The Displacement of Native Ant Species by the Introduced Argentine Ant, Iridomyrmex humilis Mayr.
Psyche 78:257-266, 1971.
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THE DISPLACEMENT OF NATIVE ANT SPECIES BY THE INTRODUCED ARGENTINE ANT
IRIDOMYRMEX HUAdILIS MAYR*
Department of Entomology and Limnology
Cornell University, Ithaca, New York 14850 Many authors have described how Iridonzyrmex hunzilis Mayr has become a major pest throughout the world (Brun, 1924; Zimmei- man, 1940; Smith, 1947; Morley, 1953; Skaife, 1961; Pasfield, 1968). Once these ants become established in a locality they will not tolerate the existence of any other species of ants, and as the populations of each colony build up in density, they emigrate in all directions, consolidating as they go and driving other species before them. Not only does I. humilis displace native ant species, but it has been shown to displace other introduced tramp species. The ant
Pheidole megacephala F. is apparently a native to Africa and has been spread by commerce to almost all of the more humid parts of the world. It too is a serious pest and displaces native species. HOW- ever, in 1852, in Funchal, the capital of Madiera, this species was itself displaced by I. humilis (Stoll, 1898; Wheeler, 1906). The displacement of P. megacephala by I. humilis has also been observed in the Hawaiian Islands (Wilson and Taylor, 1967; Flukes and Beardsley, 1970) and in Bermuda (Haskins and Haskins, 1965; Crowell, 1968). Wilson (1951) reports that a local naturalist in Mobile, Alabama observed I. humilis displacing the imported fire ant Solenopsis saevissima richteri Forel, and Fluker and Beardsley (1970) reported the displacement of S. winata F. in Hawaii. Shapley (1920 a, b) describes an "intermittent war" between I. h1i77zi~is and the native California species which he feels would eventually eliminate most of the native ant species. Tulloch ( 1930) and Michener (1942) described the displacement of the California harvester ant Pogonomyrmex californicus by I. hunzilis. METHODS AND MATERIALS
In the present study, the displacement of three ant species, P. californlczts Buckley, Pheidole grallipes Wheeler, and Veromessor perganilei Mayr, by 7. humilis was observed for a six year period. Detailed observations of the displacement of P. californicus were *Manuscript received by the editor March 2, 1972 25 7
Figure 1. The forward advance of I. humilis as it displaces P. cali- fornicus in an old field from October 1963 to October 1968. Reference markers are located at intervals of 100 meters (0). made at six month intervals, whereas only minor observations were made of the other two species. Studies were carried out in an old field from May 1963 to October 1968 in San Luis Rey, California, two miles east of Oceanside in San Diego County. The study field consisted primarily of sandy soil with Bronzus rubens, Salsola Mi, Sonchus olcraceous, Heliotropiunz curassauicum, and Brassica nigra, the dominant plants. The study area was almost rectangular, being 300 meters wide by 500 meters long on the south side and 450 meters long on the north side. The total area of the field was 14.25 hectares. The field was bordered by California Highway 78 on the west, dirt field roads on the east and south, and by a grass lawn on the north. Two additional fields of 5 (2A)
and 7 (2B) hectares were located
at the southern edge of the main study field. Here studies on colony size, foraging distance, and food preference were carried out (Erick- son, 1972; and in manuscript).
All colonies of P. californicus and I. humilis were individually marked with color-coded wooden stakes placed one meter from the colony entrance. At intervals of approximately six months the posi- tion of each colony was noted on a large map, measured to the nearest one meter from the colony entrance using the reference
Table 1. The areas involved as I. humilis displaced native ant fauna in a southern California field. ^
Total area Total number Area added by Number of colonies Mean area per -. occupied by I. of colonies in Mean area I. humlis during in the newly colony in newly Date Jtumilis (m2) the whole field per colony time interval (m') displaced area displaced area (m2) 2 Oct. 1963
390 2 - 390 2 195
10218 6 1703
18926 15 1261 6 1385
29 Sep. 1964 w
3 May 1965 34225 2 1 1629 15299 12 1275 t*-y 3 Oct. 1965 48547 2 9 1674 14322 11 1302 r^
1 May 1966 68535 3 5 1958 19988 14 1428 .z rs
1 Oct. 1966
12058 10 1206 ^>i-
3 6 2594
12803 9 1422 o
29 Sep. 1967 106166 4'2 2527 12770 8 1596 å´-+
3 Jun. 1968 123654 47 2631 17488 12 1457 2 52 2648 13975
4 Oct. 1968 137699* 10 1398 t*s-
*Does not include 4670rn2 in the southeast corner of the field still occupied by P. californicus. Total area in-
volved therefore is 142,369 m2 or 14.24 ha. !^
**Value obtained by subtracting 14531 m2 underwater at the time of sampling from the total area involved. t-'k
260 Psyche [December
markers placed at 1co meter intervals throughout the field (see Fig. I ).
The line3 on Fig. I deliniate the limit of eastward ex- pansion of I. humilis colonies at the time the survey was taken. The new area added was then calculated by subtracting the total occupied area at the previous sampling date from the new total area occupied by I. huinilis. This area was then divided by the number of colonies in the new added territory to get the mean area per colony values. At the same time, quadrat sampling with 30 randomly placed 2 meter by 2 meter quadrats were carried out to determine the vegeta- tion characteristics. Weather data were taken from a station 3 miles northeast of the field site and averaged to get monthly mean tem- peratures and precipitation.
The displacement of the
three other ant species by I. humilis
started slowly in October, 1963, but increased to an almost constant rate from 3 May 1 964 to 4 October 1 968 (Table I, Fig. I ) . The new area added by I. humilis during each displacement interval of six months was approximately 14000 m2 ranging from 83 18 m2 to 19988 m2 in the study field. The mean area per colony of I. humilis for the whole occupied portion of the field increased during each sampling interval, whereas the mean area per colony in the newly displaced land was almost a constant 1400 m2 per colony. The num- ber of colonies of I. humilis increased during each displacement in- terval as the displacement proceeded whereas the number of colonies of P. californicus decreased except for a minor fluctuation due to flooding between 3 October 1965 and I
May 1966 (Fig. 2). By
October 1968, not a single colony of P. grallipes was observed in the study field, and by 5 March 1969 all colonies of P. californicus and 17. pergandei were located outside the boundaries of the study field. In their place remained 57 colonies of I. humilis. To explain this phenomenon of displacement, some comparison is necessary of the basic biology of the ant species involved. The nests of I. humilis are situated wherever there is sufficient moisture and where light is excluded, as under rocks and logs (Woodword, 1905, 1910 ; Eckert and Mallis, 1937, Smith, 1947) or in shallow nests in the soil (Cook, 1953). These ants occur in a wide variety of habi- tats - swamps, beaches, lawns and gardens, roadsides, houses, and various woodlands ( Crowell, 1 968). I. humihs are exceptionally rest- less ants and normally emigrate one or more times a season in search
Erickson - Displacement of Native Ant
The number of colonies in the entire field of P. californicus (0) and 1. humills (0) from October 1963 to October 1968. of more favorable habitats (Wilson, 1971). Colonies of I. humilis contain a large number of queens with thousands of workers ( Smith, 1947) and proliferate by swarming of detachments of workers who accompany secondary queens out of the nest (Wheeler, 1933 ; Wynne- Edwards, I 963 ; Crowell, I 968). They are highly omnivorous but tend to seek sweet or fatty foods (Eckert and Mallis, 1937; Creigh- ton, 1960 ; Cook, 1953), and tend aphids and scale insects in orchards and gardens (Skaife, 1961 ) .
In contrast with I. humilis, the California harvesters are large ants (4-6 mm long) which are primarily seed gatherers, but are also known to be slightly omnivorous (Van Pelt, 1966). Colonies of P. cdifornicus are small in comparison to I. humilis and contain only one queen.
Proliferation takes place by large swarms of winged reproductives.
The California harvester ant tends to nest in dryer semi-desert habitats and can tolerate much higher temperatures than I. humilis (Wheeler, I 926 ; Cole, I 932, I 968 ; Michener, I 942 ; Erickson, I 972) .
The relative reproductive potential of I. humilis is probably much higher than P. californicus. This is most likely due to the large num-
262 Psyche [December
ber of queens in each colony, the method of colony proliferation, and the omnivorous habits of these ants. These factors may also account for the great and rapid spread of I. humilis throughout the temperate regions of the world.
Basic differences in food resources limit to some extent the amount of competition between I. humilis and the three harvester ant species (Table 2). I. hu~nilis is highly omnivorous whereas the three har- vester ant species are only slightly omnivorous, being basically seed gatherers. I. humilis not only monopolizes the proven food sources but attempts to control the remaining foraging areas (Wilson, 197 I ) . In the main study field, food, especially seeds, were very abundant. The food chambers of P. californicus and I. humilis were always full when the colonies were excavated. In fields 2A and 2B, the area was supplemented with approximately five pounds of mixed grass seed per month to determine the foraging characteristics and distances for P. californicus. The seeds, colored with common food coloring, were fully acceptable to the ants, making up 43 to 59% of the P. californicz(s food stores and 9 to 17% of the I. humilis food stores. The variously colored seeds were spread in concentric circles from a nest of P. caJifornicus every 5 meters to a distance of 30 meters. The maximum foraging distance for P. californicus was about 10 meters except in areas where there was an I. humilis colony in which case the harvester ants foraged no farther than 5 meters even though the I. hzmilis colony was 20 meters away. Even though both fields were supplemented with a little over 50 pounds of mixed grass seed per year, in one year P. californicus was displaced 76 meters (2A) and 109 meters (2B) by I. hz~mIZis. It does not appear that this displacement is due to any overlap of a fundamental food dimension. At each sampling interval the mean area per colony of I. humilis in the newly displaced territory was approximately 1400 m2 whereas the mean area per colony for the entire field increased from 1400 m2 to approximately 2600 m2 during the five year period (Table I). There thus appears to be a minimal area for a colony of I. humilis in the newly acquired areas and as these colonies become established and increase in population density, the colony requires a larger area. Michenei- (1942) working with P. californicus encountered a similar displacement by I. hzunilis. He described in detail how in- dividual harvester ants would be set upon and killed by groups of I. Jnnnilis. When temperatures are cool, Pogonomyr/nex species tend to be sluggish and it is at this time that the Argentine ants torment the harvester ants as they forage around the nest (Michenes, 1942).
19711 Erickson -Displacement of Native Ant 263 Table 2. Food resources of I. humilis compared with species it has displaced throughout the world.
Main Degree of
Food Source Omnivory Reference
Iridomyrmex humilis Mayr sweet or fatty + + + Wheeler, 1910 foods, tends Eckert &
aphids scale Mallis, 1937
insects, grains Creighton, 1950
*Pogonomyrmex californicus seed gatherers + Wheeler, 1910 Buckley Forel, 1928
Van Pelt, 1966
I'he'do.'e megacQhala F. sweet or fatty + + + Wheeler, 1910 foods Fore], 1928
*Pheidole grallipes Wheeler seed gatherers + Eckert & Mallis, 1937
*Feromessor pergandei Mayr seed gatherers + Eckert & Mallis, 1937
Solenopsis saevissima Forel insects, fruits, + + + Creighton, 1950 grains, flowers, Cook, 1953
Solenopsis geminata F. insects, fruits, + + Creighton, 1950 grains Fluker &
ced in present study.
highly omnivorous; + = moderately omnivorous; = omnivorous
Should a harvester ant come upon an Argentine ant during the warmer parts of the day, the former grasps the smaller ant with its mandibles and stings it to death (Michener, 1942). At dawn, sun- set, or on a cloudy day the Argentine ants will attack and cling to the mandibles, legs, and antennae of the harvester ants and attempt to kill the larger ant. Observations made in the present study con- firm Michener's discussion of the aggressive actions between the spe- cies.
There were no significant differences in the mean monthly tem- perature or precipitation from month to month (i.e. - all the Janu- arys, etc.) over the course of the study. The vegetation studies
264 Psyche [December
similarly showed that there was no significant difference in the order of dominance of the six plants mentioned. It does not appear that P. californicus ameliorates the habitat as it does not clear vegetation as many harvester ant species do. I. hzimiizs does not utilize the same nest sites as the displaced species and in fact, not a single I. humilis colony was found within two meters of an abandoned harvester ant colony.
Pasfield ( 1968) found /. humilis displaced its neighbors at a max- imum rate of 274 meters (300 yards) per year in Australia. This value is higher than the 100 to 200 meters per year at Fort Shatter on the island of Oahu between 1940 and 1944 (Pemberton, 1944) or the average of 100 meters per year in the present study (Fig. I). Flukes and Beardsley (1970) observed I. hufnilis displace P. megace- phala in Hawaii at about 66 to 100 meters per year. All these values seem low when compared to the displacement rate of 8 kilometers (5 miles) per year for native species by the fire ant 8. saevissima in the Gulf states (Wilson and Brown, 1957). The effectiveness of competition in nature is best demonstrated by the impact of an invading species on the native fauna. It appears that here, there is a tremendous competition for nest space, which is the general case for highly aggressive territorial ant species such as Pheidole, Solenopsis, and Iridomyrmex
(Wilson, I 97 I ) . Three as-
pects of the populations biology of I. humilis gives this species a dis- tinct competitive advantage over the native harvester ants. The general aggressive nature of I. humilis as well as the large number of queens and method of proliferation allow these ants to move in and establish new colonies in a very short time. Raiding columns of
workers clear the way and pioneer groups of workers and queens follow into freshly opened nest areas.
I would like to thank my uncle, Mr. Carl Erickson, for the gen- erous use of his property and to my cousins, Bob, Tom, David. and JoAnn for their assistance in various aspects of the study. Thanks are also due to Mrs. Mudie of the Department of Botany at San Diego State College for determination of the plant species and Dr. A. C. Cole, Jr. of the University of Tennessee for identification of the ant species. Mr. F. Slansky, Jr. and Drs. W. L. Brown, Jr., R. G. Helgesen, and P. P. Feeny read the manuscript and gave many helpful comments. A grant from the Grace Griswold Fund assisted with the publication costs.
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