Natural communities are finely-tuned systems, where each species has an ecological value to the other species that are part of that ecosystem. Species diversity increases an ecosystem's stability and resilience, in particular its ability to adapt and respond to changing environmental conditions. If a certain amount, or type (such as a keystone species) of species are lost, eventually it leads to the loss of ecosystem function. Many ecosystems though have built-in redundancies so that two or more species' functions may overlap. Because of these redundancies, several changes in the number or type of species may not impact an ecosystem. However, not all species within an ecosystem are of the same importance. Species that are important due to their sheer numbers are often called dominant species. These species make up the most biomass of an ecosystem. Species that have important ecological roles that are greater than one would expect based on their abundance are called keystone species. These species are often central to the structure of an ecosystem, removal of one or several keystone species may have consequences immediately, or decades or centuries later (Jackson et al. 2001 [link]). Ecosystems are complex and difficult to study, thus it is often difficult to predict which species are keystone species. The impact of removing an individual or several keystone species from kelp forests in the Pacific is examined in Example 18.1.
Kelp forests, as their name suggests, are dominated by kelp, a brown seaweed of the family Laminariales. They are found in shallow, rocky habitats from temperate to subarctic regions, and are important ecosystems for many commercially valuable fish and invertebrates.
Vast forests of kelp and other marine plants existed in the northern Pacific Ocean prior to the 18th century. The kelp was eaten by herbivores such as sea urchins (Family Strongylocentrotidae), which in turn were preyed upon by predators such as sea otters (Enhydra lutris). Hunting during the 18th and 19th centuries brought sea otters to the brink of extinction. In the absence of sea otters, sea urchin populations burgeoned and grazed down the kelp forests, at the extreme creating "urchin barrens," where the kelp was completely eradicated. Other species dependent on kelp (such as red abalone Haliotis rufescens) were affected too. Legal protection of sea otters in the 20th century led to partial recovery of the system.
More recently sea otter populations in Alaska seem to be threatened by increased predation from killer whales (Orcinus orca) (Estes et al. 1998 [link]). It appears that whales may have shifted their diet to sea otters when populations of their preferred prey, Stellar sea lions (Arctocephalus townsendi) and Harbor seals (Phoca vitulina) declined. The exact reason for the decline in the sea lion and seal populations is still unclear, but appears to be due to declines in their prey in combination with increased fishing and higher ocean temperatures. As a result of the loss of sea otters, increased sea urchin populations are grazing down kelp beds again.
Interestingly, a similar scenario in kelp forests in Southern California did not show immediate effects after the disappearance of sea otters from the ecosystem. This is because the system was more diverse initially. Other predators (California sheephead fish, Semicossyphus pulcher, and spiny lobsters, Panulirus interruptus) and competitors (abalone Haliotis spp) of the sea urchin helped maintain the system. However, when these predators and competitors were over-harvested as well in the 1950s, the kelp forests declined drastically as sea urchin populations boomed.
In the 1970s and 1980s, a sea urchin fishery developed which then enabled the kelp forest to recover. However, it left a system with little diversity. The interrelationships among these species and the changes that reverberate through systems as species are removed are mirrored in other ecosystems on the planet, both aquatic and terrestrial.
As this example illustrates, biodiversity is incredibly complex and conservation efforts cannot focus on just one species or even on events of the recent past.
species that are important due to their sheer numbers in an ecosystem
ecological valuethe values that each species has as part of an ecosystem
keystone speciesspecies that have important ecological roles that are greater than one would expect based on their abundance