A power-efficiency trade-off in resource use alters epidemiological relationships

Trade-offs play pivotal roles in the ecology and evolution of natural populations. However, trade-offs are probably not static, invariant relationships. Instead, ecological factors can shift, alter, or reverse the relationships underlying trade-offs and create critical genotype x environment (G x E) interactions. But which ecological factors alter trade-offs or create G x E interactions, and why (mechanistically) do they do this? We tackle these questions using resource quality as the central ecological factor and a case study of disease in the plankton. We show that clonal genotypes of a zooplankton host (Daphnia dentifera) exhibit a "power-efficiency" trade-off in resource use, where powerful (fast-feeding) host clones perform well on richer algal resources, but more efficient (slow-feeding) clones perform relatively well on poorer resources. This resource-based trade-off then influences epidemiological relationships due to fundamental connections between resources and fecundity, transmission rate (an index of resistance), and replication of a virulent fungal parasite (Metschnikowia bicuspidata) within hosts. For instance, using experiments and dynamic energy budget models, we show that the power-efficiency trade-off overturned a previously detected trade-off between fecundity and transmission risk of hosts to this parasite. When poorer resources were eaten, transmission risk and fecundity were negatively, not positively, correlated. Additionally, poor resource quality changed positive relationships between yield of infectious stages (spores) and host fecundity: those fecundity-spore relationships with poor food became negative or nonsignificant. Finally, the power-efficiency trade-off set up an interaction between host clone and resource quality for yield of fungal spores: powerful clones yielded relatively more spores on the better resource, while efficient clones yielded relatively more on the poorer resource. Thus, the physiological ecology of resource use can offer potent, mechanistic insight linking environmental factors to epidemiological relationships.     

Breeding systems and seed size in a neotropical flora: testing evolutionary hypotheses

A well-known, but largely untested, prediction in plant reproductive ecology is that dioecious taxa should produce larger, more, higher-quality, or better-defended seeds than cosexual taxa. Using a data set composed of 972 species in 104 families, representing the flora of the Tambopata Wildlife Reserve (Madre de Dios, Peru), we evaluated the first component of this prediction, examining ecological and evolutionary relationships between breeding system and mean seed size with two kinds of tests. First, we conducted cross-species analyses to determine whether species with different breeding systems differed significantly with respect to mean individual seed size. Second, we used a hypothesized phylogeny to identify pairs of the most closely related taxa or clades within the Tambopata community that differed with respect to breeding system. Comparing pair members allowed us to determine whether evolutionary divergence in breeding system (between taxa with unisexual vs. cosexual individuals) was consistently associated with evolutionary change in seed size. In both analyses, we controlled for potentially confounding effects of growth form by examining these relationships within woody and nonwoody taxa. Cross-species analyses revealed that dioecious species produced larger seeds than cosexual species among woody species, shrubs, lianas (each growth form analyzed separately), and all species pooled, but not among trees. Phylogenetically independent contrasts upheld the significant association between breeding system and seed size among woody taxa, lianas, and all taxa pooled, but not among shrubs. We discuss the implications of our findings for evolutionary hypotheses regarding associations between dioecy and seed size.     

Herbivore-induced "rent rise" in the host plant may drive a diet breadth enlargement in the tenant

Inter- and intraspecies variations in host plant traits are presumably involved in many host shifts by insect herbivores, and elucidating the mechanisms involved in such shifts has been a crucial goal in insect-plant research for several decades. Here we propose that herbivore-induced evolutionary increases in host plant resistance may cause oligophagous insect herbivores to shift to other sympatric plants as currently preferred host plants become increasingly unpalatable. We tested this hypothesis in a system based on the perennial herb Filipendula ulmaria (Rosaceae), whose herbivory defense has become gradually stronger due to prolonged selection by Galerucella tenella (Coleoptera: Chrysomelidae) herbivory in a boreal archipelago. We show that Galerucella gradually increases its use of the alternative host plant Rubus arcticus (Rosaceae) in parallel to gradually increased resistance in Filipendula. Our results imply that, by driving the evolutionary increase in Filipendula resistance, Galerucella is also gradually making the original host species more unpalatable and thereby driving its own host-breadth enlargement. We argue that such self-inflicted "rent rises" may be an important mechanism behind host plant shifts, which in turn are believed to have preceded the speciation of many phytophagous insects.     

Incipient speciation within a subgenus of rockfish (<Emphasis Type="Italic">Sebastosomus</Emphasis>) provides evidence of recent radiations within an ancient species flock

The high frequency of speciation events associated with species flocks (i.e., radiations of closely related species) provides invaluable insight into the speciation process. Investigations of the speciation process in the marine environment are rare, and therefore, the genetic analysis of the rockfish genus Sebastes, considered an ancient marine species flock, provides an opportunity to investigate this process in the sea. Using both mitochondrial and nuclear markers, we analyzed five closely related species within the rockfish subgenus Sebastosomus. Our goal was to understand the evolutionary history and genetic relationships among species within this group and to provide evidence of recent speciation events within the subgenus. In the genetic analysis of the subgenus, we found different stages of the speciation process, with greater genetic divergences among three of the five species, evidence of recent divergence between two of the five species, Sebastes entomelas and S. mystinus, and significant genetic divergence between two lineages within S. mystinus revealing a signature of incipient speciation. We also found frequency differences of the two S. mystinus lineages among sample locations and found no evidence of introgression between the lineages at the location where both coexist. Although Sebastes is an example of an ancient species flock, this study provides evidence of ongoing speciation within the genus and reveals stages of this process from incipient to distinct species.     

Eggshell spotting in brood parasitic shiny cowbirds (<Emphasis Type="Italic">Molothrus bonariensis</Emphasis>) is not linked to the female sex chromosome

For avian brood parasites in which individual females are host-specialists, the arms race between hosts and parasites has favored egg color polymorphism in the parasite, with female lineages laying mimetic eggs that resemble those of the host species they parasitize. Female sex-linked inheritance of egg color fosters evolutionary stability of egg polymorphism if female lineages show both consistent eggshell color and host use. This co-evolutionary relationship is unlikely to occur if individual brood parasites use different hosts or if egg color is not maternally inherited. The shiny cowbird (Molothrus bonariensis) is an extreme generalist brood parasite that shows a very high degree of egg polymorphism. We tested whether egg spotting in this species has female sex-linked inheritance. If genetic factors controlling the expression of egg spotting were present on the female-specific W chromosome, we expected co-segregation between spotting patterns and mtDNA haplotypes, as both W and mtDNA are maternally inherited. In contrast to the known maternal inheritance of spotting patterns in great tits, we found no associations between eggshell spotting and mtDNA haplotypes, which suggests that eggshell spotting is not maternally inherited in this cowbird species.     

Infectious Diseases and Extinction Risk in Wild Mammals

Abstract:  Parasite-driven declines in wildlife have become increasingly common and can pose significant risks to natural populations. We used the IUCN Red List of Threatened and Endangered Species and compiled data on hosts threatened by infectious disease and their parasites to better understand the role of infectious disease in contemporary host extinctions. The majority of mammal species considered threatened by parasites were either carnivores or artiodactyls, two clades that include the majority of domesticated animals. Parasites affecting host threat status were predominantly viruses and bacteria that infect a wide range of host species, including domesticated animals. Counter to our predictions, parasites transmitted by close contact were more likely to cause extinction risk than those transmitted by other routes. Mammal species threatened by parasites were not better studied for infectious diseases than other threatened mammals and did not have more parasites or differ in four key traits demonstrated to affect parasite species richness in other comparative studies. Our findings underscore the need for better information concerning the distribution and impacts of infectious diseases in populations of endangered mammals. In addition, our results suggest that evolutionary similarity to domesticated animals may be a key factor associated with parasite-mediated declines; thus, efforts to limit contact between domesticated hosts and wildlife could reduce extinction risk.     

Cuckoo wasps manipulate foraging and resting activities in their hosts

Parasite-induced alterations in host behaviour have been reported in a large number of taxa. However, some parasites are better than others to exploit the resources offered by their hosts. To date, our understanding of the extent to which some obligate parasites exploit social insect colonies is still limited. In this study, we examined parasite-mediated behavioural alterations of Polistes biglumis wasps parasitized by the obligate social parasite Polistes atrimandibularis by comparing host female-activity in parasitized and non-parasitized colonies. Host foundresses foraged more and rested less in parasitized than in non-parasitized colonies (controlling for the number of larvae in the nest, the time of day, and the day in the season). Next, we used short-term parasite removal experiments to investigate how social parasites manipulate their hosts. We found that parasitized hosts foraged more and rested less when social parasites were on the nest rather than after their removal, and we tested which kind of interactions occurred between parasites and hosts. P. atrimandibularis parasites may use mainly non-aggressive interactions (such as antennation and trophallaxis) to manipulate host activities, rather than visual, acoustic or chemical signals as other parasites do.     

Neighborhood phylodiversity affects plant performance

Facilitation and competition are ecological interactions that are crucial for the organization of plant communities. Facilitative interactions tend to occur among distantly related species, while the strength of competition tends to decrease with phylogenetic distance. The balance between both types of interactions will ultimately determine the specific composition of multispecies associations. Although multispecies patches are the arena in which coexistence develops among different phylogenetic groups within communities, the specific processes that occur across life stages have not been explored. Here we study how different species, in composing discrete patches in central Mexico, exert competitive or facilitative effects on seeds and seedlings. We relate these interactions to phylogenetic relationships among nurse species and beneficiary species, and among members of the patches. Survivorship and growth rates of the columnar cactus Neobuxbaumia mezcalaensis were highly positively related to increasing phylogenetic distance to different nurse species, to the presence of related species in patches, and to mean phylogenetic distances to the rest of the species in the patch. Each of these three elements influenced N. mezcalaensis differently, with different nurse species varying substantially in their early effects on emergence, and the nearest relatives and species composition of patches varying in their late effects on survival and growth. Our results emphasize that evolutionary relationships among co-occurring species in vegetation clumps exert direct and indirect effects on plants, affecting individual performance and species coexistence.     

Dissecting the drivers of population cycles: Interactions between parasites and mountain hare demography

There is a growing awareness that cyclic population dynamics in vertebrate species are driven by a complex set of interactions rather than a single causal factor. While theory suggests that direct host-parasite interactions may destabilise population dynamics, the interaction between host and parasite may also influence population dynamics through indirect effects that result in delayed responses to either density or to life-history traits. Using empirical data on mountain hares (Lepus timidus) infected with a nematode parasite (Trichostrongylus retortaeformis), we developed an individual-based model (IBM) that incorporated direct effects and delayed life-history effects (DLHEs) of a macroparasite, alternative transmission mechanisms and seasonality in host population dynamics. The full model describes mean characteristics of observed mountain hare time series and parasite abundance, but by systematically removing model structure we dissect out dynamic influences of DLHEs. The DLHEs were weakly destabilising, increasing the propensity for cyclic dynamics and suggesting DLHEs could be important processes in host-parasite systems. Further, by modifying model structure we identify a strong influence of parasite transmission mechanism on host population stability, and discuss the implications for parasite aggregation mechanisms, host movement and natural geographical variation in host population dynamics. The effect of T. retortaeformis on mountain hares likely forms part of a complex set of interactions that lead to population cycles.     

Life history theory predicts fish assemblage response to hydrologic regimes

The hydrologic regime is regarded as the primary driver of freshwater ecosystems, structuring the physical habitat template, providing connectivity, framing biotic interactions, and ultimately selecting for specific life histories of aquatic organisms. In the present study, we tested ecological theory predicting directional relationships between major dimensions of the flow regime and life history composition of fish assemblages in perennial free-flowing rivers throughout the continental United States. Using long-term discharge records and fish trait and survey data for 109 stream locations, we found that 11 out of 18 relationships (61%) tested between the three life history strategies (opportunistic, periodic, and equilibrium) and six hydrologic metrics (two each describing flow variability, predictability, and seasonality) were statistically significant (P < or = 0.05) according to quantile regression. Our results largely support a priori hypotheses of relationships between specific flow indices and relative prevalence of fish life history strategies, with 82% of all significant relationships observed supporting predictions from life history theory. Specifically, we found that (1) opportunistic strategists were positively related to measures of flow variability and negatively related to predictability and seasonality, (2) periodic strategists were positively related to high flow seasonality and negatively related to variability, and (3) the equilibrium strategists were negatively related to flow variability and positively related to predictability. Our study provides important empirical evidence illustrating the value of using life history theory to understand both the patterns and processes by which fish assemblage structure is shaped by adaptation to natural regimes of variability, predictability, and seasonality of critical flow events over broad biogeographic scales.     

Photosynthetic resource-use efficiency and demographic variability in desert winter annual plants

We studied a guild of desert winter annual plants that differ in long-term variation in per capita reproductive success (lb, the product of per capita survival from germination to reproduction, l, times per capita reproduction of survivors, b) to relate individual function to population and community dynamics. We hypothesized that variation in lb should be related to species' positions along a trade-off between relative growth rate (RGR) and photosynthetic water-use efficiency (WUE) because lb is a species-specific function of growing-season precipitation. We found that demographically variable species have greater RGR and greater leaf carbon isotope discrimination (Delta, a proxy inversely related to WUE). We examined leaf nitrogen and photosynthetic characteristics and found that, in this system, variation in Delta is a function of photosynthetic demand rather than stomatal regulation of water loss. The physiological characteristics that result in low Delta in some species may confer greater photosynthetic performance during the reliably moist but low temperature periods that immediately follow winter rainfall in the Sonoran Desert or alternatively during cool periods of the day or early growing season. Conversely, while species with high Delta and high RGR exhibit low leaf N, they have high biomass allocation to canopy leaf area display. Such trait associations may allow for greater performance during the infrequent conditions where high soil moisture persists into warmer conditions, resulting in high demographic variance. Alternatively, high variance could arise from specialization to warm periods of the day or season. Population dynamic buffering via stress tolerance (low RGR and Delta) correlates negatively with buffering via seed banks, as predicted by bet-hedging theory. By merging analyses of population dynamics with functional trait relationships, we develop a deeper understanding of the physiological, ecological, and evolutionary mechanisms involved in population and community dynamics.     

Multiple plant exploiters on a shared host: testing for nonadditive effects on plant performance.

The combined impact of multiple plant parasites on plant performance can either be additive (the total damage equals the sum of the individual effects) or nonadditive (synergistic or antagonistic damage). Two statistical models are available for testing the independent (=additive) effects of two factors. Here we suggest that the natural history of the plant-parasite system should motivate the choice of a statistical model to test for additivity. Using in-field, manipulative experiments, we examined the interactions between the herbivorous mite Calacarus flagelliseta Fletchmann, De Moraes and Barbosa (Acari: Eriophyidae), the fungal pathogen Oidium caricae F. Noack (a powdery mildew), and their host plant Carica papaya L. in Hawaii. First, we found that herbivorous mites had a moderate negative effect on powdery mildew: when mites were absent, powdery mildew colonies were larger and more numerous. Second, we showed that each plant parasite, when evaluated alone, significantly reduced several measures of plant performance. Third, we found that the combined impact of mites and mildew on plant performance is mostly additive and, for a few variables, less than additive. Finally, we explored compensatory responses and found no evidence for nonlinearities in the relationship between plant performance and cumulative parasite impact. Plants are almost universally subject to attack by multiple herbivores and pathogens; thus a deeper understanding of how multiple plant parasites shape each other's population dynamics and plant performance is essential to understanding plant-parasite interactions.     

Arrived in the light: diel and seasonal activity patterns in wild Verreaux’s sifakas (<Emphasis Type="Italic">Propithecus v. verreauxi</Emphasis>; Primates: Indriidae)

Madagascar is characterised by pronounced annual climatic and ecological seasonality and harbours a radiation of closely related sympatric primates (Lemuriformes) that exhibit diurnal, nocturnal and cathemeral activity patterns. We collected continuous activity data over a complete annual cycle from wild diurnal Verreauxs sifakas (Propithecus v. verreauxi) to contribute detailed and comparative data: (1) to characterise their diel and seasonal activity rhythms, (2) to identify factors shaping variation in activity rhythms, and (3) to help reconstruct the evolutionary transition from nocturnal to diurnal activity. We fitted eight adult sifakas from seven different groups living in Kirindy forest with an accelerometer/data logger device and recorded their activity in 5-min bins for a total of 12 months. We characterise P. verreauxi as a strictly diurnal species with a pronounced bimodal activity pattern that briefly changed to a more unimodal pattern during their annual mating season (January to March). We documented significant annual variation in total daily activity, activity time, and activity level, as well as in most parameters characterising their bimodal activity pattern. Despite a significant positive correlation of the animals activity time with day length, pronounced annual variation in the phase position of onset and end of activity in relation to sunrise and sunset times could also be discerned. Minor enhancing effects of moonlight on nocturnal activity were only found for the first 3 h of the inactivity period. Bimodality of the activity pattern and the additional reduction of activity time during the cold and dry winter months associated with reduced food availability can be interpreted as flexible behavioural adaptations to reduce energy expenditure. We therefore propose that energetic and thermoregulatory benefits are important factors shaping these primates activity pattern.Communicated by C. Nunn     

Brood mate eviction or brood mate acceptance by brood parasitic nestlings? An experimental study with the non-evictor great spotted cuckoo and its magpie host

Some avian brood parasitic nestlings are highly virulent, destroying all host eggs or nestmates, while others accept growing up together with host nestmates. The traditional idea was that all brood parasitic nestlings would benefit from being alone in the host nest. Thus, why do nestlings of some brood parasitic species accept the company of host offspring in the nest? The trade-off hypothesis suggests that brood parasites must balance the costs and benefits of killing host young because of two major potential costs: risk of nest desertion and loss of begging assistance. Here, we test this hypothesis in a non-evictor cuckoo species, the great spotted cuckoo Clamator glandarius and its main host, the magpie Pica pica, by manipulating brood size (1–3 nestlings) and brood composition (only cuckoo, only magpie or mixed) during three consecutive breeding seasons. None of the broods were abandoned by host parents, and cuckoo nestlings alone in the nest tended to grow faster (i.e. wing length). Thus, none of the predictions of the two potential costs on which the trade-off hypothesis is based apply to the great spotted cuckoo–magpie system. Our experimental study could not directly test why chick killing has not evolved in great spotted cuckoos, but the results point in the direction of several possibilities. We suggest that chick killing in great spotted cuckoos may not be adaptive mainly because another, less costly strategy (i.e. outcompeting host nestmates for food), is efficient for successful parasitism of magpie hosts.     

Assessing self-organization of plant communities—A thermodynamic approach

Thermodynamics is a powerful tool for the study of system development and has the potential to be applied to studies of ecological complexity. Here, we develop a set of thermodynamic indicators including energy capture and energy dissipation to quantify plant community self-organization. The study ecosystems included a tropical seasonal rainforest, an artificial tropical rainforest, a rubber plantation, and two Chromolaena odorata (L.) R.M. King & H. Robinson communities aged 13 years and 1 year. The communities represent a complexity transect from primary vegetation, to transitional community, economic plantation, and fallows and are typical for Xishuangbanna, southwestern China. The indicators of ecosystem self-organization are sensitive to plant community type and seasonality, and demonstrate that the tropical seasonal rainforest is highly self-organized and plays an important role in local environmental stability via the land surface thermal regulation. The rubber plantation is at a very low level of self-organization as quantified by the thermodynamic indicators, especially during the dry season. The expansion of the area of rubber plantation and shrinkage of tropical seasonal rainforest would likely induce local surface warming and a larger daily temperature range.     

Theoretical perspectives on resource pulses

Over the last several decades, there has been a growing appreciation of the importance of nonequilibrial phenomena and transient dynamics in explaining the structure of ecological communities. This paper provides an overview of theoretical themes related to resource pulses. Theoretical models suggest short-term responses to a single pulse can qualitatively differ from longer-term responses. Recurrent resource pulses can alter community structure, permitting coexistence that otherwise would not occur, or hamper coexistence mechanisms effective in stable environments. For a given resource input, system responses can be more dramatic with short pulses. Resource pulses can cause transitions between alternative states. Dispersal permits species to exploit locally sporadic resource pulses and persist in environments that on average are unsuitable. All these issues are ripe for further theoretical explorations.     

The evolution of female social relationships in nonhuman primates

Considerable interspecific variation in female social relationships occurs in gregarious primates, particularly with regard to agonism and cooperation between females and to the quality of female relationships with males. This variation exists alongside variation in female philopatry and dispersal. Socioecological theories have tried to explain variation in female-female social relationships from an evolutionary perspective focused on ecological factors, notably predation and food distribution. According to the current “ecological model”, predation risk forces females of most diurnal primate species to live in groups; the strength of the contest component of competition for resources within and between groups then largely determines social relationships between females. Social relationships among gregarious females are here characterized as Dispersal-Egalitarian, Resident-Nepotistic, Resident-Nepotistic-Tolerant, or Resident-Egalitarian. This ecological model has successfully explained differences in the occurrence of formal submission signals, decided dominance relationships, coalitions and female philopatry. Group size and female rank generally affect female reproduction success as the model predicts, and studies of closely related species in different ecological circumstances underscore the importance of the model. Some cases, however, can only be explained when we extend the model to incorporate the effects of infanticide risk and habitat saturation. We review evidence in support of the ecological model and test the power of alternative models that invoke between-group competition, forced female philopatry, demographic female recruitment, male interventions into female aggression, and male harassment. Not one of these models can replace the ecological model, which already encompasses the between-group competition. Currently the best model, which explains several phenomena that the ecological model does not, is a “socioecological model” based on the combined importance of ecological factors, habitat saturation and infanticide avoidance. We note some points of similarity and divergence with other mammalian taxa; these remain to be explored in detail. Received: 30 September 1996 / Accepted after revision: 20 July 1997     

Generalist cuckoo bees (Hymenoptera: Apoidea: Sphecodes) are species-specialist at the individual level

Intensive and incessant arms races between a parasite and its host are generally expected to lead to parasite specialization. Nevertheless, some parasitic species still successfully attack wide spectra of hosts. One of the solutions to the evolutionary enigma of the long-term existence of generalist parasites is their specialization at an individual level, a phenomenon well known, e.g., in European common cuckoo. Over its range, it parasitizes a number of bird species; however, individual females are mostly specialists possessing adaptations to a particular host species. In this study, we test the possibility of individual specialization in generalist cuckoo bees, the insect counterparts of avian cuckoos. Females of cuckoo bees lay each egg into a single brood cell in the nests of other bee species. The host’s offspring is destroyed by the parasitic female or later by her larvae, which feed on pollen supplies accumulated by the host. Both studied cleptoparasitic bees (Sphecodes ephippius and Sphecodes monilicornis) are widely distributed in Europe, where they have been reported to use broad host spectra. We recorded several host species (including some previously unknown) for both cuckoo bee species, and confirmed that these parasites are indeed generalist even at a small local scale. However, we demonstrate that exactly as in the avian cuckoos, each female in both species of generalist bee parasites tends to attack just one host species.     

Plant secondary chemistry mediates the performance of a nutritional symbiont associated with a tree-killing herbivore

Many herbivores consume microbial food sources in addition to plant tissues for nutrition. Despite the ubiquity of herbivore-microbe feeding associations, few studies examine how host plant phenotypes affect microbial symbionts of herbivores. We tested the hypothesis that chemical polymorphism in a plant population mediates the performance of nutritional microbial symbionts. We surveyed the composition of ponderosa pine resin in northern Arizona, USA, for variation in six monoterpenes, and we approximated four chemical phenotypes. We reared populations of an herbivorous tree-killing beetle (Dendroctonus brevicomis) in ponderosa pine host material, controlling for three monoterpene compositions representing an alpha-pinene to delta-3-carene gradient. Beetles were reared in host material where the dominant monoterpene was alpha-pinene, delta-3-carene, or a phenotype that was intermediate between the two. We isolated nutritional fungal symbionts (Entomocorticium sp. B) from beetle populations reared in each phenotype and performed reciprocal growth experiments in media amended to represent four "average" monoterpene compositions. This allowed us to test the effects of natal host phenotype, chemical polymorphism, and the interaction between natal host phenotype and chemical polymorphism on a nutritional symbiont. Three important findings emerged: (1) fungal isolates grew 25-32% faster when acquired from beetles reared in the intermediate phenotype; (2) the mean growth rate of nutritional fungi varied up to 44% depending on which monoterpene composition media was amended with; and (3) fungal isolates uniformly performed best in the intermediate phenotype regardless of the chemical composition of their natal host. The performance of nutritional fungi related to both the chemical "history" of their associated herbivore and the chemical phenotypes they are exposed to. However, all fungal isolates appeared adapted to a common chemical phenotype. These experiments argue in favor of the hypothesis that chemical polymorphism in plant populations mediates growth of nutritional symbionts of herbivores. Intraspecific chemical polymorphism in plants contributes indirectly to the regulation of herbivore populations, and our experiments demonstrate that the ecological effects of plant secondary chemistry extend beyond the trophic scale of the herbivore-plant interaction.