But populations and speciesdo not exist in a vacuum…

Species interact…

Community Ecology

A) Five fundamental types of species interactions:

Effect on species

A B

A B

A B

A B

A B

Competition

Predation

Mutualism

Amensalism

Commensalism

A B

B) Concept of the Niche

1) Best known definition of niche is Hutchinson (e.g., 1957)

a) role organism plays in environment

b) role can be determined by measuring all ofan organism’s activities and requirements

3) By extension… niche defined as an N-dimensional hyperspace(encompasses all effects and requirements of a species)

2) Examples 2-factors 3-factors

Wave exposurelow high

Substratum friability

low

high

B) Concept of the Niche

3) Two types of niche

a) fundamental: niche space determined by environmental factors and resource requirements. Manifest in the absence of other organisms.

b) realized: niche space determined by combined abiotic and biotic factors. Realized in presence of other organisms

fundamental realized

fundamental niche always bigger (or at least as large) -biological interactions can (usually do) limit realized niche

Defined:The common use of a resource that is in limited supply.

1) Within and between species

C) Competition

a) Intraspecific - among individuals of the same speciessource of density dependence discussed previously

b) Interspecific - among individuals of two or more species

2) Two types of competition

a) Interference

b) Exploitative

C) Competition

2) Two types of competition

a) Interference - direct competition A Bi) e.g., aggressionii) e.g., territoriality (fishes, birds, limpets)

b) Exploitation - indirect competition

i) Compete through a resource (R)ii) e.g., sessile spp. -- space, filter feeders -- plankton

A B

R

barnacles

space

mussels

C) Competition

3) Competitive exclusion principle

The more similar organisms are, the more likely they are to compete.

a) Species occupying the same niche cannot coexist.

b) The greater the niche overlap, the greater the likelihood of competitive exclusion, leading to local extinction of one species.

c) Leads to “resource partitioning”

C) Competition

4) Resource partitioning

resource gradient*

numberof

individuals

A B C ED

species “packing”A B C ED

resource gradient

adaptation

* e.g.,- seed / plankton size- elevation- height on tree / alga

C) Competition

5) Manifested in patterns

resource gradient

numberof

individuals

A B

a) non-overlapping spatial (or temporal) distribution

tidal height

reef depth

- Implication for relative competitive superiority? - Under what conditions would these patterns be most evident?

C) Competition

5) Manifested in patterns

i) gradient in density

a) negative (inverse) relationship in abundance

Abundancesp. A

Abundancesp. B

AB

AAA BB

B BB

BB

AA

A A

ii) patchy / clumped

AB

A AA

BBB B

A A A A

B

AAA

A

C) Competition

6) Competitive releasea) Change in distribution (or some other response such

as growth) when separate and together

tidal height

sympatry (together)

absence of mussels

absence of barns

Could examine observationally or experimentally, which preferred?

allopatry – separated in space

C) Competition

7) Competitive symmetrya) Relative competitive strength

b) superior, inferior (or) dominant, subordinate

A B

A B

A B

A = B

A > B

A < B

How would you assess this??

Symmetrical

Asymmetrical

C) Competition

8) Effects on measured variables

a) Individual responses:

b) Population responses:

• Behavioral (feeding rates, foraging distribution) • Physiological (growth rate, reproductive rate) • Morphological (body size, biomass)

• Abundance (density) • Distribution (zonation) • Demographic rates (population growth)

Above responses referred to as “trait-mediated” On evolutionary time-scale, manifest as

“character displacement”

C) Competition

9) Character displacement

When differences among similar species whose

distributions overlap geographically are accentuated in

regions where the species co-occur, but are minimized

or lost where the species' distributions do not overlap.

Reflects the consequences of competition in sympatry,

where species co-occur to avoid competitive exclusion.

Example: Darwin’s finch friend’s beaks

D) Predation

Consumption of one organism (prey) by another (predator), which by definition, occurs between organisms on different trophic levels (vs. competition: within same trophic level) [but murky… cannibalism, “intraguild predation” as forms of competition*]

1) diagrammatically:

Predator A

B C

D E F

Herbivore

Primary producer(plant / alga) C

A

B

food chain food web

*(Polis et al 1989 Ann Rev Ecol Syst, Arim & Marquet 2004 Ecology Letters)

D) Predation

2) Effects on prey (direct and indirect):

“Direct effects”: direct losses (removal of individuals)- death of individuals- mortality rate of population

“Indirect effects”: influence of predator on variable other than death or mortality

• behavioral (feeding rates, foraging distribution) • physiological (growth rate, reproductive rate) • morphological (body size, biomass)

More “trait-mediated responses” vs. other “indirect effects”

D) Predation

3) Effects on prey (individual and population):

Individual responses:

• behavioral (feeding rates, foraging distribution) • physiological (growth rate, reproductive rate) • morphological (body size, biomass) • oh yeah… and you can get completely or partially eaten

Population responses:

• abundance, density• distribution (habitat use)• structure (e.g., size, age, sex ratio, genetic, spatial)• dynamics and persistence (regulation)

D) Predation

4) Complex interactions (with other processes)

E.g., competition mediated by predation:

tidal height

With barnacle predators

e.g., predator that specializes on barnacles and is restricted to the mid and lower intertidal

Without barnacle predators

In absence of predator, barnacle out-competes mussels and expands distribution down into the mid intertidal

P

PP P

P

D) Predation

Apparent competition

A B

CWhere, A and B are preyand C is a common predator.

Presence of both prey increases overall predation rates, leading to negative indirecteffect on one another.

A B C

Effect on species

Trophic cascadeWhere, A is primary producer, B is an herbivore, and C is a predator.

Effect of species on adjacent trophic level has net positive indirect effect on next trophic level. A

C

B

4) More complex predation interactions:

Trophic cascades

Strong “top-down” effects that produce downward rippling effects through a food chain.

Higher tropic level predators indirectly affect plant biomass via their impacts on herbivore populations.

Strong “bottom-up” effects that produce upward rippling effects through a food chain.

Lower tropic level producers indirectly affect predator biomass via their impacts on herbivore populations.

Predator

Herbivore

Plants

A linear “food chain”

Abiotic resources(e.g., nutrients, water, light)

Trophic level Relative abundance

Predator

Herbivore

Plants

Abiotic resources(e.g., nutrients, water, light)

Oksanen/Fretwell Model:Productivity and Food Chain Length

increasing productivity

Oksanen/Fretwell Model

Biom

ass

Environmental Productivity

Herbivores

Carnivores

Plants

Predator

Herbivore

Plants

A linear food chain

Oksanen/Fretwell Model:Productivity and Food Chain Length

•Depending on productivity of community, food chains can have fewer or more than three trophic levels.

•As primary productivity increases, trophic levels will be sequentially added.

•Food chains that have an odd number of trophic levels should be filled with lush vegetation, because herbivores are kept in check by predators.

•Food chains that have an even number of trophic levels should have low plant abundance because plants are herbivore limited.

Estes, J. A. et al. Science 1998. Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems

E) Mutualism / commensalism

1) Occurs within or between trophic levels, more often between trophic levels

a) mutualisms: e.g., pollinators

b) commensalisms: e.g., facilitation

A = B

A < B

How would you assess this??

(symmetrical)

(asymmetrical)

A B

A B

mutualism

commensalism

Abundancesp. A

Abundancesp. B

obligate - required for each others existence - pollinatorsfacultative – not required - cleaner fish and parasitized hosts

F) Community metrics (w/ focus on diversity)

1) Species richness: number of species in a community

2) Species composition: identity of species that constitute a community

3) Species diversity: species richness and relative abundance

Shannon-Weiner index of diversity:

H' = -Σ pi (ln pi)Where pi is the proportion of individuals in the community that are species i

F) Community metrics

4) Illustration of diversity

Evenness: measure of the relative similarity of species abundance in a community

E= H'/(ln S) where, S is species richness

0

25

50

75

100

A B C D

No. of indiv.s

0

25

50

75

100

A B C D

0

25

50

75

100

A B C D

H'= 0.87 H'= 1.39 H'= 1.10

G) Spatial scales of species diversity

1) Alpha (α): within habitat diversity

2) Beta (β): between habitat diversity

3) Gamma (γ): the total species diversity in a landscape

Gamma diversity is the product of alpha and beta diversity:

γ = α * β

H) Components of diversity

Multiple components of diversity within a community:

i. Diversity of species within trophic levels

ii. Diversity (number) of functional groups

iii. Diversity of species within functional groupsStachowitz et al 2007 Ann Rev Ecol Syst.

Biodiversity is, broadly speaking, the variety of life.It exists at all hierarchical levels, including genes, populations, species, functional groups, or even habitats or ecosystems.

Functional group is a collection of species with similar function in a community. Can be of widely different taxonomic groups.Examples: primary producers, detritivores, herbivores, planktivores

I) Mechanisms of diversity – community stability

Ways by which a more diverse community can enhance its stability (i.e. persistence in the face of perturbations)

Generally, multiple weak interactors create greater community stability than few strong interactors.

Consider trophic cascades versus complex food webs and the consequence of losing a single species

A

B C

D E FC

A

B versusImpact of losingspecies “B” ?

I) Mechanisms of diversity – community stability

Complementarity refers to greater performance of a species in mixture than expected from its performance in monoculture.Examples: (i) facilitation, (ii) differential resource use among multiple species enhances community productivity and stability (plants and nutrients, predators on prey control)

Functional redundancy is when two or more species fulfill similar ecological functions in a community (e.g., trophic guilds such as planktivores, detritivores) that differ in their vulnerability to perturbations.

Redundancy can contribute to community stability by compensating for relative vulnerability and loss of species.

Identity and Composition effects recognize the important effects of particular species within a community and the variation among species or particular combinations of species in their influence on an ecosystem process or property.

Examples: presence of a foundation species, keystone predator, etc.

Sampling effects reflects the likelihood of including one of these important species as diversity or richness increases.

I) Mechanisms of diversity – community stability

Metapopulations create metacommunities

A set of interacting communities linked by the dispersal of multiple, potentially interacting species.

Variation in rates of movement of species between communities influence species composition, diversity and community functions (e.g., planktivory, herbivory, detritivory, predation) and ecosystem functions (e.g., productivity, nutrient cycling)

J) Metacommunities