
Current address: Bioprotection Division, Department of Primary Industries,
PO Box 48, Frankston, Victoria, Australia
BIOTROPICA 43(1): 77–83 2011 10.1111/j.1744-7429.2010.00664.x
r2010 The Author(s) 77
Journal compilation r2010 by The Association for Tropical Biology and Conservation
Nigeria (Chapman & Chapman 2001). The three main emergent
species are Pouteria altissima (Sapotaceae), Entandrophragma ango-
lense (Meliaceae) and Newtonia buchananii (Leguminosae:Mimoso-
idae; Dowsett-Lemaire 1989) and all three were included as study
species in this research. The three other tree species investigated
were Isolona pleurocarpa (Annonaceae), Deinbollia pinnata (Sap-
indaceae) and Sterculia setigera (Sterculiaceae).
Entandrophragma angolense,D. pinnata and S. setigera were
used to test survival and leaf herbivory of artificially raised and
planted seedlings at increasing distance from parent/conspecific
adult trees, because their seeds were readily available at the begin-
ning of the study period (end of February 2005) and because they
are relatively common in Ngel Nyaki forest. Pouteria altissima,N.
buchananii and I. pleurocarpa were used to test the survival and leaf
herbivory of naturally occurring seedlings at increasing distance
from parent/conspecific adult trees because their seedlings are mor-
phologically distinct and thus can be identified with confidence.
Moreover, the seedlings of all three species are common in the forest,
especially P. altissima that produces dense understories of seedlings.
SURVIVAL,GROWTH AND LEAF HERBIVORY OF ARTIFICIALLY RAISED AND
PLANTED SEEDLINGS.—The seedling survival and herbivory hypoth-
eses were both tested using experimental plantings of a minimum of
540 seedlings of each of the three species E. angolense,D. pinnata
and S. setigera. Seeds were collected from at least five individual
trees and then sown in polypots in forest soil and grown in a forest
nursery to an average height of 15 cm. The seedlings were then
planted out in the forest in rows of ten individuals 20 cm apart, at
1 m, 12.5m and 25 m from conspecific adult trees. Before planting,
each seedling was scored for leaf loss by visually estimating the per-
cent of the total leaf area of a seedling that was missing in 5 percent
intervals. The loss of whole leaves was included in the estimate of
whole leaf tissue loss. Loss of whole leaves was evident from leafless
petioles (Stotz et al. 2000). The number of replicates for each spe-
cies was limited by the number of surviving potted seedlings, so our
sample sizes were N= 18, 19 and 20 conspecific adult trees for S.
setigera,D. pinnata and E. angolense (540, 570 and 600 seedlings),
respectively. The individual conspecific adults were chosen so that
they were at least 50 m from any conspecifics to avoid any overlap-
ping of seed shadows and the potential sharing of seedling predators
among them, as predicted by the Janzen–Connell model (Janzen
1970).
After planting in the forest, seedling survival and heights were
measured and leaf herbivory was estimated (as described above)
once a month for 3 mo. Seedling survival to month 3 and seedling
height growth from month 1 to 3 were used to measure plant per-
formance following the method of Howe (1990), while amount of
herbivory at month 3 measured herbivore impacts. Remains of pet-
ioles, which indicated that whole leaves had been lost, were in-
cluded in the herbivory estimate. Seedlings were measured in the
same order they were planted in to keep time intervals between
measurements as consistent as possible.
HEIGHT AND LEAF HERBIVORY OF NATURALLY OCCURRING SEEDLINGS.—
The first hypothesis (seedling height as an estimate of survival
increases with distance from conspecific adult trees) was tested us-
ing naturally occurring seedlings around 20 adult trees of each of
the three tree species P. altissima,N. buchananii and I. pleurocarpa.
As in the previous experiment, all of the conspecific adult trees un-
der which seedlings were measured were at least 50 m apart. None
of the adult trees were located near light gaps.
All naturally occurring seedlings were measured within plots at
five distance intervals away from the adult trees (0–5, 5–10, 10–15,
15–20 and 20–25 m). We used a maximum distance of 25 m away
from the adult tree because at greater distances it became difficult to
ensure that plots were not closer to a different conspecific. At the
first three distance intervals (0–5, 5–10 and 10–15 m), 5 5-m
plots were used. At 15–20 m distance, plot size was increased to
510 m, and at 20–25 m distance, further increased to 515 m,
to obtain a better representation of seedling height distributions
where conspecific seedling densities are lower. Within the
plots, seedling height classes were used as a proxy for seedling age
(Webb & Peart 1999). Seedling heights were measured to the near-
est centimeter and later grouped into height categories, depending
on the relative abundance of seedlings within each species (see
below).
The second hypothesis (seedling leaf herbivory decreases with
distance from conspecific adult trees) was tested in these same plots
by visually estimating seedling leaf herbivory as described above
(Stotz et al. 2000) on up to 20 seedlings in each plot. Although this
method has limitations (Clark & Clark 1985), it has been shown to
be effective in field situations (Sessions & Kelly 2001).
DATA ANALYSIS
SURVIVAL AND HERBIVORY OF EXPERIMENTALLY PLANTED SEED-
LINGS.—Seedling survival was analyzed by calculating the propor-
tion of the ten seedlings in a row that survived to month 3 at each
distance (1, 12.5 and 25 m). Any differences between distances in
seedling mortality were expected to be greatest after 3 mo when
they had been exposed to mortality factors for the maximum time
available in the experiment. We used one generalized linear mixed
model (GLMM) with a binomial distribution and a logit link
function to test for effects of species, distance from conspecific
adult and the species distance interaction. The R statistics pro-
gram (v. 2.10.1; R: A Language and Environment for Statistical
Computing 2009) with library lme4 was used for all analyses.
Parent tree was entered as a random effect. If Janzen–Connell
effects are consistent, we would predict a significant positive dis-
tance effect on survival and no significant species distance in-
teractions.
Seedling height growth in surviving seedlings was used as a
second estimate of seedling performance (Howe 1990). A second
GLMM was used to detect whether a significant proportion of the
variance in height increase from month 1 to 3 was explained by
species, distance and the interaction species distance, again using
parent tree as a random effect. The GLMM was run with a Gauss-
ian distribution and an identity link function. Probability values for
the parameters from the Gaussian GLMM were estimated using
Monte Carlo Markov chain (MCMC) methods implemented in
the ‘pvals.fnc()’ command in the ‘languageR’ library for R.
78 Matthesius, Chapman, and Kelly
Similarly, the mean percentage leaf area per plot lost to herb-
ivory at month 3 was also investigated using a Gaussian GLMM.
We tested the proportion of leaf area lost to herbivores (arcsin
square root transformed) against species, distance and species dis-
tance with parent tree as a random effect, and MCMC probability
values from pvals.fnc().
HEIGHT AND LEAF HERBIVORY OF NATURALLY OCCURRING SEED-
LINGS.—Tests for changes in the relative distributions of various-
sized seedlings with distance used linear mixed models to test the
proportion of seedlings in each plot which were in the largest size
categories. The three species varied in the numbers of seedlings in
different size classes, so the size classes were chosen for each species
to ensure that each size class contained reasonable numbers of seed-
lings. We initially used four size classes for P. altissima (0–10,
11–20, 21–40, 41–200cm) and I. pleurocarpa (0–20, 21–40,
41–70, 71–200 cm), and three for N. buchananii (0–10, 11–20,
21–200cm). For analysis, in all species, this was subsequently sim-
plified to the proportion of total seedlings that were in the largest
size class. In each case, the same five distance categories were retained
(0–5, 5–10, 10–15, 15–20 and 20–25m). For each species, a bi-
nomial GLMM run using lme4 in R tested for an effect of distance
on the proportion of seedlings per plot which were in the largest size
class. As before, parent tree was entered as a random effect.
A general linear model (GLM) was used to investigate the
effect of distance from the parent tree on the average percent leaf
herbivory of each species of seedlings. The response variable was the
mean proportion lost to herbivory for all the seedlings in each plot,
and the data were arcsin square root transformed before analysis.
Parent tree was included as a block effect in the GLMs. We did also
run these analyses as linear mixed models using lme4 (treating par-
ent as a random effect rather than a block effect) and the conclu-
sions were identical.
RESULTS
GROWTH AND HERBIVORY OF EXPERIMENTALLY PLANTED SEEDLINGS.—
The proportion of the planted seedlings of all three species that
survived decreased over the 3-mo period. By the end of the 3 mo
across all three species, there were significant differences among
species (Table S1) but there was no overall trend in the linear mixed
model for an increase in survival with increasing distance from the
conspecific adult trees (distance was nonsignificant, Table S1).
There was one modestly significant species distance interaction,
for S. setigera (P= 0.038) but with a small negative slope (Fig. 1A),
contrary to predictions. Similarly, seedling height growth showed
no support for Janzen–Connell effects, with significant variation
among species but no variation with distance, and no species dis-
tance interactions (Table S1; Fig. 1B). Herbivory decreased signifi-
cantly for all three species (D. pinnata,S. setigera and E. angolense)
with distance from conspecific adults. There was no significant spe-
cies distance interaction (Table S1; Fig. 1C).
HEIGHT AND HERBIVORY OF NATURALLY OCCURRING SEEDLINGS.—A
total of 4031 P. altissima, 4037 N. buchananii and 383 I. pleuro-
carpa seedlings were measured within all of the plots. As predicted
by the Janzen–Connell hypothesis, the proportion of taller seed-
lings increased significantly with distance from conspecific adults
FIGURE 1. The performance of experimentally planted seedlings (mean
SEM) at three distances away from conspecific adult trees (1, 12.5 and 25 m)
for Deinbollia pinnata (Dein, circles), Entandrophragma angolense (Enta, trian-
gles) and Sterculia setigera (Ster, squares). (A) Proportion of seedlings surviving
to 3mo, (B) seedling height growth from month 1 to 3, and (C) mean seedling
herbivory (% leaf area loss) at month 3.
Testing Janzen–Connell Model in West Africa 79
for all three species (Fig. 2). The larger seedling size classes were
much more common at greater distances from the parent, and after
allowing for parent tree in the linear mixed models, there were
highly significant effects of distance in all three species (Table 1). In
each case, the sign was positive, i.e., there was an increase in the
relative abundance of larger seedling classes with increasing dis-
tance, consistent with higher survival of seedlings further away.
The average percent leaf area lost to herbivory in P. altissima
seedlings varied significantly among parent trees and decreased sig-
nificantly with distance from parent trees (Table 2). The mean
herbivory reduced from a fitted value of 34.9 percent of leaf area at
2.5 m to 29.0 percent of leaf area at 22.5m distance from the parent
tree (Fig. 3). For N. buchananii, there was a significant parent effect
but no distance effect, and in I. pleurocarpa neither the parent or
distance effects were significant (Table 2).
DISCUSSION
EVIDENCE FOR JANZEN–CONNELL EFFECTS.—Our results are the first
tests of the Janzen–Connell hypothesis from an Afromontane forest
that we are aware of. As such, they make a valuable contribution to
the understanding of latitudinal gradients in species richness.
As in many studies from lowland tropical and, to a lesser ex-
tent, temperate forests (Carson et al. 2008), our results were mixed
in terms of support for the Janzen–Connell predictions we tested.
Our strongest supporting evidence came from the natural popula-
tions of P. altissima in which both predictions: (1) increased pro-
portion of larger seedlings and (2) decreased herbivory, with
distance from parent tree were fulfilled. In the case of naturally oc-
curring I. pleurocarpa and N. buchananii while prediction (1) was
supported, no significant distance effects on levels of seedling herb-
ivory were found. Similar mixed results from studies using compa-
rable methodologies in a range of studies around the world are
summarized in Carson et al. (2008). Several tropical studies (Augs-
purger 1984, Clark & Clark 1984, Webb & Pert 1999, Harms
et al. 2000) found increasing numbers of large seedlings (and/or
seedling survival) with distance from the parent tree was consistent
across species. Other tropical studies, however, have found such
patterns to be species (Chapman & Chapman 1996, Cintra 1997)
or scale (Queenborough et al. 2007) dependent. They are far less
common in temperate forests (Hyatt et al. 2003). Decreased host-
specific herbivory with distance from the parent tree has been re-
corded many times before in the tropics (Janzen 1980, Augspurger
1984, Clark & Clark 1987, Howe 1990, Gilbert et al. 1994, Nor-
ghauer et al. 2006) and it was surprising to us that this was not the
case for all three of our naturally occurring seedling species, partic-
ularly in view of the fact that survival of larger seedlings was appar-
ently higher with increasing distance away from the parent tree.
Decreasing herbivory with distance, however, was found in all three
species of the experimentally planted seedlings (D. pinnata,E. an-
golense and S. setigera). Possible explanations for why we did not
find evidence of decreased herbivory with distance for I. pleurocarpa
and N. buchananii are discussed below.
In the case of the planted seedlings, there was no significant
distance effect on seedling survival or height growth with distance
from conspecific adults over the 3 mo of the experiment, but as
mentioned above, there was a significant decrease in herbivory with
FIGURE 2. The percentage (mean SEM) of all naturally occurring seedlings
of (A) Isolona pleurocarpa, (B) Pouteria altissima and (C) Newtonia buchananii
which were in the largest height class (71–200, 41–200 and 21–200cm, respec-
tively, see text) at various distances (0–5, 5–10, 10–15, 15–20 and 20–25 m)
from the parent tree. There was a significant increase in the percentage of large
seedlings at greater distances for all three species (see Table 1).
80 Matthesius, Chapman, and Kelly
increasing distance from the parent tree for all three species. The
apparent contradiction between lower herbivory with distance, yet
no distance effect on height growth or seedling survival, might be
due to the short duration of the planting experiment (3 mo) not
leaving time for herbivory to affect seedling vital rates. The fact that
the three species included in the experiment (D. pinnata,E. ango-
lense and S. setigera) are so different in terms of ecology and yet all
failed to show demographic effects of herbivory makes this a pos-
sible explanation. Alternatively, it is possible that microhabitat
preferences could negate the benefits of reduced herbivory away
from the parent tree. For example, farther-away seedlings may be
outside their optimal habitat, or light gaps could obscure Jan-
zen–Connell effects (Clark & Clark 1984, 1987; Cintra 1997;
Cintra & Horna 1997). Light gaps have been shown to locally re-
duce pathogen loads (Augspurger 1984) and may allow for com-
pensatory growth in response to herbivory relative to seedlings
outside of gaps (Blundell & Peart 2001). While light gaps were not
measured in this study, there was environmental heterogeneity
within the experimental plots. However, it may be that leaf herb-
ivory does not affect survival and growth in these species (Blundell
& Peart 2001); Chapman and Chapman (1996) found a distance-
survival response in experimentally planted seedlings in Uganda to
be species specific.
The lack of distance effect of herbivory on seedlings of N. bu-
chananii and I. Pleurocarpa suggests that their apparently higher
survival at greater distances may have been driven by other density/
distance factors such as pathogens (Augspurger 1984, Coley & Bar-
one 1996, Bell et al. 2006, Norghauer et al. 2008) or intraspecific
competition (Comita & Hubbell 2009). Possible nondensity de-
pendent explanations include, for example, allelopathic affects be-
tween conspecific parent trees and seedlings (Janzen 1970, Connell
1971, Wright 2002) and leaf litter under a conspecific adult (Mo-
lofsky & Augspurger 1992) can reduce survival rates in conspecific
seedlings. In this study, litter may have contributed to seedling
death under parent trees of P. altissima. Seedlings of this species
were often absent from areas with heavy leaf litter (A. Matthesius,
pers. obs.) and P. altissima does periodically shed all of its leaves
(Chapman & Chapman 2001).
Alternatively, the possibility exists that we simply did not de-
tect density dependent host-specific leaf herbivory in N. buchananii
and I. Pleurocarpa because we measured herbivory at a single point
in time. This approach could hide the pattern predicted by Jan-
zen–Connell effects for some species simply because host-specific
seedling predators were not abundant at the time of the experiment
(Connell 1971) or because their host seedlings were in short supply
TABLE 1. Generalized linear mixed model analyses of the effect of distance away
from the parent tree (0–25 m) on the proportion of plants in each plot
which were in the largest size class for Isolona pleurocarpa,Pouteria
altissima and Newtonia buchananii. In each case the analysis was a
linear mixed model with parent plant as a random effect and distance as
a fixed effect, using a binomial error distribution with a logit link.
Fixed effect Estimate SE Z-value P
Isolona
Intercept –2.7189 0.4250 –6.397 o0.001
Distance 0.0810 0.0234 3.456 o0.001
Pouteria
Intercept –4.712 0.5285 –8.916 o0.001
Distance 0.0910 0.0155 5.860 o0.001
Newtonia
Intercept –5.0588 0.6215 –8.140 o0.001
Distance 0.1075 0.0210 5.126 o0.001
TABLE 2. GLM analyses of effects of distance from the parent tree on herbivory in
naturally occurring seedlings, in Isolona pleurocarpa,Pouteria altissi-
ma and Newtonia buchananii. The GLMs used parent plant as a block
effect and distance (0–25 m) as a variate, in Gaussian GLMs with mean
percentage leaf loss per plot (arcsin square root transformed) as the re-
sponse variable.
Predictor Deviance df F P
Isolona
Parent tree 0.419 19 1.22 0.272
Distance 0.0032 1 0.179 0.674
Residual 1.07 59
Pouteria
Parent tree 1.33 19 8.54 o0.001
Distance 0.0495 1 6.00 0.0165
Residual 0.646 79
Newtonia
Parent tree 0.565 20 5.15 o0.001
Distance 0.0007 1 0.131 0.718
Residual 0.368 67
FIGURE 3. The change in mean percent leaf area lost to herbivory per plot
against distance from conspecific adult (0–5, 5–10, 10–15, 15–20 and 20–25 m)
for Pouteria altissima. Shown are residuals from a GLM fitting a parent plant
effect. The distance effect was significant in a GLM with parent plant fitted first
(see Table 2).
Testing Janzen–Connell Model in West Africa 81
at some point in time before making the measurements (Janzen
1970). Another difficulty in determining herbivory rates at one
point in time in the observational datasets is that there is no infor-
mation on leaf turnover, which makes accurate determination of
the time scale over which damage has accumulated very difficult
(Coley & Barone 1996) (this was less of a problem in our 3-mo-old
planted seedlings). In addition, not all insects leave obvious evi-
dence of leaf consumption (e.g., phloem feeders; Coley & Barone
1996) and we may have sometimes underestimated herbivory
through loss of whole leaves in cases where the petiole was also lost.
In summary, while our results were mixed, we did find some
Janzen–Connell effects in Ngel Nyaki forest, which suggests that
such effects cannot be discounted in our understanding of species
diversity in Afromontane forests more generally. As pointed
by Carson et al. (2008), Janzen–Connell effects on just a few of
the common species are sufficient to lead to increased species
diversity.
FUTURE WORK.—Considering the paucity of tests of the Jan-
zen–Connell hypothesis in African forests, this is an important first
step. More African studies need to be undertaken so that more di-
rect comparisons between latitudes can be made. Such investiga-
tions will need to be longer term and on a larger scale than this
present study. They should include more life stages such as seeds as
well as seedlings.
Research to determine whether natural enemies (both herbi-
vore and pathogen) inflict density dependent damage and mortality
are required. Manipulative experiments, along the lines of Freckle-
ton and Lewis (2006) and Augspurger and Wilkinson (2007) will
be important. Such manipulations should involve the addition and
removal of pests to communities over sufficient time periods for
their absence or increased abundance to have an effect (Carson et al.
2008). Only once such studies are available will it be really possible
to include Africa in latitudinal comparisons of distance/density
effects on species diversity.
ACKNOWLEDGMENTS
We thank U. Usuf and M. Zubairu and other staff of the Nigerian
Montane Forest Project for invaluable field assistance. Very helpful
comments on this manuscript were gratefully received from W. P.
Carson, the editor, and two anonymous reviewers. We are grateful
to the Taraba State Forest Service for their invitation and permis-
sion to conduct research. Financial support came from The North
of England Zoological Society, Nexen Inc. and the A. P. Leventis
Foundation.
SUPPORTING INFORMATION
Additional Supporting Information may be found in the online
version of this article:
TABLE S1. GLMM mixed model analyses of means of experimen-
tally planted seedling survival, height growth and seedling herbivory at
three distances away from conspecific adult trees for Deinbollia pin-
nata, Entandrophragma angolense and Sterculia setigera.
Please note: Wiley-Blackwell is not responsible for the content
or functionality of any supporting materials supplied by the au-
thors. Any queries (other than missing material) should be directed
to the corresponding author for the article.
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Testing Janzen–Connell Model in West Africa 83
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... Despite the major threat to its flora and fauna by human encroachments through cattle grazing, hunting, burning, and selective logging of trees for firewood and fence posts, the forest reserve is described as an Important Bird and Biodiversity Area by Birdlife International. This is because of its restricted range of bird species such as the Bannerman's weaver ( Ploceus bannermani ) and the Yellow-breasted boubou ( Laniarius atroflavus ) [19] . ...
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Birds provide ecosystem services that play crucial role in maintaining forest ecosystems. However, the knowledge of the relationship between different bird assemblages and different forest types is still scanty. We analyzed the impacts of different tree species of different functional traits on visitation preferences by birds in Ngel Nyaki Forest Reserve, Nigeria (7o30’N, 11o30’E). We monitored visitation of different bird species to trees using the point-count method. A total of 32 bird species were recorded. Must of bird species (28 species) were insectivores, while four species are frugivores. The rate of visitation of different bird species differs significantly among the different tree species. Correlation analyses showed that the frequency of birds’ visits depend on abundance of individuals of a tree species. However, some of the tree species are more frequently visited by different species of birds, while other trees are peculiar in terms of visitation preferences by some of the bird species. More important tree species as the frequency of birds’ visitation is concern include M. lanceolata (205), P. fulva (134), and B. speciosa (113). Trees that were visited by the highest number of species of birds include B. speciosa (28), M. lanceolata (20), and T. aurentalis (19). There was also a positive effect of trees' functional traits on the total number of birds’ visits, but there was no correlation between the trees' functional traits with the abundance of individual bird species visitation. We hypothesized that small morphological differences among trees will have no impact on bird's visitation as found in this study. These results and interpretations have implications for sustainable management and conservation of montane forest and possibly elsewhere.
... Whatever the reason, and even with the possibility that we missed rare longer distance events, dispersal distances are short in this montane habitat, which may make it unlikely that ants help seeds escape intra-specific, negative density-dependent mortality (Connell 1971, Janzen 1970) and gain associated benefits (Andersen 1988, Cain et al. 2000, Gallegos et al. 2014. It is worth noting that in Afromontane forests, tree species tend to show more local clumping than in lowland tropical forests (Abiem et al. 2020, Matthesius 2006) and Janzen-Connell effects may not be as pervasive (Matthesius et al. 2011). With short-distance movement of Figure 3. (A) shoot length (cm), (B) wet weight (g) and (C) root length (cm) of seedlings from cleaned and intact seeds of P. pinnata (n = 18 for cleaned seeds, n = 18 for intact seeds). ...
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... The distribution of maximum number mainly kept in a range, rather than in their immediate vicinity, the same result was reported in previous research (Schupp and Jordano, 2011). In addition, maximum number of juveniles of four dominant species was distributed from 4.5 m to 10.5 m, and this finding can be illustrated in Janzen-Connell theory in a certain extent, which high mortality is likely to occur near the mature trees (Matthesius et al., 2011;Steinitz et al., 2011). Meanwhile, the distribution of number of juveniles were influenced significantly by the crown widths, because of the canopy inhibited the growth of shrubs and herbs, more understory trees, therefore, can grow well in upper height trees (Pretzsch and Dieler, 2012). ...
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Zhang et al.: Spatial associations and species collocation of dominant tree species in a natural spruce-fir mixed forest of Changbai Mountains in northeastern China-6213-APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 17(3): 6213-6225. Abstract. In order to achieve of the spatial association and optimum collocation, a 1-ha natural spruce-fir mixed forest plot which was established on Changbai Mountains, PR China. Diameter at breast height, height, and the crown width of the standing free trees with >1 cm DBH trees were measured, and species were identified. O-ring statistics and the nearest neighbor analysis were applied to test the spatial correlations and distribution of number of dominant species at different vertical layers. We found that: (1) according to the importance values, the top four species were Abies nephrolepis (Trautv.) Maxim., Picea koraiensis Nakai, Pinus. koraiensis Sieb. et Zucc. and Tilia. amurensis Rupr.; (2) most of species pairs have positive associations at small scales, while few are no relevance or have negative associations; (3) the number of juveniles and lower layer trees showed positive skewness under the upper layer trees and the maximum number for coniferous species was at a distance 4.5-6.5 m, while it was 6.5-8.5 m for broadleaved species. Our results provide a new insight into the development of reforestation technique in spruce-fir forests in the Changbai Mountains.
... It is likely that the main reasons behind the repulsion patterns are both the amount of litterfall that accumulates around the parent trees and in their understory (Muhamed et al., 2018) and the reduced availability of light that reaches the forest oor (Puerta-Piñero et al., 2007). This result is congruent with the Janzen-Connell theory explaining high o spring mortality rates near parent trees, which might also partly contribute in explaining the segregation patterns detected in this study (Matthesius et al., 2011). The spatial association pattern between shrubs and seedlings is comprehensible since shrubs like Quercus species with a relatively small canopy can represent a bene cial shady environment (e.g., by reducing the soil temperature and atmospheric moisture stress) and at the same time, provide su cient light during the early establishment stages (Muhamed et al., 2018). ...
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... The plateau, at an altitude of between 1500 and 1900 m in elevarion (Chapman and Chapman, 2001) is part of the volcanic mountain chain running along the Nigerian-Cameroon border in Western Africa (Burgess et al., 2007) degraded riparian forest fragments, grasslands and two large contiguous forest patches (Chapman et al., 2010). There are two main climatic seasons, wet (temperature range: 26°C-13°C) and dry (temperature range: 23°C-16°C) (Matthesius et al., 2011). The wet season typically occurs between April and October. ...
Thesis
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The survival of most species depends on their network of mutualistic partnerships with other species. Network structure - the pattern and arrangement of species in a given interaction, can reveal predictable outcomes on the fate of species and network stability. Therefore understanding what makes networks stable is extremely important. However current investigations into network stability have mainly been through theoretical and simulation studies and often based on data from plant-insect visitation networks. Empirical testing is now imperative and networks other than just plant-insect ones need to be incorporated into such studies. Moreover, the use of visitation frequency as a proxy for pollination frequency in plant-pollination network studies needs to be evaluated. In this thesis, I tested theoretical views and models on network structure and species survival using empirical data from a sunbird-tree pollination network in a remote montane forest reserve in Nigeria. First I investigated how changing landscapes and habitats affected sunbird distribution in the reserve. Secondly I compared a sunbird-tree visitation with a sunbird-tree pollination network to determine how good a proxy bird visitation is for bird pollination. Two network properties which affect network stability and underpin this work are i) connectance/ interaction strength and ii) nestedness. I examined how species relative abundance influences their interaction strength in the network and whether a species contribution to nestedness determines its survival probability. This latter point was of special interest as theoretical studies on networks have suggested that species which contribute the most to nestedness are most prone to extinction, which seems counterintuitive. The visitation frequencies of sunbird species on flowers of as many tree species as was logistically possible were observed, and sunbird species were trapped to determine the amount of pollen they transported. Using these data, I developed the flower visitation (FVN) and pollen transport networks (PTN). To determine how FVN reflects PTN, I compared the two networks using null models that controlled for species’ degree (number of links) and network size. Differences in observed nestedness differed significantly from null model expectations. I worked out an extinction proneness based on IUCN criteria for determining species at risk of extinction using rarity as a measure of vulnerability. An assessment of species abundance and diversity in the reserve and nearby fragments provided the standard for risk categorisation and evaluation of species’ robustness to changing landscapes. FVN correlated significantly with PTN, despite 64 % dissimilarity in species composition. The PTN had fewer species than FVN, but was more nested and specialised than the FVN, indicating that analyses of FVN often overestimate pollination through the inclusion of interactions with variable effects such as nectar robbing and insectivory. Although some species were relatively stronger interactors in both networks, the strongest contributor for FVN was not the strongest for PTN. FVN is therefore an inadequate predictor of efficient pollinators and a poor reflection of PTN. Abundant species had a higher interaction strength overall. Strong contributors to nestedness were the rare species, which explains why they are more prone to extinction. In my empirical network, nestedness will decrease overall through the loss of rare species, but in accordance with network theory, this will not collapse the network, because it is the abundant species with the most links which maintain network stability. Although fragmentation is not yet a challenge to sunbird distribution, anthropogenic disturbance such as indiscriminate burning of grassland to stimulate re-growth, may alter crucial habitats for sunbird survival.
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(1) The deciduous forests and the riparian evergreen vegetation that they include in the lowlands of Guanacaste Province, Costa Rica, contain at least 975 species of dicotyledonous plants. At least 110 species of beetles (Coleoptera) whose larvae are seed-predators were reared from more than 3700 samples of fruits and seeds of these plants. (2) At least 100 species of these plants usually had beetle seed-predators (Bruchidae, Curculionidae, Cerambycidae) in their mature or nearly mature fruits or seeds. (3) Most (75%) of the species of beetles were specific to a particular plant species; 14 preyed on two plant species, 9 on three, and 2 on four. The bruchid Stator pruininus preyed on six species and S. limbatus on eight species. (4) Of the 100 species of plants whose seeds were preyed on regularly by beetles, 63 were in the Leguminosae, 11 in the Convolvulaceae, and the remainder were spread among sixteen other plant families. (5) Of these 100 prey species, 59 were fed on by a single species of beetle, 25 by two species of beetles, 11 by three, 4 by four and one, Cassia leptocarpa, by five species of beetles. (6) In at least 90% of seed or fruit samples, all species of beetles attacking that species in the study area were present. Of the 100 species of beetles, eighty were found in the first sample of the appropriate fruit or seeds. The prey of ten additional species of Bruchidae in the study area is unknown, but will be other than the prey species listed here. (7) With some striking exceptions, the prey species of those beetle species which preyed on more than one plant species were closely related. In contrast, in those cases where there were two or more congeneric plant species in the study area, the species of beetle which attacked one or more of them left unattacked an average of 5.8 of the congeneric plant species. (8) There were five unambiguous cases of a prey species that occurred throughout the study area and that had two or more species of beetle seed-predator whose distributions did not overlap at all. (9) Hymenopterous parasitoids were uncommon in most of the samples and were absent from large samples of a number of common tree species whose seeds were heavily preyed on by beetles. Of the 157 predator-prey pairs reported here, 57% of the beetle species were unattacked by hymenopterous parasitoids. (10) The distribution of beetle predator species among the plant prey-species was conspicuously neither random nor uniform. Unexpectedly large numbers of species were either unattacked, or preyed on by two to five species of beetles, while unexpectedly small numbers were attacked by a single species of beetle. (11) The prey-specificity of most of these seed-predators in a species-rich flora is of great importance in understanding the potential impact of animals on plant species-richness, but is not proof that seed predation by animals causes extreme plant species-richness. Furthermore, these beetles are only a small fraction of the animals that kill or weaken plants in a manner that may influence their abundance and spatial distribution.
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1 The number of woody species in tropical forests tends to increase with precipitation, forest stature, soil fertility, rate of canopy turnover and time since catastrophic disturbance, and decrease with seasonality, latitude, altitude, and diameter at breast height (d.b.h.).2 A model is presented to account for these trends. Novel hypotheses include how increased rainfall and substrate fertility, and decreased seasonality, might (i) increase attacks by natural enemies, and thus the overall level of density-dependent plant mortality; (ii) increase shade tolerance, canopy turnover, and stem density of the species-rich understorey; and (iii) increase reliance on relatively sedentary forest-interior birds for seed dispersal, fostering high rates of speciation in understorey genera.3 High rainfall and low seasonality in the tropics favour two key groups of natural plant enemies – insects and fungi – that are directly responsible for promoting high rates of density-dependent plant mortality. Lower rainfall, greater seasonality, soil infertility, or unfavourable rooting conditions favour greater allocation to anti-herbivore defences, and thus lead to lower rates of such mortality and thence to lower tree diversity. The increased number of individuals on rainier sites is a minor contributor to increased tree diversity, accounting for only about 17% of the 8.3-fold increase with rainfall in the lowland Neotropics.4 Predictions of the model are consistent with many ecological patterns of variation in tropical tree diversity within regions, and may help explain the decrease in tree diversity with elevation and the accompanying decrease in horizontal patchiness (within-habitat  diversity).5 Random drift over evolutionary time in the relative effectiveness of density-dependent control of individual tree species by specialized natural enemies may better account for the observed distribution of tropical tree abundance than a random walk of species abundance through ecological time.