Philip S. Ward.
Ecology and Life History of the Rhytidoponera impressa Group (Hymenoptera: Formicidae). I. Habitats, Nest Sites, and Foraging Behavior.
Psyche 88:89-108, 1981.

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ECOLOGY AND LIFE HISTORY OF THE
RHYTIDOPONERA IMPRESSA GROUP
(HY MEN0PTERA:FORMICIDAE)
I. HABITATS, NEST SITES, AND FORAGING BEHAVIOR By Philip S. ward'
Department of Zoology, University of Sydney, N.S. W. 2006, Australia
The ponerine ants of the genus Rhytidoponera constitute a rich assemblage of species, widespread throughout Australia, with lesser representation in Melanesia and adjacent regions (Brown, 1 958; Wilson, 1958). On the Australian mainland they have collectively occupied a broad range of habitats, and often rank among the more abundant members of an ant community. Considerable interest centers on the unusual habit, apparently widespread in the genus, of reproduction by mated "workers" in lieu of a morphologically differentiated dealate queen (Brown, 1953, 1954; Whelden, 1 957, 1960; Haskins & Whelden, 1965).
The Rhytidoponera impressa group consists of a small, distinctive cluster of species occurring in mesic habitats (mostly rainforest and wet sclerophyll) along the east coast of Australia and in New Guinea. Until recently, the impressa group was thought to comprise no more than three species, all reproducing by means of distinct winged queens (Brown, 1953, 1954; Haskins & Whelden, 1965). However, recent studies of systematic relationships and colony structure in the impressa group have revealed the presence of at least 5 closely related species and the occurrence of reproduction by both queens and mated workers (Ward, 1978, 1980). There is a notable paucity of detailed ecological studies on
rainforest ponerines in general, and there have been no extensive field studies on Rhyzidoponera. This paper summarizes information on habitat and nest site preferences, colony densities, and various aspects of foraging, in the impressa group. A second paper describes life cycle and reproductive patterns (Ward, 1981). 'Present address: Department of Entomology, University of California, Davis, California 95616
Manuscript received by the editor April 15, 1981.



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Psyche
Data were gathered during a survey of the Rhytidop- impressa group from approximately 100 mesic forest sites in e; s
Australia and New Guinea. A detailed tabulation of these colle - sites is given in Ward (1978). Field work was carried out e October, 1974 to October, 1978, with a few additional collect!- IB May-July, 1980. Voucher specimens from these collections 1 been deposited in the Australian National Insect Collection (A- CSIRO, Canberra.
In rainforest and wet sclerophyll forest the collection proc- 4 was as follows: colonies of the impressa group were s o u g l - examining all rotting logs, loose stones and other potential n e s c which were encountered during a more or less random undirected) walk through a tract of suitable forest. In most loc- 3 a tally was kept of the number of "potential nest sites" (log: stones) sampled. The "rotting log" count was confined to rotten logs in middle to late stages of decay, with num-
preformed cavities (corresponding roughly to the "zorapteran- - b'passalid'7 stages of Wilson, 1959), since field observations sh - that recently fallen or dessicated logs were rarely inhabited. single large log was dissected in two places more than 1 meter - it was counted as two potential nest sites. Records from rottin-z include a few instances where ants also nested in soil below th- Stones ranging in areal size from about 100 to 1500 cm2 -
recorded as potential nest sites if they rested completely o: ground and could be easily overturned. Fallen epiphytic fern rrÌö- - on the rainforest floor were also considered potential nest s i t e s were examined and counted in areas where they occurred. AX = invariably, a single colony occupied only one nest site, so the - "colony" and "nest" are used in equivalently in this paper. When an impressa group colony was located, an attempt usually made to collect the entire colony contents, i.e. all WOI 3
reproductives, and brood. This entailed considerable excavati - rotting wood and/or soil. Where only colony fragments 7 believed to be collected, this was noted. Collected colonies were returned to the lab and their c o i - enumerated. A few were maintained in modified Janet or L u b nests. The majority were frozen for electrophoresis. Field observations of foraging behavior, colony movement, a




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19811 Ward- Rhytidoponera impressa. I 9 1 dispersal, and mating behavior were also made. In addition, field observations and collections of related Rhytidoponera species from Australia, New Guinea, and New Caledonia provided some com- parative data.
Habitat Preferences
The known members of the Rhytidoponera impressa group and their respective distributions are as follows (Ward, 1980): chalybaea Emery (= cyrus Forel), New South Wales, southern Queensland, New Zealand (introduced); confusa Ward, Victoria, New South Wales, southern Queensland; enigmatica Ward, New South Wales; impressa Mayr, Queensland; and purpurea Emery (= splendida Forel), northern Queensland, New Guinea. Most species in the impressa group occupy a considerable range of latitude, altitude and forest types; and all species show partial sympatry with at least one other species (Table 1). In this context, a sympatric association is defined as the occurrence of two (or more) species within the dispersal range of their alates. In all cases of sympatry, non-conspecific nests were located within several hun- dred meters of one another, and in most instances within 50 meters. Despite the overlap between species, differences in habitat prefer- ences are apparent.
R. confusa is essentially a species of wet sclerophyll forest and temperate rainforest. In Victoria and southern New South Wales it is principally confined to lowland wet sclerophyll, and does not occupy cool temperate rainforest of the type dominated by such trees as Nothofagus, Quintinia, and/or Atherosperma. At the northern limit of its range, confuia is restricted to temperate and subtropical rainforest at moderate to high elevations. Thus, there is an inverse relationship between elevation and latitude (Figure l), and the regression of altitude on latitude indicates an average shift of about 70m per degree latitude.
In contrast to confusa, chalybaea is common in subtropical rainforest of northern New South Wales and southern Queensland (where confusa is rare or absent). At the southern limit of its distribution, chal-ybaea is confined to disturbed lowland habitats. Thus, in the Sydney region, it occurs commonly in well-watered parks and gardens, and only penetrates wet sclerophyll and



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Table 1.
Summary of habitats occupied by 1 15 Rhytidoponera impressa group populations sampled during the present study. Figures in parentheses refer to the number of populations occurring sympatrically with at least one other species. Vegetation categories correspond approximately to those of Specht et al. (1 974, Tables 2.1 and 4.1). "Subtropical rainforest" is equivalent to the warm subtropical rainforest of Webb (1978), while "temperate rainforest" corresponds to Webb's (1978) submontane and cool subtropical types.
- -
confusa
57(9) 27'-38's
5- lOOOm
16(4) 30(3)
8(2)
- 2 - 1
chalybaea
34(9) 21'-34OS
5-lOOOm
4(3) 5(3)
15(1)
- 2 2
- -
6(2)
enigmatica 5(5) 34's 10- 180m 4(4) - - I(]) impressa 7(1) 17O-27OS 350-1050m - - 5 - 1 - purpurea 12(1) 7'- 18's 30- 1250m - - 2 9(1) - 1 - impressa
group 1 15(25) 7O-38OS 5- 1 250m 24( 1 1) 35(6) 30(3) 1 O(2) 4 4 8(3)



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Ward- Rhytidoponera impressa. I
DEGREES LATITUDE (South)
Figure 1.
Altitude and latitude of 57 populations of confusa (open circles) and 34 populations of chalybaea (closed circles). Regressions of altitude on latitude for confusa (upper line) and chalybaea (lower line) are highly significant (p < .001). rainforest gullies which are ecologically very disturbed, i.e. heavily encroached with introduced weeds such as Lantana, Ligustrum and Tradescantia.
Sympatric associations between chalybaea and its sibling species, confusa, occur in some of these disturbed gully sites, with confusa preferentially occupying the vegetationally less disturbed portions of the gully. These two species also occur sympatrically in stands of undisturbed temperate and subtropical rainforest in northern New South Wales and southern Queensland. In this region chalybaea tends to occupy more xeric microhabitats than confusa, but in one locality (an isolated patch of rainforest at Boonoo Boonoo Falls, N.S. W.) no obvious nest site or microhabitat differences were found between the two species, which nested within a few meters of one another.
R. chalybaea also shows an altitudinal shift with increasing latitude (Figure 1) and tends to occur at lower elevations than confusa. The general picture is one of partial ecological differentia- tion between these two species despite their very close morpho- logical resemblance (cf. Ward, 1980).




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94 Psyche [Val- 88
Table 2. Nest site records for the Rhytidoponera impressa group, excluding small, incipient colonies (5 20 workers). Figures in parentheses represent the percentages (for each species) of colonies occupying a given type of nest site. Rotten Fallen
Species Logs Stones Epiphytes Total
confusa 258 143 11 412
(62.6) (34.7) (2.71
chalybaea 145 19 1 165
(87.9) (I 1.5) (0.6)
impressa 13 1 0 14
(92.9) (7.1) (0.0)
purpurea 34 0 0 34
(100.0) (0.0) (0.0)
enigmat ica 0 2 1 0 2 1
(0.0) (100.0) (0.0)
all species 450 184 I2 646
(69.7) (28.5) ( 1.9)
R. enigmatica is a localized species, known only from wet sclerophyll vegetation in sandstone gullies (6 sites, including two ANIC records) and urban parkland (1 site), the latter record coming from an area where the original habitat would have been sandstone gully vegetation. The range of elevation from which it has been recorded is 10 to 180 meters. Thus, with regard to habitat preference enigmatica is the most stenotopic species. Most of the known populations are in sympatry with, or in close proximity to, populations of confusa and/ or chalybaea. The 7 impressa populations studied come from tropical rainforest (I), subtropical rainforest (3, and dry rainforest (1). These data, along with 30 other collection records in the ANIC, indicate that impressa is confined to Queensland rainforest at altitudes ranging from 30m to 1050m.
Based on the 12 populations studied here plus additional records from the ANIC and from Wilson (1958),purpurea is recorded from subtropical and tropical rainforest (and one population from dry microphyll rainforest on the Mt. Windsor Tableland) in northern Queensland (30m to 1200m), and from tropical montane rainforest (600m to 1300m) in Papua New Guinea. In north Queensland it occurs in both primary-growth and partially disturbed rainforest,



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198 11 Ward- Rhytidoponera impressa. I 95 while New Guinea records indicate a predilection for second-growth montane rainforest.
Nest Site Preferences and Densities
Members of the impressa group are found nesting mostly in rotten logs and under stones. Nests are multi-chambered, but not highly fragmented, seldom penetrating deeper than 15-2Ocm into soil, or occupying more than lm length of rotting log. Nest entrances are cryptic, without conspicuous mounds of excavated material.
Fallen epiphytes on the rainforest floor are occasionally utilized as nest sites by confusa and chalybaea. Duringthe present study no colonies were found in living epiphytes on trees, although there are single records of a colony-founding purpurea queen (Brown, 19 54) and a mature purpurea colony (Wilson, 1958) from fern epiphytes on rainforest trees.
Nest site records from the present study are summarized in Table 2 which lists, for each species, the number of colonies collected from rotten logs, under stones, and in fallen epiphytes. Excluded from this table are a small number of single records from other nest sites. Thus confusa was also found nesting in a Banksia lignotuber, in a rotting bracket fungus, directly in the soil, and (twice) in an
abandoned termite mound in rainforest. A chalybaea colony was located under the bark sheath of an Archontophoenix palm, and in urban areas this species occupied less orthodox nest sites (e.g. in and under rusting metal, under concrete slabs, and in crevices along a stone wall). Threepurpurea colonies (two in north Queensland, one in Papua New Guinea) were observed nesting in cavities in the trunks of living rainforest trees, and in New Guinea this species may be primarily an arboreal nester (Wilson, 1958; records in ANIC). Table 2 shows that there is a clear trend towards greater specialization in the rotten log nest site in species of more tropical latitudes. The difference between confusa and chalybaea with respect to numbers of logs and stones utilized is highly significant (xf = 33.0, p < .OOl) and the difference between chalybaea and purpurea is also significant (xi = 4.4, p < -05). In contrast to all others, enigmatz'ca (the localized species of wet sclerophyll gullies) appears to nest exclusively under stones. In 70 populations (from 63 localities, due to some sympatry) a tally was kept of the number of "potential" nest sites (rotten logs,



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[Vol. 88
FREQUENCY OF POTENTIAL NEST SITES
Figure 2. Within-species frequencies of utilized nest sites as a function of potential nest site frequencies, for 5 irnpressa group species. Closed circles refer to log nest sites, open circles to stones.
stones, fallen epiphytes) encountered as well as the number of actual nest site occupancies (Table 3). It seems clear that nest site availability varies from species to species. For both rotten log and stone nest sites there are positive correlations (r = 0.94, p < -02, in both instances, arcsine transformed data) between the proportion of a species' colonies found in a particular nest site and the relative frequency of that nest site for the species (Figure 2). This suggests that species-specific preferences are partly a function of nest site availability. (No such correlation is found for fallen epiphytes- confusa showns the highest preference for this nest site despite its relative rarity in the southern rainforests; however, the numbers are in all instances rather low.)
The relative abundances of species can be crudely compared by



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19811 Ward- Rhytidoponera impressa. I 97 Table 3. Numbers of potential nest sites (pns) sampled and actual nests encoun- tered, for 70 impressa group populations. No.
Species populations Logs Stones Epiphytes Total confusa 37 no. pns I838 2984 92 49 14
no. nests 227 9 8 8 333
nests/ pns . I24 .033 .087 .068
chalybaea 22 no. pns l I64 I I36 70 23 70 no. nests 14 I I7 I 159
nests/ pns .I21 .0 I5 .0 I4 .067
impressa 4 no. pns 260 126 7 393
no. nests 8 I 0 9
nests/ pns .03 I ,008 .000 .023
purpurea 5 no. pns 404 I09 24 537
no. nests 2 I 0 0 2 1
nests/ pns .052 .000 .000 .039
enigmaiica 2 no. pns I05 56 I - 666
no. nests 0 I5 15
nests/ pns .000 .027 -027
all species 70 no. pns 377 I 4916 193 8880 no. nests 397 131 9 537
nests / pns .I05 .270 .047 .060
examining the proportion of potential nest sites which are occupied. (The desirable complementary data on absolute densities of poten- tial nest sites for different geographical regions and habitats are not available). Comparing the density figures (Table 3) for confusa and chalybaea, the former occupies a significantly greater proportion of stone nest sites than chalvbaea (xf = 9.7, p < .Ol), but no differences exist in the proportion of suitable rotten logs occupied, and the
overall nest densities (considering all potential nest sites) are the same for the two species. Nest densities are considerably lower for impressa, purpurea, and enigmatica. Rhytidoponera confus~ and
chalybaea utilize a significantly greater proportion of rotten logs
than impressa and purpurea (contingency x2, p <.OOl, for all four comparisons), despite the greater importance of rotting logs as nest sites in the more northerly (tropical) species. This may be partly the result of greater competition for nest sites in the species-rich tropical rainforests. R. confusa and chalj~baea are often common and dominant ants in temperate and subtropical rainforests, respec- tively, of New South Wales and southern Queensland where the



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98 Psyche [Vol. 88
numbers of sympatric rainforest ant species are probably about one- quarter to one-half that experienced by purpurea in north Queens- land rainforest.
It is unclear why there is a disproportionate decline in the utilization of stones as nest sites in the more tropical members of the impressa group (Table 3) and perhaps for tropical rainforest ants in general (cf. Wilson, 1959, p. 440). One possibility is that in subtropical and tropical rainforests on well-drained soils, stones frequently lie on subsoil below the thin organic horizon and offer an environment poorer in immediate food resources and more de- manding for nest excavation than rotting logs. In temperate and some subtropical rainforests of New South Wales, soil horizons tend to be less sharply stratified and/or litter decomposition is slower, so that humic material extends below the level of loose stones.
Effects of Sympatry
Nest site densities for sympatric and allopatric populations of confusa and chalybaea are given in Table 4. Both species occupy a significantly greater proportion of log nest sites in allopatric populations (contingency x2, p < .O 1 and p < .OOl , for confusa and chalybaea respectively) and confusa inhabits a greater proportion of stone nests sites allopatrically (x? = 5.4, p < -05). The lower sympatric densities of confusa and chalybaea could be a result of sympatric associations occurring in more marginal environments. However, the combined sympatric nest densities are very similar to the allopatric densities of both species. There are no significant differences between the total proportion of rotting logs occupied sympatrically and the proportion utilized allopatrically by either confusa (x? = 0.7) or chalybaea (x? = 1.8). The combined sympatric nest density under stones is the same as that for allopatric confusa populations. While these results could be coincindental, it seems more reasonable to conclude that sympatry has a depressant effect on relative abundance, and that competition for nest sites, food, or foraging space is important.
Other Sympatric Congeners
Other, more distantly related Rhytidoponera species also co- occur with members of the impressa group. R. victoriae (s.1.) is a common species (or complex of species) present in rainforest and other mesic habitats along the entire east coast of Australia. R.



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Table 4. Number of potential nest sites sampled and the proportion occupied, for sympatric and allopatric populations of confusa and chalybaea.
Rotten Logs Stones
No. No. potential Proportion No. potential Proportion Species Populations nest sites occupied nest sites occupied confusa (sympatric with chalyhaea)
5 280 07 1 650 018
confusa (allopatric)
3 2 1558 133 2334 037
chalybaea (sympatric with confusa)
5 280 043 650 0 15
chalybaea (allopatric)
17 884 146 486 0 14




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100 Psyche [Val. 88
victoriae is considerably smaller than the impressa group species, and nests preferentially under stones.
In some north Queensland localities, purpurea or impressa coexist with one of several small Rhytidoponera species (e.g. chnoopyx and kurandensis nesting in logs and under stones) and with one of several larger species (scaberimma and related species, nesting in logs and directly in the soil). There are no rainforest Rhytidoponera of comparable size to the impressa group species that regularly coexist with the latter with the exception of croesus (s.l.), which nests in rotten logs and in tree trunks in rainforest and wet sclerophyll of New South Wales and southern Queensland. R. croesus appears to be generally uncommon, and in fact averages slightly smaller than chalybaea, confusa and impressa to a degree which may significantly reduce prey size overlap (see below). Colonies of other Rhytidoponera species are virtually never found occupying the same nest site as an impressa group colony even though other medium to large ponerines such as Amblyopone australis, Leptogenys hackeri and Prionogenys podenzanai are occasionally found nesting in close proximity to an impressa group colony (e.g. under the same stone, or in adjacent cavities in a log). Colony Movement
It appears that species in the impressa group are prone to move colonies from one nest site to another rather frequently. For example, in one rainforest population of confusa (Royal Natio nal Park, N.S.W.) eight stones under which colonies had been briefly located and otherwise left undisturbed were examined one week later: half were unoccupied. Three weeks later, only two colonies remained under the stones. While the censussing no doubt consti- tuted a disturbance conducive to nest-movement, it demonstrates nevertheless the readiness with which colony movement is carried out.
During the course of field collections, vacated nest chambers were occasionally encountered (under stones or in rotten logs) whose previous occupants could be traced to an impressa group species on the basis of cocoon remains in the middens. Moreover, colony movement involving transport of brood and other workers was observed several times in chalybaea (and in other Rhytidoportera species outside the impressa group) (Ward, 198 1).



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19811 Ward- Rhytidoponera impressa. I
Foraging and Food- Retrieval
Members of the Rhytidoponera impressa group are partly predacious on other arthropods, but also scavenge for dead insects, seeds, animal feces, etc. Capture of live prey is achieved by a short lunge forward, coincindent with rapid closure of the outstretched mandibles. Prey thus captured are subdued by stinging. In most species, foraging occurs principally on the ground, among leaf litter and rotting logs. However,purpurea workers were frequently observed foraging on low foliage of understorey plants, as well as on the rainforest floor, in north Queensland. In Papua New Guinea this species nests (at least partly) arboreally, but limited observations (Wau; September, 1975) suggests that it tends to