Lauren E. Culler

Evidence for divergent selection between the molecular forms of Anopheles gambiae: role of predation

BackgroundThe molecular forms of Anopheles gambiae are undergoing speciation. They are characterized by a strong assortative mating and they display partial habitat segregation. The M form is mostly found in flooded/irrigated areas whereas the S form dominates in the surrounding areas, but the ecological factors that shape this habitat segregation are not known. Resource competition has been demonstrated between species undergoing divergent selection, but resource competition is not the only factor that can lead to divergence.ResultsIn a field experiment using transplantation of first instar larvae, we evaluated the role of larval predators in mediating habitat segregation between the forms. We found a significant difference in the ability of the molecular forms to exploit the different larval sites conditioned on the presence of predators. In absence of predation, the molecular forms outcompeted each other in their respective natural habitats however, the developmental success of the M form was significantly higher than that of the S form in both habitats under predator pressure.ConclusionOur results provide the first empirical evidence for specific adaptive differences between the molecular forms and stress the role of larval predation as one of the mechanisms contributing to their divergence.

In a warmer Arctic, mosquitoes avoid increased mortality from predators by growing faster.

Climate change is altering environmental temperature, a factor that influences ectothermic organisms by controlling rates of physiological processes. Demographic effects of warming, however, are determined by the expression of these physiological effects through predator–prey and other species interactions. Using field observations and controlled experiments, we measured how increasing temperatures in the Arctic affected development rates and mortality rates (from predation) of immature Arctic mosquitoes in western Greenland. We then developed and parametrized a demographic model to evaluate how temperature affects survival of mosquitoes from the immature to the adult stage. Our studies showed that warming increased development rate of immature mosquitoes (Q10 = 2.8) but also increased daily mortality from increased predation rates by a dytiscid beetle (Q10 = 1.2–1.5). Despite increased daily mortality, the model indicated that faster development and fewer days exposed to predators resulted in an increased probability of mosquito survival to the adult stage. Warming also advanced mosquito phenology, bringing mosquitoes into phenological synchrony with caribou. Increases in biting pests will have negative consequences for caribou and their role as a subsistence resource for local communities. Generalizable frameworks that account for multiple effects of temperature are needed to understand how climate change impacts coupled human–natural systems.

Weak Relationships Between Environmental Factors and Invertebrate Communities in Constructed Wetlands

Growing awareness of essential wetland functions is providing support for wetland construction projects. Biomonitoring using invertebrates is a common way to evaluate project success, but relationships between wetland invertebrates and environmental factors are often weak. In recently constructed wetlands on Maryland’s Eastern Shore, we tested the hypothesis that focusing on predator and primary consumer invertebrate assemblages versus the entire community would elucidate stronger relationships with environmental factors. Despite variation in factors that are hypothesized to control wetland invertebrates (e.g., vegetation and tendency to dry), our results indicated weak relationships between environmental factors and the structure and composition of the entire community as well as predator and primary consumer assemblages separately. Examining the entire community and individual assemblages, however, showed that invertebrates were influenced by temporal factors. We propose that a complex interaction between wetland shape/size, local weather, and seasonal changes may have driven invertebrate community patterns among wetlands. Such interactions would complicate bioassessments of wetlands that differ in size, hydrology, and local weather conditions. Further study of specific factors controlling wetland invertebrates and developing new metrics that incorporate seasonal environmental change could improve biomonitoring results and thus management strategies aimed at enhancing wetland function.

Predator-Prey Interactions of Dytiscids

As conspicuous predators throughout ontogeny, dytiscids are central to freshwater food webs, particularly in lentic systems such as wetlands and ponds. Adult and larval dytiscids are considered to be generalists, feeding on zooplankton, aquatic invertebrates, larval amphibians, and fish, but some dytiscid species selectively feed on certain prey types relative to others. Selective predation, cannibalism, intraguild predation, and non-consumptive effects on prey are attributes of dytiscid feeding that are known to shape food web structure and composition and influence species coexistence. Larval and adult dytiscids are also predators of mosquito larvae and thus frequently investigated as potential agents for mosquito suppression, particularly in northern areas and in areas where mosquitoes vector diseases. The effects of dytiscid predation on food webs and mosquito populations are dependent on several abiotic and biotic conditions, including vegetation structure, habitat complexity, and temperature. Dytiscids are also food for other organisms. Odonate nymphs, fish, amphibians, reptiles, birds, and mammals are known predators of dytiscids, although the extent to which these organisms rely on dytiscids for food remains unclear. Given the prominent role of dytiscids in freshwater food webs, future research should be aimed at improving basic knowledge of dytiscid feeding ecology, using dytiscids to test predator-prey and trophic theory, and examining how environmental change affects the role of dytiscids as predators of vector and nuisance species.

Host Suitability and Gas Exchange Response of Grapevines to Potato Leafhopper (Hemiptera: Cicadellidae)

ABSTRACT Although potato leafhopper, Empoasca fabae (Harris) (Hemiptera: Cicadellidae), is highly polyphagous, classic host studies do not recognize grapevines (Vitis spp.), as suitable hosts. Recently, injury has been reported and reproduction documented within grape vineyards, suggesting a host expansion for the leafhopper. To document this apparent expansion in host use, we determined whether grape plants were suitable hosts for potato leafhopper reproduction, measured the consequence of feeding injury on gas exchange rates of grape leaves, and compared the susceptibility to feeding injury among cultivars. We found that potato leafhopper adults survived equally well on grape (Vitis vinifera L.), alfalfa (Medicago sativa L.), and fava bean (Vicia faba L.). The total number of offspring was greater on fava bean but did not differ between alfalfa and grape. Injury to grapevines was assessed by measuring gas exchange responses of leaves in field cages and in greenhouse tests. We found marginally significant declines in photosynthesis and transpiration rates in the field (9.6 and 13.2%, respectively), and much stronger effects in greenhouse tests (ranging between 22 and 52%). Our results verify that Vitis is a suitable host, and that potato leafhopper is capable of injuring its gas exchange physiology. We discuss possible explanations for the host expansion, and its potential to damage commercial grapevines.

Rates and processes of aeolian soil erosion in West Greenland

In arid landscapes across the globe, aeolian processes are key drivers of landscape change, but arid Arctic regions are often overlooked. In the Kangerlussuaq region of West Greenland, strong katabatic winds have removed discrete patches of soil and vegetation, exposing unproductive glacial till and bedrock. Although lake-sediment records suggest that landscape destabilization began approximately 1000 years ago, the upland soil erosion has never been directly dated. We use a novel application of lichenometry to estimate the rates and timing of soil erosion. We show that the formation of deflation patches occurred approximately 800–230 years ago, in general agreement with lake-sediment records. In West Greenland, the ‘Little Ice Age’ (AD 1350–1880) was characterized by a cold and arid climate, conditions that increased susceptibility to erosion. On average, deflation patches are expanding at a rate of 2.5 cm yr−1, and variation in the rate of patch expansion cannot be explained by proximity to the Greenland Ice Sheet (GrIS), slope, aspect, elevation, or patch size. An erosional threshold exists in this aeolian system, with climate conditions necessary for patch formation likely harsher than those necessary for continued patch expansion, a result that has implications for land management in arid regions. Currently, deflation patches are expanding throughout the study region and are forming in areas close to the GrIS, but future deflation rates are dependent on projected climate and potential land-use changes. Our results stress the importance of aeolian processes in arid polar landscapes such as Kangerlussuaq, and demonstrate the use of aeolian landforms in paleoclimate reconstructions and predicting future landscape change.

No indication of arthropod-vectored viruses in mosquitoes (Diptera: Culicidae) collected on Greenland and Svalbard

Viruses transmitted to vertebrates via arthropod vectors (so-called arboviruses) include many important pathogens such as dengue virus, Zika virus, and Sindbis virus. Mosquitoes represent the major vectors of many of these arboviruses and occur in all climatic zones, including the Arctic. The focal species, Aedes nigripes (Diptera: Culicidae), is the most widely distributed mosquito species in the Arctic. We screened over 11,000 specimens collected between 2012 and 2016 on Greenland (Kangerlussuaq) and Svalbard (Petuniabukta) for the presence of arboviruses which have previously been reported in latitudes up to 70°N. Assays for arbovirus detection using RT-PCR with primers specific for the genera Alphavirus (family Togaviridae), Orthobunyavirus, Phlebovirus (Bunyaviridae), Flavivirus (Flaviviridae), and Orbivirus (Reoviridae) were negative for all specimens. Similar results were recently obtained in a screening focused on tick-borne pathogens on Svalbard. The findings suggest that the circulation of arboviruses at studied localities is currently negligible or nonexistent, possibly due to dispersal, climate, or biotic restrictions. However, global climate change could enhance vector abundance and activity, introduction of invasive host species, and increase in tourism which then could lead to emerging arbovirus outbreaks in the future, with considerable impact on local ecosystems.