Age-structured Demographic Model Research Articles

Demographic models predict end-Pleistocene arrival and rapid expansion of pre-agropastoralist humans in Cyprus

The antiquity of human dispersal into Mediterranean islands and ensuing coastal adaptation have remained largely unexplored due to the prevailing assumption that the sea was a barrier to movement and that islands were hostile environments to early hunter-gatherers [J. F. Cherry, T. P. Leppard, J. Isl. Coast. Archaeol. 13, 191-205 (2018), 10.1080/15564894.2016.1276489]. Using the latest archaeological data, hindcasted climate projections, and age-structured demographic models, we demonstrate evidence for early arrival (14,257 to 13,182 calendar years ago) to Cyprus and predicted that large groups of people (~1,000 to 1,375) arrived in 2 to 3 main events occurring within <100 y to ensure low extinction risk. These results indicate that the postglacial settlement of Cyprus involved only a few large-scale, organized events requiring advanced watercraft technology. Our spatially debiased and Signor-Lipps-corrected estimates indicate rapid settlement of the island within <200 y, and expansion to a median of 4,000 to 5,000 people (0.36 to 0.46 km-2) in <11 human generations (<300 y). Our results do not support the hypothesis of inaccessible and inhospitable islands in the Mediterranean for pre-agropastoralists, agreeing with analogous conclusions for other parts of the world [M. I. Bird et al., Sci. Rep. 9, 8220 (2019), 10.1038/s41598-019-42946-9]. Our results also highlight the need to revisit these questions in the Mediterranean and test their validity with new technologies, field methods, and data. By applying stochastic models to the Mediterranean region, we can place Cyprus and large islands in general as attractive and favorable destinations for paleolithic peoples.

A demographic model for the conservation and management of the European eel: an application to a Mediterranean coastal lagoon

AbstractWe present a general size- and age-structured demographic model of the continental phase of catadromous eels’ life cycle, allowing for the incorporation of (i) fluctuations of juvenile recruitment, (ii) density-dependent settlement and sex determination, (iii) plastic body growth and sexual dimorphism, and (iv) vital rates driven by body size rather than age. The model can be used to assess (i) standing stock, (ii) fishing yield, and (iii) spawner production under different recruitment and management scenarios. We demonstrate the model by applying it to a European eel stock (Camargue lagoons, southern France) that was continuously monitored for 17 years (1993–2009). Results indicate that the upper limit to settlement in the Camargue water system is ∼419 eels ha−1 per year. A balanced sex ratio occurs for a settlement of ∼94 eels ha−1, shifting towards males at higher densities. Catchability is higher in adult silver eels than in sub-adult yellow eels, possibly due to behavioural differences. Estimated standing stock ranged between 7 and 19 kg ha−1. Fishing yield was 5–7 kg ha−1 per year, while spawner escapement ranged between 0 and 13 kg ha−1 per year (&lt;30% of what would occur in the absence of fishing and obstacles to migration).

Can We Sustainably Harvest Ivory?

Despite the 1989 ivory trade ban, elephants continue to be killed to harvest their tusks for ivory. Since 2008, this poaching has increased to unprecedented levels driven by consumer demand for ivory products. CITES is now considering the development of a legal ivory trade [1, 2]. The proposal relies on three assumptions: (1) harvest regulation will cease all illegal activities, (2) defined sustainable quotas can be enforced, and (3) we can define meaningful sustainable quotas that come close to the current demand. We know that regulation of harvest does not stop illegal takes. Despite whaling regulation after World War II, illegal whaling continued for decades [3]. The introduction of wolf culls in the US actually increased poaching activities [4], and one-off ivory sales in 1999 and 2008 did nothing to halt elephant poaching. Governance issues over the ivory supply chains, including stockpiling, make enforcing quotas challenging, if not impossible [5, 6]. We have not yet adequately assessed what could be a sustainable ivory yield. To do so, we develop a compartmental model composed of a two-sex age-structured demographic model and an ivory production and harvest model. We applied several offtake and quota strategies to define how much ivory could be sustainably harvested. We found that the sustainability space is very small. Only 100 to 150kg of ivory could be removed from a reference population of 1,360 elephants, levels well below the current demand. Our study shows that lifting the ivory ban will not address the current poaching challenge. We should instead focus on reducing consumer demand.

Comparative Demography of Skates: Life-History Correlates of Productivity and Implications for Management

Age-structured demographic models were constructed based on empirical estimates of longevity and maturity for five deepwater Bering Sea skates to investigate how observed differences in life history parameters affect population growth rates. Monte Carlo simulations were used to incorporate parameter uncertainty. Estimated population growth rates ranged from 1.045 to 1.129 yr−1 and were lower than those reported for other Alaskan skates and most chondrichthyans. Population growth rates of these and other high-latitude skates increased with relative reproductive lifespan, but displayed no significant relationship with body size or depth distribution, suggesting that assemblage shifts may be difficult to predict for data-poor taxa. Elasticity analyses indicated that juvenile and adult survival had greater per-unit effects on population growth rates than did egg-case survival or fecundity. Population growth rate was affected more by uncertainty in age at maturity than maximum age. The results of this study indicate that if skates are deemed to be a management concern, gear modifications or depth-specific effort controls may be effective.

Perspectives on International Trends and Dynamics in Population and Consumption

There are increasing concerns that global environmental limits may soon be met as a result of increasing numbers of people coupled with increasing consumption of resources. However, the current level and rates of growth in both consumption and population vary systematically among countries grouped according to income levels. Many high income countries have population growth rates at close to replacement levels, but their per capita consumption is consistently several times higher than low income countries. Low income countries need to grow out of poverty and have high population growth rates. Using current population structures for India and the USA in an age-structured demographic model, and simple projections of annual reductions in fertility or consumption per capita over the next 50 years, we show that while reductions in both consumption and fertility are necessary to stabilize impacts, there are short term gains from consumption reductions in high income countries such as the USA, and long term gains from early fertility reductions in growing economies such as India.

ESTIMATING THE FUNCTIONAL FORM FOR THE DENSITY DEPENDENCE FROM LIFE HISTORY DATA

Two contrasting approaches to the analysis of population dynamics are currently popular: demographic approaches where the associations between demographic rates and statistics summarizing the population dynamics are identified; and time series approaches where the associations between population dynamics, population density, and environmental covariates are investigated. In this paper, we develop an approach to combine these methods and apply it to detailed data from Soay sheep (Ovis aries). We examine how density dependence and climate contribute to fluctuations in population size via age- and sex-specific demographic rates, and how fluctuations in demographic structure influence population dynamics. Density dependence contributes most, followed by climatic variation, age structure fluctuations and interactions between density and climate. We then simplify the density-dependent, stochastic, age-structured demographic model and derive a new phenomenological time series which captures the dynamics better than previously selected functions. The simple method we develop has potential to provide substantial insight into the relative contributions of population and individual-level processes to the dynamics of populations in stochastic environments.

Demography and elasticity of the diamond stingray, Dasyatis dipterura: parameter uncertainty and resilience to fishing pressure

Despite their abundance in near-shore tropical and subtropical marine environments, which support much of the world’s elasmobranch fisheries, population dynamics and impacts of fisheries on stingrays are poorly documented. Age-structured demographic models were developed using empirical estimates of fecundity, longevity and maturity to project population growth parameters and potential responses to fishing mortality of Dasyatis dipterura from the Bahía Magdalena lagoon complex, México. Monte Carlo simulation was incorporated to include uncertainty in life history parameters into model projections. Six models were developed using deterministic and probabilistic approaches under unexploited and exploited (fishing mortality = 0.05 year–1) conditions. Mean annual population growth rates (λ) of 1.05–1.06 (5–6% increase), net reproductive rates of 2.3–2.4 and generation times of 14.9–16.5 years were projected from simulations. The introduction of a low fishing mortality into probabilistic models produced λ of 1.01 year–1. Elasticity analysis indicated that population growth rates for D. dipterura are more strongly influenced by the survival of juvenile and adult stages than by survival of neonates or changes in fecundity. Demographic analyses indicated that D. dipterura has a low intrinsic growth potential and limited resilience to fishing pressure. Localised depletion or population collapses are therefore likely to occur through unrestricted, unmonitored fishing effort.

DemogR: A Package for the Construction and Analysis of Age-structured Demographic Models inR

The analysis of matrix population models has become a fundamental tool in ecology, conservation biology, and life history theory. In this paper, I present demogR, a package for analyzing age-structured population models in R. The package includes tools for the construction and analysis of matrix population models. In addition to the standard analyses commonly used in evolutionary demography and conservation biology, demogR contains a variety of tools from classical demography. This includes the construction of period life tables, and the generation of model mortality and fertility schedules for human populations. The tools in demogR are generally applicable to age-structured populations but are particularly useful for analyzing problems in human ecology. I illustrate some of the capabilities of the package by doing an evolutionary demographic analysis of several human populations.

Patrons de reproduction des femelles d’isard (Rupicapra pyrenaica pyrenaica) dans une population non chassée et conséquences démographiques

From the long-term monitoring of isard females ( Rupicapra pyrenaica pyrenaica Bonaparte, 1845) marked in two areas of the Parc National des Pyrénées (France) with a quite constant population size for 20 years, we have conducted an analysis of age variation in recruitment (estimated as the proportion of offspring surviving through the winter per female). As predicted for a well-established high-density population, recruitment rates were low for young females (0.156 at 3 years of age and 0.221 at 4 years of age), but also for prime-aged females (0.414 per year for 5–16 year old females). No female older than 16 that we monitored recruited in any year. Recruitment rates were highly variable among females, among years (0.183 in 2002 vs. 0.635 in 1996 for prime-age females), and among areas (0.562 kid at Mayouret vs. 0.359 at Péguère). The use of an age-structured demographic model allowed us then to compare population dynamics between the well-established isard population in the Park and the colonizing population of isards at Bazès, in the eastern part of the Pyrénées. As predicted, the natural rate of increase of the isard population was much higher at Bazès (1.262) than in the Park (1.037). Likewise, the age structure of isard in the Park markedly differed from that obtained at Bazès (50% of females were younger than 5 years of age, and 19% were older than 10 years of age in the Park versus 67% and only 3.6%, respectively, at Bazès). Such differences in both population growth and age structure between isard populations with contrasted demographic regimes were well illustrated by marked differences in generation time (4.97 years at Bazès vs. 8.25 years in the Park). Our comparative study therefore demonstrates that generation time can vary markedly in relation to the demographic status within a given species.

Evolutionary demographic models for mortality plateaus.

Plateaus in the age pattern of hazard functions at extreme ages have been discovered in large populations of medflies, Drosophila, nematodes, and people. Mueller and Rose [(1996) Proc. Natl. Acad. Sci. USA 93, 15249-15253] have proposed several age-structured demographic models to represent effects of mutation accumulation and antagonistic pleiotropy on randomly evolving schedules of demographic rates. They assert that "evolutionary theory [as embodied in their models] predicts late-life mortality plateaus." This paper defines a class of Markovian models that includes those of Mueller and Rose and obtains a characterization of the possible limiting states. For the basic model, the result implies that schedules with late-life mortality plateaus above a minimal threshold are not limiting states. The models fail, but not for reasons previously conjectured. Transient states, visited early by the process, do display mortality plateaus. Other models from this class may have a role to play in reconciling observed plateaus with evolutionary theory.

A demographic model to predict future growth of the Addo elephant population

An age-structured demographic model of the growth of the Addo elephant population was developed using parameters calculated from long-term data on the population. The model was used to provide estimates of future population growth. Expansion of the Addo Elephant National Park is currently underway, and the proposed target population size for elephant within the enlarged park is 2700. The model predicts that this population size will be reached during the year 2043, so that the Addo elephant population can continue to increase for a further 44 years before its target size within the enlarged park is attained.