Ant-plant Interactions Research Articles

Canopy Openness Enhances Diversity of Ant–Plant Interactions in the Brazilian Amazon Rain Forest

AbstractIn closed‐canopy tropical forest understory, light availability is a significant determinant of habitat diversity because canopy structure is highly variable in most tropical forests. Consequently, variation in canopy cover affects the composition and distribution of plant species via creating variable light environments. Nevertheless, little is known about how variation in canopy openness structures patterns of plant–animal interactions. Because of the great diversity and dominance of ants in tropical environments, we used ant–plant interactions as a focal network to evaluate how variation in canopy cover influences patterns of plant–insect interactions in the Brazilian Amazon rain forest. We observed that small increases in canopy openness are associated with increased diversity of ant–plant interactions in our study area, and this change is independent of plant or ant species richness. Additionally, we found smaller niche overlap for both ants and plants associated with greater canopy openness. We hypothesize that enhanced light availability increases the breadth of ant foraging sources because variation in light availability gives rise to plant resources of different quality and amounts. Moreover, greater light availability promotes vegetative growth in plants, creating ant foraging ‘bridges’ between plants. In sum, our results highlight the importance of environmental heterogeneity as a determinant of ant–plant interaction diversity in tropical environments.

Foliar endophytic fungi as potential protectors from pathogens in myrmecophytic Acacia plants

In defensive ant-plant interactions myrmecophytic plants express reduced chemical defense in their leaves to protect themselves from pathogens, and it seems that mutualistic partners are required to make up for this lack of defensive function. Previously, we reported that mutualistic ants confer plants of Acacia hindsii protection from pathogens, and that the protection is given by the ant-associated bacteria. Here, we examined whether foliar endophytic fungi may potentially act as a new partner, in addition to mutualistic ants and their bacteria inhabitants, involved in the protection from pathogens in myrmecophytic Acacia plants. Fungal endophytes were isolated from the asymptomatic leaves of A. hindsii plants for further molecular identification of 18S rRNA gene. Inhibitory effects of fungal endophytes were tested against Pseudomonas plant pathogens. Our findings support a potential role of fungal endophytes in pathogen the protection mechanisms against pathogens in myrmecophytic plants and provide the evidence of novel fungal endophytes capable of biosynthesizing bioactive metabolites.

Ant-plant interaction in a tropical savanna: may the network structure vary over time and influence on the outcomes of associations?

Plant-animal interactions occur in a community context of dynamic and complex ecological interactive networks. The understanding of who interacts with whom is a basic information, but the outcomes of interactions among associates are fundamental to draw valid conclusions about the functional structure of the network. Ecological networks studies in general gave little importance to know the true outcomes of interactions and how they may change over time. We evaluate the dynamic of an interaction network between ants and plants with extrafloral nectaries, by verifying the temporal variation in structure and outcomes of mutualism for the plant community (leaf herbivory). To reach this goal, we used two tools: bipartite network analysis and experimental manipulation. The networks exhibited the same general pattern as other mutualistic networks: nestedness, asymmetry and low specialization and this pattern was maintained over time, but with internal changes (species degree, connectance and ant abundance). These changes influenced the protection effectiveness of plants by ants, which varied over time. Our study shows that interaction networks between ants and plants are dynamic over time, and that these alterations affect the outcomes of mutualisms. In addition, our study proposes that the set of single systems that shape ecological networks can be manipulated for a greater understanding of the entire system.

Ant dominance hierarchy determines the nested pattern in ant-plant networks

Extrafloral nectar (EFN) is a predictable and renewable resource for many ant colonies, and different ant species compete strongly to obtain and monopolize this highly nutritious food resource. Despite the importance of competition in structuring patterns of ant–plant interactions, this biological mechanism has been largely ignored in studies involving ant–plant networks. In this study we investigate the role of ant dominance hierarchy in structuring an ecological network involving ants and EFN-bearing plants in a tropical coastal environment in Mexico. We show that within a nested ant–plant network, ant species found in the central core of highly interacting species were competitively superior, showing massive recruitment and resource domination, compared with peripheral species with fewer interactions. Moreover, we also observed that both central and peripheral ant species have the ability to quickly find the food resource. However, after 2 h of observation, central ant species are more frequently collected on the food resource when compared with peripheral species. We hypothesize that the existence of a central core of competitive ant species may indicate that most plant species found within ant–plant networks could be better protected against herbivory by these dominant ant species. In short, our results highlight the importance of competition and monopolization in the resource use by ants in the maintenance of the nested pattern in ant–plant mutualistic networks. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 405–414.

English

Castor bean (Ricinus communis L.) is a myrmecophytic plant species with specialized extrafloral nectar (EFN) glands that serve to attract predatory insects, which in return defend plant-tissues against herbivores. The EFN glands on castor bean plants are located along the leaf petioles as well as on the peduncles of its imperfect (unisexual) flowers. This field-project evaluates the richness, diversity, and species assemblage of insects visiting EFN glands located on (female and male) flower peduncles and leaf petioles on castor bean plants growing in a Southern California coastal landscape. We detected that EFN glands on female-flower peduncles were visited by an insect community that was distinct from that of the other two EFN gland types on castor bean. Additionally, the insects visiting EFN glands on male-flower peduncles more closely resembled those observed visiting EFN glands on leaf petioles. We conclude that the observed differences in the biotic defense of foliar and unisexual floral tissues on castor bean are congruent with the optimal plant-defense strategy of a monoecious pioneer species. Key words: Ant-plant interactions, monoecious, mutualism, myrmecophyte, plant-defense optimality, sexual dimorphism.

Individual-based ant-plant networks: diurnal-nocturnal structure and species-area relationship.

Despite the importance and increasing knowledge of ecological networks, sampling effort and intrapopulation variation has been widely overlooked. Using continuous daily sampling of ants visiting three plant species in the Brazilian Neotropical savanna, we evaluated for the first time the topological structure over 24 h and species-area relationships (based on the number of extrafloral nectaries available) in individual-based ant-plant networks. We observed that diurnal and nocturnal ant-plant networks exhibited the same pattern of interactions: a nested and non-modular pattern and an average level of network specialization. Despite the high similarity in the ants’ composition between the two collection periods, ant species found in the central core of highly interacting species totally changed between diurnal and nocturnal sampling for all plant species. In other words, this “night-turnover” suggests that the ecological dynamics of these ant-plant interactions can be temporally partitioned (day and night) at a small spatial scale. Thus, it is possible that in some cases processes shaping mutualistic networks formed by protective ants and plants may be underestimated by diurnal sampling alone. Moreover, we did not observe any effect of the number of extrafloral nectaries on ant richness and their foraging on such plants in any of the studied ant-plant networks. We hypothesize that competitively superior ants could monopolize individual plants and allow the coexistence of only a few other ant species, however, other alternative hypotheses are also discussed. Thus, sampling period and species-area relationship produces basic information that increases our confidence in how individual-based ant-plant networks are structured, and the need to consider nocturnal records in ant-plant network sampling design so as to decrease inappropriate inferences.

Temporary sterilization behavior of mutualistic partner ants in a Southeast Asian myrmecophyte

AbstractObligate ant–plant interactions are known to be mutualistic but plant‐ants that destroy flowers of their hosts have been reported. They were regarded as parasites in myrmecophytic systems. The mechanisms that lead to flower damage (sterilization) by plant‐ants are not easy to understand as most sterilizing ants are actually regular colonizers of their plants and normally offer protection against herbivores and/or plant competition. It is difficult to find general patterns of ant or plant traits even in the few yet known associations of flower sterilization. We here present the first study from Southeast Asia where flower sterilizing occurs in the complex mutualistic Macaranga–Crematogaster system that differs from other cases. Flowers of M. hullettii in the Gombak Valley were destroyed by all three associated specific and otherwise protective Crematogaster species. The hypotheses that limitation of nesting space or food are main proximate factors for flower destruction were not strongly supported in our study system. Ants are even attracted to flowers by special food bodies produced by the plants. Only younger, not yet reproductive colonies were found to destroy flowers but not colonies with alates, indicating that flower sterilization behavior may only occur when the onset of host reproduction precedes ant reproduction, perhaps leading to a change in ant behavior. Fruit set always occurred in larger trees, and saplings for colonizing ant queens were therefore always present in the local population, stabilizing the association.

Context dependency of rewards and services in an Indian ant–plant interaction: southern sites favour the mutualism between plants and ants

Abstract:Protection-based ant–plant mutualisms may vary in strength due to differences in ant rewards, abundance of protective ants and herbivory pressure. We investigated geographical and temporal variation in host plant traits and herbivory pressure at five sites spanning the distribution range of the myrmecophyteHumboldtia brunonis(Fabaceae) in the Indian Western Ghats. Southern sites had, on average, 2.4 times greater abundance of domatia-bearing individuals, 1.6 times greater extrafloral nectary numbers per leaf, 1.2 times larger extrafloral nectary sizes, 2.2 times greater extrafloral nectar (EFN) volumes and a two-fold increase in total amino acid and total sugar concentrations in EFN compared with northern sites. A strong protection-based mutualism with ants occurred at only one southern site where herbivory was highest, suggesting that investments in attracting ants correlate with anti-herbivore benefits gained from the presence of protective ants. Our results confirm a temporally stable north–south gradient in myrmecophytic traits in this ant-plant as several of these traits were re-sampled after a 5-y interval. However, the chemical composition of EFN varied at both spatial and short-term temporal scales suggesting that only repeated measurements of rewards such as EFN can reveal the real spectrum of trait variation in an ant–plant mutualistic system.

Do ant visitors to extrafloral nectaries of plants repel pollinators and cause an indirect cost of mutualism?

Plants and ants have widespread relationships that are commonly mediated by the offer of extrafloral nectar (EFN) to ants that protect plants against herbivores. However, these ant–plant interactions are highly facultative and vary in time and space, mainly depending on the characteristics of the ant species, such as density and aggressiveness. In general, the outcomes of these relationships are positive, but in some cases, the presence of ants is neutral or negative to plants. Some studies suggest that aggressive attacks or merely the presence of ants might reduce the visitation rate of insect pollinators, such as bees, to flowers. We used experimental manipulation in natural conditions to test the hypothesis that ants on flowers of EFN-bearing plants might be recognized as a danger by pollinators (bees) and reduce the plant fitness (fruit-set). Our results show that the avoidance that ant bodyguard species feeding on EFNs of the Malpighiaceae Heteropterys pteropetala cause in pollinators, is not enough to decrease plant fruit-set. However, ants were indeed identified as a danger to pollinators as hypothesized and as suggested for other plant–pollinator relationships: flowers with plastic ants placed on the petals produced significantly fewer fruits than other treatments (using instead a plastic circle) or the control (natural condition). Indirect costs of facultative mutualisms are the focus of few studies and have been performed only rarely in the Neotropics; our results show that mutualism must be considered in multitrophic interactions studies for a better understanding of the functioning of the system.

Comparative development of the rattan ocrea, a structural innovation that facilitates ant–plant mutualism

The ocrea is an extension of the leaf sheath that occurs in a wide range of angiosperms, including some rattan palms (Arecaceae/Palmae). In some rattan species, the ocrea is an inflated sac-like structure that acts as a domatium. Typically, ants occupy this domatium, tending their young and husbanding aphids, and potentially providing a defence mechanism for the rattan. We present a comparative study of early leaf development in five palm species, including both ocreate and non-ocreate taxa. Early leaf development was examined using SEM in Calamus longipinna, which has a pronounced ocrea, and compared with two other Calamus species with an inconspicuous ocrea, and two non-ocreate palms, Chamaedorea pochutlensis and Rhapis humilis. All three Calamus species examined develop an extension from the top of the leaf sheath, in contrast with the non-ocreate palms examined for comparison. The ocrea is a vascularized outgrowth from the top of the leaf sheath, initiated shortly after differentiation of the lamina and sheath. Calamus longipinna differs from the other two Calamus species in that plications develop on the surface of the ocrea, and persist as folds on the ocrea surface. The highly unusual ocreate plications of C. longipinna form axial folds that permit the sac-like ocrea to become inflated, and give it the properties of a domatium, thus facilitating the ant–plant interaction in this species. The homologies of the ocrea in rattans require further review because the interpretation of leaf-derived appendages such as ligules and stipules remains controversial based on contradictory evidence from location and vasculature.

Nutritional benefits from domatia inhabitants in an ant–plant interaction: interlopers do pay the rent

Summary How a symbiosis originates and is maintained are important evolutionary questions. Symbioses in myrmecophytes (plants providing nesting for ants) are believed to be maintained by protection and nutrients provided by specialist plant‐ants in exchange for nesting spaces (called domatia) and nourishment offered by ant‐plants. However, besides the benefits accrued from housing protective ants, the mechanisms contributing to the fitness advantages of bearing domatia have rarely been examined, especially because the domatia trait is usually constitutively expressed, and many myrmecophytes have obligate mutualisms with single ant species resulting in invariant conditions. In the unspecialized ant‐plant Humboldtia brunonis (Fabaceae) that offers extrafloral nectar to ants, only some plants produce domatia in the form of hollow internodes. These domatia have a self‐opening slit making them more prone to interlopers and are occupied mostly by non‐protective ants and other invertebrates, especially arboreal earthworms. The protection mutualism with ants is restricted in geographical extent, occurring only at a few sites in the southernmost part of this plant's range in the Western Ghats of India. We examined nutrient flux from domatia residents to the plant using stable isotopes. We found that between 9% (earthworms) and 17% (protective or non‐protective ants) of nitrogen of plant tissues nearest the domatium came from domatia inhabitants. Therefore, interlopers such as earthworms and non‐protective ants contributed positively to the nitrogen budget of localized plant modules of this understorey tree. 15N‐enriched feeding experiments with protective ants demonstrated that nutrients flowed from domatia inhabitants to nearby plant modules. Fruit set did not differ between paired hand‐pollinated inflorescences on domatia and non‐domatia bearing branches. This was possibly due to the nutrient flux from domatia to adjacent branches without domatia within localized modules. This study has demonstrated the nutritive role of non‐protective ants and non‐ant invertebrates, hitherto referred to as interlopers, in an unspecialized myrmecophyte. Our study suggests that even before the establishment of a specialized ant–plant protection mutualism, nutritional benefits conferred by domatia inhabitants can explain the fitness benefits of bearing domatia, and thus the maintenance of a trait that facilitates the establishment of a specialized ant–plant symbiosis.

Current issues in the evolutionary ecology of ant–plant symbioses

Ant-plant symbioses involve plants that provide hollow structures specialized for housing ants and often food to ants. In return, the inhabiting ants protect plants against herbivores and sometimes provide them with nutrients. Here, we review recent advances in ant-plant symbioses, focusing on three areas. First, the nutritional ecology of plant-ants, which is based not only on plant-derived food rewards, but also on inputs from other symbiotic partners, in particular fungi and possibly bacteria. Food and protection are the most important 'currencies' exchanged between partners and they drive the nature and evolution of the relationships. Secondly, studies of conflict and cooperation in ant-plant symbioses have contributed key insights into the evolution and maintenance of mutualism, particularly how partner-mediated feedbacks affect the specificity and stability of mutualisms. There is little evidence that mutualistic ants or plants are under selection to cheat, but the costs and benefits of ant-plant interactions do vary with environmental factors, making them vulnerable to natural or anthropogenic environmental change. Thus, thirdly, ant-plant symbioses should be considered good models for investigating the effects of global change on the outcome of mutualistic interactions.

Extrafloral nectar fuels ant life in deserts.

Interactions mediated by extrafloral nectary (EFN)-bearing plants that reward ants with a sweet liquid secretion are well documented in temperate and tropical habitats. However, their distribution and abundance in deserts are poorly known. In this study, we test the predictions that biotic interactions between EFN plants and ants are abundant and common also in arid communities and that EFNs are only functional when new vegetative and reproductive structures are developing. In a seasonal desert of northwestern Argentina, we surveyed the richness and phenology of EFN plants and their associated ants and examined the patterns in ant-plant interaction networks. We found that 25 ant species and 11 EFN-bearing plant species were linked together through 96 pairs of associations. Plants bearing EFNs were abundant, representing ca. 19 % of the species encountered in transects and 24 % of the plant cover. Most ant species sampled (ca. 77 %) fed on EF nectar. Interactions showed a marked seasonal pattern: EFN secretion was directly related to plant phenology and correlated with the time of highest ant ground activity. Our results reveal that EFN-mediated interactions are ecologically relevant components of deserts, and that EFN-bearing plants are crucial for the survival of desert ant communities.

Importance of interaction frequency in analysis of ant-plant networks in tropical environments

Abstract:Several studies have shown that qualitative (binary) ant-plant networks are highly nested in tropical environments, in which specialist species (with fewer interactions) are connected with generalists (with the most interactions) in cohesive subgroups. Interactions occur in both qualitative and quantitative networks, however, how their frequency may structure the nestedness in ecological networks involving these organisms is, we believe, unknown. Based on this perspective, we used nestedness analysis to address the effect of interaction frequency on ant-plant networks (n = 14 networks). Unlike binary networks, quantitative networks are often significantly non-nested. In addition, species with a higher interaction frequency have a higher number of links, indicating that these species are possibly more abundant and/or competitive. Moreover, different biological parameters can change the nature of ant-plant interactions, as a plant can be a good resource for one ant and a ‘bad’ resource for another. Thus, this suggests a new perspective for the study of interaction networks in the tropics, since species with lower interaction frequency are not necessarily subsets of species with higher frequency, and consequently generate the non-nested pattern in quantitative networks.

Nocturnal resource defence in aphid‐tending ants of northern Patagonia

Interspecific competition plays a key role in the organisation of ant communities. In ant–plant interactions, individual host plants are usually occupied by a single ant colony, and co‐occurring ant species compete for hosts. Here indirect evidence of competition between three dominant ant species that tend aphids on two biennial thistles in northern Patagonia is described, and a novel defensive behaviour in temperate ant assemblages is reported. This study has found that: (i) dominant ant species were not spatially segregated, thus enhancing the probability of fights and invasions of host plants; (ii) ant species did not show preferences for a thistle species or for any plant characteristic, and thus all plants have similar chances of being colonised by all dominant ant species; (iii) the resident ant species remained on the same plant during the whole plant life cycle, monopolising plant resources (aphids); and (iv) all dominant species, whose nests are on the ground, assigned some ants to stay on the host plant during the night, when the low temperatures typical of this temperate environment greatly reduce foraging activities. When these ‘nocturnal guards’ were experimentally removed from the host plant, other ants from the same colony rapidly appeared showing aggressive behaviours. Taking all these findings together, it is suggested that interspecific competition influences the distribution of ants on their host plants and involves nocturnal defensive behaviours despite unfavourable thermal conditions. This illustrates how habitat features, such as the short life cycle of thistles and the low night‐time temperatures that reduce ant foraging and thus make plants more vulnerable to invasion, might affect the distribution and behaviour of ants.

The Ecological Effects of Ant-Aphid Mutualism on Plants at a Large Spatial Scale

The protective ant-plant interaction has been considered as a model system in studying mutualistic interactions, but we know little about the ecological effects of the mutualism at relatively larger spatial scales. In this study, by excluding an aphid-tending ant species (Lasius fuliginosus) from all host oak trees (Quercus liaotungensis) in 20x20 m plots, we evaluated the effects of ants on herbivory, fruit production and leaf toughness of the host tree. Through a two years study, we found that ants have a significant anti-herbivory effect on the host tree, with no effects on fruit production. At the end of the growing season, leaf toughness for plants without ants increased significantly. This suggests that ants are reliable and effective bodyguards for plants at larger spatial scales. For plants, the possible tradeoff between different defensive strategies at larger scale should be focused in further works.

Influence of extrafloral nectary phenology on ant–plant mutualistic networks in a neotropical savanna

Temporal variation has been one remarkable feature of ecological interactions. In ant–plant mutualism, this variation is widely known, although little is understood about the mechanisms that shape these variations. This study tested whether or not the phenology of extrafloral nectaries (EFNs) influences the temporal variation of two properties of an ant–plant interaction network. The network under investigation exhibited a nested pattern and low specialisation over most months. Monthly nestedness and specialisation in the network were negatively correlated, both being influenced by temporal variations in extrafloral nectar production of the plant community. The months of highest activity in the nectaries (August–November) were those when the level of generalisation in the network was at its highest. Although there were temporal variations in the properties of the network, the generalist core of the species remained the same over time. The stable core enhances the coevolutionary importance of ant–plant interactions for the community. Thus, it can be concluded that the phenology of EFNs is one effective mechanism shaping the temporal variation in ant–plant interaction.

Long-term temporal variation in the organization of an ant–plant network

Functional groups of species interact and coevolve in space and time, forming complex networks of interacting species. A long-term study of temporal variation of an ant-plant network is presented with the aims of: (1) depicting its structural changes over a 20-year period; (2) detailing temporal variation in network topology, as revealed by nestedness and modularity analysis and other parameters (i.e. connectance, niche overlap); and (3) identifying long-term turnover in taxonomic structure (i.e. switches in ant resource use or plant visitor assemblages according to taxa). Fieldwork was carried out at La Mancha, Mexico, and ant-plant interactions were observed between 1989 and 1991, between 1998 and 2000, and between May 2010 and 2011. Occurrences of ants on extrafloral nectaries (EFNs) were recorded. The resulting ant-plant networks were constructed from qualitative presence-absence data determined by a species-species matrix defined by the frequency of occurrence of each pairwise ant-plant interaction. Network variation across time was stable and a persistent nested structure may have contributed to the maintenance of resilient and species-rich communities. Modularity was lower than expected, especially in the most recent networks, indicating that the community exhibited high overlap among interacting species (e.g. few species were hubs in the more recent network, being partly responsible for the nested pattern). Structurally, the connections created among modules by super-generalists gave cohesion to subsets of species that otherwise would remain unconnected. This may have allowed an increasing cascade-effect of evolutionary events among modules. Mutualistic ant-plant interactions were structured 20 years ago mainly by the subdominant nectarivorous ant species Camponotus planatus and Crematogaster brevispinosa, which monopolized the best extrafloral nectar resources and out-competed other species with broader feeding habits. Through time, these ants, which are still present, lost their position as network hubs and diminished in their importance in structuring the network; simultaneously, plants gained in importance. The long-term network analysis reveals a decrease in attended plant species richness, a notable increase in plant species participation from 1990 to 2010 (sustained by less plant taxonomic similarity in the older 1990 network), an increase in the number of ant species and a diminishing dominance of super-generalist ants. The structure of the community has remained highly nested and connected with low modularity, suggesting overall a more participative, homogeneous, cohesive interaction network. Although previous studies have suggested that interactions between ants and EFN-bearing plants are susceptible to seasonality, abiotic factors and perturbation, this cohesive structure appears to be the key for biodiversity and community maintenance.

A Novel Type of Nutritional Ant–Plant Interaction: Ant Partners of Carnivorous Pitcher Plants Prevent Nutrient Export by Dipteran Pitcher Infauna

Many plants combat herbivore and pathogen attack indirectly by attracting predators of their herbivores. Here we describe a novel type of insect–plant interaction where a carnivorous plant uses such an indirect defence to prevent nutrient loss to kleptoparasites. The ant Camponotus schmitzi is an obligate inhabitant of the carnivorous pitcher plant Nepenthes bicalcarata in Borneo. It has recently been suggested that this ant–plant interaction is a nutritional mutualism, but the detailed mechanisms and the origin of the ant-derived nutrient supply have remained unexplained. We confirm that N. bicalcarata host plant leaves naturally have an elevated 15N/14N stable isotope abundance ratio (δ15N) when colonised by C. schmitzi. This indicates that a higher proportion of the plants’ nitrogen is insect-derived when C. schmitzi ants are present (ca. 100%, vs. 77% in uncolonised plants) and that more nitrogen is available to them. We demonstrated direct flux of nutrients from the ants to the host plant in a 15N pulse-chase experiment. As C. schmitzi ants only feed on nectar and pitcher contents of their host, the elevated foliar δ15N cannot be explained by classic ant-feeding (myrmecotrophy) but must originate from a higher efficiency of the pitcher traps. We discovered that C. schmitzi ants not only increase the pitchers' capture efficiency by keeping the pitchers’ trapping surfaces clean, but they also reduce nutrient loss from the pitchers by predating dipteran pitcher inhabitants (infauna). Consequently, nutrients the pitchers would have otherwise lost via emerging flies become available as ant colony waste. The plants’ prey is therefore conserved by the ants. The interaction between C. schmitzi, N. bicalcarata and dipteran pitcher infauna represents a new type of mutualism where animals mitigate the damage by nutrient thieves to a plant.

Soil and vegetation features determine the nested pattern of ant–plant networks in a tropical rainforest

Recently, several studies have focused on structural properties of ant–plant networks. However, little is known about the role of abiotic factors on these networks. As a result of different abiotic factors that can affect the patterns of ant–plant interactions, it was tested whether soil pH and canopy cover contribute to variation in the nestedness of mutualistic (plants with extrafloral nectar–EFN) and neutral (plants without EFN) ant–plant networks. It was shown that only mutualistic networks were affected by soil pH. It was suggested that this may occur because the variation in soil pH directly influences the secreted nectar, and as there is a preference for nectar composition by ants, this could change the patterns of interaction in mutualistic networks. As prey availability is possibly the main factor influencing ants' presence on plants without EFN, soil pH should have little or no influence on the patterns of interaction in neutral networks. On the other hand, nestedness was not affected by canopy cover in mutualistic and neutral networks. In spite of that canopy cover (light availability) is directly related to the amount of nectar secreted, the volume of nectar may not be important for the structure of the networks. However, canopy cover varied little in this study site. This small variation could not be enough to change the nested pattern in mutualistic and neutral networks. In short, the present results show that the abiotic factors that affect the availability and quality of food resources may have important effects on the structure of trophic interactions in non‐symbiotic ant–plant networks.