Browsing by Author "Bibian, Andrew J."

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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 Host Demographic Storage in the Ecological Dynamics of Heritable Symbionts
    (The University of Chicago Press, 2016) Bibian, Andrew J.; Rudgers, Jennifer A.; Miller, Tom E.X.
    Heritable symbioses are widespread and ecologically important. Many host organisms have complex life cycles that include diverse opportunities for symbionts to affect their host and be lost during development. Yet, existing theory takes a simplified view of host demography. Here, we generalize symbiosis theory to understand how demographic “storage” in the form of dormant or prereproductive life stages can modify symbiosis dynamics. Using grass-endophyte symbioses as context, we developed models to contrast the role of the seed bank (a storage stage) against the reproductive stage in symbiont persistence and prevalence. We find that the seed bank is as important as or more important than the reproductive stage in driving symbiont dynamics, as long as passage through the seed bank is obligate. Flexible entry to the seed bank substantially weakens its influence on symbiont persistence but can modify prevalence in counterintuitive ways. Our models identify a role for legacy effects, where hosts that lose symbionts retain their demographic influence. The retention of benefits via legacy effects can reduce symbiont prevalence and even cause prevalence to decline with increasing benefits to hosts because symbiont-free hosts carry those benefits. Our results resolve connections between individual-level host-symbiont interactions and population-level patterns, providing guidance for empirical studies.