Community ecology:
community - groups of organisms living close enough together for interactions to occur.
Some features of communities include:
Biodiversity - the variety of organisms that make up a community. This is composed of:
a) species richness - how many different species are there?
b) relative abundance of each species.
measures of diversity usually include both features (there are many different ways
to measure diversity [Fig., similar to 37.10 A & B, p. 748].
For example:
# of individuals of each species
species A species B species C species D species E
community 1: 25 25 25 25 0
community 2: 97 1 1 1 0
community 3: 20 20 20 20 20
Community 1 is obviously more diverse than community 2 (right out of the text),
but community 3 is more diverse than community 1.
Competition
- this is when organisms compete for the same resources. There are two types of
competition:
- interspecific (between different species)
- An example [Fig., not in book (in 5th ed. on p. 717)]:
- two species of barnacle. One is found further up on rocks
than the other (it can survive drying out at low tide better
than the other).
- if the species that lives lower down in the water is
removed, the upper species moves down and takes
over - it CAN live lower down if the other one isn’t
there.
- if the species from lower down is put back, then it
quickly drives the species from the upper zone to
extinction in the lower area - it outcompetes the
species form the upper area in it’s preferred habitat
(the lower area).
- this gave rise to the “Competitive exclusion” principle.
- two species in the same place cannot coexist if they are
competing for the same resources.
- one will do better than the other.
- intraspecific competition (within the same species)
- example: plants all need water & nutrients from the soil. If there
are too many individuals in the area, the plants will be stunted and
not grow well. This is very important in agriculture.
- competition has given rise to the concept of “niche”
- niche is the overall biotic and nonbiotic factors that an organisms
needs to survive.
- biotic - biological factors such as food availability
- abiotic - non biological factors such as temperature or
rainfall.
Predation
- often leads to the evolution of characteristics that help animals catch prey, or
help animals avoid becoming prey.
- camouflage (blending into the surroundings)
- mimicry (looking like something deadly so you don’t get bothered)
- predation can actually increase diversity:
- example [Fig., not in book (on p. 720 in 5th ed.)]:
- kelp forests of the West Coast.
- sea otters feed on sea urchins.
- sea urchins feed on kelp
- kelp supports a large diverse community of other organisms.
- when sea otters were seriously endangered, sea urchins had a
field day, and kelp forests disappeared in many areas.
- as sea otter populations recovered (they had been decimated by
the fur trade), kelp forest started coming back.
- in some areas, though, killer whales are now eating sea otters,
and kelp forests are disappearing in those areas.
- (killer whales are eating more otters because seals and sea
lion populations have declined.)
Symbiotic relationships:
- interactions between two or more species. Three kinds (actually, competition
and predation could be lumped here as well) [Fig., similar to p. 742 (some
examples in the table are a bit silly)]:
- parasitism - one species is hurt, one comes out ahead.
- tape worms, ticks, round worms, pinworm, etc.
- true parasites rarely kill their host (or they’re dead, too).
- commensalism - one species gets a benefit, the other is unhurt (rare)
- few real examples, but they might include:
- algae on turtle shells, barnacles on whales, birds
following large ungulates (they get insects scared up by the
ungulates).
- mutualism - both species come out ahead
- Sea anemone & clown fish:
- sea anemone gets food from the fish, fish gets protection
- Acacia & ants:
- ants get shelter & food from plant, ants in turn fiercely
protect the tree (if ants are removed, tree often dies).
Disturbance:
- things like fire, hurricanes, overgrazing, human activities, etc., can all disrupt a
community [Fig., not in book]
- not all disturbances are bad
- some communities rely on these to perpetuate themselves
- some pine trees will only release their seeds after fire has caused
their cones to open.
- after a disturbance, a community goes through what is called
“succession”
- succession - starting with a life-less area, and “succeeding” through
various stages until we get our community back [Fig., 37.12, p. 750].
- e.g., grasses --> shrubs --> pine trees --> deciduous trees
Trophic levels:
- the energy that is available to a certain grouping of organisms (see below).
- this explains why there are more herbivores than carnivores.
- essentially, all life is supported by autotrophs (= producers). These are mostly
plants and algae.
- they provide energy for other organisms (obviously, this is usually not
voluntary)
- herbivores eat producers, carnivores eat herbivores, other carnivores eat the first
carnivores, and so on [Fig. 37.8, p. 746].
- animals are classified as “primary consumers” (herbivores), secondary
consumers (carnivores that eat herbivores), tertiary consumers (they eat
the previous carnivores), and so on.
- there are also detritivores, which eat dead stuff (vultures, many fungi, dung
beetles, etc.)
- put all this together, and we get food webs [Fig., not in book and 37.9, p. 747].
- but, this can’t go on forever. Energy is limited.
- only about 10% (on average) of the energy at one level (say, producers,
makes it to the next level (say, primary consumers).
- [OVERHEAD, fig. 37.16, p. 753]
- incidentally, only about 1% of sunlight is captured by photosynthesis
- this makes it difficult to support too many trophic levels (e.g., too many
quaternary consumers)
- note that this also explains why meat (incl. fish) is not the most efficient way to
feed humans:
- [Fig. 37.17, p. 754]
- cows only provide 10% of the energy to humans that plants can.
Finally, just a quick word on chemical cycles.
The book only lists four of the most important ones, but there are many more:
Water, Carbon, Nitrogen & Phosphorus
- in all of these, the elements (or compounds) are recycled through the
ecosystem.
- we don't have the time to look at these in detail, but just be aware they
exist.