Statistical inference of prehistoric demography from frequency distributions of radiocarbon dates: a review and a guide for the perplexed

The last decade saw a rapid increase in the number of applications where time-frequency changes of radiocarbon dates have been used as a proxy for inferring past population dynamics. Although its simple and universal premise is appealing and undoubtedly offers some unique opportunities for research on long-term comparative demography, practical applications are far from trivial and riddled by challenges. Here I review: 1) the most common criticisms concerning the nature of radiocarbon time-frequency data as a demographic proxy; 2) the statistical nature of the problem; and 3) three classes of inferential approaches proposed so far in the literature.

Life history of the common blacktip shark, Carcharhinus limbatus, from central eastern Australia and comparative demography of a cryptic shark complex

Common and Australian blacktip sharks (Carcharhinus limbatus and Carcharhinus tilstoni) occur sympatrically in Australia, where they are reported as a complex because of their morphological similarities. This study provides the first description of the life history of C. limbatus using samples from central eastern Australia, where C. tilstoni is rare. Females (68–267cm total length (TL); n=183) and males (65–255cm TL; n=292) both matured at 8.3 years and 200cm TL, which exceeds the maximum length of C. tilstoni. Vertebral ageing revealed that female and male C. limbatus lived to 22 and 24 years respectively, exceeding known longevity in C. tilstoni. The mean (±s.d.) intrinsic rate of population increase calculated using a Euler–Lotka demographic method was 0.11±0.02year–1 for C. limbatus, compared with 0.17±0.02 and 0.20±0.03year–1 for two C. tilstoni stocks. Despite their similar appearance, these species differed in both their biological productivity and susceptibility to fishing activities. Monitoring of relative abundance should be a priority given they are likely to have divergent responses to fishing.

Comparative demography elucidates the longevity of parasitic and symbiotic relationships

Parasitic and symbiotic relationships govern vast nutrient and energy flows, yet controversy surrounds their longevity. Enduring relationships may engender parallel phylogenies among hosts and parasites, but so may ephemeral relationships when parasites colonize related hosts. An understanding of whether symbiont and host populations have grown and contracted in concert would be useful when considering the temporal durability of these relationships. Here, we devised methods to compare demographic histories derived from genomic data. We compared the historical growth of the agent of severe human malaria, Plasmodium falciparum , and its mosquito vector, Anopheles gambiae , to human and primate histories, thereby discerning long-term parallels and anthropogenic population explosions. The growth history of Trichinella spiralis , a zoonotic parasite disseminated by swine, proved regionally specific, paralleling distinctive growth histories for wild boar in Asia and Europe. Parallel histories were inferred for an anemone and its algal symbiont ( Exaiptasia pallida and Symbiodinium minutum ). Concerted growth in potatoes and the agent of potato blight ( Solanum tuberosum and Phytophthora infestans ) did not commence until the age of potato domestication. Through these examples, we illustrate the utility of comparative historical demography as a new exploratory tool by which to interrogate the origins and durability of myriad ecological relationships. To facilitate future use of this approach, we introduce a tool called C-PSMC to align and evaluate the similarity of demographic history curves.

Comparative demography of commercially-harvested snappers and an emperor from American Samoa

The age-based life history of two commercially-important species of snapper (Lutjanidae) and one emperor (Lethrinidae) were characterized from the nearshore fishery of Tutuila, American Samoa. Examination of sagittal otoliths across multiple months and years confirmed the annual deposition of increments and highlighted marked variation in life-history patterns among the three meso-predator species. The humpback red snapper Lutjanus gibbus is a medium-bodied gonochoristic species which exhibits striking sexual dimorphism in length-at-age and consequent growth trajectories and has a life span estimated to be at least 27 years. The yellow-lined snapper Lutjanus rufolineatus is a small-bodied gonochore with weak sexual dimorphism, early maturation, and a short life span of at least 12 years. The yellow-lip emperor Lethrinus xanthochilus is a large-bodied species with a moderate life span (estimated to be at least 19 years in this study), rapid initial growth, and a more complex sexual ontogeny likely involving pre- or post-maturational sex change, although this remains unresolved at present. Ratios of natural to fishing mortality indicate a low level of prevailing exploitation for all three species, which is supported by low proportions of immature female length classes captured by the fishery. However, considerable demographic variability among the three species highlights the value of detailed age-based information as a necessary component for informing monitoring efforts and future management decisions.

Comparative demography elucidates the longevity of parasitic and symbiotic relationships

Parasitic and symbiotic relationships govern vast nutrient and energy flows1,2, but controversy surrounds their longevity. Enduring relationships may engender parallel phylogenies among hosts and parasites3,4, but so may more ephemeral relationships when parasites disproportionately colonize related hosts5. When considering these relationships’ temporal durability, it would be useful to understand whether parasite and host populations have grown and contracted in concert. Here, we devised methods to compare demographic histories, derived from genomic data6. We used these methods to compare the historical growth of the agent of severe human malaria, Plasmodium falciparum, to human and primate histories7,8 and to that of their mosquito vector Anopheles gambiae9, thereby discerning long-term parallels and anthropogenic population explosions10,11. The growth history of Trichinella spiralis, a zoonotic parasite disseminated by swine domestication12,13, proved regionally-specific, paralleling distinctive growth histories for wild boar in Asia and Europe14. Parallel histories were inferred for an anemone and its algal symbiont (Aiptasia pallida15 and Symbiodinium minutum16). Concerted growth in potatoes and the agent of potato blight (Solanum tuberosum17 and Phytophthora infestans18) did not commence until the age of potato domestication, helping date the acquisition of this historically consequential fungal plant pathogen. Therefore, comparative historical demography provides a powerful new means by which to interrogate the history of myriad ecological relationships, enriching our understanding of their origins and durability.