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Lecture -6:
Ecosystem and Ecological
Communities
ENV 107: Introduction to Environmental Science
Dr. A.K.M. Saiful Islam
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
The Ecosystem
† The ecosystem concept is that the heart of the
management of natural resources.
† An ecosystem consists of several species – at least
one species that produces its own food from
inorganic compounds in its environment and one
species that decomposes the wastes of the first
species – plus a fluid medium (air, water or both).
† Two basic kinds of process must occur in the
ecosystem:
„ a cycling of the chemical elements and
„ a flow of energy.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
Basic Characteristics of Ecosystem
† Ecosystem have several characteristics.
† First, an ecosystem has structures: nonliving,
living parts.
„ Nonliving parts include rocks, water, air.
„ Living parts is the community, which is a set of
interactive species.
† Second, an ecosystem has processes, including the
two already mentioned.
† Third, an ecosystem changes over time and can
undergo development through a process called
succession.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
Food Chains and Trophic Levels
† Food Chain – the linkage of who feeds on
whom
† Food web- more complex cases of Food
chain
† Trophic Level – Ecologist group the
organisms in a food web into Trophic levels.
† Some complicated Food chains
„ A Terrestrial Food Chain
„ An Ocean Food Chain
„ The Food Web of the Harp Seal
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
A Terrestrial Food Chain
† An example, north temperate woodland food
web that existed in North America before
European Settlement and includes human
being.
„ First trophic level includes grasses, herbs,
trees.
„ Second trophic level includes mice, the pine
borer insect, and other animals.
„ Third trophic level, includes foxes, and wolves.
„ In Fourth, people would be listed
A Typical Terrestrial food web
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
A oceanic Food Chain
† In a tropical ocean ecosystem, microscopic single-cell
plankton algae are in the first trophic level.
† Small invertebrates called zoo-plankton and some fish
feed on the algae, forming the second trophic level.
† Other fish and invertebrates feed on these herbivores
and form the third trophic level. The great baleen
whales filter seawater for food, feeding primarily on
small herbivorous zooplankton and thus the baleen
whales are also in the third level.
† Some fish and marine mammals such as killer whales
feed on predatory fish and form higher trophic levels.
An Ocean Food Chain
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
The Food Web of the Harp Seal
† The harp seal is shown at the fifth level. It feeds
flatfish which feed on sand lances which feed on
euphausiids which feed on phytoplankton.
† But the harp seal feeds at several trophic levels, from
the second through the fourth, and it feeds on
predators of some of its prey and thus is a competitor
with some of its own food.
† Since, a species that feeds on several trophic levels
typically is classified as belonging to the trophic level
above the highest from which it feeds, we consider
the harp seal on the fifth trophic level.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
Food web of the Harp Seal
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
Ecosystem Types
† Ecosystem can be natural or artificial or combined
of both.
„ An artificial pond that is a part of waste
treatment plant is an example of artificial
ecosystem.
† Ecosystem can be managed or unmanaged.
„ Agriculture can be thought of as partial
management of certain kinds of ecosystem, as
can forests managed for timber production.
„ Wildlife perseverance are example of partially
managed ecosystem.
„ Zoo, is a managed ecosystem.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
The Community Effect
† Indirect and more complicated interactions are
referred to as community level interactions.
† Sea Otter and the community Effect
„ One of the preferred food of Sea otter is Sea
urchin.
„ Sea urchin feeds on Kelp, the large brown algae
that form undersea forests.
„ Sea urchin only feeds the base of kelp located at
the bottom on the beds.
† Sea otters have community level affect because it
effects the growth of kelp which is a inhabitant for
many other species.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
(a) Sea Otter on Kelp
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
(b) Abundant Sea Otters
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
(c) Sea Urchins are abundant
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
Holistic View of the community
† In any community, species association with each
other are not constant; they vary from site and they
also change over time.
† Nevertheless, every additional species in a community
adds more species interactions: it may be prey or
predator or contribute to chemical cycling: it may be
partner in a symbiosis or a competitor with other
species.
† In this sense, even in a more realistic individualistic
view of the community, the whole is indeed more than
just sum of its parts. This view is often called the
holistic view of the community.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam
Watershed
† A common used practical delineation of the
boundary of an ecosystem on land is the
watershed.
† A watershed is defined most simply as follows:
within a watershed, any drop of rain that
reaches the ground flows out in the stream.
† Topography (the lay of ground) determines the
watershed.
† When water shed is used to define the
boundaries of an ecosystem.
ENV 107:Introduction to Environmental Science © Dr. Akm Saiful Islam