Solenopsis impact ants Multivariate Tamaulipas Mexico

8/11/2019 Solenopsis impact ants Multivariate Tamaulipas Mexico

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Initial Assessment of the Impact of the Recent

Invader, Solenopsis invictaBuren, on Resident Ant

Assemblages in Matamoros, Mexico

Author(s): Jorge Quezada-Martnez, Elsy Maria Delgado-Garca,

Sergio Snchez-Pea, Heriberto Daz-Sols and Alejandro A.Calixto

Source: Southwestern Entomologist, 36(1):61-76. 2011.

Published By: Society of Southwestern Entomologists

DOI: 10.3958/059.036.0106

URL: http://www.bioone.org/doi/full/10.3958/059.036.0106

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VOL. 36, NO. 1 SOUTHWESTERN ENTOMOLOGIST MAR. 2011

Initial Assessment of the Impact of the Recent Invader, Solenopsis invictaBuren, on Resident Ant Assemblages in Matamoros, Mexico

Jorge Quezada-Martnez1, Elsy Maria Delgado-Garca1, Sergio Snchez-Pea1,Heriberto Daz-Sols1, and Alejandro A. Calixto2

Resumen. La hormiga de fuego roja importada, Solenopsis invicta Buren, esextica para Mxico. Ha sido detectada recientemente al menos tres kilmetros alsur de la frontera Texas-Mxico, en Matamoros, Tamaulipas, Mxico. Estudios

previos en los Estados Unidos han demostrado que la llegada y establecimiento deS. invicta ha resultado en reducciones tanto en densidad relativa como endiversidad de las poblaciones de hormigas residentes; tambin modifica lasasociaciones entre otras especies de hormigas. Este estudio inicial fue conducidode 2007 a 2009 para estimar el impacto de S. invicta en parmetros ecolgicos:sobre la densidad relativa, diversidad y las asociaciones interespecficas de lashormigas locales en reas infestadas y no infestadas en Matamoros. El objetivodel estudio fue determinar el impacto de S. invicta sobre las dems hormigasresidentes, en dos tipos de habitats (campos agrcolas y habitats naturales) enreas infestadas y no infestadas adyacentes. Se usaron trampas de cada paraestimar la abundancia y la diversidad de las hormigas locales. Se colect un totalde 31,838 hormigas (12,515 en el rea no infestada y 19,323 en el rea infestada),distribuidas en 6 subfamilias y 25 gneros. Los taxones ms abundantes fueronPheidole spp. en el rea no infestada y S. invicta en el rea infestadarespectivamente. En ambas reas, la asociacin prevalente estuvo constituidageneralmente por Pheidole spp., Forelius spp., la hormiga cosechadoraPogonomyrmex barbatus Smith y Dorymyrmex spp.; estas hormigas tambin estnasociadas de manera positiva entre ellas, y de manera fuertemente negativa con S.invicta, como se ha reportado en estudios previos. Hubo un cambio aparente ygneral de dominancia, donde S. invicta es antagnica y desplaza las hormigaspreviamente dominantes. La diversidad de hormigas no fue diferente entre las

reas infestadas y no infestadas por S. invicta, pero la abundancia de hormigas no-S. invictafue menor en las reas infestadas.

Abstract. The red imported fire ant, Solenopsis invicta Buren, is exotic to Mexico.It has been detected recently at least 3 km south of the Texas-Mexico border, inMatamoros, Tamaulipas, Mexico. Previous studies in the USA have indicated thatthe arrival and establishment of the red imported fire ant has resulted in reductionsof density and diversity of resident ant populations; S. invicta also modifies theassociations with and among other ant species. This initial ecological study was________________________1

Depto. de Parasitologa, Universidad Autnoma Agraria Antonio Narro, Saltillo, Coahuila 25315,Mexico. E-mail: [email protected], [email protected] of Entomology, Texas A&M University, College Station, TX 77843; E-mail:

[email protected].


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conducted from 2007 to 2009 in Matamoros, Mexico, to assess the impact of thered imported fire ant on relative density, diversity, and interspecific associations ofother resident ants. The goal of the study was to determine the impact of theimported fire ant infestation on remaining resident ants in two habitat types:(agricultural fields and natural habitats) in adjacent S. invicta infested and non-

infested areas. Pitfall traps were used to estimate ant abundance and diversity. Atotal of 31,838 ants (12,515 in the non-infested area and 19,323 in the infestedarea) was collected, distributed among 26 genera in six subfamilies. The mostabundant taxa in both areas (infested and non-infested) were the red imported fireant and Pheidole spp., respectively. In both areas, the prevailing associations weregenerally constituted by Pheidole spp., Forelius spp., the red harvester ant,Pogonomyrmex barbatus Smith, and the pyramid ant, Dorymyrmex spp. Theseants are positively associated among them, but they are strongly and negativelyassociated with the red imported fire ant, as reported in previous studies. Thereappeared to be a general shift in dominance with S. invictaoften antagonizing and

displacing the previously dominant ants. Diversity of ants was not different amongS. invicta-infested and non-infested areas, but non-S. invictaants abundance waslower in infested areas.

Introduction

The red imported fire ant, Solenopsis invicta Buren, is one of the mostimportant and invasive ant pests in the world. Fire ant workers are voraciouspredators and, upon arriving at new areas, their abundance frequently has negativeeffects on native ants (Wilson 1971, Morrison and Porter 2003). In invaded areas,the red imported fire anthasa varied diet and dense populations; it readily adaptsto diverse ecosystems, commonly after alterations by natural phenomena or humanactivity (Kaspari 2000, Schultz and McGlynn 2000). Invasive species can causeserious ecological disruptions. Several studies have reported the effect of fire antson resident ant populations, as well as on the diversity and abundance of otherarthropods, invertebrates, and even some vertebrates (Porter and Savignano 1990,Wojcik et al. 2001, Delgado-Garca et al. 2009). Through interactions such asstrong interspecific competition and predation, the red imported fire ant has beenassociated with reductions in diversity and abundance of resident ants. However,some species of ants recover rapidly and increase in abundance after reductions innumbers of fire ants. Dorymyrmex flavus McCook is a North American species

capable of becoming abundant after numbers of sympatric red imported fire ant arereduced, and afterward is significantly associated with low fire ant populations, byinterspecific competition and predation, and by forming positive associations withother species that naturally compete with fire ants for space or food (Calixto et al.2008). In 2005, the red imported fire ant was found for the first time in two Mexicanstates bordering Texas: Coahuila and Tamaulipas (Snchez-Pea et al. 2005).The goal of this study was to determine the impacts of red imported fire ants onresident ant diversity, relative abundance, and structure of ant taxa assemblages inareas relatively recently infested by this ant and compare those to adjacent non-infested areas in Matamoros, Tamaulipas, Mexico. This area was selected because

it is the largest known S. invictainfestation (at least 25 square km) in Mexico.


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Materials and Methods

Samples of ants were collected between March 2007 and October 2009 fromtwo sites: Area 1: N 255138; W 972437; and Area 2:N 255013; W 972356.The areas were in northeastern Mexico adjacent to the Texas-Mexico border and

the Rio Grande. The weather is warm with summer rains; average temperaturesare between 10 and 34C; the mean average annual precipitation is 600 mm (INEGI2008). Ants were collected from two ecosystems at each area: 1) very disturbedareas: agricultural areas (dryland sorghum, Sorghum bicolor (L.) Moench, andmaize, Zea mays L.) and areas adjacent to rural roads, and 2) relatively non-disturbed natural or weedy vegetation consisting of grasses and shrubs or smalltrees (mainly Fabaceae: huisache,Acacia farnesiana (L.) Willd.; mesquite,Prosopisglandulosa Torrey; leadtree,Leucaena sp., retama or palo verde, Parkinsonia spp.;and Texas ebony, Ebenopsis ebano (Berl.) Barneby & Grimes).

Samples were collected twice in 2007 (20 March and 31 August), 18 March

2008, and 15 October 2009. The first and second sampling dates corresponded toplots infested to a variable degree with red imported fire ants (Area 1). Area 2consisted of two plots (one infested and one not infested with red imported fire ants)(third and fourth sampling). To verify the presence of fire ants, a preliminarysampling with hot dog bait was used (Calixto et al. 2007, Snchez-Pea et al.2009).

Pitfall traps were used to estimate ground activity, relative abundance, anddiversity of red imported fire ants and of other resident ant taxa. Traps consisted ofplastic containers with green household shampoo as the lethal agent (Sanchez-Pea, unpublished observations). Traps were set up 30 m from each other onlinear transects; these were laid as the terrain characteristics (fences, ditches, etc)permitted. A total of 731 pitfall traps was used. On the first sampling date, 225traps were set up in eight transects having 20-33 traps each. Traps were plasticcontainers 9 cm deep and 4 cm in diameter (113 ml). On the second samplingdate, 131 traps were set up in five transects having 22-29 traps each; traps were75-ml plastic containers (6 cm deep and 5 cm of top diameter). On the thirdsampling date, 188 traps were used (98 arranged in two transects in the infestedarea and 90 in two transects in the noninfested area). On the fourth sampling date,187 traps were used (106 arranged in two transects in the infested area and 81 intwo transects in the noninfested area). On the third and fourth sampling dates,containers were 500 ml, 7 cm deep, and 11 cm diameter at the opening, with

slightly slanted sides. In all samplings, traps were retrieved 72 hours after set upand taken to the laboratory for identification and counting of insects. For thesampling data of the first and second dates, traps were sorted into two groupsbased on presence or absence of red imported fire ants in them, thus defininginfested and noninfested areas. On these two dates and sites, these noninfestedareas indicated spatial gaps in the fire ant population, thus representing a small-scale mosaic or patchy pattern of fire ant presence, within a generally infested area.These gaps in the presence of red imported fire ant within generally infested areaswere apparently in the range of 100 m across.

Ants were identified with the aid of a stereomicroscope (Stemi DRC Zeiss,

Germany) and taxonomic keys and descriptions by Holldobler and Wilson (1990),Plowes and Patrock (2000), Cook at al. (2002), and Antweb (2011). Ants wereidentified to genera, and to species when possible. The number of individuals wasdetermined for ant taxa per trap at all sites.


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Preliminary accumulation curves of traps versus taxa (Seymour 2001)indicated that asymptotic or nearly-asymptotic numbers had been reached at allsites (data not shown). The number of ant genera listed by OKeefe et al. (2000),for adjacent Cameron County was also comparable to our data (see Discussion).Thus, this sampling was considered as indicative in general of the diversity of

genera present in the Matamoros areas surveyed. Numbers of genera for infestedand noninfested areas were recorded and compared by using the distribution-freeWilcoxon test.

Ant assemblages in areas infested and not infested by red imported fire antswere determined using multivariate factor analysis (Krzanowski 1988) within thesampling dates, not among dates, for a data matrix containing abundance data(number of ants per pitfall trap) (Table 1). Trap sizes were different amongsampling dates, preventing direct comparisons among these. The taxaBrachymyrmex sp., Cardiocondyla sp., Cephalotes sp., Formica sp.,Myrmecocystus placodopsForel, and Pachycondyla harpax(F.), listed in Table 2,

were not included in the analysis because they were identified posterior to it.However, their numbers were five, one, one, five, one, and 41 specimens,respectively (54 ants, or 0.16% of a total of 31,838), and thus their weight in theanalysis would have been rather small. In multivariate factor analysis, factors arecomposite variables derived from, and made up, of original variables that meet thecriteria for maximum variability and orthogonality (Manly 1992). For analysis of thefirst and second sampling dates, data were sorted as either from infested ornoninfested points (traps), based on the absence of red imported fire ants inindividual pitfall traps. Data from traps were grouped whether red imported fire antwas present or not in individual traps. Multivariate analysis was performed on datafrom traps to determine the lineal combinations of small numbers of variables (antgenera and species), F1, F3, F3.Fn, known as index factors, to explain theassociation among different ant taxa. Abundances of ant taxa were used asvariables. The analysis measures the correlation between variables (number ofindividuals for each taxon), their association or assemblages (factors), and theirrelation in each factor. The analysis determines the contribution of the factors thatbest explain the structure of the ant population. Factors F1 and F2 were mostimportant because they explained the greatest variance, generating a bidimensionalspace. By convention, the two factors that explained the greatest proportion of thevariance in the data are discussed in the results. All analyses were by using thestatistical package Statistica 6.0 (1998).

Table 1. Total Number of Ants Trapped on Four Sampling Dates in Matamoros,Tamaulipas, Mexico

First sampling Second sampling Third sampling Fourth sampling

Non-infested

areaInfested

area

Non-infested

areaInfested

area

Non-infested

areaInfested

area

Non-infested

areaInfested

area

Total ants 984 4,351 513 3,160 7,450 4,090 3,568 7,718


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Results

Ant Abundance and Diversity. A total of 31,838 ants was captured in pitfalltraps: 5,335 on the first, 3,673 on the second, 11,544 on the third, and 11,285 onthe fourth sampling dates (Table 1). Six ant subfamilies and 32 taxa were collected

(Table 2). Myrmicinae was the most diverse subfamily (14 taxa) followed byFormicinae (six), Ponerinae (five), Dolichoderinae and Ecitoninae (three taxa each),and Pseudomyrmicinae (one taxon). In noninfested areas, the most abundanttaxon on the first, second, and third sampling dates was Pheidole spp. (64, 46, and32% of the ants collected); on the fourth sampling the most abundant wasSolenopsis geminata (Fabricius). It must be mentioned that many of the nativeSolenopsis fire ants collected in this work and listed here as S. geminata, areactually hybrids of S. geminataand Solenopsis xyloniMcCook (Sanchez-Pea et al.2009). The most abundant ant in infested areas was the red imported fire ant,representing 50, 55, 55, and 63% of the ants collected, respectively, in the

successive sampling dates, always followed by Forelius spp. Red imported fire antswere never collected in areas determined as noninfested.Number of genera collected were as follows: for the infested areas in each

of the four consecutive samplings: 16, 14, 17 and 19; for the infested areas, thesenumbers were 14, 10, 21 and 22, respectively (Table 2). Thus, there was not aclear trend regarding number of genera and infestation status. On the third andfourth sampling dates, the noninfested area yielded the greatest number of genera(24), while the infested areas yielded 21 and 23, respectively. More taxa wereobtained on the third and fourth sampling dates for both infested and noninfestedareas, than on the first or second sampling date (Tables 1 and 2); this could haveresulted from the use of bigger traps in the latter collections. There were nosignificant differences in value size (number of genera) between infested andnoninfested areas (p < 0.943, W = 17.5, Wilcoxon test).

Ant Taxa Associations. After multivariate analysis, the first two factorsrepresented 32% of the total variance in noninfested areas of the first sampling data(Table 3). The first factor explained 17% of the total variance and represented apositive correlation among ants from open, usually grassy spaces: Foreliussp., thered harvester ant, Pogonomyrmex barbatus, and the pyramid ant, D. flavus(Fig. 1,group A). The second factor explained 15% of the total variance and wasrepresented by a positive correlation among ants from areas with taller vegetation(brush or trees): Leptothorax sp., Camponotus sp., and Tetramorium spinosum

(Pergande) (Fig. 1, group B), and a negative correlation with S. geminata.Analysis of the infested areas at the first sampling date showed the first two

factors represented 23% of the total variability (Table 3). The first factor explained12% of the variance, and represented a positive correlation among DorymyrmexbicolorWheeler,P. barbatus, and T. spinosum (Fig. 2, group A), but there was nonegative correlation with other variables (ants). The second factor explained 11%of the total variance and represented a positive correlation between ants ofdisturbed areas: the red imported fire ant and Nylanderia terricola(Buckley) (Fig. 2,group B). Most other resident ants clustered away from these two ants.

Analysis of the noninfested areas of the second sampling showed that thefirst two factors represented 55% of the total variance (Table 3). The first factorexplained 36% and represented again a positive correlation among Forelius sp.,P.barbatus, andD. flavus(Fig. 3, group A). The second factor explained 19% of the


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total variance and represented a positive correlation between Monomoriumminimum(Buckley), and Crematogaster sp. in addition to a negative correlation withS. geminata (Fig. 3, group B).

Fig. 1. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 forants found on the first sampling date in the area not infested by Solenopsis invicta.Group C formed by the following taxa: Pheidolespp. andN. terricola. See text fordetails.

Fig. 2. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 forants found on the first sampling date in the Solenopsis invicta infested area. GroupC formed by the ants Forelius sp. andM. minimum.

Crematogaster

Cyphomyrmex

D. flavus

D. bicolor

Neivamyrmex

N. terr icola

PheidoleP. barbatus

S. geminata

S. invicta

Strumygenys

Tetramorium

Lepthothorax

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Factor 1

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Facto

r2

Group A

Group B

Group C

Camponotus

D. flavus

D. bicolor

Neivamyrmex

Forelius

M. minimum

P. barbatus

S. geminata

Tetramorium

Lepthotorax

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.

Factor 1

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Fac

tor

2

Group A

Group B

Group C


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Fig. 3. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 for

ants found on the second sampling date in the area not infested by Solenopsisinvicta. See text for details.

The analysis of data from the infested area on the second sampling dateshowed that two factors represented 35% of the total variance (Table 3). The firstfactor explained 19% and represented a positive correlation between D. flavus andM. minimum (Fig. 4, group A) and a negative correlation with Cyphomyrmexrimosus (Spinola). The second factor explained 16% of the total variance and

represented a positive correlation among P. barbatus, D. bicolor, andPheidole spp.(Fig. 4, group B), in addition to a negative correlation with red imported fire ant. On the third sampling date, two factors represented 77% of the total varianceat the non-infested area (Table 3). The first factor explained 70% and representeda positive correlation among a diverse assemblage of 13 taxa (Fig. 5, group A).The second factor explained 7% of the total variance, represented by Forelius sp.(Fig. 5, Group B).

The data from the infested area on the third sampling date showed that thefirst two factors represented 85% of the total variance (Table 3). The first factorexplained 79% and represented a positive correlation among 15 taxa (Fig. 6, groupA). The second factor, constituted by the red imported fire ant, explained 6% of the

total variance (Fig. 6, group B).On the fourth sampling date, two factors in the noninfested area represented

26% of the total variance (Table 3). The first factor explained 14% and representeda positive correlation among 13 taxa including P. barbatus andNylanderiasp.(Fig.7, group A). The second factor explained 12% of the total variance and wasrepresented by Pheidole(Fig. 7, group B).

Two factors represented 12% of the total variance in the infested area on thefourth sampling date (Table 3). The first factor explained 11% and represented apositive correlation between D. flavus andPheidole(Fig. 8, group A). The secondfactor was M. minimum and explained 1% of the total variance (Fig. 8, group B).

Cyphomyrmex

D. flavus

Forelius

M. minimum

P. barbatus

Pheidole

S. geminata

-0.2 0.0 0.2 0.4 0.6 0.8 1.

Factor 1

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Factor 2

Group A

Group B


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Fig. 4. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 forants found on the second sampling date in the red imported fire ant-infested area.

Fig. 5. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 forants found on the third sampling date in the area not infested by Solenopsis invicta.Group A formed by the ants: Camponotus,Crematogaster, C. rimosus, Labidus,Leptogenys, M. minimum, N. terricola, P. barbatus, Solenopsis molesta, andTetramorium.

Pheidole

Forelius

D. bicolor

D. flavus

S. gemina

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

Factor 1

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Factor2

Gruup A

Group B

Camponotus sp

Cyphomyrmex

D. bicolor

D. flavus

Forelius sp

M. minimum

P. barbatus

Pheidole sp

S. geminata

S. invicta

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Factor 1

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Factor2

Grupo A

Grupo B


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Fig. 6. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 fortaxa found on the third sampling date in the Solenopsis invicta-infested area. GroupA formed by the following taxa:Camponotus,Crematogaster, C. rimosus, D. bicolor,D. flavus, Labidus coecus (Latreille), Leptogenys, Leptothorax, M. minimum, P.barbatus, Pheidole, S. molesta, and Tetramorium spinosum.

Pheidole

Nylanderia sp

aratrechina spForelius sp

D. flavus

M. minimum

Tetramorium sp

Camponotus sp

Cyphomyrmex sp

P. barbatus

Leptothor ax spCrematogaster sp

Pachycondyla villosa

Pseudomyrmex brunneus

S. geminata

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Factor 1

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

.

Fac

tor

2

Group A

Group B

Fig. 7. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 forants found on the fourth sampling date in the area not infested by Solenopsis

invicta. Paratrechina = Nylanderia.

Forelius sp

S. geminata

S. invicta

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

Factor 1

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Factor2

Group A

Group B


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Pheidole sp

6

Nylanderia sp

4

D. flavus

M. minimum

Tetramor ium sp

Trachymyrmex sp

Camponotus sp

Cyphomyrmex sp

1 3

5 Crematogaster sp7

2S. geminata

S. invicta

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Factor 1

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Factor2

Gro up A

Group B

Fig. 8. Correlation analysis for variables (ant taxa) based on Factors 1 and 2 fortaxa found on the fourth sampling date in the Solenopsis invicta-infested area.Numbers correspond to: (1)P. barbatus, (2)Pseudomyrmex brunneus (Smith) (3)Leptogenys sp., (4) Forelius sp, (5) Leptothorax sp., (6) N. terricola, and (7)Pachycondyla villosa(Fabr.).

Discussion

Reliability of collection data depends on sampling method. Luff (1975)reported that the efficiency of pitfall traps depended on the size, shape, and kind ofmaterial. Small- and large-sized traps efficiently captured small and large ants,respectively (van den Bergh 1992). In particular, Luff (1975) recommended usinglarge-sized pitfall traps. Trapping of arthropods is dependent also on factors suchas individual activity and abundance (Briggs 1961, Greenslade 1964),which in turncan be affected by vegetation and the kind of soil in the area (Greenslade 1964,Baars 1979). In this work, slightly more ant taxa were found at areas infested byred imported fire ants, than in non-infested areas in some sampling dates (Table 1);this could be because of more trap transects and traps in some of the infested

areas. On the second sampling date, ant diversity apparently did not decrease inresponse to fewer traps, most likely because the traps were of a larger diametercompared to the first sampling. More taxa were collected with the larger traps (thirdand fourth samplings); this could be either because of trap size and/or morediversity in these plots.

Total ants collected included 25 of 28 (89%) of the ant genera [as identifiedand listed in OKeefe et al. (2000)] collected historically in Cameron County, TX,which is adjacent to the location sampled herein, just across the Rio Grande. Thisnumber of genera (28) is similar also to that of ant faunas at temperate and tropical(dry seasonal) locations (Folgarait 1998, Vasconcelos et al. 2008).

There were no significant differences when comparing the number of generain infested and noninfested areas, taken collectively (Wilcoxon test). This probablyindicated no major differences in diversity of genera among infested andnoninfested areas. However, further analysis of abundance is required. There are


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heterogeneous reports on the impact of red imported fire ant on resident antcommunities (Camilo and Philips 1990, Porter and Savignano 1990, Jusino-Atresinoand Philips 1994, Seymour 2001, Morrison and Porter 2003). In general, fromthese reports, the effects of habitat disturbance and red imported fire ant are notalways simple to discern. Also, the impact of red imported fire ant appears to be

more severe a few years after its establishment at a given location; ant communitiesat several localities appear to be somewhat resilient and eventually ant diversity atsome sites is reestablished considerably, with red imported fire ant, however,remaining as very strongly dominant in terms of abundance. The reports onabundance of red imported fire ant and particularly comparison of pre- and post-invasion data indicate red imported fire ant causes radical changes in the structureof ant (and arthropod) communities (Porter and Savignano 1990, Jusino-Atresinoand Philips 1994, Wojcik et al. 2001, Morrison and Porter 2003, Calixto 2004).Calixto (2004, 2007) reported the abnormal absence of as many as eight species ofants in some plots from areas in central Texas long colonized by the red imported

fire ant. Some previous work indicated that abundance of Monomorium minimum,P. barbatusand Pheidolespp. can be strongly impacted by S. invicta (Camilo andPhilips 1990, Jusino-Atresino and Philips 1994, Cook 2003). In this work, M.minimum and P. barbatus showed similar numbers between infested andnoninfested areas. On the other hand, Pheidolespp. were >60% less abundant ininfested plots. In this area, invasion by red imported fire ant is relatively recent(probably a few years), as indicated by its absence in areas a few kilometers to thesouth, west, and east. Thus, the proposed displacement of ant taxa has possiblynot taken place yet at these Matamoros locations.

In infested areas, the most abundant taxa on the four sampling dates wasthe red imported fire ant, with 50, 55, 55, and 63% of the total ants captured,respectively; next in abundance was Forelius. These results are consistent withthose reported by Camilo et al. (1990), where red imported fire ants accounted for42% of the ants captured using pitfall traps in central Texas. Cherry (2001)reported red imported fire ant was the most abundant ant in a study in Florida. Thered imported fire ant made up to 71% of ants collected at bait traps in Gainesville,FL (Wojcik 2001). In the present work, numbers of Foreliuswere greater in infestedareas. A similar situation was reported by Camilo and Philips (1990) and Jusino-Atresino and Philips (1994). Calixto (2002) also reported that abundance ofForeliuswas not changed after drastic artificial reductions of red imported fire ant.This abundance might result from its ability to efficiently compete for food and space

(Camilo et al. 1990, Wojcik et al. 2001, Cook 2003, Calixto et al. 2007). InMatamoros, in noninfested areas the most abundant ants were Pheidolespp. (32-64%), followed by S. geminataand Forelius. Factorial multivariate analysis showed that red imported fire ant-infested andnoninfested areas were different in the abundance and spatial distribution ofresident ants and their associations. P. barbatus and Dorymyrmex spp. werestrongly associated in four of the eight samplings, similar to the report of Calixto andHarris (2002). The diversity of resident ant populations was largely unmodified bythe fire ant infestation, indicating that red imported fire ants and the other residentants possibly used different resources while at the same responding similarly to

factors like general ecosystem productivity (Morrison and Porter 2003). In infestedareas, red imported fire ant was the most abundant taxon, and the analysisindicated it had a negative interaction with S. geminata. Most other taxa (Group A)showed positive associations among them in the presence of red imported fire ant.


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Similarly, in the noninfested area, there was a positive association among Group Aants in response to the dominant ants S. geminata, Dorymyrmex spp., and Pheidolespp. Previous reports indicated a dramatic shift in dominance, with the dominantants in noninfested areas becoming direct competitors and interacting negativelywith S. invicta when this species invades an area (Porter and Savignano 1990;

Calixto 2004, 2007). Camilo et al. (1990) reported that red imported fire antinteracted with nine species of native ants in central Texas, and had a negativeimpact on Pogonomyrmexand Pheidole. Likewise, in our analysis, the interactionof Pheidole, P. barbatus, and Dorymyrmexwith S. invictawas usually very negative.The red imported fire ant eliminated P. barbatusat a central Texas site (Porter andSavignano 1990). In the present work, P. barbatus is still common, possiblyindicating the invasion by red imported fire ant is relatively recent in the area.Calixto (2007) reported that D. flavus, P. barbatus, and Pheidole had conflictinginteractions with S. invicta, and that reductions in the abundance of imported fireants resulted in greater ant abundance and diversity, confirming a direct effect of S.

invictapresence upon these ants; D. flavuswas the most responsive ant of the localfauna. The data in the present report might provide local baseline data and aninitial reference for studies of the red imported fire ant and its developinginteractions with communities of Mexican and Mesoamerican ants.

Acknowledgment

We thank Ausencio Azuara, Paulina Vega, Mara Fbregas and all peoplewho helped in collections and other phases of this work. This study was supportedby CONACYT and by Direccin de Investigacin, UAAAN (Project 0203-0202-2521).

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