Disturbance alters transience but nutrients determine equilibria during grassland succession with multiple global change drivers.

Disturbance and environmental change may cause communities to converge on a steady state, diverge towards multiple alternative states or remain in long-term transience. Yet, empirical investigations of successional trajectories are rare, especially in systems experiencing multiple concurrent anthropogenic drivers of change. We examined succession in old field grassland communities subjected to disturbance and nitrogen fertilization using data from a long-term (22-year) experiment. Regardless of initial disturbance, after a decade communities converged on steady states largely determined by resource availability, where species turnover declined as communities approached dynamic equilibria. Species favoured by the disturbance were those that eventually came to dominate the highly fertilized plots. Furthermore, disturbance made successional pathways more direct revealing an important interaction effect between nutrients and disturbance as drivers of community change. Our results underscore the dynamical nature of grassland and old field succession, demonstrating how community properties such as ß diversity change through transient and equilibrium states.

Increasing shrub damage by invertebrate herbivores in the warming and drying tundra of West Greenland.

Rapid warming is predicted to increase insect herbivory across the tundra biome, yet how this will impact the community and ecosystem dynamics remains poorly understood. Increasing background invertebrate herbivory could impede Arctic greening, by serving as a top-down control on tundra vegetation. Many tundra ecosystems are also susceptible to severe insect herbivory outbreaks which can have lasting effects on vegetation communities. To explore how tundra-insect herbivore systems respond to warming, we measured shrub traits and foliar herbivory damage at 16 sites along a landscape gradient in western Greenland. Here we show that shrub foliar insect herbivory damage on two dominant deciduous shrubs, Salix glauca and Betula nana, was positively correlated with increasing temperatures throughout the first half of the 2017 growing season. We found that the majority of insect herbivory damage occurred in July, which was outside the period of rapid leaf expansion that occurred throughout most of June. Defoliators caused the most foliar damage in both shrub species. Additionally, insect herbivores removed a larger proportion of B. nana leaf biomass in warmer sites, which is due to a combination of increased foliar herbivory with a coinciding decline in foliar biomass. These results suggest that the effects of rising temperatures on both insect herbivores and host species are important to consider when predicting the trajectory of Arctic tundra shrub expansion.

Quantifying the nature and strength of intraspecific density dependence in Arctic mosquitoes.

Processes that change with density are inherent in all populations, yet quantifying density dependence with empirical data remains a challenge. This is especially true for animals recruiting in patchy landscapes because heterogeneity in habitat quality in combination with habitat choice can obscure patterns expected from density dependence. Mosquitoes (Diptera: Culicidae) typically experience strong density dependence when larvae compete for food, however, effects vary across species and contexts. If populations experience intense intraspecific density-dependent mortality then overcompensation can occur, where the number of survivors declines at high densities producing complex endogenous dynamics. To seek generalizations about density dependence in a widespread species of Arctic mosquito, Aedes nigripes, we combined a laboratory experiment, field observations, and modeling approaches. We evaluated alternative formulations of discrete population models and compared best-performing models from our lab study to larval densities from ponds in western Greenland. Survivorship curves from the lab were the best fit by a Hassell model with compensating density dependence (equivalent to a Beverton-Holt model) where peak recruitment ranged from 8 to 80 mosquitoes per liter depending on resource supply. In contrast, our field data did not show a signal of strong density dependence, suggesting that other processes such as predation may lower realized densities in nature, and that expected patterns may be obscured because larval abundance covaries with resources (cryptic density dependence). Our study emphasizes the importance of covariation between the environment, habitat choice, and density dependence in understanding population dynamics across landscapes, and demonstrates the value of pairing lab and field studies.

Parasitoids indicate major climate-induced shifts in arctic communities.

Climatic impacts are especially pronounced in the Arctic, which as a region is warming twice as fast as the rest of the globe. Here, we investigate how mean climatic conditions and rates of climatic change impact parasitoid insect communities in 16 localities across the Arctic. We focus on parasitoids in a widespread habitat, Dryas heathlands, and describe parasitoid community composition in terms of larval host use (i.e., parasitoid use of herbivorous Lepidoptera vs. pollinating Diptera) and functional groups differing in their closeness of host associations (koinobionts vs. idiobionts). Of the latter, we expect idiobionts-as being less fine-tuned to host development-to be generally less tolerant to cold temperatures, since they are confined to attacking hosts pupating and overwintering in relatively exposed locations. To further test our findings, we assess whether similar climatic variables are associated with host abundances in a 22 year time series from Northeast Greenland. We find sites which have experienced a temperature rise in summer while retaining cold winters to be dominated by parasitoids of Lepidoptera, with the reverse being true for the parasitoids of Diptera. The rate of summer temperature rise is further associated with higher levels of herbivory, suggesting higher availability of lepidopteran hosts and changes in ecosystem functioning. We also detect a matching signal over time, as higher summer temperatures, coupled with cold early winter soils, are related to high herbivory by lepidopteran larvae, and to declines in the abundance of dipteran pollinators. Collectively, our results suggest that in parts of the warming Arctic, Dryas is being simultaneously exposed to increased herbivory and reduced pollination. Our findings point to potential drastic and rapid consequences of climate change on multitrophic-level community structure and on ecosystem functioning and highlight the value of collaborative, systematic sampling effort.

Consumer-resource dynamics in Arctic ponds.

Population dynamics are shaped by species interactions with resources, competitors, enemies, and environmental fluctuations that alter the strength of these interactions. We used a food web approach to investigate the population dynamics of an abundant Arctic mosquito species, Aedes nigripes (Diptera: Culicidae). Specifically, we evaluated the importance of bottom-up variation in aquatic biofilms (food) and top-down predation from diving beetles (Colymbetes dolabratus, Coleoptera: Dytiscidae) on mosquito population performance. In spring 2018, we tracked mosquito and predator populations across eight ponds in western Greenland, measured biofilm productivity with standardized samplers, and estimated grazing pressure by invertebrate consumers with an in situ exclosure experiment. We also assessed the quality of biofilms as nutrition for mosquito larvae and evaluated pond attributes that might influence biofilm productivity and food quality. Our results indicated that mosquito population dynamics were more related to resource quality and intraspecific competition than to the density of predaceous diving beetles. Ponds with better quality biofilm tended to have more hatching larvae and those populations experienced higher per capita mortality. This aggregation of larvae in what would otherwise be the best mosquito ponds was enough to produce relatively low fitness. Thus, the landscape would support more mosquitoes if they instead distributed themselves to match predictions of the ideal free distribution. Although mortality rates were highest in ponds with the highest initial densities, the increased mortality was not generally enough to compensate for initial abundance, and 78% of the variation in the density of mosquitoes emerging from ponds was explained by the initial number of larvae in a pond. Resource quality was a strong predictor of consumer abundance, yet there was no evidence that biofilm grazing pressure was greater in ponds where mosquito density was higher. Collectively, our results suggest that mosquito ponds in western Greenland are a mosaic of source and pseudo-sink populations structured by oviposition tendencies, biofilm resource quality, and density-dependent larval mortality.