The Effects of Invasive Ants on Prospective Ant Mutualists

Ants are recognized for their abilities both to engage in mutualistic interactions with diverse taxa, and to invade and dominate habitats outside their native geographic range. Here, we review the effects of invasive ants on three guilds of mutualists: ant-dispersed plants, ant-tended arthropods, and ant-tended plants. We contrast how those three guilds are affected by invasions, how invasive ants differ from native ants in their interactions with those guilds, and how the seven most invasive ant species differ amongst themselves in those interactions. Ant-dispersed plants typically suffer from interactions with invasive ants, a result we attribute to the small size of those ants relative to native seed-dispersing ants. Effects on the ant-tended arthropods and plants were more frequently positive or non-significant, although it is unclear how often these interactions are reciprocally beneficial. For example, invasive ants frequently attack the natural enemies of these prospective mutualists even in the absence of rewards, and may attack those prospective mutualists. Many studies address whether invasive ants provide some benefit to the partner, but few have asked how invasives rank within a hierarchy of prospective mutualists that includes other ant species. Because ant invasions typically result in the extirpation of native ants, this distinction is highly relevant to predicting and managing the effects of such invasions. Interspecific comparisons suggest that invasive ants are poorer partners of ant-dispersed plants than are most other ants, equally effective partners of ant-tended arthropods, and perhaps better partners of ant-tended plants. Last, we note that the invasive ant taxa differ amongst themselves in how they affect these three mutualist guilds, and in how frequently their interactions with prospective mutualists have been studied. The red imported fire ant, Solenopsis invicta, appears particularly likely to disrupt all three mutualistic interactions, relative to the other six invasive species included in this review.     

Insights into a novel three-partner interaction between ants,coreids (Hemiptera: Coreidae) and extrafloral nectaries: implications for the study of protective mutualisms

Extrafloral nectar of plants and honeydew of hemipterans are the common mediators of facultative interactions that involve ants as a mobile strategy of defence. The outcome of these interactions can vary from mutualistic to commensalistic or even antagonistic, depending on the ecological context and the interacting species. Here, we explore a novel, three-partner interaction involving ants, the coreid Dersagrena subfoveolata (Hemiptera) and the extrafloral nectaries (EFNs) bearing plant Senna aphylla (Fabaceae) in semi-arid Northwest Argentina. We surveyed natural areas and conducted ant exclusion experiments, to understand how each pairwise interaction influences the overall outcome among the three interacting parts. The outcome of the interactions was assessed for experimental plants as the reproductive output and herbivore abundances and for coreids as predator abundances. We found that the coreids occurred exclusively on S. aphylla plants and that at least nine ant species interacted with the EFNs as well as with the coreids. Coreid occurrence and abundance depended on ant densities, which in turn, was determined by the presence of actively secreting EFNs. Coreid and ant presence did not influence plant reproductive success, and ants provided to coreids some biotic defence, mainly against vespid wasp predators, but had no effect on non-coreid herbivores. We conclude that the interaction outcome is commensalistic between ants and plants (assuming that EF nectar is not costly for the plant), antagonistic between coreids and plants, and mutualistic between coreids and ants. The sum of all outcomes is net positive effect for ants and coreids, and net slightly negative to neutral for plants.     

Argentine ants displace floral arthropods in a biodiversity hotspot

Argentine ant (Linepithema humile (Mayr)) invasions are often associated with the displacement of ground‐dwelling arthropods. Argentine ant invasions can also exert other effects on the community through interactions with plants and their associated arthropods. For example, carbohydrate resources (e.g. floral or extrafloral nectar) may influence foraging behaviour and interactions among ants and other arthropods. In South Africa's Cape Floristic Region, Argentine ants and some native ant species are attracted to the floral nectar of Leucospermum conocarpodendron Rourke (Proteaceae), a native tree that also has extrafloral nectaries (EFNs). Despite having relatively low abundance in pitfall traps, Argentine ants visited inflorescences more frequently and in higher abundance than the most frequently observed native ants, Camponotus spp., though neither native nor Argentine ant floral foraging was influenced by the EFNs. Non‐metric multidimensional scaling revealed significant dissimilarity in arthropod communities on inflorescences with Argentine ants compared to inflorescences with native or no ants, with Coleoptera, Diptera, Hymenoptera, Arachnida, Orthoptera, and Blattaria all being underrepresented in inflorescences with Argentine ants compared to ant‐excluded inflorescences. Native honeybees (Apis mellifera capensis Eschscholtz) spent 75% less time foraging on inflorescences with Argentine ants than on inflorescences without ants. Neither Argentine ant nor native ant visits to inflorescences had a detectable effect on seed set of Le. conocarpodendron. However, a pollen supplementation experiment revealed that like many other proteas, Le. conocarpodendron is not pollen‐limited. Flower predation was negatively associated with increased ant visit frequency to the inflorescences, but did not differ among inflorescences visited by native and Argentine ants. Displacement of arthropods appears to be a consistent consequence of Argentine ant invasions. The displacement of floral arthropods by Argentine ants may have far‐reaching consequences for this biodiversity hotspot and other regions that are rich in insect‐pollinated 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.     

Ant seed removal in a non-myrmecochorous Neotropical shrub: Implications for seed dispersal

This study investigated ant seed removal of Piper sancti-felicis, an early successional Neotropical shrub. Neotropical Piper are a classic example of bat-dispersed plants, but we suggest that ants are underappreciated dispersal agents. We identified eleven ant species from the genera Aphaenogaster, Ectatomma, Paratrechina, Pheidole, Trachymyrmex, and Wasmannia recruiting to and harvesting P. sancti-felicis seeds in forest edge and secondary forest sites at La Selva, Costa Rica. We also tested for differences in ant recruitment to five states in which ants can commonly encounter seeds: unripe fruit, ripe fruit, overripe fruit, bat feces, and cleaned seeds. Overall, ants harvested more seeds from ripe and overripe fruits than other states, but this varied among species. To better understand the mechanisms behind ant preferences for ripe/overripe fruit, we also studied how alkenylphenols, secondary metabolites found in high concentrations in P. sancti-felicis fruits, affected foraging behavior in one genus of potential ant dispersers, Ectatomma. We found no effects of alkenylphenols on recruitment of Ectatomma to fruits, and thus, these compounds are unlikely to explain differences in ant recruitment among fruits of different maturity. Considering that P. sancti-felicis seeds have no apparent adaptations for ant dispersal, and few ants removed seeds that were cleaned of pulp, we hypothesize that most ants are harvesting its seeds for the nutritional rewards in the attached pulp. This study emphasizes the importance of ants as important additional dispersers of P. sancti-felicis and suggests that other non-myrmecochorous, vertebrate-dispersed plants may similarly benefit from the recruitment to fruit by ants.     

Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Dispersal and the dynamics of the soil seed-bank

Genet survival in seeds of Acacia suaveolens was examined through both dispersal and dormancy in the soil in populations near Sydney. Following initial passive seed-fall, the majority of seeds lie within a 1 m radius of the stem of the parent. Further dispersal is predominately mediated by ants. A. suaveolens seeds possess an elaiosome which attracts ants. When elaiosomes are removed, the potential for further dispersal of seeds is greatly reduced. Three species of ant disperse seeds of A. suaveolens and the fate of seeds following ant dispersal was observed to depend on the particular species of ant involved. Ants of both Iridomyrmex sp. and Pheidole sp. B are too small to drag seeds and, instead, ants of these species usually remove the elaiosome in situ, with little dispersal of the-seed resulting. Ants of Pheidole sp. A are larger and disperse seeds further, frequently taking them into their nests where the elaisosome is removed. Seeds are retained inside the nests and incorporated into the floors and walls of passageways and chambers. Several supposed ‘advantages’ of myrmecochory were examined but none were verified. Instead, two distinct ‘disadvantages’ were identified. These were: burial of seeds by ants of Pheidole sp. A into ‘unsafe sites’; and too deep a burial of seeds in nests for seeds to receive a stimulus to germinate during fires, and for seedlings to emerge successfully. Outside nests of Pheidole sp. A. seeds are concentrated in the top 5 cm of the soil, whilst within nests of these ants, seeds are found up to 15 cm deep. The dynamics of various components of the soil seed-bank were examined using seeds buried in nylon mesh containers. The seed-bank is persistent without annual recruitment to seedlings, enabling a population to persist as seeds after all above-ground plants have perished.     

Ant attendance of the cotton aphid is beneficial for okra plants: deciphering multitrophic interactions

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Seed predators are undeterred by nectar-feeding ants on Chamaecrista nictitans (Caesalpineaceae)

There are many examples of mutualistic interactions between ants and plants bearing extrafloral nectaries (EFN). The annual legume Chamaecrista nictitans (Caesalpineaceae) secretes nectar from EFN, specialized structures that attract ants, spiders, and other arthropods. The effects of manipulated C. nictitans patch size and location on plant-ant interactions were tested. Defense from herbivores was not detected; plants with ants did not set significantly more fruit or seed than plants with ants excluded. On the contrary, in one year, plants without ants set more fruit and seed than C. nictitans with ants. The cause of this was not determined. Furthermore, insect herbivore damage was low during three years of observations. Sennius cruentatus (Bruchidae), a specialist seed predator beetle, escaped ant defense despite the presence of numerous ants. Beetle progeny are protected during development by living inside maturing C. nictitans fruit and preventing fruits from dehiscing before emerging as adults. Although ants reduced percent of infestation in 1995, the total number of S. cruentatus per plant was not affected by ants in years of infestation. Overall, larger experimental C. nictitans patches attracted more ants, parasitoid wasps, and percent infestation by S. cruentatus while insect herbivores declined with increasing patch size. Location of patches within fields, however, did not affect numbers of arthropod visitors. Similar to manipulated populations, very little insect herbivory occurred in four reference populations. In contrast to the experimental populations, no S. cruentatus were recovered in reference populations of C. nictitans. Herbivory by insects may not always depress seed set by C. nictitans or may not exceed a threshold level. Thus, herbivory-reduction by ants may not have been detectable in these results. Seed predation may be more influential on C. nictitans reproduction. This revised version was published online in August 2006 with corrections to the Cover Date.     

No Defensive Role of Ants throughout a Broad Latitudinal and Elevational Range of a Cactus

Defensive mutualisms mediated by extrafloral nectaries are particularly variable; their net results may change with seasons, communities and environmental contexts. Particularly, an environmental factor that can promote changes in outcomes of ant‐plant interactions is elevation in mountainous regions. We tested whether (1) the interaction between the cactus Opuntia sulphurea and ant visitors of extrafloral nectaries is a defensive mutualism; and (2) ant‐plant interaction outcomes vary with elevation as a result of changes in herbivory rate and ant activity. To evaluate if the outcome of interactions was consistent at two extremes of the range distribution of O. sulphurea, we performed an ant‐exclusion experiment with plants at two growth conditions (natural or potted) in two sites with contrasting elevation (1235–1787 m asl) in a temperate region (Villavicencio Nature Reserve, Mendoza, Argentina), and in a tropical region (Huajchilla, La Paz, Bolivia). Although herbivory rate and ant visitation frequency increased with elevation, herbivore damage, plant reproductive success, and cladode growth rate were similar between plants excluded and non‐excluded from ants among sites, geographic regions and growth conditions. These results do not support the hypotheses that the interaction between O. sulphurea and ants is a defensive mutualism, and that elevation affects the net outcome of this ant‐plant interaction.     

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.     

Plant genotype shapes ant-aphid interactions: implications for community structure and indirect plant defense

Little is known about the mechanisms by which plant genotype shapes arthropod community structure. In a field experiment, we measured the effects of milkweed (Asclepias syriaca) genotype and ants on milkweed arthropods. Populations of the ant-tended aphid Aphis asclepiadis and the untended aphid Myzocallis asclepiadis varied eight- to 18-fold among milkweed genotypes, depending on aphid species and whether ants were present. There was no milkweed effect on predatory arthropods. Ants increased Aphis abundance 59%, decreased Myzocallis abundance 52%, and decreased predator abundance 56%. Milkweed genotype indirectly influenced ants via direct effects on Aphis and Myzocallis abundance. Milkweed genotype also modified ant-aphid interactions, influencing the number of ants attracted per Aphis and Myzocallis. While ant effects on Myzocallis were consistently negative, effects on Aphis ranged from antagonistic to mutualistic among milkweed genotypes. As a consequence of milkweed effects on ant-aphid interactions, ant abundance varied 13-fold among milkweed genotypes, and monarch caterpillar survival was negatively correlated with genetic variation in ant abundance. We speculate that heritable variation in milkweed phloem sap drives these effects on aphids, ants, and caterpillars. In summary, milkweed exerts genetic control over the interactions between aphids and an ant that provides defense against foliage-feeding caterpillars.     

Production of food bodies on the reproductive organs of myrmecophytic Macaranga species (Euphorbiaceae): effects on interactions with herbivores and pollinators

In protective ant–plant mutualisms, plants offer ants food (such as extrafloral nectar and/or food bodies) and ants protect plants from herbivores. However, ants often negatively affect plant reproduction by deterring pollinators. The aggressive protection that mutualistic ants provide to some myrmecophytes may enhance this negative effect in comparison to plant species that are facultatively protected by ants. Because little is known about the processes by which myrmecophytes are pollinated in the presence of ant guards, we examined ant interactions with herbivores and pollinators on plant reproductive organs. We examined eight myrmecophytic and three nonmyrmecophytic Macaranga species in Borneo. Most of the species studied are pollinated by thrips breeding in the inflorescences. Seven of eight myrmecophytic species produced food bodies on young inflorescences and/or immature fruits. Food body production was associated with increased ant abundance on inflorescences of the three species observed. The exclusion of ants from inflorescences of one species without food rewards resulted in increased herbivory damage. In contrast, ant exclusion had no effect on the number of pollinator thrips. The absence of thrips pollinator deterrence by ants may be due to the presence of protective bracteoles that limit ants, but not pollinators, from accessing flowers. This unique mechanism may account for simultaneous thrips pollination and ant defense of inflorescences.     

Ant visitation to extrafloral nectaries decreases herbivory and increases fruit set in Chamaecrista debilis (Fabaceae) in a Neotropical savanna

Studies of ant–plant relationships elucidate how top-down effects of the third trophic level can affect the biomass, richness, and/or species composition of plants. Although widespread in the neotropics, few studies have so far examined the direct effects of ants on plant fitness. Here, through experimental manipulation (ant-exclusion) under natural conditions, we examined the effect of ant visitation to extrafloral nectaries on leaf herbivory and fruit set in Chamaecrista debilis in the Brazilian savanna. As opposed to other Chamaecrista species, our results showed that visiting ants (15 species) significantly reduce herbivory and increase fruit set by more than 50% compared to plants from which ants were excluded. This mutualistic system is facultative in nature, and corroborates the potential beneficial role of exudate-feeding ants as anti-herbivore agents of tropical plants.     

Altitudinal variation in ant–aphid mutualism in nitrogen transfer of oak (<Emphasis Type="Italic">Quercus liaotungensis</Emphasis>)

Ant–plant relationship is a model for the study of the ecology and evolution of interspecific interactions. In direct ant–plant mutualism (i.e., plants providing food or nesting places for ants, and ants protecting the plants in return) ants provide nutrients to plants. However, whether a similar mechanism exists in indirect ant–plant mutualism (i.e., an ant–aphid–plant system) remains unknown. In this work, we used the ¹⁵N stable isotope method to study altitudinal variations in the roles of ants in the nutrient transfer of oak (Quercus liaotungensis). Our work shows that ants deliver nitrogen in indirect ant–plant interactions, and that the effect of nutrient transfer differed significantly with altitude. Ants’ trophic level at high altitudes was significantly lower than that at low altitudes, indicating that the degree of ant–aphid mutualism was greater at high altitudes, which may be beneficial in nitrogen transfer. Our work suggests that ant–aphid mutualism might be context dependent, such that it affects nutrient transfer in the food web, and that this context dependency is an important factor that influences altitudinal variation in nutrient transfer.     

Coexistence through mutualist‐dependent reversal of competitive hierarchies

Mechanisms that allow for the coexistence of two competing species that share a trophic level can be broadly divided into those that prevent competitive exclusion of one species within a local area, and those that allow for coexistence only at a regional level. While the presence of aphid‐tending ants can change the distribution of aphids among host plants, the role of mutualistic ants has not been fully explored to understand coexistence of multiple aphid species in a community. The tansy plant (Tanacetum vulgare) hosts three common and specialized aphid species, with only one being tended by ants. Often, these aphids species will not coexist on the same plant but will coexist across multiple plant hosts in a field. In this study, we aim to understand how interactions with mutualistic ants and predators affect the coexistence of multiple species of aphid herbivores on tansy. We show that the presence of ants drives community assembly at the level of individual plant, that is, the local community, by favoring one ant‐tended species, Metopeurum fuscoviride, while preying on the untended Macrosiphoniella tanacetaria and, to a lesser extent, Uroleucon tanaceti. Competitive hierarchies without ants were very different from those with ants. At the regional level, multiple tansy plants provide a habitat across which all aphid species can coexist at the larger spatial scale, while being competitively excluded at the local scale. In this case, ant mutualist‐dependent reversal of the competitive hierarchy can drive community dynamics in a plant–aphid system.     

Limited gene dispersal and spatial genetic structure as stabilizing factors in an ant‐plant mutualism

Comparative studies of the population genetics of closely associated species are necessary to properly understand the evolution of these relationships because gene flow between populations affects the partners' evolutionary potential at the local scale. As a consequence (at least for antagonistic interactions), asymmetries in the strength of the genetic structures of the partner populations can result in one partner having a co‐evolutionary advantage. Here, we assess the population genetic structure of partners engaged in a species‐specific and obligatory mutualism: the Neotropical ant‐plant, Hirtella physophora, and its ant associate, Allomerus decemarticulatus. Although the ant cannot complete its life cycle elsewhere than on H. physophora and the plant cannot live for long without the protection provided by A. decemarticulatus, these species also have antagonistic interactions: the ants have been shown to benefit from castrating their host plant and the plant is able to retaliate against too virulent ant colonies. We found similar short dispersal distances for both partners, resulting in the local transmission of the association and, thus, inbred populations in which too virulent castrating ants face the risk of local extinction due to the absence of H. physophora offspring. On the other hand, we show that the plant populations probably experienced greater gene flow than did the ant populations, thus enhancing the evolutionary potential of the plants. We conclude that such levels of spatial structure in the partners' populations can increase the stability of the mutualistic relationship. Indeed, the local transmission of the association enables partial alignments of the partners' interests, and population connectivity allows the plant retaliation mechanisms to be locally adapted to the castration behaviour of their symbionts.     

Ants climb plants because they cannot swim: ant presence on flowers during the flood season reduces the frequency of floral visitors

1. Seasonal changes in environments may not only affect habitat connectivity but may also affect its use by species and their interactions. Thus, during the flood season, ants are forced to develop survival strategies such as vertical plant migration.
2. According to this, it has been hypothesized that the presence of ants may directly affect plant-pollinator interactions.
3. Thus, we asked the following questions: (i) Are floral visitors of Hyptis brevipes expelled due to ant presence on inflorescences during the flood period? (ii) Is the ant effect mediated by the abundance of ants foraging on inflorescences? And, (iii) Does flower abundance predict the abundance of floral visits and ants?
4. We experimentally sampled 59 H. brevipes plants with and without ants during the flooded season, and observed no differences in flower abundance between ant treatments.
5. The probability of detaining floral visitors on H. brevipes increased with ant abundance and exceeded 50% possible repellency, but the probability of visitor deterrence was not related to flower abundance. Furthermore, the abundance of flowers did not predict the number of ants on H. brevipes individuals or the frequency of floral visits.
6. Consequently, ant repelling effects are pronounced when there are more ants foraging on plants. However, the ant repelling effect can be mitigated when plants flourish all year-round and exhibit higher concentrations of flowers in the dry months. Additionally, the different sexual functions of plants may present specific responses due to the explosive pollination mechanism associated with ant effects.

     

Chemical warfare between fungus-growing ants and their pathogens

Fungus-growing attine ants are under constant threat from fungal pathogens such as the specialized mycoparasite Escovopsis, which uses combined physical and chemical attack strategies to prey on the fungal gardens of the ants. In defence, some species assemble protective microbiomes on their exoskeletons that contain antimicrobial-producing Actinobacteria. Underlying this network of mutualistic and antagonistic interactions are an array of chemical signals. Escovopsis weberi produces the shearinine terpene-indole alkaloids, which affect ant behaviour, diketopiperazines to combat defensive bacteria, and other small molecules that inhibit the fungal cultivar. Pseudonocardia and Streptomyces mutualist bacteria produce depsipeptide and polyene macrolide antifungals active against Escovopsis spp. The ant nest metabolome is further complicated by competition between defensive bacteria, which produce antibacterials active against even closely related species.     

Water Stress Strengthens Mutualism Among Ants,Trees, and Scale Insects

Abiotic environmental variables strongly affect the outcomes of species interactions. For example, mutualistic interactions between species are often stronger when resources are limited. The effect might be indirect: water stress on plants can lead to carbon stress, which could alter carbon-mediated plant mutualisms. In mutualistic ant–plant symbioses, plants host ant colonies that defend them against herbivores. Here we show that the partners'' investments in a widespread ant–plant symbiosis increase with water stress across 26 sites along a Mesoamerican precipitation gradient. At lower precipitation levels, Cordia alliodora trees invest more carbon in Azteca ants via phloem-feeding scale insects that provide the ants with sugars, and the ants provide better defense of the carbon-producing leaves. Under water stress, the trees have smaller carbon pools. A model of the carbon trade-offs for the mutualistic partners shows that the observed strategies can arise from the carbon costs of rare but extreme events of herbivory in the rainy season. Thus, water limitation, together with the risk of herbivory, increases the strength of a carbon-based mutualism.     

Ecology of the pteridophytes on the southern slopes of Mt. Kilimanjaro – I. Altitudinal distribution

140 taxa of 61 genera in 24 families of pteridophytes were recorded on the southern slopes of Mt. Kilimanjaro. These represent about one third of the entire pteridophyte flora of Tanzania. The families richest in species are the Aspleniaceae, the Adiantaceae, the Dryopteridaceae, the Thelypteridaceae and the Hymenophyllaceae. Due to its luxuriant montane rain forest, which receives a precipitation of up to over 3000 mm, Mt. Kilimanjaro is distinctly richer in pteridophyte species than other volcanoes in East Africa. However, compared with the mountains of the Eastern Arc, the number of pteridophytes on Mt. Kilimanjaro is smaller. This can be explained by the widely destroyed submontane (intermediate) forest rather than by the higher age of the Eastern Arc Mts.The altitudinal distribution of all ferns was investigated in 24 transects. On the southern slopes of Mt. Kilimanjaro they were found in an altitudinal range of 3640 m. Cyclosorus quadrangularis, Azolla nilotica, Azolla africana andMarsilea minuta are restricted to the foothills, while Polystichum wilsonii, Cystopteris nivalis and Asplenium adiantum-nigrum are species found in the highest altitudes.Based on unidimensionally constrained clustering and on the analysis of the lowermost and uppermost occurrence of species, floristic discontinuities within the transects were determined. From these data and from an evaluation of the distribution of ecological groups and life forms, 11 altitudinal zones could be distinguished: a colline zone (–900 m asl), a submontane zone (900–1600 m asl) with lower and upper subzones, a montane zone (1600-2800 m asl) divided into 4 subzones, a subalpine zone (2800–3900 m asl) with lower, middle and upper subzones, and finally a (lower) alpine zone above 3900 m. The highest species numbers were observed in the lower montane forest belt between 1600 and 2000 m altitude. The zonation of ferns found at Mt. Kilimanjaro corresponds well with the vegetational zonation described by other authors using bryophytes as indicators in different parts of the humid tropics.