Interspecific variation and ontogenetic change in antiherbivore defense in myrmecophytic Macaranga species

The present study examined whether or not coexisting congeneric plant species have different defense strategies against herbivores, and the intensity of defense changes ontogenetically. We focused on nine myrmecophytic Macaranga species and estimated the intensity of non-biotic and biotic defense by the degree of leaf damage in ant-free and ant-occupied plants, respectively. Ant colonization of myrmecophytic Macaranga species occurred in the early stage of plant development (5–50 cm-tall seedlings). Following the colonization, damage by leaf eaters was minimized and stable during the ontogenetic development of the host plants due to protection by ants. In ant-free trees, however, herbivore damage was immense in seedlings and decreased as trees grew. Interspecific comparison of leaf damage and herbivore fauna supported that coexisting congeneric plants differ in their types of non-biotic (chemical/structural) defense: without ant protection, Macaranga beccariana, for example, was somewhat resistant to leaf eaters but susceptible to gall-makers, Macaranga trachyphylla was heavily infested by generalist leaf eaters, and Macaranga winkleri was exploited by ant-predatory birds. Despite these variations in chemical/structural defense, ant-colonized plants were generally well defended by ants against all kinds of herbivores. This suggests that the individual host-specific ant mutualists are well adapted to deter the chemically or structurally adapted herbivores. These results imply that in the history of diversification in the Macaranga–ant–herbivore system, a sequence of mutual counter adaptation took place not only between plants and herbivores but also between ants and herbivores.     

Studies of a South East Asian ant-plant association: protection of Macaranga trees by Crematogaster borneensis

Summary In the humid tropics of SE Asia there are some 14 myrmecophytic species of the pioneer tree genus Macaranga (Euphorbiaceae). In Peninsular Malaysia a close association exists between the trees and the small, non-stinging myrmicine Crematogaster borneensis. These ants feed mainly on food bodies provided by the plants and have their colonies inside the hollow internodes. In a ten months field study we were able to demonstrate for four Macaranga species (M. triloba, M. hypoleuca, M. hosei, M. hulletti) that host plants also benefit considerably from ant-occupation. Ants do not contribute to the nutrient demands of their host plant, they do, however, protect it against herbivores and plant competition. Cleaning behaviour of the ants results in the removal of potential hervivores already in their earliest developmental stages. Strong aggressiveness and a mass recruiting system enable the ants to defend the host plant against many herbivorous insects. This results in a significant decrease in leaf damage due to herbivores on ant-occupied compared to ant-free myrmecophytes as well as compared to non-myrmecophytic Macaranga species. Most important is the ants' defense of the host plant against plant competitors, especially vines, which are abundant in the well-lit pioneer habitats where Macaranga grows. Ants bite off any foreign plant part coming into contact with their host plant. Both ant-free myrmecophytes and non-myrmecophytic Macaranga species had a significantly higher incidence of vine growth than specimens with active ant colonies. This may be a factor of considerable importance allowing Macaranga plants to grow at sites of strongest competition.     

Effects of food rewards offered by ant–plant Macaranga on the colony size of ants

Myrmecophytes (ant–plants) have special hollow structures (domatia) in which obligate ant partners nest. As the ants live only on the plants and feed exclusively on plant food bodies, sap-sucking homopterans in the domatia, and/or the homopterans honeydew, they are suitable for the study of colony size regulation by food. We examined factors regulating ant colony size in four myrmecophytic Macaranga species, which have strictly species-specific association with Crematogaster symbiont ants. Intra- and interspecific comparison of the plants showed that the ant biomass per unit food biomass was constant irrespective of plant developmental stage and plant species, suggesting that the ant colony size is limited by food supply. The primary food offered by the plants to the ants was different among Macaranga species. Ants in Macaranga beccariana and Macaranga bancana relied on homopterans rather than food bodies, and appeared to regulate the homopteran biomass and, as a consequence, regulate the ants own biomass. In contrast, ants in Macaranga winkleri and Macaranga trachyphylla relied primarily on food bodies rather than homopterans, and the plants appeared to manipulate the ant colony size. Per capita plant investment in ants (ant dry weight plant dry weight^–1) was different among the four Macaranga species. The homoptera-dependent M. beccariana and M. bancana harbored lower biomass of ants than the food-body dependent M. winkleri, suggesting that energy loss is involved in the homoptera-interposing symbiotic system which has one additional trophic level. The plants investment ratio to the ants generally decreased as plants grew. The evolution of the plant reward-offering system in ant–plant–homopteran symbioses is discussed with an emphasis on the role of homopterans.     

Diversity of ant-plant interactions: protective efficacy in Macaranga species with different degrees of ant association

The pioneer tree Macaranga in SE Asia has developed manyfold associations with ants. The genus comprises all stages of interaction with ants, from facultative relationships to obligate myrmecophytes. Only myrmecophytic Macaranga offer nesting space for ants and are associated with a specific ant partner. The nonmyrmecophytic species are visited by a variety of different ant species which are attracted by extrafloral nectaries (EFN) and food bodies. Transitional Macaranga species like M. hosei are colonized later in their development due to their stem structure. Before the colonization by their specific Crematogaster partner the young plants are visited by different ant species attracted by EFN. These nectaries are reduced and food body production starts as soon as colonization becomes possible. We demonstrated earlier that obligate ant partners can protect their Macaranga plants against herbivore damage and vine cover. In this study we focused on nonspecific interactions and studied M. tanarius and M. hosei, representing a non-myrmecophyte and a transitional species respectively. In ant exclusion experiments both M. tanarius and M. hosei suffered significantly higher mean leaf damage than controls, 37% versus 6% in M. hosei, 16% versus 7% in M. tanarius. M. tanarius offers both EFN and food bodies so that tests for different effects of these two food rewards could be conducted. Plants with food bodies removed but with EFN remaining had the lowest mean increase of herbivore damage of all experimental groups. Main herbivores on M. hosei were mites and caterpillars. Many M. tanarius plants were infested by a shootborer. Both Macaranga species were visited by various ant species, Crematogaster spp. being the most abundant. We found no evidence for any specific relationships. The results of this study strongly support the hypothesis that non-specific, facultative associations with ants can be advantageous for Macaranga plants. Food bodies appear to have lower attractive value for opportunistic ants than EFN and may require a specific dietary adaptation. This is also indicated by the fact that food body production in the transitional M. hosei does not start before stem structure allows a colonization by the obligate Crematogaster species. M. hosei thus benefits from facultative association with a variety of ants until it produces its first domatia and can be colonized by its obligate mutualist.     

Timing of butterfly parasitization of a plant–ant–scale symbiosis

In the Southeast Asian tropics, Arhopala lycaenid butterflies feed on Macaranga ant-plants inhabited by Crematogaster (subgenus Decacrema) ants tending Coccus-scale insects. A recent phylogenetic study showed that (1) the plants and ants have been codiversifying for the past 20–16 million years (Myr), and that (2) the tripartite symbiosis was formed 9–7 Myr ago, when the scale insects became involved in the plant–ant mutualism. To determine when the lycaenids first parasitized the Macaranga tripartite symbiosis, we constructed a molecular phylogeny of the lycaenids that feed on Macaranga by using mitochondrial and nuclear DNA sequence data and estimated their divergence times based on the cytochrome oxidase I molecular clock. The minimum age of the lycaenids was estimated by the time-calibrated phylogeny to be 2.05 Myr, about one-tenth the age of the plant–ant association, suggesting that the lycaenids are latecomers that associated themselves with the pre-existing symbiosis of plant, ant, and scale insects.     

Assessing ant seed predation in threatened plants: a case study

Erodium paularense is a threatened plant species that is subject to seed predation by the granivorous ant Messor capitatus. In this paper we assessed the intensity and pattern of ant seed predation and looked for possible adaptive strategies at the seed and plant levels to cope with this predation. Seed predation was estimated in 1997 and 1998 at the population level by comparing total seed production and ant consumption, assessed by counting seed hulls in refuse piles. According to this method, ant seed predation ranged between 18% and 28%. A more detailed and direct assessment conducted in 1997 raised this estimate to 43%. In this assessment spatial and temporal patterns of seed predation by ants were studied by mapping all nest entrances in the studied area and marking the mature fruits of 109 reproductive plants with a specific colour code throughout the seed dispersal period. Intact fruit coats were later recovered from the refuse piles, and their mother plants and time of dispersal were identified. Seeds dispersed at the end of the dispersal period had a greater probability of escaping from ant seed predation. Similarly, in plants with late dispersal a greater percentage of seeds escaped from ant predation. Optimum dispersal time coincided with the maximum activity of granivorous ants because, at this time, ants focused their harvest on other plant species of the community. It was also observed that within-individual seed dispersal asynchrony minimised seed predation. From a conservation perspective, results show that the granivorous ant–plant interaction cannot be assessed in isolation and that the intensity of its effects basically depends on the seed dispersal pattern of the other members of the plant community. Furthermore, this threat must be assessed by considering the overall situation of the target population. Thus, in E. paularense, the strong limitation of safe-sites for seedling establishment reduces the importance of seed predation.     

Chemical composition of leaf volatiles in Macaranga species (Euphorbiaceae) and their potential role as olfactory cues in host-localization of foundress queens of specific ant partners

Host-plant finding by foundress queens is an important step in the establishment of ant–plant symbioses and olfactory cues may play a crucial role in the Macaranga–Crematogaster ant–plant system for attracting foundresses over longer distances. MicroSPE was used to investigate leaf volatiles of 11 myrmecophytic and non-myrmecophytic Macaranga species. Chemical analysis (GC–MS) yielded a total of 114 compounds comprising a great diversity, including aliphatic compounds, aromatics, mono- and sesquiterpenoids. An analysis of the volatile data using the CNESS distances of the chemical profiles, followed by visualization of the data with non-metric multidimensional scaling (NMDS) showed that even closely related species sharing the same ant partners have clearly different scent patterns. Comparison of spectra of volatile compounds between obligate myrmecophytic Macaranga species and myrmecophilous species that are only facultatively associated with unspecific arboreal ants did not reveal general differences. Choice experiments conducted with foundresses revealed that the ants have the capacity to distinguish between different host species. However, the behavior of the foundresses following surface contact with saplings indicates that other cues, like surface structure, may play a more important role in host-recognition over short distances than volatile compounds. We discuss alternative hypotheses for the possible role of leaf volatiles in the examined Macaranga species as chemical defense against herbivores.     

Domatia as most important adaptations in the evolution of myrmecophytes in the paleotropical tree genusMacaranga (Euphorbiaceae)

The paleotropical tree genusMacaranga (Euphorbiaceae) comprises all stages of interaction with ants, from facultative associations to obligate myrmecophytes. In SE.-Asia food availability does not seem to be the limiting factor for the development of a close relationship since all species provide food for ants in form of extrafloral nectar and/or food bodies. Only myrmecophyticMacaranga species offer nesting space for ants (domatia) inside internodes which become hollow due to degeneration of the pith. Non-myrmecophytic species have a solid stem with a compact and wet pith and many resin ducts. The stem interior of some transitional species remains solid, but the soft pith can be excavated. The role of different ant-attracting attributes for the development of obligate ant-plant interactions is discussed. In the genusMacaranga, the provision of nesting space seems to be the most important factor for the evolution of obligate myrmecophytism.     

Congruence of Microsatellite and Mitochondrial DNA Variation in Acrobat Ants (Crematogaster Subgenus Decacrema,Formicidae: Myrmicinae) Inhabiting Macaranga (Euphorbiaceae) Myrmecophytes

A previously reported mitochondrial DNA (mtDNA) phylogeny of Crematogaster (subgenus Decacrema) ants inhabiting Macaranga myrmecophytes indicated that the partners diversified synchronously and their specific association has been maintained for 20 million years. However, the mtDNA clades did not exactly match morphological species, probably owing to introgressive hybridization among younger species. In this study, we determined the congruence between nuclear simple sequence repeat (SSR, also called microsatellite) genotyping and mtDNA phylogeny to confirm the suitability of the mtDNA phylogeny for inferring the evolutionary history of Decacrema ants. Analyses of ant samples from Lambir Hills National park, northeastern Borneo, showed overall congruence between the SSR and mtDNA groupings, indicating that mtDNA markers are useful for delimiting species, at least at the local level. We also found overall high host-plant specificity of the SSR genotypes of Decacrema ants, consistent with the specificity based on the mtDNA phylogeny. Further, we detected cryptic genetic assemblages exhibiting high specificity toward particular plant species within a single mtDNA clade. This finding, which may be evidence for rapid ecological and genetic differentiation following a host shift, is a new insight into the previously suggested long-term codiversification of Decacrema ants and Macaranga plants.     

Ant species confer different partner benefits on two neotropical myrmecophytes

The dynamics of mutualistic interactions involving more than a single pair of species depend on the relative costs and benefits of interaction among alternative partners. The neotropical myrmecophytes Cordia nodosa and Duroia hirsuta associate with several species of obligately symbiotic ants. I compared the ant partners of Cordia and Duroia with respect to two benefits known to be important in ant-myrmecophyte interactions: protection against herbivores provided by ants, and protection against encroaching vegetation provided by ants. Azteca spp., Myrmelachista schumanni, and Allomerus octoarticulatus demerarae ants all provide the leaves of Cordia and Duroia some protection against herbivores. However, Azteca and Allomerus provide more protection than does Myrmelachista to the leaves of their host plants. Although Allomerus protects the leaves of its hosts, plants occupied by Allomerus suffer more attacks by herbivores to their stems than do plants occupied by other ants. Relative to Azteca or Allomerus, Myrmelachista ants provide better protection against encroaching vegetation, increasing canopy openness over their host plants. These differences in benefits among the ant partners of Cordia and Duroia are reflected in the effect of each ant species on host plant size, growth rate, and reproduction. The results of this study show how mutualistic ant partners can differ with respect to both the magnitude and type of benefits they provide to the same species of myrmecophytic host.     

Friend or foe? A behavioral and stable isotopic investigation of an ant–plant symbiosis

In ant–plant symbioses, the behavior of ant inhabitants affects the nature of the interaction, ranging from mutualism to parasitism. Mutualistic species confer a benefit to the plant, while parasites reap the benefits of the interaction without reciprocating, potentially resulting in a negative impact on the host plant. Using the ant–plant symbiosis between Cordia alliodora and its ant inhabitants as a model system, I examine the costs and benefits of habitation by the four most common ant inhabitants at La Selva Biological Station, Costa Rica. Costs are measured by counting coccoids associated with each ant species. Benefits include patrolling behavior, effectiveness at locating resources, and recruitment response. I also compare the diets of the four ant species using stable isotope analysis of nitrogen (N) and carbon (C). Ants varied in their rates of association with coccoids, performance of beneficial behaviors, and diet. These differences in cost, benefit, and diet among the ant species suggest differences in the nature of the symbiotic relationship between C. alliodora and its ants. Two of the ant species behave in a mutualistic manner, while the other two ant species appear to be parasites of the mutualism. I determined that the mutualistic ants feed at a higher trophic level than the parasitic ants. Behavioral and dietary evidence indicate the protective role of the mutualists, and suggest that the parasitic ants do not protect the plant by consuming herbivores.     

Relationships between the ant Brachymyrmex obscurior (Hymenoptera, Formicidae) and Acacia pennatula (Fabaceae)

Summary. In central Mexico, the ant Brachymyrmex obscurior Forel feeds on nectar produced by extrafloral nectaries of Acacia pennatula (Schlecht. & Cham.) Benth. However, no studies have determined whether the ants visitation is related to plant nectar availability and whether ants protect A. pennatula from herbivory. The objectives of this 2-yr study (2000–2001) were to assess whether seasonal changes in ant visitation coincide with extrafloral nectar productivity in A. pennatula and to determine whether ants protect the plant. At the end of the dry season (April–June) B. obscurior was the only ant species on A. pennatula and extrafloral nectar production is limited to this period. Exclusion experiments, performed at the end of the dry season showed that A. pennatula did not receive a protective benefit when visited by ants. Branches with ants and branches where ants are excluded had similar numbers of the nonmyrmecophile leafhopper Sibovia sp. which was the only herbivore observed under natural conditions.Received 24 March 2004; revised 4 September 2004; accepted 8 September 2004.     

Host‐plant use by two Orthomeria (Phasmida: Aschiphasmatini) species feeding on Macaranga myrmecophytes

Myrmecophytes depend on symbiotic ants (plant‐ants) to defend against herbivores. Although these defensive mechanisms are highly effective, some herbivorous insects can use myrmecophytes as their host‐plants. The feeding habits of these phytophages on myrmecophytes and the impacts of the plant‐ants on their feeding behavior have been poorly studied. We examined two phasmid species, Orthomeria alexis and O. cuprinus, which are known to feed on Macaranga (Euphorbiaceae) myrmecophytes in a Bornean primary forest. Our observations revealed that: (i) each phasmid species relied on two closely‐related myrmecophytic Macaranga species for its host‐plants in spite of their normal plant‐ant symbioses; and (ii) there was little overlap between their host‐plant preferences. More O. cuprinus adults and nymphs were found on new leaves, which were attended by more plant‐ants than mature leaves, while most adults and nymphs of O. alexis tended to avoid new leaves. In a feeding choice experiment under ant‐excluded conditions, O. alexis adults chose a non‐host Macaranga myrmecophyte that was more intensively defended by plant‐ants and was more palatable than their usual host‐plants almost as frequently as their usual host‐plant, suggesting that the host‐plant range of O. alexis was restricted by the presence of plant‐ants on non‐host‐plants. Phasmid behavior that appeared to minimize plant‐ant attacks is described.     

Macaranga and Mallotus species (Euphorbiaceae) as indicators for disturbance in the mixed lowland dipterocarp forest of East Kalimantan (Indonesia)

The indicator value (IV) of Macaranga and Mallotus species (Euphorbiaceae) for different types of disturbance in lowland dipterocarp forest was assessed by counting and identifying all individuals of species of these genera taller than 30 cm in 45 (10 m×300 m) plots at nine locations. Twelve Macaranga and nine Mallotus species were found. The main forest disturbance types (primary forest, secondary forest, selectively logged forest, forest burned once, and repeatedly burned forest used for shifting-cultivation) each had their own set of indicator species. The level of disturbance in the forest types was assessed by measuring nine forest structural parameters. The occurrence of Macaranga and Mallotus species was closely related to the level of disturbance in a forest. Most Macaranga species were characteristic of high disturbance levels, while most Mallotus species preferred intermediate to low levels of disturbance. However, both genera had species at both disturbance extremes. Using multiple regression analysis, combinations of Macaranga and Mallotus species were formed and used to predict the separate forest structural parameters and the general level of disturbance of a forest. The Macaranga and Mallotus species could be grouped into (1) primary forest ‘remnant’ species; (2) generalist pioneer species; and (3) high disturbance pioneer species.     

Herbivory-induced extrafloral nectar increases native and invasive ant worker survival

Ascertaining the costs and benefits of mutualistic interactions is important for predicting their stability and effect on community dynamics. Despite widespread designation of the interaction between ants and extrafloral nectaries (EFNs) as a mutualism and over 100 years of studies on ant benefits to plants, the benefits to ants have never been experimentally quantified. The success of invasive ants is thought to be linked to the availability of carbohydrate-rich resources, though reports of invasive ant visits to EFNs are mixed. In two laboratory experiments, we compared worker survival of one native (Iridomyrmex chasei) and two invasive ant species (Linepithema humile and Pheidole megacephala) exposed to herbivorized or non-herbivorized EFN-bearing plants (Acacia saligna) or positive and negative controls. We found that non-herbivorized plants did not produce any measurable extrafloral nectar, and ants with access to non-herbivorized plants had the same survival as ants with access to an artificial plant and water (unfed ants). Ants given herbivorized plants had 7–11 times greater worker survival relative to unfed ants, but there were no differences in survival between native and invasive ants exposed to herbivorized plants. Our results reveal that ants cannot induce A. saligna extrafloral nectar production, but workers of both native and invasive ant species can benefit from extrafloral nectar as much as they benefit from sucrose.     

Extrafloral nectaries in the genus Macaranga (Euphorbiaceae) in Malaysia: comparative studies of their possible significance as predispositions for myrmecophytism

Some species of the paleotropical tree genus Macaranga (Euphorbiaceae) live in close association with ants. The genus comprises the full range of species from those not regularly inhabited by ants to obligate myrmecophytes. In Malaysia (Peninsular and Borneo) 23 of the 52 species are known to be ant-associated (44%). The simplest structural adaptation of plants to attract ants are extrafloral nectaries. We studied the distribution of extrafloral nectaries in the genus Macaranga to assess the significance of this character as a possible predisposition for the evolution of obligate myrmecophytism. All species have marginal glands on the leaves. However, only the glands of non- myrmecophytic species function as nectaries, whereas liquids secreted by these glands in myrmecophytic species did not contain sugar. Some non-myrmecophytic Macaranga and transitional Macaranga species in addition have extrafloral nectaries on the leaf blade near the petiole insertion. All obligatorily myrmecophytic Macaranga species, however, lack additional glands on the lamina. The non-myrmecophytic species are visited by a variety of different ant species, whereas myrmecophytic Macaranga are associated only with one specific ant-partner. Since these ants keep scale insects in the hollow stems, reduction of nectary production in ant-inhabited Macaranga seems to be biologically significant. We interpret this as a means of (a) saving the assimilates and (b) stabilization of maintenance of the association's specificity. Competition with other ant species for food rewards is avoided and thereby danger of weakening the protective function of the obligate ant- partner for the plant is reduced. A comparison with other euphorb species living in the same habitats as Macaranga showed that in genera in which extrafloral nectaries are widespread, no myrmecophytes have evolved. Possession of extrafloral nectaries does not appear to be essential for the development of symbiotic ant-plant interactions. Other predispositions such as nesting space might have played a more important role.     

Experimental evaluation of the impacts of two ant species on banana weevil in Uganda

The banana weevil, Cosmopolites sordidus (Germar), is an important pest of bananas. Predatory ants are increasingly being viewed as possible biological control agents of this pest because they are capable of entering banana plants and soil in search of prey. We studied ant predation on banana weevil in Uganda in crop residues and live plants in both laboratory and field experiments. Field studies with live plants used chemical ant exclusion in some plots and ant enhancement via colony transfer in others to measure effects of Pheidole sp. 2 and Odontomachus troglodytes Santschi on plant damage and densities of immature banana weevils.In crop residues, an important pest breeding site, twice as many larvae were removed from ant-enhanced plots as in control plots. In young (2 month) potted suckers held in shade houses, ant ability to reduce densities of banana weevil life stages varied with the weevil inoculation rate. At the lowest density (2 female weevils per pot), densities of eggs, larvae, and pupae were reduced by ants. At higher rates there was no effect. In older suckers (5–11 months) grown in larger containers, banana weevil densities were not affected by ants, but damage levels were reduced. In a field trial lasting a full crop cycle (30 months), we found that the ants tested reduced the density of banana weevil eggs in suckers during the crop, but did not affect larval densities in the sampled suckers. However, most larvae occur in the main banana plants, rather than associated suckers. Nevertheless, levels of damage in mature plants at harvest did not differ between Amdro-treated and ant-enhanced plots, suggesting the ant species studied were not able to provide economic control of banana weevil under our test conditions.     

Monophyletic clades of Macaranga‐pollinating thrips show high specificity to taxonomic sections of host plants

Thrips (Thysanoptera) have been recorded as pollinators of various plant species, but they are mostly regarded to be of low ecological relevance. In Southeast Asia, thrips were recently discovered to pollinate flowers of several taxonomic sections of the pioneer tree genus Macaranga (Euphorbiaceae), which is particularly well known as an ant‐plant, and for its importance in early forest succession. The lack of taxonomic treatment and of knowledge about systematic relationships among extant thrips, however, has prevented firm conclusions on the specificity of this plant–pollinator interaction. Here, results from sequencing of the mitochondrial cytochrome oxidase subunit support our previous morphospecies concept of Macaranga flower thrips, and confirm the genetic identity of five recently described species. They were remarkably all assigned to the genus Dolichothrips (Phlaeothripidae), which typically consists of phytophagous species. In addition, the molecular data revealed one cryptic species. A first phylogenetic tree of the Dolichothrips associated with Macaranga provides insights into their systematic position. In particular, we identify monophyly of all important Macaranga pollinator species, all species being largely specific to particular taxonomic host plant sections. Our results suggest a closely matched diversification of pollinating thrips with Macaranga trees. This adds a novel type of association to thrips pollinator–plant interactions, which have been so far documented as single‐species interactions or generalist thrips species visiting multiple plant taxa.     

Differential host use in two highly specialized ant-plant associations: evidence from stable isotopes

Carbon and nitrogen stable isotopes were used to examine variation in ant use of plant resources in the Cecropia obtusifolia / Azteca spp. association in Costa Rica. Tissue of ants, host plants and symbiotic pseudococcids were collected along three elevation transects on the Pacific slope of Costa Ricas Cordillera Central, and were analyzed for carbon and nitrogen isotopic composition. Worker carbon and nitrogen signatures were found to vary with elevation and ant colony size, and between Azteca species groups. Ants in the A. constructor species group appear to be opportunistic foragers at low elevations, but rely more heavily on their host plants at high elevations, whereas ants in the A. alfari species group consume a more consistent diet across their distribution. Further, isotope values indicate that both ant species groups acquire more nitrogen from higher trophic levels at low elevation and when ant colonies are small. Provisioning by the host is a substantial ecological cost to the interaction, and it may vary, even in a highly specialized association. Nonetheless, not all specialized interactions are equivalent; where interaction with one ant species group appears conditional upon the environment, the other is not. Differential host use within the Cecropia-Azteca association suggests that the ecological and evolutionary benefits and costs of association may vary among species pairs.     

Molecular phylogeny of the Acre clade (Crassulaceae): Dealing with the lack of definitions for Echeveria and Sedum

The phylogenetic relationships within many clades of the Crassulaceae are still uncertain, therefore in this study attention was focused on the “Acre clade”, a group comprised of approximately 526 species in eight genera that include many Asian and Mediterranean species of Sedum and the majority of the American genera (Echeveria, Graptopetalum, Lenophyllum, Pachyphytum, Villadia, and Thompsonella). Parsimony and Bayesian analyses were conducted with 133 species based on nuclear (ETS, ITS) and chloroplast DNA regions (rpS16, matK). Our analyses retrieved four major clades within the Acre clade. Two of these were in a grade and corresponded to Asian species of Sedum, the rest corresponded to a European–Macaronesian group and to an American group. The American group included all taxa that were formerly placed in the Echeverioideae and the majority of the American Sedoideae. Our analyses support the monophyly of three genera – Lenophyllum, Thompsonella, and Pachyphytum; however, the relationships among Echeveria, Sedum and the various segregates of Sedum are largely unresolved. Our analyses represents the first broad phylogenetic framework for Acre clade, but further studies are necessary on the groups poorly represented here, such as the European and Asian species of Sedum and the Central and South American species of Echeveria.