Earlier

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Chapter 53 COMMUNITY ECOLOGY
Earlier Unlimited populations grow
exponentially.
Density-dependent regulation of demographic
rates (b d) limits population growth (r or
mu) if the pop growth rate (mu) is low, N
approaches a carrying capacity K, or if pop
growth rate is higher, dynamics get cyclic
and even chaotic.
Next begin building community ecology by looking
at how pair-wise interactions with other
species influence demographic rates (bd)
commensalism (/0, skip this) interspecific
competition (-/-) predation/parasitism/herbivory
(/-) mutualism (/)
Then consider existence and properties of
multispecies communities.
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A community is any assemblage of populations in
an area or habitat.
How can we account for the species found together
as members of a community? Two different views on
this question emerged among ecologists.
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The exponential growth model dN/dt N r
How can we add a little more realism to this
model by incorporating Interspecific
Competition?
Suppose we can measure the impact of 1
individual of species 2 on the pop growth rate of
species 1 relative to the impact of 1
additional individual of species 1?
Call this rate of substitution the interspecific
competition coefficient a12 (the
intraspecific coefficients a11 a22 are
implicitly 1)
If we do the math we find stable coexistence of
both species 1 2 only if a12 lt 1 a21 lt
1,meaning intraspecific competition gt
interspecific competition, implying diff
species use resources differently different
niches.
Otherwise, expect competitive exclusion of one of
the species.
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When grown separately each species does
fine (note P. aurelia has smaller r but
larger K)
When grown in competition, P. aurelia drives P.
caudatum out of the system
Competitive exclusion is expected from details of
L-V competition model, but there is a simpler
interpretation (from D.Tilman)
Note that P. aurelia has a bigger carrying
capacity K than P. caudatum.
This means that each P aurelia can survive
reproduce w/ less resources ((1/KPa)th of
the resource base) lt ((1/KPc)th of the resource
base).
The growing pop of P.aurelia can drive resources
below the minimun required for individual P.
caudatum to reproduce.
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A classic illustration of competitive exclusion
in the field (involving interference/aggression)
from Connell, Fig 53.2
Even though Chthamalus is concentrated primarily
on the upper strata of rocks, when
ecologist Joseph Connell removed Balanus from the
lower strata, the Chthamalus population
spread into that area. Thus, Chthamalus could
survive lower on the rocks than where it is
generally found, were it not for competition
from Balanus . Its realized niche is only a
fraction of its fundamental niche.
Note coexistence results from tradeoffs each
species better at something.
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My enemys enemy is my friend! - recall
vaginal yeast infection story.
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There are two possible outcomes of competition
between species w/ identical niches either
the less competitive species will be driven to
local extinction, or one of the species may
evolve to use a different set of resources. This
differentiation of niches is called resource
partitioning.
We can think of resource partitioning as "the
ghost of competition past - circumstantial
evidence of earlier interspecific competition
resolved by the evolution of niche
differentiation.
Niche differentiation is often evident from
morphological character displacement.
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the parasite can change its surface coat at
about the same frequency as the host can
mount an immune response to new antigens.
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In ecology, as w/ gravity, everything is
connected to everything else - the trick is
figuring out which interactions are trivial
can be ignored.
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Competition between the gypsy moth, Lymantria
dispar, and the northern tiger swallowtail,
Papilio canadensis interactions mediated by
host plant chemistry, pathogens, and
parasitoids.Redman AM, Scriber JM - OECOLOGIA
125 (2) 218-228 OCT 2000
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
interspecific competition among insect
herbivores probably involves indirect, or
"apparent", competition more frequently than
direct battles over food Typically, indirect
competition between species occurs when the
activity of one species induces a chemical
response in its host plant, affecting other
species feeding on the same plant.
Examples of indirect competition mediated by
pathogens have been reported for a great
many species but not, as far as we know, for
herbivorous insects.
predators and parasitoids may also mediate
indirect competition The attraction of
polyphagous natural enemies to visual cues or
host damage can negatively affect neighboring
herbivores.
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
We investigated interspecific competition in a
study system involving the northern tiger
swallowtail and the gypsy moth, feeding on
quaking aspen. The northern tiger swallowtail,
Papilio canadensis is native to North America.
P. canadensis typically exhibits a strong
preference for quaking aspen, Populus
tremuloides Females oviposit on newly
expanded leaves during the first few weeks of
June, and larvae grow throughout the
summer, pupating in mid- to late August.
Quaking aspen, which supports a large number of
herbivores, is defended primarily by the
phenolic glycosides salicortin and tremulacin,
and secondarily by tannins. when leaf tissue
is removed refoliation replaces nearly 90 of
damaged foliage with leaves that tend to
be rich in phenolic glycosides. Damaged leaves
themselves also tend to contain elevated
levels of these compounds.
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
we conducted a long-term growth trial to
determine if defoliation by gypsy moths decreased
the quality of quaking aspen leaves for P.
canadensis. This assay consisted of 40 P.
canadensis neonates on each of four treatments
(1) leaves from undefoliated stands, (2)
leaves from resistant trees in defoliated stands,
(3) regrowth leaves from defoliated stands,
and (4) partially eaten leaves from
defoliated stands. ("Resistant trees" had
suffered no discernible gypsy moth damage
although they were surrounded entirely by
defoliated trees)
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
we tested for the presence of pathogenic agents
associated with gypsy moth-damaged foliage. P.
canadensis neonates (n25) were placed on the
following treatments (1) leaves from
undefoliated stands, (2) surface-sterilized
leaves from undefoliated stands, (3)
regrowth leaves from defoliated stands, and
(4) surface-sterilized regrowth leaves from
defoliated stands.
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
We also tested for the presence of pathogenic
substances in gypsy moth body fluids
themselves. we collected the cadavers of gypsy
moth larvae from an intensely defoliated
quaking aspen stand We ground the cadavers
with a mortar and pestle, diluted them with
distilled water and painted approximately
1 ml of the resulting solution on quaking aspen
leaves
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
we set up a field experiment to test whether
gypsy moth larvae attracted parasitoids that
attacked P. canadensis larvae as well.
Swallowtails were subjected to two treatments
quaking aspen stands artificially infested
with gypsy moth larvae, and paired
uninfested control stands.
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Redman Scriber - OECOLOGIA 125 (2) 218-228 OCT
2000
Our data suggest that indirect competition
between gypsy moths and tiger swallowtails can
occur. gypsy moth-defoliation significantly
depressed the quality of quaking aspen leaves for
Papilio caterpillars, even when leaves were
sterilized thus, leaf chemistry appears to have
reduced swallowtail performance.
pathogens or other lethal agents associated
with gypsy moth body fluids could
potentially devastate wild tiger swallowtail
populations. We observed 84 mortality in a
very short period of time on unsterilized leaves
and 100 mortality on leaves painted
directly with gypsy moth body fluids.
. as a result of proximity to gypsy moths,
parasitism rate was significantly altered.
Studies such as this stand not only to
illuminate the basic mechanisms and consequences
of interspecific interactions but also to
enhance our ability to manage pests responsibly.
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Effects of a biological control introduction
on three nontarget native species of saturniid
moths.Boettner et al. 2000. Conservation Biology
141798-1806.
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Boettner et al.
We simultaneously deployed cohorts (n 100)
of each of the first three instars to
measure the effect of parasitoids during each
stage of development. C. concinnata was
responsible for 81 of H. cecropia mortality in
the first three instars.
We deployed semigregarious C. promethea in
aggregations of 1-100 larvae in the field
and recorded high rates of parasitism by C.
concinnata among C. prometheca larvae
exposed for 6 days (69.8) and 8 days (65.6).
We discovered a wild population of a third
species of silk moth, the state-listed
(threatened) saturniid hemileuca maia maia, and
found that C. concinnata was responsible
for 36 (n 50)
mortality in the third instar.
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Consumer versus resource control of species
diversity and ecosystem functioning. Worm et
al. 2002. NATURE 417 848-851.The most striking
feature of life on Earth is its diversity.
Consequently, the most fundamental question in
ecology is which factors maintain diversity
in ecological communities. we analyse the
combined impacts of consumers and nutrient
resources on plant diversity.
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Worm et al. 2002 We tested these models in a
food-web context by experimentally
manipulating consumer pressure (absent, present)
and nutrient supply (no, low, medium, high
nutrient enrichment) in two wave-sheltered
rocky shore communities. We chose the two sites
to test for consumernutrient interactions
under contrasting conditions of background
nutrient supply and productivity (Open
bars Bald Rock low filled bars Maasholm
high).
As predicted, changes in nutrient supply and
consumer pressure had interactive effects on
species diversity.
In Maasholm high productivity filled bars,
the reverse applied nutrient enrichment
decreased diversity and consumers increased
diversity. these effects were interactive
consumers reduced diversity under ambient
conditions, but enhanced it under enriched
conditions. Peak diversity was found in
treatments without consumers in Bald Rock low
productivity,
but in treatments with consumers in
Maasholm high productivity.
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Worm et al. 2002 The effects of nutrient
enrichment depend on consumer pressure and vice
versa. Both factors have strong and opposing
effects on diversity, which change in sign
among low-productivity and high-productivity
ecosystems. When consumers are present, peak
diversity shifts towards higher levels of
nutrient supply. These results have important
implications for conservation of
biodiversity and environmental management
because they strongly suggest the potential for
synergistic interactions among the most
common human impacts on ecosystems. Human
alterations of the nitrogen and phosphorus cycles
continue to increase nutrient supply and
productivity in ecosystems worldwide. At the
same time, consumer pressure is altered through
overharvesting of herbivore and predator
populations, habitat fragmentation and
destruction. We conclude that it is not
meaningful to assess or manage these impacts in
isolation. Rapid change in species composition
and loss of diversity will occur when the
dynamic balance of consumer and resource control
is distorted, especially when consumer
removals and resource enrichment occur at the
same time.