Recruitment rates in forest plots: Gf estimates using growth rates and size distributions

1 In censuses of tree populations in permanent plots, short census intervals and small population size lead to uncertainty in the observed recruitment rate of a minimum size. Increasing the census interval, however, underestimates the rate because of unrecorded 'recruit and die' events.
2 We propose a new Gf procedure for estimation of recruitment rates. Recruitment rate per area is obtained by multiplication of the density in the smallest size class (f) and the average size growth rate in that class (G) divided by the width of the class. This procedure is valid when the size distribution of the population examined is continuous with size.
3 When tree size structure is negative‐exponentially distributed, as is often the case in natural rain forest populations, the Gf estimate of the recruitment rate for a given size class was least biased close to the midpoint size of this class.
4 Gf estimates agreed well with census estimates of recruitment rate from permanent plots in rain forests. A tendency for Gf estimates to be larger than census estimates disappeared when census estimates were corrected for mortality after recruitment.
5 The effects of plot size, census interval and variation in growth rate on estimates of recruitment rate were simulated using model populations. Small plot size caused substantially more among‐plot deviation for the census count of recruitment events than for the Gf estimate. The census recruitment rate also showed larger variation among plots for shorter intervals than the Gf estimate, which was independent of census interval. The Gf estimates were therefore more accurate than census counts in many situations. More than several tens of trees were needed in a size class to allow a reliable Gf estimates.     

Integral projection models perform better for small demographic data sets than matrix population models: a case study of two perennial herbs

1. Matrix population models are widely used to describe population dynamics, conduct population viability analyses and derive management recommendations for plant populations. For endangered or invasive species, management decisions are often based on small demographic data sets. Hence, there is a need for population models which accurately assess population performance from such small data sets.
2. We used demographic data on two perennial herbs with different life histories to compare the accuracy and precision of the traditional matrix population model and the recently developed integral projection model (IPM) in relation to the amount of data.
3. For large data sets both matrix models and IPMs produced identical estimates of population growth rate (λ). However, for small data sets containing fewer than 300 individuals, IPMs often produced smaller bias and variance for λ than matrix models despite different matrix structures and sampling techniques used to construct the matrix population models.
4. Synthesis and applications . Our results suggest that the smaller bias and variance of λ estimates make IPMs preferable to matrix population models for small demographic data sets with a few hundred individuals. These results are likely to be applicable to a wide range of herbaceous, perennial plant species where demographic fate can be modelled as a function of a continuous state variable such as size. We recommend the use of IPMs to assess population performance and management strategies particularly for endangered or invasive perennial herbs where little demographic data are available.     

Causes of early human population growth

The archaeological record indicates large increases in human population coincident with the emergence of food production about 10,000 years ago. The cause of the growth is unclear. Extreme views attribute the change to increases in the birth rate or to decreases in the death rate. Many argue that sedentism led to improved ovarian function and higher fertility through higher caloric intakes or reduced activity levels. Similarly, shortened lactation periods may have reduced birth spacing and increased fertility. Others attribute the rise in population to decreases in mortality, arguing that the evidence from skeletal populations indicates improvements in health and the expectation of life at birth, though others use the same evidence to argue that mortality increased. An analysis presented here draws on findings that indicate substantial increases in the survival of young children as populations switch from nomadic to sedentary lives. Projections indicate that this improvement in child survival is so critical that it may be followed by substantially larger decreases in survival at later ages, yet result in higher population growth rates and reduced expectation of life at birth. Increases in the birth rate are not necessary for population growth, even when overall mortality increases. Large increases in overall mortality can be associated with large increases in population. Because positive population growth can occur while the expectation of life at birth declines, this analysis shows that this summary statistic is not an appropriate indicator of population fitness. © 1996 Wiley-Liss, Inc.     

Can population growth rates vary with the spatial scale at which they are measured?

1.  The ratio of successive population censuses is often assumed to reflect population growth rates. We identify three simple potential sources of bias in the estimation of population growth rates that relate to either the total number of censused individuals or the spatial areas over which censuses are conducted.
2.  The commonly used method of adding a constant to time series data to avoid problems caused by division by zero can lead to underestimation of growth rates at low densities in increasing populations.
3.  Variances associated with density estimates can lead to positive bias in estimation of growth rates when populations are distributed in ephemeral patches. The spatial variance and spatio-temporal covariance in bank vole census data suggest that this bias could be severe when small trapping grids are used. Use of logged estimators of growth rate avoids this problem.
4.  Using census data from non-randomly placed trapping grids that are smaller than twice the maximum range of natal dispersal to estimate population growth rates can lead to negatively biased estimates, particularly at low population densities.
5.  These three sources of bias are evaluated as explanations for scale-dependent changes in the estimates of growth rates identified in populations of snowshoe hare ( Lepus americanus ), bank voles ( Clethrionomys glareolus ) and lemmings ( Lemmus lemmus ).     

Partial life cycle analysis: a model for pre-breeding census data

Matrix population models have become popular tools in research areas as diverse as population dynamics, life history theory, wildlife management, and conservation biology. Two classes of matrix models are commonly used for demographic analysis of age‐structured populations: age‐structured (Leslie) matrix models, which require age‐specific demographic data, and partial life cycle models, which can be parameterized with partial demographic data. Partial life cycle models are easier to parameterize because data needed to estimate parameters for these models are collected much more easily than those needed to estimate age‐specific demographic parameters. Partial life cycle models also allow evaluation of the sensitivity of population growth rate to changes in ages at first and last reproduction, which cannot be done with age‐structured models. Timing of censuses relative to the birth‐pulse is an important consideration in discrete‐time population models but most existing partial life cycle models do not address this issue, nor do they allow fractional values of variables such as ages at first and last reproduction. Here, we fully develop a partial life cycle model appropriate for situations in which demographic data are collected immediately before the birth‐pulse (pre‐breeding census). Our pre‐breeding census partial life cycle model can be fully parameterized with five variables (age at maturity, age at last reproduction, juvenile survival rate, adult survival rate, and fertility), and it has some important applications even when age‐specific demographic data are available (e.g., perturbation analysis involving ages at first and last reproduction). We have extended the model to allow non‐integer values of ages at first and last reproduction, derived formulae for sensitivity analyses, and presented methods for estimating parameters for our pre‐breeding census partial life cycle model. We applied the age‐structured Leslie matrix model and our pre‐breeding census partial life cycle model to demographic data for several species of mammals. Our results suggest that dynamical properties of the age‐structured model are generally retained in our partial life cycle model, and that our pre‐breeding census partial life cycle model is an excellent proxy for the age‐structured Leslie matrix model.     

Effects of intraspecific and interspecific density on the demography of a perennial herb, Sanicula europaea

We examined the effects of intraspecific and interspecific competition on demographic processes in the perennial herb Sanicula europaea by manipulating the density of neighbouring plants. We followed the response in terms of survival, growth and reproduction and in terms of seedling recruitment. The demographic data from all phases of the life cycle enabled us to assess also the overall effects of treatments on population growth rate (λ) by transition matrix models. We also decomposed the differences in λ between control and treatments, using life table response experiments (LTRE). To study the effects of competition on recruitment in more detail and to evaluate the role of seed availability, we sowed seeds at different densities with or without vegetation removal.
Vegetative growth and flowering frequency of established individuals was not significantly affected by removal treatments, which suggest no, or a delayed response to released competition. Neighbour removal had no effect on seedling emergence but enhanced recruitment through a higher seedling survival. Conspecific and simultaneous conspecific and heterospecific removal of plants led to an increase in population growth rate (λ), whereas heterospecific removal alone led to a decrease. Emergence of seedlings and fate of vegetative established individuals contributed most to differences in λ between the control and the different treatments. Seed addition enhanced seedling emergence but, as seedling and juvenile survival were density dependent, densities of established individuals appear not to be seed limited.
In S. europaea removal treatments had different effects on established individuals and recruitment. This suggests that studies quantifying the effects of competition over the entire life cycle and performed in a natural environment are necessary to assess the importance of competition in perennial plant populations.     

Linking demographic responses and life history tactics from longitudinal data in mammals

In stochastic environments, a change in a demographic parameter can influence the population growth rate directly or via a resulting impact on age structure. Stochastic elasticity of the long‐run stochastic growth rate λ_s to a demographic parameter offers a suitable way to measure the overall demographic response because it includes both the direct effect of changing the demographic parameter and its indirect effect through changes in the age structure. From 25 mammalian populations with contrasting life histories, we investigated how pace of life and population growth rate influence the demographic responses (measured as the relative contributions of the direct and indirect components of stochastic elasticity on λ_s). We found that in short‐lived species, the change in population structure resulting from an increase in yearling survival leads to an additional increase in λ_s, whereas in long‐lived species, the same change in population structure leads to a decrease. Short‐lived species thus display a boom‐bust life history strategy contrary to long‐lived species, for which the long lifespan dampens the demographic consequences of changing age structure. Irrespective of the species’ life history strategy, the change in population age structure resulting from an increase in adult survival leads to an additional increase in λ_s due to an increase of the proportion of mature individuals in the population. On the contrary, a change in population age structure resulting from an increase of reproductive performance leads to a decrease in λ_s that is due to the increase of the proportion of immature individuals in the population. Our comparative analysis of stochastic elasticity patterns in mammals shows the existence of different demographic responses to changes in age structure between short‐ and long‐lived species, which improves our understanding of population dynamics in variable environments in relation to the species‐specific pace of life.     

Cladoceran birth and death rates estimates: experimental comparisons of egg-ratio methods

I. Birth and death rates of natural cladoceran populations cannot be measured directly. Estimates of these population parameters must be calculated using methods that make assumptions about the form of population growth. These methods generally assume that the population has a stable age distribution.
2. To assess the effect of variable age distributions, we tested six egg ratio methods for estimating birth and death rates with data from thirty-seven laboratory populations of Daphnia pulicaria. The populations were grown under constant conditions, but the initial age distributions and egg ratios of the populations varied. Actual death rates were virtually zero, so the difference between the estimated and actual death rates measured the error in both birth and death rate estimates.
3. The results demonstrate that unstable population structures may produce large errors in the birth and death rates estimated by any of these methods. Among the methods tested, Taylor and Slatkin's formula and Paloheimo's formula were most reliable for the experimental data.
4. Further analyses of three of the methods were made using computer simulations of growth of age-structured populations with initially unstable age distributions. These analyses show that the time interval between sampling strongly influences the reliability of birth and death rate estimates. At a sampling interval of 2.5 days (equal to the duration of the egg stage), Paloheimo's formula was most accurate. At longer intervals (7.5–10 days), Taylor and Slatkin's formula which includes information on population structure was most accurate.     

Demographic models of the northern spotted owl (Strix occidentalis caurina)

Summary Calassical demographic methods applied to life history data on the northern spotted owl yield and estimate of the annual geometric rate of increase for the population of λ=0.96±0.03, which is not significantly different from that for a stable population (λ=1.00). Sensitivity analysis indicates that adult annual survivorship has by far the largest influence on λ, followed by the probability that juveniles survive dispersal, and the adult annual fecundity. Substantial temporal fluctuations in demographic parameters have little effect on the long-run growth rate of the population because of the long adult life expectancy. A model of dispersal and territory occupancy that assumes demographic equilibrium is evaluated using data on the amount of old forest habitat remaining in the Pacific Northwest and the current occupancy of this habitat by northern spotted owls. This model is employed to predict the effect of future habitat loss and fragmentation on the population, implying that extinction will result if the old forest is reduced to less than a proportion 0.21±0.02 of the total area in a large region. The estimated minimum habitat requirement for the population is greater than that allowed in management plants by the USDA Forest Service.     

New approaches to the pre- and post-contact history of Arctic peoples.

The last few years have witnessed the addition of new techniques and research strategies to the study of the population history of Arctic peoples. Osteon-photon analysis of bone cores provides an improved method of assigning age at death to skeletons. Consequently, it is possible to improve calculations of life expectancy and relate them to pathological correlates such as osteoporosis, separate neural arches, spina bifida and arthritis along with regular growth changes. This capability enables much better utilization of pre-contact skeletons and therefore of the numbers, density and composition of populations before European contact. The general picture emerging from skeletal studies, census records and living populations is, in Arctic Eskimos, one of high fertility, high mortality and short length of life, with a slow population growth rate. Aleuts show lower fertility, lower mortality and longer length of life, also with a low population growth rate.     

Using elasticity analysis of demographic models to link toxicant effects on individuals to the population level: an example

1. A simple two-stage population model was applied to data from a previously published life-table response experiment (LTRE), which examined the toxicity of 4- n -nonylphenol to life-history traits of the polychaete Capitella sp. I. Population growth rates ( λ ) and the relative sensitivities (= elasticities) of λ to changes in each of the individual life-history traits were calculated.
2. In the present study, the life-history parameters measured in laboratory-reared individuals were manipulated to simulate potential effects of competition and predation on fecundity, time to reproductive maturity and juvenile survival to explore how such factors might influence the sensitivity of population growth rate to toxicant-caused changes in individual life-history traits.
3. Dramatic changes in elasticity patterns among simulations indicate that population growth rates may respond very differently to toxicant exposure depending on the extent to which other demographically limiting factors (e.g. competitors and/or predators) are operating on the population.
4. Effectively predicting the population-level consequences arising from toxicant effects measured on individuals can be improved by exploring the elasticity pattern of λ for the population over a range of realistic ecological situations.     

Plant Demography and Habitat: A Comparative Approach

Abstract Progress in plant demography will depend upon being able to synthesize a large body of data and this requires a means of comparing populations between sites and species. We have employed a comparative technique using elasticity analysis of stage-projection matrices to partition the contributions of fecundity (F), survival (L) and growth (G) to the finite rate of increase λ. Ordination of populations of 77 perennial herbs and trees in G-L-F space has shown that species segregate in this triangular space according to their life history and habitat. In the present paper we use the correspondence between demographic parameters and habitat revealed by this method to predict how succession and a variety of environmental factors such as grazing and fire are likely to alter the demography of populations and ultimately to change the composition of communities.     

Prediction vs. reality: Can a PVA model predict population persistence 13 years later?

The challenge of conservation biology is to make models that predict population dynamics and have a high probability of accurately tracking population change (increase, decrease, constancy). In this study we tested whether the transition model is accurate enough to predict population persistence and size 13 years down and whether after 13 years populations had achieved a stable stage distribution. We modeled 6 small populations of an epiphytic orchid using a Lefkovitch type analysis to predict population growth pattern based on monthly surveys for approximately 1.5 years. In addition, sensitivity and elasticity analyses were used to identify life stages with high sensitivity or elasticity that have the largest influence on population growth rate. We re-censused the populations 13 years after the first study and compared the structure of the populations to predictions based on the earlier census data. Effective population growth rates were similar to those expected except for one where the population went extinct. The prediction slightly (but not significantly) overestimated the actual population growth rates of some populations. Elasticity analysis revealed that the adult stage is critical in the life cycle. The observed stage distributions of the populations were not stable at the beginning of the survey and neither were they after 13 years. We suggest that this might be caused by external perturbations that result in unequal mortality between life stages and stochastic recruitment events. The ability of the matrices to predict population size approximately eight generations in the future is encouraging and warrants the continued use of these approaches for PVA.     

Local population dynamics and the impact of scale and isolation: a study on different little owl populations

The understanding of how variation of demographic rates translates into variation of population growth is a central aim in population ecology. Besides stochastic and deterministic factors, the spatial extent and the isolation of a local population may have an impact on the contribution of the different demographic components. Using long-term demographic data we performed retrospective population analyses of four little owl ( Athene noctua ) populations with differential spatial extent and degree of isolation to assess the contribution of demographic rates to the variation of the growth rate (λ) of each local population and to the difference of λ among populations. In all populations variation of fecundity contributed least to variation of λ, and variation of adult survival contributed most to variation of λ in three of four populations. Between population comparisons revealed that differences mainly stem from differences of immigration and juvenile local survival. The relative importance of immigration to λ tended to decrease with increasing spatial extent and isolation of the local populations. None of the four local populations was self-sustainable. Because the local populations export and import individuals, they can be considered as open recruitment systems in which part of the recruited breeding birds are not produced locally. The spatial extent and the degree of isolation of a local population have an impact on local population dynamics; hence these factors need to be considered in studies about local population dynamics and for deriving conservation measures.     

The interactive effects of climate, life history, and interspecific neighbours on mortality in a population of seed harvester ants

Abstract.  1. Fifteen years of census data from a population of the red harvester ant ( Pogonomyrmex barbatus ), 5 years of census data from populations of Aphaenogaster cockerelli and Myrmecocystus mimicus at the same site, and long-term precipitation data from a nearby weather station were used to examine how nearest interspecific neighbours, climate, and life history influence P. barbatus colony mortality.
2. It was found that more P. barbatus colonies die following dry summers than following wet summers, but there is no relationship between the establishment of new colonies and rainfall.
3.  Aphaenogaster cockerelli , but not M. mimicus , nearest neighbours influence P. barbatus colony mortality in some years and for some ages.
4. The effects of nearest A. cockerelli neighbours are usually most severe following dry summers, when resource availability is reduced.     

Effect of metal stress on life history divergence and quantitative genetic architecture in a wolf spider

Effects and consequences of stress exposure on life history strategies and quantitative genetic variation in wild populations remain poorly understood. We here study whether long-term exposure to heavy metal pollution may result in alternative life history strategies and alter quantitative genetic properties in natural populations of the wolf spider Pirata piraticus. Offspring originating from a reference and a metal contaminated population and their reciprocal hybrid cross were bred in a half-sib mating scheme and subsequently reared in cadmium contaminated vs. clean environment. Results from this experiment provided evidence for a genetically based reduced growth rate and increased egg size in the contaminated population. Growth rate reduction in response to cadmium contamination was only observed for the reference population. Animal model analysis revealed that heritability for growth rate was large for the reference population under reference conditions, but much lower under metal stressed conditions, caused by a strong decrease in additive genetic variance. Heritability for growth of the metal contaminated population was very low, even under reference conditions. Initial size of the offspring was primarily determined by maternal effects, whereas egg size produced by the offspring was determined by both sire and dam effects, indicating that egg size determination is under control of the female genotype. In conclusion, these results show that metal stress can not only affect life history variation in natural populations, but also decreases the expression as well as the of the amount of genetic variation for particular life history traits.     

The analysis of plant growth in ecological and evolutionary studies

Plant growth rate is often assumed to be an ecologically important life history trait. However, conventional plant growth analysis, while providing a useful accounting of rates of weight gain and its components, is ill-suited for testing relationships between growth and fitness, particularly in natural populations. Two new approaches that are suitable for testing such relationships have evolved over the past several years. The first - the population biology of plant parts, or 'modular demography' - permits non-destructive measures of growth rate in natural field populations. When modular demography is performed using matrix population models, controls over growth rate can be examined, as well as consequences of growth variation for reproduction. The second - demographic growth analysis - provides growth parameters analogous to those of conventional growth analysis, but can be performed in natural field populations. Demographic growth analysis allows measures of individual growth-rate variation, which, in turn, can be related to plant performance.     

Demography of the vegetable ivory palm Phytelephas seemannii in Colombia, and the impact of seed harvesting

1. The demography of the vegetable ivory palm Phytelephas seemannii was studied on the Pacific coast of Colombia and a female-based matrix model was used to determine the proportion of seed that can be sustainably harvested from the population.
2. The density of the palm stands ranged from 240 to 420 adult palms ha⁻¹. The sex ratio was 1:1 and palms of both sexes produced leaves at the same rate. Seedlings produced 1·2 leaves per year on average, juveniles 1·8, and adults 6·1–7·4.
3. Adult females had fewer leaves than males (18·5 vs. 21·3 on average). Leaves of females lasted about 2·7 years in the crown, those of males about 3·2 years.
4. Stems creep on the ground, growing at the apex and often dying behind, the extant portion not always reflecting the palm's total age. The longest stem of a female palm in the study plots was 2·5 m, corresponding to an 'extant' age of 85 years; the longest stem recorded was that of a male (outside the plots) 10·5 m long, corresponding to an 'extant' age of 184 years. These figures reflect differences in growth habit, not in longevity.
5. Reproduction began at about 24 years, when the palm still lacked an above-ground stem.
6. The population growth rate λ was 1·059, and was most sensitive to changes in survival of juveniles and adults, and relatively insensitive to changes in fecundity and growth.
7. River channel migration is the most important cause of adult mortality. Phytelephas seemannii is apparently an efficient colonizer of the understorey in the late phases of riverine forest succession.
8. The population can tolerate a harvest intensity of 86% of all seeds before λ decreases to the equilibrium level of 1·00. Monitoring of the populations under intensive harvesting is required.     

A meta‐analysis of latitudinal variations in life‐history traits of roach,Rutilus rutilus,over its geographical range: linear or non‐linear relationships?

1. We collated information from the literature on life history traits of the roach (a generalist freshwater fish), and analysed variation in absolute fecundity, von Bertalanffy parameters, and reproductive lifespan in relation to latitude, using both linear and non-linear regression models. We hypothesized that because most life history traits are dependent on growth rate, and growth rate is non-linearly related with temperature, it was likely that when analysed over the whole distribution range of roach, variation in key life history traits would show non-linear patterns with latitude.
2. As fecundity depends strongly on length, and the length structure of females varied among populations, latitudinal patterns in fecundity were examined based on residuals from the length–fecundity relationship. The reproductive lifespan of roach was estimated as the difference between age at maturity and maximum age of females in each population.
3. The three life history traits of roach analysed all varied among populations and were non-linearly related to latitude. Only the relationship between reproductive lifespan and latitude was a better fit to a linear that to a quadratic model, although Loess smoothing curves revealed that this relationship was actually closer to biphasic than linear in form. A latitude of 50°N formed a break point in all three life history traits.
4. The negative relationships we have described between (i) fecundity and reproductive lifespan and (ii) fecundity and egg mass suggest that lower fecundity is compensated for by longer lifespan, while lower fecundity is compensated for by an increased egg mass, when analysed independently of location.     

Regulation of damseifly populations: the effects of larval density on larval urvival, development rate and size in the field

SUMMARY. 1. The popuhttion density of Coenagrion pttella larvae was monitored in five populations, and of Ischntira elegans in two populations, between October 1982 and May 1983.
2. There was no measurable mortality of larvae over winter and no larval growth until April. Larvae in high density populations were smaller than those in low density populations and were more likely to have a semi- voltine life history.
3. The population density of C. ptiella was also monitored (more frequently) in two populations with differenl initial densities between July and November 1983. In the high density population there was a constant rate of larval mortality, while in the low density population there was no detectable larval mortality, indicating that larval mortality may be density dependent. Larvae in the high density population were again smaller, and more likely to be semi-voitine, than those in the low density population.
4. The role of density dependent larval growth, development and mortality in the regulation of damseifly populations is discussed.