The diversity of population responses to environmental change

The current extinction and climate change crises pressure us to predict population dynamics with ever‐greater accuracy. Although predictions rest on the well‐advanced theory of age‐structured populations, two key issues remain poorly explored. Specifically, how the age‐dependency in demographic rates and the year‐to‐year interactions between survival and fecundity affect stochastic population growth rates. We use inference, simulations and mathematical derivations to explore how environmental perturbations determine population growth rates for populations with different age‐specific demographic rates and when ages are reduced to stages. We find that stage‐ vs. age‐based models can produce markedly divergent stochastic population growth rates. The differences are most pronounced when there are survival‐fecundity‐trade‐offs, which reduce the variance in the population growth rate. Finally, the expected value and variance of the stochastic growth rates of populations with different age‐specific demographic rates can diverge to the extent that, while some populations may thrive, others will inevitably go extinct

Evolutionary ecology of endocrine-mediated life-history variation in the garter snake Thamnophis elegans

The endocrine system plays an integral role in the regulation of key life-history traits. Insulin-like growth factor-1 (IGF-1) is a hormone that promotes growth and reproduction, and it has been implicated in the reduction of lifespan. IGF-1 is also capable of responding plastically to environmental stimuli such as resource availability and temperature. Thus pleiotropic control of life-history traits by IGF-1 could provide a mechanism for the evolution of correlated life-history traits in a new or changing environment. An ideal system in which to investigate the role of IGF-1 in life-history evolution exists in two ecotypes of the garter snake Thamnophis elegans, which derive from a single recent ancestral source but have evolved genetically divergent life-history characteristics. Snakes from meadow populations near Eagle Lake, California (USA) exhibit slower growth rates, lower annual reproductive output, and longer median adult lifespans relative to populations along the lakeshore. We hypothesized that the IGF-1 system has differentiated between these ecotypes and can account for increased growth and reproduction and reduced survival in lakeshore vs. meadow snakes. We tested for a difference in plasma IGF-1 levels in free-ranging snakes from replicate populations of each ecotype over three years. IGF-1 levels were significantly associated with adult body size, reproductive output, and season in a manner that reflects established differences in prey ecology and age/size-specific reproduction between the ecotypes. These findings are discussed in the context of theoretical expectations for a trade-off between reproduction and lifespan that is mediated by pleiotropic endocrine mechanisms

Complex Interplay of Body Condition, Life History, and Prevailing Environment Shapes Immune Defenses of Garter Snakes in the Wild

The immunocompetence “pace-of-life” hypothesis proposes that fast-living organisms should invest more in innate immune defenses and less in adaptive defenses compared to slow-living ones. We found some support for this hypothesis in two lifehistory ecotypes of the snake Thamnophis elegans; fast-living individuals show higher levels of innate immunity compared to slow-living ones. Here, we optimized a lymphocyte proliferation assay to assess the complementary prediction that slowliving snakes should in turn show stronger adaptive defenses. We also assessed the “environmental” hypothesis that predicts that slow-living snakes should show lower levels of immune defenses (both innate and adaptive) given the harsher environment they live in. Proliferation of B- and T-lymphocytes of free-living individuals was on average higher in fast-living than slow-living snakes, opposing the pace-of-life hypothesis and supporting the environmental hypothesis. Bactericidal capacity of plasma, an index of innate immunity, did not differ between fast-living and slow-living snakes in this study, contrasting the previously documented pattern and highlighting the importance of annual environmental conditions as determinants of immune profiles of free-living animals. Our results do not negate a link between life history and immunity, as indicated by ecotype-specific relationships between lymphocyte proliferation and body condition, but suggest more subtle nuances than those currently proposed.Fil: Palacios, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentina. Iowa State University. Department of Ecology, Evolution and Organismal Biology; Estados UnidosFil: Cunnick, Joan E.. Iowa State University. Department of Animal Science; Estados UnidosFil: Bronikowski, Anne M.. Iowa State University. Department of Ecology, Evolution and Organismal Biology; Estados Unido

Aging and Its Demographic Measurement

This case study highlights the general issues raised earlier. First, that maximum lifespan is not an easily obtainable metric. Specifically, it is unambiguous in the sense that once the last animal dies, it is most definitely dead. But to estimate the variance in maximum lifespan, many replicate populations would need to be followed for each treatment group (with each replicate providing a single observation of maximum lifespan). Second, median lifespan, although measurable from a single population, provides no information on the age-specificity and patterns in age-specific vital rates that are contributing to differences in "aging" (i.e., differences in physiological frailty and rates of increasing mortality across the adult lifespan). Finally, our partitioning of aging into two components — IMR and RoA — allows us to unravel causation in a demographic sense. Specifically, it allows us to specify an aging rate that is separate from its starting value (IMR), independent of fluctuations in survival due to temporary experimental impacts, and not necessarily equivalent to expectations due to median or maximum lifespan

Gene Expression of Components of the Insulin/Insulin-Like Signaling Pathway in Response to Heat Stress in the Garter Snake, \u3ci\u3eThamnophis Elegans\u3c/i\u3e

The insulin/insulin-like signaling (IIS) pathway is an evolutionary conserved molecular signaling pathway that regulates growth, reproduction, stress resistance, and longevity in response to nutrition and external stress. While the constituents of this pathway and their functions are relatively well understood in laboratory model animals, they have not been explored in many other organisms, with notable exceptions in the fisheries literature. We tested for the gene expression of four key components of this pathway in the garter snake (Thamnophis elegans) liver, and determine how the transcription of these components responds to heat stress. We found that the two insulin-like growth factor ligands (IGF-1 and IGF-2) and the receptors (IGF-1 Receptor and M6P/ IGF-2 Receptor, or IGF-1R and IGF-2R) are expressed in garter snake liver tissue. Under normal laboratory conditions, IGF-2 and IGF-2R are expressed at a higher level than IGF-1 and IGF-1R. In response to heat stress, IGF-1 expression remained the same, IGF-2 expression decreased, and the expression of both receptors increased. These results demonstrate that elements of the IIS pathway are responsive to heat stress in snakes. Further studies are needed to fully understand the biological consequences of this response