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CHAPTER 1
Background: Habitat Loss, Fragmentation, and Climate Change
Protected areas have been increasing in number and expanse globally, yet biodiversity continues to be imperiled. This is in part because, while protected areas are critical for the conservation of biodiversity, they are not sufficient. Conserving biodiversity, especially given unprecedented rates of climate change, requires conservation planning and implementation across large areas. This means that protected areas need to be framed in the context of a larger landscape or seascape and in relation to other protected areas. Accordingly, protected areas are significantly more effective in conserving biodiversity if they are part of an ecological network. For this to occur, areas that facilitate connectivity between protected areas must be managed to maintain or restore this connectivity. Such an approach to conservation is especially important during this time of climate change.
Global, national, and regional policy is increasingly calling for improvements to habitat connectivity. At the global level, the call for connectivity can be found in many major conventions and documents such as the Aichi Biodiversity Targets, the Convention for Biological Diversity guidance documents, and the World Business Council for Sustainable Development's Call to Action for Landscape Connectivity 2017. Here we see that connectivity conservation to address the biodiversity crisis is a priority for conservation globally.
This chapter provides a coarse overview of rapid changes occurring across our planet that contribute to the need for wildlife corridors to conserve connectivity, including habitat loss and fragmentation as well as climate change. We also discuss the importance of connectivity as a component of ecological networks for conservation and to support the global shift toward large-scale conservation. Finally, we provide basic definitions related to connectivity and corridors.
Human-Induced Change and Habitat Loss
Habitat loss and associated species loss are primarily a result of the acceleration of human-induced changes that occurred over the past century. The human population has increased six-fold since the 1800s, and the earth has been transformed to accommodate human habitation and rising consumption (Ehrlich and Ehrlich 2004). A human footprint is detectable across 75 percent of the land area in the world, an increase of 9 percent between 1993 and 2009 (Sanderson et al. 2002; Venter et al. 2016; fig. 1.1). Likewise, human influence in the marine environment is significantly impacting the chemistry and pollution of the ocean and is having a negative impact across marine ecosystems (Doney 2010). Only one-third of the world's 177 largest rivers are free flowing (https://www.internationalrivers.org/resources/where-riversrun-free-1670).
Attempts to map large undeveloped or wilderness areas (greater than 4,000 square kilometers, or about 1,500 square miles) globally estimated that such areas only constitute somewhere between 16 and 25 percent of the land on Earth outside the polar regions (McCloskey and Spalding 1989; Venter et al. 2016). The paucity of wilderness on Earth restricts the space suitable for persistence of some species, especially those that require intact ecosystems or have large ranges. In more human-impacted areas, mammal movement has been reduced by as much as half compared to equivalent areas with a much smaller human footprint (Tucker et al. 2018). This demonstrates that human presence and activities impact many species' ability to move. Where wilderness does remain, it is often in isolated patches. These isolated patches of natural habitat rarely contain the biodiversity that existed in the region prior to fragmentation. Smaller patches simply do not provide sufficient habitat for some species (Laurance et al. 2002).
Widespread habitat loss and fragmentation due to human activities clearly threatens species survival. As a result, extinction rates are 1,000 times higher than the historic background rates documented in the fossil record (Millenium Ecosystem Assessment 2005). If the current rate of biodiversity loss continues, we will experience the most extreme extinction event in the past 65 million years (Wilson 1988; Ripple et al. 2017). These losses will be devastating for humans, given that we depend on the goods and services that intact ecosystems offer. Many of our medicines and fibers, and all of our food, the basis for our economies and our survival, are derived from the nonhuman species with which we share this planet. Complex ecosystems are also responsible for many of the natural processes on which we depend, such as maintaining air quality, soil production, chemical and nutrient cycling; moderating climate; providing freshwater, fish and game, and pollination services; breaking down pollutants and waste; and controlling parasites and diseases (for more in-depth discussion, see Jacobs et al. 2013). An interdisciplinary field of study has arisen around trying to understand ecological resilience, which is defined as the capacity of a system to withstand changes to the processes that control its structures (Holling and Gunderson 2002). One of the primary observations is that disturbing ecosystems can reduce their resilience and result in dramatic shifts to less desirable states that weaken the ecosystem's capacity to provide goods and services (Folke et al. 2004).
Some scientists have quantified ecosystem services in financial terms (Daily and Ellison 2002). Studies estimate that Earth's biosphere provides up to US$72 trillion worth of goods and services per year that we currently do not pay for (Costanza et al. 1997; Corporate Ecoforum and The Nature Conservancy 2012). While quantifying ecosystem services may help enlighten people to the importance of natural systems in their daily life, it is difficult to quantify the value to humans of each of the 10 to 30 million or so species inhabiting Earth. The ecological roles of most species are unknown to us, but we do know the key roles that some species play in the normal functioning of biotic communities. For example, beavers are ecosystem engineers that create wetlands, which increases species richness at the landscape scale (Wright et al. 2002). A study of the functional role of species in ecological communities reveals that while frequently multiple species contribute to ecosystem process and function in the same way, referred to as redundancy (Walker 1992), ecosystem resilience may depend on that redundancy. In other words, even removing redundant species could have long-term consequences (Naeem 1998).
While the cost of species extinction to ecosystems and the goods and services that they provide should be one of today's primary concerns, what appears to motivate many people to conserve nature is the intrinsic value of biodiversity (Rolston 1988; Vucetich et al. 2015). With the loss of biodiversity, we are losing important opportunities for personal inspiration and cultural enrichment, whether by bird watching, fishing, or enjoying a natural scenic view. There is increasing evidence that spending time in nature has significant health benefits for young and old alike. To ignore the emotional and, for some, spiritual connections to nature and focus solely on the goods and services people rely on is a mistake. We must acknowledge our ethical and moral responsibility to prevent irreversible change to Earth's systems so that we do not harm other species and our own future generations.
Climate Change Overview
Global annual average surface air temperature has increased by about 1.0°C (1.8°F) from 1901 to 2016. Earth is currently experiencing the warmest period in the history of modern civilization. Many of us have experienced the implications of this such as record-breaking, climate-related weather extremes causing flooding and fires. As we move into the middle of this century, annual average temperatures are expected to rise, such as another 1.5°C for the United States, relative to the recent past under all plausible future climate scenarios (Wuebbles et al. 2017). It is increasingly unlikely that the global average temperature rise will remain below a 2.0°C change, a global temperature target, by the end of the century (IPCC 2014). This target was recommended as a level that would have global impacts but allow stabilization of natural systems.
It is clear that climate change is impacting the conservation of global biodiversity and will radically alter ecosystems and result in species' extinctions (Watson et al. 2012; Jantz et al. 2015). Today's climate change models forecast that the global climate and sea level are on a trajectory to change through the end of the century and beyond even if we successfully curbed greenhouse gas emissions in the immediate future (Ripple et al. 2017). While climate change is not unprecedented in Earth's history, today's rates of climate change far exceed anything previously experienced.
During the Quaternary (the last 2.6 million years), there were many periods of cooling interspersed by times when the climate warmed, bringing about expansions and contractions of ecosystems. Species have three mechanisms for adapting to climate change. A species might adapt within its current range through phenotypic plasticity (observable characteristics of an individual resulting from the interaction of its genotype and its environment); adaptive evolution (evolutionary changes that are adaptive to a given environment); or a species' range might shift in elevation, latitude, longitude, or aspect (Hilty et al. 2012; Pecl et al. 2017). Paleoecological studies have documented that during the Quaternary, in response to changing environmental conditions, populations increased or decreased in abundance, species shifted their ranges, and new species evolved while only a few species went extinct. Different types of range shifts have been documented. The entire range of some taxa, for example, spruce (Picea ssp.) in eastern North America, has shifted to higher latitudes. Other taxa, for example, pine trees (Pinus ssp.) in Nevada, shifted their ranges up in elevation, but the amount of shift and even the direction have varied. Contraction to or expansion from refugial populations are other patterns that have been observed (Davis and Shaw 2001). Because species respond individualistically to climate change, shifting ranges at different rates and in different directions, community turnover historically was common, and frequently resulted in novel associations (Gill et al. 2015).
In current times, synergistic effects between habitat loss, habitat fragmentation, and global climate warming (Ripple et al. 2017) are compounding the effects of habitat loss on biodiversity. Some species may not be able to shift their distributions or evolve new adaptations fast enough to accommodate global climate change. Fragmented landscapes decrease the opportunity for movements that could result in range shifts. Numerous studies document the historical movement of species as the climate changed in the past (e.g., DeChaine and Martin 2004), but the unprecedented speed of modern climate change combined with habitat loss could make historical processes of adaptation less applicable today (Steffen et al. 2015).
Models can predict the effects of climate change on vegetation communities. While predictions depend on the model specifications, the climate models, and greenhouse gas concentration trajectories used, the general message is that the natural vegetation in its current state is at risk from climate stress. In Europe, models predict that up to 80 percent of the vegetation types will be replaced by a different vegetation type by the end of the century if carbon emissions are not drastically curbed (Hickler et al. 2012). Without effective carbon mitigation in the Mediterranean Basin, increased temperatures and water deficits will cause the desertification of large parts of the landscape (Guiot and Cramer 2016). In western North America, models predict a widespread shift from tree-dominated landscapes to shrub- and grass-dominated landscapes (Jiang et al. 2013; Thorne et al. 2017). However, at high elevations, forests are simulated to move upward causing the contraction of alpine grass and shrub vegetation (Shafer et al. 2015).
Climate change research suggests that a critical factor for species and system adaptation is resilience. Terrestrial and aquatic ecosystems tend to be more resilient if they are conserved in large and unfragmented areas such that the ecological processes that sustain these systems can continue unhindered (Walker and Salt 2006). Climate change can directly affect species populations and communities through, for example, higher average or extreme temperatures, increased water deficit, or more severe flooding. Other stressors can exacerbate negative effects of climate change, and vice versa (Staudt et al. 2013). An increase in disturbances such as fires, flooding, windstorms, or insect outbreaks can decrease system stability (Buma and Wessman 2011). Resource extraction, such as timber harvest, can also decrease ecosystem resilience making the forest more susceptible to climate change (Staudt et al. 2013). Higher temperatures can magnify the adverse environmental effects of pollutants by increasing their availability in the environment and amplifying their negative effects. Biological disturbances can reach catastrophic dimensions, for example through the invasion of nonnative species that have a competitive advantage, or the emergence of pest species or pathogens that have expanded their range or are more numerous due to milder winters. Since the late 1990s, mountain pine beetles (Dendroctonus ponderosae) have caused pine mortality throughout millions of acres in North America, and the outbreak has been attributed to warmer temperatures, increased drought stress, and generally unhealthy growing conditions (West et al. 2014).
Lessons learned from previous climate changes in Earth's history, observations of the effects in the first decades of human-induced climate changes on species and ecosystems, and predictions of the magnitude of and biological responses to climate change give us a foundation for thoughtful adjustments to conservation planning and action, including corridor conservation. Climate change is literally forcing us to reorient our approach to conservation as we are no longer managing to maintain a historical reference point, but rather are managing for change (Hilty et al. 2012). The fields of protected-area design, connectivity modeling, restoration, risk assessment, and assisted migration have evolved a good deal over the past fifteen years. This includes even the abandonment of long-existing principles, such as the requirement to source seeds for restoration from local populations (Havens et al. 2015). The enormous scale at which climate change impacts are happening reinforces the importance of planning conservation at larger spatial scales. It is only by achieving conservation at such scales that we have a chance of conserving otherwise increasingly imperiled biodiversity around the world. Integrated landscape conservation needs to design and implement conservation networks connected by corridors and embedded in a permeable landscape. Planning and managing ecosystems in these protected areas for resilience is essential to reduce stressors that exacerbate the effects of climate change. Species will be able to persist in landscapes with large, connected natural areas where pressures from overharvesting, invasive species, and pollutants are low (fig. 1.2).
In corridor conservation, we need to shift from planning habitat connectivity for a few charismatic focal species to an ecosystem approach where the entire biota can move through the landscape in response to climate change. This can be accomplished by applying coarse-filter strategies designed to conserve the majority of biodiversity (Lawler, Watson, et al. 2015; Anderson et al. 2016). Complementary fine-filter approaches may be necessary to focus on conserving individual species that slip through the coarse filter and require specific conservation action (Hunter 2005). We will discuss these different approaches in more detail when we consider climate-wise connectivity in chapter 8. Assisted migration, translocating species to locations where the climate is suitable, is an example of a fine-filter approach. Originally this strategy was mostly rejected based on the great uncertainty of the ecological effects of moving a species into formerly unoccupied habitat. Now the concept has gained greater acceptance especially among the forestry industry with numerous models predicting the need for arid-adapted seed stock and relocation of species that cannot adapt to changing local conditions (Lawler, Ackerly, et al. 2015; Thomas 2011; Seddon et al. 2014).
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Excerpted from "Corridor Ecology"
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Copyright © 2019 Jodi A. Hilty, Annika T. H. Keeley, William Z. Lidicker Jr., and Adina M. Merenlender.
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