LOCAL FORMICA ULKEI MOUND- WITH Departmentant Formica ulkei Emeryat Palos Park, Illinois, has been knownfor several decades. In recent years various phases of ecology and physiology

1932] Distribution of Formica ulkei 127

LOCAL DISTRIBUTION OF FORMICA ULKEI MOUND-NESTS WITH REFERENCE TO CERTAIN

ECOLOGICAL FACTORS

BY W. A. DREYER

Department of Zoology, University of Cincinnati

and

THOMAS PARK

Department of Zoology, University of Chicago

Introduction

The existence of a number of mound-nest colonies of theant Formica ulkei Emery at Palos Park, Illinois, has beenknown for several decades. In recent years various phasesof ecology and physiology of this ant and its nests havebeen analyzed and published. Holmquist (1928) reportedat length on its life-history and hibernation; Orlando Park(1929) discussed certain myrmecocoles associated with itin the nests; Thomas Park (1929) reported the occurrenceand behavior of a thief-ant intimately found in the mounds,and Dreyer (1932) has analyzed certain aspects of the res-piratory behavior of this ant in relation to its hibernation.Up to the present time no accurate description of the ex-tent and distribution of these mounds has been attempted.In this paper they will be discussed with reference to theirindividual form, their distribution, and certain environ-mental factors correlated with this distribution.

1 The authors take pleasure in acknowledging their appreciation toProfessor W. C. Allee, of the University of Chicago, for his kindlyaid and criticism of this work.

128 Psyche December

Field Survey

The Palos Park region lies in the Valparaiso moraineabout twenty miles southwest of Chicago. It is a rollingupland, situated to the south and east o the Des Plainesriver, traversed by the Sag, a small stream flowing in awesterly direction into the Des Plaines. Most o the regionis covered with a thick growth o the mesic oak-hickorytype of subclimax forest characteristic o the morainic up-lands o the Chicago region. (Cowles 1901). The mound-nests o Formica ulkei ound here are concentrated in anarea o approximately one quarter mile square in whichthe forest is cut up by a number of open lanes of swampymeadow providing for local drainage towards the west.Several temporary ponds are present in the spring but byearly summer there is no surface water except at times ofrain. The substratum consists of a thick layer of glacialdrift, yellow clay and boulders, with a thin layer, two tosix inches, o black humus surface soil.A careful survey o this area in which all the mounds

were tabulated according to size, location, and activity, re-veals the presence o 435 mounds of Formica ulkei. Thesevary in size from mounds about one foot in diameter andsix inches in height to five feet in diameter and three feetin height. Typically, the mounds have a somewhat mattedthin surface layer composed of finely divided soil and or-ganic debris such as twigs, stems, bits of leaves, and frag-ments of insects. The surface is perforated with many en-trances to the interior which is finely ramified with welldefined galleries, one to two centimeters in diameter. Theseextend throughout the mound and into the substratum downto the soil-water level from one to five or six feet belowthe surface. (Holmquist 1928). In most cases the moundsare free from living vegetation. The smaller mounds areround to oval at the base and bluntly conical in shape.The larger mounds are in general somewhat elongated andconsequently show a definite orientation in that the longaxis is east and wes:t. In addition, the south face has alonger more gentle slope with considerably greater surface

At the .resent time the Sag canal is used for drainage.

1932] Distr’ibution of Formica ulkei 129

area (sometimes double that o the north slope) than theshorter, steeper north slope. Among the 435 mounds tabu-lated, sixteen are dead or inactive colonies. In these casesthe mounds possess a weather-beaten appearance. The sur-face layer is not so evident, and vegetation has begun toinvade the soil o the mound.

In the enumeration and mapping o the mounds it wasobserved that their distribution in the area is definitely

Fi’. 1 Sample of typical distribution of Formicaulkei mound nests. Small, medium, and large moundsare indicated by the size of the black dots. The forestedarea is ruled with parallel lines. Clearings are shown bythe circles around the dots in the interior of the forest.Scale is 1/ inch 25 yards.

limited. The majority, 48.7%, are located along the margino the orest where it is traversed by open lanes or whereit adjoins open meadow to the east, north, west, and south-west. This number includes mounds o all sizes, classifiedas small, medium, and large, although the two latter classespredominate. A small percentage, 17%, nearly all oi thesmall type, are located in the open, i. e. 10 to 40 yards romthe orest margin. The remainder, 34.3%, invariably oc-cur in small open clearings in the interior o the orest.As in the case o the marginal nests the medium and largesizes predominate. The ollowing table gives the complete

130 Psyche [December

classification of the mounds as to size, and distribution onthe margin, open, or in a clearing. The accompanyingfigure, showing a typical portion of the map on which allthe mounds are located, clearly illustrates this sort of dis-tribution.

Margin Open Clearing TotalSmall 54 56 33 143Medium 74 11 48 133Large 84 7 68 159Total 212 74 149 435Per cent 48.7% 17.% 34.3% 100.%

Relation of Mound Distribution to Certain EnvironmentalFactors

In addition to mapping and classifying the mounds ofthis particular region, some attention was directed towardsthe status of the local environmental factors in order todetect any correlation between the particular conditionsand the distribution of these nests. Inspection shows arelation between the nest distribution and light since allcolonies are situated to receive direct sunlight for thegreater part of the day. Also, the mounds themselves areso constructed that the maximal surface area is exposedto the sun rays while the perpetually shaded north side ofthe mound becomes, in many instances, a steep slope fromthe apex of the nest to the ground. This suggestion of lightas a factor is further substantiated by the fact thatwherever nests appear within the forest, rather than alongthe margins, they are located within an actual clearing,which permits practically the same amount of light to fallupon the mounds as does on the marginally located nestsand those of the open territory. Using the Macbeth Illumi-nometer, the intensity of the light was measured in oot-candles, to ascertain if the margins and clearings, wheremounds are found, are actually receiving more light thanthe shaded 2orest floor. This is summarized in the secondtable.

Small mounds are those 6 inches to 1 1-2 feet in diameter; mediummounds, 1 1-2 to 3 feet in diameter; large mounds, 3 to 5 feet indiameter.

1932] Distribution of Formic ukei 131

Test-location Date TimeOpen margin 8-21-31 9"45 A. M.Open margin 8-26-31 9:45 A. M.F’orest clearing 8-21-31 9:55 A. M.Forest clearing 8-26-31 9:55 A. M.Shaded forest 8-21-31 10:05 A. M.Shaded orest 8-26-31 10:05 A. M.

Foot-candles240017502601665

4418

The data of the table indicate that both margins andforest clearings are receiving essentially similar sun-radia-tions, while the shaded forest floor receives very low foot-candle intensities. There is no difference between the openmeadow and the open margin of the forest. Relating thesedata to the fact that active nests are abundantly foundalong the open forest margins, in the open, or in clearingsand are not located in the shaded forest areas, the conclu-sion is suggested that light intensity plays a definitely posi-tive role in that a certain range of light intensity, i. e. ofunobstructed sun, is favorable to mound building.

Such a distribution of the nests with respect to light inthe visible range, also connotes a correlation between nestlocation and temperature; the latter being, in part, anexpression of sunlight. The relation of temperature tomound nests of Formica exsectoides which are essentiallysimilar to those of Formica ulkei has been treated by An-drews (1927). This investigator found that the tempera-tures of the upper part of the mound were higher thanthose of the lower part, and that these internal tempera-tures vary daily being due to heat received from the sun.These observations are in accord with our own. Andrews(1926) has also noted that Formica exsectoides nests in-crease in sunny regions and decrease in shaded areas; afurther indication of the role of sunlight and temperaturein connection with ant mound distribution.The effect of soil moisture content on the nest distribu-

tion becomes a limiting factor only in the region of themarshlands which exist in some of the lanes between theforested areas. Here the surrounding substratum, for somethirty feet about its center, is too moist to permit nestbuilding. Apparently the greater soil moisture of the lower

132 Psyche December

land confines the nest building to the narrow strip alongthe orest margin. It thus exerts a negative or phohibi-rive influence which prevents the urther spread o themounds into the open where similar sunlight intensity pre-vails. It may be urther noted in this connection that thegreater number o mounds in the open are o the smalltype, the galleries o which do not extend ar below thesurface of the ground. Inspection shows soil conditions,other than moisture, to be quite similar in various parts ofthe area. There has been little erosion or deposition andthe morainal drift is uniform in character. The ants buildmounds and subterranean galleries with facility in boththe loose surface soil and the hard clay below.The relation between the presence of a colony and dam-

age to surrounding vegetation has not been studied but apossible correlation is suggested in the well developed clear-ing surrounding all colonies in the interior of the forest.Andrews (1928) report.s damage of mound building ants tovegetation. In this instance, however, consideration of thelight conditions and the complete absence of either moundsor clearings in the denser’ parts of the forest indicate thatthe clearing is a cause rather than an effect of mound pres-ence.An interesting aspect of the problem not yet investigated,

is the extent to which Formica ulkei modify the fauna ofthe region, or the extent to which certain species are absentor present here in comparison to similar areas not so popu-lated by these ant colonies. Finally, observations on theunequal activity of mounds of the various sizes recorded in-dicate the need for careful analysis of the relations betweensize, age, and population of ant mounds. Such an analysisdepends on subsequent periodic observations of thesemounds, announcement of which will be made when theirsignificance becomes apparent.

Summary1. A survey of Palos Park, Illinois region records the

presence of 435 mound-nests of Formica ulkei Emery in anarea one quarter mile square.

2. Measurement of light intensity with the Macbeth II-

1932] Distribution of Formica ukei 133

luminometer shows an average o 2047 oot candles on theopen margin and 1963 oot candles in the orest clearing asopposed to 31 oot candles on he shaded orest floor.

3. The distribution oi the mounds is directly correlatedwith light intensity and attendant temperature conditionsApparently a certan range o light intensity is a positivelylimiting actor in he location o hese ant mounds.

4. Soil moisture limits the distribution o the mounds insuch a way that it is considered in the role o a negativelylimiting actor.

5. Several other interesting aspects o the complete eco-logical analysis o this unusual group o ant mounds aresuggested or urther study.

LITERATURE CITED

Andrews, E. Ao 1926. Sequential distribution o ’ormicaexsecoides F. Pyche, xxxii, pp. 127-150.

1927. Ant-mound as o emperature andsunshine. Journal o Morphology and Physiology, xliv,pp 1-21.

1928. Injuries to vegetation by mound-building ants. American Naturalist, lxii, pp. 6375.

Cowles, H. Co 1901. The plant societies o Chicago andvicinity. Bull. Geog. Sc. Chicago;also Botanical Ga-zette, xxxi.

Dreyer, W. A. 1932. The effect of hibernation and seasonalvariation o.f temperature on the respiratory exchangeo Formica ulkei Emery. Physiological ZoOlogy, v, No.2, pp. 301-331.

Holmquist, A. M. 1928. Noes on the life-history and hab-its o the mound-building ant Formica u[kei Emery.Ecology, ix, No. 1, pp. 70-87.

Park, Orlando 1929. Ecological observations upon themyrmecocoles o Formica ukei Emery, especially Lep-tinus testaceus Mueller. Psyche, xxxvi, No. 3, pp.195-215.

Park, Thomas 1929. Notes on the relationship betweenFormica ukei Emery and Soenopsis moesta Say. En-tomological News, xl, No. 10, pp. 325-326.

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LOCAL FORMICA ULKEI MOUND- WITH Departmentant Formica ulkei Emeryat Palos Park, Illinois, has been knownfor several decades. In recent years various phases of ecology and physiology