Community Structure Optional Reading: Fish interact with bears!
The optional reading for Professor Allan’s “Community Structure” lecture is a paper Called Anadromous Fish as Keystone Species in Vertebrate Communities, by Wilson and Halupka. It appeared in the journal Conservation Biology in 1995.
The article is about interactions between aquatic and terrestrial communities in the Southwest coast of Alaska. The authors review the literature and suggest that the interactions are highly important to ecological analyses for conservation of both fish and terrestrial vertebrates yet there is a lack of studies that quantify these interactions.
Anadromous fish are fish that return from the sea to fresh water in order to spawn—the most well known being salmon. The article suggests that the biology of vertebrate predators is closely linked to their anadromous fish prey. It argues that the overharvest of fish stocks could impact the population biology of terrestrial predators such as bears, otters and eagles.
This is a poorly studied case and most previous studies have been from the perspective of fisheries biology. Papers have considered terrestrial fish predators as competition to humans. A fisheries mindset shapes how science looks at salmon populations. One study suggests killing mergansers (a duck-like waterfowl that eats Atlantic Salmon, even when it was clear that the real reason for fish decline was overexploitation by humans. Although a shift in perception to include fish-wildlife interactions complicates the system, doing so could bring things out that were previously unseen. Think about the complex systems literature we’ve read for both 509 and 510.
There are 15 species of anadomous fish on the coast of the Pacific Northwest, including seven species in the Salmon family but also species of lamprey and smelt. Several species often spawn in the same space and time but the diversity and abundance varies highly in time and space, and from year to year. High variability of prey density is reflected in opportunistic foraging of many wildlife consumers. But in many places the presence of anadomous fish species is reliable enough to factor into terrestrial species’ biology.
In the region of study in Southwest Alaska, all species of carnivorous mammal and many birds take advantage of anadomous fish as a food source. Bears, wolves, red fox, and mink are among mammalian predators while bird predators include Bald Eagles, Red-tailed Hawks, and Winter Wren. Juvenile fish are eaten by otters and birds. Many species of fish die shortly after spawning (called semelparous), and their carcasses are eaten, even by herbivores like squirrels and deer! Fish eggs are eaten by many bird species.
Few studies have been done on the impacts of fish populations on animal biology. It is often supposed that the availability of anadromous fish to the coast-dwelling brown bear results in its significant larger size than the inland grizzlies. Salmon fat content has been shown to boost bear lipid stores during torpor as well as reproductive success in females. Similarly, salmon consumption is shown to aid wintering Bald Eagles. During river otters’ energetically costly lactation periods juvenile salmon provide an important food source. Marine mammals often follow anadromous fish far upriver; beluga whales, seals, and sea lions have been found hundreds of kilometers upstream, having followed the salmon run.
The paper suggests that animal distribution and breeding biology may alter in response to abundance of anadromous fish. Bear activity often correlates with density of fish in a stream. Similarly, eagle numbers correlate with availability of fish. Predators also affect prey distribution and behavior, for example the Arctic charr has a trade-off between foraging and predation risks.
Fisheries have potential to seriously alter anadromous fish distribution. Fisheries are managed regionally and in many cases salmon are harvested before they reach their spawning grounds. Small-stream populations are at particular risk of extirpation, which this paper postulates is harmful for three main reasons: 1) fish are easier for predators to catch when in small, shallow streams; 2) existence of fish species at multiple tributaries enables intraspecific spacing among predator populations (eg. Newly independent bear cubs and females with cubs would not forage near male bears). 3) over a larger scale, affecting temporal and spatial availability of fish could not match predator biology and life history.
This is an interesting preliminary study, but I would have to agree with the authors that quantitative research is necessary.