COMMUNITY ECOLOGY
A community is defined as an assemblage of species living close enough together for
potential interaction
PATTERN AND PROCESS
• Pattern is what we can easily observe directly - vegetation zonation, species lists, seasonal distribution of activity.
• Process gives rise to the pattern- Succession, Interaction, nutrient availability, patterns of disturbance, energy flow, history, and evolution.
CHARACTERISTICS OF COMMUNITY
• Scale
• Spatial and Temporal
Structure Life form)
• Species Diversity
• Trophic structure
• Succession and Disturbance
Community Structure; Limited Membership concept Proposed by Elton
• Scale is the size of a community.
• Provided that the area or habitat is well defined, a
community can be a system of almost any size, from a rotting log, to a forest, to the surface of the Pacific Ocean.
COMMUNITY BOUNDARIES
• Study of community often based on the main life-form type.
Community is discrete But community can be a diffuse or mosaic one
• Spatial Structure is the way species are
distributed relative to each other.
• Some species provide a framework that creates habitats for other species.
These species, can in turn create habitats for others.
HOW DOES COMMUNITY NAMED?
MAINSTREAM CONCEPTS
TANSLEY AND CLEMENT
Community is a super-organism
Community as fundamental unit of nature. Community can be
classified
GLEASON
Vegetational continuity and species individuality
Individualistic concept; that every species has its own range of tolerance Classification is possible, but any
classification is for the convenience of the human observer and is not a
description of the obligate interactions that tie the community together
Community as a Super-organism
• Population should be bound together in a network and
organized by an
obligate interaction.
• How strong are the connection between species?
Rivet Model
• Species in
community are like the rivets in the
wing of airplanes
• If one species is reduced or
increased, many other are affected
REDUNDACY MODEL
• The web of life is very loose. An increase or decrease in one
species has little effects on other
species in community
• If predator disappear, another species will take place
Note: We human, are lack of complete
information about nature. Never try to do fool action to destroy the balance of the nature
• Temporal structure is the timing of the appearance and activity of species.
Some communities e.g.
paddy field performed a temporal pattern.
• Example: Plankton community is found only during the
submerged period
• Diversity is combination of species richness i.e. number of species in the community, and their relative abundances
(eveness).
• Species are not equally
abundant, some species occur in large percentage of samples, others are poorly represented (dominance).
• Some communities, such as tropical rainforests, are much more diverse than others, such as the great basin desert.
• Species Diversity is often
expressed using diversity index
Diversity and Abundance
• Relative abundance (% each species
contributes to the total number of individuals)
Simpson’s diversity index
• D = 0 - 1
– 0: high diversity – 1: low diversity
• Stand one (Table 16.1): D=0.13
• Stand two (Table 16.2): D=0.36
• Tropical habitats support much larger numbers of species of organisms than do temperate and
polar regions.
Species richness generally declines along an equatorial-polar gradient
Latitudinal Diversity
Species richness generally change trough environmental gradient
RIVERS
The Sea
Up-stream Arus kuat
Dangkal Sempit Jeram, riam,
Berbatu Slope dan elevasi
Erosi
Produktivitas rendah Organisme sedikit Middle
Arus melemah Luas dan dalam
Slope menurun Erosi sedikit Mulai Sedimentasi
Produktivitas naik Diversitas organisme naik
Arus paling lemah Luas dan dangkal Sedimentasi partikel kecil
Produktivitas tertinggi Ekstrim
Diversitas rendah
Down-stream
The species-area curve quantifies what may seem obvious: the larger the geographic area, the greater the number of species.
Species richness is related to a community’s geographic size
• Because of their size and isolation, islands
provide great opportunities for studying some of the biogeographic factors that affect the species diversity of communities.
– Imagine a newly formed island some distance from the mainland.
Species richness on islands depends on island size and distance from the mainland
DIVERSITY-STABILITY
Types of stability:
1. Resilient
community: returns to former state after disturbance
2. Resistant
community:
changes little in response to
disturbance
• Trophic structure is the hierarchy of feeding. It describes who eats whom
• (a trophic interaction is a transfer of energy: i.e., eating, decomposing, obtaining energy via
photosynthesis).
• For every community, a diagram of trophic interactions called a food web.
• Energy flows from the bottom to the top.
TROPHIC STRUCTURE
It is a key factor in community dynamics
Quaternary consumers
Tertiary consumers Carnivore
Carnivore Carnivore
Carnivore
Secondary consumers
Carnivore Carnivore
Primary consumers
Zooplankton Herbivore
Primary producers
Phytoplankton Plant
A terrestrial food chain A marine food chain
• Keystone species are
disproportionately important in communities.
• Generally, keystone species act to maintain species diversity.
• The extinction of a keystone species eliminates the niches of many other species.
• Frequently, a keystone species modifies the
environment in such a way that other organisms are able to live, in other cases, the keystone species is a predator that maintains diversity at a certain trophic level.
Without Pisaster (experimental) With Pisaster (control)
1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 20
15 10 5 Number of species present 0
K E Y S T O N E S P E C I E S
Examples of Keystone Species
• California Sea Otters: This species preys upon sea urchins, allowing kelp forests to become established.
• Pisaster Starfish: Grazing by Pisaster prevents the establishment of dense mussel beds, allowing other species to colonize rocks on the pacific coast
• “Mangrove” trees: Actually, many species of trees are called mangrove
trees. Their seeds disperse in salt water.
They take root and form a dense forest in saltwater shallows, allowing other species to thrive
100 80 60 40 0 20
Sea otter abundance Otter number (% max. count)
400 300 200 0 100
Sea urchin biomass Grams per 0.25 m2
10 8 6 4 0 2
Total kelp density Number per 0.25 m2
1997 1993 1989
1985 1972
Year Food chain before
killer whale involvement in chain
Food chain after killer whales started preying on otters
Observation of sea otter populations and their predation shows how otters affect ocean
communities
Trophic Cascades
• Species at one trophic level influence species at other levels; the addition or subtraction of species affects the entire food web.
– This causes positive effects for some species, and negative effects for others. This is called a trophic cascade. For instance, removing a secondary
consumer might positively affect the primary
consumers they feed upon, and negatively affect the producers that are food for primary consumers.
Top down vs. Bottom up
• Most biological communities have both top-down and bottom-up effects on their structure and composition.
– In a well known study of ponds by Matthew Leibold, it was demonstrated that the biomass of herbivores
(zooplankton) was positively correlated to the biomass of producers (algae), indicating a top down effect.
– He intentionally introduced fish to some ponds, The result was a decrease in zooplankton and increase in producers, indicating a top down effect.
Species with a Large Impact
• Certain species have a very large impact on community structure
• Such species are highly abundant or play a pivotal role in community dynamics
• Dominant species are those that are most abundant or have the highest biomass
• They exert powerful control over the
occurrence and distribution of other species
• One hypothesis suggests that dominant species are most competitive in exploiting resources
• Another hypothesis is that they are most successful at avoiding predators
• The niche concept is very important in community ecology.
• A niche is an organism’s habitat and its way of making a living.
• An organism’s niche is reflected by its place in a food web: i.e, what it eats, what it competes with, what eats it.
• Each organism has the potential to create niches for others.
• Disturbance creates opportunities for new species to invade an area and establish themselves.
• These species modify the environment, and create opportunities for other species to invade. The new species eventually displace the original ones.
Eventually, they modify the environment enough to allow a new series of invaders, which ultimately
replace them, etc.
Disturbance , Invasion, Succession
Disturbances are events such as floods, fire,
droughts, overgrazing, and human activity that damage communities, remove organisms from them, and alter
resource availability.
• Fire
• Floods
• Drought
• Large
Herbivores
• Storms
• Volcanoes
• Human Activity
Succession
• Disturbance of a community is
usually followed by recovery, called ecological succession.
• The sequence of succession is driven by the interactions among dispersal,
ecological tolerances, and competitive ability.
–Primary succession-the sequence of species on newly exposed landforms that have not previously been influenced by a community, e.g., areas exposed by glacial retreat.
–Secondary succession occurs in cases which vegetation of an area has been partially or completely removed, but where soil, seeds, and spores remain.
·A climax community is a more or less permanent and final stage of a particular
succession, often
characteristic of a restricted area.
·Climax communities are characterized by slow rates of change, compared with more dynamic, earlier
stages.
·They are dominated by species tolerant of
competition for resources.
