Joseph S. Phillips

Biological diversity across hierarchical levels (e.g. genes and populations to communities and ecosystems) is the result of dynamic interactions between ecological, developmental and evolutionary processes. Detailed long-term studies of ecosystems in the wild are important for a holistic understanding of how biological diversity and ecosystem function are affected by natural and anthropogenic changes, Lake Mývatn, Northeast Iceland, is an exceptionally well-studied ecosystem with decades long time series data across multiple trophic levels, including threespine stickleback ( Gasterosteus aculeatus ), a keystone fish species in this ecosystem. Here we synthesise knowledge of ecological importance and evidence for intraspecific diversity of stickleback in Mývatn, and discuss its role as a potential mediator of eco-evolutionary feedbacks. In terms of ecology, the population density of stickleback fluctuates strongly over time and varies across different habitats in the lake. Stickleback have a key role across different trophic levels: they are important prey for birds and fish, hosts for various parasite, as well as effective predators on invertebrates and likely have considerable impact on invertebrate and vertebrate populations of Mývatn. The stickleback of Mývatn is a generalist species, which shows evidence of subtle phenotypic and genetic divergence across habitats (likely due to periodically strong dispersal and subsequent gene flow), but substantial phenotypic and genetic change across years (indicating potential for microevolution in response to strong temporal environmental changes). We discuss different sources of variation in stickleback ecology, as well as the potential for stickleback to mediate feedbacks between ecological and evolutionary processes in this spatio-temporally dynamic lake ecosystem.

Ecosystem engineers impact ecosystems by changing the physical environment in which species interact. In Lake Mývatn, Iceland, larvae of the midge Tanytarsus gracilentus function as ecosystem engineers by building tubes in the sediment that solidify the benthic surface and create a 3-dimensional structure on which microalgae can grow. To assess the ecosystem-scale effect of midge engineering, we took advantage of the natural high-amplitude fluctuations in T. gracilentus over a 13-year time series. We combined larval abundance data with benthic primary production under saturating light conditions (P _max ) and pelagic in situ gross primary production (GPP) measured on average 8.4 times per summer. T. gracilentus abundance had a positive effect on benthic P _max , as expected from their ecosystem engineering, while pelagic GPP had a negative effect on benthic P _max , reflecting competition from phytoplankton. A complementary analysis of five replicate sediment cores taken on each of 26 sampling days of the time series shows a quantitatively similar positive effect of local-scale variation in larval midge abundance on benthic net ecosystem production (NEP) in space. Finally, we reanalyzed data from an experiment manipulating light levels and the presence/absence of midge larvae to show that larval engineering increased benthic NEP more at low than at high light levels. This indicates that at least part of the positive engineering effect of midges on benthic P _max arises from amelioration of light limitation through self-shading, likely due to the structure created by midge tubes. These results show that Mývatn's high benthic primary productivity is due not only to high inputs of nutrients but also to the ecosystem engineering of tube-forming midges.

Thermal stratification, which is a common feature of lentic freshwater systems, has extensive effects on ecological interactions and ecosystem function, including processes that may determine which systems can support fish populations and affect growth, phenology, and metabolism where populations exist. Because these habitats are important for Northern freshwater fishes, improvement of our ability to forecast thermal stratification and associated ecological processes, like dissolved oxygen dynamics, could increase the accuracy of occupancy and distribution modeling, inform conservation strategies, and predict contemporary evolutionary patterns. Although thermal regimes in temperate systems are well-characterized, the irregular thermal regimes that are often present in small Arctic and subarctic lakes and ponds are more poorly understood. In a unique cave pond system near Mývatn Iceland, where conditions shaped by thermal stratification may be acting as selective agents on divergence of Arctic charr populations, we found differences in thermal stratification regimes related to the orientation of cave openings and the highly irregular catchment topography. In particular, while greater exposure to warm air temperatures can facilitate summer stratification and results in more temporally and spatially variable temperatures, exposure to wind—which is modulated on a small scale by the terrain—can facilitate mixing. These patterns caused only the more sheltered of the two ponds remain continuously isothermic. We also found that growth rates and body condition in the ponds’ Arctic charr populations ( Salvelinus alpinus ) are consistent with constraints on growth and metabolism imposed by low temperature in the cooler, continuously isothermal pond, although we cannot rule out the effects of prey limitation.

Understanding which factors shape and maintain biodiversity is essential to understand how ecosystems respond to crises. Biodiversity observed in ecological communities is a result of the interaction of various factors which can be classified as either neutral‐ or niche‐based. The importance of these processes has been debated, but many scientists believe that both processes are important. Here, we use unique ecosystems in groundwater‐filled lava caves near Lake Mývatn, to examine the importance of neutral‐ versus niche‐based factors for shaping invertebrate communities. We studied diversity in benthic and epibenthic invertebrate communities and related them to ecological variables. We hypothesized that if neutral processes are the main drivers of community structure we would not see any clear relationship between the structure of community within caves and ecological factors. If niche‐based processes are important we should see clear relationships between community structure and variation in ecological variables across caves. Both communities were species poor, with low densities of invertebrates, showing the resource limited and oligotrophic nature of these systems. Unusually for Icelandic freshwater ecosystems, the benthic communities were not dominated by Chironomidae (Diptera) larvae, but rather by crustaceans, mainly Cladocera. The epibenthic communities were not shaped by environmental variables, suggesting that they may have been structured primarily by neutral processes. The benthic communities were shaped by the availability of energy, and to some extent pH, suggesting that niche‐based processes were important drivers of community structure, although neutral processes may still be relevant. The results suggest that both processes are important for invertebrate communities in freshwater, and research should focus on understanding both of these processes. The ponds we studied are representative of a number of freshwater ecosystems that are extremely vulnerable for human disturbance, making it even more important to understand how their biodiversity is shaped and maintained.

Non-biting midges (family Chironomidae) are found throughout the world in a diverse array of aquatic and terrestrial habitats, can often tolerate harsh conditions such as hypoxia or desiccation, and have consistently compact genomes. Yet we know little about the shared molecular basis for these attributes and how they have evolved across the family. Here, we address these questions by first creating high-quality, annotated reference assemblies for Tanytarsus gracilentus (subfamily Chironominae, tribe Tanytarsini) and Parochlus steinenii (subfamily Podonominae). Using these and other publicly available assemblies, we created a time-calibrated phylogenomic tree for family Chironomidae with outgroups from order Diptera. We used this phylogeny to test for features associated with compact genomes, as well as examining patterns of gene family evolution and positive selection that may underlie chironomid habitat tolerances. Our results suggest that compact genomes evolved in the most recent common ancestor of Chironomidae and Ceratopogonidae, and that this occurred mainly through reductions in non-coding regions (introns, intergenic sequences, and repeat elements). Gene families that significantly expanded in Chironomidae included biological processes that may relate to tolerance of stressful environments, such as temperature homeostasis, inflammatory response, melanization defense response, and trehalose transport. We identified a number of genes with evidence for positive selection in Chironomidae, notably sulfonylurea receptor, peroxiredoxin-1, and protein kinase D. Our results help to understand the genomic basis for the small genomes and extreme habitat use in this widely distributed group. Significance Statement: Chironomid midges are known for having small genomes and being able to tolerate many forms of environmental stress, yet little is known of the shared features of their genomes that may underlie these traits. We found that reductions in non-coding regions coincide with small chironomid genomes, and we identified duplicated and/or selected genes that may equip chironomids to tolerate harsh conditions. These results describe the key genomic changes in chironomid midges that may explain their ability to inhabit a range of extreme habitats across the world.

Many consumers depend on the contemporaneous growth of their food resources. For example, Tanytarsus gracilentus midges feed on algae, and because midge generation time is much longer than that of algae, individual midges benefit not just from the standing stock but also from the growth of algae during their lifespans. This implies that an intermediate consumption rate maximizes midge somatic growth: low consumption rates constrain midge growth because they do not fully utilize the available food, whereas high consumption rates suppress algal biomass growth and consequently limit future food availability. An experiment manipulating midge presence and initial algal abundance showed that midges can suppress algal growth, as measured by changes in algal gross primary production (GPP). We also found a positive relationship between GPP and midge growth. A consumer–resource model fit to the experimental data showed a hump‐shaped relationship between midge consumption rates and their somatic growth. In the model, predicted midge somatic growth rates were only positively associated with GPP when their consumption rate was below the value that optimized midge growth. Therefore, midges did not overexploit algae in the experiment. This work highlights the balance that consumers which depend on contemporaneous resource growth might have to strike between short‐term growth and future food availability, and the benefits for consumers when they ‘manage' their resources well.

Population cycles can be caused by consumer–resource interactions. Confirming the role of consumer–resource interactions, however, can be challenging due to an absence of data for the resource candidate. For example, interactions between midge larvae and benthic algae likely govern the high‐amplitude population fluctuations of Tanytarsus gracilentus in Lake Mývatn, Iceland, but there are no records of benthic resources concurrent with adult midge population counts. Here, we investigate consumer population dynamics using the carbon stable isotope signatures of archived T. gracilentus specimens collected from 1977 to 2015, under the assumption that midge δ¹³C values reflect those of resources they consumed as larvae. We used the time series for population abundance and δ¹³C to estimate interactions between midges and resources while accounting for measurement error and possible preservation effects on isotope values. Results were consistent with consumer–resource interactions: high δ¹³C values preceded peaks in the midge population, and δ¹³C values tended to decline after midges reached high abundance. One interpretation of this dynamic coupling is that midge isotope signatures reflect temporal variation in benthic algal δ¹³C values, which we expected to mirror primary production. Following from this explanation, high benthic production (enriched δ¹³C values) would contribute to increased midge abundance, and high midge abundance would result in declining benthic production (depleted δ¹³C values). An additional and related explanation is that midges deplete benthic algal abundance once they reach peak densities, causing midges to increase their relative reliance on other resources including detritus and associated microorganisms. Such a shift in resource use would be consistent with the subsequent decline in midge δ¹³C values. Our study adds evidence that midge–resource interactions drive T. gracilentus fluctuations and demonstrates a novel application of stable isotope time‐series data to understand consumer population dynamics.

Coevolution of competitors can lead to niche partitioning promoting coexistence or to heightened conflicts promoting competitive exclusion. If both are possible, when should coevolution favor coexistence versus exclusion? We investigated this question with a general eco-evolutionary model in which species can reduce the interspecific competition they experience through evolutionary investments in two types of competitive traits: partitioning traits that promote coexistence and conflict traits that promote exclusion. We found that communities were generally mixed, consisting of species investing in both trait types or mixtures of species specializing in one type. For each species, its competitors’ abundances and investments determined its experienced competition, and stronger competition begot greater competitive trait investment. Species investing in conflict traits strengthened competition for other species both directly and indirectly, whereas partitioning traits only weakened competition via direct effects. Conflict traits were therefore the stronger driver of community-wide investments in all traits. However, species investing most in conflict traits experienced less competition, so they ultimately evolved least investment, making them most likely to be excluded by the next invader. Thus, coevolution may provide an open door for species that play nice and a revolving door of exclusion for those that do not.

Quantifying temporal variation in demographic rates is a central goal of population ecology. In this study, we analyzed a multidecadal age-structured time series of Arctic char (Salvelinus alpinus) abundance in Lake Mývatn, Iceland, to infer the time-varying demographic response of the population to reduced harvest in the wake of the fishery’s collapse. Our analysis shows that while survival probability of adults increased following the alleviation of harvesting pressure, per capita recruitment consistently declined over most of the study period, until the final three years when it began to increase. The countervailing demographic trends resulted in only limited directional change in the total population size and population growth rate. Rather, the population dynamics were dominated by large interannual variability and a shift towards an older age distribution. Our results are indicative of a slow recovery of the population after its collapse, despite the rising number of adults following relaxed harvest. This underscores the potential for heterogeneous demographic responses to management efforts due to the complex ecological context in which such efforts take place.