Browsing by Author "Sneck, Michelle E."

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    The effect of demographic correlations on the stochastic population dynamics of perennial plants
    (Ecological Society of America, 2016) Compagnoni, Aldo; Bibian, Andrew J.; Ochocki, Brad M.; Rogers, Haldre S.; Schultz, Emily L.; Sneck, Michelle E.; Elderd, Bret D.; Iler, Amy M.; Inouye, David W.; Jacquemyn, Hans; Miller, Tom E.X.
    Understanding the influence of environmental variability on population dynamics is a fundamental goal of ecology. Theory suggests that, for populations in variable environments, temporal correlations between demographic vital rates (e.g., growth, survival, reproduction) can increase (if positive) or decrease (if negative) the variability of year-to-year population growth. Because this variability generally decreases long-term population viability, vital rate correlations may importantly affect population dynamics in stochastic environments. Despite long-standing theoretical interest, it is unclear whether vital rate correlations are common in nature, whether their directions are predominantly negative or positive, and whether they are of sufficient magnitude to warrant broad consideration in studies of stochastic population dynamics. We used long-term demographic data for three perennial plant species, hierarchical Bayesian parameterization of population projection models, and stochastic simulations to address the following questions: (1) What are the sign, magnitude, and uncertainty of temporal correlations between vital rates? (2) How do specific pairwise correlations affect the year-to-year variability of population growth? (3) Does the net effect of all vital rate correlations increase or decrease year-to-year variability? (4) What is the net effect of vital rate correlations on the long-term stochastic population growth rate (λs)? We found only four moderate to strong correlations, both positive and negative in sign, across all species and vital rate pairs; otherwise, correlations were generally weak in magnitude and variable in sign. The net effect of vital rate correlations ranged from a slight decrease to an increase in the year-to-year variability of population growth, with average changes in variance ranging from −1% to +22%. However, vital rate correlations caused virtually no change in the estimates of λs (mean effects ranging from −0.01% to +0.17%). Therefore, the proportional changes in the variance of population growth caused by demographic correlations were too small on an absolute scale to importantly affect population growth and viability. We conclude that, in our three focal populations and perhaps more generally, vital rate correlations have little effect on stochastic population dynamics. This may be good news for population ecologists, because estimating vital rate correlations and incorporating them into population models can be data intensive and technically challenging.
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    The role of environmental variation and host gene flow on the vertical transmission and population prevalence of heritable symbionts
    (2017-11-17) Sneck, Michelle E.; Miller, Tom; Egan, Scott P.; Kohn, Michael H.; Bartel, Bonnie
    Heritable microbial symbionts, vertically transmitted from maternal host to offspring, have made an indelible contribution to the ecology and evolution of life on earth. For instance, the fixation of symbionts in hosts contributed to pivotal biological shifts, such as the evolution of vascular plants and eukaryotic cells. Vertically transmitted symbionts are often specialized to host genotypes and confer fitness benefits to hosts, including protection against abiotic and biotic stress. Despite their ubiquity and strong influence on hosts, our understanding of what drives the prevalence and persistence of heritable symbionts lags behind that of macro-organisms. Two factors are theorized to determine equilibrium frequencies of heritable symbionts: 1) symbiont vertical transmission rates, and 2) the relative fitness of symbiotic and non-symbiotic hosts. Therefore, characterizing when and how these factors vary in host populations are necessary first steps to predicting the population dynamics of heritable symbionts. Here, I used large-scale field surveys, greenhouse and common garden experiments, as well as demographic modeling approaches to test the hypothesis that outcrossing (i.e., gene flow) between genetically distant hosts disrupts symbiosis. Specifically, host outcrossing is hypothesized to create genetic incompatibilities between sexually reproducing hosts and their specialized clonal symbionts, which may reduce both vertical transmission rates and symbiont mediated mutualistic benefits. First, I found that symbiont prevalence in one host species negatively associated with drought, while symbiont genotype explained residual variation in vertical transmission rates. These results suggest that symbiont genotype, and to a lesser extent, climate variables play roles in shaping symbiont population dynamics, but substantial variability was unexplained. Second, I manipulated gene flow between hosts along a gradient of genetic distances and determined that symbiont vertical transmission was robust to host outcrossing, which remained high for several host generations. Lastly, I quantified the net effect of host outcrossing on symbiont population dynamics. Contrary to our hypothesis, host outcrossing did not disrupt mutualistic benefits of symbiosis, and instead, buffered hosts against deleterious effects of outbreeding depression. Together, my work provides strong evidence that host outcrossing does not disrupt symbiosis, and alternatively demonstrates that heritable symbionts are important players in the population dynamics of outcrossing hosts.