"Symbiosis increases population size and mitigates environmental fluctuations in a physiologically-structured model parameterized for bivalves"
As a nutritional strategy, symbiosis increases the metabolic capabilities of the host. In thyasirid clams, it has been shown that trophic symbiosis can alter the energy allocation pattern of a host. However, the possible role of symbiosis as a life history strategy that can shape population dynamics remains unknown. Here, we show how nutritional symbiosis and the abundance of and dependence on symbionts can modulate the host's population dynamics and buffer resource limitation. We used Dynamic Energy Budget (DEB) theory to build a physiologically-structured population model that explicitly includes energy acquisition and allocation at different stages of an organisms' life cycle. We formulated the model deriving the demographic rates from a DEB model and assuming equal mortality rates in both populations. We parameterized the model for two cohabiting clam species: asymbiotic (specialists that feed on free-living bacteria) and symbiotic thyasirids (generalists that gain nutrients from both free-living and symbiotic bacteria). We demonstrate that, without seasonal fluctuations, symbiotic thyasirids have higher abundances than asymbiotic thyasirids since the symbiotic bacteria act as an energy reserve allowing for higher energy allocation to reproduction. In a seasonal environment, when temperatures are low and resource is limiting, symbiotic and asymbiotic thyasirids have similar population sizes; nonetheless, the symbiotic population is less prone to extinction. Our findings suggest different adaptations to resource fluctuation: asymbiotic thyasirids depend on a larger energy reserve, while symbiotic thyasirids rely on symbiont assimilation. Our results highlight the relevance of linking individual energetics and life-history traits to population dynamics and are the first step towards a general understanding of the role of symbioses in populations' resilience.
Utah State University
"Modeling phenological consequences of warming climate for a southern population of mountain pine beetle"
The mountain pine beetle (MPB, Dendroctonus ponderosae Hopkins) attacks living Pinus trees, and reproduces in the phloem. Adults must attack a host simultaneously to overwhelm host defenses and successfully colonize. Temperatures directly but non-linearly affect MPB progress through life stages and the phenology of adult emergence. MPB are successful in a thermal niche where they are univoltine and synchronize emergence. Changing temperatures have broadened that niche geographically, leading to tree mortality of over 5.2 Mha in the western US. Successful bivoltine MPB have not been observed in the field, although a phenology model parameterized for northern US MPB populations suggests bivoltinism is possible in the southern MPB range under future warming scenarios. Bivoltinism could have devastating impacts on pine forests. However, northern and southern MPB are genetically different in response to temperature, requiring geographic-specific model parameters. Using rate curves parameterized with developmental observations from MPB in Arizona we have constructed a predictive cohort model for a southern MPB population. Initiating the model with field attack data and using temperature data recorded under the bark of attacked hosts, we simulated warming scenarios by adding to the yearly mean to test thermal regimes that would result in a bivoltine MPB population. We successfully constructed a predictive cohort model for a southern MPB population. A key result is a new method for projecting observed variability in oviposition, through multiple larval instars, into emergence distributions. Comparison of the cohort model with field emergence data allows us to infer developmental rates for unwitnessed pre-ovipositional adults and also validate model predictions. Model responses to simulated temperatures highlight thermal regimes that promote bivoltinism for the southern MPB population.
University of British Columbia Okanagan
"Mathematical insights into mechanisms leading to coexistence and competitive exclusion among mutualist guilds"
Microbial inoculants have been used as organic fertilizers worldwide. One of the most widely used commercial products are arbuscular mycorrhizal (AM) fungi, as these fungi can associate with a vast variety of crops. Despite the potential benefits for soil quality and crop yield associated with the use of AM fungi, experiments assessing the effective establishment of the fungi in the field have given inconsistent results, where some observations show field establishment and improved crop yield, while other studies show poor establishment of the inoculated species. Additionally, it is not yet clear whether the introduction of commercial inoculants could lead to a biodiversity loss in the native fungal community, and ultimately have a negative impact on plant growth. Here we develop a series of ordinary and partial differential equation models to study the spatio-temporal dynamics of a guild of mutualists (the fungal species) sharing a resource provided by the same partner (the host plant), constituing a shared resource for all fungi. Our results allow to assess the risks and benefit of inoculation, by identifying under which conditions inoculation can effectively boost productivity, when it has no significant effect on plant growth and on the native fungal community, and when it represents an invasion risk. More generally, our models provide important ecological insights into the mechanisms responsible for coexistence and competitive exclusion among mutualist guilds, and constitute a framework to predict the consequences of species manipulation in mutualist communities. Indeed, the models are simple enough to apply to a broad range of mutualisms found in nature, such as pollination or seed dispersal.