Chapter 5 - Biodiversity, Species Interactions, and Population Control
Core Case Study: Southern Sea Otter
Southern sea otters, an endangered species, live in giant kelp beds in a small area of
coastal California. These otters use stones to crack open shellfishes from underwater rockbeds,
eating ¼ their weight daily in clams, mussels, crabs, urchins, and abalone. In the early 1900s,
sea otters were hunted almost to extinction for their fur and to stop competition with humans for
abalone. They have increased from 50 otters in 1937 to over 3,000 in 2007. Why should we be
concerned about this species? 1) They generate millions of dollars a year in ecotourism dollars;
2) Ethics; many people believe it is wrong to cause the premature extinction of a species; and 3)
. The otter’s diet restricts sea urchins and other kelp-eating
species, allowing development of kelp habitats that support large numbers of organisms.
Species interact through:
: when two or more species interact to gain access to the same
, such as food, light, or space.
: when one species, the predator, feeds directly on another species, the prey.
: when one organism, the parasite, feeds on the body of another organism, the
host, usually by living on or in the host.
: when the interaction of two species is beneficial to both, providing food, shelter,
or other resource.
: when the interaction benefits one species, while having little effect on the
These interactions affect the resource base, population size, and natural selection process
(survival and reproduction) of organisms in an ecosystem.
resources is the most common interaction. Evolving better abilities to
acquire food or other resources over another species is the most common aspect of
competition, not direct fighting.
A species’ unique role in an ecosystem (or the resources it
uses, remember the n-dimensional hypervolume) is its ecological niche
, some species are
, with broad niches, and some are specialists
, with very narrow niches, at least along
one niche dimension.
No two species can occupy the exactly same ecological niche, a concept
known as competition exclusion principle
Humans compete with many species for food and
habitat as we convert the earth to our uses and degrade, destroy, or fragment natural habitats.
are plants and phytoplankton, which make their own food by
(feed on plants), carnivores
(feed on the
flesh of other animals), omnivores
(feed on plants and animals), and decomposers (detritivores
and saprovores, which feed on detritus and dead bodies of organisms).
interactions can include grizzly bears feeding on cutworm moths, but usually involve pursuit and
, such as a cheetah running down prey.
Predators use camouflage and chemical