Topics for the chapter were selected to improve the consistency of coverage of the report and to standardize the assessment process for ecosystems and biodiversity. Chapter leads went through the detailed technical input for the Third National Climate Assessment and pulled out key issues that they felt should be updated in the Fourth National Climate Assessment. The chapter leads then came up with an author team with expertise in these selected topics. To ensure that both terrestrial and marine issues were adequately covered, most sections have at least one author with expertise in terrestrial ecosystems and one with expertise in marine ecosystems.

Monthly author calls were held beginning in December 2016, with frequency increasing to every other week as the initial chapter draft deadline approached. During these calls, the team came up with a work plan and fleshed out the scope and content of the chapter. After the outline for the chapter was created, authors reviewed the scientific literature, as well as the technical input that was submitted through the public call. After writing the State of the Sector section, authors pulled out the main findings to craft the Key Messages.

Key Message 1: Impacts on Species and Populations Climate change continues to impact species and populations in significant and observable ways (high confidence). Terrestrial, freshwater, and marine organisms are responding to climate change by altering individual characteristics, the timing of biological events, and their geographic ranges (likely, high confidence). Local and global extinctions may occur when climate change outpaces the capacity of species to adapt (likely, high confidence). Description of evidence base Changes in individual characteristics: Beneficial effects of adaptive capacity depend on adequate genetic diversity within the existing population and sufficient population sizes. In addition, successful adaptive responses require relatively slow or gradual environmental change in relation to the speed of individual or population-level responses.13 Empirical evidence continues to suggest that plastic changes and evolution have occurred in response to recent climate change10,11,12,233 and may be essential for species’ persistence.186,234,235 However, adaptation is only possible if genetic diversity has not already been eroded as a result of non-climate related stressors such as habitat loss.15 Additionally, projections suggest that climate change may be too rapid for some species to successfully adapt.35,236 Adaptive capacity, and by extension the ability to avoid local or even global extinctions, is likely to vary among species and even populations within species. Changes in range: Shifts in species’ ranges have been documented in both terrestrial and aquatic ecosystems as species respond to climate change.35,39 Approximately 55% of terrestrial and marine plant and animal species studied in temperate North America have experienced range shifts.35 Climate change has led to contractions in the latitudinal or elevational ranges of 41% (97 of 238) of studied terrestrial plant and animal species in North America and Hawaiʻi in the last 50–100 years.35 Range shifts in terrestrial animal communities average 3.8 miles per decade.107 In marine communities, range shifts of up to 17.4 miles per decade have been documented.17 Planktonic organisms in the water column (that is, passively floating organisms in a body of water) more closely track the trajectory of preferred environmental conditions, resulting in more extensive range shifts; these organisms have exhibited rates of change from 4.3 miles per decade for species with broad environmental tolerances to 61.5 miles per decade for species with low tolerance of environmental change over a 60-year period.237 Walsh et al. 2015 38 documented significant changes in the center of distribution over two decades of 43% of planktonic larvae of 45 fish species. These shifts have been linked to climate velocity—the rate and direction of change in temperature patterns.30,39,238,239 Marked differences in observed patterns of climate velocity in terrestrial and aquatic ecosystems have been observed.29,240 Climate velocity in the ocean can be greater than that on land by a factor of seven.17 Changes in phenology: In marine and freshwater systems, the transition from winter to spring temperatures is occurring earlier in the year, as evidenced by satellite measures of sea surface temperature dating back to 1981.23 In addition, the timing of sea ice melt is occurring earlier in the spring at a rate of about 2 days per decade and has advanced by 25–30 days since 1979 in some regions.24 Shifts in phenology have been well documented in terrestrial, marine, and freshwater systems.113 As with range shifts, changes to phenology are expected to continue as the climate warms.114 Extinction risks: The rate and magnitude of climate impacts can exceed the abilities of even the most adaptable species, potentially leading to tipping points and abrupt system changes. In the face of rapid environmental change, species with limited adaptive capacity may experience local extinctions or even global extinctions.126,127 Major uncertainties Changes in individual characteristics: Species and populations everywhere have evolved in response to reigning climate conditions, demonstrating that evolution will be necessary to survive climate change. Nonetheless, there is very limited evidence for evolutionary responses to recent climate change. As reviewed by Crozier and Hutchings 2014,10 only two case studies document evolutionary responses to contemporary climate change in fish, as opposed to plasticity without evolution or preexisting adaptation to local conditions, and both cases involved the timing of annual migration.241,242 In the case of the sockeye salmon, for example, nearly two-thirds of the phenotypic response of an earlier migration date was explained by evolutionary responses rather than individual plastic responses.241 Changes in range: Although the evidence for shifting ranges of many terrestrial and aquatic species is compelling, individual species are responding differently to the magnitude and direction of change they are experiencing related to their life history, complex mosaics of microclimate patterns, and climate velocity.243,244,245,246,247 Additionally, projections of future species distributions under climate change are complicated by the interacting effects of multiple components of climate change (such as changing temperature, precipitation, sea level rise, and so on) and effects from non-climate stressors (such as habitat loss and degradation); these multiple drivers of range shifts can have compounding or potentially opposing effects, further complicating projections of where species are likely to be found in the future.41 Description of confidence and likelihood There is high confidence that species and populations continue to be impacted by climate change in significant and observable ways. There is high confidence that terrestrial, freshwater, and marine organisms are likely responding to climate change by altering individual characteristics, the timing of biological events, and their geographic ranges. There is high confidence that local and global extinctions are likely to occur when climate change outpaces the capacity of species to adapt.

Key Message 2: Impacts on Ecosystems Climate change is altering ecosystem productivity, exacerbating the spread of invasive species, and changing how species interact with each other and with their environment (high confidence). These changes are reconfiguring ecosystems in unprecedented ways (likely, high confidence). Description of evidence base Primary productivity: Diverse observations suggest that global terrestrial primary production has increased over the latter 20th and early 21st centuries,48,49,50,51 and climate models project continued increases in global terrestrial primary production over the next century.130,131 Modest to moderate declines in ocean primary production are projected for most low- to midlatitude oceans over the next century,143,144,145 but regional patterns of change are less certain.60,143,145 Projections also suggest that changes in productivity will not be equal across trophic levels: changes in primary productivity are likely to be amplified at higher levels of the food web;149,150,151 for example, small changes in marine primary productivity are likely to result in even larger changes to the biomass of fisheries catch.152 Changes in phenology: Synchronized timing of seasonal events across trophic levels ensures access to key seasonal food sources,25,248 particularly in the spring, and is especially important for migratory species dependent on resources with limited availability and for predator–prey relationships.29 The match–mismatch hypothesis249 is a mechanism explaining how climate-induced phenological changes in producers and consumers can alter ecosystem food web dynamics.114 For example, Chevillot et al.250 found that reductions in temporal overlap of juvenile fish and their zooplankton prey within estuaries, driven by changes in temperature, salinity, and freshwater discharge rates, could threaten the sustainability of nursery functions and affect the recruitment of marine fishes. Secondary consumers may be less phenologically responsive to climate change than other trophic groups,114 causing a trophic mismatch that can negatively impact reproductive success and overall population levels by increasing vulnerability to starvation and predation.16,155 Long-distance migratory birds, which have generally not advanced their phenology as much as lower trophic levels,113 can be particularly vulnerable.27 A recent study found that 9 out of 48 migratory bird species examined did not keep pace with the changing spring phenology of plants (termed green-up) in the period 2001–2012.28 Trophic mismatch and an inability to sufficiently advance migratory phenology such that arrival remains synchronous with peak resource availability can cause declines in adult survival and breeding success.28,155 Invasive species: Changes in habitat and environmental conditions can increase the viability of introduced species and their ability to establish.69,75,76 Climate change may be advantageous to some nonnative species. Such species are, or could become, invasive, as this advantage might allow them to outcompete and decimate native species and the ecosystem services provided by the native species. Invasive species’ impacts on ecosystems are likely to have a greater negative impact on human communities that are more dependent on the landscape/natural resources for their livelihood and cultural well-being.251,252 Thus rural, ranching, fishing, and subsistence economies are likely to be negatively impacted. Some of these communities are economically vulnerable (for example, due to low population density, low median income, or reduced tax revenues) and therefore have limited resources and ability to actively manage invasive species.253,254 Climate change and invasive species have both been recognized as two of the most significant issues faced by natural resource managers.61,62 For example, the invasive cheatgrass (Bromus tectorum) is predicted to increase in abundance with climate change throughout the American West, increasing the frequency of major economic impacts associated with the management and rehabilitation of cheatgrass-invaded rangelands.255,256 Ecological and economic costs of invasive species are substantial, with global costs of invasive species estimated at over $1.4 trillion annually.61 Annual economic damages from climate change are complex and are projected to increase over time across most sectors that have been examined (such as coral reefs, freshwater fish, shellfish) (Ch. 29: Mitigation, Figure 29.2). Species interactions and emergent properties: Human-caused stressors such as land-use change and development can also lead to novel environmental conditions and ecological communities that are further degraded by climate impacts (Ch. 11: Urban, KM 1) .13,163 Studies of emergent properties have progressed from making general predictions to providing more nuanced evaluations of behavioral mechanisms such as adjusting the timing of activity levels to avoid heat stress 6,81,87 and predation,88 tolerances to variable temperature fluctuations and water availability,79,80,82,257 adaptation to changes,82,258 turnover in community composition,259,260 and specific traits such as dispersal ability.67,85 Changes in community composition vary relative to invasion rates of new species, local extinction, and recruitment and growth rates of resident species, as well as other unknown factors.260 In some cases, such as Pacific Northwest forests, community turnover has been slow to date, likely due to low exposure or sensitivity to the direct and indirect impacts of climate change,259 while in other places, like high-latitude systems, dramatic shifts in community composition have been observed.261 Differential responses within and across communities are expected due to individual sensitivities of community members. For example, as a result of the uncertainties associated with range shifts, the impact of individual species’ range shifts on ecosystem structure and function and the potential for the creation of novel community assemblages have medium certainty. The interplay of physical drivers resulting in range shifts and the ways in which interactions of species in new assemblages shape final outcomes affecting ecosystem dynamics is uncertain, although there is more certainty in how ecosystem services will change locally. There is still high uncertainty in the rate and magnitude at which community turnover will occur in many systems; still, there is widespread agreement of high turnover and major changes in age and size structure with future climate impacts and interactions with other disturbance regimes.259,260,261 Climate-induced warming is predicted to increase overlaps between some species that would normally be separated in time. For example, tree host species could experience earlier bud burst, thus overlapping with the larval stage of insect pests; this increase in synchrony between normally disparate species can lead to major pest outbreaks that alter community composition, productivity, ecological functioning, and ecosystem services.262 Direct climate impacts, such as warmer winters and drought-induced stress on forests, can interact with dynamics of pest populations to render systems more susceptible to damage in indirect ways. In the case of the bark beetle, for example, forests that have experienced drought are more vulnerable to damage from beetle attacks.138,263 Other potential outcomes of novel species assemblages are changes in energy and nutrient exchange (for example, altered carbon use in streams as new detritus-feeding or predator communities emerge)193 and respiration89 within and among ecological communities. Abrupt and surprising changes or the disruption of trophic interactions have the potential for negative and irreversible impacts on food webs and ecosystem productivity that supports important provisioning services including fisheries and forest harvests for food and fiber. Abrupt changes in climate have been observed over geological timescales and have resulted in mass extinctions, decreased overall biodiversity, and ecological communities largely composed of generalists.67 Major uncertainties Primary productivity: There is still high uncertainty in how climate change will impact primary productivity for both terrestrial and marine ecosystems. For terrestrial systems, this uncertainty arises from an incomplete understanding of the impacts of continued carbon dioxide increases on plant growth;132,133,134 underrepresented nutrient limitation effects;135 effects of fire136 and insect outbreaks;137 and an incomplete understanding of the impacts of changing climate extremes138,139 on primary production. Direct evidence for declines in marine primary production is limited. The suggestion that phytoplankton pigment has declined in many ocean regions,55 indicating a decline in primary production, was found to be inconsistent with primary production time series59 and potentially sensitive to analysis methodology.56,58,264 Subsequent work accounting for methodological criticisms still argued for a century-scale decline in phytoplankton pigment but acknowledged large uncertainty in the magnitude of this decline and that some areas show marked increases.54 There is growing consensus for modest to moderate productivity declines at a global scale in the marine realm.143,144,145 Considerable disagreement remains at regional scales.143 For both the terrestrial and marine case, however, projections clearly support the potential for marked primary productivity changes. Phenology: Models of phenology, particularly those leveraging advanced statistical modeling techniques that account for multiple drivers in phenological forecasts,265 enable extrapolation across space and time, given the availability of gridded climatological and satellite data.21,266,267,268 However, effective characterization of phenological responses to changes in climate is often constrained by the availability of adequate in situ (ground-based) organismal data. Experimental manipulation of ecological communities may be insufficient to determine sensitivities; for example, E. M. Wolkovich et al. 2012269 compared observational studies to warming experiments across four continents and found that warming predicted smaller advances in the timing of flowering and leafing by 8.5- and 4.0-fold, respectively, than what has been observed through long-term observations. The majority of terrestrial plant phenological research to date has focused on patterns and variability in the onset of spring, with far fewer studies focused on autumn.270 However, autumn models have large biases in describing interannual variation.271,272 Additional research is needed on autumnal responses to environmental variation and change, which would greatly expand inferences related to the carbon uptake period, primary productivity, nutrient cycling, species interactions, and feedbacks between the biosphere and atmosphere.273,274,275,276 While broad-based availability of phenological data has improved greatly in recent years, more extensive, long-term monitoring networks with consistently implemented protocols would further improve scientific understanding of phenological responses to climate change and would better inform management applications.277 Invasive species: There is some uncertainty in knowing how much a nonnative species will impact an environment, if and when it is introduced, although there are methods available for estimating this risk.278,279 For example, the U.S. Department of Agriculture conducts Weed Risk Assessment,280 and the U.S. Fish and Wildlife Service publishes Ecological Risk Screening Summaries (https://www.fws.gov/fisheries/ans/species_erss_reports.html). New technologies, such as genetic engineering, environmental DNA, and improved detection via satellites and drones, offer promise in the fight against invasive species.281 New technologies and novel approaches to both invasive species management and mitigation and adapting to climate change could reduce negative impacts to livelihoods, but there is some uncertainty in whether or not the application of new technologies can gain social acceptance and result in practical applications. Species interactions and emergent properties: Climate change impacts to ecosystem properties are difficult to assess and predict, because they arise from interactions among multiple components of each system, and each system is likely to respond differently. One generalization that can be made arises from fossil records, which show climate-driven mass extinctions of specialists followed by novel communities dominated by generalists.67 Although there is widespread consensus among experts that novel interactions and ecosystem transitions will result from ecological responses to climate change,85 these are still largely predicted consequences, and direct evidence remains scarce; thus, estimates of how ecosystem services will change remain uncertain in many cases.13,67,84,128,159,161,162,163,258,282,283 Modeling and experimental studies are some of the few ways to assess complicated ecological interactions at this time. New and more sophisticated models that can account for multispecies interactions, community composition and structure, dispersal, and evolutionary effects are still needed to assess and make robust predictions about system responses and transitions.161,258,282 High uncertainty remains for many species and ecosystems due to a general lack of basic research on baseline conditions of biotic interactions; community composition, structure, and function; and adaptive capacity; as well as the interactive, synergistic, and antagonistic effects of multiple climate and non-climate stressors.67,128,283 Improved understanding of predator–prey defense mechanisms and tolerances are key to understanding how novel trophic interactions will manifest.257 Description of confidence and likelihood There is high confidence that climate-induced changes are occurring within and across ecosystems in ways that alter ecosystem productivity and how species interact with each other and their environment. There is high confidence that such changes can likely create mismatches in resources, facilitate the spread of invasive species, and reconfigure ecosystems in unprecedented ways.