Abundance and distribution of organic mound-building ants of the Formica rufa group in Yellowstone...

Abundance and distribution of organic mound-building antsof the Formica rufa group in Yellowstone National ParkA. C. Risch1, M. F. Jurgensen2, A. J. Storer2, M. D. Hyslop2 & M. Schutz1

1 Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland

2 Ecosystem Science Center, Michigan Technological University, School of Forest Resources and Environmental Sciences, Houghton, MI, USA

Introduction

Red wood ants (Formica rufa group) are ubiquitous

in many conifer and mixed-conifer forests of Europe

and Asia and build large, organic nests (mounds)

that can be 2 m in height and 4 m in diameter (e.g.

Gosswald 1989a; b). Because of their wide occur-

rence in Eurasian forested ecosystems, these ants

have been the focus of extensive research on their

social structure (e.g. Crozier and Pamilo 1996;

Pamilo et al. 1997), geographical distribution and

density (e.g. Kissling 1985), population dynamics

Keywords

ant mounds, Formicidae, North America, red

wood ants

Correspondence

Anita C. Risch, Swiss Federal Institute for

Forest, Snow and Landscape Research,

Zuercherstrasse 111, 8903 Birmensdorf,

Switzerland. E-mail: [email protected]

Received: May 31, 2007; accepted: September

26, 2007.

doi: 10.1111/j.1439-0418.2007.01243.x

Abstract

Red wood ants (Formica rufa group) are ubiquitous in many conifer

and mixed-conifer forests of northern Europe and Asia. In contrast,

relatively little is known about the abundance and distribution of the

24 North American F. rufa group species. As ants are important com-

ponents of most soil invertebrate communities and are considered eco-

system engineers that alter the flow of energy and nutrients through

terrestrial systems, it is important to gain information on their distribu-

tion and abundance. We conducted a survey for red wood ant mounds

in Yellowstone National Park (YNP), Wyoming/Montana, USA, where

human disturbance has been kept to a minimum for over 130 years.

We found a total of 85 red wood ant mounds (0.11 to 0.17 mounds/

ha) on 327 km of roads and 180 km of the hiking trails we surveyed.

The occurrence of ant mounds was higher then expected by random

distribution at elevations between 1600 and 2400 m, annual precipita-

tion of 250 to 760 mm, middle and late successional lodgepole pine,

late successional Douglas fir forest and non-forested grassland/sage-

brush prairie vegetation. Additionally, mounds were clustered in gently

sloped not north-exposed locations and in areas that had not recently

burned. Most of the mounds detected were inhabited by Formica ob-

scuripes Forel, which occupied 94% of the mounds sampled. Based on

a multi-criteria binary Geographic Information System model that we

developed, we found that ant mounds were to be expected with a

high probability in less then 1% of the YNP area. These results

together with the detected low density and small size of the red wood

ant mounds within the study area suggest that these insects have a

much lower impact on invertebrate biodiversity and ecosystem pro-

cesses, such as forest productivity and carbon and nutrient cycling on

the ecosystem scale compared with their counterparts in European or

Asian systems.

J. Appl. Entomol.

326J. Appl. Entomol. 132 (2008) 326–336 ª 2008 The Authors

Journal compilation ª 2008 Blackwell Verlag, Berlin

and behaviour (e.g. Klimetzek 1981), and effect on

invertebrate biodiversity (e.g. Laakso and Setala

1997, 1998, 2000; Hawes et al. 2002; Reznikova and

Dorosheva 2004). Other studies have investigated

the impacts of these ants on tree growth through

their interactions with defoliators and aphids (e.g.

Laakso and Setala 2000), and assessed their role in

forest ecosystem carbon and nutrient cycling (e.g.

Frouz et al. 1997; Lenoir et al. 2001; Risch et al.

2005; Domisch et al. 2006; Kilpelainen et al. 2007).

Twenty-four Formica species are placed within the

North American F. rufa group (Hedlund 2002), five of

which build large organic mounds like the Eurasian

species, while the others use smaller and variable

amounts of ‘thatch’ (organic debris) to cover their

nests (cf. Jurgensen et al. 2005). In contrast to the

Eurasian red wood ants, relatively little is known

about the abundance, distribution and ecology of the

F. rufa ants in North America, which occur in various

vegetation types ranging from meadows to sagebrush

and subalpine forests (e.g. Weber 1935; Gregg 1963;

Wheeler and Wheeler 1986; Jurgensen et al. 2005).

However, ants are important components of most

invertebrate communities (Holldobler and Wilson

1990; Folgrait 1998; Agosti et al. 2000) and are con-

sidered ecosystem engineers that alter the flow of

energy and nutrients through terrestrial systems

(Jones et al. 1994). Thus, it is important to gain a bet-

ter understanding of the distribution of red wood ants

and the role they play in North American ecosystems.

Some studies have investigated the habitat conditions

for some F. rufa group species (e.g. Weber 1935; Con-

way 1996, 1997; Lindgren and MacIsaac 2002). How-

ever, this information is not sufficient to develop

models to predict under which conditions these ants

would possibly occur and to estimate the role they

might play in North American ecosystems. To our

knowledge, no baseline information is available on

environmental factors that might constrain or benefit

the occurrence of these mound-building ants in

North America. Consequently, we conducted a sur-

vey on red wood ants in one of the most pristine eco-

systems in the United States, Yellowstone National

Park (YNP) in the states of Wyoming and Montana,

where human disturbance has been kept to a mini-

mum for over 130 years. More specifically, we were

interested in: (i) the density of red wood ant mounds

along selected trails and roads in YNP; (ii) the envi-

ronmental factors that correlate with mound occur-

rence and size; (iii) the occurrence of different red

wood ant species in YNP; and (iv) predicting which

areas of YNP would be most suitable for ant occur-

rence by developing a GIS-based habitat model.

Study area

Yellowstone National Park is located in the north-

western corner of Wyoming and the southwestern

corner of Montana, USA. Founded in 1872, YNP

encompasses nearly 9000 km2 of land. Fifty-five per-

cent of the land is covered by different aged lodge-

pole pine (Pinus contorta var. latifolia Engelm.)

stands, and approximately 26% by different aged

Douglas fir [Pseudotsuga menziesii (Mirb.) Franco],

subalpine fir [Abies lasiocarpa (Hook.) Nutt.], Engel-

mann spruce (Picea engelmannii Parry) and/or white-

bark pine (Pinus albicaulis Engelm.) stands. Thirteen

per cent of the land is non-forested vegetation

mainly dominated by grasslands or sagebrush prairie,

and the rest is covered by water.

Elevations within Yellowstone range from roughly

1500 to 3500 m. The climate features long cold win-

ters and short warm summers with an average

annual temperature of approximately 1�C. The aver-

age annual precipitation ranges from 250 mm per

year at the north boundary to 2050 mm per year in

the south west corner of the Park. Wild fires are fre-

quent in YNP and have return intervals of roughly

100 to 300 years. In the extremely dry year of 1988,

over 200 000 ha of forest were destroyed represent-

ing the largest fire event in the past 100 years.

Methods

Ant mound survey

During the summer of 2005, we surveyed for red

wood ant mounds along YNP’s roads and trails. All

paved roads with the exception of the two stretches

between Fishing Bridge Campground and the East

entrance and between Grant Village and the South

entrance were surveyed. The trails were randomly

chosen, but stratified by accessibility from the North

entrance, which lead to a higher proportion of the

trails surveyed in the northern and central part of

the park. Overall, we surveyed 327 of 431 km of the

paved roads (76%) and 180 of the 1634 km hiking

trails (11%). The roads were approximately 7 m, the

trails 1.5-m wide, and we surveyed a strip of 2.5 to

5 m on both sides of the roads and trails, respec-

tively. Consequently, a total area of 510–765 ha was

surveyed within the park. Road surveys were con-

ducted by car with a travel speed not exceeding

30 km/h. One observer in addition to the driver was

always present to ensure that all mounds were

found on both sides of the road. Upon locating

a mound along a road or a trail, we recorded its

A. C. Risch, M. F. Jurgensen, A. J. Storer, M. D. Hyslop and M. Schutz Red wood ants in Yellowstone National Park

J. Appl. Entomol. 132 (2008) 326–336 ª 2008 The Authors

Journal compilation ª 2008 Blackwell Verlag, Berlin 327

https://www.researchgate.net/publication/254148570_Impacts_of_wood_ants_Formica_aquilonia_Yarr_on_the_invertebrate_food_web_of_boreal_forest_floor?el=1_x_8&enrichId=rgreq-544a3bd2-d80b-41ae-83ca-a44bec520dd6&enrichSource=Y292ZXJQYWdlOzIyOTY0NzQ1ODtBUzoxNzI0NDA1MTY4MzMyODRAMTQxODEyNDQzMTY5Nw==

https://www.researchgate.net/publication/233078977_Red_wood_ants_in_North_America?el=1_x_8&enrichId=rgreq-544a3bd2-d80b-41ae-83ca-a44bec520dd6&enrichSource=Y292ZXJQYWdlOzIyOTY0NzQ1ODtBUzoxNzI0NDA1MTY4MzMyODRAMTQxODEyNDQzMTY5Nw==

https://www.researchgate.net/publication/227230204_Population_studies_on_hill_building_wood-ants_of_the_Formica_rufa-Group?el=1_x_8&enrichId=rgreq-544a3bd2-d80b-41ae-83ca-a44bec520dd6&enrichSource=Y292ZXJQYWdlOzIyOTY0NzQ1ODtBUzoxNzI0NDA1MTY4MzMyODRAMTQxODEyNDQzMTY5Nw==

coordinates with a Garmin eTrek Summit Global

Positioning System (GPS) unit (Garmin International

Inc., Olathe, Kansas, USA). Two perpendicular diam-

eters and four heights (in all four cardinal directions)

were measured on each mound. In addition, we

collected 10–15 large worker ants from each mound

and put them in 100% alcohol to preserve them for

species identification. When more than three mounds

were present in a radius of 15 m, we assumed them

to be colonies and only two of the mounds were

sampled for species identification. The ants were

identified to species, and selected specimens were

sent to Dr Andrew Suarez at the University of Illinois,

Champagne, Urbana, for confirmation. Six mound

sightings were called in by other researchers working

in YNP, and only GPS coordinates and elevation were

recorded for these mounds. No diameter measure-

ments were conducted on these mounds and no

specimens were collected.

Data analysis

The coordinates of each mound were imported into

ArcGIS 9.1 (ESRI, Redlands, California, http://

www.esri.com/) as text data and converted to a point

layer. Existing Geographic Information System (GIS)

layers were provided by the National Park Service

(NPS). These layers contained information on the

park boundary (1995, 1:62 500), 30-year average of

annual precipitation (1990; 1:125 000), elevation

above sea level [from digital elevation model (DEM),

30 m], slope and aspect (derived from DEM, 30 m),

vegetation type (1999, 1:62 500 m), and fire history

perimeters for fires larger then 0.4 km2 (2004;

1:62 500). All layers were then converted into 30-m

raster cells. The average annual precipitation was

divided into five groups (250–510 mm/year, 510–

760 mm/year, 760–1010 mm/year, 1010–1270 mm/

year, >1270 mm/year). The overstory vegetation was

grouped into ‘vegetation types’: (i) non-forest/sage-

brush; (ii) post-disturbance lodgepole stands (natural

regeneration after fire); (iii) early successional lodge-

pole pine stands; (iv) middle successional lodgepole

pine stands; (v) late successional lodgepole pine

stands; (vi) Douglas fir forest; (vii) white-bark pine

forest; (viii) Engelmann spruce forest; and (ix)

subalpine fir stands. Using this data, we calculated

the total hectares and frequency (in %) of each of

the classes for our variables: (i) average annual

precipitation; (ii) elevation; (iii) slope; (iv) aspect;

and (v) vegetation type for the entire park.

In a next step, we performed a spatial join of the

mound locations with each of the layers provided by

the NPS. This allowed us to determine the habitat

characteristics at each mound location. Following

this join, we calculated the frequencies (in %) of ant

mounds in each class of each feature. To test

whether the ant mounds were clustered or distrib-

uted randomly, we compared the frequencies of

each feature class (in %) with the frequencies of ant

mounds in each feature class (in %) using X2-test

statistics. Additionally, we used linear regression

analyses to assess whether there were relationships

between the dependent variables mean mound

height, mound diameter and mound volume (calcu-

lated from diameter and height measurements using

the equation of half an ellipsoid; Risch et al. 2005),

and each of the independent continuous variables

average annual precipitation, elevation and slope.

Analyses of variance (anova) followed by Tukey post-

hoc test for pair-wise comparisons were used to

assess whether there were relationships between the

dependent variables mean mound height, mound

diameter and mound volume and the independent

nominal variables aspect and vegetation type.

We determined the precipitation, the ranges of

elevation, slope, aspect, vegetation types and time

since last fire where ant mounds were clustered and

constructed a multi-criteria binary model for predict-

ing where red wood ants would most likely occur

within YNP. Areas that did not meet the criteria

were excluded from consideration, and areas that

met the criteria were combined to determine areas

most likely to contain ant mounds.

Results

Mound density and distribution

We located a total of 85 mounds along the 327 km

of road (51 mounds) and 180-km trail (34 mounds)

covered in our survey (fig. 1). As we surveyed a 5–

10 m wide strip along trails and roads (2.5 to 5 m on

each side), we covered an area of 510 to 765 ha.

Consequently, the density of mounds were estimated

to be roughly 0.11–0.17 mounds/ha. The mounds

showed a non-random distribution related to the

average annual precipitation (X2 = 135, df = 4,

P < 0.001; fig. 2a) with 73% of the mounds (62

mounds) located in areas with average annual pre-

cipitation of 250 to 760 mm. None of the mounds

was found in areas that received more then 1270-

mm precipitation per year. All mounds were located

at elevations between 1600 and 2650 m, with 47

(55%) of the 85 mounds detected between 2200 and

2400 m. The ant mounds also were clustered with

Red wood ants in Yellowstone National Park A. C. Risch, M. F. Jurgensen, A. J. Storer, M. D. Hyslop and M. Schutz



regard to elevation as significantly more mounds

were found between 1600 and 2400 m elevations

than would be expected by random distribution

(X2 = 474, df = 12, P < 0.001; fig. 2b).

Eighteen (21%) of the mounds were located in

flat terrain (slope 0%); overall, only 15 (18%) of the

mounds were found on slopes steeper than 15%

(fig. 2c). Roughly, half of the mounds were located

in south-eastern to western exposures (44 mounds

or 51%); a total of 28 mounds (33%) on north-west,

north or north-east exposures (fig. 2c). The mounds

also showed a clustered distribution with regard to

slope and aspect. They were found more frequently

then expected by chance in terrain with slope

between 0% and 15% (X2 = 28, df = 5, P < 0.001)

and in locations with south-eastern to western and

flat conditions (X2 = 29, df = 8, P < 0.001). No infor-

mation on slope and aspect was available for six

(7%) of the 85 mounds.

Fifty-one of the 85 mounds (59%) were found in

forested habitat: six mounds (7%) in late succes-

sional Douglas fir stands, one in post-disturbance,

two in early, 24 in middle and 18 in late succes-

sional lodgepole pine stands (fig. 2d). No mounds

were detected in Engelmann spruce, subalpine fir or

Fig. 1 Locations of ant mounds (dots) found within Yellowstone

National Park. The surveyed roads (solid black lines) and trails (dashed

black lines) and non-surveyed roads (solid grey lines) and trails (dashed

grey lines), respectively, are shown. A total of 510 to 765 ha (2.5–5 m

on both sides of the roads and trails) were surveyed in our study.

(a) (b)

(c) (d)

Fig. 2 Frequency of the mounds related to the frequencies of different biotic and abiotic parameters: (a) average annual precipitation (mm/year);

(b) elevation above sea level (m); (c) slope (%) and aspect; and (d) vegetation type.




white-bark pine stands. Thirty-one mounds (36%)

were located in non-forested areas. For four mounds

(5%), no information on vegetation type was avail-

able. Again, the occurrence of ant mounds was clus-

tered with regard to vegetation type: significantly,

more mounds then expected were detected in late

successional Douglas fir, middle and late-successional

lodgepole pine and non-forested areas (X2 = 133,

df = 7, P < 0.001). In particular, post-disturbance

lodgepole pine stands seemed to be avoided by the

insects. A total of 59 mounds (69%) were located in

areas that were not subject to forest fires or did burn

before 1988, while 26 mounds (31%) were located

in forests that were subject to the large 1988 fires

that burned 240 km2 of the YNP area. Ten and 14 of

these 26 mounds were, however, less then 1000 and

2000 m, respectively, from unburned habitat.

The model we developed to predict where red

wood ant mounds would occur with the highest

likelihood within YNP showed that only limited

areas would be suitable habitat for these insect spe-

cies (fig. 3). Locations that met all six criteria (pre-

cipitation, elevation slope, aspect, vegetation, fire

history) totalled 7969 ha or 0.89% of the park’s

889 394 ha only.

Mound size and volume

For six (7%) of the 85 mounds, no information on

mound size was collected. The diameter of the

remaining 79 mounds ranged from 10 to 228 cm,

the height from 5 to 76 cm, and volume from 1 and

1960 dm3, respectively (fig. 4). The average mound

diameter of the 79 mounds was 78 cm, average

height 28 cm and average volume 175 dm3 (table 1).

No relationships were detected between the size of

the mound (height, diameter or volume) and the

environmental parameters, such as precipitation, ele-

vation, slope, aspect and vegetation type (P > 0.05

for all linear regressions and anova).

Ant species

Overall, we collected worker ants from a total of 50

of the 85 mounds for species identification. Forty-

seven (94%) of these 50 mounds were inhabited by

Formica obscuripes Forel. Two mounds (4%) were

inhabited by Formica oreas Wheeler and one mound

(2%) by Formica puberula Emery, which is not con-

sidered a F. rufa group species, but placed within the

Formica sanguinea group (Hedlund 2002). In five of

Fig. 3 Potential habitat map for mound-building red wood ants in Yellowstone National Park. The black areas (0.89% or 7969 ha of the entire park)

represent the locations where all six input criteria were met and where ant mound occurrence were expected with a maximum likelihood. The

input criteria were the following: (i) elevation 250–760 mm/year (28.5% or 635 538 ha of the park); (ii) elevation 1600–2400 m (40.6% or

361 271 ha); (iii) slope 0–15% (59% or 364 644 ha); (iv) aspect flat, southeast–southwest (SE–SW) (24.5% or 218 370 ha); (v) vegetation type, middle

and late successional lodgepole pine, late successional Douglas fir forest and non-forested grassland/sagebrush prairie vegetation (40.2% or

532 076 ha); and (vi) time since last fire; never burned or burned before 1988 (44.1% or 391998 ha).




the 47 mounds (11%) that were inhabited by F. ob-

scuripes, we additionally identified several specimens

of Formica dakotensis Emery workers, indicating that

these mounds were co-inhabited by the two species.

Discussion

Mound density

The number and density of mounds detected along

roads and trails surveyed in YNP (85 mounds; 0.11–

0.17 mounds/ha) are much lower than what has

been reported from the few studies conducted on

North American F. rufa group species we found in

the literature (table 1). However, all these studies

were conducted in very small areas (0.3 ha to

5.2 km2) and the information did not come from

systematic or random surveys. The YNP red wood

ant mound densities are also much lower then most

values reported from European ecosystems, where

considerable information from systematic and non-

random (e.g. super-colony) surveys is available

(overview on European studies in Risch et al. 2005).

In European ecosystems, low mound densities were

generally found as a result of forest management

(e.g. Domisch et al. 2005; Kilpelainen et al. 2007),

air pollution or destruction of mounds by people

(Kneitz 1965; Gosswald 1989b; Travan 1998; see also

Risch et al. 2005), especially after World War II. For

North America, Jurgensen et al. (2005) suggested

that both natural parameters, such as environmental

conditions (temperature, moisture), disturbances

(fire, human), predation (e.g. bears, woodpeckers),

and competition with other ant species (e.g. car-

penter ants in the genus Camponotus), could be

responsible for the overall low density and abun-

dance of mound-building ant species. In YNP, we

found only 31% of the mounds in areas that burned

in 1988 or more recently, and most of these mounds

were located fairly close to unburned patches. These

findings suggest that fire could have a destructive

effect on organic mounds within our study area.

A similar observation was reported by Marcot et al.

(1997), who indicated that fire destroyed over 80%

of the F. obscuripes mounds in southern Idaho. In his

overview of the biology of F. obscuripes in North

Dakota, Weber (1935) concluded that fire not only

can severely damage mounds, but also destroys the

ant’s food sources, which can result in serious col-

ony set-back and even death.

(a) (b)

(c)

Fig. 4 Size distribution of red wood ant mounds in Yellowstone National Park: (a) average diameter (cm); (b) average height (cm); and (c) average

volume (dm3).




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Similar to fire, the sizeable grizzly bear population

in YNP [350 individuals in 2005; Interagency Grizzly

Bear Study Team (IGBST) 2005] might affect the

abundance of the mound-building ants. Mattson

(2001) showed that F. rufa group ants contributed

between 4.2% and 4.7% in July, and 22–27% in

August, respectively, to the grizzly bears’ diet (aver-

age values for the years 1977 to 1992). In 1977, they

even reported 28% of the bear’s diet to be composed

of red wood ants for July and 35% for August.

North American black bears have also been shown

to consume red wood ants (Colorado: Conway 1997;

Minnesota: Noyce et al. 1997; Oregon: Bull et al.

2001; Utah: Auger et al. 2004); however, no data on

black bears diet are available for YNP.

Our results obtained from a survey method that

used non-randomly distributed linear features

instead of random or systematically selected plots,

may not mirror the ‘true’ density of red wood ant

mounds in YNP. However, given the large area sur-

veyed and talking with YNP employees who work

and hike on and off-trail, we believe that the general

results of our survey are representative of F. rufa

group ants within the park. Our low estimate of

mound density rather is an over- than an underesti-

mation, as YNP fire crews would extinguish more

fires along the roads, and bears are disturbed by

tourists along the road side. Both these factors could

lead to ‘low disturbance’ zones for the ants, and

actually favour the formation and survival of

mounds. Further, ants have been shown to favour

forest edges (e.g. Sudd et al. 1977; Punttila 1996), as

more sunlight can penetrate the ground and the cli-

matic condition are more favourable compared with

the closed forest. Consequently, it is possible that

the nests of mound-building ants in YNP were clus-

tered along trails and roads, which would again lead

to an overestimation of mound density in our study.

Overall, the low densities of F. rufa group ant

mounds that we detected in YNP suggest that these

ants do not play a similar dominant role within this

ecosystem compared with European and Asian spe-

cies that have been reported to have a large effect

on: (i) the spatial heterogeneity of carbon and nutri-

ent stores and fluxes (e.g. Frouz et al. 1997; Lenoir

et al. 2001; Risch et al. 2005; Domisch et al. 2006;

Kilpelainen et al. 2007); (ii) the invertebrate biodi-

versity (e.g. Laakso and Setala 2000; Hawes et al.

2002; Reznikova and Dorosheva 2004); and (iii) tree

growth (e.g. Rosengren and Sundstrom 1991). How-

ever, in a next step, we would like to survey those

areas within YNP for which our model predicted ants

to occur with a high probability. This would help to

provide knowledge on whether these insects occur

in high abundances at specific locations. If they do

so, it is possible that they affect the ecosystem prop-

erties and processes at very small spatial scales, simi-

lar to findings by Heikkinen (1999). This author

showed that F. obscuripes had a negative effect on

the spider population of the sagebrushes located

immediately around mounds in a shrub-steppe habi-

tat in Utah.

Mound distribution and size

We detected that the ant mounds found within YNP

were clearly clustered with regard to average annual

precipitation, elevation, slope, aspect and vegetation

type. These conditions likely provide a favourable

temperature and moisture regime and/or better food

resources for the insects under study. Above-ground

mounds have the benefit of heating up more rapidly

on a daily and/or seasonal basis; however, they may

be vulnerable to temperature extremes (Holldobler

and Wilson 1990). Thus, building nests within a cer-

tain range of elevation and exposure might help

optimize temperature conditions for the mounds.

Similarly, building mounds in areas that receive

a certain amount of precipitation might also help the

insects to maintain an environment that is not too

dry, but also to assure that the mounds do not get

flooded. Weber (1935), for instance, suggested that

by placing thatch over the nest, F. obscuripes may

reduce the need for reliable rainfall; however, it is

not known to what extent.

Little information is available on the conditions in

which red wood ant mounds have been detected in

North America. Our literature survey revealed that

elevation ranges from 1300 to 2600 m seemed to be

dominating (table 1); however, some taxonomists

also detected mounds at elevations of up to 3200 m

(Gregg 1963; Wheeler and Wheeler 1986). Weber

(1935) reported F. obscuripes to occur in locations

with mean annual precipitation of 130 to 890 mm,

and some authors showed that F. obscuripes prefers

open areas and were rarely detected in dense woods

(Weber 1935; MacKay and MacKay 1984; McCahon

and Lookwood 1990; Conway 1996). However, for

all these studies, no qualitative or quantitative data

was presented. Only MacKay and MacKay (1984)

showed that most of their 108 Formica haemorrhoidal-

is Emery mounds detected were located on warmer

east, south-east and south exposed sites, which is

similar to our results. Furthermore, European F. rufa

group species have been reported to prefer warmer

south-southeast exposed slopes over slopes with




cooler northern exposures, as temperature

conditions are more favourable (Forel 1920;

Klimetzek 1970; Bretz 1971; Sossna 1973; Travan

1998). Most of the studies listed in table 1 were con-

ducted in either sagebrush habitat or other open

areas, and only a few in forests. Yet, our results

suggest that besides the non-forested habitat, the

older forest stands are also preferred by the ants.

This would be similar to what has been reported

from Europe, where red wood ant mounds were

numerous in late- compared to early-succession

Swiss conifer forests, and in 100- compared with

5-year old Norway spruce stands in Finland (Risch

et al. 2005; Kilpelainen et al. in press).

Similar sized F. rufa group mounds to those we

located were found in studies conducted in other

North American ecosystems (see table 1). However,

we could not find any relationship between the ant

mound size and the environmental parameters we

tested. From Europe, it is known that mound sizes

tended to be somewhat larger in older forests, sug-

gesting that mound size might be positively corre-

lated to the lack of disturbance (Domisch et al. 2005;

Risch et al. 2005; Kilpelainen et al. 2007). In Euro-

pean forests, researchers also found a tendency

towards larger mounds on north compared with

east, west and south slopes (Klimetzek 1970; Sossna

1973; Gosswald 1989a; Risch et al. 2005). The likely

reason is that, in these locations, temperatures tend

to be cooler, and more insulation is needed for the

mound to maintain temperature.

Ant species

Formica obscuripes was the most abundant F. rufa

group species in YNP. This result is similar to what

Jurgensen et al. (2005) indicated in their review of

North American F. rufa group species, and is also

mirrored by the studies summarized in table 1.

Formica dakotensis is also reported to have a wide

distribution in the United States (e.g. Hedlund

2002; Jurgensen et al. 2005). However, in our

study, we did not find any F. dakotensis mounds,

but only specimen of this species as co-inhabitants

of F. obscuripes mounds. It is known that some

European F. rufa group species can form mixed

mounds when one species takes over the mound

of another species, but does not kill the former

queen(s) (Collingwood 1979; pers. comm. P. Pun-

tilla, L. Sundstrom). Czechowski and Radchenko

(2006) suggested that the mixed mounds inhabited

by Formica aquilonia Yarrow, Formica polyctena Forst

and F. rufa L. they found in southern Finland

could have been formed in two ways: (i) an

already mixed group of fundatrices (colony foun-

ders) was present when a colony of Formica fusca

(slave species) was taken over; or (ii) individuals

of one species were adopted by an existing mound

of another species, especially if that colony has lost

its own queen(s). We could not find any literature

that would have described such mixed-mound

behaviour for the two species that were associated

with each other in YNP. Yet, given that we found

11% of the mounds sampled to be housed by the

two species, this phenomenon may be fairly fre-

quent.

The other two species detected in our study,

F. oreas and F. puberula, inhabited only three of

the mounds. Based on Hedlund (2002), F. oreas’

range of distribution is in most states of the north-

western USA, while F. puberula can occur all over

the western USA from Washington State to Texas.

However, we did not find any information on the

distribution or ecology of these two species in the

literature.

Conclusions

Overall, our results suggest that red wood ants

likely would not dominate the invertebrate commu-

nity at the ecosystem scale within YNP, and there-

fore, would have a lower impact on the overall

biodiversity compared with their counterparts in

Eurasian ecosystems. Further, they are likely to be

less important for increasing the spatial heterogene-

ity of carbon and nutrient flows as has been shown

from European ecosystems. Based on our review of

the literature, it is likely that this would be similar

for other temperate ecosystems in North America.

Acknowledgements

We are grateful for the support by the YNP Service

administration. We also like to thank Heidi Anderson,

Vicky Regula, Kerey Barnowe-Meyer, Georgina Ob-

rist, Monika Wysser, Vincent Green and Christine

Smith for reporting GPS coordinates from mounds

they found during their work. Justin Rosemier and

Andrew Suarez assisted with species identification.

This research was founded by Swiss-National Science

Foundation fellowships PBEZ-104320 and PBEZA-

104320 grant, by funding provided by Michigan

Technological University, School of Forest Resources

and Environmental Sciences and by the vegetation-

soils sub-project of the Swiss-NFI (National Forest

Inventory).




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