Ocean food web.
Scanning electron micrograph of foraminifera.
Ocean acidification, increased sea surface and air temperatures, and changes to ocean circulation will all have significant impacts on food webs and ecosystem function across marine habitats. Although it is uncertain how these changes will play out, there is increasing evidence that no habitat or life stage will be unaffected by global climate change.
Food webs describe the relationship between organisms in a habitat in terms of who is eating whom. Trophic levels within a food web are based on what an organism eats. For example, phytoplankton are primary producers, mussels and clams are basal consumers (lowest consumers in the food web), crabs are secondary consumers, sea otters are primary predators and great white sharks are top predators. In addition to providing food for higher consumers, species also perform other jobs, or functions, in their habitat. Organisms that have the same function in a habitat make up functional groups. Functional groups perform jobs such as water filtration, wave buffering, and nutrient recycling. These ecological functions are important for maintaining balance within a habitat and for providing humans with valuable services such as clean water, storm protection and climate regulation. Each ecosystem in the ocean and along the coast has its own set of species that make up different trophic levels and functional groups that co-exist in a healthy, functioning ecosystem. Species composition changes can affect this balance, and alter system functioning.
In the eastern Pacific, the species composition of foramnifera (small hard-shelled protists) at the base of the food chain shifted from cold to warm water species within the last century.1 This change in composition is highly correlated with rising ocean temperature, and there is no evidence of similar shifts – from cold tolerant to warm tolerant species – within the last 1,400 years. It is yet unclear how this shift will impact the food web.
Functional groups that provide essential ecosystem functions such as water filtration are also being affected. Pacific oyster (Crassostrea gigas) hatcheries along the US west coast, which grow many of the shellfish that are consumed throughout the world, have seen an 80% reduction in production over the last several years.2 Losses in production, which are mainly due to the failure of larval recruitment, are correlated with influxes of low pH water during the upwelling season (spring and early summer). Such water is capable of dissolving the fragile shells of juvenile shellfish. The delivery of low pH water along the Washington and Oregon coast has increased since 2005 and has been attributed to stronger upwelling events and increased atmospheric CO2 that has changed ocean chemistry.3 Changes in the success of these species that form the bottom of the food chain will cascade to higher trophic levels, impacting the food web and ecosystem functioning.
The most pervasive changes to the marine food web are likely to occur as species move to find a more suitable habitat and invade new spaces, or go extinct if they are unable to find suitable habitat. Species are adapted to the physical and biological features of their environment. As temperature, salinity, pH, habitat and/or currents change, species must be able to adapt to the changes (i.e. acclimate or rapidly evolve) or move to areas that are favorable for reproduction and survival. These large scale changes in distribution will alter the ways organisms interact with one another, the types of biological jobs they perform, and the dominant ecological players in the community. Tropical and sub-polar regions are expected to see the most species extinctions, while the Arctic and Southern Ocean are expected to see the greatest rise in species invasions as a result of pole-ward migration by temperate species.4
