BiologicalJournal ofthe Linnean Society (1995),55: 29-44. With 4 figures
Convergence and historical effects in harvester
ant assemblages of Australia, North America, and
South America
RODRIGO G. MEDEL
Departamento de Ciencim Ecohgicas, Facultad de Ciencias, Universidad de Chile,
Cmilla 653, Santiago, Chile
Received 1 1 May 1994; acceptedforpublication 7 September 1994
In this paper I examine the extent to which contemporary ecological patterns in 42 harvester
ant assemblages of three continents can be explained as a result of present-day environments
or from differences in the history of each ant biota. The contribution of each factor to the
overall variability of six community characters was evaluated by the ANOVA procedure.
The method revealed absence of convergence in three-continent and pairwise-continent
analyses in almost every community attribute that was measured. Significant convergence was
detected only in the foraging score for the North America-South America comparison. This
implies that the foraging mode used by ants for searching seeds is more similar within similar
environments in the two continents than between different environments in the same continent.
Significant historical effects were much more prevalent than convergence both in threecontinent and pairwise-continent comparisons. Abundance at baits, body size, and foraging
distance were more variable within similar environments in different continents than between
different environments in the same continent. The overall absence of convergence documented
in this study suggests that constraints related to the evolutionary history of each species
assemblage have inhibited convergent evolution in response to local selective pressures.
ADDITIONAL KEY WORDS:-ANOVA - desert - convergent evolution - phylogeny.
CONTENTS
Introduction . . . . . . . .
Material and methods . . . . . .
Study sites and sampling methodology .
Statistical procedure . . . . .
Results
. . . . . . . . .
Convergence . . . . . . .
Historical effects . . . . . .
. . . . . . . .
Discussion
Acknowledgements . . . . . .
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References
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INTRODUCTION
It is now widely accepted that species inhabiting similar habitats but different
regions may evolve in similar ways, originating morphological and ecological
0024-4066/95/050029+16 $08.00/0
29
0 1995 The Linnean Society of London
R.G. MEDEL
30
equivalents among independently evolved biotas. Most evidence giving support
to the idea of convergence comes from tests on morphological and behavioural
traits of individual species (e.g. Cody, 1974; Mares, 1976; Ricklefs & Travis,
1980), but analyses at the level of guilds or communities are more equivocal
(Terborgh & Robinson, 1986).
One of the most critical problems for testing convergence directly is that
similarity and convergence are not equivalent concepts (Schluter, 1986). To
demonstrate similarity among two independently evolved assemblages does
not imply that they underwent convergence, because the same degree of
similarity might have been attained from a wide range of initial ancestral
states. Consequently, in the absence of knowledge about ancestral assemblages
it may be difficult to conduct an unequivocal test for convergence. Instead,
two indirect approaches have been used. The ‘outgroup’ approach contrasts
the level of similarity observed among assemblages in similar habitats but in
different regions with the level of similarity among the focal assemblage with
a ‘control’ assemblage in a different habitat but in the same region. If the
similarity among the hypothesized convergent species assemblages is higher
than their similarity to the ‘control’ assemblage, it suggests that convergent
evolution produced the actual similarity (e.g. Fuentes, 1976; Blonde1 et aL,
1984). The problem with this approach is that it is very sensitive to which
assemblage is selected as ‘control’ (Orians & Paine, 1983; Wiens, 1991). A
second approach involves quantifylng the relative contribution of habitat and
history to the overall variation in the metric of interest by means of an
analysis of variance (ANOVA) procedure (Schluter, 1986). This method has
at least two important advantages over the ‘outgroup’ approach. First, by
knowing the amount of variation required for convergence to be statistically
significant, it is possible to assess degrees of convergence rather than all-ornone results. Because conclusions about convergence may differ depending
upon which variable is analysed, the ANOVA procedure provides information
about the relative sensitivity of each variable to exhibit convergence. Second,
it permits one to quantify the relative contribution of habitat or some abiotic
variable and historical factors to the overall variation in the character selected
for analysis. These factors deserve special attention in view of recent assertions
that studies of convergence may benefit from including the phylogeny of
species and the history of different continents (Westoby, 1988; Mares, 1993).
Despite the advantages of the ANOVA approach, its most severe shortcoming
is the large amount of comparable field measurements of independently
evolved biotas needed to carry out the analyses. Ideally, assemblages should
be sampled in a variety of habitats or abiotic conditions using the same
methodology. These requirements are rarely met and lack of replication
within habitats and continents continues to be an important obstacle to
estimating convergence by the ANOVA procedure.
Harvester ants are among the most important granivorous taxa in deserts
of the world (Brown et aL, 1979; Davidson et aZ., 1980). Previous global
comparisons of the ant biotas of desert and Mediterranean ecosystems have
failed to reveal similarities attributable to convergent evolution (e.g. Kusnezov,
1957; Hunt, 1973; Orians et aZ., 1977). Even more specific tests of convergence
carried out in desert harvester ant assemblages have failed to reveal
convergence between Australia and North America concluding that historical
’
CONVERGENCE IN HARVESTER ANT ASSEMBLAGES
31
differences among continents are more important in determining ant
community structure (Morton & Davidson, 1988). The notion of historical
effects has been used in the literature to denote either the influence of past
local or regional contingencies on contemporaneous ecological patterns (e.g.
Gould & Woodruff, 1990; Herrera, 1992), or as a synonym for ‘phylogenetic
constraint’ to denote that the time-course of phylogeny sets limits on the
evolution of individual species (e.g. Felsenstein, 1985; Brown & Maurer,
1987). Although the conceptual limits between these two categories of
phenomena are difficult to trace unequivocally, in the context of this article
I will consider historical effects in relation to the former meaning, that is,
in relation to the regional or continental contingencies outside the realm of
local ecological processes that potentially affect the structure of species
assemblages.
The aim of this paper is to test the hypothesis of convergence in harvester
ant communities of Australian, North American and South American arid
zones by examining the relative contribution of present-day environments
and historical effects to community characters. I apply Schluter’s ANOVA
method to test if the variability in the structure of harvester ant assemblages
is greater between localities with different mean annual precipitation within
a continent than that with similar precipitation on different continents, as
expected from the hypothesis of convergence. Ideally, the hypothesis should
be tested with reference to the past states of communities, but in the absence
of such information, I will interpret the results as indirect evidence of
community-level convergence. According to the ANOVA procedure, the
absence of convergence need not imply that historical factors have been
important. Instead, history will be important when the variability among
localities that differ in precipitation within continents is significantly lower
than the variability of localities with similar precipitation on different
continents.
MATERIAL AND METHODS
Study sites and sampling methodology
The data base included a total of 42 harvester ant communities inhabiting
deserts of Australia, North America and South America (Fig. 1). Information
for Australian and North American ant communities was gathered from
Morton & Davidson (1988) and Davidson (1977), respectively. I obtained
data for 16 South American ant communities, including 11 from the Monte
desert (Argentina) and 5 from Chilean arid zones. The total number of
communities considered for analyses by continent was 16 in Australia, 10 in
North America and 16 in South America (Table 1). Field work in South
America was carried out during the summer seasons of 1990 and 1991. It
has long been recognized that in deserts, seed production and primary
productivity are best estimated by the amount of precipitation (Beatley,
1969; Rosenzweig, 1968). Although seasonal and interannual variability in
precipitation may be at least as important as the mean value to characterize
habitats in arid zones (Crawford & GOSZ,1982; Ezcurra & Rodrigues, 1986),
granivorous species richness and population density tend to be positively
1500
Figure 1. Map indicating the geographical regions and study sites in arid and semiarid zones of Australia, North America, and South America. south
American communities are M y named in the Appendix.
1200
-
AUSTRALIA
'
'm
CONVERGENCE IN HARVESTER ANT ASSEMBLAGES
33
TABLE1. Number of harvester ant communities used in this
work, by continent and precipitation intervals
Continent
Mean annual
precipitation
Australia
North America
South America
50-99
100-149
150-199
200-249
250-299
300-349
2
6
4
2
2
3
2
3
2
-
6
3
3
2
2
correlated with the mean annual rainfall in arid zones in the range of 0300 mm (Brown, 1975; Davidson, 1977; but see Abramsky & Rosenzweig,
1984). Several other variables such as vegetation structure and soil type may
contribute importantly to characterize habitats in arid zones. However, the
absence of truly comparable field measurements for the 42 communities in
the three continents restricted the analysis to the local mean annual
precipitation as a macroscopic habitat descriptor.
South American study sites were selected after inspection of precipitation
data files in the Ministerio de Obras Piiblicas for Chile and in the Servicio
Meteorologico Nacional for Argentina. The range of precipitations included
in the analyses was 119-338 mm for Australian localities, 76-276 for North
American localities, and 43-332 for South American localities (see Appendix).
Previous inspection of data revealed that intervals of 50 mm of mean annual
precipitation were appropriate to evaluate variability between continents
without losing replicates within continents. Consequently, each assemblage
was assigned to a precipitation interval of 50 mm according to the mean
annual precipitation recorded for the site. With this classification, all categories
were represented in at least two continents, as required for analysis, except
for the first precipitation interval (0-49) where only one assemblage was
present. I decided to include such locality (Caldera, South America, 43 mm)
in the next precipitation interval (50-99) (Table 1).
In order to make the data from South America comparable with those of
the other two continents, I used a sampling procedure similar to that used
by Morton & Davidson (1988). At each study site, a grid of 80 petri dishes
in a configuration of 8 x 10 was laid out. Dishes were buried at ground level
and spaced regularly 5 m one from each other. Each petri dish was filled
with cracked millet seeds and four strips of masking tape were set up to
facilitate the access of ants to seeds. Baits were offered to ants approximately
2 h before the first census. Baits were replenished as needed. To sample ant
species that were active at different temperatures, censuses were conducted
at soil temperatures of 25", 35", 45" and 50°C. A fifth census was made 2
h after sunset to include ants with nocturnal activity. Each census consisted
of the visual inspection during 60 s of the species foraging at each bait. I
considered an ant to be a seed-eater when it was observed foraging in the
seed baits or transporting seeds to the nest. Ant species observed at seed
baits were therefore included as part of the harvester ant assemblage, even
34
R. G . MEDEL
though some species were not primarily seed-eating (e.g. Acromyrmex).
Identification of species followed Kusnezov (1978) for Argentinean ants, and
Snelling & Hunt (1975) for Chilean ants.
Six community attributes were singled out for analyses. (1) Species richness,
calculated simply as the number of species present in the assemblage. (2)
Alpha diversity, calculated by the Shannon diversity index, H’ = - Cpilnpi,
where pi is the relative abundance of each species. Abundance was measured
as the number of baits occupied by species i as a proportion of the total
number of baits occupied by all species in the grid. A bait was considered
occupied by species i when it was present in the bait at any census period.
Although most time individual colonies occupied only one bait, this measure
gives weight to species with large colonies foraging in several baits
simultaneously (e.g. Acromyrmex). (3) Number of occupied baits, calculated as
the sum of the occupied baits by the total of species in the assemblage. (4)
Community body size, calculated from the mean size of each species. Body
size was estimated indirectly by measuring the head width of ants with a
binocular microscope (precision 0.05 mm). Because size polymorphism is
variable among communities (Davidson, 1978; Oster & Wilson, 1978), I
measured the size of every species, regardless of whether they had been
measured in other localities. An average number of 13 workers of each
species was measured. Community body size was calculated by the index,
BS = C$pi, where ti is the mean body size of species i, and pi represents its
relative population abundance as previously defined. (5) Community foraging
distance, measured as the linear distance from the nearest nest entrance to
the baits foraged by workers. This community character was calculated by
the index, D = Cdipi, where di is the average foraging distance of species i,
and pi is the relative abundance of species i as before. (6) Community
foraging score reflects the foraging mode used by any species while searching
and collecting seeds. A score of 1 was assigned to species with individual
foraging and a score of 0 to species with columnar foraging mode. A score
of 0.5 was assigned to species having a mixed foraging. This assignment
permits calculation of a foraging score as, S = Cfipi, where fi is the foraging
score of the species i, and pi its relative abundance as previously defined.
Statistical procedure
Following Schluter (1986), the total variance of each community attribute
was partitioned into the three components:
KQtd=
vp+V+V
where
is the total variance of the community character, V, is the
fraction of total variance attributable to effects of precipitation, V, is the
portion of total variance attributable to continental or historical effects, and
V, is the random error component, measured as the mean square error in
ANOVA. Because of the unbalanced sampling design, data were analysed
within the framework of GLM for higher efficiency. I used an additive model
with no interaction term among continent and precipitation intervals assuming
that variance components were additive when ant assemblages were replicated
within cells. This assumption proved to be reasonable after comparison of
CONVERGENCE IN HARVESTER ANT ASSEMBLAGES
3.5
the MSe in a first ANOVA that included missing values and the MSe in a
second ANOVA with missing values replaced with their predicted values
from the first ANOVA (Schluter & Ricklefs, 1993). Variance components for
each attribute were computed from the two-way ANOVA using community
values as replicates (Sokal & Rohlf, 1981, see details in Schluter & Ricklefs,
1993). Since replicates in some cells included communities from the same
continent, unknown effects of spatial autocorrelation among localities may
influence the final results. In order to assess such possibility, I performed a
second analysis that included the average of their community characters. I
calculated convergence according to the index
where C represents the fraction of the total variance in the community
character that is among precipitation categories. C ranges from 0 to 1. When
C = 0, it reflects total absence of convergence, when C = 1, it indicates
complete convergence and 0 error within precipitation intervals. Similar to
convergence, historical effects were calculated by
where H reflects the proportion of the total variance that is among historical
groups or continents. V, and Kod are defined as before. H may vary
between 0 and 1 indicating from nil to complete importance of historical
effects. The statistical significance of convergence and historical values was
assessed by standard two-way ANOVA. I made three-continent and pairwisecontinent analysis for every community attribute.
RESULTS
Convergence
The analysis of all three continents combined revealed non-significant C
values for every community character both in replicated communities as in
the average values (Table 2). Painvise continent analyses revealed that
Australian and North American communities exhibited low and non-significant
C values for every attribute in the replicate and the average analysis (Table
2). This finding gives support to previous conclusions about the absence of
convergence in the structure of harvester ant communities in these two
continents (Morton & Davidson, 1988). A similar situation was observed in
the comparison between Australia and South America (Table 2). Similarly,
ant communities in North America and South America, had low C values
in most attributes except in foraging score, which had a significant 0.327 C
value in the analysis including replicated communities and a significant 0.843
in the average community analysis (Table 2). Overall, these results revealed
absence of convergence in the simultaneous three-continent comparison and
in the three pairwise comparisons both in the replicate as in the average
analysis.
R. G . MEDEL
36
TABLE
2. Values of convergence (C) and historical effects (H) of harvester ant communities
of Australia, North America, and South America. Indexes range from 0 to 1 indicating nil
and complete convergence, and historical effect, respectively. R = analysis with replicated
community characters within continents and precipitation interval. A = analysis with averaged
community characters within continent and precipitation interval
~
~~
~
Australia
North America
South America
Community variable Analysis
Species richness
Alpha diversity
Number of occupied
baits
Body size
Foraging
distance
Foraging
score
R
A
R
A
R
A
R
A
R
A
R
A
C
0.048
0.079
0.015
0.142
0.016
0.088
0.035
0.047
0.019
0.182
0.039
0.394
H
0.077t
0.400*
0.033
0.030
0.140*
0.203t
0.491***
0.651***
0.232**
0.379**
0.034
0.220'
Australia
North America
C
0.064
0.115
0.088
0.195
0.167t
0.149
0.012
0.073
0.139
0.369
0.044
0.4707
H
0.132t
0.429*
0.051
0.022
0.044
0.039
0.392**
0.716**
0.032
0.256*
0.032
0.154t
Australia
South America
C
0.051
0.577t
0.012
0.645t
0.053
0.124
0.077
0.280
0.033
0.065
0.069
0.119
H
0.002
0.089t
0.043
0.023
0.164*
0.430.
0.038
0.050
0.039
0.000
0.003
0.280*
North America
South America
C
0.089
0.097
0.093
0.100
0.060
0.060
0.002
0.009
0.059
0.177
0.327*
0.843*
H
0.122t
0.230t
0.012
0.016
0.009
0.109
0.728***
0.844***
0.497***
0.485**
0.047
0.012
t P < 0.1, * P <0.05, **P<0.01, ***P<0.001
Historical effects
I7i?-ee-continent analyses
The overall analyses of the three continents revealed a marginally significant
difference in species richness. This trend was amplified in the average analysis
where assemblages revealed a significant historical effect (Table 2). In contrast,
low and non-significant historical values in alpha diversity were detected
(Table 2). A significant historical effect was found in the number of occupied
baits with South American communities attaining the highest values in almost
every precipitation interval (Table 2, Fig. 2). Because the sampling procedure
gives weight to large colonies however, it is possible that these differences
tend to reflect larger colonies rather than high population abundance.
Community body size showed significant differences between the three
continents in the two analyses which can be attributed to the high values
exhibited by North American ant species (Table 3). This is graphically
depicted in Figure 3 where North American communities conspicuously fall
above the average line. Regarding foraging distance, significant historical
effects were detected which can be attributed to the high distance covered
by North American species while searching for and collecting seeds (Table
3, Fig. 4). Finally, ant assemblages did not show significant H values in
community foraging score which suggests that ants utilize roughly similar
foraging modes to search for seeds on a per-locality base. However, the
average analysis revealed significant differences in the community values
which suggests that on the average Australian species tend to forage
individually rather than in a columnar form (Table 3).
CONVERGENCE IN HARVESTER ANT ASSEMBLAGES
400
v)
c
.0
n
U
.-
P)
5 200
37
0
t8
0
0
El
0
0
0
0
'c
0
0
z
0
I00
200
300
Mean annual precipitation (mm)
Figure 2. Abundance (as reflected by the number of occupied baits) of Australian (A), North
American (n),and South American ( 0 )harvester ant communities. The straight line connects
the mean values by precipitation interval.
TABLE3. Mean values of each community attribute in Australia, North
America, and South America. Figures indicate meanfSE
Continent
Community variable
Australia
Species richness
Alpha diversity
Number of occupied baits
Community body size
Community foraging distance
Community foraging score
>r
'D
0
1.2
8.50f 1.04
1.54f 1.96
106.06f25.28
0.85 f0.12
2.95* 1.04
0.61 +0.18
A
0
North America South America
5.20* 1.40
1.30f0.32
115.70& 58.88
1.25 f0.12
4.92f 1.48
0.53f0.12
7.62f 1.76
1.58f0.28
204.06 f54.34
0.83 f0.06
2.19f0.60
0.50f0.08
El
0
0
n
.->r
C
n
t
0.0
A
0
- 8
0
E
0
A
ha
0
0
0
6
A
0
A
0.4
I
I00
I
I
200
I
I
300
Mean annual precipitation (mm)
Figure 3. Body size values of Australian, North American, and South American harvester ant
communities. The straight line connects the mean values by precipitation interval. Symbols
as in Fig. 2.
38
R.G. MEDEL
Mean annual precipitation(mm)
Figure 4. Foraging distance values of Australian, North American, and South American
harvester ant communities. The straight line connects the mean values by precipitation
interval. Symbols as in Fig. 2.
Pairwise-continent analyses
The comparison between Australia and North America generally revealed
low and non-significant H values except for some values in species richness,
body size, and foraging distance (Table 2). There was a weak historical effect
for species richness ( P = 0.067), with Australian communities having more
species coexisting locally than North American communities (Table 3). As in
the simultaneous three-continent analysis, this trend was amplified in the
average analysis. Body sizes had high H-components of variance in the
replicate and average analysis, with North American ants being significantly
larger than their Australian counterparts (Table 3). This results gives support
to previous conclusions of Morton & Davidson (1988) that Australian ants
are smaller and more tightly distributed along the size gradient than North
American ants. Foraging distance exhibited significant differences on the
average but not on a per-locality base (Table 2).
The comparison between Australia and South America revealed nine nonsignificant H values and only three significant historical values (Table 2).
South American ants had almost two-fold greater abundance at baits than
Australian ants (Table 3). Because these two continents did not show
significant differences in species richness, results are consistent with previous
findings that Australian ants tend to be locally more rare than ants of other
continents (Morton & Davidson, 1988; Morton, 1993). Assemblages differed
in the foraging score only in the average analysis (Table 2).
Comparing North America with South America, significant historical effects
were detected in body size and foraging distance (Table 2). However, South
American communities had a marginally significant higher species richness
than North American ones which is consistent in the replicate and average
analysis (Table 2). North American ants exhibited significantly higher values
of body size and foraging distance that South American ants (Table 3). This
effect is sufficiently intense to be reflected on a per-locality base and on the
CONVERGENCE IN HARVESTER ANT ASSEMBLAGES
39
average between continents as judged by the highly significant H values in
the two analyses.
DISCUSSION
Community characters were more variable within precipitation intervalsbetween continents than between precipitation intervals-within the same
continent, suggesting that the mean annual precipitations that characterize
environments have a small importance in determining harvester ant community
structure relative to the history of continents. But why has convergence not
been detected in this particular study? The reasons may lie in the methodology
utilized in testing convergence as well as historical biotic and abiotic
differences among continents. Although the large number of communities
within precipitation intervals and continents used in this study suggests that
the observed variation is real, implicit in the ANOVA procedure is that any
factor leading to an increase in the pooled variance of the community
character will tend to underestimate convergence and historical effects. One
such factor might be the geographical complexity within continents (Medel,
unpublished manuscript). Both North American and South American arid
and semiarid zones are disrupted by mountain ranges that create substantial
local and regional variation in climate and vegetation. For instance, the data
base of South American communities consists of two faunas separated by
the Andes Mountain Range since approximately the Miocene when the
Andes experienced its major uplift. The Andes Mountain Range has caused
striking differences in the harvester ant fauna of the two regions, such as
the higher seed removal rate and the four-fold greater species richness in
the Monte desert in comparison to Chilean arid zones (Medel & Vasquez,
1994). Differences among the two deserts may be related to the high
diversification of harvester ant genera such as Pogonomyrmex in Argentinian
arid zones (Kuznezov, 1963), and to the colonization of other granivorous
genera such as Pheidole and Ehmopheidole from tropical and subtropical areas
to the Monte desert (Kusnezov, 1951a, 1951b). The geographical complexity
occasioned by the Andes Mountain Range within South America may have
increased the pooled variance in the ANOVA, rendering low C values in
continent comparisons.
Another potential factor for the absence of convergence is that localities
of different continents, although similar in mean annual precipitation, may
not be strictly comparable in other abiotic variables (Niemi, 1985; Wiens,
1991). Even though habitats appear similar regarding one abiotic factor, they
differ importantly in other variables. It is quite possible that characterization
of habitats based on mean annual precipitation has obscured more meaningful
abiotic variables for arid and semiarid zones. For instance, Australian ants
inhabit far more habitats than North American ants within a similar range
of precipitation (Morton, personal communication), which suggests that there
may be no necessary connection between precipitation intervals and habitats
for harvester ants. In addition, rather than long-term averages of annual
precipitation, the coefficient of variation within and between years may be
more meaningful when rainfall is temporally and spatially unpredictable in
40
R. G . MEDEL
arid regions (Crawford & GOSZ,1982; Ezcurra & Rodrigues, 1986; Armesto
et al., 1993).
Edaphic variables have been previously suggested as the most important
abiotic factor underlying differences in the ant fauna between continents
(Morton & Davidson, 1988). Australian soils tend to be less coarse and fertile
than the soils of North American arid zones (Morton & James, 1988;
Westoby, 1988). South American arid zones have been described as having
infertile soils, ranging from clayish to rocky depending on the locality (Beek
& Bramao, 1969). Chilean lowland arid zones are characterized by an
assortment of broad valleys and mixed landscapes which have mainly infertile
sandy soils devoid of vegetation (Killingbeck, 1993). At higher altitudes, soils
are predominantly derived from volcanic ash, which also retain little water
(Arroyo et al., 1988). The Argentinian Monte desert has landscape units of
deep valleys surrounded by low mountains called ‘bolsones’. Each landscape
unit consists of a variety of soils such as badlands, muddy spots, dunes, and
small salt marshes, that create substantial variation across short distances
(Morello, 1958; Mares et al., 1985). Although differences in edaphic conditions
may be important in generating the observed differences among the three
continents, in the absence of quantitative measures it is not currently possible
to evaluate the effect of these abiotic variables by Schluter’s (1986) procedure.
Contrary to the convergence effects, historical effects were statistically
significant in several pairwise and three-continent comparisons. Phylogeny
and degree of specialization of ants in seed consumption must have been
important historical factors as suggested by differences in the taxonomic
membership at each continent. In general, most harvester ant genera are
concentrated in the subfamily Myrmicinae (Holldobler & Wilson, 1990). This
situation is well represented in North American deserts where all species
sampled belong to this subfamily. North American harvester ants consist of
the genera Messor, Novomessor, Pheidole, Pogonomyrmex, and Solenopsis. A
somewhat more diversified harvester ant fauna was observed in Australian
arid zones where 66% of the species reported in Morton & Davidson (1988)
belong to the Myrmicinae, and 34% to the Formicinae. Australian genera
consist of Chelaner, Melophorus, Meranoplus, Monomorium, Pheidole, and
Tetramorium, all classified as specialist granivores by Andersen (1991). South
America had the taxonomically most diversified harvester ant fauna, with
52% belonging to the Myrmicinae, 24% to the Dolichoderinae, 19% to the
Formicinae, and 5% to the Pseudomyrmecinae. The four genera that evolved
a granivorous form in arid South America are Elasmopheidole, Pheidole,
Pogonomymzex, and Solenopsis (Kusnezov, 1956). However, in addition to these
genera, in South America I detected seed removal by Amomyrmex, Araucomyrmex,
Brachymyrmex, Dorymyrmex, Forelius, Prenolepis, and Pseudomymzex (see Appendix).
Although the biology of South American desert ants is poorly known, it is
quite possible that most species inhabiting arid zones tend to be omnivorous,
feeding on seeds opportunistically. Differences in the taxonomic diversity of
ants involved in seed removal and the variable degree of specialization of
ants may thus represent an important historical constraint precluding
convergent evolution.
Differences in the accompanying fauna may also be an important historical
factor precluding convergence among continents. For instance, experimental
CONVERGENCE IN HARVESTER ANT ASSEMBLAGES
41
studies among harvester ants, rodents and birds in North American deserts
provide convincing evidence of intense resource limitation (Brown & Davidson,
1977; Davidson, 1985; Brown et aL, 1986; Thompson et aL, 1991). On the
contrary, it has been suggested that seed limitation is not an important factor
in determining the ant community organization in arid Australia (Briese,
1982; Westoby et aL, 1982) presumably due to the presently depauperate
fauna of granivorous rodents (Morton, 1979; Morton & Baynes, 1985), and
to the low importance of seed-eating birds (Morton, 1985). A relatively
similar biotic setting is observed in South America. Absence of granivorous
rodents and birds comparable to the specialized seed-eating species of North
America characterizes South American deserts (Orians et aL, 1977; Capurro
& Bucher, 1982; Brown & Ojeda, 1987). However, in the past, the Monte
desert in South America did support bipedal and presumably seed-eating
marsupials (Argyrolagidae), which became extinct in Pleistocene times
(Simpson, 1970; Mares, 1976; Mares et aL, 1985). The involvement of
different granivore taxa in the three continents suggests that ant community
evolution might be subject to dissimilar selective pressures and proceed in
different directions rather than toward similar states as predicted by the
convergence hypothesis. Consequently, contemporary patterns may be the
result of different species associations that evolved in the past under different
circumstances and are not related to present-day environments.
In this study I have examined the roles of convergence and history in
determining the structure of harvester ant communities inhabiting deserts of
Australia, North America and South America by using Schluter’s ANOVA.
The overall absence of convergence documented in this study suggests that
constraints related to the evolutionary history of each biota have inhibited
convergent evolution in response to local selective pressures in harvester ant
assemblages. Results give support to the assertion that the hypothesis of
convergence should be tested mainly at the level of individual species rather
than at the community or ecosystem level (Peet, 1978; Orians & Paine,
1983; Terborgh & Robinson, 1986). However, this does not imply that studies
directed to test convergence are meaningless. On the contrary, the hypothesis
continues to be of value for comparative studies because it permits
separation of present-day environmental from historical factors. Although
many regularities among communities can in principle be attributed to similar
environments, perhaps attempts to derive generalizations such as convergence
among independently evolved biotas run the risk of ignoring the importance
of unique evolutionary events (Wiens, 1991). Future studies of convergence
that quantify the contribution of historical factors such as phylogeny to the
overall variance in ANOVA, would greatly increase our ability to understand
the role of evolutionary history in the structure of present-day communities.
ACKNOWLEDGEMENTS
This paper has benefited greatly from comments made by Francisco
Bozinovic, Mauricio Canals, Diane Davidson, Fabitin Jaksic, Pablo Marquet,
Stephen Morton and Dolph Schluter. D. Davidson and S. Morton are also
acknowledged for providing information of their work in Australian and
North American deserts. Field work in South America was aided by Sergio
42
R. G. MEDEL
Herrera and Rodrigo Vasquez. Financial support was provided by Fundacion
Andes and grants FONDECYT 0821-90 and DTI (Universidad de Chile) B
3539-9312.
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APPENDIX
South American ant species involved in seed removal from baits. Figures indicate the abundance
of each species by locality, measured as the number of occupied baits when the five censuses are
considered simultaneously (see Materials and methods for details).
Localities
Species
Acromyrmex lobicornis
Acromyrmex mpersus
Araucomyrmex tener
Araucomyrmex goetschi
Brachymyrmex giardii
Brachymyrmex laevis
Brachymyrmex longicornis
Dotymyrmex enrifer
Dotymyrmex exsanguis
Ehopheidole
subaberrans
Forelius grandis
Pheidole sp.
Pogonomyrmex brevibarbis
Pogonomyrmex
cunicularius
Pogonomyrmex inermis
Prenolepis longicornis
Pseudomyrmex denticollis
Sohopsis gayi
Solenopsis granivora
Solenopsis saevissima
Solenopsis sp.
1
2
0
0
0
19
0
0
0
0
0
66
0
0
0
0
17
0
0
25
0
0
0
0
0
0
0
0
54
0
0
0
3
22
0
0
0
0
0
0 38
0
3
13 0
46
0
3
0
23
0
0
0
1 4 0
69 12
7 15
0
0
79
0
0
3
42
0
4
0
0
6
2
0
1
0
0
5
6
13 0
0
4
5
5
0
0
0
0
25 51
8
4
0
0
42 35
7
8
9 1 0 1 1 1 2 1 3 1 4 1 5 1 6
0
0 2 2
0
0 1 8
5
0
0
4 1 2
0
36
0
0 27
1
0 0 1 3
0 10 55
0
0
5
1
0
0 27 18 0
0 40 40
0
0
0
0
0
0
66
0
0
0
4
0
10
0
0
0
0
50
0
0
0
0
0
63
0
0
55
24
0
0
0
0
0
0
0
0
4
0
0
0
0
0 5 8
0
2 1 0 0 0 0
0 27
0
0 40
0
0
0
0
0
69
0
3 58
0
0
0
4
0
0
0
0
0
0 3 1
3
0
0 1 8
0 26 30
Localities
1. Caldera, Chile (27"10'S, 70"50'W), 43 mm.
2. El Balde, Argentina (30°40S, 68"30'W), 66 mm.
3. Villa Mazh, Argentina (28"40'S, 66"30W), 80 mm.
4. Castilla, Chile (27"50'S, 70"35'W), 84 mm.
5. Retamito, Argentina (32"04'S, 68O35'W), 87 mm.
6. Zonda, Argentina (31"40S, 68"45'W), 87 mm.
7. Cachiyuyo, Chile (29"04'S, 7O05O'W), 123 mm.
8. San Jose de Jachal, Argentina (30"10S, 68"40'W), 128 mm.
9. Huaco, Argentina (3O0O5'S, 68°25'W), 128 mm.
10. La Serena, Chile (29"52'S, 71"15'W), 162 mm.
11. Agua del Medio, Argentina (29"30'S, 68"25'W), 194 mm.
12. Villa Uni6n, Argentina (29"40'S, 68°10W), 194 mm.
13. Mascasin, Argentina (31"30'S, 67"10W), 270 mm.
14. Auc6, Chile (31"45'S, 7l0O0'W), 297 mm.
15. Andalgala, Argentina (27"45'S, 66'25'W), 307 mm.
16. Uspallata, Argentina (32"40'S, 69"25'W), 332 mm.
0
0
0
0
0
30
0
0
0
3
0
24
28
27 58
0
0
0
0
0
0
0
0
5 9 2
0
4
21
3
7
1
0
9
0
0
0
0
4
0
8
0
0 4
0
0
0 57
0
3
0
56
0
0
0
0
20
24
0
0
37
0
0
0
2
51
77
0
0
0
0
0
0
65
45
3
21
0
61
0
0
0
0
24
0
0
0
0 1 4
67
0
0
52
0
0
0
0
8
0
0
9
5
0
53
0
39
0
0
65
0
0
0
0
0
0
11 40
62
0
0
0
8 38
55 65
0
0
0
9
1
0