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Arizona. 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,

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Earlier unlimited populations grow exponentially




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 cyclicand even chaotic.

Next: begin building community ecology by looking at how pair-wise interactions

with other species influence demographic rates (b&d):

commensalism (+/0, skip this)

interspecific competition (-/-)

predation/parasitism/herbivory (+/-)

mutualism (+/+)

Then: consider existence and properties of multispecies communities.

Earlier unlimited populations grow exponentially

The individualistic hypothesis of community structure

was enunciated by H. A. Gleason in the early 1900s.

It depicted the plant community as

a chance assemblage of species in the same area

simply because

they happen to have similar abiotic requirements

- for example, for temperature, rainfall, and soil type.

The interactive hypothesis

was advocated by F. E. Clements in the early 1900s.

… the community as a group of closely linked species,

locked into association by mandatory biotic interactions

that cause the community to function

as an integrated unit - as a superorganism.

In most actual cases

where there are gradients of environmental variation,

the composition of plant communities

seems to change continuously,

with species more or less independently distributed.

… support the view of plant communities as

loose associations without discrete boundaries.

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.

Earlier unlimited populations grow exponentially


Intraspecific competition

w/ density-dependent

-FB on growth rate r

The logistic growth model: dN/dt = N [rmax (1 - N/K)]

The Lotka-Volterra interspecific competition model:

dN1/dt = N1[r1max (1 - N1/K1- a12 N2/K1)]

dN2/dt = N2[r2max (1 - N2/K2- a21 N1/K2)]

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< 1 & a21< 1,meaning: intraspecific competition > interspecific competition,

implying: diff species use resources differently {different niches}.

Otherwise, expect competitive exclusion of one of the species.

Earlier unlimited populations grow exponentially

Competitive exclusion is often found in simple lab experiments

where 2 diff species are forced to scramble for one limiting resource.

(it’s difficult to est stable pairs in simple environments w/ exploitation competition).

A classic illustration is from Gause’s exp’s w/ paramecia

competing for bacteria in chemostats (w/ fixed input of food).

When grown separately

each species does fine

(note: P. aurelia has

smaller r but larger K)

When grown in competition,

P. aureliadrivesP. 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) < ((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.

Earlier unlimited populations grow exponentially


tolerates a broad range

of exposure to air

(broad fun. niche),

but is a poor sumo wrestler.


cannot tolerate

long exposure to air

(narrow fun. niche),

but in lower inter-tidal,

is very aggressive

- pries off Chthamalus.

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.

Earlier unlimited populations grow exponentially

Prospects for 'competitive exclusion' treatment

to control salmonellas and other foodborne pathogens in poultry.Mead GC. Veterinary Journal 159: (2) 111-123 MAR 2000.

In newly hatched chicks,

the rapid establishment of an adult-type intestinal microflora,

via the oral route, produces almost immediate resistance to colonization

by any food poisoning salmonellas that gain access to die rearing environment.Exploitation of the 'competitive exclusion' effect

is now an accepted part of the overall strategy

by which poultry-associated salmonellas are being controlled in some countries.

My enemy’s enemy is my friend!

- recall vaginal yeast infection story.

Earlier unlimited populations grow exponentially

The Galápagos finches provide a good example

of character displacement in beak sizes

and, presumably,

in the seeds that they can eat most efficiently.

Allopatric populations {in different places}

of Geospiza fuliginosa and G. fortis

have beaks of similar size,

but on an island where both species occur,

a significant difference in beak depth has evolved.

It is a challenge to develop an appropriate null model

for differences expected w/o past competition & character displacement.

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.

Earlier unlimited populations grow exponentially

It is apparent that predation

influences the distribution and abundance

of prey species - most obviously by

killing some and scaring the rest.

Predators can actually increase biodiversity,

by disturbing competitive exclusion:

A keystone predator blockscompetitive exclusion

by preferentially killing the most abundant

(dominant) competitor. (R. Payne, Fig 53.14)

w/o starfish

eating mussels,

mussels displace

everything else

Earlier unlimited populations grow exponentially

Parasites & disease influence the distribution and abundance of organisms

The African trypanosome, Trypanosoma brucei,

is an important pathogen of humans and livestock.

… the parasite and the subsequent disease in cattle

renders much of Central Africa refractory to raising

high-yielding beef and dairy cows;

indirectly, this has the most profound effect

on the human economy and nutrition in the area.

{& explains why cows haven’t yet displaced wildlife}

The reservoir hosts are wild game animals (antelope and deer).

Humans become infected when they invade the territory of the tsetse fly.

Consider chestnut blight & Dutch elm disease.

… the parasite can change its surface coat at about the same frequency

as the host can mount an immune response to new antigens.

Earlier unlimited populations grow exponentially

In ecology, as w/ gravity, ‘everything is connected to everything else’

- the trick is figuring out which interactions are ‘trivial’ & can be ignored.

Chain reactions linking acorns to gypsy moth outbreaks and Lyme disease risk.Jones et al. 1998. Science 279:1023-1026.

Borrelia burgdorferi


(Lyme disease)






deer ticks







gypsy moths



oak trees

Earlier unlimited populations grow exponentially

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

Earlier unlimited populations grow exponentially

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.

Earlier unlimited populations grow exponentially

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.

Earlier unlimited populations grow exponentially

Redman & Scriber - OECOLOGIA 125: (2) 218-228 OCT 2000

The European gypsy moth, Lymantria dispar was brought to Massachusetts from

France in 1868, and has since radiated … over most of the eastern United States.

Gypsy moth outbreaks are ecologically dramatic events, and it is difficult to imagine

that many species remain unaffected …

Gypsy moth defoliation affects the chemistry of primary host plants …

and gypsy moth outbreaks are associated with

high densities of pathogens, predators, andparasitoids.

At least some of these natural enemies … are known to use alternative hosts.

Earlier unlimited populations grow exponentially

Redman & Scriber - OECOLOGIA 125: (2) 218-228 OCT 2000

P. canadensis larvae grew significantly

more slowly on leaves from resistant trees, regrowth leaves, and partially eaten leaves

from gypsy moth-defoliated areas, than did larvae fed leaves from undefoliated areas

All defoliation treatments severely reduced survival

… 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)

Earlier unlimited populations grow exponentially

Redman & Scriber - OECOLOGIA 125: (2) 218-228 OCT 2000

… increased survival when leaves from defoliated areas were sterilized,

whereas sterilization of leaves from undefoliated stands had no effect on survival.

… we tested for the presence of pathogenic agents

associated with gypsy moth-damaged foliage.

P. canadensis neonates (n=25) 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.

Earlier unlimited populations grow exponentially

Redman & Scriber - OECOLOGIA 125: (2) 218-228 OCT 2000

Leaves painted with gypsy moth body fluids and water

resulted in 100% mortality of swallowtail caterpillars …

However, when we autoclaved the body fluid solution,

the effect on survival nearly disappeared.

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

Earlier unlimited populations grow exponentially

Redman & Scriber - OECOLOGIA 125: (2) 218-228 OCT 2000

When we placed swallowtail caterpillars on trees in areas that had been artificially infested with gypsy moths …

the incidence of parasitism

increased significantly

with the presence of gypsy moths.

… 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.

Earlier unlimited populations grow exponentially

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-defoliationsignificantly depressed the quality of quaking aspen leaves forPapilio 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 withgypsy 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.

Earlier unlimited populations grow exponentially

Abstract:We examined the nontarget effects of

a generalist parasitoid fly, Compsilura concinnata (Diptera: Tachinidae),

that has been introduced repeatedly to North America from 1906 to 1986

as a biological control agent against 13 pest species.

We tested the effect of previously established populations of this fly

on two native, nontarget species of moths (Lepidoptera: Saturniidae),

Hyalophora cecropia and Callosamia promethea,

in Massachusetts forests.

We estimated survivorship curves for newly hatched H. cecropia larvae (n = 500),

placed five per tree in the field and found no survival beyond the fifth instar.

Figure 1. 

Survivorship curves for deployed H. cecropia

(numbers plotted on a log scale vs. time)

recorded from daily observations.

Dotted line represents an estimate of the required survival

for a population to experience no change in density

Effects of a biological control introduction

on three nontarget native species of saturniid moths.Boettner et al. 2000. Conservation Biology 14:1798-1806.

Earlier unlimited populations grow exponentially

Our results suggest that

reported declines of silk moth populations in New England

may be caused by the importation and introduction of C. concinnata.

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.

Earlier unlimited populations grow exponentially

The supply of limiting resources, such as nutrients,

controls primary productivity; that is, the rate of production of organic matter.

On local scales, productivity and diversity

are often unimodally related (Fig. 1a),

such that peak diversity is observed

at intermediate productivity8.

Declining diversity at higher levels of productivity

is thought to be due to competitive exclusion.

Exclusion can be prevented

by periodic mortality events,

caused by consumers or physical disturbance.

These factors also show unimodal relationships (Fig. 1b).

In an attempt to unify these patterns theoretically,

one study explored how traditional Lotka–Volterra competition models

respond to increases in productivity and disturbance frequency9.

The study predicted that the effects of disturbance on diversity

depend strongly on productivity, and vice versa (for details see Fig. 1c).

Physical disturbance and consumer pressure were predicted to give similar patterns.

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.

Earlier unlimited populations grow exponentially

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 consumer–nutrient interactions

under contrasting conditions of background nutrient supply and productivity

(Open bars = Bald Rock = low; filled bars = Maasholm = high).

In Bald Rock {low productivity; open bars},

nutrient enrichment increased diversity

and consumers decreased diversity.

consumers had strong negative effects under ambient conditions,

but weak effects under enriched conditions.

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 enrichmentdecreased diversity and consumersincreased diversity.

… these effects were interactive:

consumers reduced diversity under ambient conditions,

but enhanced it under enriched conditions.

Peak diversity was found in treatmentswithout consumers in Bald Rock {low productivity},

but in treatments with consumers in Maasholm {high productivity}.

Earlier unlimited populations grow exponentially

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.