Begon et al 1986

Community Ecology

A community is an assemblage of species’ populations which occur together in space and time and therefore have the potential for interaction.

Communities can be recognized and studied at any number of levels, scales and sizes.

In terms of trophic (feeding) relationships, species comprising communities “function” as:•photosynthesizers -- producers•herbivores -- primary consumers•carnivores -- secondary, tertiary consumers)•decomposers•omnivores -- obtain food from more than one trophic level)

Emergent properties of communities include:

•species diversity•limits of similarity of competing species•food web structure•community biomass & productivity

Coral reef community, Indian Ocean, Figi

Community structure: patterns and underlying processes

A central goal in the field of community ecology is to understand the processes that explain the “structure” (pattern) of a community, ie the composition of species, and their abundances and distributions?

Processes underlying those patterns involve interactions between species and the influence of abiotic factors.

Important questions in community ecology address the degree to which organismal and population level interactions explain community structure

Coral reef community, Indian Ocean, Figi

Redwood Community

The redwood forest of coastal California and southwestern Oregon is “dominated” by the redwood trees – this community is named for its most conspicuous member species

Raven and Johnson 1999

Sword Fern

Redwood Sorrel

Ground beetle feeding on slug on sword fern

Communities can be characterized in terms of their Species richness and species diversity

Species richness; total number of species

Species diversity; relative abundance of species in a community

The relative abundance of species is very significant in terms of community structure and function

Species diversity includes information both on number of species and on their relative abundance

Raven and Johnson 1999

By what, if any, processes does community structure arise?

Observation: Particular associations of species (especially plants) co-occur over extensive geographic areas; examples in eastern deciduous forest include:

•beech maple forests•oak hickory forests

Early thoughts focused, somewhat narrowly, on two hypotheses

•Individualistic hypothesis (Gleason): chance assemblage of species that occur together because of similar abiotic requirements

•Interactive hypothesis (Clements): closely linked assemblage of species linked into association by mandatory biotic interactions that cause community to function as an integrated unit

http://home.carolina.rr.com/httpd/hikepisgah/hikepisgah.html

Pisgah National Forest is in Transylvania County, near Brevard, North Carolina.

Individualistic hypothesis predicts communities should generally lack discrete geographic boundaries because species each have independent distributions along environmental gradients (tolerance ranges for abiotic factors)

Interactive hypothesis predicts species should be clustered into discrete communities with distinct boundaries because presence/absence of species is strongly influenced by presence/absence of other species; species should usually co-occur

Results of Robert Whitaker’s research Each tree species at one elevation in Santa Catalina Mountains of Arizona supports individualistic hypothesis; independent distributions for species – apparently due to moisture tolerances; species co-occur where environment meets their moisture tolerances

Predictions of the individualistic and interactive hypotheses, and Whitaker’s test

each colored curve represents abundance of a single species

Contemporary thinking on explanations of community structure

•Empirical evidence indicates that in most cases composition of plant communities appears to change on a continuum; species’ distributions seem to be independent by and large

•Especially true for plant species over large regions over which environmental variation occurs on a smooth gradient, not in abrupt steps

•Individualistic hypothesis is probably not as broadly applicable to animal species as it is to plant species - often linked more closely to other organisms

•Simple generalizations on processes governing community structure do not have broad explanatory power; distributions of most populations in communities are affected to some extent by both abiotic factors and biotic interactions

•Processes that disturb and destabilize existing relationships among organisms (eg fire, flood, storm) are probably among the most significant abiotic factors affecting community structure; disturbance may be the single most important factor affecting structure of many communities

The limpkin is widespread in the American tropics, but occurs in the U.S. only in Florida, where it can satisfy its dietary requirement for a certain freshwater snail.

The eastern grey squirrel is not linked strongly to a single food; its found in eastern deciduous forests from Florida to Canada in many habitat types including pine dominated forests, but is most common in mature hardwood forests

Sharp community boundaries may exist where environmental factors change abruptly – Mg content of soil explains this abrupt change in coastal California

Interactions between populations of different species Ecologists recognize five major classes of interactions among organisms, based on the effect each one has on the other

•Competive interactions mutually harmful interaction arising when two organisms use one or more of the same resources, the availability of which is insufficient to meet their combined needs

•Predator-prey or host-parasite interactions One organism benefits at the expense of another, by eating or otherwise using that organism as a resource

•Symbiotic Interactions

•Mutualism Interaction in which both organisms benefit

•Commensalism (?)Interaction in which one organism benefits and the other is unaffected

•Amensalism (?)Interaction in which one organism is harmed and the other is unaffected

Interacting species may coevolve

•Coevolved species have mutually influenced one another’s evolution in some manner; Ecological interactions among species may influence evolution of species’ traits

•Coevolution may result in result in striking reciprocoal evolutionary adaptations and counter-adaptations, but not necessarily

Adaptation of organisms to other species is regarded as a fundamental characteristic of life, despite difficulties in assessing evolutionary relationships

•Implicit though, is some measure of reciproal genetic change, which has not been demonstrated for most putative instances of co-evolution

•Difficult to establish that evolutionary change in one species is the selective force that drives evolutionary change in an interacting species

Coevolution may be diffuse or species-specific

•Diffuse co-evolution species traits evolve as a consequence of interactions multiple species, perhaps involving multiple types of interactions

•Regarded as much more common than species-specific coevolution

•Species-specific coevolution species traits evolve as a consequence of interactions with a single species

Coevolution of traits bearing on interactions

Species in mustard family produce mustard oils Mustard oils protect cabbage from many, but not all, herbivorous insects; the caterpillar of the cabbage butterfly feeds on a cabbage leaf

Solomon et. al., 1999

Competition

The Concept of the Ecological Niche

•“Niche” refers to sum total of the way an organism uses its environment, biotic and abiotic resources, to live

•For each species of tropical tree lizard, niche includes

•temperature, humidity range

•size, orientation of hunting branches

•hunting times

•size, species of insect prey

•many, many more “niche dimensions”

Campbell 1993

Percent waterSpecies’ niche is related to acceptable limits and optimum values of variables that affect the species. Two dimensions of a caterpillar’s niche; % water & % nitrogen content of its food

Per

cent

nitr

ogen

Keeton & Gould 1994

Relative Growth Rates (%)

•Fundamental Niche

Entire niche a species (or population, or organism) is theoretically capable of occupying

•Realized Niche

The actual niche the organism is able to occupy in the presence of competitors

•Niche Overlap

Niche overlap refers to the degree of similarity in the fundamental niche of two species

Competition and Ecological Niches

Green Anole Brown Anole

Green Anoles are native to S.E. United States. The species has become rare in parts of Florida in recent years, apparently as a consequence of interspecific competition with the introduced Brown Anole.

Unit 6 Ecological Patterns and Processes

Apr 17 W Population structure and models of population growth 959-965

Apr 19 F* Influence of density, disturbance &life history on populations 965-973

Apr 22 M Biotic interactions and community structure 974-987

Apr 24 W Disturbance, succession and community structure 987-990

Apr 26 F Historical and ecological biogeography 1007-1014

Apr 29 M Conservation and decline of biodiversity 1031-1044

Effects of Competition and its Importance in Community Organization

Laboratory Experiments early last century

•Competitive exclusion experiments

More recent lines of reasoning and natural experiments

•Resource partitioning

•Character displacement

Re l

ativ

e po

pula

ti on

den s

it ies

Gause’ test of the effect of interspecific competition

•Supports hypothesis that similar species with similar needs for same limiting resource can not coexist

•Later termed “competitive exclusion principle” and reinforced with other experimental work

Gause’s principle, restated in the context of fundamental and realized niches; two species can not coexist if their niches are identical; ecologically similar species can coexist, given one or significant differences in their niches.

Paramecium caudatum alone

Paramecium aurelia alone

Both grown in mixed culture

•When grown together with constant food (bacteria), caudatum driven to extinction.

Competition and the Consequences of Competition in Nature

•Competition in natural communities is much more complex, and more difficult to study, than in laboratory experiments

•The possible consequences of interspecific competition (consider two species) are either; extirpation of one species or coexistence of both species

•Coexistence happens when competition is reduced, minimized,

•Niche differentiation is the process by which species partition the use of resources in a shared habitat

•Catch 22 for researchers;

•competition in nature is difficult to demonstrate because its been reduced through niche differentiation.

•What we are able to study in nature, typically, is not intense competition, but the “ghost of competition past”

•Demonstration of niche differentiation is taken as strong evidence that competition is an important ecological force

demonstrate resource partitioning

demonstrate character displacement; evolutionary character divergence that reduces competition

We’ll look at comparative and experimental approaches

Resource Partitioning

•Similar, coexisting species often have niche differences; implies competition has operated in the past as an evolutionary-ecological force

•Resource partitioning refers to (often slightly) different use of resources among similar species in sympatry.

•Differences in habitat use (location within area of the community) is also regarded a potential consequence of resource partitioning.

Competition as an Ecological and Evolutionary Force: Strong Circumstantial Evidence I

Raven and Johnson 1999

Resource partitioning in sympatric Anolis lizards in the Caribbean.

•Jonathan Losos at Washington U. found that species variously use upper canopy, lower branches, trunk, or grass below tree for hunting areas.

•When two species do occupy same part of tree, they either eat different size insects or occupy different thermal microhabitats.

•Same pattern of partitioning independently evolved on other Caribbean Islands.

Character Displacement

•Character displacement refers to presumed divergence in morphology or other trait (behavior, etc,) and related divergence in resource use, in sympatric populations compared to allopatric populations

•Comparison of closely related species occurring both sympatrically and and allopatrically

Competition as an Ecological and Evolutionary Force: Strong Circumstantial Evidence II

Character displacement in Geospiza. These two species have similar bill size among allopatric populations, but different size when populations are sympatric

(Raven and Johnson 1999)

Controlled Field Experiments Provide Direct Evidence of the Importance of Competition

Controlled Field Experiments make for a strong inference regarding cause and effect

•Barnacles (Joseph Connell)

•Sticklebacks (Donald Schluter)

•Joseph Connell demonstrated competitive exclusion within a community in a rocky intertidal zone

•The species coexist by partitioning the habitat.

Competition among two species of barnacles limits niche use Chthamalus can live in deep and shallow zones (its fundamental niche), but Balanus forces Chthamalus out of the part of its fundamental niche that overlaps the realized niche of Balanus.

Experimental Demonstration of Competitive Exclusion

Benthic three-spined stickleback from Paxton Lake, attending nest (photo by Matt Mcleod, 1996)

Part of Shluter’s wet lab & one of the experimental ponds

Photo’s from Shluter’s webpage at UBC:

Dolph Schluter,Department of Zoology, University of British Columbia http://www.zoology.ubc.ca/~schluter/

Solomon et al 1999

Predation

•ecological relationships in which one organism (population, species) benefits and the other is harmed

•generally, feeding relationships where individuals of one species directly provide nourishment for individuals of another species

•includes predation, herbivory, parasitism, parasitoidism

•objectives; examine nature of these ecological relationships especially predation, and the consequences of these relationships on community structure

Herbivory Animal eats a plant, perhaps killing the organism, eg mouse eating a seed), perhaps not, eg, grazing by cattle – the latter being akin to parasitism

Predation (Hunting): predator kills and eats its prey

Parasitoidism Insects, usually small wasps, lay eggs on living hosts; on hatching, larvae feed in boty of host, eventually causing its death

animals have many defensive adaptations to avoid being eaten

•behavioral

•fighting/defense

•concealment

•congregating

•fleeing

•morphological

•shells, exoskeletons

•spines, spinous fins

•size

•chemical, coloration

•toxins

•cryptic coloration

•warning coloration

•mimicry

Bombadier beetle ejects a noxious spray at the temperature of boiling water at a predator

Indo-Pacific lionfish, one of the most toxic reef fishes. Posion glands at base of spines, and warning coloration

Crows mobbing barn owl

cryptic coloration in canyon tree frog, on

granite substrate

Bluejay vomiting after eating a noxious monarch

butterfly

Dendrobatid frogs of Latin America are highly toxic to vertebrates. Over 200 alkaloids isolated from Dendrobatid mucus; some are so toxic that 2-3 micrograms in bloodstream will kill a human being.

Milkweed toxins are poisonous to many herbivores, but not Monarch caterpillars

Monarchs metabolize toxins, thereby becoming unpalatable themselves

Aposematic (warning) coloration has evolved many times in unpalatable lineages

Aposematic (warning) coloration is common in species that uses poisons and stings to repel predators.

Mullerian and Batesian mimicry among species of Costa Rican butterflies

Some species have a disproportionately large influence on community composition and structure

•Through their ecological interactions, many if not all species have an effect, to some degree, on various components of community organization, including

•species richness

•microclimate, soil structure, soil chemistry

•resource abundance and distribution

•flow of energy, nutrients

•Some species have major influences on community organization by virtue of their number, biomass, size

•in terrestrial communities, plants constitute much of the structural environments, strongly modify the physical environment, and are channels for input of energy and nutrients

•Some species are “keystone” members of communities in that they have a disproportionately large effect compared to their representation (abundance) in the community

•Starfish (hunters)

•Bison (grazers)

By grazing preferentially on grasses, bison increase the density of forbs and overall plant productivity and species richness

•30 bison introduced into Konza Prairie research Natural Area in Kansas (experimental plot); plant community compared to control plot

•grasses fertilized by bison urine photosynthesize faster (nitrogen becomes available quickly to plants)

Images from Begon et al 1986, Campbell 2000, Purves et al 2000

Paine’s (1966) manipulation experiment shows the influence a top carnivore may have on community structure (species’ richness)

•The main influence of the starfish was to make space available for competitively subordinate species. It created areas free of barnacles and, most importantly, free of the dominant mussels which would otherwise outcompete other invertebratres and algae for space.

•Overall the removal of starfish led to a reduction in number of species from fifteen to eight.

Disturbance, Succession, Non-equilibrium, and Community Structure

Alder, cottonwood and willow on a glacial moraine, perhaps 100 years after the glacier had retreated from this area

Succession is a process of change that results from disturbance in communities

•Many if not most communities are characterized by periodic disturbances that affect structure and composition, such as fire, floods, storms, freezes, volcano

•Effects of such disturbances may variously include

- “knock back” many if not all popul’s to low levels -remove all vegetation from terrestrial or aquatic community -scour the soil, streambed, etc

•These disturbances occur at various scales; they may be localized and patchy or geographically extensive;

•Such disturbances create the conditions for “ecological succession”

•disturbed area is colonized by a variety of species (often time those with life history traits that give them a competitive advantage in a low-competition environment)

•with time, growth of populations of colonizers, etc., ecological conditions change and colonizers are eventually replaced by a succession of other species

•“Ecological succession” refers to transitions in species composition over ecological time

Succession: Vegetation colonizes a moraine near McBride Glacier, AK. Copyright Bruce F. Molnia/TP.

Succession: Fireweed after fire in Yellowstone National Park. Copyright Jon Mark Stewart/BPS.

barren landscape exposed after

retreat of the glacier is initially colonized

by lichens, then mosses

at a later time, dwarf trees and shrubs

(alder, cottonwood, willow) colonize the

area

Still later, hemlocks and spruces

dominate the community

Primary succession after retreat of glaciers

Alder and Dryas (an herbaceous plant) have nitrogen-fixing bacteria in root nodules, which “improves soil for other species

Solomon et al 1999, Purves et al 200

In the 1960’s and 1970’s, community structure was explained in terms of “stability”, “equilibrium” and climax communities

•By the early 1900’s, the idea of “climax communities” began to gain acceptance; succession leads to a stable endpoint, a climax community; stable climax predicted when web of interspecific interactons became so intricate that the community was”saturated” -- no more species could colonize except after a localized or extensive extinction of species

•The “balance of nature” view held that communities exist, normally, in a “state of equilibrium”, unless they are significantly “disturbed”

•“Stability” was regarded as tendency for community to reach and maintain equilibrium (relatively constant condition) in spite of disturbance

•Interspecific interactions were regarded as agents of stability; maintained stability, or returned communities to equilibrium following disturbance

•This “balance of nature” model is now regarded as having limited explanatory power with respect to community structure

Mature northern hardwood forest dominated by sugar maple; Upper Peninsula, MI. Copyright J. Robert Stottlemyer/BPS.

Tropical evergreen forest (rainforest), Mt. Spec National Park, Australia. Copyright BPS.

Contemporary nonequilibrial model views communities as mosaics of patches at different stages of succession

•Succession is a highly variable and virtually perpetual process – no longer understood as an orderly, linear progression driven mainly by interspecific interactions;The course of successional change depends on size, frequency and severity of disturbance.

•Most communities are routinely disturbed by outside factors during the course of succession; few if any communities “reach”, much less persist in a climax state; growing body of research indicates that disturbance is the main force driving successional changes.

•Disturbance keeps communities in a constant state of flux, rendering them mosaics of patches at different successional stages and preventing them from ever achieving a state of “equilibrium”

Chaparral biome in Monterrey, CA. Copyright Edward Ely/BPS.

Communities are composed of populations living and interacting in a given environment

Producers

Decomposers

Death, waste products

Death, waste productsDeath

Community Ecology

Primary Consumers

Secondary Consumers

Primary consumers

Interaction Effect on Species 1 Effect on Species 2

Competion

between sp. 1 and sp. 2 harmful harmful

Predation

by sp. 1 on sp. 2 beneficial harmful

Symbiosis

Mutualism of sp. 1 and sp. 2 beneficial beneficial

Commensalism of sp. 1 w/ sp. 2 beneficial no effect

Parasitism by sp. 1 on sp. 2 beneficial harmful

Ecological Interactions among organisms

Competion among two species of barnacles limits niche use

Chthamalus can live in deep and shallow zones (its fundamental niche), but Semibalanus forces Chthamalus out of the part of its fundamental niche that overlaps the realized niche of Semibalanus.

Predation

Predator-Prey Interactions

•Interactions in which one organism uses another one for food

•Animals that capture live animals and eat them

•Animals that eat live plants

•Among most conspicuous of community interactions

•Interactions drives evolution of adaptations in both prey and predator species

•Interactions drive causally related patterns of population dynamics

North American bobcat, a solitary hunter, feeding on a mouse.

(Solomon et. al., 1999)

Giant Panda of mountainous China feeding on bamboo

Plants evolve defenses against herbivores

Morphological defenses

•Thorns, spines

•Hairs; glandular and sticky distally, deters herbivorous insects

•Intracellular silica deposition; renders plant (esp. grasses) tough to eat

Plants evolve defenses against herbivores

Chemical defenses: secondary chemical compounds

• Widely occurring among plant lineages

•Generally either toxic or impede development by disrupting metabolic pathways

Plants evolve defenses against herbivores

Chemical defenses: secondary chemical compounds

•Milkweed family and the related dogbane family produce milky sap that deters herbivores; also produce cardiac glycosides, which impair vertebrate heart function

•Species in mustard family produce mustard oils

Mustard oils protect cabbage from many, but not all, herbivorous insects; the caterpillar of the cabbage butterfly feeds on a cabbage leaf (Solomon et. al., 1999)

Aposematic (warning) Coloration. Dendrobatid frogs of Latin America are highly toxic to vertebrates. Over 200 alkaloids isolated from Dendrobatid mucus; some are so toxic that 2-3 micrograms in bloodstream will kill a human being.

Animal defenses against predators

Milkweed toxins are poisonous to many herbivores, but not Monarch caterpillars

Monarchs metabolize toxins, thereby becoming unpalatable themselves

Aposematic (warning) coloration has evolved many times in unpalatable lineages

Animal defenses against predators

Animal defenses against predators

Aposomatic (warning) coloration; common in species that uses poisons and stings to repel predators. (Solomon et. al. 1999)

Mullerian and Batesian mimicry among species of Costa Rican butterflies

Interaction Effect on Species 1 Effect on Species 2

Competion

between sp. 1 and sp. 2 harmful harmful

Predation

by sp. 1 on sp. 2 beneficial harmful

Symbiosis

Mutualism of sp. 1 and sp. 2 beneficial beneficial

Commensalism of sp. 1 w/ sp. 2 beneficial no effect

Parasitism by sp. 1 on sp. 2 beneficial harmful

Ecological Interactions among organisms

Competiton

•Competition occurs when two organisms attempt to use the same limiting resource; resource availability can not satisfy both individuals

•Interference competition; individuals “fight” over resource

•Exploitative competition; individuals simply consume resource

•Interspecific competition; among individuals of differing species

•intensity correlates with similarity of organisms (similar niches)

•Intraspecific competition; among conspecifics (very similar niches!)

Competiton

•Competition occurs when two organisms attempt to use the same limiting resource; resource availability can not satisfy both individuals

•Interference competition; individuals “fight” over resource

•Exploitative competition; individuals simply consume resource

•Interspecific competition; among individuals of differing species

•intensity correlates with similarity of organisms (similar niches)

•Intraspecific competition; among conspecifics (very similar niches!)

Predator-Prey Interactions

•Interactions in which one organism uses another one for food

•Animals that eat live plants, fungi, etc.

•Animals that capture live animals and eat them

•Interactions drive evolution of adaptations in both prey and predator species

•Interactions drive causally related patterns of population dynamics

North American bobcat, a solitary hunter, feeding on a mouse.

Giant Panda of mountainous China feeding on bamboo

(Solomon et. al., 1999)

Pelican catching surface

feeding fish

Plants defenses against herbivores

Morphological defenses

•Thorns, spines

•Hairs; glandular and sticky distally, deters herbivorous insects

•Intracellular silica deposition; renders plant (esp. grasses) tough to eat

Plants evolve defenses against herbivores

Chemical defenses: secondary chemical compounds

•Milkweed family and the related dogbane family produce milky sap that deters herbivores; also produce cardiac glycosides, which impair vertebrate heart function

•Species in mustard family produce mustard oils

Mustard oils protect cabbage from many, but not all, herbivorous insects; the caterpillar of the cabbage butterfly feeds on a cabbage leaf (Solomon et. al., 1999)

Predators can alter community structure by moderating competition among prey species

Predators moderate competition among prey species

•One important effect of a predator on community structure

Herbivory

Some lineages of plants defend against feeders with chemical protection

Predation

Plants evolve defenses against herbivores

Chemical defenses: secondary chemical compounds

• Widely occurring among plant lineages

•Generally either toxic or impede development by disrupting metabolic pathways

Animal defenses against predators

Aposomatic (warning) coloration;