The next 9 blog posts will summarize my reading assignments for the EBIO 3rd semester exam. The exam is scheduled for 3 hours and involves my four committee members asking me questions about anything at all! I was required to put together a reading list covering 4 main topics: biological soil crusts & drylands, microbial ecology, ecosystem services, and community, restoration, and disturbance-succession ecology. Obviously, I actually have 7 topics, which I managed to squeeze into "4". The reading list is a guide for the exam. To help me through this exam preparation process, I will use these blogs to summarize what I am learning over the next 9 weeks.
Ecosystem resilience: "the magnitude of a perturbation that a system can withstand before undergoing a shift", or sometimes, "the time it takes for a system to recover from a perturbation". My reading this week addressed all sorts of factors related to this resilience concept. Specifically for soil microbial communities, Shade et al. explained individual-, population-, and community-level characteristics (dormancy, dispersal rate, turnover rate) that contribute to microbial community resilience and thus stability over time. Viewed from a different perspective, ecosystems can also be resilient to restoration (Suding et al. 2004). Sometimes there are feedbacks that increase the resilience of degraded systems: species effects, trophic interactions, landscape connectivity and seed source, and long-term change. As an example, Suding et al. talk about how grasses invaded Hawaiian shrublands, making the landscape more fire-prone, which allows more fires and thus more grasses. This positive feedback makes the ecosystem more resilient to your restoration efforts because it is internally-reinforcing. Getting the shrubland back in a self-sustaining form may be very tricky. Two papers by Suding et al. set up an important framework for understanding restoration through alternative states, positive feedbacks, and threshold models. These theories are quite complex with mathematical underpinnings and applications through models, but I constantly remind myself of the simplicity of the ideas when boiled down...for example, do we add gravel, grass-clippings, or living plants to stabilize a soil surface and increase soil moisture availability? The theory becomes important when you have asked this same question over and over again in different contexts and you'd probably prefer to know for sure which option is the best one for your particular context. The theory is also important when our restoration efforts start to falter due to complexities that we did not account for. Restoration is going to continue to be extremely important into the future and hopefully it will play a big role in my work, so it is critical for me to understand the fundamental theory in the field. Succession is the dominant model used in restoration. Succession is the orderly progression of community shifts toward a climax community (often discussed in terms of vegetation) with early successional species facilitating the colonization of later successional species. If ecosystems all follow successional patterns, it makes sense that you could encourage ecosystem recovery through restoration actions by following the known sequence of succession for that environment. In Read et al., this idea was tested specifically in biocrusts. They used a space-for-time substitution (they found crusts that were 0-50+ years old) and looked at the community composition, species traits, and functions to show that there were specific species traits that made them good at being early or late successional species. They also found that biocrust cover increased (after grazing disturbance) up to 20 years later, but that the community composition never stabilized (reached a climax community). Therefore, the successional model does not quite fit. Many other researchers have found fault with a successional model for restoration. For instance, an alternative perspective would be the stepwise degradation cycle (King & Hobbs 2006) which divides degradation into abiotic, biotic, structure, and function categories. The degradation spiral they show indicates important points of action for restoration which push back against the positive feedbacks that cause the spiral, hopefully by pushing on multiple steps at once and in a way that minimizes the spatial scale of interactions. Building on this perspective, Suding et al. prefer to think of a degraded state as an alternative state (two or more different communities can exist on the same patch of land equivalent environmental conditions). This is important to distinguish from a gradual or threshold type of degradation. In a threshold model, there is a steep point at which the community shifts due to environmental factors, but the way back to the prior state is along a continuous known trajectory. In alternative state models, hysteresis occurs such that there may not be an easy route back to the prior state. This is important to know if you are a land manager spending a lot of money on a restoration project because the best path forward may be quite complex. A land manager might know if thresholds or alternative states are occurring using monitoring, identifying examples of rapid change through measurements, and looking at the shape of the stress/time curve. Another way suggested in the literature is to look for sharp spatial boundaries in community composition or function without a measurable boundary in the environmental parameters. There are other factors that may deter a well-planned restoration effort. Stuble et al. 2017 show that the outcomes of restoration efforts are highly dependent on the spatial and temporal context in which they were implemented. In their experiment, 3 different sites were "restored" in the exact same way for 4 years. After two years of growth, each plot's community stabilized and that final community composition was highly dependent on the site and year. This is something dryland restorationists know all too well because the success of their efforts highly depends on how good a water-year it is at their site. In another study, Winkler et al. (2018) further explain how restoration on the Colorado Plateau can be unique. Not only are their diverse and intense land uses across many different land ownership agencies, but there are also strong abiotic gradients and heterogeneous geomorphology which all impact restoration efforts. In this paper, two successful restoration efforts on the Colorado Plateau are described (one for graminoid grass establishment and one for native grass seeding to prevent invasive species growth). They predict that we can have more successful restoration there if we use sustained, strategic programs, targeted and evolving restoration practices, long-term monitoring with adaptive management, land-scape scale collaboration, and a willingness to change old practices. These final ideas are contested in the next blog post. References Community Ecology. Second Edition. Gary G. Mittelbach and Brian J. McGill 2019 Oxford University Press. King, E.G. & Hobbs, R.J. 2006. Identifying linkages among conceptual models of ecosystem degradation and restoration: Towards an Integrative Framework. Restoration Ecology 14(3), 369-378. https://doi.org/10.1111/j.1526-100X.2006.00145.x Read, C. F. et al. 2016. Testing a model of biological soil crust succession. Journal of Vegetation Science, 27, 176-186. https://doi.org/10.1111/jvs.12332 Shade, A. et al. 2012. Fundamentals of microbial community resistance and resilience. Frontiers in Microbiology, 3(417). https://doi.org/10.3389/fmicb.2012.00417 Stuble, K.L. et al. 2017. Every restoration is unique: Testing year effects and side effects as drivers of initial restoration trajectories. Journal of Applied Ecology 54, 1051-1057. https://doi.org/10.1111/1365-2664.12861 Suding, K.N. et al. 2003. Alternative states and positive feedbacks in restoration ecology. TRENDS in Ecology and Evolution 19(1). https://doi.org/10.1016/j.tree.2003.10.005 Suding, K.N. & R. J. Hobbs. 2009. Threshold models in restoration and conservation: a developing framework. Trends in Ecology & Evolution 24: 271– 279. https://doi.org/10.1016/j.tree.2008.11.012 Winkler, D.E. et al. 2018. Beyond traditional ecological restoration on the Colorado Plateau. Restoration Ecology 26(6), 1055-1060. https://doi.org/10.1111/rec.12876
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AuthorSierra is a graduate student in the Barger Lab at CU Boulder studying microbial ecology for dryland restoration. Archives
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