Big Idea 4 - Ecology

Ch. 53 – community ecology

Levels of Organization• To study relationships within the biosphere,

ecologists have organized it into smaller pieces.

• The levels are:1. Organism (a single individual – one fish,

for example)2. Population (when organisms of a single

species share the same geographic location at the same time)

3. Biological community (a group of populations that interact and occupy the same space at the same time)

4. Ecosystem (a biological community plus all of the abiotic factors that affect it)

ex – trees plus water and soil5. Biome (a large group of ecosystems that

share the same climate and have similar communities)

Ex – marine or desert biomes6. Biosphere (all the biomes on Earth

combined)

Communities

• A community is a group of organisms of different species that live in a particular area

Overview

• Ecologists call relationships between species in a community interspecific interactions

• Interspecific interactions affect species survival and reproduction

• Examples are competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and disease

Ecological Niches• An organism’s niche is the specific

role it plays in its environment– All of its uses of biotic and abiotic

resources in its environment– Example: oak tree in a deciduous forest

• Provides oxygen to plants, animals, etc.• Home for squirrels• Nesting ground for blue jays• Takes water out of the soil• Etc., etc.

• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

Fundamental v. Realized Niche

• Fundamental Niche includes resources an organism could theoretically use (if no competition)

• Realized Niche includes resources it actually does use given competition from other species.

Resource partitioning

• Resource partitioning is differentiation of ecological niches – enables similar species

to coexist in a community

• Similar species develop ways to partition/divide resources in order to coexist.

Interspecific Competition

• Competition between organisms of different species

• occurs when species compete for a resource in short supply

• can lead to competitive exclusion (local elimination of a competing species)

• The Competitive Exclusion Principle:– Two species with similar needs for the same limiting

resources cannot coexist in the same place– Niches may overlap but they may not be identical.

Interspecific Interactions

• There are 4 major interspecific interactions (symbiotic relationships):– Parasitism (includes

predation and herbivory)– Competition– Commensalism– Mutualism

Predation (and Parasitism)

• (+ -)– The interaction is

beneficial to one species and detrimental to the other

• Predation:– When a predator eats its

prey– Some feeding

adaptations of predators are claws, teeth, fangs, stingers, and poison

– Example in picture:

Herbivory

• Herbivory refers to an interaction in which an herbivore eats parts of a plant or alga

• It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

Plant Defenses Against Herbivores

• “Plants Fight Back!”• Plants have 2 major

mechanisms by which they defend themselves against being eaten– Mechanical

Defenses• Thorns, hooks, etc.

Plant Defenses Against Herbivores

• Chemical Defenses– Produce chemicals

that are distasteful or harmful to an herbivore

– Morphine (opium poppy)

– Nicotine (tobacco)

Animal Defenses Against Predators

• Animals defend themselves against predators passively (hiding) or actively (fleeing)

• Cryptic coloration (camouflage) makes prey difficult to spot

• Aposematic coloration (warning coloration)warns predators not to each animals that may be toxic or may sting.

Animal Defenses Against Predation

• Mimicry– When one species “imitates” or “mimics” another– Batesian mimicry

• When one edible or harmless species mimics an bad-tasting (unpalatable) or harmful species

• Example: hawkmoth mimics a snake

LE 53-7

Hawkmoth larva

Green parrot snake

Animal Defenses Against Predation

• Mimicry– Mullerian mimicry

• Two species, both of which are unpalatable (taste bad) or harmful, resemble each other

• Example: monarch butterfly (unpalatable) and queen butterfly (unpalatable) resemble each other

Parasitism

• One organism (the parasite) gets its nourishment from another organism (the host), which is harmed in the process

• Usually do not kill their hosts• Endoparasites:

– Live within host tissues (tapeworms)

• Ectoparasites:– Feed on external

surfaces (mosquitoes)

Mutualism(++) relationshipBoth partners benefit from the

relationship“You scratch my back, I’ll scratch

yours”Examples:

Ants & acacia treetree provides high

protein food in beltian bodies & habitat for nests inside thorns; ant protects against predators

Mycorrhizae-fungal extentions on plant roots

Plant gets increased water/nutrition, fungi gets food

Hummingbirds & flowersHummingbirds get food, flowers

can reproduce

Commensalism• Commensalism – one organism

benefits and the other is neither helped nor harmed– Ex – shark and remora fish;

sea anemones and clownfish

Dominant & Keystone Species• In general, a few species in a community exert

strong control on that community’s structure• Two fundamental features of community

structure are species diversity and feeding relationships

• Dominant Species:– Species in a community that have the

highest abundance or highest biomass– One hypothesis suggests that dominant

species are most competitive in exploiting resources

– Another hypothesis is that they are most successful at avoiding predators

– American Chestnut tree and the chestnut blight

• Keystone Species:– Important to a community because of their

ecological roles (niches), not by numbers– In contrast to dominant species, keystone

species are not necessarily abundant in a community

– Wolves control deer populations– Bees help pollinate local plants– Elephants eat small trees, which preserve

grasslands– Prarie dogs eat grass which keeps water in

the soil and prevents evaporation

LE 53-17

100

80

60

40

0

20

Sea otter abundance

Ott

er n

umbe

r(%

max

. cou

nt)

400300200

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Sea urchin biomass

Gra

ms

per

0.25

m2

10864

02

Total kelp density

Num

ber p

er0.

25 m

2

19971993198919851972Year

Food chain beforekiller whaleinvolvement inchain

Food chain afterkiller whales startedpreying on otters

Ecosystem “Engineers” (Foundation Species)

• Some organisms exert influence by causing physical changes in the environment that affect community structure

• Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community

• For example, beaver dams can transform landscapes on a very large scale

LE 53-19

Num

ber o

f pla

nt s

peci

es

8

6

4

0

2

Conditions

WithJuncus

WithoutJuncusSalt marsh with Juncus

(foreground)

Ecological Succession• Ecological succession is a

change in the species that live in a given area over a period of time– One community replaces

another• Primary succession = occurs

in places where soil is not yet formed (bare bedrock)

• Secondary succession = occurs in places where there is soil, but where some disturbance has eliminated the previous community

Ecological Succession• The first organisms to inhabit

an area undergoing succession are known as pioneer organisms– These are usually small

organisms (bacteria, lichens, algae, etc.)

• The ecosystem goes through a number of stages, with each new stage usually consisting of larger organisms than the last one

• Once a community has become stable and is not changing much, it is known as a climax community

Ecological Succession cont.

• Early-arriving species and later-arriving species may be linked in one of three processes:– Early arrivals may facilitate

appearance of later species by making the environment favorable

– They may inhibit establishment of later species

– They may tolerate later species but have no impact on their establishment

Causes of Ecological Succession

• There are 3 major causes of ecological succession:1. Human Activities

- logging, mining, development, etc.2. Natural Disasters/Disturbances

- fires, volcanic eruptions, etc.3. Natural Competition Among Species

- Fictitious example:- turtles and frogs both eat crickets- frogs are faster, turtles are slower- frogs eat more crickets, turtles starve- turtle population dies out, frog population

gets bigger

Species Diversity

• Species diversity of a community is the variety of organisms that make up the community

• It has two components: – species richness– relative abundance

• Species richness is the total number of different species in the community

• Relative abundance is the proportion each species represents of the total individuals in the community

Species diversity cont.

• Two communities can have the same species richness but a different relative abundance

• A community with an even species abundance is more diverse than one in which one or two species are abundant and the remainder are rare

Trophic Structure

• Trophic structure is the feeding relationships between organisms in a community

• It is a key factor in community dynamics

• Food chains link trophic levels from producers to top carnivores

LE 53-12

Quaternaryconsumers

Tertiaryconsumers

Carnivore

Carnivore

Carnivore

Carnivore

Secondaryconsumers

CarnivoreCarnivore

Primaryconsumers

ZooplanktonHerbivore

Primaryproducers

PhytoplanktonPlant

A terrestrial food chain A marine food chain

Food Webs

• A food web is a series of connected food chains with complex trophic interactions

Limits on Food Chain Length

• Each food chain in a food web is usually only a few links long

• Two hypotheses attempt to explain food chain length: – the energetic hypothesis and – the dynamic stability hypothesis

Limits on food chain links cont.

• The energetic hypothesis suggests that length is limited by inefficient energy transfer– Most data support the

energetic hypothesis

• The dynamic stability hypothesis proposes that long food chains are less stable than short ones

Bottom-Up and Top-Down Controls

• The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels

• In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers

• The top-down model proposes that control comes from the trophic level above

• In this case, predators control herbivores, which in turn control primary producers

• Long-term experimental studies have shown that communities can shift periodically from bottom-up to top-down controls

• Pollution can affect community dynamics• Biomanipulation can help restore polluted

communities

What Is Disturbance?

• A disturbance is an event that changes a community, removes organisms from it, and alters resource availability

• Fire is a significant disturbance in most terrestrial ecosystems

• It is often a necessity in some communities

LE 53-21

Before a controlled burn.A prairie that has not burned for several years has a high propor-tion of detritus (dead grass).

During the burn. The detritus serves as fuel for fires.

After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

• The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster higher diversity than low levels of disturbance

• The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

LE 53-22

Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.

One year after fire. This photo of the same general area taken the following year indicates how rapidly the com-munity began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

Human Disturbance

• Humans are the most widespread agents of disturbance

• Human disturbance to communities usually reduces species diversity

• Humans also prevent some naturally occurring disturbances, which can be important to community structure

Biogeographic factors affect community diversity

• Equatorial-Polar Gradients• Species richness generally

declines along an equatorial-polar gradient and is especially great in the tropics

• The greater age, climate, and amount of light found in tropical environments may account for the greater species richness

Biogeographic factors affect community diversity cont.

• Climate is likely the primary cause of the latitudinal gradient in biodiversity

• Two main climatic factors correlated with biodiversity are:– solar energy and – water availability

• They can be considered together by measuring a community’s rate of evapotranspiration

• Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

Area Effects

• The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species

• Species richness on islands depends on island size, distance from the mainland, immigration, and extinction

Rivet and Redundancy Models

• The rivet model suggests that all species in a community are linked in a tight web of interactions– It also states that loss of even a single species has

strong repercussions for the community• The redundancy model proposes that if a species

is lost, other species will fill the gap• Community hypotheses and models represent

extremes; most communities probably lie somewhere in the middle