Download Lecture23_2011_foodwebs

23rd Lecture – November 22, 2011
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I will post the last assignment in the next few days.
Assignment 3 is due on Tuesday.
-- There are no seminars for this week.
IV. Community ecology
A. What is community ecology?
B. How do we quantify communities?
1. Tools of community ecology
a.
b.
c.
d.
growth form and structure
diversity or species number
dominance/ relative abundance:
rank abundance curves
Most of the above stuff is best for guilds, but may not be
informative about entire communities.
e. food or trophic webs
In this food web of the
community that occurs in
the water filled leaves of
pitcher plant, the top
predator is a mosquito
larvae (W. smithii) that eats
rotifers (H. rosa) and
protozoa. The rotifers and
protozoa in turn eat
bacteria, as do the mites (S.
gibsoni). The bacteria are
basal species, as they
consumer dead ants in this
brown food web. There are
a couple of more species of
flies, whose larvae also feed
directly on the ants.
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Top predators -- species that get eaten by
nothing else in the food web
Basal species -- species that feed on nothing
within the web (usually plants)
Omnivores -- species that feed at more than
one trophic level
Trophic species -- groups of species that have
the same predator and prey
Cannibalism -- a cycle in which a species
feeds upon itself
Connectance -- number of actual interactions
divided by the number of possible interactions
Compartments -- suites of species with strong
linkages among group members but weak
linkages to other species
Green (plant-based) vs. Brown (detritus-based) food webs and their linkages
Bob Paine worked on intertidal food webs in Washington state. In this example, he
considers a suite of diatoms and algal species that are consumed by urchins and mollusks
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--Acmaea pelta is a limpet, simple cone-shaped snails
--Strongulocentrotus purpuratus is the purple sea urchin
--Katharina and Tonicella are chitons, primitive molluscs
with plates
--Alaria is a brown algae (seaweed)
--Hedophyllum is a similar brown algae, called “sea cabbage”
--Corralina and Bossiella are coralline red alga
--Lithothamnium is another red algae.
IV. Community ecology
….
e. food or trophic webs
Robert Paine suggests there are three kinds of food webs:
connectedness webs emphasize the feeding relationships among
organisms
energy flow webs represent an ecosystem viewpoint in which
energy flow between resource and consumers is emphasized
functional webs the importance of each population in maintaining
the integrity of the community reflects
connectedness webs emphasize the feeding relationships among organisms
energy flow webs show energy flow between resource and consumers
functional webs links that maintain the integrity of the community
Note in particular, the large effect of S. purpuratus on
Hedophyllum, even though it doesn’t eat it! This is due to
indirect effects of the urchin Strongulocentrotus in eating
competitors of the algae Hedophyllum. So, removing
urchins results in a dramatic increase in the algae.
IV. Community ecology
….
e. food or trophic webs
Many ways to quantify food webs include:
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food chain length (number of trophic levels) corn - pigs – man
-- ratios of mass or energy between trophic levels (10%
biomass rule of thumb)
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connectence (# actual links / # of possible links)
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compartmentalization (subdivision)
-- ratios of mass or energy between trophic levels (10%
biomass rule of thumb)
(By the way, bleak
are small fish in the
cyprinid family)
IV. Community ecology
….
e. food or trophic webs
Many ways to quantify food webs include:
--
food chain length (number of trophic levels) corn - pigs – man
-- ratios of mass or energy between trophic levels (10%
biomass rule of thumb)
--
connectence (# actual links / # of possible links)
--
compartmentalization (subdivision)
Compartmentalization example
Do food webs consist of clear
trophic levels with simple
connections?
Gary Polis’ research where
he attempted to construct
the full food web for a
“simple” real community in
a California desert. This is
only part of the food web.
He argued that food webs
were way too complicated to
apply simplistic measures
such as compartments or
even trophic levels.
So, as Polis suggests, are food webs too complicated to be viewed as simple chains of
interactions with discrete trophic levels? Maybe . . but, remember Paine’s three types of
food webs. If we consider the energy flow or functional foodwebs, each of which considers
the strengths of the interactions, then many of the links in Polis’ complicated foodwebs will
disappear. So, we generally feel that trophic levels still have some meaning and that,
functionally, communities are really quite as complex as suggested by Polis.
Gilbert, et al. found that:
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connecting patches increased alpha richness(within
patches).
connecting patches homogenized richness across
patches, decreasing beta richness.
gamma richnessincreases with alpha richness, despite the
decreased beta richness. So, corridors help preserve
richness.
predators are more sensitive to fragmentation that
consumers.
IV. Community ecology
A. What is community ecology?
B. How do we quantify communities?
1. Tools of community ecology
2. the organization of communities - are communities a natural
unit of organization?
a. Yes - this was the view of F. Clements who thought of communities
as discrete units with sharp boundaries (superorganism view resulting in a closed
community). A species within a community can be thought of as an organ inside
an organism -- its functioning and dynamics cannot be understood without
considering the whole organism
b. No- Gleason and Cooper viewed communities as a chance or
fortuitous association of organisms whose adaptations enabled them to life
together under the particular physical and biological conditions found at the
particular location (individualistic view leading to an open community). (similar to
Redundancy Model in Krebs)
Clement’s superorganism,
closed communities
Individualistic, open
communities
Idealized Niche communities
with resource partitioning
Resource partitioning with
several strata
IV. Community ecology
2. the organization of communities - are communities a natural
unit of organization?
a. Yes - Clements’ superorganism, closed, communities
b. No- Gleason and Cooper’s individualistic, open
communities
c.
Evidence
-continuity and discontinuity of stands
-gradient analysis, ordination (Robert Whittaker)
Species populations of
forest trees along a
moisture gradient in the
Siskyou Mountains,
Oregon (above) and the
Santa Catalina Mountains,
Arizona. (below)
IV. Community ecology
2. the organization of communities - are communities a natural
unit of organization?
a. Yes - Clements’ superorganism, closed, communities
b. No- Gleason and Cooper’s individualistic, open
communities
c. Evidence
-- continuity and discontinuity of stands
-- gradient analysis and ordination show individualistic
-- dynamic relations between species populations: at some
level, species must be organized by obligate interrelationships.
To some degree, predators depend on certain prey or plants depend on
certain pollinators, etc., which requires that species distributions are nonindependent. So, both “superorganism” and “individualistic” views may
apply, although most patterns suggest individualism.
Remember, you were supposed to have read this article:
Study Guide Items from Lecture 23
Terms:
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Top predators
Basal species
Omnivores
Connectance
Food chain length
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cannibals
Trophic levels
Trophic species
Compartmentalization
Brown food webs
Concepts:
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Brown (based on detritus) vs. green (based on plants) food webs
Paine’s three types of foodwebs (connectedness, energy, and functional)
Ways to quantify and compare foodwebs (e.g., food chain length, % predators)
Loss of mass as one goes up trophic levels and 10% rule of thumb
Polis’ view that communities are too complex to identify trophic levels
Case Studies:
•Pitcher plant food webs
•Paine’s intertidal system (we will talk about this again with keystone predation)
•Gilbert et al.’s study of organisms found in moss patches with and without corridors (in
particular, note how corridors affected alpha, beta, and gamma diversity.