Christian Che-Castaldo

Research Scientist, Mount St. Helens Institute

Primary succession structures amphibian population dynamics

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Amphibians play a crucial role in primary succession, as they efficiently transfer energy across trophic levels to bridge the gap between invertebrates and reptile and mammalian predators. While the assembly of amphibian communities in nascent landscapes has been shown to be disturbance-mediated, we know little about how successional processes affect the demographics of its constituent members. Relying on detailed investigations of the natural history of lentic amphibian species colonizing the Mount St. Helens debris avalanche following the 1980 eruption, we are using long-term monitoring data in conjunction with remote sensing imagery to better understand how the geological impacts of the eruption, and subsequent ecological recovery, alter amphibian species interactions, population dynamics, and community-level biodiversity.

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Continental models of Pygoscelid penguin abundance

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Colonial seabirds have long served as indicator species for the health of the Southern Ocean. Demographic data for these species are very sparse, and counts repeatedly collected at a small number of breeding sites are often used for the adaptive management of marine resources at very large spatial scales, even though their suitability for this purpose is unknown. To overcome this limitation, we created MAPPPD, an open-access, web-based decision-support tool and database providing all publicly available Pygoscelid and emperor penguin count data. By fitting a Bayesian population dynamics model to all known Adélie penguin abundance data, we find that aggregating abundances across space provides the fastest and most reliable signal of population changes at the regional or species-level scale. Remote satellite imagery represents a crucial but underutilized tool for filling the large gaps in penguin count data, and we are currently developing statistical methods to integrate Landsat-derived colony size estimates into penguin population model forecasts.


Stress-mediated herbivore phenology directs succession

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Variation in plant quality across space and time is a driving force behind the heterogeneous distribution of herbivorous insects on their host plants. At the same time, herbivory itself can mediate nutrient cycling, causing feedbacks that affect plant quality. We examined both of these processes during the ecological recovery occurring on Mount St. Helens following the 1980 eruption. We used Bayesian dynamic site-occupancy models to show that herbivory by the poplar-willow weevil on Sitka willow, a dioecious pioneering shrub colonizing Mount St. Helens pyroclastic flows, prefer or perform best on vigorously growing willows that are seasonally water stressed. This work highlight the common but often overlooked phenological basis of the plant stress and plant vigor hypotheses, which both focus on how stress changes the quality of plant resources available to immature insects. Weevil herbivory was associated with large reductions in willow leaf phosphorus content and decelerated phosphorus cycling, despite the fact that weevil larvae are stem-borers and do not consume leaves. Taken together, these findings document how weevil herbivory forestalls willow colonization of drier areas through host plant juvenilization, while having a small effect in riparian zones where willows tolerate herbivory. This seral delay has promoted alternative successional trajectories, such as the moss and herb dominated communities seen today on the Mount St. Helens pyroclastic flows.

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Causes of female-biased adult sex ratios in willow

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The mechanisms behind biased sex-ratios in dioecious plants are often not well understood and represent the product of various genetic and ecological factors acting sequentially throughout the life-cycle of a plant. On Mount St. Helens, we observed a 2:1 female-bias in Sitka willow genet populations, sex ratios that are consistent with those reported from other Salix populations around the world. The causes of female-bias in Salix is an open question, as females presumably pay higher reproductive costs than males and willows lack of well-defined sex chromosomes precludes most genetic processes known to cause female bias. In primary successional landscapes, costs of reproduction in plants is exacerbated due to resource scarcity while vegetative, as opposed to sexual, reproduction is often a preferred life history strategy. Given this, we measured whole-plant reproductive allocation for male and female willows on Mount St. Helens and created artificial streams on pyroclastic flows to test for sex-biased willow establishment from stem fragments. Despite females investing more heavily than males in sexual reproduction, we found no evidence of sexual dimorphism in adult mortality or in vegetative propogation. Combined with the observation that female Sitka willows suffer higher rates of weevil herbivory than do males, these results implicate early‐acting genetic factors as the cause for female-biased Sitka willow genet sex ratios, a hypothesis supported by recent molecular work.