Browsing by Subject "Life history"

The evolution of reproductive strategies involves a complex calculus of costs and benefits to both parents and offspring. Many marine animals produce embryos packaged in tough egg capsules or gelatinous egg masses attached to benthic surfaces. While these egg structures can protect against environmental stresses, the packaging is energetically costly for parents to produce. In this series of studies, I examined a variety of ecological factors affecting the evolution of benthic development as a life history strategy. I used marine gastropods as my model system because they are incredibly diverse and abundant worldwide, and they exhibit a variety of reproductive and developmental strategies.

The first study examines predation on benthic egg masses. I investigated: 1) behavioral mechanisms of predation when embryos are targeted (rather than the whole egg mass); 2) the specific role of gelatinous matrix in predation. I hypothesized that gelatinous matrix does not facilitate predation. One study system was the sea slug Olea hansineensis, an obligate egg mass predator, feeding on the sea slug Haminoea vesicula. Olea fed intensely and efficiently on individual Haminoea embryos inside egg masses but showed no response to live embryos removed from gel, suggesting that gelatinous matrix enables predation. This may be due to mechanical support of the feeding predator by the matrix. However, Haminoea egg masses outnumber Olea by two orders of magnitude in the field, and each egg mass can contain many tens of thousands of embryos, so predation pressure on individuals is likely not strong. The second system involved the snail Nassarius vibex, a non-obligate egg mass predator, feeding on the polychaete worm Clymenella mucosa. Gel neither inhibits nor promotes embryo predation for Nassarius, but because it cannot target individual embryos inside an egg mass, its feeding is slow and inefficient, and feeding rates in the field are quite low. However, snails that compete with Nassarius for scavenged food have not been seen to eat egg masses in the field, leaving Nassarius free to exploit the resource. Overall, egg mass predation in these two systems likely benefits the predators much more than it negatively affects the prey. Thus, selection for environmentally protective aspects of egg mass production may be much stronger than selection for defense against predation.

In the second study, I examined desiccation resistance in intertidal egg masses made by Haminoea vesicula, which preferentially attaches its flat, ribbon-shaped egg masses to submerged substrata. Egg masses occasionally detach and become stranded on exposed sand at low tide. Unlike adults, the encased embryos cannot avoid desiccation by selectively moving about the habitat, and the egg mass shape has high surface-area-to-volume ratio that should make it prone to drying out. Thus, I hypothesized that the embryos would not survive stranding. I tested this by deploying individual egg masses of two age classes on exposed sand bars for the duration of low tide. After rehydration, embryos midway through development showed higher rates of survival than newly-laid embryos, though for both stages survival rates over 25% were frequently observed. Laboratory desiccation trials showed that >75% survival is possible in an egg mass that has lost 65% of its water weight, and some survival (<25%) was observed even after 83% water weight lost. Although many surviving embryos in both experiments showed damage, these data demonstrate that egg mass stranding is not necessarily fatal to embryos. They may be able to survive a far greater range of conditions than they normally encounter, compensating for their lack of ability to move. Also, desiccation tolerance of embryos may reduce pressure on parents to find optimal laying substrata.

The third study takes a big-picture approach to investigating the evolution of different developmental strategies in cone snails, the largest genus of marine invertebrates. Cone snail species hatch out of their capsules as either swimming larvae or non-dispersing forms, and their developmental mode has direct consequences for biogeographic patterns. Variability in life history strategies among taxa may be influenced by biological, environmental, or phylogenetic factors, or a combination of these. While most prior research has examined these factors singularly, my aim was to investigate the effects of a host of intrinsic, extrinsic, and historical factors on two fundamental aspects of life history: egg size and egg number. I used phylogenetic generalized least-squares regression models to examine relationships between these two egg traits and a variety of hypothesized intrinsic and extrinsic variables. Adult shell morphology and spatial variability in productivity and salinity across a species geographic range had the strongest effects on egg diameter and number of eggs per capsule. Phylogeny had no significant influence. Developmental mode in Conus appears to be influenced mostly by species-level adaptations and niche specificity rather than phylogenetic conservatism. Patterns of egg size and egg number appear to reflect energetic tradeoffs with body size and specific morphologies as well as adaptations to variable environments. Overall, this series of studies highlights the importance of organism-scale biotic and abiotic interactions in evolutionary patterns.

The pace of life history is highly variable across mammals, and several evolutionary biologists have theorized that the tempo of a species' life history is set by external factors. These factors, such as food availability and predation pressure, determine mortality rates. In turn, mortality rate determines the age at maturity. High mortality rate results in early age at maturity; individuals must grow and reproduce quickly because of the high risk of death. Conversely, a low mortality rate is allows individuals to prolong their growth period and reproduce slowly. This theory assumes that growth rates are constant across species, and thus body size is determined by mortality rates.

This project posits that the intrinsic characteristics of species set the pace of life history. Among anthropoids, there is a great deal of variation in growth rates and the pace of life history relative to body size. The hypotheses proposed by this project state that the degree of encephalization in a species determines the growth rates, the length of the growth period, and the adult lifespan. Growing a large brain is costly and requires a prolonged period of development. However, a large brain has the benefit of reducing mortality by facilitating cognitive strategies for food procurement and predator avoidance. This cost/benefit balance results in the pattern of life-history variation in which mortality rates are correlated with the length of the growth period. However, the causal arrows are reversed; instead of the mortality rate determining the age at maturity and consequently the size of the species, the relative brain size of the anthropoid determines the mortality rate and the age maturity.

These hypotheses were tested by determining the body and brain growth trajectories of thirteen anthropoids, and compiling life-history data from long-term studies of these species in the wild. Multi-variate analyses demonstrated that extensive brain growth, whether through prolonged duration or rapid growth rates, results in slow body-growth rates during the juvenile period and delayed age at maturity. In addition, encephalization results in longer adult lifespan. Therefore, this project demonstrated that intrinsic characteristics of anthropoid species determine the pace of their life histories.

This dissertation focuses on the implications of habitat change in freshwater ecosystems. Habitat change has three primary components that are inextricably connected; habitat loss, alteration, and fragmentation. Habitat loss is the physical removal and/or replacement of “core” habitat such that a new “matrix” habitat exists in its place. Habitat alteration is the modification of core habitat that causes a quality change (either positive or negative depending on the target species). Habitat fragmentation affects the connectivity between core habitat patches in the landscape. Rivers are highly fragmented both naturally and anthropogenically, so they represent a system readily available to study the impacts of habitat change on ecosystems.

Four approaches were used to evaluate the impacts of habitat change on freshwater ecosystems. First (Chapter 1 as published in the Annals of the New York Academy of Sciences with co-authors Dr. Martin Doyle and Dr. David Strayer; see Fuller et al. (2015)), a review of the major causes and consequences of habitat change in river networks was conducted with the goal of also bridging a theoretical gap between terrestrial and freshwater systems related to habitat change ecology. Second (Chapter 2), an empirical evaluation of a fragmented (dammed) river reach was used to evaluate the local impacts of habitat loss and alteration on physical (sediment), biogeochemical (dissolved oxygen), and biological (freshwater mussels) response variables. Third (Chapter 3), gene flow model simulations were used to identify the genetic impacts of habitat fragmentation at the river-network scale. This simulation effort contrasted the impact of habitat fragmentation with species longevity to see how organisms using different life history strategies related to lifespan respond genetically to habitat fragmentation. Fourth (Chapter 4), an empirical landscape genomics evaluation of a species of freshwater mussel (Elliptio complanata) was conducted to identify its genetic response to a river network with a long history of habitat change.

Conclusions from this research make several contributions to the ecological theory of habitat change. First, by applying the habitat change lexicon in terrestrial systems to freshwater systems, sharing results and theory across the terrestrial-aquatic literature becomes simple and may advance the theory behind habitat change ecology more rapidly with more empirical results to draw upon. Second, temporally dynamic matrix habitat and species capitalizing on altered edge habitat were identified surrounding a local habitat fragmentation agent (a dam), suggesting some species may strongly benefit from the presence of edge habitat in river networks. Third, from the gene flow model simulations, the life history of a species played an important role in how organisms respond genetically to habitat fragmentation where long-lived species appear buffered from the genetic diversity loss caused by habitat fragmentation. Finally, the empirical evaluation of a freshwater mussel species that has experienced a long history of anthropogenic-driven habitat change via water quality alterations, inundation losses, and dam fragmentation appears to have maintained a population genetic structure unrelated to the expected habitat change in the system.

Starvation during early human development produces epigenetic effects that could be adaptive if famine persists. We modeled the response to early starvation exposure in C. elegans using `L1 arrest,' a reversible developmental arrest in the first larval stage brought on by hatching in the absence of food. We found lifelong developmental effects following recovery from extended L1 arrest. Remarkably, some epigenetic effects persist for multiple generations. After extended starvation, development is delayed, producing smaller adults, fertility is reduced, and stress resistance increases. Starvation causes a striking amount of phenotypic variation among isogenic individuals, and those that develop slowest are least fertile and most stress resistant. However, increased stress resistance appears to be characteristic of recovering animals following any amount of L1 arrest but decreasing as the animal grows, possibly by size-dependent dilution. We assessed starved animals for signs of autophagy-related feeding defects and found low rates of pumping and feeding as well as grinding defects. A retrospective pumping assay revealed that after extended starvation, animals with lower pumping rates at the L1 stage tended to grow slower. Our work shows that environmental conditions and life history have transgenerational life history effects on several organismal traits, and that these traits appear to be rooted in nutrient-deprivation secondary to autophagy-related feeding defects. However, the manner by which these effects are transmitted transgenerationally remains an open and interesting question.

Understanding the relationships between lifestyle, ecology and physiology is essential for understanding variation in health, life history, and subsistence practice among populations. Previous work has investigated human behavioral ecology and life history across a wide range of human populations, but study with populations experiencing changes to their lifeways remains particularly important. Work with populations that traditionally practice nomadic pastoralism as a subsistence strategy and are experiencing encroaching market pressures offers the opportunity to investigate the effects of stark subsistence and market transitions across a variety of lifestyle factors (e.g., nutrition, physical activity, healthcare, socioeconomic status).Using data collected with the Daasanach Health and Life History Project, this dissertation applies a broad approach to test whether changes in lifestyle (e.g., market integration and sedentarization) affect health and patterns of early childhood growth within a human population through the framework of life history theory. Health, physical activity, growth, nutrition, reproduction, and community composition data have been synthesized to test the effects of life history tradeoffs that arise through socioecological variation. As semi-nomadic pastoralists who currently face the encroaching pressure of sedentarization, the Daasanach living in and around the town of Illeret are well suited to test this hypothesis. In addition, this project will expand the existing body of work concerning life history and health variation in non-industrial populations, specifically adding a population with a subsistence pattern that is currently underrepresented. This addition allows for a new level comparison between the variation in ecology, life history, health, and behavior that characterize our species, advancing our understanding of difference between industrialized and non-industrialized populations, and the breadth of variation in the variables across human populations.