CHAPTER 3
Climatic Influences on Ecosystems
William R. L. Anderegg and Terry L. Root
Abstract. Climate drivers act on ecosystems and species in multiple, interconnected and synergistic ways. In this chapter, we describe the primary climatic influences on species and ecosystems around the globe. Within the context of these drivers, the observed trends in species’ response to recent climate change are examined, focusing on western North America. We highlight issues and systems particularly pertinent to California, but provide a global framework and context for a mechanistic understanding of many of the major effects of climate on ecosystems and species. Extinction of the most vulnerable species is already occurring. Global policies of mitigation and adaption will likely be necessary to stave off future species extinctions and significant changes in ecosystems.
INTRODUCTION
All across the planet, from the shallow waters of tropical coral reefs to boreal forests and arctic tundra, species are already changing in response to anthropogenic climate change. Birds and butterflies are moving poleward and higher in elevation with warmer temperatures (Root et al. 2003). Amphibians in tropical cloud forests have gone extinct with warming, drying, and having no habitat into which to expand (Pounds et al. 2006). Alpine mammal populations in North America have been disappearing at the lower edges of their ranges due to temperature stress (Beever et al. 2003). The observed impacts of climate change on ecosystems presage more dramatic impacts in the future, depending on the magnitude, rate, and pathway of future warming (Root et al. 2005, Parmesan 2006, IPCC 2014). Future anthropogenic rapid changes in the climate are expected to have profound consequences for the earth’s biota. Our ability to predict changes, facilitate adaptation, and lessen future ecological disasters and extinctions will be aided by understanding of how climate affects species, populations, communities, and ecosystems.
In this chapter, we describe the primary climatic influences on ecosystems around the globe and, within the context of these drivers, the observed trends in species’ response to recent climate change, focusing on western North America. We highlight issues and systems particularly pertinent to California, but provide a global framework and context for a mechanistic understanding of many of the major effects of climate on ecosystems and species.
In many cases, non-climate human-caused influences, such as habitat loss and fragmentation, have already compromised the potential resilience and resistance of ecosystems to future change, rendering the systems more vulnerable to shifting climate (Scholes et al. 2014). Various facets of development in California over the past 150 years have threatened large numbers of species (Myers et al. 2000). Habitat loss and fragmentation due to urban growth, agricultural expansion, forestry activities, and water diversion and damming, coupled with competition and other pressures from nonnative species, have placed up to 35% of the state’s flora (1700 species) and 15% of the state’s vertebrate fauna (111 species) at increased risk of extinction (Stein et al. 2000, Greer 2004, OEHHA 2004, Bunn et al. 2007). These stressors are likely to act in synergistic ways with pressures from climate change, for instance, by restricting the ability of species to move in response to temperature changes (Loarie et al. 2008, Burrows et al. 2014).
CLIMATE INFLUENCES IN ECOSYSTEMS—PRIMARY DRIVERS
Climate has always been a major driver in shaping and changing ecosystems throughout the earth’s history (Schneider and Londer 1984). Both the general climatic conditions and patterns of extreme events in an area influence species and ecosystems in many ways. They affect individual fitness, population dynamics, species ranges and densities, community assembly, and ecosystem structure and function, among many others. Variance in climate over regional spatial scales can drive locally adapted physiologies, morphologies, and behavioral patterns. We provide an overview of the broad climatic driving forces that act on ecosystems including temperature, precipitation, atmospheric carbon dioxide, and other secondary drivers (i.e., climate-sensitive disturbance factors or processes) such as fire frequency. Additionally, we provide a brief overview of how species can and have been observed to respond to changing climate. While many of the climate drivers presented in this chapter can affect species, we primarily provide examples of the breadth of possible species’ responses to temperature changes because the vast majority of research has focused on temperature and because predictions of future temperature changes have relatively less uncertainty than precipitation, for instance.
Temperature
Temperature plays a critical role in determining the geographical distribution of ecosystems, as well as distribution and abundance of many species. In terrestrial systems, temperature range constraints have been well documented in many plant, insect, bird, and mammal species (Graham 1986, Grace 1987, Root 1988, Lenoir et al. 2008). While suitable resources often exist outside of a species’ range, individuals closer to the range boundaries are generally closer to the edge of their physiological tolerances and therefore face higher environmental stresses (Brown et al. 1996, but see Root 1988 and Chapter 9).
Species generally respond to changes in temperature in any combination of four possible manners: (1) Changing the timing of life-history events (e.g., migration, blooming) called phenology; (2) changing their distributions or ranges to more climatically favorable areas; (3) changing aspects of behavior, morphology, reproduction, or genetics; or (4) changing abundance with the potential of undergoing extirpations (local extinctions) or extinctions (Parmesan 2006, Barnosky et al. 2011). Furthermore, because each species has its own individual tolerances and responses to changing climatic conditions, species are affected differentially and exhibit different responses. This can lead to large-scale disruption of ecologic communities and trophic interactions (Stralberg et al. 2009).
Phenology Shifts
Changes in the timing of species’ life-history events, or phenology, have been the most comprehensive and widespread of all sets of observations with climate change. Many species of plants have been flowering earlier and growing seasons have lengthened over recent decades. Globally, species that responded with phenology changes to climate change saw an average advance of 5.1 days per decade over the previous 30 or more years (Root et al. 2003). Phenological advances have been largest in early parts of spring (Badeck et al. 2004). A follow-up study of Aldo Leopold’s 1930s and 1940s observations on a Wisconsin farm revealed that, while only one species shifted later, 35% of the 55 species (plants and birds) studied showed earlier phenology events in the 1990s, associated with spring temperature increases of 2.8°C (Bradley et al. 1999). Growing season has increased in length in the northern parts of the United States since 1966 (White et al. 1999, Scholes et al. 2014).
Birds provide striking examples of changes in life-history events. Laying date in tree swallows (Tachycineta bicolor) has been correlated to May temperature and has gotten earlier by about nine days from 1959 to 1991 over its entire breeding range in the contiguous United States (Dunn and Winkler 1999). In California, of the migratory birds found to have their first arrival date changing significantly (p < 0.1), 100% (8 of 8) are arriving earlier over time (MacMynowski et al. 2007). Phenological responses to recent climate change are apparent with many other taxa as well. In 70% of the California butterfly species studied, first-flight date has advanced by an average of 24 days over 31 years, and winter temperature and precipitation explained 85% of the variation in flight date (Forister and Shapiro 2003).
Trophic Interactions and Asynchrony
Differences in life-history strategies and physiological tolerances can lead to species exhibiting vastly different responses when exposed to similar amounts of warming and responses can even differ within species depending on complex interactions of temperature with other drivers such as soil moisture (Wolkovich et al. 2012). In some species, factors other than temperature, such as photoperiod, may trigger phenological events, which can lead to a severe mismatch in trophic interactions among species if a focal species responds to temperature or a temperature-related factor (e.g., snowmelt timing). This asynchrony of life-cycle events can result in, for instance, predators without prey, or herbivorous and egg-laying insects without host plants. While some species have proven able to track such differential shifts and avoid the damages of asynchrony (Charmantier et al. 2008), the majority of cases show that interacting species were out of synchrony due to climate change, leading to decreased fitness in many cases (Visser and Both 2005). For instance, population crashes and local extinctions have been shown to be a direct result of butterfly-host plant asynchrony in especially warm or dry years (Ehrlich et al. 1980, Thomas et al. 1996, McLaughlin et al. 2002).
Distribution Shifts
Species have exhibited changes in distributions, also known as species’ ranges, primarily poleward and upward in elevation in response to recent anthropogenic warming. In the Northern Hemisphere, the upper elevational boundaries of species ranges moved an average of 11 m per decade higher and the northern boundaries 17 km per decade north over the past 20–140 years (Chen et al. 2011). The differential spatial movement of various species in a community will likely stress many biotic interactions in the future and could contribute to species occurring together that are not currently together, thereby disrupting current ecological communities and forming “noanalogue” communities (Rosenzweig et al. 2007, Williams and Jackson 2007, Stralberg et al. 2009). Invasion of nonnative species may also be favored by warming temperatures (Pauchard et al. 2009).
Substantial changes in bird, insect, and plant ranges with climate change have been documented in western North America. Bird species in the Sierra Nevada Mountains have moved upward in elevation tracking their temperature envelope (Tingley et al. 2009, Tingley et al. 2012). Large proportions of extirpations along southern range boundaries of Edith’s checkerspot butterfly (Euphydrias editha) in North America have shifted the mean location of living populations 92 km northward (Parmesan 2006). Dominant vegetation and bird species in Southern California shifted in abundance uphill with warming (Kelly and Goulden 2008, Chapter 8). In Yosemite National Park, alpine mammals have largely moved upward in elevation in response to the 2–3°C increases in temperature (Moritz et al. 2008). Across the western United States, many alpine pika (Ochotono princeps) populations at lower elevations of their range have been extirpated between 1930 and present day (Beever et al. 2003).
Marine systems exhibit strong changes in community composition and species abundances with recent warming. In Monterey Bay, California, Sagarin et al. (1999) document a significant increase in southern-range plankton species and decrease of northern-range plankton species between 1931 and 1996, which were accompanied by a 2°C sea surface temperature rise. Similar shifts in fish communities in kelp-forest habitat off the California coast have been found (Holbrook et al. 1997). Using models, Dorman (Chapter 4) found that in years when the coastal upwelling is delayed, the abundance of krill plummets, which in turn results in decreased predator survival (salmon).
Across species, the local velocity of climate change—how fast an organism would need to travel to stay in the same temperature conditions—appears to be the most important predictor of range shifts, although dispersal ability and available habitat will also likely prove important (Pinsky et al. 2013). Synergistic impacts of land-use that impairs species movement and climate that necessitates movement will likely threaten many less-mobile species. The ability of species to move to more suitable climates, the presence of suitable habitat, and the extent of species current range all have strong conservation implications for the ability of a species to adapt and persist (Loarie et al. 2010). Because extant habitat fragmentation and disturbance may compromise the ability of species to disperse with climate changes, managed relocation may be necessary in some cases to allow species to colonize new locations and avoid extinctions, though this could carry substantial risks as well (McLachlan et al. 2007).
Behavior and Genetics
Species have exhibited several other different types of adaptations, including changes in behavior and morphology, but there are many fewer studies on these. Desert lizard species in Mexico have reduced their foraging activities and remain in the shadows longer than before the temperatures increased. As a consequence, reduced access to food has decreased their reproductive output (Sinervo et al. 2010). The reproduction of sea turtles is being affected because sex determination occurs in the egg and it is temperature dependent (Janzen 1994). The genetics of the pitcher plant mosquito has shown striking changes, which can be linked directly with warming temperatures (Bradshaw and Holzapfel 2006). The average mass and wing length of songbirds in central California has increased over the last 40 years, perhaps due to changing climate variability (Goodman et al. 2011).
Abundance Changes and Extinctions
When species are confronted with a habitat where the conditions are inhospitable, meaning outside their physiological environmental tolerances, and phenological dispersal, behavioral, and genetic mechanisms cannot help the species to sufficiently adapt to the environment, changes in species abundance and, at the extreme, species extirpations or extinctions can result. Species near the poleward edges of continents or near mountaintops will have no habitats into which they can disperse as their environment warms (e.g., Beever et al. 2003). Roughly 20–30% of known species will likely be at increasingly high risk of extinction if global mean temperatures exceed 2–3°C (3.6–5.4°F) above preindustrial temperatures [1.2–2.2°C (2.2–4°F) above current] (Thomas et al. 2004, IPCC 2007a). If the global average temperature goes above 4°C (7.2°F), this would likely commit 40–50% of known species to extinction (IPCC 2007a). Species extinction jeopardizes a large number of ecosystem services such as crop pollination and pest control, and the loss of any species is irreversible.
Prominent examples of climate-induced extirpations and extinctions have been documented in amphibians and reptiles. Wetland desiccation and drought tied to climate change led to local extirpations and rapid population declines in amphibians in Yellowstone National Park (McMenamin et al. 2008). Resurveys of 200 mountaintop sites in Mexico in 2009 reveal that 12% of lizard populations were not found and presumed to have gone extinct due to high ambient air temperatures since 1975 (Sinervo et al. 2010). These probable extinctions have been accurately simulated by modeling the fundamental trade-off these ectotherms (animals with body temperatures determined by ambient temperatures) face in sheltering in shade to avoid hot ambient temperatures versus foraging time. As discussed above, restricted foraging time translates into reduced population growth rates and survival. These models suggest that global extinction of lizards reached 4% by 2009 and will likely reach 16% by 2050 and 30% of lizard populations by 2080 (Sinervo et al. 2010).
Effects on California
Mediterranean-type ecosystems, which cover most of California, are likely to be strongly influenced by temperature increases (Sala et al. 2000). With only a 2°C (3.6°F) increase, models suggest a substantial reduction of alpine and subalpine habitat, a shift from conifer /evergreen to mixed-evergreen forest, expansion of desert and grassland at the expense of shrubland, and substantial decrease of many endemic species’ range (Hayhoe et al. 2004, Loarie et al. 2008).
Precipitation
Along with temperature, precipitation shapes many ecosystems. Precipitation often determines the type, structure, density, and diversity of vegetation and ecosystem boundaries (Holdridge 1947, Whittaker 1975). For example, in the tropics rainfall is one of the best predictors of species diversity and biomass in a site (Gentry 1982). Precipitation also plays a vital role in replenishing groundwater, soil moisture, and sustaining lakes, rivers, and other freshwater systems, although both the quantity and timing of precipitation matter greatly, especially for freshwater systems (Trenberth et al. 2007). A critical source of water for many plants around the world is fog (Bruijnzeel et al. 2011). Along with fog, overcast skies are known to significantly reduce drought stress in plants (Fischer et al. 2009). While precipitation is an important driver in ecosystems, changes in precipitation with climate change are harder to predict and have greater spatial heterogeneity than changes in temperature. These two elements make it difficult to generate and test hypotheses regarding climate change alterations in precipitation patterns.
In terrestrial ecosystems, precipitation often limits plant species’ ranges and abundances. The response to changes in precipitation is largely system-dependent. Drought plays an important role in forest dynamics in many temperate forests, including the Argentinian Andes, Rocky Mountains, North American woodlands, and the Eastern Mediterranean (Rosenzweig et al. 2007). Severe droughts can trigger widespread tree mortality (Anderegg et al. 2013) and increase vulnerability to pest, fire, and other disturbances (e.g., Breshears et al. 2005). Conversely, increases in precipitation, especially in arid systems, can increase vulnerability to plant diseases, fire risk through increased biomass yielding higher fuel loads, and alien species invasion (Dukes and Mooney 1999, Westerling et al. 2006).
Effects on California
Future changes in precipitation in California are unclear. Models provide mixed projections of future trends with a majority indicating slight decreases in precipitation in southern parts of the state and slight increases in northern parts (IPCC 2013). The most dramatic effects of precipitation changes will likely arise as temperature and rainfall interact, producing extreme events (Box 3.1) such as drought and increased fire risk, or high-volume storm events that scour streams. Even with little change in precipitation volume, changes in precipitation form (rain vs. snow), timing, and variability can still greatly influence ecosystems. For instance, increased precipitation variability in central coastal California has been linked to the local extinction of the Bay checkerspot butterfly (Euphydryas editha bayensis) (McLaughlin et al. 2002). With an increasing number and severity of droughts, cloud patterns along the coast will likely change (Still et al. 1999), putting dozens of drought-sensitive plants, many of which are rare, within the fog-dependent ecosystems in jeopardy (Fischer et al. 2009).
BOX 3.1 • Extreme events
Extreme weather events, such as heat waves, droughts, floods, wildfires, and severe storms, can be very damaging to ecosystems and are likely to be some of the most profound impacts of anthropogenic climate change (IPCC 2011). Some extreme events, such as heat waves, have already changed in frequency and severity in recent decades and are expected to change much more in years to come, even with relatively small changes in the mean global temperature (Meehl et al. 2007, Trenberth et al. 2007, IPCC 2011).
Changes in extreme events are likely to cause large changes in terrestrial ecosystems. Extremes in temperature or precipitation can act on individuals by influencing fitness, species populations by changes in abundance, phenology or reproduction, and ecosystems through major structural change such as hurricanes, floods, or fire. Past extreme events in temperature and drought have been linked to population crashes or local extinction in several butterfly species (Parmesan et al. 2000). Mass die-offs of multiple tree species, including piñon pine (Pinus edulis) and trembling aspen (Populus tremuloides), across the southwestern United States in recent years have been tied to severe droughts in the region (Breshears et al. 2005, Anderegg et al. 2012b). These widespread forest mortality events were triggered by extreme “climate change-type drought,” in which severe drought is exacerbated by higher summertime temperatures. This indicates that even if drought intensity or severity does not increase, these systems will be vulnerable due to the temperature increases alone (Adams et al. 2009, Williams et al. 2013). Widespread forest mortality events triggered by extreme climate events can alter ecosystem structure, function, and severely impact biodiversity (Allen et al. 2010, Anderegg et al. 2012a, Anderegg et al. 2013).
Atmospheric Carbon Dioxide
Increased atmospheric carbon dioxide (CO2) concentrations can directly affect global vegetation distribution, structure, and productivity (Rosenzweig et al. 2007). Carbon, taken up from the atmosphere as CO2 in photosynthesis through the Calvin Cycle, forms the backbone of organic compounds, especially sugars, synthesized by plants that fuel survival and growth. Opening pores on leaves known as stomata to take up CO2, however, has an inherent trade-off of losing water from the plant. Increased atmospheric CO2 should, in theory, enhance tree water-use efficiency—how much carbon a tree gets for a certain amount of water loss—and thereby increase growth, net primary productivity, and carbon storage, all of which is termed CO2 fertilization. In reality, however, studies have found increases in water-use efficiency in forests globally (Keenan et al. 2013) but more cryptic and less consistent increases in growth or carbon storage (e.g., Shaw et al. 2002, Morgan et al. 2004, Peñuelas et al. 2011). Thus, the magnitude and sign of CO2 enrichment will likely vary vastly among ecosystems. The largest CO2-fertilization influence has been seen in grassland systems and semiarid ecosystems and is likely due to the benefit of increased water savings from lower stomatal water loss (Morgan et al. 2004, Donohue et al. 2013). Other constraints such as nutrient limitation, water or temperature stress, and changing community assembly could limit carbon fertilization in terrestrial systems (Oren et al. 2001). Observed decreases in terrestrial net primary productivity between 2000 and 2009 due to regional droughts emphasize how other climate stresses may limit or even overwhelm the expected CO2-fertilization effect (Zhao and Running 2010).
In addition to the effect of increased atmospheric CO2 on terrestrial plants, high atmospheric CO2 concentrations have and will significantly alter ocean chemistry through ocean acidification. Since the industrial revolution, dissolved CO2 from human emissions has already significantly affected ocean chemistry, leading to a decrease of mean surface-ocean pH by 0.1 pH units. Lower oceanic pH decreases the concentration of dissolved calcium carbonate minerals, primarily aragonite and calcite, which holds dire consequences for marine ecosystems (IPCC 2014). As carbonate concentrations drop, coral reef-building organisms will have more difficulty calcifying and creating the physical structures that define coral reefs. Several modeling studies suggest that at atmospheric CO2 concentrations of 480 parts per million (ppm), erosion could outstrip calcium carbonate buildup, and coral reefs will slowly dissolve (Orr et al. 2005). In addition to lowered calcification rates from decreasing pH, direct effects of acidity and temperature severely threaten coral reefs and other marine ecosystems (Doney et al. 2009).
PROMINENT SECONDARY DRIVERS
Sea-level rise
Global sea levels rise with warmer temperatures due to the thermal expansion of the ocean and melting of continental ice in places like Greenland and the Antarctic. Higher water levels combined with increased storm severity can lead to many more damaging floods and extreme storm impacts in coastal regions. Increasing the number of major storms can more permanently damage coastal ecosystems including coral reefs by reducing recovery time and resilience (Forbes et al. 2004). In addition, rising water tables, levels, and salinity can reduce wetland, estuary, and coastal habitat. This further reduces habitat for many species in systems already pressured by human development (Field et al. 2007).
Effects on California
A 140-year log in San Francisco suggests that major winter storms have become more frequent since 1950 (Bromirski et al. 2003). Primary effects of rising sea level include bluff erosion, loss of beaches and wetlands, increased incidence of severe floods, and salinity movement inward in deltas, such as the Sacramento–San Joaquin delta area, which can severely alter vegetation and species habitat (Lund et al. 2007). High sea-level rise is expected to lead to large losses in tidal marsh in the San Francisco Bay, though all sea-level rise scenarios suggest a decrease of high marsh habitat and an increase in low marsh and mudflat habitats (Stralberg et al. 2011).
Snowpack and runoff
Changes in precipitation patterns and winter–spring temperatures may lead to decreased snowpack, runoff, or changes in runoff timing in many parts of the world. Above average warming on mountain peaks will likely lead to earlier and shorter runoff periods, rapid water release and possible downstream floods, and water shortages during summer growing seasons (Mote et al. 2005, Fischlin et al. 2007). This change in hydrological-cycle timing can affect downstream vegetation, as well as agriculture and human water supplies, and cause increases in flooding. In addition to reducing springtime and summer water availability, earlier snowmelt and runoff in alpine systems exposes plants and animals to frost, increases susceptibility to fire, alter phenology, and can disrupt animal movements, potentially raising wildlife mortality (Inouye et al. 2000, Keller et al 2005, Westerling et al. 2006).
Effects on California
Decreases in mountain snowpack have already been observed in many of the mountains in Northern California (Mote et al. 2005). Mountain snowpack and runoff timing are predicted to change by many climate models and these predicted changes will likely add more stress to the state’s social and natural systems that depend on downstream water. Hayhoe et al. (2004, p.12426) suggest that, “Declining Sierra Nevada snowpack, earlier runoff, and reduced spring and summer streamflows will likely affect surface water supplies and shift reliance to groundwater resources, already overdrafted in many agricultural areas in California.” Winter floods are more probable with increased runoff that have the potential to damage riparian and wetland systems. Furthermore, earlier snowmelt in the Sierras has been found to lead to increased vulnerability to wildfires in mountain forests (Westerling et al. 2006).
Fire
While it is often assumed that temperature and precipitation determine the boundaries of the world’s ecosystems, fire is also a strong driving force in several biomes (Bond et al. 2005). Natural fire cycles have kept some fire-sensitive ecosystems far from the physiological constraints of only temperature and precipitation. For instance, many fire-prone grassland, shrubland, and savannah systems transition to forest ecosystems in fire-exclusion experiments (Bond and van Wilgen 1996). Conversely, anthropogenic fires induced in island ecosystems have successfully transformed forested areas into grasslands (D’Antonio and Vitousek 1992, Ogden et al. 1998).
Climate change has already and will likely continue to intensify fire regimes in many different types of ecosystems, driving large changes in vegetation and ecosystems (Gillett et al. 2004, Westerling et al. 2006). Fire alters community structure by favoring species that survive fire or spread with fire (Bond and Keeley 2005). Intensified fire regimes across boreal forests, especially in North America, may drive changes in vegetation structure and composition, as witnessed in the shift from Picea-dominant to Pinus-dominant forests in Eastern Canada (Lavoie and Sirois 1998).
Effects on California
Frequency and intensity of wildfires with climate change is projected to increase in many areas of California (Westerling et al. 2006). Alpine forests and Mediterranean-type vegetation, two major types of Californian vegetation, are commonly identified as vulnerable systems to wildfires (Fischlin et al. 2007). Increasing summer wildfires could lead to changes in dominant vegetation types or changed community structure. Land management, such as grazing and fire suppression, will also interact strongly with wildfire probability and, in places like Sierra Nevada mixed conifer forest with a natural cycle of small and non-crown fire regime, increase the likelihood of massive crown fires (Westerling et al. 2006).
CONCLUSION
Climate drivers act on ecosystems and species in multiple, interconnected, and synergistic ways. Changes in temperature, precipitation, carbon dioxide concentrations, extreme events, sea-level rise, snowpack, and fire regimes all can ripple through ecosystems and cause profound changes. These changes will likely interact synergistically with other human-caused environmental changes such as habitat fragmentation. California’s boreal forest and Mediterranean habitats are vulnerable to a series of climate-induced changes and much of the state’s unique flora is at risk of large range contractions.
Species across the globe have already responded to levels of recent anthropogenic warming. They provide a coherent signal, independent climate attribution test, and indication of future trends and possible asynchronies that could drive large numbers of species to extinction. Already, extinction has hit the most vulnerable sets of species, range-restricted amphibians, and global policies of mitigation and adaption will likely be necessary to stave off future species extinctions and significant changes in ecosystem.
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