BIOL 4120: Principles of EcologyBIOL 4120: Principles of Ecology

Lecture 16: Community Lecture 16: Community EcologyEcology

Dafeng HuiDafeng Hui

Office: Harned Hall 320Office: Harned Hall 320

Phone: 963-5777Phone: 963-5777

Email: dhui@tnstate.eduEmail: dhui@tnstate.edu

Outline (Chapter 17)Outline (Chapter 17)Factors influence community structureFactors influence community structure

17.1 Fundamental niche constrains community 17.1 Fundamental niche constrains community structurestructure

17.2 Species interactions are diffuse17.2 Species interactions are diffuse17.3 Food web illustrate indirect interactions17.3 Food web illustrate indirect interactions17.4 Food web suggest controls of community 17.4 Food web suggest controls of community

structurestructure17.5 Species interactions along environmental 17.5 Species interactions along environmental

gradients involve both stress tolerance and gradients involve both stress tolerance and competitioncompetition

17.6 Environmental heterogeneity influences 17.6 Environmental heterogeneity influences community diversitycommunity diversity

17.7 Resource availability can influences plant 17.7 Resource availability can influences plant diversity within a communitydiversity within a community

17.1 Fundamental niche constrains 17.1 Fundamental niche constrains community structurecommunity structure

All organisms can live and reproduce over a specific range of environmental condition

These conditions differ from organism to organism

The conditions under which an organism function well are the consequence of physiological, morphological and behavioral adaptations.

These same adaptations also limit its ability to perform equally well under different conditions.

Plants adapted to shade, high nutrient

The environmental conditions vary in time and space, and fundamental niche of species vary, thus fundamental niche difference among species help explore the processes that structure communities.

Fundamental niche constrains Fundamental niche constrains community structurecommunity structure

Fundamental niches of hypothetical species along environmental gradient (e.g. T, Moisture, elevation)

•All species have bell-shaped niches

•Niche overlap

•Each species has limits beyond which it can’t survive

•For any given range of environment, only a subset of species can survive

•As environments change, abundance of species will change.

Distribution of three species

Geographic distribution of three tree species that are part of the two forest communities presented in Tables 16.1 and 16.2.

Distribution of these three species overlap in West Virginia.

As we move across eastern North America, the set of tree species whose distributions overlap will change, and therefore so will the species composition of the forest communities.

Null modelNull model

Null model: assume that presence and abundance of the individual species found in a

given community are a result of the independent responses of the individual species to the prevailing physical environment.

Interactions among species have no significant influence on the structure

Null model provides basis for experimental study

Physically remove one species and examine the population response of the other (competition, predation, parasitism, mutualism)

If no change, accept null model (no interaction).

But many evidences show that species interactions do influence both the presence and abundance of species within communities.

Example, competition influence realized niches. Mutualism enhances the presence and distribution.

17.2 Species interactions are 17.2 Species interactions are diffusediffuse

Species interactions exist, but the importance are often underestimated.

because• such interactions are many and relatively diffuse• Involve a number of species

Competition Experiment by Norma Fowler (UT Austin)Usually the removal of a single species will have very limited effectsRemoval of group of species can have large effect

this makes it difficult to determine the effect of any given species on another.

But collectively, competition may be an important factor limiting the abundance of all species involved.

Species interactions are diffuseSpecies interactions are diffuse

Pollination:

A single plant species may be dependent on a variety of animals species for successful reproduction.

Predator-prey:

Predator species (lynx, coyote, horned owl etc.) and snowshoe hareFood web chart (on next slide), 11 of 12 predators prey on snowshoe

hares.

Any single predator species may have a limited effect on the snowshoe hare population; together, they regulate its population.

17.3 Food webs illustrate indirect 17.3 Food webs illustrate indirect interactionsinteractions

Food websProvide

information on indirect effects

See lynx and white spruce

Presence of lynx is good for white spruce due to survival of seedlings

Another example of predation in shaping structure of communities

Starfish prey on mussels, barnacles, limpets, and chitons

Remove starfish, what would happen?

Species diversity increase or decrease?

Why?

Keystone predation• Predation can also influence outcome of

interactions between prey species• The starfish preys on many species of mussels,

barnacles, limpets, etc• Remove starfish from experimental plots and

compare to normal situation• Number of prey species in experimental plots

was halved• Diversity was decreased as better competitors

excluded other species

Apparent competition• In the absence of predator, the population of

each prey is regulated by purely intraspecific density-dependent mechanisms

• Neither prey species compete, directly or indirectly, with each other

• Predator abundance depends on the total abundance of prey

• Under these conditions, the combined population abundance of two prey species will support a higher predator density.

Apparent competition

Experimental supports:

Nettle aphid, grass aphid and ladybug beetle (page 359, textbook)

Brought nettle aphid plants to grass aphid plants together suppressed both population, as a results of larger ladybug beetle population.

Combined populations of two prey species support a larger predator population neither can support alone. As a result, two prey populations reduced, gives outward appearance of interspecific competition.

Indirect commensalismTwo species of herbivorous species Daphnia (water fleas)

Two predators: Midge larva and Larval salamander

Each predator prey on one species

In a pond Where salamander larval were present, # of large Daphnia was low, small was high; where absent, small Daphnia were absent, midges could not surviveBenefit midge larva, neutral to

salamander

Indirect mutualism• When indirect interaction is beneficial to both

(predator) species.

Indirect interactions play an important roleRemoving top predators from community could have

some unforeseen consequences (conservation and management)

An Example: Predator control in Greater Yellowstone ecosystem (Joel Berger from UN Reno)

Predator control, decrease in Grizzly bear and wolf increase in moose population decrease in willow and other woody species along

riverline decrease in birds (even local extinction for some

species)

17.4 Food webs suggest controls of 17.4 Food webs suggest controls of community structurecommunity structure

Wealth of experimental evidence illustrates the importance of both direct and indirect interactions on community structure.

How do you tell which ones are important in controlling community structure? Are all interactions important?

Hypothesis one:

all species interactions are important; remove any one of these species may have a cascading effect on all others.

Hypothesis two:

only a smaller subset of species interactions are controlling community structure.

System stable until enough species are lost to make whole system collapse

Difficult to study (there are some dominant species like starfish, but majority is mystery)

One approach is splitting species into functional groups• Each group has a similar function and perhaps can replace each

other

Trophic levels1.Primary producers2.Herbivores3.Carnivores

Bottom-up control• Plant population control herbivore populations, which in turn

control the diversity and population density of carnivore population

Top-down control• Predator (carnivore) populations control the diversity of prey

species, and the prey of the prey, and so on.

Bottom-up control is very common. Mostly, community structure is regulated by bottom-up control.

Examples support top-down control

• Large-mouth bass experiment by Mary Power at OU

• Bass (Predator) prey on minnows (herbivore), minnows graze on algae

• Remove bass vs control• Pools with bass had low minnow population and a luxuriant

growth of algae• Pools with bass removed had high minnow populations and low

populations (biomass) of algae

• Top predator control plant population indirectly through their direct control on herbivores.

“The world is green”: predators will keep herbivores in control

17.5 Species interactions along environmental 17.5 Species interactions along environmental gradients involve both stress tolerance and gradients involve both stress tolerance and

competitioncompetitionBiological structure of a community is constrained by environmental

tolerances of the species (fundamental niche). Those tolerances are often modified through both direct and indirect interactions with other species (realized niche).

Competitors and predators can function to restrict a species in a community and mutualists can function to facilitate a species’s presence and abundance within a community.

Results: a pattern of species distribution across an environmental gradient

This is due to trade-offSome species that can grow well are at an advantage when

resource at bestSome species than can survive when resource is at worst can

survive where previous species cannot.

General pattern of trade-off between a species’ ability to survive and grow under low resource and maximum growth achieved under high resource availability.

The resulting outcome of competition will be a pattern of zonation.

Salt provides stress

Level of salt varies with distance from low water mark

Lower boundary is determined by stress

Upper boundary is determined by competition

Note also that nutrition can change situation

Increase fertilization results in change in species at boundary

Tradeoff• Competitive

ability• Tolerance of

stressClassic examples of zonation occur in salt marshes

17.6 Environmental heterogeneity influences 17.6 Environmental heterogeneity influences community diversitycommunity diversity

Biological structure of a community reflects both the ability of the component species to survive and grow in the prevailing environmental conditions and their interactions.

As environmental changes from location to location, so will the species and their interaction.

But how does local environmental heterogeneity within a community influence patterns of diversity?

Relationship between bird species diversity and foliage height diversity for deciduous forest communities in eastern North America. (x is not height)

Late Robert MacArthur

13 communities in northeastern US

Bird species diversity

Foliage height diversity

17.7 Resource availability can influence plant 17.7 Resource availability can influence plant diversity within a communitydiversity within a community

High nutrients will support high rates of photosynthesis, plant growth, and a high

density of plants.

How does nutrient availability influence plant diversity in communities?

Why?

Michael Huston, ORNL, TN (Texas State Uni.)

Relationship of tree species richness to a simple index of soil fertility for 46 forest communities in Costa Rica.

Hypothesis

Inverse relationship results from reduced competitive displacement under low

nutrient availability

Low nutrient availability reduces growth rates and supports a lower density and biomass of vegetation. Species that might dominate under higher nutrient availability cannot realize their potential growth rates and biomass and as a result are unable to displace slower growing, less competitive species.

Supported by many other experiments

(Rothamsted Experimental Station in Great Britain, 1859-)

Increasing nutrient availability has been to decrease diversity

What processes cause the decrease in diversity What processes cause the decrease in diversity with increasing nutrient availability? with increasing nutrient availability?

J. Cahill (University of Alberta, Canada) examined J. Cahill (University of Alberta, Canada) examined how competition in grassland communities shifts how competition in grassland communities shifts along a gradient of nutrient availabilityalong a gradient of nutrient availability• A shift in the importance of belowground and A shift in the importance of belowground and

aboveground competition and the nature of aboveground competition and the nature of their interaction under varying levels of their interaction under varying levels of nutrient availabilitynutrient availability

Competition for belowground and aboveground Competition for belowground and aboveground resources differs in an important wayresources differs in an important way

Competition for belowground resources is Competition for belowground resources is size size symmetricsymmetric because nutrient uptake is because nutrient uptake is proportional to the plant’s root sizeproportional to the plant’s root size• Symmetric competition occurs when Symmetric competition occurs when

individuals compete in proportion to their individuals compete in proportion to their sizesize

Competition for aboveground resources is Competition for aboveground resources is size size asymmetricasymmetric — — larger plants have a larger plants have a disproportionate advantage by shading smaller disproportionate advantage by shading smaller ones.ones.

Under low nutrient availability, plant Under low nutrient availability, plant growth rate, size, and density are low for growth rate, size, and density are low for all speciesall species• Competition primarily occurs Competition primarily occurs

belowground belowground symmetric symmetric Growth rate, size, and density increase as Growth rate, size, and density increase as

nutrient availability increasesnutrient availability increases• As faster-growing species overtop the As faster-growing species overtop the

others, creating a disparity in light others, creating a disparity in light availability availability asymmetric competition asymmetric competition

Fertilization results in an increase in the Fertilization results in an increase in the species richness of autotrophs in both species richness of autotrophs in both freshwater and marine communitiesfreshwater and marine communities

Why the difference between patterns in Why the difference between patterns in terrestrial versus aquatic communities?terrestrial versus aquatic communities?• Differences in the role of competitionDifferences in the role of competition

Reduced competition results from fertilizationReduced competition results from fertilization Limited by more than one nutrient, no single Limited by more than one nutrient, no single

spp has a competitive advantage.spp has a competitive advantage.

Freshwater and marine communities

END

14 circular mescosms,

1.6 m in diameter, 1.5 m height

Three years