Download Liana cooccurrence patterns in a temperate rainforest

Journal of Vegetation Science 22 (2011) 868–877
Liana co-occurrence patterns in a temperate rainforest
R.A.J. Blick & K.C. Burns
Keywords
Compartments; C-score; Liana; Negative
co-occurrence; Network; Null model.
Nomenclature
Allan (1961), Brownsey & Smith-Dodsworth
(2000), Moore & Edgar (1970)
Received 31 March 2010
Accepted 28 February 2011
Co-ordinating Editor: Tom Philippi
Blick, R.A.J. (corresponding author,
[email protected]): School of Biological
Sciences, Victoria University of Wellington, P.O.
Box 600, Wellington, New Zealand, Evolution &
Ecology Research Centre, School of Biological,
Earth and Environmental Sciences, University of
New South Wales, NSW 2052, Sydney, Australia
Burns, K.C. ([email protected]): School of
Biological Sciences, Victoria University of
Wellington, P.O. Box 600, Wellington, New
Zealand
Abstract
Questions: Are liana–host interactions structured at the community level? Do
liana–host interactions differ between species, growth form guilds or habitats?
Location: Otari-Wilton’s Bush, on the southern tip of North Island, New Zealand.
The forest contains 75 ha of mature and regenerating conifer–broadleaf forest.
Methods: Nine liana species were quantified among 217 trees to test for
negative co-occurrence patterns. We also conducted additional analyses within
and among compartments embedded in the community-level matrix. Liana
and host abundance distributions were assessed across two contrasting habitats.
Results: Community-level analyses revealed negative co-occurrence patterns.
Positive, neutral and negative co-occurrence patterns were found among
compartments within the community-level matrix. Host species compartments
were consistent with randomized expectations, while positive co-occurrence
patterns were found within the host species matrix. Negative co-occurrence
patterns were found inconsistently among lianas that share the same region of
host space, and those that do not.
Conclusions: Overall, results indicate the liana community is structured nonrandomly. Liana–host interactions appear to follow an opportunistic growth
strategy and interactions are due mostly to habitat partitioning.
Introduction
Species interaction matrices provide a useful method for
assessing deterministic, neutral or random associations of
species assemblages (Gotelli & Graves 1995; Gotelli 2000;
Bascompte et al. 2003; Vázquez 2005; Almeida-Neto et al.
2007; Guimarães et al. 2007; Hausdorf & Hennig 2007;
Ings et al. 2009). If communities are assembled deterministically, similar species should replace one another
among communities (Diamond 1975). Furthermore,
co-evolved traits or geographically isolated individuals
may generate compartments (e.g. modules, blocks or
species subgroups) within ecological networks (Prado &
Lewinsohn 2004; Lewinsohn & Prado 2006; Olesen et al.
2007). Typically, large pollination networks consist of
compartments with highly ‘nested’ organization and are
linked by generalist species (Olesen et al. 2007). However,
there remains little consensus on the processes that generate compartments among species assemblages. Few
studies have used an individual-based approach (i.e.
matrices of interactions between individuals rather than
868
species) to identify the processes structuring communitylevel interactions.
Here we investigate liana–host interactions in a temperate New Zealand forest. Patterns in the distribution of
lianas have been studied extensively in tropical (Talley et
al. 1996b; Clark & Clark 2000; Chittibabu & Parthasarathy
2001; Carsten et al. 2002) and temperate systems (Talley
et al. 1996a; Allen et al. 1997, 2005; Baars et al. 1998;
Malizia et al. 2010) and their role in ecosystem functioning has recently been emphasized (Schnitzer & Bongers
2002; Schnitzer 2005). Lianas represent a significant
proportion of vascular plants in tropical regions. Their
distribution in cooler climates may be limited due to an
increased rate of embolism, therefore temperate regions
consist of few species capable of withstanding harsh
environmental conditions (Schnitzer 2005). The influence of environmental conditions on liana species composition have been identified (DeWalt et al. 2000, 2006;
Burnham 2002; Ibarra-Manriquez & Martinez-Ramos
2002; Burnham 2004; Malizia et al. 2010); however, the
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Blick, R.A.J. & Burns, K.C.
mechanisms responsible for community organization are
less clear when structural support requirements are considered (Putz 1984; Balfour & Bond 1993; Campbell &
Newbery 1993; Carsten et al. 2002).
Two mechanisms may structure liana–host interactions. First, lianas may undergo ‘vertical’ partitioning of
host tree space. The primary stems of lianas are typically
small in diameter and incapable of supporting their own
weight. Therefore lianas require neighbouring trees to
provide structural support. Host tree structural characteristics, such as branch height or bark type, may alter
liana–host interactions by influencing colonization rates
(Nabe-Nielsen 2001; Campanello et al. 2007), altering
liana distribution (DeWalt & Chave 2004; Senbeta et al.
2005) and maintaining liana diversity (Schnitzer &
Carson 2001; also see Putz 1984; Balfour & Bond 1993;
Campbell & Newbery 1993; Wright et al. 1997; Malizia &
Grao 2008; Watanabe & Suzuki 2008). Alternatively,
‘horizontal’ partitioning among spatially distinct habitats
may also influence liana–host interactions. Liana distribution can change in response to abiotic conditions (Baars
& Kelly 1996; Burrows 1996; Hättenshwiler 2002; Schnitzer & Bongers 2002; Schnitzer 2005; Swaine & Grace
2007; Foster et al. 2008; but see DeWalt & Chave 2004;
van der Heijden & Phillips 2008), which can simultaneously affect the distributions of host trees (DeWalt et al.
2006). Relationships between liana structural requirements and host characteristics (i.e. ‘host preferences’
Burns & Dawson 2005) could lead to community-level
patterns, such as negative co-occurrence patterns and
host specialization.
We investigated these hypotheses by documenting the
lianas growing among host tees in a temperate New
Zealand forest. In an effort to pinpoint the mechanisms
responsible for negative co-occurrence patterns, we followed up community-level analyses with additional analyses of compartments within the community-level matrix.
Our aims are to (1) characterize general patterns of lianas
and their hosts, (2) identify whether these underlying
patterns of distribution promote negative co-occurrence
patterns, and (3) investigate compartmentalization in a
temperate plant–plant interaction network. Overall, this
study highlights the importance of abundance and distribution in structuring interaction networks.
Methods
Sampling
Liana distributions were quantified in Otari-Wilton’s
Bush, which is located on the southern tip of the North
Island, New Zealand (41114 0 S, 174145 0 E). The area
experiences a mild climate with approximately 1249 mm
of annual rainfall and an average monthly temperature of
approximately 12.8 1C. The forest contains 75 ha of
mature and regenerating conifer–broadleaf forest. There
are five dominant canopy tree species, Elaeocarpus dentatus, Prumnopitys ferruginea, Beilschmiedia tawa, Dysoxylem
spectabile and Melicytus ramiflorus, and two canopy emergent trees, Dacrydium cupressinum and Knightia excelsa.
Easily accessible regions of old-growth forest, and the
presence of a canopy walkway made this site an ideal
location for assessing epiphyte and liana communities.
Burns & Dawson (2005) provide a more detailed description of the site and its epiphyte and liana assemblages.
Ten randomly located 10 m 10-m plots were established in old-growth forest. Five plots were situated on a
slope with northern exposure (north) and five plots were
situated on an opposite valley slope with southern
exposure (south), thus ensuring an even distribution
between the two major forest habitats within the reserve.
All woody plants within each plot were inspected for
lianas, and species identities of liana–host interactions
were noted. Host size has been found to influence liana–
host interactions (Chittibabu & Parthasarathy 2001). We
included all host species in our analysis, as Burns &
Dawson (2005) showed that host size had little effect on
liana distribution at this site. Most liana species were
attached to the forest floor by multiple stems, which were
typically intertwined and difficult to distinguish individually. Therefore, accurate counts of interactions between
individual plants were not possible. Instead, the frequency of liana occurrence was quantified as the number
of host trees occupied by each liana species following
Burns (2007). Furthermore, all liana–host interactions
were recorded individually, regardless of multiple host
interactions, as primary and secondary host trees could
not be distinguished in such cases in which ‘canopy
bridges’ had formed.
Liana guilds and habitat associations
Liana species were allocated to guilds a priori according to
their vertical distribution of host trees. Three guilds were
established. (1) The ‘canopy’ guild included all species
that grew to the top of host trees. (2) The ‘trunk’ guild
encompassed species that produce the majority of their
foliage in close association with the main stem of host
trees, directly below the canopy. All species within this
guild were in the genus Meterosideros and climbed hosts
using specialized roots. (3) The ‘base’ guild encompassed
species that typically grew along the forest floor and climb
only the base of host trees. All members of this guild were
ferns that climb hosts with specialized roots, and seldom
grow above 3 m. To test the accuracy of guild designations, we measured the maximum heights of all lianas
encountered during sampling. One-way fixed factor
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ANOVA assessed differences in maximum height obtained
from each plant guild.
To characterize the general patterns of lianas and their
hosts we used a series of individual analyses. We tested
whether liana and host distributions differed among habitats using individual one-tailed exact non-parametric
Mann-Whitney U-tests for each species. Separate chisquared tests were conducted to test whether lianas from
the same guild were partitioning habitats. For all lianas, we
assessed whether the number of host interactions differed
between habitats using simple logistic regression. Tests for
interspecific differences in habitat preferences differ from
tests for guild-based negative co-occurrence patterns (see
co-occurrence analyses below) in that they investigate
interspecific differences in horizontal distributions, rather
than vertical differences in distribution. However, in both
instances, differences in distribution would promote negative co-occurrence patterns. All data were transformed to
meet normality assumptions when required and all analyses were conducted using the R environment.
Co-occurrence analyses
First, a community-level matrix was constructed, which
assessed broad-scale liana–host interactions. This matrix
was then subdivided to assess structural patterns displayed
by compartments within the matrix. All compartments
were generated by subdividing the rows and columns of
the community-level matrix by known biological information (Fig. 1). As such, we investigated both quantitative
Fig. 1. An example of matrices defined at five levels (i.e. community-level matrix, host species compartments, host species matrix, liana guild
compartments and liana guild matrix). Host tree species are differentiated by symbols (square, triangle or circle: e.g. the first three columns represent
three individuals of the same species) and liana species are differentiated by letters (rows A to I). Liana species are labelled on each host tree to indicate
their position of occurrence. Dashed lines with attached arrows indicate the approximate region of host colonization and guild classification. In all
matrices, interactions are represented by 1 in the corresponding matrix cell. Absences are represented by empty cells for clarity.
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and binary matrices to resolve the patterns of interactions
found among lianas and host trees.
We first subdivided the columns (individual host trees)
of the community-level matrix to create compartments for
each host species. Here, we defined these as ‘host species
compartments’ and they represent binary interactions of
individual trees with each liana species. The number of
host species compartments was dependent on the number
of host species observed. Therefore matrix size differed
between compartments, and the patterns that they might
display indicate the role of host preferences in communitylevel interactions. In the final step of analysing columns of
the community-level matrix, we amalgamated all ‘binary’
columns of each host species compartment to create the
‘host species matrix’. The host species matrix represents a
typical species interaction matrix without interaction frequency between species pairs.
Second, we subdivided rows (liana species) of the
community-level matrix to create compartments of liana
species that are typically found on the same regions of
host space. Here, we defined these as ‘liana guild compartments’ that contained three groups corresponding
with the liana guilds defined above. Null model results
indicate whether lianas within each guild co-occur nonrandomly. In the final step of analysing rows of the
community-level matrix, we amalgamated all rows of
each liana species compartment to create the ‘liana guild
matrix’. The liana guild matrix now consisted of three
rows representing presence or absence of each guild. Null
model results indicate whether liana species that utilize
disparate regions of host space co-occur together.
In all matrices, when an interaction between liana
species and a host tree was observed the cell was given
a value of ‘1’; otherwise it was given a value of ‘0’.
Null model simulations re-arrange these 1 and 0 values
according to null model constraints. Binary matrices
were assessed using fixed-equiprobable null model
constraints (i.e. fixed rows and relaxed column constraints) and randomized using the sequential swap
method of matrix fill. Quantitative matrices followed a
similar procedure while accounting for liana and host
abundance distributions. The total number of lianas for
each species, in each plot, was fixed. Hosts were randomly
sampled without replacement from a list of pooled host
frequencies found in each habitat. Each list included
host interaction frequencies, rather than the number of
individuals, to account for multiple interactions with
each host tree. Matrices generated for north and south
habitats were merged to create the community matrix.
Null model simulations of liana guild compartments and
the liana guild matrix were also derived from the community matrix. This is a quantitative approach that
accounts for the underlying distribution of abundances
and provides a set of constraints that accounts for habitat
partitioning.
We used the c-score to calculate negative co-occurrence patterns among liana species at each level. The
c-score calculates the average checkerboard distribution
of all species interactions within each liana–host interaction matrix. The c-score index was obtained by calculating
the number of checkerboard units ‘cu’ for each species
pair: cu = (Oi S)(Oj S), where Oi is the total number of
host species occupied by liana species i, Oj is the total
number of host species occupied by liana species j, and S is
the number of host species occupied by both species
(Stone & Roberts 1990).
The observed c-score was compared to 5000 simulation
replicates, and all observed values greater than null model
expectations indicated support for negative co-occurrence
patterns. All c-score simulations were carried out using
ECOSIM version 7 (Gotelli & Entsminger 2001) for binary
matrices or the R environment for quantitative matrices.
All results were assessed using a Bonferoni correction
(a/5 = 0.01) to adjust for exploratory analyses of different
compartments.
Null model simulations are susceptible to matrix size and
fill (Gotelli 2000). Therefore, to account for small matrix
size in compartments, all species recording less than five
interactions were omitted from analysis. Therefore, two
dicotyledonous lianas (Clematis paniculata and Rubus cissoides), one canopy emergent (Podocarpus totara), one canopy tree (Pittosporum eugenioides), six shrub species (Alectryon
excelsus, Brachyglottis repanda, Coprosma lucida, Myrsine australis, Pseudopanax arborius, Olearia rani), two tree ferns
(Cyathea medulla, Cyathea dealbata), two liana species represented as hosts (Metrosideros fulgens and Ripogonum scandens)
and two unknown host species were omitted. To ensure all
matrices were comparable, the standardized effect size
(SES) was assessed in each null model simulation. Matrix
size (number of cells) was transformed to meet normality
assumptions and correlated with SES values using linear
regression. If a relationship was not found, these matrices
were considered comparable.
Results
A total of 217 trees occurring in all plots met the criteria
for analysis. Approximately 60% of trees hosted (n = 125)
lianas and the remainder (n = 92) lacked lianas. Two
dicotyledonous lianas, Parsonsia heterophylla (A. Cunn.),
Passiflora tetrandra, and one monocotyledonous liana,
Ripogonum scandens (G. Forst.) were assigned to the ‘canopy’
guild. Three dicotyledonous lianas, Metrosideros fulgens
(G. Forst.), M. perforata (A. Rich.) and M. diffusa (G. Forst.)
were assigned to the ‘trunk’ guild. Three climbing ferns,
Blechnum filiforme (A. Cunn.), Microsorum pustulatum
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Liana co-occurrence patterns
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125
3
3
5
7
9
4
8
6
8
14
11
36
11
68
14
8
45
21
11
36
24
10
237
1
0
0
2
0
0
0
2
0
5
2
1
0
0
0
0
1
0
2
6
3
1
0
2
0
0
1
0
0
7
7
0
2
0
0
0
0
0
0
9
6
0
4
1
0
0
1
0
0
12
2
0
0
4
3
0
0
3
0
12
0
0
0
7
2
0
1
3
0
13
1
1
0
4
4
2
2
1
0
15
4
1
0
7
2
0
3
2
1
20
7
3
1
2
2
3
0
3
0
21
7
5
0
4
3
1
3
1
3
27
5
1
1
7
4
1
2
7
0
28
Canopy 23
Canopy
1
Canopy
0
Trunk
5
Trunk
1
Trunk
4
Base
22
Base
2
Base
4
62
Ripogonum scandens
Passiflora tetandra
Parsonsia herterophylla
Metrosideros fulgens
Metrosideros perforata
Metrosideros diffusa
Blechnum filiforme
Microsorum pustulatum
Microsorum scandens
Frequency of host
interactions
Number of host trees
Frequency
of liana
occurrence
Cyathodes
fasciculata
Corynocarpus
laevigatus
Hedycarea
arborea
Geniostoma
rupestrae
Dacridium Macropiper
cupressium excelsum
Dicksonia
squarosa
Prumnopitys
ferruginea
Melicytus Knightia
ramiflorus excelsa
Beilschmidia
tawa
Elaeocarpus
dentatus
Dysoxylum
spectabile
Guild
Tree density differed among habitats (north = 77,
south = 140); although the distribution of individuals did
not differ (Mann Whitney U-test: z = 0.437, P = 0.662).
At the species level, four out of 13 host tree species
showed significant differences between habitats (one
tailed exact Mann-Whitney U: E. dentatus, z = 2.300,
P = 0.032; G. rupestre, z = 2.635, P = 0.008; M. ramiflorus,
z = 2.652, P = 0.008; and D. spectabile, z = 2.795,
P = 0.008). All remaining host tree species showed no
significant differences (n = 8; P 4 0.134). This result indicates that there is an overlap in tree species distribution;
however, certain species characterize the north- and
south-facing habitats in this reserve.
Liana density was similar among habitats (north = 106,
south = 131; Mann-Whitney U: z = 0.408, P = 0.683),
however, when we considered each species individually,
two out of nine lianas showed greater abundance in the
south-facing habitat (one-tailed exact Mann-Whitney U:
R. scandens, z = 2.220, P = 0.032; and B. filiforme, z =
2.471, P = 0.016) and one liana species showed greater
abundance in the north-facing habitat (M. pustulatum,
z = 2.471, P = 0.016). All remaining liana species showed
no significant differences between habitats (n = 6; z 4
1.928, P 4 0.095).
Height measurements generally support quantitative
guild designations. There was a significant difference in
maximum height obtained by each climbing guild (oneway ANOVA; f2, 211 = 19.97, P o 0.001). The canopy guild
reached an average maximum height of 7.52 m; the trunk
guild reached an average maximum height of 5.07 m; and
the base guild reached an average maximum height of
2.2 m. M. diffusa and P. heterophylla recorded different
overall heights to other members in their respective
guilds. In this study, these species were observed as
Host tree species
Liana guild and habitat associations
Liana species
(G. Forst.) and M. scandens (G. Forst.) were assigned to the
‘base’ guild.
Lianas occurred on two canopy emergent trees (Dacrydium cupressinum and Knightia excelsa), six canopy trees
(Melicytus ramiflorus, Elaeocarpus dentatus, Beilschmiedia
tawa, Dysoxylem spectabile, Corynocarpus laevigatus, Prumnopitys ferruginea), one sub-canopy tree species (Hedycarya
arborea), three shrub species (Macropiper excelsum, Geniostoma rupestre, Cyathodes fasciculata) and one tree fern
(Dicksonia squarrosa) (Table 1). D. spectabile, G. rupestre and
M. excelsum were almost exclusively found in the southfacing habitat and collectively represented the highest
number of individual trees without lianas (n = 62). M.
excelsum and G. rupestre occurred in the north-facing
habitat, however they typically remained free of lianas
(n = 6 and n = 4, respectively).
Table 1. Host tree and liana species with 5 or more interactions are ordered by frequency of occurrence in descending order. Rows are further divided into guild categories. All host trees and liana
species registering less than 5 interactions were omitted from analysis and not included in this table.
Blick, R.A.J. & Burns, K.C.
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60
South
North
31
30
22
19
20
10
Trunk
Canopy
Base
Trunk
6
Par het
2
2
Met ful
0
2
Met per
Guild
Pas tet
Par het
Rip sca
Guild
Met ful
Met per
Met dif
Guild
Ble fil
Mic sca
Mic pus
Base
Mic pus
2
0
77
65
Pas tet
10
2
18
14
Met dif
4
34
Ble fil
6
40
Mic sca
Observed abundance
Mean maximum plant height
8
0
50
50
10
Rip sca
12
Canopy
Fig. 2. Mean maximum height (m) obtained by three liana guilds and
their host trees. Bars with open fill represent liana species. Bars with
closed fill represent liana guilds. Error bars indicate standard errors.
Species names abbreviated to first three letters of genus and species.
Fig. 3. Observed abundance of each liana in the north-facing habitat
(closed fill) and south-facing habitat (open fill). Species names abbreviated to first three letters of genus and species.
juveniles and were encountered less frequently than all
other species. For all remaining species, our method for
classifying lianas into guilds accurately accounted for host
tree partitioning (Fig. 2).
When we considered guild associations, we found
that liana species in the base guild and the trunk guild
differed significantly between habitats (x2 = 53.479, df = 2,
P o 0.001; x2 = 7.512, df = 2, P = 0.023, respectively). This
result was not consistent with the canopy guild (x2 = 3.165,
df = 2, P = 0.205) (Fig. 3). These results indicate that lianas
typically grow in preferred habitats and that lianas with
similar growth strategies grow in divergent locations. However, lianas that use twinning or tendrils to reach the
canopy do not partition the habitat clearly.
Not all trees were occupied by lianas. Trees that lacked
lianas (n = 92) were generally small (mean height = 2.9 m,
mean DBH = 23.6 mm), although some trees were from
larger size classes (max height = 15.1 m, max DBH = 199
mm). The number of occupied hosts was similar between
habitats (n = 53, n = 72, respectively; one-tailed exact
Mann-Whitney U: z = 0.335, P = 0.738). However, five
out of nine liana species occupied host trees more
frequently in their preferred habitat (simple logistic
regression: M. perforata, score = 30.713, df = 1, P o 0.001;
M. fulgens, score = 27.674, df = 1, P o 0.001; M. pustulatum,
score = 37.207, df = 1, P o 0.001; M. scandens, score = 5.776,
df = 1, P = 0.016; B. filiforme, score = 16.887, df = 1, P o 0.001.
R. scandens showed a weak significant difference in host use
between habitats (score = 3.514, df = 1, P = 0.061). All remaining liana species did not occupy host trees preferentially
in either habitat (n = 3, score o 0.721, df = 1, P 4 0.396).
Co-occurrence analyses
Co-occurrence patterns differed strongly between the
community-level matrix and generated compartments.
Negative co-occurrence patterns were found in the
community-level matrix as the observed c-score was
significantly higher than randomized expectations
(CS = 4.472, ZS = 6.543, P o 0.001) (Fig. 4). However, the
individual matrices generated for north and south habitats revealed contrasting results to the community-level
matrix (CS = 0.286, ZS = 4.702, P o 0.001; CS = 1.583,
ZS = 0.302, P = 0.381, respectively).
Host species compartments revealed results consistent
with null model expectations for all species (P 4 0.041).
The host species matrix revealed positive co-occurrence
patterns, as the observed c-score was significantly lower
than expected by chance (CS = 4.47, P = 0.004).
Liana guild compartments and the liana guild matrix
gave a range of results. The observed c-score for the base
guild (CS = 5.000, ZS = 1.974, P = 0.024) showed some
evidence for negative co-occurrence, while the trunk and
canopy guilds were not significantly different to randomized expectations (CS = 0.000, ZS = 4.795, P o 0.001;
CS = 3.330, ZS = 0.498, P = 0.309, respectively). The liana
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Blick, R.A.J. & Burns, K.C.
Fig. 4. Null model simulations for the community and associated compartments following the layout of Fig. 1. Matrix evaluations failed for the liana
guild matrix and one host species compartment (Geniostoma rupestrae) and are omitted here. If the observed c-score (indicated by arrows on the x-axis)
is significantly greater than 5000 simulations, it would indicate negative co-occurrence patterns. Host species names are abbreviated to the first letter of
genus and species.
guild matrix gave a null result (CS = 1.000), as matrix
evaluations gave a single index of 0.816. This may have
occurred due to an inflated number of links (i.e. 1 values) in
the amalgamated matrix.
No significant correlation was found between matrix
size and SES (r2 = 0.102, P = 0.668). The results were
interpreted as matrix size was not responsible for differences in null model simulations.
Discussion
Negative co-occurrence patterns were observed at the
community level, and analyses of compartments within
the community-level matrix provided some resolution of
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the mechanisms underpinning community-level analyses. Null model simulations of each habitat and each
host species compartment revealed habitat-level divergence in species interactions. Analysis of liana and host
abundance distributions support the hypothesis that negative co-occurrence in the community matrix is derived
from lianas that interact with host species in preferred
habitats. When we generated liana guild compartments,
we found negative co-occurrence patterns in the base
guild. This was due to differences in horizontal partitioning of habitat space and interactions with host trees that
grow in the same location.
The community-level matrix revealed significant negative co-occurrence patterns indicating the presence of
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Blick, R.A.J. & Burns, K.C.
host-specific interactions. This type of community structure is observed when many species interact in reciprocal
pairs, with the absence of generalist species interactions
(Bascompte & Jordano 2007). In plant–plant interactions,
two contrasting community-level patterns are recognized
(Blick & Burns 2009). Mistletoe–host interactions often
reveal a high degree of host specificity, in which establishment success appears to be host limited (Norton & De
Lange 1999; Mathiasen et al. 2008; Watson 2009). Alternatively, epiphyte–host interactions reveal nested patterns (Burns & Dawson 2005; Burns 2007) similar to
those seen in plant–pollinator and plant dispersal networks; in which the community is structured with a core
set of generalist species and relatively few specialists
(Olesen et al. 2007). Here, the community-level matrix
indicates similar structural patterns to those seen in
mistletoe–host interactions, indicating host preferences
in the community.
In contrast to the community matrix, each habitat
alone and all host species compartments revealed results
consistent with random expectations. This suggests that
lianas colonize host trees randomly within each habitat,
and negative co-occurrence patterns found in the community-level matrix are derived from divergent species
assemblages. Liana and host abundance distributions
support the notion that geographically separated species
constrain the number of possible liana–host interactions.
Similarly, ‘functional biogeographical’ units have been
found embedded in the community matrix of nectarivorous
birds (Carstensen & Olesen 2009) and in marine food webs
(Rezende et al. 2009). These functional units consist of
spatially organized species assemblages; however, the processes underlying these patterns remain largely unresolved.
Here, we suggest that liana–host interactions are structured
through indirect processes relating to habitat; however,
further testing is required to validate this hypothesis.
Liana guild compartments showed varying results. The
base guild revealed negative co-occurrence patterns,
while the trunk and canopy guilds revealed neutral or
positive results. Alone, negative co-occurrence patterns
indicate that the base guild is partitioning host space
vertically through unshared allocation of resources (competitive exclusion principle) (Hardin 1960). However, if
we consider abundance distributions across the northand south-facing habitats we find an alternative explanation. The base guild partitioned habitat almost exclusively, while the trunk and canopy guilds showed less
marked differences. Therefore, co-occurrence patterns
reflect abundance distributions in each compartment and
the community-level matrix. More broadly, this suggests
that one guild may be influencing the outcome at the
community level. We do not preclude structural characteristics as a mechanism altering liana–host interaction;
however, our results indicate that spatial organization of
individuals among habitats is important. Further testing is
required to identify the specific processes that lead to
these habitat associations.
The host species matrix revealed positive co-occurrence
patterns – a result opposite to that of the community
matrix. This result suggests that lianas are co-occurring on
host species (e.g. facilitation). Facilitative interactions have
been recorded previously (Putz 1984; Pérez-Salicrup & de
Meijere 2005), and community-level positive co-occurrence patterns have been found in other ecosystems (see
Sfair et al. 2010). However, this result does not support
what was found using an individual-based approach to
analyse the community-level matrix or host preferences
previously found at this site (Burns & Dawson 2005). This
illustrates the necessity for investigating species interactions
using individual-based null models that account for abundance or intensity of interactions among individuals (Krishna et al. 2008; Blick & Burns 2009).
Liana–host interactions are an important part of tropical and temperate ecosystems (Schnitzer 2005). Lianas
compete for nutrients and exploit other trees to obtain
available light in the forest canopy. Accessing available
light may influence co-occurrence of liana species (Putz
1984; Pérez-Salicrup & de Meijere 2005). However, our
results indicate that habitat preference is an important
component underlying community-level patterns of liana–host interactions in Otari-Wilton’s Bush Reserve.
Further investigation into liana–host interactions should
account for habitat preferences, which accurately characterize host availability and host use.
In summary, we found habitat preferences among liana
species that would have otherwise been interpreted as
partitioning of host space (liana guild compartments).
Overall, liana–host interactions found in this study system
follow an opportunistic growth strategy, and interactions
are due mostly to habitat partitioning. While we have
used a relatively species-poor plant–plant interaction network, we have illustrated how the underlying patterns of
distribution can affect negative co-occurrence patterns.
Acknowledgements
We thank Tom Philippi, Bruce Allen and two anonymous
reviewers for their helpful comments. We also thank
Victoria University of Wellington for financial assistance
and the Otari-Wilton’s Bush Reserve staff for permission
to conduct this study.
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