Multiple- species interactions

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Multiple- species interactions
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Food Webs
Studies of food webs trophic (energy
nutrition) relationships among organisms in all
or part of a community originated with Elton
(1927)
Nodes Taxonomic or functional categories
Links Pathways indicating the flow of energy or
nutrients
Paine, R. T. (1966) Food webs are the
ecologically flexible scaffolding around which
communities are assembled and structured
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Food Webs
Elton observed that predators tended to be larger
and less numerous than their prey described as
the pyramid of numbers (a.k.a. Eltonian
pyramid)
Elton hypothesized that this occurred because
predators are generally larger than prey to
subdue them
Pyramid could represent numbers, biomass, energy
consumed per year, etc.
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Food Webs
Lindeman (1942) developed some of the modern
concepts of food webs and introduced the energy
efficiency hypothesis the fraction of energy
entering one trophic level that passes to the
next higher level is very low (5-15)
The first and second laws of thermodynamics
predict inefficient transfer of energy between
trophic levels 1st Law Conservation of
Energy 2nd Law All energy transformations
result in an increase in entropy, i.e., only a
fraction of the energy conserved under the
first law is available to do work within the
system of interest
Inverted pyramids of biomass can occur (e.g.,
whales, krill, phytoplankton in southern oceans),
but only when productivity and turnover of
producers is extremely high
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Food Webs
Levin, S. A. (1992) Is a taxonomic subdivision
most appropriate, or would a functional one serve
better? Should subdivision stop at the species
level, consider different demographic classes, be
partitioned according to genotype, etc.?
2? Consumers
Green, living food web
1? Consumers
Trophic levels within a simple food chain donor
levels supply energy or nutrients to recipient
levels
1? Producers
1? Consumers
Brown, detrital food web
2? Consumers
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Food Webs
Web jargon Connectance (c) Number of links
(L) or connections between species (S) or nodes
that exist in the community expressed as a
proportion of maximum connectance c L /
S(S-1)/2 Maximum connectance S(S-1)/2
Linkage density (L/S) Average number of trophic
links per species
Compartmentation Extent to which a food web
contains relatively isolated subwebs expressed
as the number of species that interact with any
given pair of species versus those that interact
with only one of the pair
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Food Webs
Web jargon Omnivory Feeding on prey of more
than one trophic level
Same-chain omnivory
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Food Webs
Web jargon Omnivory Feeding on prey of more
than one trophic level.
Same-chain omnivory
Different-chain omnivory
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Food Webs
Web jargon Cycles and loops Connectance
patterns when species have reciprocal feeding
relationships
Cycle Species A eats B, and B eats A (e.g.,
wasps that prey on spiders that in turn catch
wasps in their webs)
A
B
Loop Species A eats B, B eats C, and C eats A
A
B
C
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Food Webs
Web jargon Circuit properties Measured as
overlaps in prey consumption among predators
Predators
Circuit properties describe the overlap in prey
consumption among predators. If every series of
three predators completes a triangle of line
segments, the predator overlap graph is said to
exhibit the rigid circuit property
Prey
Predator overlap graph that nearly exhibits the
rigid circuit property
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Food Webs
Modeling food webs What kind of food web
configurations promote stable equilibrium
population dynamics of species?
May (1973) and Pimm Lawton (1977, 1978) used
multispecies Lotka-Volterra models to determine
whether particular configurations of food webs
are stable
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0 no connection no interaction positive
effect mutualism or prey supplying
energy to predator - negative effect
predation Values corresponded to interaction
strengths
3
2
1
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Food Webs
Simulations generally examine the influence of
small changes in predator and prey populations
away from equilibrium values
Two criteria for assessing stability Do
populations return to equilibrium sizes? How
long does the system take to return to
equilibrium?
Different connection patterns of species in a
food web can be modeled using different values in
the matrix
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Food Webs
Results of these models are difficult to
interpret! The way in which the matrices are
constructed, which reflects the lengths of food
chains, connectedness, etc., determines the
degree to which food webs are stable
Real-world data are needed for connectedness and
interaction signs and strengths to determine
which food-web models will make the most accurate
real-world predictions This is a difficult, if
not impossible task! Most empirical food web
studies have therefore been done on simple,
short-lived species
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Food Webs
Do food webs yield repeated patterns? If so, do
the patterns have ecological significance?
Patterns are often subject to interpretation
because diagrams of webs are almost always
abstractions of actual feeding relationships,
e.g., Paines (1966) seven functional groups
represent 300 species
In spite of this and various other reservations
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Food Webs
Are ratios of species at different trophic levels
constant across communities?
Cohen (1978) reviewed published community webs
and found that they yielded ratios of predators
to prey of 43 (i.e., more predator species than
prey
This may simply reflect greater lumping of prey
into functional groups than predators by the
authors of the published studies
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Food Webs
How long are food chains?
As expected, they are relatively short, rarely
more than 5 species in length (Pimm Lawton
1977 Pimm 1982)
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Food Webs
Yodzis (1984) devised an insightful test of the
influence of energy efficiency on food-chain
length
Examined 34 published food webs (Briand
1983) Compared food-chain length for chains
dominated by invertebrate ectotherms vs.
vertebrate ectotherms vs. vertebrate endotherms
at trophic height 2
Invert. ectotherms are about an order of
magnitude more efficient at energy conversion
than vert. endotherms, and vert. ecotherms are
intermediate 23 of invert. ectotherms support a
consumer vs. 9 vert. ectotherms vs. 6
endotherms
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Food Webs
Jenkins et al. (1992) direct test of the
trophic efficiency hypothesis
If efficiency of energy transfer primarily
determines food chain length, then manipulating
productivity should influence food chain length
Placed plastic buckets in an Australian forest as
artificial water-filled tree-holes with
different amounts of litter added to generate a
productivity gradient
Natural tree-holes contain 4-level trophic
chains Litter -- mosquito larvae -- larvae of
predatory midge -- tadpoles Litter at natural
level (938 g/m2/yr), 10 natural level, 1
natural level Well-replicated study tracked for
48 wk
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Food Webs
Jenkins et al. (1992) direct test of the
trophic efficiency hypothesis
Decreased productivity resulted in decreased
number of coexisting species, decreased number of
trophic links, and decreased food-chain
length Averages after 48 weeks Natural litter
levels 4 spp, 5 links, 2.75 spp chains 10
litter 3 spp, 3 links, 2.5 spp chains 1
litter 1.5 spp, 1.5 links, 2 spp chains
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Food Webs
Modeling suggested that cycles, loops, and
omnivory would destabilize food webs Do cycles
and loops occur in nature? Is omnivory common?
Polis (1991) a skeptic of food web theory
characterized desert food webs in great detail
Two-species cycles and three-species loops occur,
and are especially common in communities
characterized by size-dependent predation Role
reversals between predators and prey are not
uncommon Omnivory is quite common
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Food Webs
The distribution of interaction strengths was
very important for determining modeling outcomes.
How are interaction strengths distributed in
nature?
Unlike the randomly defined interaction strengths
of earlier modeling approaches, interaction
strengths are not normally distributed they are
heavily skewed toward weak interactions This
shows that evaluating interaction strength and
not merely trophic links is essential to
understanding population dynamics and stability
within food webs
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Interaction Webs
We have been discussing food webs, but we can
broaden the concept to include all types of
interactions interaction webs
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Pairwise species interactions direct effects
Influence of species A
- (negative)
(positive)
0 (neutral/null)

-
0
A
B
A
B
A
B
-
-
-
-
Antagonism (Predation/Parasitism)
Competition
Amensalism
-

0
A
B
A
B
A
B
Influence of Species B
0
0
0
0
Neutralism (No interaction)
Amensalism
Commensalism
-

0
A
B
A
B
A
B




Antagonism (Predation/Parasitism)
Mutualism
Commensalism
From Abrahamson (1989) Morin (1999)
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Multiple-Species Interactions Direct
Indirect Effects
Dissecting exploitation competition reveals its
indirect nature
H
H
-
-
-


P
Solid arrows indicate direct effects, dotted
arrows indicate indirect effects
From Menge (1995)
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Multiple-Species Interactions Direct
Indirect Effects
Tri-trophic Interaction or Trophic Cascade
Apparent Competition
C
H
-

-
-
H




P
P
-
-

P
Solid arrows indicate direct effects, dotted
arrows indicate indirect effects
From Menge (1995) Morin (1999)
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Multiple-Species Interactions Direct
Indirect Effects
Habitat Facilitation
Keystone Predation

H
H
H
-
-

• (e.g., inhibits
• feeding)

P
P
P
-
Solid arrows indicate direct effects, dotted
arrows indicate indirect effects
From Menge (1995) found 83 distinct types of
indirect interactions in 23 communities
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Multiple-Species Interactions Direct
Indirect Effects
Indirect Commensalism
Indirect Mutualism

H

H
H
H
-

-

-


-
-

P
P
P
P
-
-
Solid arrows indicate direct effects, dotted
arrows indicate indirect effects
From Menge (1995) found 83 distinct types of
indirect interactions in 23 communities
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Multiple-Species Interactions Direct
Indirect Effects
Interaction chain indirect effect Interaction
modification indirect effect
Interaction w/o indirect effects
H
-
P
Wootton (1993)
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Multiple-Species Interactions Direct
Indirect Effects
Interaction chain indirect effect effects
mediated by changes in the abundance of an
intermediate species Interaction modification
indirect effect
Interaction w/o indirect effects
Interaction chain
-
H
H
H
-
-

P
P
Wootton (1993)
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Multiple-Species Interactions Direct
Indirect Effects
Interaction chain indirect effect effects
mediated by changes in the abundance of an
intermediate species Interaction modification
indirect effect e.g., donor species changes the
per capita effect of the transmitter species on
recipient (e.g., limpets, barnacles and
oystercatchers when barnacles are present,
limpets are harder to find)
Interaction w/o indirect effects
Interaction chain Interaction modification
-
-
H
H
H
H
H
-
-
-


P
P
P
Wootton (1993)
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Multiple-Species Interactions Direct
Indirect Effects
Distinguishing between interaction chain indirect
effects and interaction modification indirect
effects is important because they make different
predictions for responses to perturbation
Interaction chains Should be predictable if you
know how each pair of species interacts
directly Interaction modification Often not
predictable a priori
Because many indirect effects are not readily
predictable, and only become apparent as
unpredicted results of manipulative
experiments, we may have an incomplete idea of
their importance Furthermore, an indirect effect
may not be noticed at all if it acts counter to a
strong direct effect (e.g., partially canceling
it out)
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Multiple-Species Interactions Direct
Indirect Effects
Apparent competition Schmitt (1987) Prey
species Sessile bivalves (Chama and Mytilus)
are filter feeders that occur mostly in crevices.
Gastropods (Tegula and Astraea) occur on rock
surfaces and graze algae Limited opportunities
for direct competition between bivalves and
gastropods since neither diet nor space
requirements overlap greatly Common predators
Lobsters, octopi, whelks
Experiment Continually transplant bivalves to
sites with high densities of gastropods to
maintain high densities of bivalves in those
sites Predictions Predator density will
increase, gastropod density will decrease
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Multiple-Species Interactions Direct
Indirect Effects
Apparent competition Schmitt (1987) Found
higher predator density and a decline in
gastropod density over time relative to control
sites
Control sites
Sites with added bivalves
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Multiple-Species Interactions Direct
Indirect Effects
Indirect mutualisms and commensalism Dodson
(1970) noted that communities found in small
alpine ponds fall into two groups 1. Ponds
containing larval salamanders (Ambystoma) that
feed primarily on larger zooplankton and
planktivorous midges (Chaoborus) that feed on
small zooplankton (that dont normally coexist
with the large zooplankton) 2. Ponds with only
Ambystoma (ponds with only Chaoborus did not
occur) Hypothesis Size selective predation of
plankton by Ambystoma maintains the feeding niche
of Chaoborus
Hypothesis Size selective predation of plankton
by Ambystoma maintains the feeding niche of
Chaoborus
Results Removal of Ambystoma resulted in a
shift in body size of plankton and a decline in
Chaoborus abundance in the single pond that could
be manipulated (Giguere 1979)
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Multiple-Species Interactions Direct
Indirect Effects
How important are indirect effects?
Menge (1995) reviewed 23 experimental studies of
rocky intertidal habitats that were sufficiently
well replicated and long enough in duration for
indirect effects to become evident Considered
only ecologically significant effects (that
caused at least a 10 change in the abundance of
one or more species)
Found that indirect effects accounted for 40 of
the observed changes in community structure
caused by manipulations (e.g., predator or prey
removal) Most of the indirect effects were cases
of keystone predation (35) and apparent
competition (25) Exploitative competition
constituted only 3 of indirect effects!
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Multiple-Species Interactions Direct
Indirect Effects
A likely example of a trophic cascade through
prey consumption McLaren and Peterson
(1994) 500 km2 Isle Royale National Park in Lake
Superior Producer Balsam fir Herbivore
Moose (59 of winter diet of moose is
fir) Carnivore Wolf (colonized island in
1959) Measured growth rings on fir saplings over
30 yr cycles of growth suppression occur in
synchrony with changes in moose densities, but
not with climatic fluctuations
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Wolf population fluctuation 1960 - 1990
Moose population fluctuation
Fir tree mean ring width (mm) western Isle Royale
Fir tree mean ring width (mm) eastern Isle Royale
Actual evapotranspiration
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Multiple-Species Interactions Direct
Indirect Effects
Adding behavioral ecology to trophic cascades
How does the change in behavior of a top predator
cascade through a community? E.g., wolf
behavioral response to climate (Post et al.
1999) On Isle Royale, fluctuations in North
Atlantic Oscillation (NAO) result in changes in
winter snow accumulation Annual aerial surveys
of wolves show close correlation between
wolf pack size and the state of the NAO
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Multiple-Species Interactions Direct
Indirect Effects
Lower NAO index greater snow depth positive
correlation with larger pack sizes
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Multiple-Species Interactions Direct
Indirect Effects
Kill rate per pack (and per individual wolf)
increases with wolf pack size
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Multiple-Species Interactions Direct
Indirect Effects
Climatic effects on hunting behavior cascade,
influencing moose population size...
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Multiple-Species Interactions Direct
Indirect Effects
and fir tree growth... Declines in moose density
reduce browsing intensity the year after heavy
snow
Changes in wolf behavior have ecosystem level
effects in this community because moose
dramatically influence net primary production,
litter production and edaphic nutrient dynamics
(Post et al. 1999)
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Multiple-Species Interactions Direct
Indirect Effects
Trophic cascade without prey consumption
(Beckerman et al. 1997) Carnivores can affect
the impact of herbivores on producers in 2
ways 1) Interaction chain Direct consumption
of herbivores 2) Interaction modification
Behavioral mediation, i.e., the mere presence of
predators could alter prey foraging behavior by
reducing herbivore feeding time, or by inducing a
shift in herbivore diet selection Study in
old-field community in Connecticut
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Multiple-Species Interactions Direct
Indirect Effects
Constructed 3 trophic levels Grasses and herbs,
generalist leaf chewing grasshopper (Melanoplus
femurrubrum), and hunting spider (Pisaurina
mira) Problem How to create a predator that
does not have the capacity to consume prey, but,
can display hunting behavior or signal risk to
its prey? Solution Glue the spiders mouth
parts together! - No effect on spider
hunting behavior, except the spiders cannot
capture, kill and consume prey - Spiders
can survive up to 2 months with glued mouth
parts Experiment Test whether indirect effect
of predators on prey arises from density or
behavioral responses in the herbivore population
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Multiple-Species Interactions Direct
Indirect Effects
Made mesh enclosures for 3 trophic groupings
- Plants only - Plants juvenile or adult
grasshoppers - Plants grasshoppers glued
or unglued spiders Experiment was run from Aug.
to Oct. over 2 yr
Results Grasshopper densities with / without
spider treatment were not significantly
different Significant positive effect of spiders
on grass biomass was consistent with a trophic
cascade treatments containing spiders had
significantly less herbivore damage than
treatments with just grasshoppers No significant
difference between glued and unglued spiders on
grass biomass
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Multiple-Species Interactions Top-Down vs.
Bottom-Up Effects
Are abundances and distributions of organisms
controlled by resources (bottom-up processes) or
by predation and disease (top-down processes)?
Bottom-up view Organisms at each trophic level
are food limited Top-down view Top level is
food limited, lower levels are alternately
predator vs. food limited (originated with
Hairston, Smith Slobodkin 1960 HSS)
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Multiple-Species Interactions Top-Down vs.
Bottom-Up Effects
Hunter and Price (1992) argue that we should
always start with a bottom-up template the
removal of higher trophic levels leaves lower
levels present (if perhaps greatly modified),
whereas the removal of primary producers leaves
no system at all Echoed in John McPhees (1998)
Annals of the Former World, pg. 84 Break the
food chain and creatures die out above the link
Fretwell (1977) Oksanen et al. (1981) OFAN
proposed a reconciliation by suggesting that
productivity determines the number of trophic
levels that can be supported in a community
plant productivity therefore ultimately dictates
when top-down forces could cascade back down
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Multiple-Species Interactions Top-Down vs.
Bottom-Up Effects
Dyer Letourneau (2003) is an example of using
top-down and bottom-up thinking to examine the
controls on diversity at different trophic levels
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Multiple-Species Interactions Keystone species
Keystone predator a predator whose activities
maintain species diversity at lower trophic
levels by disallowing competitive exclusion
(Paine 1966)
Keystone resource first applied to plant
species that played prominent roles in sustaining
frugivores through periods of general food
scarcity in tropical forests, e.g., figs
(Terborgh 1986)
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Multiple-Species Interactions Ecosystem
engineers
Ecosytem engineer an organism that creates,
modifies, or maintains habitat (or microhabitat)
by causing physical state changes in biotic or
abiotic materials that, directly or indirectly,
modulate the availability of resources to other
species (Jones et al. 1994)
In other words, the ecosystem engineer has a
large impact beyond simply assimilating and
dissimilating material The definition is
especially useful when applied to organisms that
modify the environment through means other than
trophic activities
Ecosystem engineers may be physical ecosystem
engineers or chemical ecosystem engineers
Ecosystem engineers may be allogenic or
autogenic
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Multiple-Species Interactions Ecosystem
engineers
Allogenic engineer organisms that change the
environment by transforming living or nonliving
materials from one physical state to another, via
mechanical or other means (Jones et al.
1994) E.g., beavers
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Multiple-Species Interactions Ecosystem
engineers
Autogenic engineer organisms that change the
environment via their own physical structures,
i.e., their living and dead tissues (Jones et al.
1994) E.g., long-leaf pines