Evolution Lecture 11 (Ecology Lecture 2)

NS 102 Andrew Berry's Lectures
2. Evolution & Ecology
Lecture 6: Community Ecology
What determines how many species there are at a given location?
Many factors affect species number.
Species-Area Relation: Larger areas can support more species. This turns out to
be a surprisingly strong correlation. Large areas typically contain a greater
diversity of habitats – which support a greater diversity of species – than small
Island Biogeography Theory. This is applied not only to true islands, but to
“ecological islands” such as widely separated mountain tops – the intervening
lowland habitat is unsuitable for mountain-adapted species – and even “habitat
islands” such widely spaced trees which may be effectively islands for the insects
that feed on them. Note that the perception of the environment’s “grain” varies
from one organism to another. Two herbivores, one an insect, and the other a
large mammal, may feed on the foliage of the same tree species. Individual trees
may be 20 m apart, in which case they appear to be “islands” to the insect species,
whereas the habitat appears continuous to the mammal.
 The number of species on an island is a function of the rate of
immigration (and establishment) of new species (which itself is a function
of the distance of the island from the nearest mainland), and the rate of
extinction of established species. The equilibrium species number is
when immigration rate = extinction rate. Experimental studies in which all
the insects on islands have been eliminated and then recolonization
monitored over time support this simple ecological theory.
Latitudinal trends in species diversity. There are many more species in the
Tropics than there are in the temperate zone. Many reasons have been advanced
to explain this pattern:
 More energy input (from the Sun) in the Tropics. In effect, therefore,
the energy “pie” is larger in the Tropics, which means that it can be
divided up among more species
 Lack of marked seasonality in the Tropics. Species in the temperate
zone have to survive under extremely different conditions - summer and
winter – whereas those in the Tropics have a much more constant
environment (though there may be marked wet and dry seasons). Species
in the Tropics may therefore be better able to specialize than ones in the
temperate zone.
 Long term climatic stability in the Tropics. The temperate zones were
affected by the most recent glaciation (ice age), which ended about 10,000
years ago. Thus it is possible that the temperate zones are still undergoing
post-glacial colonization. No such interruption occurred in the Tropics.
 Species/Area Effect. Because the Tropics are contiguous – ie on both
sides of the Equator – whereas the two temperate zones, N and S, are
separated by the Tropics. The single large tropical zone is predicted by the
species-area relation to have more species.
What determines what species are present in a given community?
Organisms are adapted to certain conditions. Thus we see high densities of a
species in the region to which it is best adapted with a progressive decrease in
density as we travel away from that “ideal” area to regions where the species finds
it harder and harder to survive.
Succession. If we clear a piece of land of all plants, and then allow colonization
to take place we see first one group of plants, which is replaced by another group,
which itself is in turn replaced by another group, and so on. This process of
turnover in species composition is called succession. The first plants are specialist
colonizers, the next plants are ones that can take advantage of the shade provided
and soil consolidation caused by the first plants; under these conditions, the
second group is competitively superior to the first, and replaces it. This facilitates
the arrival of a third group, which eventually will replace the second, and so on.
Eventually, a climax community will result: the mature community that is the end
product of succession. Climax communities are in general much more species
rich than early succession communities.
Competition. From experiments on the protozoan, Paramecium, the Russian
ecologist G. F. Gause derived the Principle of Competitive Exclusion, which
states that: Two species occupying the same habitat can coexist only if they are
not competing for the same resource. Gause raised P. caudatum and P. aurelia in
separate nutrient-rich cultures, and both thrived. If however they were raised
together P. aurelia competitively excluded P. caudatum. If two species that have
slightly different resource use are raised together, on the other hand, they may coexist (as in the case of P. caudatum and P. bursaria; the latter can take advantage
of the low oxygen parts of the habitat where P. caudatum is at a disadvantage).
This principle can be observed in the non-overlapping geographical distribution of
closely related – and therefore ecologically similar – species (eg the distribution of
white-eyes, Zosterops, (birds) on islands around New Guinea). From the
competitive exclusion principle, we have the idea of the ecological niche. G E
Hutchinson (1957) defined a niche as a region (an n-dimensional hypervolume) in
a multi-dimensional space of environmental factors (food preferences, nesting site
requirements, etc etc) that affect the welfare of the species. The niche is in effect
the microhabitat in which the species lives, and the way that the species exploits
the resources of that microhabitat. Competition ensures that there is no significant
overlap of niche among species. This can be seen in nature in the way in which
closely related species divide up the available resources; they specialize on
particular aspects to reduce the effect of inter-specific competition.
Barnacles. Chthamalus and Balanus barnacles use different parts of the
intertidal zone. However, in the absence of the other species, one species will
take over the entire region. Thus when both are present, their range is
determined by a competitive interaction between them, with Chamalus being
competitively superior in upper parts of the habitat, and Balanus in lower parts
of the habitat.
Competition may be affected by predation. When R.T. Paine (1966)
removed the top predator, a star fish, Pisaster, from an inter-tidal ecosystem,
he found that the number of herbivore species declined as a result. Thus the
presence of the predator promoted diversity by ensuring that one herbivore
species did not out-compete the others – if it ever became too common, it
would be knocked back by the star fish. Removal of the predator resulted in
one herbivore being able to take over, by out-competing the other herbivores
in the ecosystem.
Character Displacement. Competition can lead to the evolution of
“character displacement” whereby competing species’s niches become,
through evolution, differentiated. This can most clearly be seen on islands. In
the Galapagos Islands, when the Darwin’s finches Geospiza fortis and G.
fuliginosa occur on separate islands they have middle-sized bills (that can be
used for eating any size of seed). However, when they occur together on the
same island, G. fortis has a large bill (specializing on large seeds) and G.
fuliginosa has a small bill (for small seeds). Thus, natural selection operating
on the species when they co-occur has caused their niches to differentiate.