Salmon are widely recognized for their ecological importance, navigating vast distances between ocean and freshwater environments. Their life cycle and movements contribute significantly to diverse ecosystems. This has led many to consider them a keystone species, a classification reserved for organisms with disproportionate ecological influence.
Understanding Keystone Species
A keystone species is an organism that has a large effect on its environment relative to its abundance. These species help maintain the structure of an ecological community, influencing other organisms and contributing to biodiversity. The removal of a keystone species can lead to significant changes, often resulting in a loss of diversity and altered habitat conditions.
Zoologist Robert T. Paine introduced the concept in 1969, based on his studies of marine invertebrates. For example, the ochre starfish preys on mussels, preventing them from dominating intertidal zones and allowing other species to thrive. Sea otters maintain kelp forest ecosystems by controlling sea urchin populations, which otherwise overgraze kelp, leading to habitat loss. Wolves influence herbivore populations like elk, which affects plant communities and other animal species.
Salmon’s Role in Ecosystems
Salmon connect marine, freshwater, and terrestrial environments through their life cycle, transporting nutrients and energy across these distinct habitats. Pacific salmon gain most of their body mass in the ocean before returning to rivers to spawn and die. Their bodies, rich in marine-derived nutrients, become available to freshwater and riparian ecosystems.
Their decaying bodies release substantial amounts of nitrogen, carbon, and phosphorus into nutrient-poor streams and adjacent land areas. This nutrient subsidy enhances the growth of riparian forests; studies show trees and shrubs near spawning streams can derive a notable percentage of their foliar nitrogen from salmon. This fertilization supports plant life and contributes to the productivity of aquatic food webs.
Salmon serve as a food source for a wide array of predators and scavengers. Bears, eagles, wolves, and marine mammals like seals and orcas rely on salmon for sustenance. When salmon return to rivers, bears often transport carcasses into surrounding forests, dispersing marine-derived nutrients. Juvenile salmon also provide food for other fish, birds, and small land animals in freshwater environments.
Beyond nutrient transport and food web support, salmon spawning activities physically alter streambeds. Females dig nests, known as redds, in gravel beds to deposit their eggs. This excavation process moves sediment downstream, influencing stream morphology and creating habitat for other species. These actions can lead to changes like channel deepening in headwaters and the gradual filling of pools downstream.
Consequences of Salmon Population Changes
Declines in salmon populations trigger a cascade of negative effects throughout dependent ecosystems. When fewer salmon return to spawn, the influx of marine-derived nutrients into freshwater and riparian zones is significantly reduced. This can lead to decreased soil fertility in adjacent forests, affecting plant growth and potentially shifting plant communities. The availability of essential nutrients like nitrogen and phosphorus becomes limited, impacting the overall productivity.
Dependent predators and scavengers face severe challenges with reduced salmon availability. Bears, eagles, and other animals that rely on salmon may experience starvation or reduced reproductive success. This scarcity can force predators to seek alternative food sources, leading to broader ecological imbalances. The ripple effect extends to smaller organisms, such as aquatic insects, which may become smaller or less abundant.
Changes in salmon populations also impact the physical structure and biodiversity of streams. Reduced spawning activity means less disturbance and transport of gravel, potentially altering the natural dynamics of streambeds. This can affect the availability of suitable habitat for other aquatic species that rely on salmon-created conditions. Ecosystems that have co-evolved with salmon become unstable when this species diminishes, leading to a decline in health and resilience.