Plant-based food resources,trophic interactions among alien species,and the abundance of an invasive ant

Recent research on invasive ants suggests that their success may be facilitated by increased resources at introduced locations stemming from the emergence of novel trophic interactions with abundant honeydew-producing Hemiptera. Moreover, those Hemiptera may themselves often be introduced or invasive. To test the importance of mutualisms for invasive species, we conducted a study in the southeastern United States of factors hypothesized to affect the abundance of an invasive ant native to South America, Solenopsis invicta. The study was conducted within grazing pastures, where S. invicta can be extremely abundant while also exhibiting substantial variability in abundance. A path analysis showed that the abundance of S. invicta was strongly and positively affected by the abundance of an invasive honeydew-producing mealybug native to Asia, Antonina graminis, and by the mealybugs’ host grasses because of their strong positive effect on mealybug abundance. Abundance of the mealybug was primarily attributable to an invasive host grass native to Africa, Cynodon dactylon. The abundance of S. invicta was also positively affected by the abundance of other arthropods that they are likely to consume, and those arthropods were positively affected by the abundance of both the A. graminis host grasses and other plants. Thus the study shows that the distribution and abundance of different plant species could have important effects on the abundance of S. invicta through their effect on the ants’ food resources. The results are also consistent with the hypothesis that the emergence of novel trophic interactions among invasive species can promote the abundance of invasive ants.     

Indirect effects of an invasive exotic species on a long-distance migratory songbird

The loss of foundational tree species to non-native pests can have far reaching consequences for forest composition and function, yet little is known about the impacts on other ecosystem components such as wildlife. We had the opportunity to observe how the loss of eastern hemlock (Tsuga canadensis), due to the invasive hemlock woolly adelgid (Adelges tsugae), influenced the population ecology of the Black-throated Blue Warbler (Setophaga caerulescens) over a 7 year period. We followed the process of adelgid infestation and subsequent hemlock loss, which allowed us to investigate the patterns and mechanisms of population change. We document a precipitous decline in breeding pairs at one site where hemlock was most abundant in the understory, but not at our other two sites. We observed no changes in reproductive output or apparent survival, yet territory size increased dramatically at the most affected site, suggesting that the decline was due to a lack of colonization by new breeders. Our results demonstrate how an invasive insect pest can indirectly influence wildlife species not believed to be vulnerable and in ways not typically investigated.     

Streetlights positively affect the presence of an invasive grass species

Anthropogenic disturbances associated with urban ecosystems can create favorable conditions for populations of some invasive plant species. Light pollution is one of these disturbances, but how it affects the growth and establishment of invasive plant populations is unknown. Cheatgrass (Bromus tectorum) is a problematic invasive species where it has displaced native grassland communities in the United States, but to our knowledge, there have been no studies of the ecological factors that affect cheatgrass presence in urban ecosystems. We conducted field surveys in urban alleys in Denver, Colorado, to compare the presence of cheatgrass at sites with and without artificial light at night (hereafter artificial light) from streetlights. These streetlights are mounted on utility poles, which cause ground disturbance when installed in alleys; we were able to test the independent effect of poles on cheatgrass establishment because not all poles have streetlights on them. We found that cheatgrass was positively associated with the presence of streetlights and to a lesser extent poles. In addition to cheatgrass, we also found that other plants were positively associated with the presence of both poles and streetlights. Our results suggest that artificial light may benefit the occurrence of cheatgrass and other plant species in urban settings. While invasive populations of cheatgrass in wild habitats attract the most attention from managers, we suggest more consideration for this grass in urban environments where its growth and establishment benefit from anthropogenic changes.     

Inducer-dependent phenotypic divergence in an embryonal-carcinoma cell line

Abstract. In monolayer cultures, embryonal carcinoma (EC) cells from the cell line Nulli-SCCI can be induced to differentiate at high efficiency by exposure to either retinoic acid or hexamethylenebisacetamide. Depending on which inducing agent is used, two distinct differentiated phenotypes result. These phenotypes resemble two of the earliest differentiated derivatives of the cells of the inner cell mass of a mouse blastocyst, i.e., parietal and visceral endoderm. Both differ from EC cells as well as from each other on the basis of their morphology, antigenic expression, secretion of plasminogen activator, and protein-synthetic profiles.     

Assortative mating but no evidence of genetic divergence in a species characterized by a trophic polymorphism

Disruptive selection is a process that can result in multiple subgroups within a population, which is referred to as diversification. Foraging‐related diversification has been described in many taxa, but many questions remain about the contribution of such diversification to reproductive isolation and potentially sympatric speciation. Here, we use stable isotope analysis of diet and morphological analysis of body shape to examine phenotypic divergence between littoral and pelagic foraging ecomorphs in a population of pumpkinseed sunfish (Lepomis gibbosus). We then examine reproductive isolation between ecomorphs by comparing the isotopic compositions of nesting males to eggs from their nests (a proxy for maternal diet) and use nine microsatellite loci to examine genetic divergence between ecomorphs. Our data support the presence of distinct foraging ecomorphs in this population and indicate that there is significant positive assortative mating based on diet. We did not find evidence of genetic divergence between ecomorphs, however, indicating that isolation is either relatively recent or is not strong enough to result in genetic divergence at the microsatellite loci. Based on our findings, pumpkinseed sunfish represent a system in which to further explore the mechanisms by which natural and sexual selection contribute to diversification, prior to the occurrence of sympatric speciation.     

Functional convergence and phenotypic divergence in two specialist species of pine-associated ladybirds

Traditionally, convergent evolution has been considered to produce phenotypic similarity in independently evolved species. By contrast, recent studies have detected morphological divergence between species even in similar selective environments when different morphological traits combine to produce a specific functional output. However, it is still unclear whether a complex combination of non-morphological phenotypic traits, such as behavioural and life-history traits, can produce a similar performance in different species. In this study, I examined prey capture performance and related phenotypes in two sympatric ladybird species, Sospita oblongoguttata and Harmonia yedoensis, which specialize on the giant pine aphid, which is known to be elusive for ladybird hatchlings. In particular, I focused on egg size and proportion of trophic eggs in the clutch, since the amount of maternal investment per offspring can contribute to prey capture performance of ladybird hatchlings. Predation success of hatchlings against the giant pine aphid was higher in both S. oblongoguttata and H. yedoensis than in Harmonia axyridis, a generalist ladybird species that feeds on various kinds of aphid species in nature. Sospita oblongoguttata females, however, produced relatively larger eggs and in most clutches provided no trophic eggs, whereas H. yedoensis females produced smaller eggs and provided more trophic eggs per clutch. Moreover, hatchling morphology in H. yedoensis more closely resembled that of its congener, H. axyridis, than that of the more distantly related S. oblongoguttata, although like H. yedoensis, S. oblongoguttata predates on the elusive pine aphid. These results in two pine-associated specialist ladybirds indicate that divergent phenotypes can nonetheless have similar prey capture performance. In conclusion, this work demonstrates that the general ultimate function can be achieved by various mechanisms through convergence that operates at different level of life.     

Testing the link between species interactions and species co‐occurrence in a trophic network

Species interactions are dynamic processes that vary across environmental and ecological contexts, and operate across scale boundaries, making them difficult to quantify. Nevertheless, ecologists are increasingly interested in inferring species interactions from observational data using statistical analyses of their spatial co‐occurrence patterns. Trophic interactions present a particular challenge, as predators and prey may frequently or rarely co‐occur, depending on the spatial or temporal scale of observation. In this study, we investigate the accuracy of inferred interactions among species that both compete and trophically interact. We utilized a long‐term dataset of pond‐breeding amphibian co‐occurrences from Mt Rainier National Park (Washington, USA) and compiled a new dataset of their empirical interactions from the literature. We compared the accuracy of four statistical methods in inferring these known species interactions from spatial associations. We then used the best performing statistical method, the Markov network, to further investigate the sensitivity of interaction inference to spatial scale‐dependence and the presence of predators. We show that co‐occurrence methods are generally inaccurate when estimating trophic interactions. Further the strength and sign of inferred interactions were dependent upon the spatial scale of observation and predator presence influenced the detectability of competitive interactions among prey species. However, co‐occurrence analysis revealed new patterns of spatial association among pairs of species with known interactions. Overall, our study highlights a limiting frontier in co‐occurrence theory and the disconnect between widely implemented methodologies and their ability to accurately infer interactions in trophically‐structured communities.     

Abiotic factors and trophic interactions affect the macroinvertebrate community of bromeliad tanks in a Neotropical Restinga

Limnology - Aquatic macroinvertebrate communities are dependent on intrinsic environmental characteristics and biological interactions in microhabitat systems. We investigated the...     

Plant–pollinator interactions: comparison between an invasive and a native congeneric species

Plant–pollinator interactions determine reproductive success for animal-pollinated species and, in the case of invasive plants, they are supposed to play an important role in invasive success. We compared the invasive Senecio inaequidens to its native congener S. jacobaea in terms of interactions with pollinators. Visitor guild, visitation rate, and seed set were compared over 3 years in three sites in Belgium. Floral display (capitula number and arrangement) and phenology were quantified, and visiting insects were individually censused, i.e. number of visited capitula and time per visited capitulum. As expected from capitula resemblance, visitor guilds of both species were very similar (proportional similarity = 0.94). Senecio inaequidens was visited by 33 species, versus 36 for S. jacobaea. For both species, main visitors were Diptera, especially Syrphidae, and Hymenoptera. Visitation rate averaged 0.13 visitor per capitulum per 10 min for S. inaequidens against 0.08 for S. jacobaea. However, insects visited more capitula per plant on S. jacobaea, due to high capitula density (886 m⁻² versus 206 m⁻² for S. inaequidens), which is likely to increase self-pollen deposition considerably. Seed set of S. jacobaea was lower than that of S. inaequidens. We suggest that floral display is the major factor explaining the differences in insect visitation and seed set between the two Senecio species.     

Inclusion of trophic interactions increases the vulnerability of an alpine butterfly species to climate change

Climate change is expected to have significant and complex impacts on ecological communities. In addition to direct effects of climate on species, there can also be indirect effects through an intermediary species, such as in host–plant interactions. Indirect effects are expected to be more pronounced in alpine environments because these ecosystems are sensitive to temperature changes and there are limited areas for migration of both species (i.e. closed systems), and because of simpler trophic interactions. We tested the hypothesis that climate change will reduce the range of an alpine butterfly (Parnassius smintheus) because of indirect effects through its host plant (Sedum sp.). To test for direct and indirect effects, we used the simulations of climate change to assess the distribution of P. smintheus with and without Sedum sp. We also compared the projected ranges of P. smintheus to four other butterfly species that are found in the alpine, but that are generalists feeding on many plant genera. We found that P. smintheus gained distributional area in climate‐only models, but these gains were significantly reduced with the inclusion of Sedum sp. and in dry‐climate scenarios which resulted in a reduction in net area. When compared to the more generalist butterfly species, P. smintheus exhibited the largest loss in suitable habitat. Our findings support the importance of including indirect effects in modelling species distributions in response to climate change. We highlight the potentially large and still neglected impacts climate change can have on the trophic structure of communities, which can lead to significant losses of biodiversity. In the future, communities will continue to favour species that are generalists as climate change induces asynchronies in the migration of species.     

Indirect interactions between two aphid species in relation to ant attendance

The influence of foraging by the ant, Lasius niger, on the population growth of two aphid species, Lachnus tropicalis and Myzocallis kuricola, on chestnut trees, Castanea crenata, was examined. The ant-tending effect was divergent depending on the aphid density per ant: it was positive when there were few aphids per ant, but negative when there were many aphids per ant. In addition, the density of one aphid species also influenced the ant-tending effect on the other aphid. Furthermore, the influences were asymmetrical: an increase in L. tropicalis density per ant reversed the ants effect on this species and on M. kuricola, while an increase in M. kuricola per ant did not significantly influence the ants effect on L. tropicalis. Thus, the ant seems to stabilize the L. tropicalis population density and keep this species from extinction, while the ants effect on M. kuricola depends on the density of L. tropicalis and may lead M. kuricola to extermination. This change in the ant-tending effect corresponds to the previously detected density-dependent change in predation activity of the ants on aphids. In contrast, the density-dependent change in the protection effect of the ants against natural enemies does not explain the results.     

Decoupling of genetic and phenotypic divergence in a headwater landscape

In stream organisms, the landscape affecting intraspecific genetic and phenotypic divergence is comprised of two fundamental components: the stream network and terrestrial matrix. These components are known to differentially influence genetic structure in stream species, but to our knowledge, no study has compared their effects on genetic and phenotypic divergence. We examined how the stream network and terrestrial matrix affect genetic and phenotypic divergence in two stream salamanders, Gyrinophilus porphyriticus and Eurycea bislineata, in the Hubbard Brook Watershed, New Hampshire, USA. On the basis of previous findings and differences in adult terrestriality, we predicted that genetic divergence and phenotypic divergence in body morphology would be correlated in both species, but structured primarily by distance along the stream network in G. porphyriticus, and by overland distance in E. bislineata. Surprisingly, spatial patterns of genetic and phenotypic divergence were not strongly correlated. Genetic divergence, based on amplified DNA fragment length polymorphisms, increased with absolute geographic distance between sites. Phenotypic divergence was unrelated to absolute geographic distance, but related to relative stream vs. overland distances. In G. porphyriticus, phenotypic divergence was low when sites were close by stream distance alone and high when sites were close by overland distance alone. The opposite was true for E. bislineata. These results show that small differences in life history can produce large differences in patterns of intraspecific divergence, and the limitations of landscape genetic data for inferring phenotypic divergence. Our results also underscore the importance of explicitly comparing how terrestrial and aquatic conditions affect spatial patterns of divergence in species with biphasic life cycles.     

Behavioral interactions of the invasive Argentine ant with native ant species

Summary: The Argentine ant, Linepithema humile, has invaded many areas of the world, displacing native ants. Its behavior may contribute to its competitive success. Staged and natural encounters were observed at food resources in the field, between Argentine ants and eight ant species native to northern California. There was no relation between the frequency of aggression by any ant species and the outcome of encounters, though Argentine ants were more likely than ants of native species to behave aggressively. When an ant of one species initiated an encounter of any kind with an ant of another species, the ant that did not initiate was likely to retreat. This was true of all species studied. Most encounters between ants were initiated by Argentine ants. Thus the native species tended to retreat more frequently than Argentine ants. Interactions between Argentine ants and native species at food resources, causing ants of native species to retreat, may help Argentine ants to displace native species from invaded areas.     

Behavioural elements reflect phenotypic colour divergence in a poison frog

The coexistence of both aposematic and cryptic morphs as different anti-predator strategies within a species seems to be an unusual phenomenon in nature. The strawberry poison frog, Oophaga pumilio, shows an astonishing colour diversity among populations in western Panama. In this study we selected a red and a green colour morph from two Panamanian islands (Isla Solarte and Isla Colón) for behavioural observations and measurements of conspicuousness. We found that red frogs were more visible to both conspecific frogs and potential predators than green frogs. Interestingly the difference in conspicuousness was most pronounced at the substrate that males used as principal calling places. Red males were more active and spent more time foraging than green males, which spent more time hidden. The association between conspicuousness of colouration and behaviour results in a more aposematic and a more cryptic anti-predator strategy. This is the first study which links differences in conspicuousness between animals on their natural backgrounds to differences in foraging as well as anti-predator behaviour and discusses the results in light of previous findings of toxicity analyses and potential costs and benefits of aposematism. To this end, our study adds a novel perspective for explaining extreme colour diversity between populations within an initially aposematic species.     

Adaptive phenotypic divergence in an annual grass differs across biotic contexts*

Climate is a powerful force shaping adaptation within species, yet adaptation to climate occurs against a biotic background: species interactions can filter fitness consequences of genetic variation by altering phenotypic expression of genotypes. We investigated this process using populations of teosinte, a wild annual grass related to maize (Zea mays ssp. mexicana), sampling plants from 10 sites along an elevational gradient as well as rhizosphere biota from three of those sites. We grew half‐sibling teosinte families in each biota to test whether trait divergence among teosinte populations reflects adaptation or drift, and whether rhizosphere biota affect expression of diverged traits. We further assayed the influence of rhizosphere biota on contemporary additive genetic variation. We found that adaptation across environment shaped divergence of some traits, particularly flowering time and root biomass. We also observed that different rhizosphere biota shifted expressed values of these traits within teosinte populations and families and altered within‐population genetic variance and covariance. In sum, our results imply that changes in trait expression and covariance elicited by rhizosphere communities could have played a historical role in teosinte adaptation to environments and that they are likely to play a role in the response to future selection.     

Genetic divergence in the small Indian mongoose (Herpestes auropunctatus), a widely distributed invasive species

The combination of founder events, random drift and new selective forces experienced by introduced species typically lowers genetic variation and induces differentiation from the ancestral population. Here, we investigate microsatellite differentiation between introduced and native populations of the small Indian mongoose (Herpestes auropunctatus). Many expectations based on introduction history, such as loss of alleles and relationships among populations, are confirmed. Nevertheless, when applying population assignment methods to our data, we observe a few specimens that are incorrectly assigned and/or appear to have a mixed ancestry, despite estimates of substantial population differentiation. Thus, we suggest that population assignments of individuals should be viewed as tentative and that there should be agreement among different algorithms before assignments are applied in conservation or management. Further, we find no congruence between previously reported morphological differentiation and the sorting of microsatellite variation. Some introduced populations have retained much genetic variation while others have not, irrespective of morphology. Finally, we find alleles from the sympatric grey mongoose (Herpestes edwardsii) in one small Indian mongoose within the native range, suggesting an alternative explanation for morphological differentiation involving a shift in female preferences in allopatry.     

Higher zooplankton species richness associated with an invertebrate top predator

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Indirect interactions between browsers and seed predators affect the seed bank dynamics of a chaparral shrub

Interactions between herbivores and seed predators may have long-term consequences for plant populations that rely on persistent seed banks for recovery after unpredictable fires. We assessed the effects of browsing by deer and seed predation by rodents, ants and birds on the densities of seeds entering the seed bank of Ceanothus cuneatus var. rigidus, a maritime chaparral shrub in coastal California. Ceanothus produced many more seeds when protected from browsers in long-term experimental exclosures than did browsed plants, but the seed densities in the soil beneath browsed and unbrowsed Ceanothus were the same at the start of an intensive one-year study. The density of seeds in the soil initially increased in both treatments following summer seed drop: while densities returned to pre-drop levels within a few weeks under browsed plants, soil seed densities remained high for 5–8 months beneath unbrowsed plants. Rodent abundance (especially deer mice) was higher near unbrowsed plants than >30 m away, and rodents removed Ceanothus seeds from dishes in the experimental plots. At least in the short term, rodent density and rates of seed removal were inversely related to the intensity of browsing. Our data have management implications for maintaining viable Ceanothus populations by regulating the intensity of browsing and the timing, intensity and frequency of fires.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .