Community Responses to Changing Conditions over Time: Succession 1511010 2020Humanu Pig Duck RattlesnakeTe TTuna Moth YYYeast00 3030 4040 5050 6060 7070
Number of nucleotide base differencesTimeThe organisms pictured here all share a
particular enzyme, but in the course of
evolution, mutations have resulted in
changes in the gene that codes for that
enzyme. This diagram shows the nucleotide
base differences in this gene among humans
and other organisms. Note that organisms
thought to be more closely related to humans
havefewer differences than organisms that
are more distantlyrelated to humans.c. Molecular BiologyAdapted from Figure 15.19 on p.247, in S. A. Alters and B. AltersBiology: Understanding Life, Hoboken, NJ:
John Wiley and Sons, Inc. ( 2009 ).
Interpreting Data
Based on the diagram, which organism shown is most
closely related to the duck?Community Responses to Changing
Conditions over Time: Succession
LEARNING OBJECTIVES
- Define ecological succession.
- Distinguish between primary and secondary
succession.
A
community of organisms does not spring into
existence full blown. By means of ecological
succession, a given community develops
gradually through a sequence of species.
Certain organisms colonize an area; over time,
others replace them, and eventually the replace-
ments are themselves replaced by still other spe-
cies. Ecologists first studied succession in three
diverse ecosystems: an abandoned field, a north-
ern freshwater bog, and sand dunes.
The actual mechanisms that underlie suc-
cession are not clear. In some cases, it may be
that a resident species modified the environment in some
way, thereby making it more suitable for a later species
to colonize. It is also possible that prior residents lived
there in the first place because there was little competi-
tion from other species. Later, as more invasive species
arrived, the original species were displaced.
Ecologists initially thought that succession inevitably
led to a stable and persistent community, known as a climax
community, such as a forest. But more recently,
this traditional view has fallen out of favor. The
apparent stability of a “climax” forest is probably
the result of how long trees live relative to the
human life span. It is now recognized that ma-
ture climax communities are not in a state of sta-
ble equilibrium but rather in a state of continual
disturbance. Over time, a mature community✓✓THE PLANNER
ecological
succession The
process of community
development over
time, which involves
species in one stage
being replaced by
different species.