Download Community Ecology Community - a group of species that live and

Community Ecology
Community - a group of species that live and interact within a
relatively well defined geographic area
Typical communities
Oak-Hickory forest
Nearshore
Cypress-Tupelo
Desert
Spartina marsh
Prairie Grassland
Mangrove
Serpentine soil
Coral reef
Peat Bog
Sometimes a community is defined by the range of one or a
few species - usually plant species (Oak-Hickory),
other times it is defined by a distinct geographic region (nearshore),
and other times it is defined by a physical parameter (e.g. desert,
serpentine soil)
Questions addressed by community ecologists:
What organizes communities? What determines what kind of
species live together in an area? Why don’t other species live
among them? How do species interact? What are the results of
their interactions?
Properties of individual species determine where each species
can live. Interactions between species may restrict where each
species is found.
The full range of resources and habitat space that a species
might potentially use is its “fundamental niche.” Species can
only exist for long periods of time within their niche.
A niche is more than just a place or habitat. It is a description
of how and where the species exists.
Species commonly do not exploit their entire niche. They utilize
only a portion of their niche. The portion they utilize is called
their “realized niche.”
Species are often restricted from utilizing their entire niche due to
the presence of other species.
Interspecific Competition is an important type of interaction
between species that organizes communities and limits the size of
a niche
Interference competition occurs when two species engage
physically while trying to secure a resource
Exploitative competition occurs when two species both
utilize the same resource but do not interact directly
Classic example - interaction between barnacles - Chthamalus
and Semibalanus
Chthamalus lives in the
intertidal zone
Semibalanus lives in the
subtidal zone
If Semibalanus is removed
Chthamalus also utilizes the
subtidal zone
Semibalanus is unable to live
in the intertidal zone due to
desiccation
Semibalanus excludes Chthamalus from the subtidal zone through
its rapid growth - undercuts or crushes Chthamalus - interference
The realized niche is smaller than the fundamental niche for
Chthamalus
Competition - Exclusion and Coexistence
Early experiments (by Gause) with cultures of more than species
of Paramecium showed that one species can cause the extinction
of another
Each species had
logistic growth
when cultured
alone
When cultured in pairs
either exclusion or
coexistence occurred
Gause’s Competitive Exclusion Principle - when two
competitors utilize the same required resource that is available in
only limited quantities, one species will cause the extinction of
the other - “complete competitors cannot coexist”
Exclusion doesn’t happen if the two species are able to exploit
portions of their niche that don’t overlap with the other’s
Gause found P. caudatum was able to
feed on bacteria at the top layers of
the culture tube and excluded P.
bursaria from that zone. P. bursaria
fed on yeast in the bottom layers of
the culture and excluded P. caudatum
from that zone.
When species coexist through utilization of different types of
resources or different ranges of a limiting resources it is called
“Resource Partitioning” or “Niche Partitioning”
Anolis lizards in Cuba live in the same area but exploit different
microhabitats
Species under continual competition will sometimes evolve
because of selection against those individuals that suffer the most
severe competition
Finches in the Galapagos
Islands feed on seeds and
their beak size determines
the size seeds they can eat
When found on different
islands (allopatric) the beak
sizes are similar
When found on the same island (sympatric) they have divergent
beak sizes - greater difference when sympatric than when
allopatric is called “Character Displacement”
Competition for seeds is highest on intermediate sized seeds - one
species specializes on large seeds, the other on small seeds
The effects of competition can be demonstrated with field
experiments
Fenced
enclosures
constructed to
exclude
Kangaroo rats
but not
exclude
smaller rodents
A larger number of smaller rodents could live inside the
enclosure when Kangaroo rats were excluded They were each exploiting some of the same resources - after
removal the small rodents did not have Kangaroo rats as
competitors
Predation is an important factor in community organization
Predators exploit prey and can cause
extinction of prey - Didinium preys
on Paramecium
But often prey have refuges where predators
can’t exploit them
The presence of a predator feeding on one or
more competing species will allow
competitors to coexist because prey numbers
are kept low and they can’t completely
exploit a resource that limits another species
Predation selects for behaviors or physical features in prey that
deter predation
In plants, physical characteristics include spines, thorns, irritating
hairs, chemical defenses such as irritants or toxins
Chemical defenses include ortho-quinone in many vegetables,
mustard oils in cabbage and relatives, opiates in poppies,
THC in marijuana, digitalis in foxgloves, cardiac glycosides
in milkweed, caffeine in many plants, ephredrine in Ephedra
Chemicals that have no metabolic function and are effective
deterrents of herbivores are called “Plant Secondary
Compounds”
The presence of secondary compounds selects for characteristics
in herbivores that reduce the effects of the toxin
Cabbage butterflies
can tolerate
mustard oils and
their caterpillars are
the only important
herbivores on many
species in the group
Monarch butterfly caterpillars feed on
milkweed and incorporate the cardiac
glycosides into their bodies. They become
toxic to any vertebrates that might eat them.
The plant evolutionary response to effective herbivores can be to
increase the concentration of toxin in their tissues - can result in a
“coevolutionary arms race”
Animals also have strategies to avoid predators
Crypsis (camouflage) is used by many
species
Chemical defenses are also used by animals and they are often
associated with aposematic (warning) coloration
Some species evolve to look like toxic or otherwise noxious
species and gain reduced predation - “Mimicry”
Batesian mimicry - a toxic
model and a nontoxic mimic the Monarch and the Viceroy
Mullerian mimicry two toxic species
evolve to look alike both benefit from the
other’s presence
Other mimics
A beetle
A fly
Coral and
King
snakes
Many species only exist due to the beneficial actions or presence
of another species
Symbiosis - “living together”
Symbiotic interactions can be defined through their effects
N am e of
interaction
Com m ensalism
M utualism
Parasitism
Effect on
species 1
Effect on
species 2
+
+
+
0
+
-
Commensalism
Many examples - humans (0) eyelash mites (+), cattle (0) and
egrets (+), anemones (0) and clownfish (+)
Mutualism
Many more examples - corals and dinoflagellates, sea squirts and
photosynthetic bacteria, trees and mycorhizzal fungi, ants and
aphids, acacias and ants, plants and pollinators and seed dispersers
Acacia provides food
from extra-floral
nectaries and Beltian
bodies, and nurseries
inside enlarged thorns
Ants provide protection
from herbivores and
clear area of competitors
Parasitism
Even more examples
Ectoparasites - lice, ticks, fleas, leeches
on humans and others, fungi like
ringworm, athletes foot, plants that
parasitize others: dodder, mistletoe
Parasitoids - insects larvae feed on living host - screw-worm fly,
bot fly, many wasp larvae feed on living but stunned caterpillars or
spiders
Endoparasites - protistans include trypanosomes, Plasmodium,
Entamoeba histolytica, worms include many nematodes and
flatworms, many fungi parasitize plants
Brood parasitism - the eggs of one species are left in the nest of
another - seen in several birds, and many fish
Cuckoos leave an
egg in another bird
species nest
Young hatch early
and expel eggs of
parent
Parent cares for
cuckoo until fledged
Some bird species have developed the ability to detect cuckoo eggs
when they don’t look like their own - In some areas cuckoos have
eggs that match the color of their favorite host - another “arms race”
Some species make conditions right for the coexistence of many
species - these are called “Keystone Species”
Predators can be keystone species because they reduce
competition and promote coexistence of prey species
Beavers create
a habitat for
many species
that would
have no habitat
otherwise
Coral polyps
create a habitat
for thousands
of species
Not all species interactions are direct one species may have an effect on
another through its interaction with a
third species - these effects are
“indirect effects”
Rodents and ants both eats seeds
Ants prefer small seeds
Rodents prefer large seeds
Ants and rodents compete for
seeds - direct effect
But the presence of rodents
benefits ants because rodents
each large seeds, leaving small
seeds to germinate and
benefiting future ant generations
Disturbance can play an important role in communities
Fires, floods, freezes, droughts, tree-falls, etc. all reduce
population sizes of competitors and promote their coexistence
Very frequent and severe
disturbance can cause species
extinctions
No disturbance can result in
competitive exclusion
Intermediate levels of
disturbance reduces competition
and maintains high diversity
This is the “intermediate
disturbance hypothesis”
Communities change and develop by passing through a series of
stages - each stage is called a “sere” - the whole process is
called “succession”
The first type of succession described was on cleared farmland
that was allowed to return to forest naturally. In such situations
the seres are often - grasses and weedy dicots (good dispersers),
followed by shrubs and larger herbs (good competitors), followed
by small trees like pines (better competitors), and finally
followed by hardwoods (the best competitors)
This type of succession - on reclaimed land - is called
“secondary succession”
The last sere is maintained until a major disturbance - the last sere
is called the “climax community”
Primary succession is the development of each sere starting
from sterile rock or soil - e.g. after glacial retreat
Lichens are the first colonizers. They help weather rock into
patches of soil. Mosses colonize the soil and improve it further
by adding organic matter. Shrubby alders colonize and make the
soil acidic, which makes it favorable for Spruce. Spruce grow
and their shade eliminates the alders.
The overall trend in succession is from r-selected to K-selected
species. The early species are tolerant, and facilitate the entry
of intermediate sere species, climax species inhibit the entry of
other species