Ecological impacts of seed harvester ants on soil attributes in a Larrea -dominated shrubland

The influence of seed harvester ant ( Pogonomyrmex rugosus ) colonies on soil properties and soil surface and moisture characteristics was investigated through comparison of adjacent, nonnest (reference, 4 m beyond ant colony) areas in Las Vegas, Nevada. Effects of ant colonies on both terrace and slope sites were investigated. Soil moisture content and soil bulk density in a creosote bush ( Larrea tridentata )--dominated shrubland were significantly lower, while soil temperature, soil organic matter, and percent pore space were significantly higher in soils with ant nests relative to adjacent reference soils. Soil pH and texture did not differ significantly between nest and reference soils. Among soil surface characteristics, percent bare soil and rock (gravel, cobble, and boulder) cover were not significantly different between nest and reference soils. In evaluating soil moisture characteristics, soils with ant nests had a significantly higher water infiltrability and greater depth of water penetration, but a significantly lower area of water spread (surface-water runoff) at both terrace and slope sites. Between the 2 geomorphic surfaces, water infiltrability and depth of water penetration were significantly greater at the terrace than at the slope. Water-borne soil movement (fluvial erosion) was significantly greater at the slope than terrace but did not differ significantly between nest and reference soils. The presence of active P. rugosus colonies in the L. tridentata -dominated shrubland altered certain soil properties and appeared to have a protective influence on the soil by fostering more infiltration and less runoff of surface water in southern Nevada.     

Predator-prey interactions between the digger wasp Clypeadon laticinctus and the harvester ant Pogonomyrmex occidentalis

Females of the digger wasp Clypeadon laticinctus prey exclusively on workers of the western harvester ant Pogonomyrmex occidentalis. There is considerable variability in the size of ant workers both within and between ant mounds, and wasps tend to select larger workers as prey. Worker ants were most likely to be outside the nest at surface temperatures of 25–35°C. Wasps were seen at ant mounds most frequently when surface temperatures were between 40 and 50°C. Wasps captured ants outside and inside ant nests. Predation attempts were equally likely to result in a successful capture in either location, but entering the ant nests was more time consuming. Approximately 63% of visits by wasps to ant mounds resulted in a successful prey capture. In general, ant mounds that were visually conspicuous or close to wasp nest aggregations were more likely to be visited by wasps. However, frequency of visitation and rates of predation at a given mound varied considerably from week to week. Agonistic interactions between female wasps involving chases and occasional fights were likely to occur whenever two wasps were simultaneously present at an ant mound, especially when the only access to prey was by entering the mound.     

Foraging ecology of the giant Amazonian ant Dinoponera gigantea (Hymenoptera,Formicidae, Ponerinae): activity schedule,diet and spatial foraging patterns

This study provides a detailed account of the natural history and foraging biology of the ponerine ant Dinoponera gigantea in a rainforest in north Brazil. The species nests on the ground and the colonies contain 70–96 workers. Ant activity is negatively correlated with temperature, and is more intense at dawn and dusk. Foragers leave the nest independently and search for food individually on the leaf litter, within ca 10 m around the nest. Workers are opportunistic feeders that collect seeds and fruits, and hunt for live prey as well as scavenge for dead animal matter. The dry weight of food items ranges from <10 mg (spiders, insect parts) to >400 mg (seeds, fruits). There is no nestmate recruitment during the search for or retrieval of food, irrespective of food type and size. Foragers have a high directional fidelity, and ants from neighbouring colonies may engage in ritualized territorial contests at the border of their foraging areas. The foraging ecology of D. gigantea is compared with other ponerine species living in tropical forests, as well as with other ant groups showing similar behavioural patterns.     

Pogonomyrmex ewyheei nest site density and size on a minimally impacted site in central Oregon

Little is known about the basic characteristics of the western harvester ant ( Pogonomyrmex owyheei ) in the absence of anthropogenic disturbances. We examined the role of P. owyheei as an agent of disturbance in an area of semiarid vegetation in central Oregon known as the Horse Ridge Research Natural Area (HRRNA) that has been largely free of livestock grazing and other significant anthropogenic influences for over 23 yr. We determined density and size characteristics of nest sites and estimate total area cleared by P. owyheei activities on HRRNA. From random sampling of twenty-five 0.04-ha plots we found a mean nest density/standard error of 1.6(± 0.16) nests/0.04 ha. Mean area cleared per nest site was 4.8 m 2 , which results in an estimated barren area of 46,080 m 2 on the 240-ha HRRNA. Comparing our findings to others on P. owyheei and P. occidentalis , we found nest density and mean cleared area to be in the middle range of reported observations under a variety of land-use influences. The literature suggests that moderate disturbance may increase nest site density, but little relationship exists between disturbance history and mean size of nest sites.     

Geobotany of the Niobrara chalk barrens in Colorado: a study of edaphic endemism

The chalk barrens of the Niobrara Formation in the Arkansas River valley of Colorado exhibit a high degree of plant endemism and rarity. We examined their geochemistry and structure as well as the reproductive ecology of their signature species Mirabilis rotundifolius to assess factors influencing endemism. While no single component consistently identified the barren soils, the natural locations that support Mirablilis can be identified using elemental profiles from a cluster analysis of soil extracts. Because seeds germinate and plants proliferate in transplant gardens with non-calcic soils, bedrock and soil chemistry do not appear to be determinant components of endemism. Mirabilis rotundifoilus utilizes diverse pollinators but is capable of autogamy when insects are excluded; seed predation by harvester ants may be a limiting factor for population spread outside the barrens habitat. Mirabilis rotundifolius is a disturbance-tolerant species, part of a barrens cohort that shares traits of suffrutescence, woody caudices, or multibranched rhizomes penetrating the chalk strata. Its presence may be enhanced when disturbance inhibits the presence of other species that compete for limited water resources. We conclude that endemism is dictated not by unusual soil chemistry requirements but rather by structural components of the barrens. In a water-limited environment, shallow soils and thin chalk strata coupled with a functionally adaptive growth habit allow M. rotundifolius to survive when other species are excluded by disturbance or the physiochemical aspects of the barrens.     

Microhabitat-specific controls on soil respiration and denitrification in the Mojave Desert: the role of harvester ant nests and vegetation

Seed harvesting ants ( Pogonomyrmex rugosus ) concentrate organic matter and nutrients near their nests and create biogeochemical hotspots in desert soil. We examined factors regulating denitrification and soil respiration in a Mojave Desert ecosystem to determine the role harvester ant colonies play in nitrogen loss and carbon mineralization. Organic matter and nutrient storage were significantly greater in colonies than under the dominant vegetation (i.e., Pleuraphis rigida , a bunch grass) and in bare soil, with standing stocks of inorganic nitrogen in colonies nearly 4-fold greater than in the other microhabitats. Soil respiration, measured with laboratory incubations, was below detection limits under ambient soil conditions. Respiration rate in soil from bare patches and under grass was limited by water and labile organic carbon, with secondary nitrate limitation evident only once carbon limitation was alleviated. Soil respiration in ant nest soil was limited by water and labile organic carbon only. Denitrification, measured by the acetylene block technique, was elevated in bare soil and under grass with the addition of nitrate. Ant nest soil also responded to a nitrate addition; however, denitrification rate was greatest with addition of glucose. Ordinarily desert soil has low rates of respiration and denitrification due to dry ambient conditions. However, ant colonies likely function as important sites for nitrogen loss and carbon mineralization following rain storms, especially if storms coincide with seed set and the temporally pulsed input of organic matter into colonies.     

Comparison between plant species in bushy-tailed woodrat middens and in the habitat

Bushy-tailed woodrats ( Neotoma cinera ) collect vegetation and store it in middens. We asked to what extent plant species collected by woodrats reflect the array of species growing in the habitat. Species composition of plant clippings at 20 bushy-tailed woodrat middens in central Colorado were compared to vegetation growing within 30 m of the dens. Amount of overlap between midden and habitat species was low (28-49%) when all taxa were included; however, if only woody taxa were considered, overlap was 71-89%. Sorensen's Index of Similarity exhibited a like pattern; the index increased markedly if only woody taxa were included. Only one plant species not found within 30 m of a den occurred in significant amounts of the middens. Bushy-tailed woodrats collected a wide array of species but were more selective the greater the habitat plant diversity. Results of this and other studies indicate the concordance between midden contents and habitat vegetation decreases with increasing habitat plant diversity. Ancient woodrat middens are nonetheless useful to paleontologists seeking to reconstruct past vegetation associations in woody vegetation is well represented in middens.     

Comparing the effects of granivorous rodents on persistence of Indian ricegrass ( Oryzopsis hymenoides ) seeds in mixed and monospecific seed patches

In desert environments seeds are often heterogeneously distributed in small patches that vary in number of seed species and in seed density. Because seed harvest by rodents is often density dependent (a larger proportion of seeds is removed from high-density seed patches than from low-density patches), the proportion of residual or post-harvest seeds should be greater in low-density patches. In addition, seed preference can affect harvest. We tested whether the residual proportion of a highly preferred seed (Indian ricegrass, Oryzopsis hymenoides ) was less when in a seed patch with a 2nd species (mixed-species patch) than when in a monospecific seed patch. We predicted that the increased overall seed density due to the presence of 2 species in a patch would result in a lower residual proportion of ricegrass seeds in the mixed-species seed patches than in the monospecific patches. As predicted, the residual proportions of Indian ricegrass seeds were less each time ricegrass was paired with one of 6 other species in mixed-species patches. Similarly, the residual proportion of each of those 6 species was less when paired with ricegrass than when in a monospecific patch. We speculate on the potential implications of these results for the population dynamics of plant species and the physical structure of plant communities.     

Burrowing Owl nest success and burrow longevity in north central Oregon

We studied nest success, burrow longevity, and rates of burrow reuse for a migratory population of Burrowing Owl ( Athene cunicularia ) in north central Oregon from 1995 to 1997. Nest success varied annually from 50% to 67%. Principal causes of nest failure were desertion (26%) and depredation by badgers ( Taxidea taxus ; 13%). Reuse of available nest and satellite burrows in subsequent years was 87% in 1996 and 57% in 1997. Reuse was highest at burrows in sandy soils, which may indicate that nest-site availability is a limiting factor in sandier soil types. Trampling by livestock resulted in the loss of 24% of all burrows between one season and the next, and natural erosion resulted in closure of 17%. Both causes of burrow failure occurred more frequently in soils with a sand component due to their friable nature. We recommend that habitat used by livestock be evaluated for use by Burrowing Owls, that occupied areas be managed to minimize destruction of burrows by livestock, and that predator-control efforts be revised to exclude mortality of badgers.     

Biological characterization of the obligate symbiosis between Acropyga sauteri Forel (Hymenoptera: Formicidae) and Eumyrmococcus smithii Silvestri (Hemiptera: Pseudococcidae: Rhizoecinae) on Okinawa Island,southern Japan

The ant Acropyga sauteri Forel has an obligate, mutualistic symbiosis with a mealybug, Eumyrmococcus smithii Silvestri, on Okinawa Island, southern Japan. The mealybugs live inside ant nests nearly all their lives, and the ants depend on them for food. Alate foundress queens carry mealybugs during their nuptial flights, using them to establish new colonies at new sites. However, important aspects of the symbiosis have not yet been elucidated. The present study characterizes the basic biology of the symbiosis and describes for the first time the morphologies of all growth stages of E. smithii. Our study suggests that E. smithii has only one nymphal stage, followed by a female pupal stage or male prepupal stage. Intensive sampling of ant nests across seasons showed that A. sauteri prefers nest sites 5–20 cm underground. Acropyga sauteri produced reproductive stages mainly in mid‐March or early April, and numbers of both ant workers and mealybugs increased from spring to summer. Experimental determination of colony identity with a method using nestmate recognition by ants suggested that each ant colony rarely has a perimeter greater than 30 cm, that the ants are monogynous, and that different ant colonies are densely aggregated along the root system of a plant, adjacent to each other but not interflowing. Both symbiotic partners were vulnerable to attacks by several common subaerial ant species following physical disturbance to their nests.     

Nest architecture in polydomous grass-cutting ants (Acromyrmex balzani)

Acromyrmex balzani is a grass-cutting ant species frequently found in Cerrado areas. However, little is known about the architecture of the polydomous nests of this ant. Fifteen A. balzani nests located in a cerrado region in Botucatu, São Paulo, Brazil, were studied. The polydomous nests were studied in three ways. First, we investigated the architecture in nests moulded with cement and without moulding. Second, we performed an aggressiveness test among workers in different subnests and nests. Third, we excavated the nest and collected the colony to measure the population, verifying the existence or not of a queen in all nests. A cement mould was made of seven nests to permit better visualization of internal structures such as chambers and tunnels. Eight nests were excavated without moulding and white neutral talc was used to highlight the parts of the nests. After excavation, the depth and dimensions (length, width and height) of the chambers were measured. The results showed that the nests had a single entrance hole whose structure consisted of straw and other plant residues in winter. Mounds of loose soil, if present, were found 6–48 cm from the hole. The number of chambers containing fungus ranged from one to five, with the first being found a few centimetres beneath the ground surface (4 cm) and the last up to a maximum depth of 160 cm. The length of the tunnels ranged from 12 to 28 cm. These tunnels were built in a vertical or inclined position, leading to the chambers. No waste chambers were found, with the waste being deposited externally. Additionally, the polydomous nests contained one to eight subnests. In the aggressiveness test, when concolonial workers were confronted, no aggressiveness was observed. In contrast, when allocolonial workers were confronted, there was a high incidence of aggression among them. Excavation of polydomous nests showed only one queen for each polydomous nest, i.e. subnests with a single queen. Our study contributes to a better understanding of the so far unknown nest architecture of the polydomous grass-cutting ant A. balzani.     

A new species of fossil Chrysothamnus (Asteraceae) from New Mexico

The new, presumably extinct species, Chrysothamnus pulchelloides, is formally described and illustrated. The plant materials came from Holocene packrat middens. Anatomical and phyletic relationships of the fossil species to extant taxa are discussed.     

Use of a secondary nest in Great Basin Desert thatch ants ( Formica obscuripes Forel)

Workers of Great Basin Desert thatch ants ( Formica obscuripes Forel) dig simple secondary nests at the base of plants upon which they tend aphids and scales. These secondary nests house only foragers, with the number of foragers occupying each nest positively correlated with the number of worker-tended Homoptera feeding on plant foliage above. Thatch ant secondary nests are cooler than 25 cm below the dome top of the primary nest and maintain a significantly more constant temperature than is observed on the ground surface or in the plant canopy. Thatch ant foragers use secondary nests for at least two purposes: as a cool refuge for Homoptera tenders when midday plant canopy temperatures rise during the summer months, and as the primary place within which Homoptera tenders transfer honeydew to larger "honeydew transporters" for ultimate transport back to the primary nest.     

Bionomics of the myrmecophilous oribatid mite Protoribates myrmecophilus (Acari: Oribatida) in the Oriental tropics

Behavioural interactions between the myrmecophilous oribatid mite Protoribates myrmecophilus and the host ant Myrmecina spp. are described and compared to those of Aribates javensis which shows a very specialized myrmecophily. The behaviour of P. myrmecophilous is similar to free-living oribatids: they can walk by themselves and can survive without ant attendance. In the ant nests, the mites are sometimes cared for by ants and they are brought by ants into a new nest site. Protoribates myrmecophilus is eaten by non-host species of Myrmecina. Protoribates myrmecophilus is a less specialized myrmecophilous oribatid species.     

Indicators of red squirrel (Tamiasciurus hudsonicus) abundance in the whitebark pine zone

We investigated occupied squirrel middens and squirrel sightings and vocalizations as indicators of red squirrel ( Tamiasciurus hudsonicus ) abundance in high-elevation whitebark pine ( Pinus albicaulis ) zone. Data were collected 1984-1989 from line transects located on 2 study sites in the Yellowstone ecosystem. We evaluated the performance of each measure on the basis of precision and biological considerations. We concluded that, of the 3 measures, active middens were the best indicator of red squirrel abundance. We also observed that the density of active middens dropped by 48%-66% between 1987 and 1989, following a severe drought and extensive wildfires that burned one of the study sites during 1988.     

Effect of perch sites on Mourning Dove nest distribution

We examined the effect of natural and artificial perch sites on Western Mourning Dove ( Zenaida macroura marginella ) nest density in shrubby habitat. Nest density in shrubs was strongly correlated with the density of natural perch sites. This relationship occurred in 2 vegetation types with differing shrub composition. The correlation was stronger in areas with a homogenous shrub layer. Nest density was also higher in plots with artificial perch sites than in adjacent control plots. Nest density increased between years in plots where artificial perch sites were constructed but remained the same in adjacent control plots. Knowledge of perch-site effects has practical management applications in areas where doves nest in shrubby habitat. The possibility exists that perch sites could be managed for nest density in these habitats.     

Seed dispersal in hybrid Salsola

The study population is a group of hybrids of Salsola iberica &times; S. paulsenii . The plants show great variation in phenotypic characteristics, and the character complexes of the parent species seem to have largely disappeared. Seeds are dispersed both locally, from intact plants, and by tumbling. Tumbling is the dominant form of dispersal in most individuals studied. Most locally dispersed seeds fall between .5 and 1.5 m from the parent plant. Despite the dominance of tumbling, large numbers of seeds are dispersed locally and probably serve to maintain the population while those dispersed by tumbling introduce the plants into new areas.     

New records and range extensions of species of Dipogon (Hymenoptera, Pompilidae) in Colorado

Four species of Dipogon are recorded from Colorado for the first time. These are hurdi Evans, pulchripennis (Cresson), graenicheri Banks, and brevis (Cresson). All are inhabitants of wooded areas and are believed to nest in cavities in trees. Notes are presented on habitat and prey of these 4 species and of the recently described species kiowa Evans.     

Patterns of prey use in four sympatric species of Philanthus (Hymenoptera: Sphecidae) with a review of prey selection in the genus

Summary

Observations were made on four sympatric species of sphecid wasps of the genus Philanthus that nest in late summer and prey upon Hymenoptera. All four species nest in a restricted locale, but with little overlap in nesting areas. Comparison of nest contents of females demonstrated limited overlap in prey utilization among the four species, but only when both size and taxonomic category of prey were considered. The ultimate cause of this resource partitioning is unknown. Review of 30 populations of 16 species of North American Philanthus reveals varying degrees of specialization on prey types. Although populations of one species, separated in either time or space, tend to have similar niche breadths, this is not always the case. Intra-and inter-populational comparisons of prey records suggest behavioural plasticity allowing response to variations in prey availability. Some of the divergence in prey use among species is due to differences in predator size.     

A preliminary study of nest structure and composition of the weaver ant Polyrhachis (Cyrtomyrma) delecta (Hymenoptera: Formicidae)

Polyrhachis weaver ants build their nests from vegetation bound together using silk produced by their larvae. Here we provide a pilot study of the composition and the physical structure of three arboreal silk nests of Polyrhachis (Cyrtomyrma) delecta based on examination of three colonies. We found broadly similar nest architecture and size of the nests with each containing six or seven identifiable chambers, and describe the distribution of ants of different castes and life stages between them. We also note the construction of silk ‘girder’ structures, which spanned larger chambers, and we hypothesize that these provide additional strength to the internal nest structure. This study highlights the importance of more detailed investigation of the internal nest structure and composition in Polyrhachis, and other weaver ant species, which will help to develop our understanding of this specialized form of nest construction and nesting habits in a diverse group of ants.