From growth to extinction : explored by life history and metabolic theory

The laws of energy and material conservation are fundamental principles across various scales and systems. Based on the conservation laws, I derive several theoretical models to understand mechanisms behind the energy budget of ontogenetic growth and the pattern of the late Pleistocene extinction of megafauna in the Americas. First, I present a model, empirically grounded in data from birds and mammals, that correctly predicts how growing animals allocate food energy between synthesis of new biomass and maintenance of existing biomass. Previous energy budget models have typically been based on rates of either food consumption or metabolic energy expenditure. The model provides a framework that reconciles these two approaches and highlights the fundamental principles that determine rates of food assimilation and rates of energy allocation to maintenance, biosynthesis, activity, and storage. The model predicts that growth and assimilation rates for all animals should cluster closely around two canonical curves. Second, the previous model, which focuses on endotherms, has been extended to understand effects of temperature on the energy budget of ontogenetic growth of ectotherms. A tendency for ectotherms to develop faster but mature at smaller body sizes in warmer environments has been studied for decades, and is called the temperature size rule (TSR). It can be explained by a simple model in which the rate of growth or biomass accumulation and the rate of development or differentiation have different temperature dependence. The model accounts for both TSR and the less frequently observed reverse-TSR, predicts the fraction of energy allocated to maintenance and synthesis over the course of development, and the temperature independent growth efficiency. It also predicts that less total energy is expended when developing at warmer temperatures for TSR and vice versa for reverse-TSR. It has important implications for effects of climate change on ectothermic animals and also provides how selection may lead to the evolution of both TSR and reverse-TSR. Finally, based on mammalian life history and life history scaling relationships, an exploitation-extinction theory has been developed for the rate of human harvest in the disappearance of the Pleistocene megafauna in the Americas. The theory demonstrates that the added mortality of human harvest on populations need not be selective to produce a size-biased extinction. The variation in the adult natural instantaneous mortality rate and/or the maximum recruitment compensation at any body mass are main components determining the probability of extinction. The theory successfully predicts the shapes of the extinction probability curves for the late Pleistocene extinction in the Americas. It provides a theoretical basis to challenge a major criticism of the overkill theory that early Paleoindian hunters had to be extremely selective to have produced the highly size-biased pattern characteristic to the late Pleistocene extinction of megafauna in the Americas

Rensch's rule in large herbivorous mammals derived from metabolic scaling

Rensch’s rule, which states that the magnitude of sexual size dimorphism tends to increase with increasing body size, has evolved independently in three lineages of large herbivorous mammals: bovids (antelopes), cervids (deer), and macropodids (kangaroos). This pattern can be explained by a model that combines allometry,life-history theory, and energetics. The key features are thatfemale group size increases with increasing body size and that males have evolved under sexual selection to grow large enough to control these groups of females. The model predicts relationships among body size and female group size, male and female age at first breeding,death and growth rates, and energy allocation of males to produce body mass and weapons. Model predictions are well supported by data for these megaherbivores. The model suggests hypotheses for why some other sexually dimorphic taxa, such as primates and pinnipeds(seals and sea lions), do or do not conform to Rensh’s rule

Human hunting mortality threshold rules for extinction in mammals (and fish)

Question: Are there general life-history rules for exploitation-caused extinction of mammal populations? Mathematical methods: A population of size N faced with the added mortality of human exploitation will deterministically go extinct if its per-capita birth rate can no longer match its per-capita mortality rate as N approaches zero. We develop exploitation-extinction theory for a mammal life history using R₀ < 1 as N goes to zero, and combine the criterion with several facts about mammal life histories. Conclusions: Extinction results if the ratio of the instantaneous mortality rate caused by hunting (F) divided by the adult instantaneous mortality rate (M, for the unexploited population) exceeds a critical value (F/M > C). The C value is determined mostly by the level of recruitment compensation as N declines, and C is likely very similar for different sized mammals. We use existing mammal life-history data to estimate C (~0.5). We then estimate the threshold of instantaneous mortality rate, F, as a function of adult body mass, W; it's a -0.25 power allometry. Finally, we extend the model to fish. C is expected to vary a lot between fish species, mostly because fish are expected to have much larger recruitment compensation than mammals, the recruitment may correlate with body size, and immature fish are often not exploited. We show how to combine these to predict C.Keywords: exploitation-caused extinction, fisheries-extinction, life span allometry, mammals, population recruitmen

Hurricanes affect diversification among individual life courses of a primate population

1. Extreme climatic events may influence individual- level variability in phenotypes, survival and reproduction, and thereby drive the pace of evolution. Climate mod -els predict increases in the frequency of intense hurricanes, but no study has measured their impact on individual life courses within animal populations. 2. We used 45 years of demographic data of rhesus macaques to quantify the influ- ence of major hurricanes on reproductive life courses using multiple metrics of dynamic heterogeneity accounting for life course variability and life-history trait variances. 3. To reduce intraspecific competition, individuals may explore new reproductive stages during years of major hurricanes, resulting in higher temporal variation in reproductive trajectories. Alternatively, individuals may opt for a single optimal life-history strategy due to trade- offs between survival and reproduction. 4. Our results show that heterogeneity in reproductive life courses increased by 4% during years of major hurricanes, despite a 2% reduction in the asymptotic growth rate due to an average decrease in mean fertility and survival by that is, shortened life courses and reduced reproductive output. In agreement with this, the population is expected to achieve stable population dynamics faster after being perturbed by a hurricane (p = 1.512 ; 95% CI: 1.488, 1.538), relative to ordi- nary years (p = 1.482; 1.475, 1.490). 5. Our work suggests that natural disasters force individuals into new demographic roles to potentially reduce competition during unfavourable environments where mean reproduction and survival are compromised. Variance in lifetime reproduc- tive success and longevity are differently affected by hurricanes, and such vari- ability is mostly driven by survival

The Macroecology of Sustainability

Global consumption rates of vital resources suggest that we have surpassed the capacity of the Earth to sustain current levels, much less future trajectories of growth in human population and economy