Rolf A. Ims

Ecological mechanisms and landscape ecology

Landscape ecology deals with the effects of the spatial configuration of mosaics on a wide variety of ecological phenomena. Because problems in many areas of conservation biology and resource management are related to landscape use, development of a rigorous theoretical and empirical foundation for landscape ecology is essential. We present an approach to research that focuses on how individual-level mechanisms operating in a heterogeneous mosaic produce ecological patterns that are spatially dependent. The theoretical framework that we develop considers the density and distribution of a population among patches as a function of (a) within-patch movement patterns of individuals; (b) emigration from patches as a function of population density, patch configuration, patch context, and within-patch movement; and (c) loss of individuals as they disperse through landscape elements

Ecological Dynamics Across the Arctic Associated with Recent Climate Change

Assessing the Arctic The Arctic is experiencing some of the most rapid climate change currently under way across the globe, but consequent ecological responses have not been widely reported. At the close of the Fourth International Polar Year, Post et al. (p. 1355) review observations on ecological impacts in this sensitive region. The widespread changes occurring in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering. At the close of the Fourth International Polar Year, we take stock of the ecological consequences of recent climate change in the Arctic, focusing on effects at population, community, and ecosystem scales. Despite the buffering effect of landscape heterogeneity, Arctic ecosystems and the trophic relationships that structure them have been severely perturbed. These rapid changes may be a bellwether of changes to come at lower latitudes and have the potential to affect ecosystem services related to natural resources, food production, climate regulation, and cultural integrity. We highlight areas of ecological research that deserve priority as the Arctic continues to warm.

Spatial population dynamics: analyzing patterns and processes of population synchrony

The search for mechanisms behind spatial population synchrony is currently a major issue in population ecology. Theoretical studies highlight how synchronizing mechanisms such as dispersal, regionally correlated climatic variables and mobile enemies might interact with local dynamics to produce different patterns of spatial covariance. Specialized statistical methods, applied to large-scale survey data, aid in testing the theoretical predictions with empirical estimates. Observational studies and experiments on the demography of local populations are paramount to identify the true ecological mechanisms. The recent achievements illustrate the power of combining theory, observation and/or experimentation and statistical modeling in the ecological research protocol.

The ecology and evolution of reproductive synchrony

The temporal pattern of breeding in populations is often characterized by a pronounced temporal clustering of births, flowering or seed set. It has long been suspected that this phenomenon is not caused by climatic seasonality alone but that reproductive synchrony represents a strategy that individuals adopt to maximize reproductive success. The classical hypotheses predicting an adaptive advantage of reproductive synchrony incorporate both sociobiological and ecological explanations. However, new theoretical and empirical analyses have shown that the predicted advantage of reproductive synchrony depends on the ecological setting in which populations reproduce, and processes earlier thought to be responsible only for synchrony may under some ecological conditions lead to asynchronous reproduction being the best strategy.

FUNCTIONAL RESPONSES IN HABITAT USE: AVAILABILITY INFLUENCES RELATIVE USE IN TRADE-OFF SITUATIONS

Current methods for evaluating habitat selection from animal space-use ob- servations ignore possible interactions between time allocation patterns relative to different resources, their relative abundance, and their spatial arrangements. Habitat selection may occur in situations in which animals experience a trade-off, e.g., between time used foraging in areas with abundant forage but poor protective cover, and time used for resting in areas with good protective cover but low forage abundance. We show how functional responses in habitat use (i.e., change in preference with availability of one of two main habitat types) may be tested. Given radio-telemetry data for a sample of individuals, binomial logit models can be used to regress proportionate use of a habitat type P(u) against the proportion of that habitat available, P(a). Given an appropriate fit to the data by a linear predictor on a logit scale, functional response will be indicated by a estimated slope parameter ? 1, while a slope = 0 will indicate a consistent use as availability changes. Habitat preference is inferred from the logit regression parameters when the fitted value of the proportion of use at a specified proportion of availability, is significantly greater than the proportional availability.

Collapsing population cycles

During the past two decades population cycles in voles, grouse and insects have been fading out in Europe. Here, we discuss the cause and implication of these changes. Several lines of evidence now point to climate forcing as the general underlying cause. However, how climate interacts with demography to induce regime shifts in population dynamics is likely to differ among species and ecosystems. Herbivores with high-amplitude population cycles, such as voles, lemmings, snowshoe hares and forest Lepidoptera, form the heart of terrestrial food web dynamics. Thus, collapses of these cycles are also expected to imply collapses of important ecosystem functions, such as the pulsed flows of resources and disturbances.

ON THE ADAPTIVE VALUE OF REPRODUCTIVE SYNCHRONY AS A PREDATOR-SWAMPING STRATEGY

The logic behind the hypothesis explaining reproductive synchrony as a strategy for reducing the predation of vulnerable offspring (predator swamping) is evaluated by means of two simple models. Predator swamping was found to be an adequate explanation for the occurrence of within-season reproductive synchrony when the predator exhibits a Holling type-II functional response (specialist predator). However, in the case of a generalist predator switching from alternative prey (Holling type-III functional response), which is expected to be a common functional-response type when particular prey are unavailable at certain times of the year, highly asynchronous reproduction may be the best reproductive strategy. In particular, when prey switching occurs at high offspring densities and/or the satiation density of the predator is high relative to the total reproduction of the prey population, the peak predation rate is expected to occur when reproduction is completely synchronous. Spatially clumped prey populations are expected to experience a dramatically increased predation rate if reproduction is synchronized within clumps but not between them. Hence, it is predicted that mechanisms promoting within-group reproductive synchrony are even less likely to have evolved as a predator-swamping strategy in patchily distributed populations, in which reproduction cannot be synchronized between groups. It is argued that the present models also may serve as a theoretical framework for studies on the evolution of fruiting synchrony in animal-dispersed plants.

Trophic Interaction Cycles in Tundra Ecosystems and the Impact of Climate Change

Abstract While population cycles are geographically widespread, it is on arctic tundra that such cycles appear to be most influential for the functioning of the whole ecosystem. We give an overview of tundra species that exhibit population cycles and describe what are currently believed to be the causal mechanisms. Population cycles most likely originate from trophic interactions within the plant-based tundra food web, where lemmings, either as prey for carnivores or as consumers of plants, play the key role. The predominance of trophic interaction cycles at northern latitudes is ultimately related to climate, and such cycles should therefore be vulnerable to climate change. Recent evidence indicates that changes have already taken place in the dynamics of some key herbivores and their predators, consistent with the expected impacts of climate change. There is a strong need for large-scale integrated monitoring and research efforts to further document such changes and their ecosystem consequences.

INTRINSIC AND CLIMATIC DETERMINANTS OF POPULATION DEMOGRAPHY: THE WINTER DYNAMICS OF TUNDRA VOLES

The relative impacts of intrinsic factors (e.g., density dependence) and ex- trinsic factors (e.g., climate) on winter demography may be critical for the generation of different population dynamic patterns (including cyclicity) in northern vole and lemming populations. However, little is known about winter demography because studies with tem- poral and spatial replication at the population level and an adequate sample of individuals with known fates within each population are rare. In this study, we monitored the winter demography of 48 local tundra vole populations introduced to experimentally enclosed plots the preceding spring for four years in Norway. The rate of population change over the winter (November-May) was density dependent due to recruitment. However, the large variation in the rate of change between the different winters was due to a density-independent, and most likely a climatically driven, variation in survival rate. In particular, mild weather that led to the formation of ice on the ground seemed to be detrimental for winter survival. We predict that if increased frequency of such events arose, due to climate change, normal cyclic dynamics of northern small rodent populations would be disrupted. We found support for the hypothesis that voles adjusted their body mass toward a certain mean during the winter so as to maximize winter survival. The survival rate of males was lower than that of females, possibly due to their larger body mass, and this resulted in female-biased population sex ratios in the spring. This result suggests a link between sexual selection (responsible for the sexual size dimorphism) and natural selection (operating though size-dependent winter survival) with implications for the demographic structure of the population.

Analysing diet of small herbivores: the efficiency of DNA barcoding coupled with high-throughput pyrosequencing for deciphering the composition of complex plant mixtures

BackgroundIn order to understand the role of herbivores in trophic webs, it is essential to know what they feed on. Diet analysis is, however, a challenge in many small herbivores with a secretive life style. In this paper, we compare novel (high-throughput pyrosequencing) DNA barcoding technology for plant mixture with traditional microhistological method. We analysed stomach contents of two ecologically important subarctic vole species, Microtus oeconomus and Myodes rufocanus, with the two methods. DNA barcoding was conducted using the P6-loop of the chloroplast trn L (UAA) intron.ResultsAlthough the identified plant taxa in the diets matched relatively well between the two methods, DNA barcoding gave by far taxonomically more detailed results. Quantitative comparison of results was difficult, mainly due to low taxonomic resolution of the microhistological method, which also in part explained discrepancies between the methods. Other discrepancies were likely due to biases mostly in the microhistological analysis.ConclusionWe conclude that DNA barcoding opens up for new possibilities in the study of plant-herbivore interactions, giving a detailed and relatively unbiased picture of food utilization of herbivores.