Behavior measurement and population change

Abstract

Selection intensity, as indicated by total pre‐reproductive mortality and fertility (Crow, 1958), was computed among three Indian tribal populations living in similar geographical environments—the Kolams, Raj Gonds, and Pardhans of Adilabad District, Andhra Pradesh. The Pardhans showed the greatest selection intensity, (1.1811) followed by the Kolams (0.8564) and Raj Gonds (0.7240). Pre‐reproductive mortality and infertility contributed equally to selection intensity in these tribal groups.     

Rate of population change in voles from different phases of the population cycle

Factors involved in causing cyclic vole populations to decline, and in preventing populations from recovering during the subsequent low density phase have long remained unidentified. The traditional view of self-regulation assumes that an increase in population density is prevented by a change in the quality of individuals within the population itself, but this is still inadequately tested in the field. We compared the population growth of wild field voles ( Microtus agrestis ) from the low phase (conducted in 1998) with that of voles from the increase phase (conducted in 1999) in predator-proof enclosures (each 0.5 ha) in western Finland. Within a few months, enclosed vole populations increased to high density, and the realised per capita rate of change over the breeding season did not differ between the populations from different cycle phases. This implies that the recovery of populations from the low phase was not hindered by an impoverishment in quality of individual voles. Accordingly, we suggest that population intrinsic factors (irrespective of the mechanisms they are based on) are unlikely to play a significant role in the generation of cyclic density fluctuations of voles. Instead, we discovered direct density-dependent regulation in the vole populations. Accurate estimates of population growth and the observed density dependence provide important information for empirically based models on population dynamics of rodents.     

An autoregressive model of population density change in an experimental population of Daphnia magna

Summary The usefulness of autoregression equations as empirical models of population change in an experimental population of Daphnia magna was studied. A two-term and a ten-term autoregression equation were fit to the data. Tests of the significance of the terms of the ten-term equation were carried out, and the fit of the second-order autoregression tested. The problem of autocorrelated error terms is discussed. The ability of the autoregressions to generate the observed changes in population density of the Daphnia magna population was studied by running Monte Carlo simulations using both the two-term and the ten-term models. Both models were fairly accurate up to a period of about 60 days. The second order autoregression model appeared to be more appropriate at high densities than the ten-term model, but the ten-term model was more effective at population densities less than 80 individuals.     

The equilibrium population size of a partially migratory population and its response to environmental change

A partially migratory population consists of non‐migrant and migrant individuals that share a common site during one period of the annual cycle. In this paper, we derive the expected equilibrium population sizes of migrants and non‐migrants and show how the abundance of one type is dependent on the other because their dynamics are coupled through density‐dependent effects. We present an approach for developing hypotheses about how changes in the environment will influence partially migratory populations and for formulating testable predictions about the effects of future changes on the proportions of migrants and non‐migrants. We apply this approach to a hypothesis put forward by Berthold that improved environmental conditions at the shared site will generally increase the number of non‐migrants and decrease the number of migrants, and to a study by Nilsson et al. which observed an increase in the number of migrants in a partially migratory blue tit Cyanistes caeruleus population in Sweden, but an overall maintenance of the proportion of migrants.     

A rough guide to population change in exploited fish stocks

Interpreting how populations will change in response to exploitation is essential to the sound management of fish stocks. While deterministic models can be of use in evaluating sustainable fishing rates, the inherent variability of fish populations limits their value. In this paper a probabilistic approach is investigated which avoids having to make strong assumptions about the functional relationship between spawning stock size and the annual number of young fish (recruits) produced. Empirical probability distributions for recruits are derived, conditioned on stock size, and used to indicate likely stock changes under different fishing mortality rates. The method is applied to cod ( Gadus morhua ) in the North Sea to illustrate how population change can be inferred and used by fishery managers to choose fishing mortality rates which are likely to achieve sustainable exploitation.     

Rapid morphological change in an insular population of feral sheep

Artificially selected qualities can reduce fitness in a wild setting, thus feral domesticates should experience strong selective forces. Domestic sheep Ovis aries have frequently become feral on islands, which differ substantially from mainland environments. We examined changes in body mass and wool traits in feral sheep inhabiting Santa Cruz Island (SCI), California for ≥90 years. To elucidate the influence of nutrition, we compared the mass of feral island sheep with that of island sheep raised in farm conditions. We found that feral sheep on SCI were smaller than purported founder breeds, and that most documented populations of insular feral sheep worldwide have converged to similar body sizes (within 6 kg). SCI rams attained greater mass in farm conditions but ewes did not, suggesting phenotypic plasticity in ram body mass. Ewes exhibited self-shedding of wool at a greater frequency than rams, and sex differences and shedding patterns were consistent with thermoregulation and the risk of fly strike disease as benefits of wool loss. Pigmentation rates did not increase, further supporting the influence of heat stress on wool traits. These changes occurred in <25 generations and may have had a genetic basis, representing a potential example of rapid evolution in insular feral sheep.     

Bird migration times, climate change, and changing population sizes

Past studies of bird migration times have shown great variation in migratory responses to climate change. We used 33 years of bird capture data (1970–2002) from Manomet, Massachusetts to examine variation in spring migration times for 32 species of North American passerines. We found that changes in first arrival dates – the unit of observation used in most studies of bird migration times – often differ dramatically from changes in the mean arrival date of the migration cohort as a whole. In our study, the earliest recorded springtime arrival date for each species occurred 0.20 days later each decade. In contrast, the mean arrival dates for birds of each species occurred 0.78 days earlier each decade. The difference in the two trends was largely explained by declining migration cohort sizes, a factor not examined in many previous studies. We found that changes in migration cohort or population sizes may account for a substantial amount of the variation in previously documented changes in migration times. After controlling for changes in migration cohort size, we found that climate variables, migration distance, and date of migration explained portions of the variation in migratory changes over time. In particular, short-distance migrants appeared to respond to changes in temperature, while mid-distance migrants responded particularly strongly to changes in the Southern Oscillation Index. The migration times of long-distance migrants tended not to change over time. Our findings suggest that previously reported changes in migration times may need to be reinterpreted to incorporate changes in migration cohort sizes.     

Different effects of climate change on the population dynamics of insects

Climate change affects the pattern of population dynamics of insects in different ways. Global warming not only leads to greater over-winter survival, earlier appearance in spring, an increase in the number of generations in a year, lengthening of the reproductive season, etc., but also affects their biotic associations as a result of changes in interspecific interactions. Changes in the density of insects in response to unusually hot summers provide us with useful indications of the potential effects of global warming. Different insect guilds respond differently to hot summers, which sometimes result in an increase in density and sometimes a decrease. These effects may occur immediately or be delayed by 1 or 2 years. As long as the regime remains unchanged, the affected population can recover sooner or later. Even a single-year change in climate, however, if it allows predators to outbreak, may be strong enough to cause a regime shift. Most insects are susceptible to heat stress between 28 and 32 °C, global warming could have a more profound impact on the population dynamics and biodiversity of arthropods than has previously been predicted.     

Demographic factors and genetic variation influence population persistence under environmental change

Population persistence has been studied in a conservation context to predict the fate of small or declining populations. Persistence models have explored effects on extinction of random demographic and environmental fluctuations, but in the face of directional environmental change they should also integrate factors affecting whether a population can adapt. Here, we examine the population‐size dependence of demographic and genetic factors and their likely contributions to extinction time under scenarios of environmental change. Parameter estimates were derived from experimental populations of the rainforest species, Drosophila birchii, held in the lab for 10 generations at census sizes of 20, 100 and 1000, and later exposed to five generations of heat‐knockdown selection. Under a model of directional change in the thermal environment, rapid extinction of populations of size 20 was caused by a combination of low growth rate (r) and high stochasticity in r. Populations of 100 had significantly higher reproductive output, lower stochasticity in r and more additive genetic variance (V_A) than populations of 20, but they were predicted to persist less well than the largest size class. Even populations of 1000 persisted only a few hundred generations under realistic estimates of environmental change because of low V_A for heat‐knockdown resistance. The experimental results document population‐size dependence of demographic and adaptability factors. The simulations illustrate a threshold influence of demographic factors on population persistence, while genetic variance has a more elastic impact on persistence under environmental change.     

The state of plant population modelling in light of environmental change

Plant population modelling has been around since the 1970s, providing a valuable approach to understanding plant ecology from a mechanistic standpoint. It is surprising then that this area of research has not grown in prominence with respect to other approaches employed in modelling plant systems. In this review, we provide an analysis of the development and role of modelling in the field of plant population biology through an exploration of where it has been, where it is now and, in our opinion, where it should be headed. We focus, in particular, on the role plant population modelling could play in ecological forecasting, an urgent need given current rates of regional and global environmental change. We suggest that a critical element limiting the current application of plant population modelling in environmental research is the trade-off between the necessary resolution and detail required to accurately characterize ecological dynamics pitted against the goal of generality, particularly at broad spatial scales. In addition to suggestions how to overcome the current shortcoming of data on the process-level we discuss two emerging strategies that may offer a way to overcome the described limitation: (1) application of a modern approach to spatial scaling from local processes to broader levels of interaction and (2) plant functional-type modelling. Finally we outline what we believe to be needed in developing these approaches towards a ‘science of forecasting’.     

Genetic change for earlier migration timing in a pink salmon population

To predict how climate change will influence populations, it is necessary to understand the mechanisms, particularly microevolution and phenotypic plasticity, that allow populations to persist in novel environmental conditions. Although evidence for climate-induced phenotypic change in populations is widespread, evidence documenting that these phenotypic changes are due to microevolution is exceedingly rare. In this study, we use 32 years of genetic data (17 complete generations) to determine whether there has been a genetic change towards earlier migration timing in a population of pink salmon that shows phenotypic change; average migration time occurs nearly two weeks earlier than it did 40 years ago. Experimental genetic data support the hypothesis that there has been directional selection for earlier migration timing, resulting in a substantial decrease in the late-migrating phenotype (from more than 30% to less than 10% of the total abundance). From 1983 to 2011, there was a significant decrease-over threefold-in the frequency of a genetic marker for late-migration timing, but there were minimal changes in allele frequencies at other neutral loci. These results demonstrate that there has been rapid microevolution for earlier migration timing in this population. Circadian rhythm genes, however, did not show any evidence for selective changes from 1993 to 2009.     

Investigating the potential impacts of climate change on a marine turtle population

Recent increases in global temperatures have affected the phenology and survival of many species of plants and animals. We investigated a case study of the effects of potential climate change on a thermally sensitive species, the loggerhead sea turtle, at a breeding location at the northerly extent of the range of regular nesting in the United States. In addition to the physical limits imposed by temperature on this ectothermic species, sea turtle primary sex ratio is determined by the temperature experienced by eggs during the middle third of incubation. We recorded sand temperatures and used historical air temperatures (ATs) at Bald Head Island, NC, to examine past and predict future sex ratios under scenarios of warming. There were no significant temporal trends in primary sex ratio evident in recent years and estimated mean annual sex ratio was 58% female. Similarly, there were no temporal trends in phenology but earlier nesting and longer nesting seasons were correlated with warmer sea surface temperature. We modelled the effects of incremental increases in mean AT of up to 7.5°C, the maximum predicted increase under modelled scenarios, which would lead to 100% female hatchling production and lethally high incubation temperatures, causing reduction in hatchling production. Populations of turtles in more southern parts of the United States are currently highly female biased and are likely to become ultra‐biased with as little as 1°C of warming and experience extreme levels of mortality if warming exceeds 3°C. The lack of a demonstrable increase in AT in North Carolina in recent decades coupled with primary sex ratios that are not highly biased means that the male offspring from North Carolina could play an increasingly important role in the future viability of the loggerhead turtle in the Western Atlantic.     

The anthropological evidence for change through Romanisation of the Poundbury population.

Poundbury Camp cemetery was in use for about 500 Years and was the burial ground for an Iron Age Durotrigian group, a rural Roman settlement and an urban Romano-British community. Low variance of metrical characters and persistence of familial traits in the three groups suggest a continuity of the population and in situ growth. However evidence for an anthropological response to the cultural romanisation of the population has been found in a number of skeletal traits including squatting which was most often adopted by Durotrigian females. Dietary changes are indicted by variation in concentrations of trace elements, including lead, in the bones.