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1. Controls of local grain size distribution, bed structure and flow conditions on sediment mobility

   https://doi.org/10.1002/esp.5599  

   Smith, Heidi E. J.; Monsalve, Angel D.; Turowski, Jens M.; Rickenmann, Dieter; Yager, Elowyn M. (May 2023, Earth Surface Processes and Landforms)

   Abstract Steep, boulder bed streams often contain sediment patches, which are areas of the bed with relatively well‐defined boundaries that are occupied by distinct grain size distributions (GSD). In sediment mixtures, the underlying GSD affects the critical Shields stress for a given grain size, which is commonly predicted using hiding functions. Hiding functions may vary with reach‐wide bed GSD, but the effect of local GSD on relative sediment mobility between sediment patches is poorly understood. We explore the effects of patch‐scale GSD on sediment mobility using tracer particles combined with local shear stresses to develop hiding functions for different patch classes within a steep stream. Hiding functions for all tested patch classes were similar, which indicates that the same hiding function can be used for different patches. However, the critical Shields stress for a given grain size generally decreased with lower patch median grain size (D₅₀) suggesting that patches control the relative mobility of each size through both the underlying GSD and local shear stresses. The effects of the underlying GSD partly depend on grain protrusion, which we measured for all grain sizes present on each patch class. Protrusion was generally greater for larger grains regardless of patch class, but for a given grain size, protrusion was increased with smaller patchD₅₀. For a given grain size, higher protrusion results in greater applied fluid forces and reduced resisting forces to partly explain our lower critical Shields stresses in finer patches. Patches therefore can importantly modulate relative sediment mobility through bed structure and may need to be included in reach‐scale sediment transport and channel stability estimates. 

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2. Tipping Cascades in a Multi-patch System with Noise and Spatial Coupling

   https://doi.org/10.1007/s11538-021-00943-y  

   Mallela, Abhishek; Hastings, Alan (September 2021, Bulletin of Mathematical Biology)

   Abstract Forecasting tipping points in spatially extended systems is a key area of interest to ecologists. A slowly declining spatially distributed population is an important example of an ecological system that could exhibit a cascade of tipping points. Here, we develop a spatial two-patch model with environmental stochasticity that is slowly forced through population collapse, in the presence of changing environmental conditions. We begin with a basic spatial model, then introduce a fast–slow version of the model using geometric singular perturbation theory, followed by the inclusion of stochasticity. Using the spectral density of the fluctuating subpopulation in each patch, we derive analytic expressions for candidate indicators of population extinction and evaluate their performance through a simulation study. We find that coupling and spatial heterogeneity decrease the magnitude of the proposed indicators in coupled populations relative to isolated populations. Moreover, the degree of coupling dictates the trends in summary statistics. We conclude that this theory may be applied to other contexts, including the control of invasive species. 

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3. Modeling effects of matrix heterogeneity on population persistence at the patch-level

   https://doi.org/10.3934/mbe.2022638  

   Fonseka, Nalin; Goddard Ⅱ, Jerome; Henderson, Alketa; Nichols, Dustin; Shivaji, Ratnasingham (September 2022, Mathematical Biosciences and Engineering)

   Yang Kuang (Ed.)

   Habitat loss and fragmentation is the largest contributing factor to species extinction and declining biodiversity. Landscapes are becoming highly spatially heterogeneous with varying degrees of human modification. Much theoretical study of habitat fragmentation has historically focused on a simple theoretical landscape with patches of habitat surrounded by a spatially homogeneous hostile matrix. However, terrestrial habitat patches are often surrounded by complex mosaics of many different land cover types, which are rarely ecologically neutral or completely inhospitable environments. We employ an extension of a reaction diffusion model to explore effects of heterogeneity in the matrix immediately surrounding a patch in a one-dimensional theoretical landscape. Exact dynamics of a population exhibiting logistic growth, an unbiased random walk in the patch and matrix, habitat preference at the patch/matrix interface, and two functionally different matrix types for the one-dimensional landscape is obtained. These results show existence of a minimum patch size (MPS), below which population persistence is not possible. This MPS can be estimated via empirically derived estimates of patch intrinsic growth rate and diffusion rate, habitat preference, and matrix death and diffusion rates. We conclude that local matrix heterogeneity can greatly change model predictions, and argue that conservation strategies should not only consider patch size, configuration, and quality, but also quality and spatial structure of the surrounding matrix. 

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4. Optogenetic stimulation of striatal patches modifies habit formation and inhibits dopamine release

   https://doi.org/10.1038/s41598-021-99350-5  

   Nadel, J. A.; Pawelko, S. S.; Scott, J. R.; McLaughlin, R.; Fox, M.; Ghanem, M.; van der Merwe, R.; Hollon, N. G.; Ramsson, E. S.; Howard, C. D. (December 2021, Scientific Reports)

   null (Ed.)

   Abstract Habits are inflexible behaviors that develop after extensive repetition, and overreliance on habits is a hallmark of many pathological states. The striatum is involved in the transition from flexible to inflexible responding, and interspersed throughout the striatum are patches, or striosomes, which make up ~15% of the volume of the striatum relative to the surrounding matrix compartment. Previous studies have suggested that patches are necessary for normal habit formation, but it remains unknown exactly how patches contribute to habit formation and expression. Here, using optogenetics, we stimulated striatal patches in Sepw1-NP67 mice during variable interval training (VI60), which is used to establish habitual responding. We found that activation of patches at reward retrieval resulted in elevated responding during VI60 training by modifying the pattern of head entry and pressing. Further, this optogenetic manipulation reduced subsequent responding following reinforcer devaluation, suggesting modified habit formation. However, patch stimulation did not generally increase extinction rates during a subsequent extinction probe, but did result in a small ‘extinction burst’, further suggesting goal-directed behavior. On the other hand, this manipulation had no effect in omission trials, where mice had to withhold responses to obtain rewards. Finally, we utilized fast-scan cyclic voltammetry to investigate how patch activation modifies evoked striatal dopamine release and found that optogenetic activation of patch projections to the substantia nigra pars compacta (SNc) is sufficient to suppress dopamine release in the dorsal striatum. Overall, this work provides novel insight into the role of the patch compartment in habit formation, and provides a potential mechanism for how patches modify habitual behavior by exerting control over dopamine signaling. 

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5. How environmental drivers of spatial synchrony interact

   https://doi.org/10.1111/ecog.06795  

   Reuman, Daniel C.; Castorani, Max C.; Cavanaugh, Kyle C.; Sheppard, Lawrence W.; Walter, Jonathan A.; Bell, Tom W. (August 2023, Ecography)

   Spatial synchrony, the tendency for populations across space to show correlated fluctuations, is a fundamental feature of population dynamics, linked to central topics of ecology such as population cycling, extinction risk, and ecosystem stability. A common mechanism of spatial synchrony is the Moran effect, whereby spatially synchronized environmental signals drive population dynamics and hence induce population synchrony. After reviewing recent progress in understanding Moran effects, we here elaborate a general theory of how Moran effects of different environmental drivers acting on the same populations can interact, either synergistically or destructively, to produce either substantially more or markedly less population synchrony than would otherwise occur. We provide intuition for how this newly recognized mechanism works through theoretical case studies and application of our theory to California populations of giant kelp. We argue that Moran interactions should be common. Our theory and analysis explain an important new aspect of a fundamental feature of spatiotemporal population dynamics. 

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