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Book Excerpt: Primer of Ecological Restoration

Tuesday, February 18, 2020  
Posted by: Alexis Gibson
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  Written by Dr. Karen Holl, the Primer of Ecological Restoration is an introduction to restoration that brings together key concepts for designing and implementing effective projects. Designed to be used as an introduction or reference, the book is supported by online materials including case studies, photographs, and discussion questions. If you are interested in learning more about the book, join SER and Dr. Holl for a webinar on March 3 (more information here). 

Chapter 1: Why Restore Ecosystems?

Why Is Restoring to a Historical State So Difficult?


The array of restoration definitions discussed above reflects that defining the historical state or range of variability is challenging, and achieving that state is even more difficult, for several reasons discussed below. Moreover, restoration is undertaken for a host of different reasons in our rapidly changing world (chap. 1).


Defining the Historical Target and Shifting Baselines


Restoration projects often aim to restore an ecosystem to a state prior to human disturbance. However, the question of what past temporal state to select is subjective (Aronson, Dhillion, and Le Floch 1995; Barak et al. 2016). Do we restore to fifty, two hundred, or one thousand years ago? In the Americas, it is common to aim to restore ecosystems to a state prior to European colonization, but that is an arbitrary point in time given that indigenous peoples influenced these landscapes for thousands of years. The question is likewise complicated in Europe, where records indicate that humans have actively manipulated ecosystems extensively for thousands of years (Backstrom et al. 2018). In these regions, traditional cultural ecosystems, which have developed with historical human land use practices, may help inform the reference model (Gann et al. 2019). In addition, ecosystems are not static and have changed in response to nonhuman-caused fluctuations in climate and weather on the time period of decades, centuries, or longer (Millar and Brubaker 2006). The specific time point selected strongly affects the restoration target.


If the goal of restoration is a historical state, then a common confounding problem is the lack of detailed information about what the ecosystem looked like. If the reference model (chap. 3) is what the site looked like a decade or two ago, then detailed information on species composition and ecosystem processes should be readily available. In contrast, if a restoration project in the Americas aims to restore to a state prior to European colonization, then knowledge on the species composition at that time relies largely on limited natural history notes of the early European explorers and, in rare cases, on ethnographic accounts and drawings from indigenous peoples. Characterizing the composition of historical ecosystems in regions with long periods of extensive agricultural use is nearly impossible. At best, one can gain a general vision of what the ecosystem looked like rather than the details needed to set specific restoration objectives.


A related issue is that what people perceive as a “historical” or “predisturbance” state is subject to human interpretation (Backstrom et al. 2018). With the unprecedented changes in the scale of human impacts on ecosystems over the past few decades (chap. 1), it is increasingly apparent that even ecosystems considered as “pristine” or “wilderness” are changing in response to anthropogenic impacts that often occur due to actions far from a given site. As that happens, we become more accustomed to the altered state, a phenomenon known as “shifting baselines” (Pauly 1995); in other words, each successive generation of people assumes that the diminished biological state is the norm rather than recognizing that this state has itself been altered by prior human activities. Pauly originally described shifting baselines in the context of fisheries where scientists compare fish declines to the abundance at the start of their careers without considering historical declines in fish populations due to overfishing over a period of centuries. One can think of numerous examples where this perception is the case, such as historical extinction of many faunal species due to forest clearing and overhunting centuries ago that have in turn affected the dispersal and distribution of plant species more recently. These changes mean that restoration practitioners in each successive generation are likely to lower their expectations for ecosystem recovery. Shifting baselines also make it difficult to judge whether a restored system is returning to the reference model (chap. 3) if the reference ecosystem is changing at the same time.


Impossibility of Controlling the Trajectory of Ecosystem Recovery


Once a reference model for restoration is chosen, another challenge is directing the trajectory of recovery toward the desired state. Even in minimally human-impacted systems, ecosystem recovery is often highly variable and unpredictable (chap. 5) as opposed to the linear trajectory of recovery that is often assumed (see fig. 2.1). Interannual weather fluctuations, natural disturbances (e.g., fire and flooding), and rare long-distance dispersal events affect the species that establish. For example, the species that colonize and survive in the first few years of dry forest restoration are determined by which plants set seed, the amount of rainfall in each year, whether a fire burns through the site, and an infinite number of other factors. The ecosystem trajectory in subsequent years is affected by which species establish initially, as well as ongoing variation in climatic variables, natural disturbances, and other stochastic events. The result is what Hilderbrand, Watts, and Randle (2005) refer to as the “myth of the carbon copy” in restoration. In other words, we can invest considerable resources to restore abiotic conditions and introduce desired species, but it is impossible to control the many variables at multiple spatial and temporal scales that influence the trajectory of ecosystem recovery and re-create a prior ecosystem exactly.


Lack of Resources and Knowledge


Billions of dollars are spent globally on restoration each year, but there will never be sufficient funds to undertake all the necessary restoration work. Limited funding makes it difficult to support projects over the multiple years necessary to ensure that the resulting ecosystems resemble the historical state that was selected. There are a number of high-profile and well-funded projects, such as restoration of 100 kilometers of the Kissimmee River in Florida, which has cost approximately $800 million (Kissimmee River case study). Most restoration projects, however, are undertaken with limited funds and are often supported by volunteer labor. There will always be trade-offs between what is desirable and what is feasible given funding availability.


Likewise, lack of knowledge limits our ability to restore ecosystems fully. Our understanding of how to restore many ecosystems has advanced substantially through a mix of scientific studies and learning from our successes and failures in restoration projects. Nonetheless, major gaps remain in our knowledge of the complex interactions between abiotic and biotic factors in nearly all ecosystems, and we know even less about how to restore them. An apt analogy is Humpty Dumpty: like an egg, it is much easier to destroy an ecosystem than to put it back together again. The act of trying to restore ecosystems is the ultimate test of our understanding of how they work (Bradshaw 1987).


Conflicting Goals


On the surface, restoring an ecosystem to a specific historical trajectory seems relatively noncontroversial in terms of an ecological goal, but conflicts often arise regarding the desired ecosystem stage or focal species. As ecosystems recover, they go through a process of succession (chap. 5) during which disturbance-adapted species become less common and other species more abundant. Likewise, active restoration actions will favor some species over others. For example, as described in the Tamarisk Removal case study, removal of invasive, nonnative tamarisk (Tamarix spp.) trees to reduce transpiration of water and restore native riparian vegetation has been highly controversial because it negatively affects an endangered bird,  the southwestern willow flycatcher (Empidonax traillii extimus), that nests in tamarisk trees, and conflicting ecological goals are only part of the challenge.


As the scale of both the human footprint on the landscape and the restoration projects being undertaken increases, balancing a host of ecological, socioeconomic, and cultural restoration goals is essential. This recognition is embedded in some recent definitions of restoration, which explicitly consider human well-being as a critical component of ecological restoration goals (Reitbergen-McCracken, Maginnis, and Sarre 2007; Suding et al. 2015). At the site scale, it might mean selecting nonnative tree species that are valued by local communities for fruit or timber as part of the planting palette for tropical forest restoration rather than only using native tree species. At the landscape scale, it means employing a mix of approaches to enhance habitat value for humans and other species. For example, forest and landscape restoration in tropical agriculture landscape mosaics often includes a mix of maintaining and restoring diversity in remnant forests, restoring low productivity agricultural lands that are important for minimizing erosion and flooding, and increasing and diversifying tree cover in actively used agricultural lands. These compromises between goals result in ecosystems that do not aim to replicate historical conditions across the entire landscape. 


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Excerpted with permission from Primer of Ecological Restoration by Karen D Holl © 2020. Published by Island Press in cooperation with SER.