BIOLOGY 1M03 Study Guide - Final Guide: Keystone Species, Pisaster, Primary Succession

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Biology Exam Notes
Chapter 53
Mutualisms
Are +/+ interactions that involve a wide variety of organisms and rewards.
Most critical of all mutualisms occur between bacteria that fix nitrogen and certain species of
plants.
Mutualism benefits both species, but the interaction does not involve individuals from different
species being altruistic.
The benefits received are a by-product of each individual pursuing its own self-interest by
maximizing its ability to survive and reproduce.
Keystone Species Structure Communities
The structure of a community can change dramatically if a single species of predator or
herbivore is removed from or added to a community.
A keystone species is a species that has a much greater impact on the surrounding species than
its abundance would suggest.
For example, the sea star Pisaster is a keystone species in some intertidal areas. When Pisaster
was removed from experimental areas, the number of species present and the complexity of the
habitat changed radically.
Disturbance and Change in Ecological Communities
Community composition and structure may change radically in response to changes in abiotic
and biotic conditions.
A disturbance is any event that removes some individuals or biomass from a community.
The important feature of a disturbance is that it alters some aspect of resource availability.
The impact of disturbance is a function of 3 factors: type of frequency, frequency of disturbance
and severity of disturbance.
Most communities experience a characteristic type of disturbance; in most cases disturbances
occur with a predictable frequency and severity. This is called a community’s disturbance
regime.
Ecologist’s use 2 approaches to determine the pattern of disturbance: inference of long-term
patterns from data obtained in short-term analysis and reconstruction of the history of a
particular site.
Biologists are now better able to manage these forests by allowing, monitoring and controlling
burns in them.
The Development of Communication after Disturbance
Succession is the recovery that follows a severe disturbance
Primary succession occurs when a disturbance removes the soil and its organisms, as well as
organisms that live above the surface.
Secondary succession occurs when a disturbance removes some or all of the organisms from an
area but leaves the soil intact.
The specific sequence of species that appears over time is called the successional pathway.
Biologists focus on 3 factors to predict the outcome of succession in a community: the traits of
the species involved, how species interact; and historical and environmental circumstances.
Pioneering species are the first organisms to arrive at a newly disturbed site. They have good
dispersal ability, being able to tolerate severe abiotic conditions, and high reproductive rates.
However, they have little competitive ability.
During succession, existing species can have one of the 3 effects on subsequent species:
facilitation, tolerance or inhibition.
Facilitation occurs when early-arriving species make conditions more favourable for the arrival
of certain later species.
Tolerance happens when existing species do not affect the probability that subsequent species
will become established.
Inhibition occurs when the presence of the one species inhibits the establishment of another.
Species Richness in Ecological Communities
Species richness is the number of species present in a given community.
Species diversity is a weighted measure that incorporates a species’ relative abundance, as well
as its presence or absence.
Larger patches of habitat generally contain more species than do smaller patches; however,
islands in the ocean have smaller numbers of species than do areas of the same size on
continents.
The number of species present on an island is a product of just 2 events: immigration and
extinction.
Species richness varies as a function of island characteristics: the number of existing species, the
island size and the remoteness of the island.
The model is called the theory of island biogeography.
Communities in the tropics have more species than communities in temperate or subarctic
environments. In general, as latitude increases, species diversity decreases.
To explain this pattern, biologists have had to consider 2 principles: the causal mechanism must
be abiotic, the species diversity of a particular area is the sum of four processes: speciation,
extinction, immigration and emigration.
Several key hypotheses have been suggested:
(1) The high- productivity hypothesis proposes that high productivity promotes high diversity.
(2) The energy hypothesis contends that high temperature increases productivity and the
likelihood that organisms can tolerate the physical conditions in a region.
(3) The time hypothesis argues that the tropical regions have had more time for speciation than
other regions.
(4) The intermediate disturbance hypothesis states that regions with a moderate type,
frequency and severity of disturbance should have high species richness and diversity.
Each of these factors may influence diversity, but no single hypothesis offers a convincing
explanation for the global diversity gradient.

Document Summary

Are +/+ interactions that involve a wide variety of organisms and rewards. Most critical of all mutualisms occur between bacteria that fix nitrogen and certain species of plants. Mutualism benefits both species, but the interaction does not involve individuals from different species being altruistic. The benefits received are a by-product of each individual pursuing its own self-interest by maximizing its ability to survive and reproduce. The structure of a community can change dramatically if a single species of predator or herbivore is removed from or added to a community. A keystone species is a species that has a much greater impact on the surrounding species than its abundance would suggest. For example, the sea star pisaster is a keystone species in some intertidal areas. When pisaster was removed from experimental areas, the number of species present and the complexity of the habitat changed radically. Community composition and structure may change radically in response to changes in abiotic and biotic conditions.