Habitat selection in a variable environment

A Monte Carlo simulation scheme was utilized to determine optimal strategies of habitat utilization in a variable environment. The model allows for differences in quality among habitats at any one time and for varying levels of environmental variance and autocorrelation. When habitats are on the average equal in quality, tracking of temporal fluctuations in environment through variable habitat selection is universally advantageous with the gain in fitness limited by environmental variance, autocorrelation, and number of available habitats. Average differences in quality among habitats will restrict the advantage of variable habitat utilization (over complete usage of the average better habitat) to cases of high environmental autocorrelation or high ratios of enviromental variance to mean habitat separation. Extending an earlier prediction of Levins (1965), the average heterozygosity per individual in a natural population should increase with increasing environmental variance.     

Fitness and physiology in a variable environment

The relationship between physiological traits and fitness often depends on environmental conditions. In variable environments, different species may be favored through time, which can influence both the nature of trait evolution and the ecological dynamics underlying community composition. To determine how fluctuating environmental conditions favor species with different physiological traits over time, we combined long-term data on survival and fecundity of species in a desert annual plant community with data on weather and physiological traits. For each year, we regressed the standardized annual fitness of each species on its position along a tradeoff between relative growth rate and water-use efficiency. Next, we determined how variations in the slopes and intercepts of these fitness-physiology functions related to year-to-year variations in temperature and precipitation. Years with a relatively high percentage of small rain events and a greater number of days between precipitation pulse events tended to be worse, on average, for all desert annual species. Species with high relative growth rates and low water-use efficiency had greater standardized annual fitness than other species in years with greater numbers of large rain events. Conversely, species with high water-use efficiency had greater standardized annual fitness in years with small rain events and warm temperatures late in the growing season. These results reveal how weather variables interact with physiological traits of co-occurring species to determine interannual variations in survival and fecundity, which has important implications for understanding population and community dynamics.     

On economically tracking a variable environment

This paper presents a simple model of how an animal should best use experience to track a changing environment. The model supposes that the environment switches between good and bad states according to a first-order Markov chain. The optimal sampling behavior is characterized in terms of the stability of runs (the probability that the environment will stay in the same state from one time to the next) and the relative costs of two kinds of errors: sampling and overrun errors. This model suggests further experimental and theoretical problems.     

Population growth and structure in a variable environment

When a population experiences temporal changes in the vital rates due to environmental or biotic variation, change is not only expected in the rate of population growth but also in the structure of the population. In this study we present a method for transforming observed patterns (notably how vital rates change with temperature) into functions that can be used in population growth models and analysis of population structure. The method is exemplified by applying it to cohort studies in different constant temperatures of four species of aphids, Lipaphis erysimi (K.), Metopolophium dirhodum (Wlk.), Rhopaliosiphum padi and Macrosiphum avenae (F.). We use piece-wise linear functions to transform the vital rates of the cohort studies. The lifespans are divided into phases, each phase having linear rates. A projection matrix is formulated, where the elements are temperature dependent fecundities, survivorships and developmental rates. The major result is, contrary to what theory predicts as reasonable (Caswell 1989), that population structure of these aphid species will become almost fixed although the temperature varies. This result is consistent with findings of earlier field studies (Wiktelius 1982). A fixed population structure implies that it is possible to calculate the population growth rate on the basis of intrinsic rates of increase. By simulating different temperature regimes we also show that initial oscillations in the population structure dampen out after a few days. After initial oscillations, calculations of population growth using intrinsic rates of increase are consistent with calculations made by a matrix model.     

Motor learning and prediction in a variable environment

Traditional studies of motor learning and prediction have focused on how subjects perform a single task. Recent advances have been made in our understanding of motor learning and prediction by investigating the way we learn variable tasks, which change either predictably or unpredictably over time. Similarly, studies have examined how variability in our own movements affects motor learning.     

Phenotypic plasticity facilitates resistance to climate change in a highly variable environment

Increased summer drought will exacerbate the regeneration of many tree species at their lower latitudinal and altitudinal distribution limits. In vulnerable habitats, introduction of more drought-tolerant provenances or species is currently considered to accelerate tree species migration and facilitate forest persistence. Trade-offs between drought adaptation and growth plasticity might, however, limit the effectiveness of assisted migration, especially if introductions focus on provenances or species from different climatic regions. We tested in a common garden experiment the performance of Pinus sylvestris seedlings from the continental Central Alps under increased temperatures and extended spring and/or summer drought, and compared seedling emergence, survival and biomass allocation to that of P. sylvestris and closely related Pinus nigra from a Mediterranean seed source. Soil heating had only minor effects on seedling performance but high spring precipitation doubled the number of continental P. sylvestris seedlings present after the summer drought. At the same time, twice as many seedlings of the Mediterranean than the continental P. sylvestris provenance were present, which was due to both higher emergence and lower mortality under dry conditions. Both P. sylvestris provenances allocated similar amounts of biomass to roots when grown under low summer precipitation. Mediterranean seedlings, however, revealed lower phenotypic plasticity than continental seedlings under high precipitation, which might limit their competitive ability in continental Alpine forests in non-drought years. By contrast, high variability in the response of individual seedlings to summer drought indicates the potential of continental P. sylvestris provenances to adapt to changing environmental conditions.     

Seed and forage yield,and forage quality determinants of nine legume shrubs in a non-tropical dryland environment

The objective was to identify legume shrub species for development of agroforestry technologies based on seed and forage (leaves and twigs < 10 mm diameter) yield, and determinants of forage quality. Ten individual plants of Bituminaria bituminosa ‘Ecotypes 1’, B. bituminosa ‘Ecotypes 2’, Medicago citrina, and M. arborea from Spain; Colutea istria and Onobrychis aurantiaca from Syria; C. istria from Jordan; Chamaecytisus mollis from Morocco; and Coronilla glauca from France were randomly selected from plots established in a non-tropical dryland environment in northwest Syria in 2000. Five individual plants of each species were cut back to 0.5 m above ground in March 2004. Coppice regrowths were pruned in December 2004 and April 2005 to determine forage yield and proportion of forage in the total above ground biomass (PEFB). Forage samples were analyzed for concentrations of crude protein (CP), lignin(sa), acid detergent fibre (ADFom), neutral detergent fibre (aNDFom), in vitro organic matter (OM) digestibility (IVOMD), and in vitro 24 h gas production (IVGP24h). Matured seeds were hand harvested from the remaining five plants of each species to estimate seed yield. Forage (21–250 kg DM/ha) and seed (0–200 kg DM/ha) yields; PEFB (0.22–0.96); and concentrations of CP (85–115 g/kg DM), lignin(sa) (14–42 g/kg DM), ADFom (94–170 g/kg DM), aNDFom (122–217 g/kg DM), IVOMD (456–617 g/kg OM), and IVGP24h (27–42 ml 200 mg/DM) varied (P<0.05) among shrub species. The IVOMD and IVGP24h were positively correlated (r = 0.75, P<0.032), whereas IVOMD and IVGP24h were negatively correlated with ADFom, lignin(sa) and aNDFom. In terms of forage and seed yields and determinants of forage quality, C. istria from Jordan, M. arborea, B. bituminosa ‘Ecotype-2’, C. istria and O. aurantiaca have higher potential than C. mollis, C. glauca and B. Bituminosa ‘Ecotype-1’ for the development of agroforestry technologies in non-tropical dry areas.     

Dynamics of a harvested moose population in a variable environment

1. Population size, calves per female, female mean age and adult sex ratio of a moose ( Alces alces ) population in Vefsn, northern Norway were reconstructed from 1967 to 1993 using cohort analysis and catch-at-age data from 96% (6752) of all individuals harvested.
2. The dynamics of the population were influenced mainly by density-dependent harvesting, stochastic variation in climate and intrinsic variation in the age-structure of the female segment of the population.
3. A time delay in the assignment of hunting permits in relation to population size increased fluctuations in population size.
4. Selective harvesting of calves and yearlings increased the mean age of adult females in the population, and, because fecundity in moose is strongly age-specific, the number of calves per female concordantly increased. However, after years with high recruitment, the adult mean age decreased as large cohorts entered the adult age-groups. This age-structure effect generated cycles in the rate of recruitment to the population and fluctuations introduced time-lags in the population dynamics.
5. An inverse relationship between recruitment rate and population density, mediated by a density-dependent decrease in female body condition, could potentially have constituted a regulatory mechanism in the dynamics of the population, but this effect was counteracted by a density-dependent increase in the mean age of adult females.
6. Stochastic variation in winter snow depth and summer temperature had delayed effects on recruitment rate and in turn population growth rate, apparently through effects on female body condition before conception.     

Learning to live in a global commons: socioeconomic challenges for a sustainable environment

Ecologists, economists and other social scientists have much incentive for interaction. First of all, ecological systems and socioeconomic systems are linked in their dynamics, and these linkages are key to coupling environmental protection and economic growth. Beyond this, however, are the obvious similarities in how ecological systems and socioeconomic systems function, and the common theoretical challenges in understanding their dynamics. This should not be surprising. Socioeconomic systems are in fact ecological systems, in which the familiar ecological phenomena of exploitation, cooperation and parasitism all can be identified as key features. Or, viewed from the opposite perspective, ecological systems are economic systems, in which competition for resources is key, and in which an evolutionary process shapes the individual agents to a distribution of specialization of function that leads to the emergence of flows and functionalities at higher levels of organization. Most fundamentally, ecological and socioeconomic systems alike are complex adaptive systems, in which patterns at the macroscopic level emerge from interactions and selection mechanisms mediated at many levels of organization, from individual agents to collectives to whole systems and even above. In such complex adaptive systems, robustness must be understood as emergent from selection processes operating at these many different levels, and the inherent nonlinearities can trigger sudden shifts in regimes that, in the case of the biosphere, can have major consequences for humanity. This lecture will explore the complex adaptive nature of ecosystems, and the implications for the robustness of ecosystem services on which we depend, and in particular examine the conditions under which cooperative behavior emerges. It will then turn attention to the socioeconomic systems in which environmental management is based, and ask what lessons can be learned from our examination of natural systems, and how we can modify social norms to achieve global cooperation in managing our common future. Of special interest will be issues of intragenerational and intergenerational equity, and the importance of various forms of discounting.     

Parental provisioning in a variable environment: Evaluation of three foraging currencies and a state variable model

We estimated the reproductive success of black terns (Chlidonias niger) based on three optimal foraging currencies (maximizing the net rate of energy intake, daily delivery rate, and efficiency, respectively) and a state variable model. There was a broad range of capture intervals (the time required for the parent to capture a single prey) when the flight speeds predicted by the three currencies were so high that they resulted in daily provisioning costs which parents could not fully recover through self-feeding. Whenever the efficiency currency produced higher estimates of reproductive success, parents lost comparatively less weight than when they foraged as rate-maximizers. If parents did not experience any weight loss, the net rate and efficiency currencies made equivalent fitness projections. However, both of these currencies provided lower fitness returns than daily delivery rate at longer capture intervals. There were a number of capture intervals when estimates of reproductive success from the state variable model and at least one of the foraging currencies were equal. Provisioning behaviour under the state variable model was much more flexible and parents were therefore able to reduce their self-feeding rate on days when food was particularly scarce, thereby increasing the total delivery to the nest. This resulted in higher fitness returns than was possible under the foraging currencies. Our results suggest that efficiency-maximizing is more likely to provide fitness returns that are equivalent to the state variable model in comparison with the rate-maximizing alternatives. Furthermore, only the efficiency currency and the state variable model made predictions of flight speed that were similar to speeds measured in black tern parents provisioning young at natural nests.     

On the fixation process of a beneficial mutation in a variable environment

A population that adapts to gradual environmental change will typically experience temporal variation in its population size and the selection pressure. On the basis of the mathematical theory of inhomogeneous branching processes, we present a framework to describe the fixation process of a single beneficial allele under these conditions. The approach allows for arbitrary time-dependence of the selection coefficient s(t) and the population size N(t), as may result from an underlying ecological model. We derive compact analytical approximations for the fixation probability and the distribution of passage times for the beneficial allele to reach a given intermediate frequency. We apply the formalism to several biologically relevant scenarios, such as linear or cyclic changes in the selection coefficient, and logistic population growth. Comparison with computer simulations shows that the analytical results are accurate for a large parameter range, as long as selection is not very weak.     

Choice in a variable environment: Effects of d-amphetamine on sensitivity to reinforcement

Four pigeons responded under a 7-component mixed schedule in which each component arranged a different left:right reinforcer ratio (27:1, 9:1, 3:1, 1:1, 1:3, 1:9, 1:27). Components were unsignaled, and the order within each session was randomly determined. After extensive exposure to these contingencies, effects of a range of doses of d-amphetamine (0.3-5.6 mg/kg) on estimates of sensitivity to reinforcement at several levels of analysis were assessed. Under non-drug conditions, the structure of choice was similar to that previously reported under this procedure. That is, responding adjusted within components to the reinforcer ratio in effect (i.e., sensitivity estimates were higher in the 2nd than in the 1st half of components), and individual reinforcers produced “preference pulses” (i.e., each food presentation produced an immediate, local, shift in preference toward the response that just produced food). Although there was a general tendency for d-amphetamine to reduce overall sensitivity to reinforcement, the size of this effect and its reliability varied across pigeons. Further analysis, however, revealed that intermediate d-amphetamine doses consistently reduced sensitivity immediately following reinforcer presentations; that is, these doses consistently attenuated preference pulses.