Ecology
Community Community definitions and concepts Types of interspecific interactions Trophic structure Top-down and Bottom-up control Diversity Succession Latitudinal gradients The equilibrium theory of island biogeography
Community Ecology
A biological community
Is an assemblage of populations of various species living close enough for potential interaction
Community concepts
Contrasting views of community structure are the subject of continuing debate Two different views on community structure
Emerged among ecologists in the 1920s and 1930s
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Integrated Hypotheses (Clements)
A community is an assemblage of closely linked species, locked into association by mandatory biotic interactions
Individualisitc hypothesis (Gleason)
Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements
The integrated hypothesis (Clements)
Predicts that the presence or absence of particular species depends on the presence or absence of other species One should see sharp ecotones between distinct communities with little overlap in the species present in alternative communities
Population densities of individual species
Environmental gradient (such as temperature or moisture)
Figure 53.29a
(a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together.
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The individualistic hypothesis (Gleason)
Predicts that each species is distributed according to its tolerance ranges for abiotic factors
Population densities of individual species
Environmental gradient (such as temperature or moisture) (b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar a biotic n eeds.
Figure 53.29b
In most studies, community composition change continuously, with each species more or less independently distributed. This favors the individualistic view.
Number of plants per hectare 600 400 200 0 Wet Moisture gradient Dry
Figure 53.29c
(c) Trees in the Santa Catalina Mountains. T he d istribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities.
Interspecific Interactions
Populations in a community are potentially linked by interspecific interactions
A communitys interactions include competition, predation, herbivory, symbiosis, and disease
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Table 53.1
Competition
Interspecific competition
Occurs when species compete for a particular resource
Strong competition can lead to competitive exclusion
The local elimination of one of the two competing species
Gause studied competition in laboratory experiments using Paramecium species
Each species, when alone, shows logistic growth to a carrying capacity When both species are present, one is driven to extinction by the other Conclusion: two species cannot coexist if their niches are too similar
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The Competitive Exclusion Principle
The competitive exclusion principle
States that two species competing for the same limiting resources cannot coexist in the same place
Ecological Niches
The ecological niche is the multidimensional space that an organism occupies. The dimensions of the niche include a biotic factors as well as biotic resources and interactions.
Two (of many) abiotic factors m ight be temperature and moisture. Fundamental niche is the space occupiable in the absence of biotic interactions Realized niche is the space the organism actually occupies
As a result of competition
A species fundamental niche may be different from its realized niche
EXPERIMENT Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides a nd Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland.
RESULTS
When Connell removed B alanus f rom the lower strata, the Chthamalus population spread into that area.
Chthamalus Balanus
High tide Chthamalus realized niche
High tide
Balanus realized niche Ocean Low tide Ocean
Chthamalus fundamental niche
Low tide
Figure 53.2
In nature, Balanus fails to survive high on the rocks because it is unable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus i s usually concentrated on the upper strata of rocks. To determine the fundamental of niche of C hthamalus, Connell removed Balanus from the lower strata.
CONCLUSION The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of C hthamalus m uch smaller than its fundamental niche.
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The niche concept allows restatement of the competitive exclusion principle
Two species cannot coexist in a community if their niches are identical
However, ecologically similar species can coexist in a community
If there is one or more significant difference in their niches
Resource Partitioning
The differentiation of niches that enables similar species to coexist in a community
Anolis lizards in Caribbean
A. insolitis perches on shady branches
A. distichus perches on sunny fence posts
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Character Displacement
The tendency for characteristics to be more divergent in sympatric populations than in allopatric populations of the same two species Evidence of divergence due to competition
(+/-)
Predator and prey populations can strongly interact (e.g. lynx and hare) Predation shapes many attributes of both predators and prey
Claws, teeth, fangs, stingers, and poison of predators crypsis, mimicry, shells, speed, sensory capabilities of prey
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Cryptic coloration, or camouflage
Makes prey difficult to spot
Figure 53.5
Aposematic coloration
Warns predators to stay away from prey
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In Batesian mimicry
A palatable or harmless species mimics an unpalatable or harmful model
(b) Green parrot snake (a) Hawkmoth larva
Figure 53.7a, b
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In Mllerian mimicry
Two or more unpalatable species resemble each other
(a) Cuckoo bee
Figure 53.8a, b
(b) Yellow jacket
Herbivory
(+/-)
Herbivory, the process in which an herbivore eats parts of a plant
Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores
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Parasitism and Disease (+/-)
In parasitism, one organism, the parasite
Derives its nourishment from another organism, its host, which is harmed in the process
The effects of disease on populations and communities
Is similar to that of parasites
My other tongue is a crustacean
Parasite grabs base of fish tongue Robs tongue of blood, tongue atrophies Parasite replaces tongue Parasite & fish share food
Cymothoa exigua and the rose snapper
Parasites can manipulate their hosts in many ways
Grasshoppers infected with fungal disease When grasshoppers close to death, climb to top of grass blades Fungal spores better dispersal
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Mutualism
Mutualistic symbiosis, or mutualism
(+/+)
Is an interspecific interaction that benefits both species
Figure 53.9
ant Acacia in Costa Rica note thorns nesting sites for ants
photos by Dan Janzen
Extra-floral n ectaries
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Beltian bodies
Tree provides housing & food for ants - ants benefit Mutualism - (+/+) What does the tree get out of it?
Ants patrol the tree day and night
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Ants attack any herbivores
Vine growing near an ant acacia
Same vine after it had touched the ant acacia
After 45 days with or without ants
Acacia with ants
Acacia with no ants
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Acacia tree 1 year after ants had been removed
Acacia tree 1 year after ants had been removed
Summary: ant - acacia mutualism tree provides food, nest sites for ants ant provides tree protection from competitors, herbivores
Figure 53.9
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Mutualisms are systems of mutual exploitation and can break down into interactions that benefit one player at the expense of the other Bee pollinating milkweed flower
(+/+)
Bee robbing nectar
(+/-)
Commensalism (+/0)
In commensalism
One species benefits and the other is not affected
Figure 53.10
Species Diversity
Two components
Number of species Relative abundance of each species (evenness)
A B C
D Community 1 B: 25% C: 25%
A: 25%
D: 25%
Figure 53.11
A: 80%
Community 2 B: 5% C: 5%
D: 10%
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Trophic Structure
Trophic structure
Is the feeding relationships between organisms in a community Is a key factor in community dynamics
Quaternary consumers
Food chains
Link the trophic levels from producers to top carnivores
Carnivore Tertiary consumers Carnivore
Carnivore
Carnivore
Secondary consumers Carnivore Carnivore
Primary consumers Herbivore Zooplankton
Primary producers Plant Phytoplankton A marine food chain
Figure 53.12
A terrestrial food chain
Food Webs
Humans
A food web
Is a branching food chain with complex trophic interactions
Baleen whales
Smaller whales
Sperm toothed whales
Crab-eater seals
Leopard seals
Elephant seals
Birds
Fishes
Squids
Carnivorous plankton
Euphausids (krill)
Copepods
Phytoplankton
Figure 53.13
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Trophic pyramids
Food chains are usually only a few links long In many studies, there is a correlation between productivity and food chain length
6 5 Number of species 4 3 2 1 0 High (control) Medium Productivity Figure 53.15 Low No. of species No. of trophic links 6 Number of t rophic l inks 5 4 3 2 1 0
Species with a Large Impact on community structure
Certain species have an especially large impact on the structure of entire communities
Either because they are highly abundant or because they play a pivotal role in community dynamics
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Dominant species
Are those species in a community that are most abundant or have the highest biomass Exert powerful control over the occurrence and distribution of other species
Redwood forest
Keystone Species
Keystone species
Are not necessarily abundant in a community Exert strong control on a community by their ecological roles, or niches
Field studies of sea stars
Exhibit their role as a keystone species in intertidal communities
Number of species present
20 15 10 5 0
With P isaster (control)
Without P isaster (experimental)
1963 64 65 66 67 68 69 70 71 72 73 (a) T he sea star P isaster ochraceous f eeds preferentially on mussels but will consume other invertebrates. (b) When P isaster w as removed from an i ntertidal z one, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which P isaster was not removed, there was little change in species diversity.
Figure 53.16a,b
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Ecosystem Engineers (Foundation Species)
Some organisms exert their influence
By causing physical changes in the environment that affect community structure
Figure 53.18
Some foundation species act as facilitators
That have positive effects on the survival and reproduction of some of the other species in the community
8 Number of plant species
6
4
2
0 Salt marsh with Juncus (foreground) With Juncus Without Juncus
Figure 53.19
Conditions
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, the presence or absence of abiotic nutrients
Determines community structure, including the abundance of primary producers
The top-down model of community organization
Proposes that control comes from the trophic level above
The effects of top down control vary with the length of the food chain
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Length = 3
100 Otter number (% max. count) 80 60 40 20 0 (a) Sea otter abundance 400 Grams per 0.25 m2 300 200 100 0 (b) Sea urchin biomass 10 8 6 4 2 0 1972 Food chain before killer whale involvement i n chain 1985 1989 Year (c) Total kelp density 1993 1997 Food chain after killer whales started preying on otters
Length = 4
Figure 53.17
What Is Disturbance?
A disturbance
Is an event that changes a community Removes organisms from a community Alters resource availability
Fire
Is a significant disturbance in most terrestrial ecosystems Is often a necessity in some communities
Figure 53.21a c
(a) B efore a controlled burn. A prairie that has not burned for several years has a high p roportion of detritus (dead grass).
Number per 0.25 m2
Increase in top predator (otters) causes increase in kelp
Increase in top predator (killer whales) causes decrease in kelp
(b) During the burn. T he detritus serves as fuel for fires.
(c) A fter the burn. A pproximately one month after the controlled burn, virtually all of the biomass in this prairie is living.
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The large-scale fire in Yellowstone National Park in 1988
Demonstrated that communities can often respond very rapidly to a massive disturbance
(a) Soon after fire. A s this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.
(b) One year after fire. T his photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.
Figure 53.22a, b
Ecological Succession
Ecological succession
Is the sequence of community and ecosystem changes after a disturbance
Primary succession
Occurs where no soil exists when succession begins
Secondary succession
Begins in an area where soil remains after a disturbance
Retreating glaciers
Provide a valuable field-research opportunity on primary succession
Canada
Mc Bri de
Grand Pacific Gl. 1940 1912
Gl. gs Rig
Alaska
Gl.
0
5 Miles
10
1 8 99 1907 1879
1941 1948 1931 1911
1935
1879 1949
1879
1900 1892
1913 1860
Reid Gl. Johns Hopkins Gl.
1879 Glacier Bay 1830 1780
1760 Pleasant Is.
Figure 53.23
McBride glacier retreating
Ca se me nt Gl.
uir M Gl. au te Pla Gl.
1948
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Succession on the moraines in Glacier Bay, Alaska
Follows a predictable pattern of change in vegetation and soil characteristics
(a) Pioneer stage, with fireweed dominant
(b) Dryas stage
60 50 Soil nitrogen (g/m2 ) 40 30 20 10 0 Pioneer Dryas Alder Spruce Successional stage
(c) Spruce stage
Figure 53.24a d
(d) Nitrogen fixation by D ryas a nd alder increases the soil nitrogen content.
Three models of succession
Facilitation: Early arriving species may facilitate the appearance of later species by making the environment more favorable Inhibition: Early arriving species may inhibit establishment of later species Tolerance: Early arriving species have no impact on later arriving species
The intermediate disturbance hypothesis
Suggests that moderate levels of disturbance can foster higher species diversity than low or high levels of disturbance Frequenty-disturbed habitats harbor primarily early successional species Rarely-disturbed habitats harbor primarily climax species Intermediate disturbance levels lead to hightest levels of diversity
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Two key factors correlated with a communitys species diversity
geographic location (for instance its latitude) size
Latitudinal Gradients in Species Richness
Species richness generally declines along an equatorial-polar (latitudinal) gradient
Diversity is usually highest near the equator (tropics) and declines toward the poles.
Many factors may contribute to this pattern including:
Greater age of tropical environments (more time for the evolution of species) Greater productivity in the tropics due to greater sunlight and water availability
The two main climatic factors correlated with biodiversity
Are temperature, solar energy input and water availability which are combined in measures of evapotranspiration
180 160 Tree species richness 140 120 100 80 60 40 20 0 100 (a) Trees 900 300 500 700 Actual e vapotranspiration ( mm/yr) 1,100 (b) Vertebrates 10 1 Vertebrate species richness (log scale) 100 200
50
1,000 1,500 500 Potential evapotranspiration ( mm/yr)
2,000
Figure 53.25a, b
Careful! Correlation need not imply causation!
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Area Effects
The species-area curve quantifies the idea that
All other factors being equal, the larger the geographic area sampled, the greater the number of species
1,000 Number of species (log scale)
Breeding birds in North America
100
10
1 1 10 100 103 104 105 106 107 108 109 1010 Area (acres)
Figure 53.26
Equilibrium Model of Island Biogeography
Species richness on islands represents a balance between the rates of immigration and extinction
Size of island affects both immigration and extinction rates
Larger islands have higher rates of immigration and lower rates of extinction
Distance from the source (mainland) affects immigration rates.
Immigration more frequent if island nearer mainland
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Species richness increases with island size in the Galapagos and many other archipelagos.
FIELD STUDY Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galpagos Islands, which vary greatly in size, in relation to the area of each island. RESULTS 400 200 Number of plant species (log scale) 100 50 25
10 5 0 0.1 1 10 Area of island (mi2 ) (log scale) 100 1,000
Figure 53.28
CONCLUSION
The results of the study showed that plant species richness increased with island size, supporting the species-area theory.
Summary I
Communities are made up of a set of organisms, some of which may interact strongly Certain species within the community may play a strong role in its structure Trophic structure may be summarized by food chains, food webs, or trophic pyramids Communities can be subject to bottom-up or top-down control (or both)
Summary II
Diversity is a measure of both species richness and evenness of species abundances Succession is the orderly build-up of a community following disturbance and may proceed by any or a combination of tolerance, facilitation, and inhibition
Diversity might be highest at intermediate rates of disturbance
Latitudinal gradients in species richness may have both historical and ecological determinants The theory of island biogeography explains variation in the species diversity of islands through the effects of island size and distance from source populations.
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