The dynamics of a succulent karoo vegetation

Strong inference techniques were used in a preliminary study of the structure and dynamics of the vegetation the Prince Albert region of the Great Karoo, South Africa. Seedling emergence studies indicate that open areas in the interstices of the existent vegetation are colonized by mound-building species of the Mesembryanthemaceae. Later, these species serve as sites of establishment for seedlings of several species of woody shrubs. Eventually, the woody shrubs replace the mound-building mesems through interspecific competition. The woody shrubs persist in the community until they reach senescence and die or are removed through overgrazing. Superimposed on this dynamic pattern is a further temporal pattern involving a combination of disturbance and subsequent soil changes. Much of this effect is caused by fossorial animals, which are associated with large circular (10–20 m diameter) mounds. Burrowing by these animals changes the soil characteristics and establishes an additional sequence of vegetation succession in which the mounds serve as refuges from which non-palatable species, such as Pteronia pallens, can invade the rangeland. Later, as the mounds are abandoned and disturbance effects cease, more palatable species, such as P. empetrifolia, will replace P. pallens, overcoming finally the effects of the adjacent mounds on the surrounding vegetation.     

The role of ant nest-mounds in maintaining small-scale patchiness in dry grasslands in Central Germany

Nest-mounds of Lasius flavus, Lasius alienus and Formica rufibarbis are mainly on north-facing slopes with a markedly patchy distribution at Gimritz, northwest of Halle (Saale). Nest-mound soils had higher sodium and potassium levels but lower phosphorus and nitrogen levels than soils away from mounds. Moisture content was lower but soil pH was significantly higher in nest-mound soils. However, there was no difference in the relative abundance of plants of alkaline or acid soils growing on mounds compared to plants off mounds and no significant difference between the numbers of nitrophilous plants on and off mounds. Fewer plant species (35 spp.) in total occurred on mounds than off mounds (42 spp.). Plant species richness and total plant cover was significantly less on the nest-mounds than off the mounds. Similarly, the mean number of grass (1.39 spp dm−2) and forb species (1.13 spp dm−2) on mounds was lower than the mean number of grass (1.99 spp dm−2) and forb species (1.91 spp dm−2) off mounds. Some shrubs and forbs had higher occurrence and cover on the mounds, although only in Calluna vulgaris, Thymus serpyllum and Cerastium arvense was this difference particularly marked. Ant nest-mounds may favour plant species that cannot compete with tall grasses on unmanaged, set-aside meadows. This revised version was published online in November 2006 with corrections to the Cover Date.     

Rare sesquiterpenes from South African Pteronia species

The genus Pteronia consists of approximately 80 species which are widely distributed in southern Africa. The essential oils isolated from the aerial parts of eleven species, analyzed by GC-MS varied both qualitatively and quantitatively. In Pteronia pallens, Pteronia empetrifolia and Pteronia flexicaulis uncommon sesquiterpenes such as presilphiperfolol-7-ene, 7-α-(H)-silphiperfol-5-ene, 7-β-(H)-silphiperfol-5-ene, α-campholene aldehyde, silphiperfol-5-ene, cameroonan-7-α-ol, silphiperfol-7-β-ol, presilphiperfolan-9-α-ol and presilphiperfolan-8-ol (a major compound in P. pallens) were identified. Cluster analysis based of the chemical composition of the oils revealed that individual plants of Pteronia camphorata collected in the same population had similar oil profiles with a high correlation coefficient (S_corr ≈ 0.98). Similarly, the essential oil composition of P. pallens collected from two distinct localities also showed high levels of congruency (S_corr ≈ 0.99).     

Effect of three mound-building ant species on the formation of soil seed bank in mountain grassland

The effect of three ant species (Lasius flavus, Formica spp., Tetramorium caespitum) on soil seed bank formation was studied in temperate mountain grassland. Seed removal experiments, analysis of soil seed content and seed survival experiments were carried out to evaluate the influence of ground ants on the seed fate. In the seed removal experiment seeds of 16 species, including 5 species with elaiosome-bearing seeds (myrmecochores), were exposed and their removal followed for 39 h. On average, ants removed 63.8% of myrmecochorous seeds and 10.9% of seeds without adaptation to ant dispersal. Analysis of soil seed content revealed that myrmecochores, in spite of expectations that they would accumulate in nests of seed dispersing ants, were most abundant in the soil of control plots. Evidence on seed relocation to the ant nests was obtained from a comparison of mounds of seed dispersing and seed non-dispersing ant species, as more seeds were found in the mounds of Formica spp. and Tetramorium caespitum (seed dispersers) in comparison with the mounds of Lasius favus (non-disperser).The soil seed bank of the compared microhabitats (control plots and mounds of 3 ant species) differed in their species composition, seed abundance and vertical distribution. The most distinct qualitative differences were between seed flora of control plots and mounds of Tetramorium caespitum. Control plots had approximately 30,000 propagules per m², which was double the number of seeds found in the ant mounds. In control plots, abundance and diversity of seeds steeply declined with depth; this trend was not observed in the mounds probably due to bioturbation. In the seed survival experiment, more seeds (2 out of 3 species) survived in control plots, which may also contribute to the higher seed abundance in this microhabitat.This study showed that seed relocation by ants does not contribute significantly to seed bank build-up at this study site. Ants may, however, increase the regeneration success of myrmecochores, mainly by dispersal for distance and placement in a larger spectrum of microsites, in contrast to species not adapted for myrmecochory.     

Small-scale disturbances spread along trophic chains: leaf-cutting ant nests, plants, aphids, and tending ants

Small-scale disturbances caused by animals often modify soil resource availability and may also affect plant attributes. Changes in the phenotype of plants growing on disturbed, nutrient-enriched microsites may influence the distribution and abundance of associated insects. We evaluated how the high nutrient availability generated by leaf-cutting ant nests in a Patagonian desert steppe may spread along the trophic chain, affecting the phenotype of two thistle species, the abundance of a specialist aphid and the composition of the associated assemblage of tending ants. Plants of the thistle species Carduus nutans and Onopordum acanthium growing in piles of waste material generated by leaf-cutting ant nests (i.e., refuse dumps) had more leaves, inflorescences and higher foliar nitrogen content than those in non-nest soils. Overall, plants in refuse dumps showed higher abundance of aphids than plants in non-nest soils, and aphid colonies were of greater size on O. acanthium plants than on C. nutans plants. However, only C. nutans plants showed an increase in aphid abundance when growing on refuse dumps. This resulted in a similar aphid load in both thistle species when growing on refuse dumps. Accordingly, only C. nutans showed an increase in the number of ant species attending aphids when growing on refuse dumps. The increase of soil fertility generated by leaf-cutting ant nests can affect aphid abundance and their tending ant assemblage through its effect on plant size and quality. However, the propagation of small-scale soil disturbances through the trophic chain may depend on the identity of the species involved.     

Effects of ants on plant diversity in semi-natural grasslands

Ants (L. niger and L. flavus) build conspicuous mounds that are covered with vegetation. The aim of this study was to investigate whether the vegetation on ant mounds in semi-natural grasslands differed from that around the mounds. Another aim was to investigate whether the changes in the vegetation on ant mounds were influenced by grazing management or by habitat characteristics, semi-dry versus moist. Here, the total number of plant species and total plant cover were lower on ant mounds than in patches off-mound. The plant cover of perennials that form rosettes was twice as high on mounds inhabited by L. niger than on those inhabited by L. flavus. Only a few plant species were restricted to either ant mounds or adjacent field and the effects of ants on the plant diversity in semi-natural grasslands seemed to be low. Grazing management did not affect the differences in the vegetation on ant mounds and in equal-sized patches off-mound, whereas habitat characteristics affected ant-induced changes in vegetation cover of some plant species.     

Interactive effects of soil-dwelling ants, ant mounds and simulated grazing on local plant community composition

Interactions between aboveground vertebrate herbivores and subterranean yellow meadow ants (Lasius flavus) can drive plant community patterns in grassland ecosystems. Here, we study the relative importance of the presence of ants (L. flavus) and ant mounds under different simulated grazing regimes for biomass production and species composition in plant communities. We set up a greenhouse experiment using intact soil cores with their associated vegetation.We found that plant biomass production in the short term was affected by an interaction between simulated grazing (clipping) and ant mound presence. Clipping homogenized production on and off mounds, while in unclipped situations production was higher off than on mounds. During the experiment, these differences in unclipped situations disappeared, because production on unclipped mounds increased. Plant species richness was on average higher in clipped treatments and patterns did not change significantly over the experimental period. Plant community composition was mainly affected by clipping, which increased the cover of grazing-tolerant plant species. The actual presence of yellow meadow ants did not affect plant community composition and production.We conclude that the interaction between ant mounds and clipping determined plant community composition and biomass production, while the actual presence of ants themselves was not important. Moreover, clipping can overrule effects of ant mounds on biomass production. Only shortly after the cessation of clipping biomass production was affected by ant mound presence, suggesting that only under low intensity clipping ant mounds may become important determining plant production. Therefore, under low intensity grazing ant mounds may drive the formation of small-scale plant patches.     

Effects of harvester ants on plant species distribution and abundance in a serpentine grassland

Seed harvesting ants can have important effects on the composition and structure of plant communities. We investigated two effects of Messor andrei, the black seed-harvesting ant, on a serpentine grassland plant community in northern California. First, to determine if selective seed predation by ants affects plant community composition, we excluded harvester ants from 1-mediameter circular plots of grassland. Abundances of all species on these plots and on control plots were measured before and after exclosure. Second, to determine if M. andrei nest mounds affect plant community composition, we compared plant species abundances on and off nest mounds. M. andrei deposit large amounts of organic matter on their nest mounds over a foraging season, so mounds may alter the edaphic environment. The exclusion of seed-harvesting activity did not cause changes in the plant community. Nest mounds had a strong effect on plant communities: there were many more grasses and fewer forbs on ant mounds, although at least one forb, Lepidium nitidum, produced twice as many seeds when it grew on nest mounds. We found that nest mounds formed islands of higher-temperature soil in the serpentine grassland. Received: 31 March 1997 / Accepted: 6 May 1997     

Vegetation of ant-hills in a mountain grassland:effects of mound history and of dominant ant species

Vegetation in grasslands with well-developed long-lastingant-hills in the Slovenské Rudohorie Mts., Slovakia, was studiedin relation to (i) position on the mound, (ii) ant speciesforming the mound, and (iii) history of the mound. Permanent plotrecordings of mound size and dominant ant species started fifteen years priorthe study began provided information on the history of individual mounds.The mound vegetation bears a striking similarity to vegetation insimilar habitats across a large part of Europe due to presence of species suchas Agrostis capillaris, Dianthusdeltoides, Polytrichum commune agg.,Thymus pulegioides, and Veronicaofficinalis. Out of the three major ant species-groups presentat the site (Lasius flavus, Tetramoriumcaespitum and Formica spp.), L.flavus had the most pronounced and the most lasting effect on themound vegetation. The dominance of the plant species listed above increased withthe time span over which the mound was inhabited by L.flavus. The effects of other species on vegetation composition,though discernible from short-term observation, disappeared over severalyears. The mounds proper did not differ from the undisturbed grassland in theproportion of myrmecochorous plants or plants with specific seed size ordormancy type. However, there was a highly significant concentration ofmyrmecochorous plants in the grassland patches immediately neighbouring themounds; this is likely to be due to seeds deposited there by the workers fromthe nest after the elaiosomes had been consumed. The mound vegetation wascomposed mainly of species with long stolons or rhizomes; however, there was nosignificant difference in formation type or length of stolons/rhizomesbetween mounds and the rest of the grassland or among mounds formed by differentant species.     

How plants may benefit from their consumers: leaf-cutting ants indirectly improve anti-herbivore defenses in <Emphasis Type="Italic">Carduus nutans</Emphasis> L

Although herbivores often have a negative impact on plant fitness, sometimes plants may benefit from their consumers. However, these positive interactions usually occur as a result of plant damage (e.g., overcompensation, defense induction). I present evidence of a novel way by which plants may benefit from their consumers without being eaten. Plants of Carduus nutans increased their physical defenses when grown in external refuse dumps of the leaf-cutting ant Acromyrmex lobicornis. Seedlings planted in refuse exhibited longer spines and tougher leaves than those planted in control soils. Pick-up assays with entire leaves and leaf discs demonstrated that these enhanced physical defenses prevented leaf-cutting ant harvest. Additionally, plants established in refuse dumps showed fewer insect herbivory than those in non-nest soils. The nutrient-rich refuse dump appeared to reduce the stage at which leaves are tender and thus more vulnerable to herbivory. This is the first case where plants may benefit from insect herbivores via waste products without the cost of being eaten. This illustrates how plants may plastically respond to reliable cues of the risk of herbivory.     

Development of single sequence repeat markers for the ant Aphaenogaster senilis and cross-species amplification in A. iberica, A . gibbosa, A. subterranea and Messor maroccanus

Eight polymorphic microsatellite loci were isolated and characterized for Aphaenogaster senilis, a common ant species distributed in the Western Mediterranean. Characterization of 15 individuals form southern Spain showed moderate to high allelic diversity ranging from 2 to 9 alleles per locus. Cross-species tests on 10 individuals of A. iberica, A. gibbosa, A. subterranea and Messor maroccanus revealed successful amplification for most loci. This set of markers can be useful for population genetic studies and might even prove useful in other phylogenetically close species of the subfamily Myrmicinae.     

Influence of mound construction by red and hybrid imported fire ants on soil chemical properties and turfgrass in a sod production agroecosystem

Mound construction by imported fire ants (Solenopsis spp.) actively modify the biogeochemical and physical properties of soil; however, their influence on nutrient levels in surrounding vegetation is poorly understood. Aside from the reported persistence of elevated available P and K levels in clay-rich soils one year after mound abandonment, the relative stability of nutrient concentrations from one season to the next is largely unknown. Nutrient concentrations were concurrently analysed from ant mounds and undisturbed soils as well as plant samples collected from warm-season turfgrass in a commercial sod production agroecosystem. Initial collection of soil and turfgrass samples coincided with peak annual biomass (September 2006); the second soil sample collection occurred over twelve weeks later during turfgrass dormancy and ant brood minimum (December 2006). Total C, C/N ratios, organic matter (OM), and Zn²⁺ concentrations as well as pH of ant mound soils were significantly higher than control plot soils; these trends persisted across seasons. Turfgrass harvested from ant mound perimeters in September exhibited elevated N, P, Ca²⁺, S, Cu²⁺, Fe²⁺, and Na⁺ concentrations. Evaluation of the relative stability of soil parameters across seasons revealed a significant drop in ant nest pH from September to December 2006. Total N of mound soils was distinctively greater than control soil counterparts during September only. Soil P, K⁺, Ca²⁺, Mg²⁺ and S (all macronutrients), as well as Na⁺ concentrations from ant mound soils were substantially elevated during the late Autumn to Winter transition compared to control soil locations, whereas Fe²⁺ and Mn²⁺ levels (both micronutrients) were significantly lower in ant mound soils versus control soil environments. Continuous pedoturbation by imported fire ants as well as seasonal shifts in mound soil chemistry resulting from changes in assimilation/dissimilation among mound biota may influence the site-specific effectiveness of microfaunal pathogens (e.g., Thelohania solenopsae) or parasites (e.g., Orasema spp.) identified as classical biological control agents of non-native Solenopsis spp. Therefore, further study of the intrinsic complexities of soil ecosystem dynamics of imported fire ant mounds across several seasons is warranted. Received 10 September 2007; revised 23 April 2008; accepted 1 May 2008.     

Diversity of Rhizobia isolated from various Hedysarum species

Cultural and physiological properties, serology, plasmid profiles and infective traits were determined for 23 strains of rhizobia isolated from various Hedysarum species: H. coronarium (common name: sulla) (16), H. carnosum (1), H. alpinum (3), H. mackenzii (2) and H. pallens (1) from Portugal, Spain, Tunisia, Alaska and Israel. Strains isolated from H. alpinum, H. mackenzii and H. pallens have slow growth rates on yeast-extract mannitol medium and were unable to nodulate H. coronarium plants, whereas the latter were effectively nodulated by all sixteen fast growing strains from sulla. Regardless of the country of origin all H. coronarium strains fell into one serogroup and were not serologically related with strains of other Hedysarum species. The RAPD (random amplified polymorphic DNA) fingerprinting method which was carried out on five H. coronarium and three H. alpinum strains allowed distinction to be made among serologically related rhizobia. No particular plasmid profile pattern was observed in relation to the host or geographical origin of the strains.     

Isolation of polymorphic microsatellite loci for the study of habitat fragmentation in the harvester ant <Emphasis Type="Italic">Messor structor</Emphasis>

Microsatellite sequences of the harvester ant Messor structor were identified by use of an enriched genomic library. Populations from Austria, Germany and Croatia were screened and 4–21 alleles were identified within seven microsatellite loci. Cross species amplification was evaluated for three other Messor species. ^†W. Arthofer, B. C. Schlick-Steiner and F. M. Steiner contributed equally to this work.     

Successional trends and biomass of mosses on windthrow mounds in the temperate rainforests of Southeast Alaska

We investigated successional trends on windthrow mounds in two old-growth Tsuga heterophylla-Picea sitchensis forests in northern southeast Alaska to determine the influence of windthrow disturbance on the maintenance of plant diversity. We were particularly interested in assessing the value of mosses in detecting long-term effects of disturbance in temperate rainforests. Mosses established a dense carpet on windthrow mounds within the first few decades after the disturbance. No consistent changes were noted in total moss and vascular plant cover, moss biomass, or species diversity between young mounds (±50 yrs), intermediate mounds (±150 yrs) or old mounds (> 200 yrs), or between mounds and the undisturbed forest floor, despite consistent differences in soils development.Classification and ordination of the vegetation data did not show a consistent relationship between soil surface age or soil depth and overall species composition on the two sites. Young mounds were the most compositionally distinctive, primarily due to moss species. Pogonatum alpinum var. sylvaticum, P. contortum and Polytrichum formosum were generally confined to young mounds with unstable substrata, while Dicranum majus and Sphagnum girgensohnii were associated with old soil surfaces and deep organic soils. Vascular plant species with affinities for riparian or deep shade habitats (Tiarella trifoliata, Coptis asplenifolia and Dryopteris expansa) showed a general preference for the forest floor. Gymnocarpium dryopteris was the only vascular plant with a significant association with young mounds.Mosses comprised approximately 25% of understory plant biomass and as much as 50% of understory productivity. In cool temperate forests, the inclusion of mosses in vegetation analysis may provide valuable insights into the nature of vegetation patterns over subtle environmental gradients. The distinctiveness of the temperate rainforest type and the unique ecological effects of windthrow disturbance in this type are also suggested by this study.Abbreviations HR = Heintzleman Ridge (study site) - OP = Outer Point (study site)     

Directed dispersal and decomposition drive cyclic succession in arid subtropical thicket

The influence of animal-mediated processes on the purported cyclical succession model of vegetation dynamics in arid subtropical thicket of southern South Africa was examined. We used spatially explicit sampling methods to evaluate the influence of dispersal and decomposition on plant community structure. The harvester ant Messor capensis was the only ant species recorded harvesting Portulacaria afra fruit and the elaiosome bearing seed of the tree Gymnosporia polyacantha from experimental piles. Propagules were dispersed to nest mounds positioned beneath woody shrubs. Vertebrates dispersed fleshy-fruited tree species to regeneration sites beneath P. afra clumps. The minute seeds of succulent shrubs were secondarily dispersed from beneath shrubs to ant nest sites located in the open. The results from two experiments indicate that soil nutrients and decomposition activity are spatially localised. The rate of below-ground decomposition, and the incidence of an invertebrate detritivore assemblage, was high beneath P. afra clumps compared with exposed nutrient-enriched and grassy sites. Animal dispersal vectors are integral to the regeneration of major plant groups in arid subtropical thicket by moving seeds from competitively harsh environments to safe sites beneath nurse plants. Decomposition and nutrient release are localised beneath P. afra clumps due to the unique conditions that support a suite of detritivore taxa recorded nowhere else at the study site.     

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

Summary The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha^–1 · y^–1;Messor ebeninus: 2823–4437 g · ha^–1 · y^–1;Messor arenarius: 2165–2535 g · ha^–1 · y^–1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha^–1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.     

Studies on the isolation of plumbagin from in vitro and in vivo grown Drosera species

The rapid clonal multiplication of two species of South African Drosera is described. Levels of plumbagin, (5-hydroxy-2-methyl-1,4-naphthoquinone) from in vivo and in vitro grown plants are compared to those present in Plumbago roots. P. auriculata Lam. roots contained more than twice as much plumbagin as in vivo grown D. capensis L. plants which in turn contained more than twice as much as comparable plants of D. natalensis Diels. It is concluded that the extraction of plumbagin from Drosera plants is not commercially feasible.     

Hyperaccumulation of nickel by two Alyssum species from the serpentine soils of Iran

Serpentine soils, which contain relatively high concentrations of nickel and some other metals, are the preferred substrate for some plants, especially those that accumulate Ni in their tissues. In temperate regions more Ni-hyperaccumulator plants are found in Alyssum than in any other genus. In this study, serpentine soils of two areas (Marivan and Dizaj) in the west/northwest of Iran and also perennial Alyssum plants growing on these soils were analyzed for Ni and some other metals. The highest concentrations of total metals in the soils of these areas for Ni, Cr, Co and Mn were 1,350, 265, 94 and 1,150 μg g⁻¹, respectively, while concentrations of Fe, Mg and Ca reached 3.55%, 16.8% and 0.585% respectively. The concentration of exchangeable Ni in these soils is up to 4.5 μg g⁻¹. In this study two Alyssum species, A. inflatum and A. longistylum, have been collected from Marivan and Dizaj, respectively. Analysis of leaf dry matter shows that they can contain up to 3,700 and 8,100 μg Ni g⁻¹, respectively. This is the first time that such high Ni concentrations have been found in these species. The concentrations of other metals determined in these species were in the normal range for serpentine plants, except for Ca, which was higher, up to 5.3% and 3.5%, respectively     

Interspecific variation in the defensive responses of obligate plant-ants: experimental tests and consequences for herbivory

The aggressive behavior of ants that protect plants from herbivores in exchange for rewards such as shelter or food is thought to be an important form of biotic defense against herbivory, particularly in tropical systems. To date, however, no one has compared the defensive responses of different ant taxa associated with the same plant species, and attempted to relate these differences to longer-term efficacy of ant defense. We used experimental cues associated with herbivory—physical damage and extracts of chemical volatiles from leaf tissue—to compare the aggressive responses of two ant species obligately associated with the Amazonian myrmecophyte Tococa bullifera (Melastomataceae). We also conducted a colony removal experiment to quantify the level of resistance from herbivores provided to plants by each ant species. Our experiments demonstrate that some cues eliciting a strong response from one ant species elicited no response by the other. For cues that do elicit responses, the magnitude of these responses can vary interspecifically. These patterns were consistent with the level of resistance provided from herbivores to plants. The colony removal experiment showed that both ant species defend plants from herbivores: however, herbivory was higher on plants colonized by the less aggressive ant species. Our results add to the growing body of literature indicating defensive ant responses are stimulated by cues associated with herbivory. However, they also suggest the local and regional variation in the composition of potential partner taxa could influence the ecology and evolution of defensive mutualisms in ways that have previously remained unexplored.