Climate change impacts also occur at the ecosystem scale, changing fundamental ecosystem characteristics, properties, and related ecosystem services; altering important trophic relationships; and affecting how species and populations interact with each other.
Because primary producers are the base of the food web, climate impacts to primary production can have significant effects that radiate throughout the entire ecosystem. While climate models project continued increases in global terrestrial primary production over the next century,130,131 these projections are uncertain due to a limited understanding of the impacts of continued CO2 increases on terrestrial ecosystem dynamics;132,133,134 the potential effects of nutrient limitation;135 the impacts of fire136 and insect outbreaks;137 and an incomplete understanding of the impacts of changing climate extremes.138,139 Furthermore, even without these factors, projections suggest decreasing primary production in many arid regions due to worsening droughts, similar to responses observed in the Southwest United States in recent years.140,141,142 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 Most models project increasing primary productivity in the Arctic due to decreasing ice cover. This trend is supported by satellite-based observations of the primary productivity–ice cover relationship over the last 10–15 years.146,147,148 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
Varying phenological responses to climate change can also impact the food web and result in altered species interactions and resource mismatch.17,153 Such mismatches can decrease the fitness of individuals, disrupt the persistence and resilience of populations, alter ecosystems and ecosystem services, and increase the risk of localized extinctions.16,26,113,154,155 In marine ecosystems, rapid phenological changes at the base of the food web can create a mismatch with consumers,156 disrupting the availability of food for young fish and changing the food web structure.24,156 In both terrestrial and aquatic environments, migratory species face the potential for resource mismatch. For example, a majority of migratory songbirds in North America have advanced their phenology in response to climate change, but for several species, such as the yellow-billed cuckoo and the blue-winged warbler, these changes have been outpaced by advancing vegetation in their breeding grounds and stopover sites.28 The resulting mismatch between consumers and their food or habitat resources can result in population declines.155
In addition to changes in productivity and phenology, novel species interactions as a result of climate change can cause dramatic and surprising changes. For example, range expansions of tropical herbivorous fishes have changed previously kelp-dominated systems into kelp-free sites.157 These novel combinations of species are expected to outcompete and potentially eliminate some native species, posing a significant threat to the long-term stability of iconic ecosystems and the services they provide.157 A recent survey of 136 freshwater, marine, and terrestrial studies suggests that species interactions are often the immediate cause of local extinctions related to climate change.158
Climate change impacts to ecosystem properties are difficult to assess and predict because they arise from multiple and complex interactions across different levels of food webs, habitats, and spatial scales. Modeling and experimental studies are some of the few ways to assess complicated ecological interactions, especially in marine systems where direct observations of plants, fish, and animals are difficult.67,159,160,161 There is strong consensus that trophic mismatches and asynchronies will occur, yet these are mostly predicted consequences, and few examples have been documented.13,84,162,163 While theory and management principles for novel ecosystems are new, strongly debated, and largely descriptive, they are also crucial for understanding and anticipating widespread ecosystem changes in the future.164,165,166 For example, it remains largely uncertain which members of historical ecological communities and ecosystems will adapt in place or move into new locations to follow optimal ecological and environmental conditions.167 Such uncertainties complicate management decisions regarding where and when human intervention is advisable to assist persistence.
It is also unclear how the restructuring of ecosystems will manifest in terms of the functioning and delivery of ecosystem services.167,168 For example, along the Northeast Atlantic coast, native fiddler and blue crabs have shifted their ranges north and are now found in New England coastal habitats where they were previously absent.169,170 These two species join an assemblage of native and invasive crab species, which are responding to changes in environmental and ecological conditions in different ways. In some locations, purple marsh crabs are benefiting from lower abundances of blue crabs and other predators, in part due to overfishing; this results in population explosions of purple marsh crabs that damage marsh habitats through herbivory (plant eating) and burrowing activities.171 Because salt marshes provide a range of ecosystem services, including coastal protection, erosion control, water purification, carbon sequestration, and maintenance of fisheries, marsh destruction can negatively impact human communities.172 Thus, climate impacts to ecosystems can have important consequences for ecosystem services and the people who depend on them.