Marcel Holyoak - US grants

Population density cycles that appear synchronous, or irin-phasel, over large geographic areas
are some of the most striking phenomena in population biology. In theory, such synchrony is
caused either by widespread meteorological factors, or by movement of individuals between
populations. Theory for predator-prey 'metapopulationsl' that extend over groups of patches
also links synchrony to regional persistence. The study of these phenomena in nature has been
hampered by difficulty in identifying both the cause of population cycles and the roles of
environmental factors and interpatch movement in modifying population fluctuations and
synchrony. The environmental forcing of metapopulations which is explored here is relevant to
biological control of pest species and conservation. Additionally, extreme weather events
caused by global warming have the potential to synchronize regional populations, which might
cut short regional persistence. The proposed work uses mathematical techniques focused on
phase dynamics to analyze synchrony and build precise links among environmental variability,
population dynamics and extinction. Phase dynamics have been widely used in neurobiology,
but their potential in ecology is only just beginning to be realized. This project develops new
analytic tools for an ecological audience and uses a model experimental system, bacteria and
protozoa in laboratory microcosms, to bridge the gap between populations and metapopulation
theory. The work starts with a classic model and then develops more precise and biologically
realistic models, which in turn will fuel further, more precise, experimental tests.
Two-patch predator-prey systems coupled by random dispersal will provide a link with
analytical solutions, which numerical simulations and experiments can build on to consider more
complex and realistic situations. Two patch models will be used to derive equations which relate
phase, the point in a predator-prey cycle, to population dynamic processes. The dynamics of
phase difference between two patches will be derived and used as a measure of synchrony,
which can be related to regional persistence and within-patch predator-prey dynamics. Phase
dynamics can also distinguish whether persistence is controlled not by deterministic equilibrium
dynamics (the focus of most theoretical studies), but instead by long-lived transient dynamics
which may dominate during ecologically relevant time scales; specifically regression of phase
difference through time will be used to calculate the duration of transient dynamics, when phase
difference becomes zero. Experimentally, the initial phase difference of predator-prey
oscillations in two linked patches will be manipulated by starting microcosms with different
predator and prey densities in each patch. Statistics will then quantify the phase difference
between patches and test its correlation with regional persistence time. Repeating this
procedure in microcosms with different movement rates between patches (lengths of corridors)
will test the prediction that increased movement rate between patches will reduce phase
differences and regional persistence time. Experiments with 1-8 patches will manipulate
environmental variability through temperature fluctuations and control whether this operates
uniformly across a region or just in a single patch. Quantification of regional persistence time
and phase differences between patches will then test the predictions that local variability
enhances regional persistence, but regional variability and increased movement reduce regional
variability.
This project will demonstrate how and why environmental variability influences dynamics
and extinction in regionally-distributed predator and prey systems. The techniques of phase
dynamics will be brought to a broader ecological audience, and two graduate students will be
trained with the necessary mathematical, modeling, statistical and experimental techniques that
are required to understand the links between the environment and populations. This work will
provide a paradigm on which future combined experimental and theoretical studies of population
synchrony and persistence can build.

Ecological communities are comprised of organisms that assemble over time by the colonization of a local site from the surrounding region and by subsequent species interactions that determine the fate of species. The sequence of such colonizations and subsequent changes in community composition are termed "community assembly". Because ecological communities are not static, understanding assembly is central to successfully conserving biodiversity. Unlike previous work, this project studies assembly in patchy landscapes where both within-patch assembly and between-patch movement are expected to be important. It uses a recently-developed mathematical approach and a highly-tractable laboratory system consisting of protozoa in microcosms to understand transitions in species composition. These approaches represent powerful tools for developing and testing ecological ideas that can then be tested in the field systems we wish to conserve. The approach consists of experimentally identifying all persistent species combinations and determining the frequency of invasion by previously absent species. Experiments will test the effect on the frequency distribution of species compositions of between patch movement, whether species are native or nonnative, and the distance over which species are moving in landscapes. Modeling such transitions will produce a precise, testable, general theoretical framework that can be applied to studying community assembly in many systems. The problems addressed are key to conservation of biodiversity, management of invasive species, and habitat restoration.

Ecologists work to understand population dynamics of important species. A common working model used by many ecologists considers food available to a focal species and the focal species as food for other species. This model has been inadequate to explain many field observations, including the 1000-fold differences in numbers of wooly bear caterpillars at Bodega Marine Reserve over the past 20 years. One possible solution is to make simple tritrophic models more realistic by including additional species interactions or by including spatial dynamics. The objective of this study is to provide a first field test of this these new models by conducting field experiments and surveys evaluating additional complexity at each trophic level. This research will evaluate the relative importance of: 1) added species at the resource (plant) level, 2) added species at the herbivore/omnivore level, and 3) movement and spatial dynamics at the predator level.

Most ecological research is conducted for up to three years in a single location. A longer perspective over a larger area is essential to capture a realistic view of how and why populations change. This project will provide tests of current general theories about factors that organize and stabilize terrestrial communities. This information will allow management of undesirable species and evaluations of anthropogenic changes to useful species and ecosystems.

The Earth's climate has changed continually over 4 billion years. The recent increased pace and direction of this change challenge the nation's welfare, the economics of agriculture, and the management of essential resources. This project provides a unique opportunity to identify the mechanisms by which short-term climatic effects cause long-term population changes, affect spatial exchanges between local populations, and result in the extinction of some populations. The project focuses on a common herbivore, the wooly-bear caterpillar, and results have the potential to inform conservation and management of beneficial as well as pest herbivores. More importantly, the project will be integrated into high school curricula by involving students and teachers in monitoring caterpillar abundances. Undergraduate students from groups under-represented in science and graduate students will also receive training in the practice of science through their participation in the project

The wooly-bear moth, Platyprepia virginalis, is a primary herbivore on lupines in California. The investigators have accumulated up to 30 years of abundance data for this species and for important drivers of its population dynamics, including host plants, other herbivores, predators, parasitoids, movement, and climatic conditions. The project will initiate short-term experiments to identify mechanisms that causally link climatic factors to observed patterns of abundance and distribution across habitats. These will evaluate top-down effects of climate on predators, bottom-up climate effects through a facilitative interaction involving a second herbivore species, and the effects of flooding and litter accumulation on caterpillar vital rates. Statistical analyses and mathematical models will test inferences from these experiments, evaluating whether the hypothesized mechanisms cause inter-annual fluctuations in abundance and changing spatial dynamics including local extinctions. The project's combination of manipulative experiments with model development and analyses of long-term demographic data is rare. It will provide a well-documented example of source-sink dynamics, an ecological phenomenon for which data remain scarce, and is likely to become a classic example of the consequences of climate change for population dynamics.