Fred Punzo.
Trophic and temporal niche interactions in sympatric populations of Eremobates palpisetulosus Fichter and E. mormonus (Roewer) (Solpugida: Eremobatidae).
Psyche 101(3-4):187-194, 1994.

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TROPHIC AND TEMPORAL NICHE INTERACTIONS IN SYMPATRIC POPULATIONS OF EREMOBATES
PALPISETULOSUS FIGHTER AND E. MORMONUS (ROEWER) (SOLPUGIDA: EREMOBATIDAE)
Department of Biology, Box 5 F
University of Tampa
Tampa, FL 33606, USA
Studies were conducted from April to August 1993, on the diets and dietary overlap, as well as die1 activity patterns of sympatric populations of the solpugids, Eremobates palpisetulosus and E. mormonus. Although both species are generalist predators, they do exhibit different prey preferences based on the hardness of the cuticle. Seventy eight percent of the diet of E. palpisetulosus con- sisted of hard-bodied orthopteran and coleopteran insects, whereas 77% of the diet of E. mormonus consisted of softer-bodied dipteran, heteropteran, and orthopteran insects as well as spiders. No significant dietary differences were found between sexes. Both solpugid species exhibit similar temporal patterns of activity although E. mormonus does remain active at the surface for a longer period of time.
The importance of competition as a primary factor in determin- ing the structural organization of animal communities has been the subject of intensive debate over the last two decades (Schoener, 1974; Connor and Simberloff, 1979; Dunham, 1980; Roughgarden, 1986; Punzo, 1992). It has also been suggested that various multi- dimensional niche axes (i.e. temperature, food, space, time) can have significant effects on other ecological interactions including competition (Tracy and Christian, 1986). Arthropod predators that Manuscript received 17 May 1994.
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inhabit xeric regions frequently exhibit similar temporal patterns of activity (nocturnal) and often feed on similar prey items (Cloud- sley-Thompson, 1975; Punzo, 1994a, 1994b). In view of the arid- ity, high temperatures, and fluctuating prey density levels that often characterize desert ecosystems, the potential for intra- and inter- specific competition is quite high (Dunham, 1980). Solpugids are a major component of the arachnid fauna inhabit- ing desert regions (Muma, 195 1 ; Cloudsley-Thompson, 1977; Punzo, 1993, 1994~). Although some information is available on the hunting behavior (Bolwig, 1952; Cloudlsey-Thompson, 196 1 ; Muma, 1966; Wharton, 1987) , diet (Punzo, 1994b, 1994c, 1994d) and thermal requirements (Cloudsley-Thompson, 1977; Punzo, 1994a) of Solpugida, no studies have been conducted on the possi- ble competitive interactions between sympatric species. The pur- pose of the present study was to analyze the dietary preferences and temporal patterns of activity for the solpugids, Eremobates palipsetulosus and E. mormonus.
MATERIALS AND METHODS
Using total body length as an indicator (Muma, 1951), adult males (18-23 mm, total body length) and females (19-26 mm) of E. palpisetulosus and E. mormonus were collected and marked as previously described by Punzo (1994a), as they wandered on the surface of the ground at night during April-August, 1993. All ani- mals were collected within a 10 km radius of Lajetas (Brewster Co., Texas), located within the northern region of the Chihuahuan Desert. A detailed description of the vegetational zones and geol- ogy of this region of Trans Pecos Texas is given by Tinkam (1948). Individuals of both species were collected on sandy substrates associated with a sotol-lechuguilla community, with the dominant vegetation consisting of sot01 (Dasylirion leiophyllum) creosote (Larrea divaricata), tarbrush (Flourensia cernua), mesa sacahuista (Nolina erumpens), lechuguilla (Agave lecheguilla), mesquite (Prosopis glandulosa) and scattered clumps of chino grass (Bouteloua breviseta). Collection methods included hand-collect- ing and the use of pitfall traps.
Solpugids were identified following Muma (195 1). Data on the time of day, place of capture (utilizing USGS topographical maps), total body length, sex, surface temperatures, and the presence of



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prey in the chelicerae were recorded for each animal as previously described by Punzo (1991, 1992). A total of 463 E. palpisetulosus adults were collected, 64 of which (13.8 %) were in possession of a prey item, as well as 387 E. mormonus adults, 47 of which (12.1 %) had a prey item. Prey items were carefully removed from the chelicerae using forceps and placed in 70% ethanol for subsequent identification to order or family depending on the degree of diges- tion. Levins' standardized measure of trophic niche breadth (BA) was calculated from data based on percent frequency of occurrence of food items as described by Krebs (1989), where BA values can range from 0 (prey species in only one prey category) to 1.0 (prey species represented equally in all resource categories). Dietary and temporal niche overlap (D) were calculated according to Floyd and Jenssen (1 983):
where Pxi and Pi are frequencies of prey classes or time intervals for species x and y, respectively, for the ith category. The index, D, can range from 0 (no overlap) to 1.0 (complete overlap). Frequencies (%) of occurrence for various prey taxa (trophic niche) and time intervals of ground surface activity (temporal niche) were used for the determination of D as described by Floyd and Jenssen (1983) and Schoener (1983). All statistical procedures used in data analyses followed Sokal and Rohlf (1981). E. palipsetulosus and E. mormonus are both generalist predators and will feed on a wide variety of arthropod prey species (Table 1). This is in general agreement with previous data on allopatric popu- lations of E. palpisetulosus (Punzo, 1994b) and E. mormonus (Punzo, 1994c) from different study sites, as well as for several other solpugid species (Muma, 1966; Punzo, 1994d). Diets of males and females appear to be similar (ANOVA, p > 0.5). E. remobates palpisetulosus feeds more extensively on hard-bodied arthropods such as coleopterans and orthopterans, whereas E. mormonus captured a higher proportion of soft-bodied prey such as blattellid and gryllid nymphs as well as spiders. This is reflected



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Table 1. Number of prey items captured by Eremobates palpisetulosus (N = 64 solpugids and prey items) and E. rnor- monus (N = 47) during field observations. Percentage (fre- quency of occurrence) of total prey given in parentheses. Development stage of prey: A (adult), N (nymph), L (larva). - -
Prey taxon E. palpisetulosus E. rnormonus Insecta
Coleoptera
Carabidae (A)
Scarabaeidae (A)
Tenebrionidae (A)
Undetermined (A)
Dictyoptera
Blattellidae (N)
Diptera
Muscidae (A)
Undetermined (A)
Heteroptera
Lygaeidae (A)
Undetermined (A)
Lepidoptera
Noctuidae (A)
Nymphalidae (A)
Orthoptera
Acrididae (A)
(N)
Gryllidae (A)
Undetermined (A)
(N)
Arachnids
Araneae (A)
Scorpiones (A)
in the relatively low degree of dietary overlap (D = 0.56). Several nocturnal arthropod species such as velvet ants (Mutillidae), blister beetles (Meloidae), and millipedes, characterized by well known chemical defense mechanisms and common in the study area, were not captured by either of these solpugid species. Measurements of trophic niche breadth (Table 2) indicate that E. palpisetulosus chooses prey from a narrower resource range than E. mormonus. No evidence of cannibalism was found although it has been reported for other solpugids (Bolwig, 1952; Muma, 1966).



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Table 2. Levins' standardized measure of trophic niche breadth (En) based on the percent frequency of occurrence of various prey items.
Percent frequency of occurrence
Prey taxon E. pulpisetulosus E. mormonus Coleoptera
Dictyoptera
Diptera
Heteroptera
Lepidoptera
Orthoptera
Arachnida
Both solpugid species exhibit strong nocturnal patterns of activity (Table 3) which is common for desert arachnids (Cloudsley- Thompson, 1977; Wharton, 1987; Punzo, 1991, 1994a). No individu- als of either species were observed on the surface before 2100 hr in the summer and before 2000 hr in the spring. The peak period of activity for E. palpisetulosus was between 2200 and 2400 hr in the spring, and between 2200 and 0100 hr in the summer. For E. mor- monus, peak activity periods were between 2200 and 0100 hr for both spring and summer months. Calculation of temporal niche overlap (D) yielded a value of 0.67 for the spring and 0.64 for the summer, indicating that these solpugids occupy a similar temporal niche axis.
This study indicates that although these solpugids are generalist predators, they appear to select different types of prey perhaps based on the hardness of the exoskeleton. This may represent one way in which interspecific competition is reduced in these sym- patric species. With respect to temporal patterns of activity, E. mormonus adults exhibited a longer period of activity at the sur- face. Since microhabitat, food and time represent the three major categories of resource dimensions (Schoener, 1974; Roughgarden, 1986), a difference in any one of these dimensions could effec- tively reduce competition. Future studies should focus on possible species differences in foraging (hunting behavior, prey prepara-



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tion), microhabitat preference, the role of soil hardness in habitat selection (especially burrowing sites), fecundity, age and body size of the female at first reproduction, mating behavior, longevity, and responses to themal stress.
I thank James Botrell and my son, Thomas Punzo, for assistance in the collection of solpugids, and Brian Garman, Dept. of Mathematics (University of Tampa), for consultation on statistical analyses. I also thank the University of Tampa for a Faculty Development Grant which provided funds for travel and supplies, and the comments of anony- mous reviewers on an earlier draft of the manuscript. Bolwig, N. 1952. Observations on the behavior and mode of orientation of hunting Solifugae. J. Ent. SOC. S. Afr. 15:239-240. Cloudsley-Thompson, J. L. 1961. Some aspects of the physiology and behavior of Galeodes arabs. Ent. exp. appl. 4:257-263. Cloudsley-Thompson, J. L. 1975. Adaptations of Arthropoda to arid environments. Annu. Rev. Ent. 20:261-283.
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