Quantity over quality: light intensity,but not red/far‐red ratio,affects extrafloral nectar production in Senna mexicana var. chapmanii

Extrafloral nectar (EFN) mediates food‐for‐protection mutualisms between plants and insects and provides plants with a form of indirect defense against herbivory. Understanding sources of variation in EFN production is important because such variations affect the number and identity of insect visitors and the effectiveness of plant defense. Light represents a potentially crucial tool for regulating resource allocation to defense, as it not only contributes energy but may help plants to anticipate future conditions. Low red/far‐red (R/FR) light ratios can act as a signal of the proximity of competing plants. Exposure to such light ratios has been shown to promote competitive behaviors that coincide with reduced resource allocation to direct chemical defenses. Little is known, however, about how such informational light signals might affect indirect defenses such as EFN, and the interactions that they mediate. Through controlled glasshouse experiments, we investigated the effects of light intensity, and R/FR light ratios, on EFN production in Senna mexicana var. chapmanii. Plants in light‐limited conditions produced significantly less EFN, and leaf damage elicited increased EFN production regardless of light conditions. Ratios of R/FR light, however, did not appear to affect EFN production in either damaged or undamaged plants. Understanding the effects of light on indirect defenses is of particular importance for plants in the threatened pine rockland habitats of south Florida, where light conditions are changing in predictable ways following extensive fragmentation and subsequent mismanagement. Around 27% of species in these habitats produce EFN and may rely on insect communities for defense.     

The Influence of Host Plant Extrafloral Nectaries on Multitrophic Interactions: An Experimental Investigation

A field experiment was conducted with outplantings of the native perennial shrub Senna mexicana var. chapmanii in a semi-natural area adjacent to native pine rockland habitat in southern Florida. The presence of ants and the availability of extrafloral nectar were manipulated in a stratified random design. Insect communities were monitored and recorded over a period of six months with a view to addressing three main questions. Do ants provide biotic defense against key herbivores on S. chapmanii? Is the presence of ants on S. chapmanii mediated by EFN? Finally, are there ecological costs associated with the presence of ants on S. chapmanii, such as a reduction in alternative predator or parasitoid numbers? Herbivores on S. chapmanii included immature stages of three pierid butterflies, and adult weevils. Eight species of ants were associated with the plants, and other predators included spiders, ladybugs, wasps, and hemipterans. Parasitic, haemolymph-sucking midges (Ceratopogonidae) and parasitoid flies were also associated with the caterpillar herbivores, and possibly the extrafloral nectaries of the plants. The presence of ants did not appear to influence oviposition by butterflies, as numbers of lepidopterans of all developmental stages did not differ among treatments. Significantly more late instar caterpillars, however, were observed on plants with ants excluded, indicating that ants remove small caterpillars from plants. Substantially more alternative predators (spiders, ladybugs, and wasps) were observed on plants with ants excluded. Rates of parasitization did not differ among the treatments, but there were substantially fewer caterpillars succumbing to virus among those collected from control plants.We provide a rare look at facultative ant-plant mutualisms in the context of the many other interactions with which they overlap. We conclude that ants provide some biotic defense against herbivores on S. chapmanii, and plants benefit overall from the presence of ants, despite negative impacts on non-ant predators.     

A facultative mutualism between aphids and an invasive ant increases plant reproduction

1. The consequences to plants of ant–aphid mutualisms, particularly those involving invasive ants, are poorly studied. Ant–aphid mutualisms may increase or decrease plant fitness depending on the relative cost of herbivory by ant‐tended aphids versus the relative benefit of increased ant suppression of other (non‐aphid) herbivores. 2. We conducted field and greenhouse experiments in which we manipulated the presence and absence of cotton aphids (Aphis gossypii) on cotton plants to test the hypothesis that a mutualism between cotton aphids and an invasive ant, the red imported fire ant (Solenopsis invicta), benefits cotton plants by increasing fire ant suppression of caterpillars. We also manipulated caterpillar abundance to test whether the benefit of the mutualism varied with caterpillar density. 3. We found that more fire ants foraged on plants with cotton aphids than on plants without cotton aphids, which resulted in a significant reduction in caterpillar survival and caterpillar herbivory of leaves, flower buds, and bolls on plants with aphids. Consequently, cotton aphids indirectly increased cotton reproduction: plants with cotton aphids produced 16% more bolls, 25% more seeds, and 10% greater seedcotton mass than plants without aphids. The indirect benefit of cotton aphids, however, varied with caterpillar density: the number of bolls per plant at harvest was 32% greater on plants with aphids than on plants without aphids at high caterpillar density, versus just 3% greater at low caterpillar density. 4. Our results highlight the potential benefit to plants that host ant–hemipteran mutualisms and provide the first experimental evidence that the consequences to plants of an ant–aphid mutualism vary at different densities of non‐aphid herbivores.     

Abiotic mediation of a mutualism drives herbivore abundance

Species abundance is typically determined by the abiotic environment, but the extent to which such effects occur through the mediation of biotic interactions, including mutualisms, is unknown. We explored how light environment (open meadow vs. shaded understory) mediates the abundance and ant tending of the aphid Aphis helianthi feeding on the herb Ligusticum porteri. Yearly surveys consistently found aphids to be more than 17‐fold more abundant on open meadow plants than on shaded understory plants. Manipulations demonstrated that this abundance pattern was not due to the direct effects of light environment on aphid performance, or indirectly through host plant quality or the effects of predators. Instead, open meadows had higher ant abundance and per capita rates of aphid tending and, accordingly, ants increased aphid population growth in meadow but not understory environments. The abiotic environment thus drives the abundance of this herbivore exclusively through the mediation of a protection mutualism.     

Plant Ontogeny as a Conditionality Factor in the Protective Effect of Ants on a Neotropical Tree

The outcome of any interspecific interaction is often determined by the ecological context in which the interacting species are embedded. Plant ontogeny may represent an important source of variation in the outcome of ant–plant mutualisms, as the level of investment in ant rewards, in alternative (non‐biotic) defenses, or both, may be modulated by the plant's developmental stage. In addition, the abundance and identities of the ants involved in the interaction may change during ontogeny of the host‐plant. Here, we evaluated if plant ontogeny affects the interaction between ants and a savanna tree species (Caryocar brasiliense) that produces extrafloral nectar. We found fewer ants per branch and fewer species of ants per tree in juvenile than in reproductive trees of medium and large size. In addition, large‐sized reproductive trees were more likely to host more aggressive ants than were medium‐sized reproductive or juvenile trees. Such differences strongly affected the outcome of the interaction between ants and their host‐plants, as the magnitude of the effect of ants on herbivory was much stronger for large trees than for juvenile ones. The fact that we did not find significant ontogenetic variation in the concentration of leaf tannins suggests that the observed differences in herbivory did not result from a differential investment in chemical defenses among different‐sized plants. Overall, the results of our study indicate that the developmental stage of the host plant is an important factor of conditionality in the interaction between C. brasiliense and arboreal foraging ants.     

Intraspecific competition favors ant–plant protective mutualism

The dependency of the anti-herbivore defense on ant–plant protective mutualism often varies depending on abiotic and biotic conditions. Although intraspecific competition is a primary interaction between neighboring plants, its effects on ant–plant mutualisms have yet to be sufficiently elucidated. In order to determine the effects of intraspecific competition and competitor genotype on ant–plant mutualisms, I conducted competition and ant-removal experiments and examined their effects on damage to the leaves of Urena lobata var. tomentosa plants. I found that larger numbers of worker ants visited the plants growing with non-siblings than plants growing alone and that plants growing with non-siblings had a higher shoot to root ratio and secreted greater volumes of extrafloral nectar than plants growing alone and/or with siblings. Under the presence of both sibling and non-sibling competitors, I observed that when ants were removed from plants, those grown with conspecific neighbors were characterized by a higher percentage of damaged leaf area than plants harboring ants. The effect of ant exclusion on leaf damage was more pronounced in plants grown with non-siblings than those grown near siblings. However, when the plants were grown alone, I detected no significant difference in percentage leaf damage between the ant-excluded and ant-harboring plants. The results indicate that neighboring plants can exert strong effects on ant–plant protective mutualisms, thereby highlighting the need to take into consideration plant–plant interactions in studies on these mutualistic associations.     

Ants on plants: a meta-analysis of the role of ants as plant biotic defenses

We reviewed the evidence on the role of ants as plant biotic defenses, by conducting meta-analyses for the effects of experimental removal of ants on plant herbivory and fitness with data pooled from 81 studies. Effects reviewed were plant herbivory, herbivore abundance, hemipteran abundance, predator abundance, plant biomass and reproduction in studies where ants were experimentally removed (n = 273 independent comparisons). Ant removal exhibited strong effects on herbivory rates, as plants without ants suffered almost twice as much damage and exhibited 50% more herbivores than plants with ants. Ants also influenced several parameters of plant fitness, as plants without ants suffered a reduction in biomass (−23.7%), leaf production (−51.8%), and reproduction (−24.3%). Effects were much stronger in tropical regions compared to temperate ones. Tropical plants suffered almost threefold higher herbivore damage than plants from temperate regions and exhibited three times more herbivores. Ant removal in tropical plants resulted in a decrease in plant fitness of about 59%, whereas in temperate plants this reduction was not statistically significant. Ant removal effects were also more important in obligate ant–plants (=myrmecophytes) compared to plants exhibiting facultative relationships with hemiptera or those plants with extrafloral nectaries and food bodies. When only tropical plants were considered and the strength of the association between ants and plants taken into account, plants with obligate association with ants exhibited almost four times higher herbivory compared to plants with facultative associations with ants, but similar reductions in plant reproduction. The removal of a single ant species increased plant herbivory by almost three times compared to the removal of several ant species. Altogether, these results suggest that ants do act as plant biotic defenses, but the effects of their presence are more pronounced in tropical systems, especially in myrmecophytic plants. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. N. P. de U. Barbosa, L. Diniz, Y. Oki and F. Pezzini contributed equally to this work and are listed in alphabetical order.     

Plant defense, herbivory, and the growth of Cordia alliodora trees and their symbiotic Azteca ant colonies

The effects of herbivory on plant fitness are integrated over a plant??s lifetime, mediated by ontogenetic changes in plant defense, tolerance, and herbivore pressure. In symbiotic ant?Cplant mutualisms, plants provide nesting space and food for ants, and ants defend plants against herbivores. The benefit to the plant of sustaining the growth of symbiotic ant colonies depends on whether defense by the growing ant colony outpaces the plant??s growth in defendable area and associated herbivore pressure. These relationships were investigated in the symbiotic mutualism between Cordia alliodora trees and Azteca pittieri ants in a Mexican tropical dry forest. As ant colonies grew, worker production remained constant relative to ant-colony size. As trees grew, leaf production increased relative to tree size. Moreover, larger trees hosted lower densities of ants, suggesting that ant-colony growth did not keep pace with tree growth. On leaves with ants experimentally excluded, herbivory per unit leaf area increased exponentially with tree size, indicating that larger trees experienced higher herbivore pressure per leaf area than smaller trees. Even with ant defense, herbivory increased with tree size. Therefore, although larger trees had larger ant colonies, ant density was lower in larger trees, and the ant colonies did not provide sufficient defense to compensate for the higher herbivore pressure in larger trees. These results suggest that in this system the tree can decrease herbivory by promoting ant-colony growth, i.e., sustaining space and food investment in ants, as long as the tree continues to grow.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

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.     

Benefits for plants in ant-plant protective mutualisms: a meta-analysis

Costs and benefits for partners in mutualistic interactions can vary greatly, but surprisingly little is known about the factors that drive this variation across systems. We conducted a meta-analysis of ant-plant protective mutualisms to quantify the effects of ant defenders on plant reproductive output, to evaluate if reproductive effects were predicted from reductions in herbivory and to identify characteristics of the plants, ants and environment that explained variation in ant protection. We also compared our approach with two other recent meta-analyses on ant-plant mutualisms, emphasizing differences in our methodology (using a weighted linear mixed effects model) and our focus on plant reproduction rather than herbivore damage. Based on 59 ant and plant species pairs, ant presence increased plant reproductive output by 49% and reduced herbivory by 62%. The effects on herbivory and reproduction within systems were positively correlated, but the slope of this relationship (0.75) indicated that tolerance to foliar herbivory may be a common plant response to absence of ant guards. Furthermore, the relationship between foliar damage and reproduction varied substantially among systems, suggesting that herbivore damage is not a reliable surrogate for fitness consequences of ant protection. Studies that experimentally excluded ants reported a smaller effect of ant protection on plant reproduction than studies that relied upon natural variation in ant presence, suggesting that study methods can affect results in these systems. Of the ecological variables included in our analysis, only plant life history (i.e., annual or perennial) explained variation in the protective benefit of mutualistic ants: presence of ants benefitted reproduction of perennials significantly more than that of annuals. These results contrast with other quantitative reviews of these relationships that did not include plant life history as an explanatory factor and raise several questions to guide future research on ant-plant protection mutualisms.     

Conservation of an Ant–plant Mutualism in Native Forests and Ecologically‐managed Tree Monocultures

The Brazilian Atlantic Forest has been replaced by homogeneous tree monocultures with potentially drastic effect on ecological interactions. We expect that ecologically‐managed tree monocultures, however, can help to mitigate this impact. Here, we carried out an experiment with Inga vera (Fabaceae), an extrafloral nectary bearing plant, to test if the efficiency of ants as anti‐herbivory defense is affected by the replacement of its natural habitat (Araucaria Forest) by ecologically‐managed tree monocultures (plantations of Araucaria, Pinus, and Eucalyptus). Seedlings of Inga vera were transplanted to three patches of each habitat and ants were excluded from half of the plants. The abundance of ants and herbivores was low, similar among habitats, and exhibited temporal asynchrony. Number of herbivores and accumulated herbivory levels were lower in plant with ants. Rates of herbivory were extremely low and lower for young leaves than for mature leaves. The presence of ants did not affect plant performance traits measured by their growth in height, and their final numbers of leaves and leaflets. Contrary to what might be expected, ant‐protected plants produced fewer leaves and leaflets than unprotected ones. In conclusion, Inga vera‐ant interaction was similar between its natural habitat and the tree monocultures, indicating that potentially both species diversity and ecological processes can be conserved in ecologically‐managed tree monocultures.     

Ants indirectly reduce the reproductive performance of a leafless shrub by benefiting aphids through predator deterrence

Ant–aphid mutualisms can generate cascade effects on the host plants, but these impacts depend on the ecological context. We studied the consequences of ant–aphid interactions on the reproductive performance of a Mediterranean leafless shrub (Retama sphaerocarpa), through direct and indirect effects on the arthropod community. By manipulating the presence of ants and aphids in the field, we found that ants increased aphid abundance and their persistence on the plant and reduced aphid predators by nearly half. However, the presence of ants did not affect the abundance of other plant herbivores, which were relatively scarce in the studied plants. Aphids, and particularly those tended by ants, had a negative impact on the plant reproductive performance by significantly reducing the number of fruits produced. However, fruit and seed traits were not changed by the presence of aphids or those tended by ants. We show that ants favoured aphids by protecting them from their natural enemies but did not indirectly benefit plants through herbivory suppression, resulting in a net negative impact on the plant reproductive performance. Our study suggests that the benefits obtained by plants from hosting ant–aphid mutualisms are dependent on the arthropod community and plant traits.

     

The effects of ants on herbivory and herbivore numbers on foliage of the mallee eucalypt,Eucalyptus incrassata Labill.*

Although there has been much recent interest in ant-plant mutualisms, few data are available on the effects of foraging ants on herbivore numbers and levels of herbivory on plants that do not offer specific inducements to attract ant visitation. In forestry plantations and tropical crops, ants have erratic but sometimes dramatic effects on the numbers of insect herbivores but, in more natural habitats, their effects on levels of herbivory appear to be largely unknown. In Australia, where ants and Eucalyptus woodlands are ubiquitous and abundant and where considerable debate has occurred regarding levels of herbivory in Eucalyptus forests, very little work has been done to examine the effects of ants on densities of insect herbivores on eucalypts. In this study, ants were experimentally excluded from mature and immature foliage of saplings of the mallee Eucalyptus incrassata in South Australia, and herbivore numbers and levels of leaf herbivory were assessed during the next 6 months. No significant differences in herbivory were found between ant-access and ant-exclusion treatments. In spring and early summer, ants were found in associations with aggregations of eurymelid bugs on young foliage, and the effects of ants on bug densities were experimentally investigated. Bug densities decreased rapidly in ant-exclusion treatments compared with ant-access controls. Ants also quickly removed seeds of E. incrassata from experimental caches. The potential of ants to limit the numbers of insect herbivores on eucalypts seems limited given their tendency to form mutualistic associations with sap-feeding Homopterans and because of a lack of other herbivores that are particularly vulnerable to ant predation.     

Multiple effects of mutualistic ants improve the performance of a neotropical ant-plant: A long-term study with the Cecropia-Azteca system

Comprehension of the benefits involved in mutualisms is crucial to disentangle the role of interactions in the structure and functioning of populations, communities and ecosystems. In ant-plant mutualisms, benefits provided by plants to ants are immediately recognizable, but reverse benefits are less obvious, conditional and accumulate over longer time spans. Here we tested the hypothesis that the ant Azteca muelleri simultaneously provides multiple benefits to its host plant (Cecropia glaziovii), ultimately increasing plant performance. We planted seedlings and experimentally prevented ant colonization for half of them. Over 4.5 years we quantified the effects of ant presence or absence on plant growth, herbivory levels, fungal infection, fertilization via ant debris and changes in defense strategies. Ant colonization increased plant height by 125% compared to ant-free plants. Such an improvement in plant performance can be explained because plants with ants faced less herbivory, lower prevalence of pathogenic fungi, invested less in foliar trichomes and had more foliar nitrogen. We thus confirmed that ant mutualists provide cumulative benefits including nutritional benefits, effective defense and lower investment into other defenses – which result in increased plant growth. We highlight the importance of long-term experiments that simultaneously evaluate a multiplicity of potential ant effects to better understand their relative contribution to the performance of the mutualistic partner.     

Maladaptation beyond a geographic range limit driven by antagonistic and mutualistic biotic interactions across an abiotic gradient

Species’ geographic range limits often result from maladaptation to the novel environments beyond the range margin. However, we rarely know which aspects of the n‐dimensional environment are driving this maladaptation. Especially of interest is the influence of abiotic versus biotic factors in delimiting species’ distributions. We conducted a 2‐year reciprocal transplant experiment involving manipulations of the biotic environment to explore how spatiotemporal gradients in precipitation, fatal mammalian herbivory, and pollination affected lifetime fitness within and beyond the range of the California annual plant, Clarkia xantiana ssp. xantiana. In the first, drier year of the experiment, fitness outside the range edge was limited mainly by low precipitation, and there was some evidence for local adaptation within the range. In the second, wetter year, we did not observe abiotic limitations to plant fitness outside the range; instead biotic interactions, especially herbivory, limited fitness outside the range. Together, protection from herbivory and supplementation of pollen resulted in three‐ to sevenfold increases in lifetime fitness outside the range margin in the abiotically benign year. Overall, our work demonstrates the importance of biotic interactions, particularly as they interact with the abiotic environment, in determining fitness beyond geographic range boundaries.     

The effects of fire and fragmentation on occurrence and flowering of a rare perennial plant

The pine rocklands of southern Florida are a fire-dependent forest associated with outcroppings of limestone. Pine rockland plants have several adaptations to fire, and for many species burns increase plant growth, flowering, and seedling establishment. The pine rockland forest has been reduced and fragmented in recent decades. Outside of Everglades National Park, only two percent of the original pine rocklands remain, and are in the form of small fragments. Habitat fragmentation may have a negative effect on the biology of plants; we investigated the effects of both fire and fragmentation on Angadenia berteroi (A.DC.) Miers, a threatened species of the southern Florida pine rockland. We estimated the density and flowering of A. berteroi using adaptive cluster sampling in six study sites with different fire and disturbance histories. A. berteroi is more abundant in the largest fragments, and those having experienced fire most recently. However, fragmentation and lack of fire did not appear to have a great impact on flowering or fruit production. Insights from this threatened species may provide impetus not only to conserve, but to properly manage remaining pine rocklands in south Florida.     

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.     

Mutualistic acacia ants exhibit reduced aggression and more frequent off‐tree movements near termite mounds

In many ant–plant mutualisms, ants establish colonies in hollow thorns, leaf pouches, or other specialized structures on their host plants, which they then defend from herbivores. Resource heterogeneity could affect the maintenance of these mutualisms if it leads to one or both partners altering their investment in the interaction. Such a phenomenon may be especially pertinent to the Acacia–ant mutualism found in East African savannas, where termite mounds have a profound effect on the spatial structuring of resources used by both plants and ants. Here, we examined whether the proximity to termite mounds of Acacia drepanolobium trees is associated with variation in the behavior of one of their ant associates, Crematogaster nigriceps. We found that ant colonies near termite mounds had decreased aggressive responses to simulated herbivory as well as increased off‐tree movement. We hypothesize that these changes are the result of resident ant colonies near termite mounds shifting investment from defense of their host plant to foraging for nearby resources.     

Mutualism fails when climate response differs between interacting species

Successful species interactions require that both partners share a similar cue. For many species, spring warming acts as a shared signal to synchronize mutualist behaviors. Spring flowering plants and the ants that disperse their seeds respond to warming temperatures so that ants forage when plants drop seeds. However, where warm‐adapted ants replace cold‐adapted ants, changes in this timing might leave early seeds stranded without a disperser. We investigate plant seed dispersal south and north of a distinct boundary between warm‐ and cold‐adapted ants to determine if changes in the ant species influence local plant dispersal. The warm‐adapted ants forage much later than the cold‐adapted ants, and so we first assess natural populations of early and late blooming plants. We then transplant these plants south and north of the ant boundary to test whether distinct ant climate requirements disrupt the ant–plant mutualism. Whereas the early blooming plant's inability to synchronize with the warm‐adapted ant leaves its populations clumped and patchy and its seedlings clustered around the parents in natural populations, when transplanted into the range of the cold‐adapted ant, effective seed dispersal recovers. In contrast, the mutualism persists for the later blooming plant regardless of location because it sets seed later in spring when both warm‐ and cold‐adapted ant species forage, resulting in effective seed dispersal. These results indicate that the climate response of species interactions, not just the species themselves, is integral in understanding ecological responses to a changing climate. Data linking phenological synchrony and dispersal are rare, and these results suggest a viable mechanism by which a species' range is limited more by biotic than abiotic interactions – despite the general assumption that biotic influences are buried within larger climate drivers. These results show that biotic partner can be as fundamental a niche requirement as abiotic resources.