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Ecological impacts of seed-harvester ants on the phenological events of seven Mojave Desert shrub species in Southern Nevada.

Abstract.--Ecological impacts of a seed-harvester ant (Pogonomyrmex rugosus) infestation on the phenological characteristics of surrounding seven Mojave Desert shrub species were quantitatively investigated in Las Vegas Valley of southern Nevada. These shrub species were Shockley's goldenhead (Acamptopappus shockleyi), white bursage (Ambrosia dumosa), shadscale (Atriplex confertifolia), Nevada ephedra (Ephedra nevadensis), winterfat (Krascheninnikovia lanata), range ratany (Krameria erecta), and creosote bush (Larrea tridentata). The initiation of leafing, flowering, and fruiting phenologies was significantly delayed, and flowering success was significantly reduced in all shrub species occurring within and around the nest area compared to the adjacent non-nest area. The interval between initial dates of each major phenophase (leafing, flowering, and fruiting) of all shrub species became larger in nest relative to non-nest areas. The infestation of seed-harvester ants had a detrimental effect on the phenological events of seven Mojave Desert shrub species in southern Nevada.

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The seed-harvester ant (Pogonomyrmex rugosus) occurs in arid and semiarid plant communities throughout much of southwestern United States (Carlson and Whitford 1991). Nests of harvester ant species are generally clear of plants at least in the central area surrounding the single nest entrance where ant activities are most intense (Beattie and Culver 1977; Lei 1999). Previous studies have found changes in plant diversity and composition (Beattie and Culver 1977; Lei 1999), population densities (Hobbs 1985; Lei 1999), and vegetation production (Rogers and Lavigne 1974) associated with ant nests.

Studies of plant phenology are crucial to understand the resource base of populations, communities, and ecosystems. Phenological events of selected Mojave Desert shrub species were studied in Rock Valley of southern Nevada (Wallace and Romney 1972; Ackerman and Bamberg 1974; Beatley 1974; Rundel and Gibson 1996). Yet, ecological impacts of seed-harvester ants on the phenological attributes of the surrounding shrub species have not been documented in Las Vegas Valley. Rock and Las Vegas Valleys are similar in climatic conditions, species composition, vegetation cover, community structure, soils, geology, and topography. The objective of this study was to quantify leafing, flowering, and fruiting phenologies of all seven Mojave Desert species--Shockley's goldenhead (Acamptopappus shockleyi), white bursage (Ambrosia dumosa), shadscale (Atriplex confertifolia), Nevada ephedra (Ephedra nevadensis), winterfat (Krascheninnikovia lanata), range ratany (Krameria erecta), and creosote bush (Larrea tri-dentata), occurring in the nest area compared to the adjacent non-nest area in Las Vegas Valley.

Methods

Study Site

The field study was conducted in the southwestern part of Las Vegas Valley in southern Nevada (36[degrees]04'N; 115[degrees]10'W). The climate is arid with hot, dry summer and cool, episodic wet winters. The mean monthly air temperature and precipitation for the years 2000 through 2003 are shown in Table 1 (NOAA, Las Vegas). The mean length of the frost-free period is over 350 days, and frequently an entire winter month will pass without a subfreezing air temperature. The mean annual rainfall is 10.1 cm (NOAA, Las Vegas), and most of the precipitation results from monsoonal thunderstorms in summer and drizzles in winter months. Often, a single major thunderstorm will contribute the rainfall for an entire month.

Vegetation of Las Vegas Valley was classified as creosote bush-white bursage association. Other woody taxa that were present in the nest area included Shockley's goldenhead, shadscale, Nevada ephedra, winterfat, and range ratany. Soils, derived from limestone-dolomite mountains and hills, are sandy in texture with loose rocks on the surface (Lei 2001a). Multiple caliche layers are found in the subsoil. Organic decomposition and soil formation are slow due to the arid nature of this area.

Field Survey

All 187 seed-harvester nests were identified in a 5-ha study plot. Each ant nest had multiple entrances, with a construction of subterranean chambers (cavities) and runways. Diameters of the exposed soil surface (disc) at each ant nest were measured in centimeters by computing the average of length and width of the nest.

Phenological observations were taken weekly in the nest and adjacent non-nest (control) areas from late January 2000 through the growing season in July 2003. A two-lane road separated the nest and non-nest areas. For each of the seven shrub species, initial dates of each phenophase (appearance of new leaves, flowers, and mature fruits) were observed among any of the 40 randomly chosen individual plants in the study plots. Each individual plant was marked with brightly colored flagging for ease of visualization in subsequent months and years. Subsequent periods of each phenophase later in the season were not evaluated or considered.

Moreover, within each shrub species, flowering success (percentages of mature flower cover) was assessed in these 40 randomly chosen individuals. Percentages of flower cover were visually quantified in 10% point increments (Lei 2001b).

Statistical Analysis

Mean diameter of ant nests was computed in centimeters. Significant differences in mean initial dates of phenological events and flowering success were tested with one-way Analysis of Variance (ANOVA; Analytical Software 1994) between nest and adjacent non-nest areas. Julian day calendar, a calendar system that numbers days consecutively instead of using cycles of days and months, was used for ease of computation. Nevertheless, results were converted back to the standard calendar system for ease of interpretation and comprehension. Mean values were expressed with standard errors, and statistical significance was determined at the 5% level.

Results

The mean diameter of ant nests (disc) was 85.6 [+ or -] 9.4 cm (n = 187). In general, initial dates of phenophases between shrubs occurring in nest and non-nest areas ranged over periods of 2-5 weeks. All seven shrub species consistently exhibited a significant delay in vegetative, floral, and reproductive phenologies in the nest area compared to the adjacent non-nest area (Tables 2-4). With the presence of active ant nests, the average time between first appearance of leaves and first mature flowers, as well as between first appearance of flowers and first mature fruits of these shrub species was considerably longer.

Among all shrub species, Nevada ephedra exhibited the earliest mean initial dates of phenophases irrespective of ant infestation (Tables 2-4). Creosote bush, shadscale, and Nevada ephedra grew continuously when most shrubs underwent summer dormancy in typical dry years. Range ratany balanced its late growth with an initial leafing approximately 4-7 weeks later than all other shrub species (Table 2). Range ratany did not develop mature fruits until the end of June (Table 4).

All seven shrub species initiated flowers eight weeks after the initial date of leafing irrespective of ant infestation (Tables 2-3). Flowering of range ratany and creosote bush was considerably later than the other five shrub species (Table 3). The interval between first appearance of leaves and first flowers was five weeks in Nevada ephedra. Creosote bush and white bursage formed leaves early, but flowered approximately eight weeks after leaf initiation in both nest and adjacent non-nest areas (Tables 2-3). In all seven shrub species, leaf and flower production overlapped during the spring months. Ripe fruits were first observed approximately 2-3 weeks after flower initiation in the non-nest area, and 3-4 weeks in the nest area (Tables 3-4).

Furthermore, flowering success was significantly reduced in all seven shrub species occurring within and around disc perimeters compared to the non-nest area (Table 5). Mean flower cover was under 20% for shrubs occurring in the nest area, while mean flower cover was between 30 to 45% for shrubs occurring in the non-nest area. Percent flower cover ranged from 12.1% in Nevada ephedra in the nest area to 40.8% in creosote bush in the non-nest area (Table 5).

Discussion

Initiation dates of three important phenophases--appearance of leaves, appearance of flowers, and development of fruits--of seven Mojave Desert shrub species were quantitatively investigated in Las Vegas Valley of southern Nevada. Observations of phenological patterns were considered typical for the species in this study. This study demonstrated a substantial variation in phenological timing among seven shrub species occurring in the non-nest area, and phenological events of these species were significantly delayed by as many as 3-4 weeks due to ant infestation.

In this study, seed-harvester ants locally influenced plants beyond the nest (disc) area as evidenced by delayed phenological events in the nest area compared to the adjacent non-nest area. From casual observations, numerous ants were found on stems, leaves, and flowers of shrubs occurring within and around the periphery of nest discs. Through time, portions of shrub leaves were defoliated by ants. Rissing (1988) also observed the defoliation of creosote bush leaves by seed-harvester ants in southern Nevada. Clark and Comanor (1975) stated that the majority of harvester ant species in the genus Pogonomyrmex may actively defoliate leaves and destroy plants growing on and near their nests in order to reduce shade because high nest temperatures are required for brood development. Plants must allocate their limited energy to growth, survival, and reproduction. Because of an extensive defoliation by ant activities, shrubs must expend some energy to withstand ant infestation and grow additional leaves prior to flowering and fruiting development. Consequently, the flowering and reproductive success of these seven shrub species is substantially reduced. No comparative data are available since ecological impacts of ant infestation on shrub phenology have not been quantitatively examined elsewhere in the Mojave Desert.

The disc is a visually obvious nest structure, but not the limits of the nest itself, which may extend below the soil surface at a distance beyond the disc (Lei 1999). Furthermore, seed-harvester ant nests influence the soil surface beyond the limits or physical structure of the nest disc as well.

Variations have been documented in phenological timing among shrub species and from year to year in Rock Valley of southern Nevada (Ackerman and Bamberg 1974; Beatley 1974; Rundel and Gibson 1996) regardless of seed-harvester ant infestation. In springs of 2000-2003, mean leaf initiation of range ratany was 4-7 weeks later than other shrub species in Las Vegas Valley. Flowering in most species was commenced about 4-6 weeks following leaf initiation, and fruiting was commenced approximately 2-3 weeks following flower initiation (Rundel and Gibson 1996), which concur with this study. Moreover, the interval between initial dates of each major phenophase of all seven shrub species became larger due to ant infestation in this study.

This study examines how active seed-harvester ant colonies have adversely affected the phenology of Mojave Desert shrub species. In order to completely understand the phenology of Mojave Desert shrub species, additional field research studies are required when attempting to formulate generalizations based on the phenological data of the Mojave Desert shrub species in southern Nevada.
Table 1. Mean monthly air temperature and precipitation of Las Vegas
Valley from the years 2000-2003 (NOAA, Las Vegas). Official weather
data were obtained from the McCarran Airport in Las Vegas, near my
study site. The letter "T" indicates trace precipitation, an amount
greater than zero but less than the lowest reportable value.

 Air temperature Precipitation
Month ([degrees] C) (mm)

January 10.3 T
February 11.3 1.4
March 14.7 3.7
April 19.9 3.3
May 25.6 T
June 31.2 T
July 34.4 13.6
August 32.5 7.6
September 28.3 7.1
October 21.1 10.4
November 11.8 6.1
December 9.0 9.1

Table 2. Mean initial dates of leafing in seven Mojave Desert shrub
species occurring in the nest and adjacent non-nest (control) areas
in Las Vegas of southern Nevada. Mean dates are presented with
standard errors, and statistical significance is determined at p
[less than or equal to] 0.05 using one-way ANOVA.

Species Non-nest Nest

Creosote bush March 6 [+ or -] 4 March 27 [+ or -] 5
Shockley's goldenhead March 11 [+ or -] 4 March 29 [+ or -] 5
Nevada ephedra February 19 [+ or -] 3 March 3 [+ or -] 4
Range Ratany April 2 [+ or -] 5 April 28 [+ or -] 5
Shadscale February 26 [+ or -] 4 March 28 [+ or -] 4
Winterfat March 7 [+ or -] 4 March 29 [+ or -] 6
White bursage February 17 [+ or -] 4 March 9 [+ or -] 4

Species F-value p-value

Creosote bush 13.29 0.0109
Shockley's goldenhead 9.72 0.0206
Nevada ephedra 17.15 0.0063
Range Ratany 20.28 0.0041
Shadscale 27.00 0.0020
Winterfat 14.52 0.0089
White bursage 12.00 0.0134

Table 3. Mean initial dates of flowering in seven Mojave Desert shrub
species occurring in the nest and adjacent non-nest (control) areas in
Las Vegas of southern Nevada. Mean dates are presented with standard
errors, and statistical significance is determined at p < 0.05 using
one-way ANOVA

Species Non-nest Nest

Creosote bush May 2 [+ or -] 4 May 28 [+ or -] 5
Shockley's goldenhead April 26 [+ or -] 5 May 15 [+ or -] 4
Nevada ephedra March 27 [+ or -] 4 April 19 [+ or -] 4
Range ratany May 15 [+ or -] 4 June 3 [+ or -] 5
Shadscale April 6 [+ or -] 5 May 12 [+ or -] 4
Winterfat April 19 [+ or -] 3 May 5 [+ or -] 6
White bursage April 14 [+ or -] 3 May 17 [+ or -] 5

Species F-value p-value

Creosote bush 20.18 0.0041
Shockley's goldenhead 10.83 0.0166
Nevada ephedra 15.87 0.0073
Range ratany 6.34 0.0473
Shadscale 38.88 0.0008
Winterfat 7.68 0.0324
White bursage 16.43 0.0067

Table 4. Mean initial dates of fruiting in seven Mojave Desert shrub
species occurring in the nest and adjacent non-nest (control) areas
in Las Vegas of southern Nevada. Mean dates are presented with standard
errors, and statistical significance is determined at p [less than or
equal to] 0.05 using one-way ANOVA.

Species Non-nest Nest

Creosote bush May 13 [+ or -] 4 June 10 [+ or -] 5
Shockley's goldenhead April 24 [+ or -] 5 June 9 [+ or -] 4
Nevada ephedra April 19 [+ or -] 4 May 16 [+ or -] 4
Range ratany June 7 [+ or -] 4 June 30 [+ or -] 5
Shadscale April 24 [+ or -] 5 May 29 [+ or -] 4
Winterfat May 8 [+ or -] 5 June 4 [+ or -] 6
White bursage May 9 [+ or -] 3 June 12 [+ or -] 5

Species F-value p-value

Creosote bush 23.52 0.0029
Shockley's goldenhead 60.75 0.0002
Nevada ephedra 23.85 0.0028
Range ratany 7.31 0.0407
Shadscale 36.75 0.0009
Winterfat 20.08 0.0042
White bursage 32.51 0.0013

Table 5. Flowering success (mean percent flower cover per individual
plant) of seven Mojave Desert Shrub species occurring in the nest and
adjacent non-nest (control) areas in Las Vegas of southern Nevada. Mean
values are presented with standard errors, and statistical significance
is determined at p [less than or equal to] 0.05 using one-way ANOVA.

Species Non-nest Nest

Creosote bush 40.8 [+ or -] 4.3 17.6 [+ or -] 2.9
Shockley's goldenhead 30.3 [+ or -] 3.5 13.9 [+ or -] 3.4
Nevada ephedra 25.5 [+ or -] 3.2 12.1 [+ or -] 3.0
Range ratany 31.2 [+ or -] 3.4 18.8 [+ or -] 2.6
Shadscale 30.2 [+ or -] 3.7 12.4 [+ or -] 1.9
Winterfat 33.1 [+ or -] 3.4 19.3 [+ or -] 2.8
White bursage 39.4 [+ or -] 3.7 15.3 [+ or -] 3.2

Species F-value p-value

Creosote bush 47.21 0.0001
Shockley's goldenhead 10.62 0.0115
Nevada ephedra 19.71 0.0022
Range ratany 9.54 0.0149
Shadscale 26.19 0.0009
Winterfat 32.06 0.0005
White bursage 24.74 0.0011


Acknowledgment

I gratefully acknowledge Steven Lei for observing the initiation of leafing, flowering, and fruiting phenologies in seven Mojave Desert shrub species and for statistical analysis. David Charlet and two anonymous reviewers provided helpful comments to greatly improve this manuscript.

Literature Cited

Ackerman, T.L. and Bamberg, S.A. 1974. Phenological studies in the Mojave Desert at Rock Valley (Nevada Test Site). Pp. 215-226 in: Lieth, Helmet (ed.). Phenology and seasonality modeling. Springer-Verlag. New York.

Analytical Software. 1994. Statistix 4.1, an interactive statistical program for microcomputers. Analytical Software. St. Paul, Minnesota. 329 pp.

Beatley, J.C. 1974. Phenological events and their environmental triggers in a Mojave Desert ecosystem. Ecology 55:856-863. Beattie, A.J. and Culver, D.C. 1977. Effects of the mound nests of ant, Formica obscuripes, on the surrounding vegetation. American Midland Naturalist 97:390-399.

Carlson, S.R. and Whitford, W.G. 1991. Ant mound influence on vegetation and soil in a semiarid mountain ecosystem. American Midland Naturalist 126:125-139.

Clark, W.H. and EL. Comanor. 1975. Removal of annual plants from the desert ecosystem by western harvester ants, Pogonomyrmex occidentalis. Environmental Entomology 4:52-56.

Hobbs, R.J. 1985. Harvester ant foraging and plant species distribution in annual grassland. Oecologia 67:519-523.

National Oceanic and Atmospheric Administration. 2000-2003. Local climatological data: Annual summary with comparative data, Las Vegas, Nevada. National Climatic Data Center. Ashville, North Carolina.

Lei, S.A. 1999. Ecological impacts of Pogonomyrmex on woody vegetation of a Larrea-Ambrosia shrubland. Great Basin Naturalist 59:281-284.

Lei, S.A. 2001a. Ecological impacts of seed-harvester ants on soil attributes in a Larrea-dominated shrubland. Western North American Naturalist 60:439-444.

Lei, S.A. 200lb. Diversity of parasitic Cuscuta and their host plant species in a Larrea-Atriplex ecotone. Southern California Academy of Sciences 100:36-43.

Rissing, S.W. 1988. Seed-harvester ant association with shrubs: competition for water in the Mojave Desert? Ecology 69:809-813.

Rogers, L.E. and Lavigne, R.J. 1974. Environmental effects of western harvester ants on the shortgrass prairie ecosystem. Environmental Entomology 3:994-997.

Rundel, P.W. and Gibson, A.C. 1996. Ecological communities and processes in a Mojave Desert ecosystem: Rock Valley, Nevada. University Press. Cambridge, Massachusettes. 369 pp.

Wallace, A. and E. Romney. 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. TID-25954. US AEC Technical Information Center. Oak Ridge, Tennessee.

Accepted for publication 26 May 2004.

Simon A. Lei

Department of Biology, Community College of Southern Nevada, WDB, 6375 West Charleston Boulevard, Las Vegas, NV 89146-1139 E-mail: salei@juno.com
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Author:Lei, Simon A.
Publication:Bulletin (Southern California Academy of Sciences)
Geographic Code:1U8NV
Date:Dec 1, 2004
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