Lecture 6 Notes
Dispersal; biological realms: past and present patterns
The development of a group of organisms,
whether a species or family or order appears to consist of:
(1) a localized beginning,
(2) an expansive phase with adaptive radiation, and
(3) ultimately a regression with or without the formation of a new and more
Of basic importance to the second
phase and therefore to the study of the distribution of organisms is the
concept of species dispersal or spread. Dispersal can be divided into
three main modes, all of which result in changes in the geographic range
of a species:
(1) Jump-dispersal is the movement of individual organisms across
great distances, followed by the successful establishment of a population
of the original disperser's descendents at the destination. This usually
takes place over a time period less than the life span of the individual
and often over inhospitable terrain. For example - spiders are carried
across the open sea by air currents.
(2) Diffusion is the gradual movement of populations across hospitable terrain
for a period of many generations. Species that steadily expand their
ranges can be said to be diffusing. For example - killer bees spread northward
in South America and into North America following their introduction into
(3) Secular migration is diffusion taking place so slowly that the diffusing
species undergoes appreciable evolutionary change during the process.
The range of the species expands or shifts over long time intervals (thousands
or millions of years). The environments themselves may change and natural
selection acts on the descendant populations For example, hte llamas and vicunas
of South America are descended from now extinct North American members of
the camel family that migrated during the pliocene over the Isthmus of Panama.
Acting in conjunction with the spread
mechanisms to effect changes in range is local extinction, so that ranges
can expand, contract or "creep" (expand in one direction while contracting
in another). In practice spread often consists of a mixture of the above
modes across terrain with various degrees of hospitableness. Any terrain
which is inhospitable enough to prevent or slow spread of the species can
be termed a barrier. For example, a body of water is a barrier to a
terrestrial organism, just as dry land is a barrier to a marine organism.
In particular, Beringia, the area between Alaska and Siberia, has functioned
simultaneously as a bridge for terrestrial animals and a barrier for marine
ones during periods of marine regression and vice versa during periods of
marine transgression. Moreover, one terrain type may be a barrier to
one mode of dispersal but not to another - for example, a desert may be an
impassable barrier to slow diffusion of a mesophytic plant (plant with moderate
moisture requirements) but offer no resistence to the rapid passage of wind-blown
seeds across it (jump dispersal). So the second principle is
that barriers can cause long delays in dispersal.
The third principle is that environments
are constantly changing and affecting dispersal, colonization, speciation
and extinction. Climate change, for example, also changes terrestrial or marine
routes from barriers to bridges or vice versa for different organisms, either
directly through the organisms’ temperature tolerance limits or indirectly
through effects on vegetation that may serve as food or as barriers to the
spread of seeds.
The fourth principle is that new species
which survive successfully are generally better adapted to their surroundings
than their predecessors, although there are exceptions including many domesticated
The fifth principle is that the present-day
distributions of species do not necessarily reflect the areas of origin.
The sixth principle is that organisms
with a previously wide distributions may become restricted eventually leaving
a relict population, or become extinct in the face of increasing competition
and/or adverse changes in the environment.
In general, plants spread more
rapidly than animals and as a result, the modern distributions of plant species
tend to reflect current conditions of climate and soil, while those of animals
are more likely to reflect the geographic and geologic history of a region.
Keep these 6 principles in mind as
we go on to consider:
The effects of continental drift on species spread:
Continental drift affects all three
modes of species spread. Of itself, it accomplishes (for terrestrial organisms)
the slowest form of secular migration, by gradually rafting species - and
even whole communities - across enormous distances. Organisms whose
observed distributions are directly attributable to this rafting are likely
to be those for which jump dispersal is improbable, thus the evidence of slow
drift has not been obscured. For example, large flightless vertebrates, freshwater
fishes, or invertebrates that cannot survive drying, or exposure to salt
The present-day distribution of earthworms
is the obvious outcome of continental drift. There are several Gondwanaland
genera that occur in southern South America, South Africa and Australasia,
whereas further north, another group is common to West Africa and Central
America. Further north still, the majority of earthworm species are
the same on both sides of the Atlantic. This preservation of past continental
distribution in earthworm distributions is not surprising, since earthworms
cannot survive submersion in salt water, they live in the soil where their
eggs are unlikely to become attached to ducks feet, and they are particularly
susceptible to desiccation.
Indirectly, continental drift has
affected the spread of plants and animals by diffusion and secular migration,
by breaking and creating land and marine bridges such as Beringia, or by
shifting land masses into different climatic zones. For example, the mountains
of Malaysia and New Guinea, uplifted in Pliocene-Pleistocene time, provided
a cool, high altitude route for the diffusion of Northern Hemisphere plants
into Australia. The dispersal of terrestrial organisms depends not only on
the existence of land bridges or stepping stone routes, but also on those
routes having climates that the dispersing organisms can endure.
Besides causing great change in the
temperature and moisture regimes of an area, the rearrangement of the continents
also has brought changes in the strength and direction of prevailing winds
with important consequences for wind-dispersed organisms.
For organisms capable of jump dispersal,
as continents drift towards one another, the number of organisms that can
jump disperse gradually increases as the gap narrows until contact is made
and diffusion can also take place. As land masses drift away from each
other, jump dispersal becomes less frequent. The clearest indication
of non-occurrence of spread is obtained when sequences of fossil biotas are
found that become steadily more different from each other the more recent
they are, such as the post Cretaceous floras of South America and Africa.
The effects of continental drift on
species distribution (and diversity) are by no means straightforward and I
want, now, to return to a historical approach to explore some of the theories
about events and interactions after the formation of Pangea.
Evolution and dispersal during the Mesozoic
At the end of the Paleozoic (that
is at the Permian/Triassic boundary), a wave of extinctions is believed to
have greatly reduced the diversity of the marine fauna of the continental
shelves. One theory to explain this, suggests that after the formation
of Pangea, the crustal plates were for a time motionless - consequently,
the formation of new oceanic crust at the mid-ocean ridges stopped and the
ridges collapsed. This caused a large drop in sea level, converting
much of the continental shelf into dry land. Reduction in extent of
their environment therefore reduced the numbers of individuals and taxa of
shallow water marine forms.
An alternative explanation is that
because the formation of Pangea resulted in one long unbroken and relatively
uniform continental shelf, the spread of species resulted in competative exclusion
and therefore extinctions. Yet another explanation suggests that this
decrease in diversity is simply an artifact of the fact that not much sediment
was laid down in the many cool , dry areas, so that few fossil samples are
available and those that are known do not therefore represent the entire
Pangea remained intact for the major
part of the Triassic Period, the first period of the Mesozoic Era, and throughout
this period the landmass moved slowly northward and so continents remained
elevated, and climates remained cool, dry and seasonal in much of their interiors.
The sea was still largely open and therefore relatively warm and therefore
contributed to relatively warm climates in high latitudes.
Triassic faunas and floras were markedly
provincial. Terrestrial biotas are commonly divided into northern (Laurasian)
and southern (Gondwanan) realms. The biotic character was probably linked
to the dry or seasonal interior climates that we have just mentioned. The
diversity of conifers, seed ferns and cycads increased. Conifers were dominant
in Laurasia while seed ferns dominated the Gondwanan realm until late in the
period when conifers became more common there as well. The mammal-like
reptiles (Pelycosaurs) that had diversified and spread throughout Pangea during
the Permian eventually gave way to mammalian anscestors (the cynodonts and
therapsids) and to the diapsids (including archosaurs) and, by the late Triassic,
the archosaur descendants, the first dinosaurs.
It is believed that the diapsids were
favoured at this time over the mammalian ancestors by their ability to excrete
uric acid rather than urea, since this is more water efficient. The dominance
and diversity of large herbivores increased during the middle and late Triassic
and amongst these, the cynodonts, dicynodonts, and rhyncosaurs (thecodonts
in the figure) were particularly common in Gondwana. The latter two groups
became extinct by the end of the Triassic.
During the late Triassic, Pangea began
to split into fragments. In the Jurassic, North America was probably moving
away from Africa and from South America. Eurasia also began to move away from
North America and finally Africa began to split from South America and Antarctica.
However, the process was very slow and separation by ocean basins probably
did not occur until the late Jurassic. Global climates continued to be generally
warm, but with strong latitudinal variation, particularly in rainfall.
During the Jurassic Period, cycads,
cycads, ferns, and conifers developed further in both the Laurasian and Gondwanan
realms. Different groups of conifers were more important in Laurasia and Gondwana.
Many groups of herbivorous insects were arose during the Jurassic.
The larger land fauna was dominated
by large herbivorous reptiles, notably the dinosaurs. The dinosaurs diversified
and spread throughout the world and to the air (pterosaurs) during the early
and mid Jurassic before extensive continental separation had occurred. The
route between North America and Asia was by way of the mild-climated Greenland
and Europe. These large animals reached Africa either directly, or via
Toothed birds (Archeopteryx) evolved
from the thecodont reptiles. By the late Jurassic, the Theria existed
in North America - these were the first small, primitive mammal ancestors
of monotremes, marsupials and placentals.
During the early Jurassic, sea level
rose and marine habitat diversity again increased. In the shallow waters
of tropical and low latitude continental shelves and slopes, fluctuations
of temperature and salinity were relatively small and here in particular,
marine invertebrates again increased in diversity and numbers. Ammonites were
at their peak and the first scleractinian (hard) corals appeared. Ichthyosaurs
and plesiosaurs reached their peak in diversity and abundance.
By the late Jurassic,
North America/Greenland, Gondwana and northern Eurasia were separate continents.
On land, distinct floral provinces developed in temperate and tropical regions
of both the northern and southern hemispheres. Conifers, cycads and other
gymnosperms continued into the Cretaceous, but it is likely that, around this
time, the first flowering plants (angiosperms) developed in tropical uplands
with a warm, seasonally-dry climate - most likely in the interior of west
Gondwana. The reason for this is thought to be that the enclosed ovules,
characteristic of angiosperms, would have been selectively favoured in a
region of seasonal drought.
Gradually, during the Cretaceous, angiosperms expanded their ecological tolerances
and hence their geographic ranges. They migrated altitudinally downward and
upward and, latitudinally, northward and southward. However, primitive
angiosperms have inefficient dispersal mechanisms and therefore cannot escape
into new regions if the climate of their ancestral region deteriorates.
Therefore, today, they persist only in regions where the climate has remained
much as it was when they first evolved - that is, southeast Asia, northeastern
Australia and India. Their confinement to unchanging environments also means
that they are not exposed to the rigours of strong selective forces - hence
they evolve slowly and remain primitive. More advanced angiosperms,
on the other hand, with efficient long-distance dispersal mechanisms often
find themselves in habitats markedly different from the ones they come from
and will therefore be subject to intense natural selection. Hence they
Along with the development of angiosperms
went further radiation of the insects whose behaviour, life cycles and distributions
are intimately connected with plant morphology and distributions - as is evident
in the Lepidoptera (butterflies and moths) and advanced bees (Hymenoptera).
They too had the advantage of efficient dispersal mechanisms including, by
this time, flight. In general, the angiosperms gradually became dominant
over the gymnosperms and ferns - such that, by the end of the Cretaceous,
flowering plants made up 50 to 80% of the species in middle to high latitudes.
Although small mammals were present,
the dinosaurs remained the dominant land vertebrates during the Cretaceous.
New types dispersed throughout the northern hemisphere while it was still
undivided by seaways. Those that evolved later had more restricted distribution
patterns because sea floor spreading increased as the continents broke apart
causing a rapid rise of the ocean ridges and a consequent rise in sea level.
As sea level rose towards its maximum, inland seas flooded low relief parts
of the continents isolating new continents in both the northern and southern
hemispheres. In addition, land barriers such as the Andes, Himalayas and
Rocky Mountains were built at this time.
Vertebrate assemblages began to show
pronounced differences between the northern and southern hemispheres. In the
south, sauropods were the dominant herbivores while, in the north, ornithopods
(circumboreal) and ceratopsians (North America only) were more abundant -
although pachycephalosaurs developed at this time in the northern hemisphere.
As the mammals evolved, the marsupials
and monotremes probably first became established in the South America/Australia/Gondwana
land mass. Marsupials dispersed to North America and throughout eastern Gondwana
and were the dominant mammal group in North and South America for most of
the Cretaceous. Climates remained mild and the mammals therefore were
able also to reach Europe via Greenland. The placentals developed first
in what is present-day Asia and reached North America via Beringia during
the Cretaceous. Some placentals reached South America but, at that time,
neither group reached Africa or India since these continents had separated
before the late Cretaceous.
The raised sea level also provided
an increased area of warm water for colonization and niche partition by marine
invertebrates. The oxygen isotope record shows that the deeper waters of these
oceans were 15˚C warmer than those of today. As sea floor spreading
progressed, deep ocean trenches became barriers causing genetic isolation
and an accelerated rate of morphological divergence of shallow water marine
From the late Cretaceous onwards,
as the continents moved further apart, the earth's climate became cooler
and more seasonal. We will look at Cenozoic climates more thoroughly in a
later lecture but at this point I want to jump to the present in order to
better describe the surviving continental configurations.
The modern world can be divided up
into regions on the basis of the present distribution of its flora and fauna,
that is, the dominant and easily visible groups of organisms occurring there.
These current assemblages of organisms are the result of the early history
of the groups, together with continental drift and climate change during the
Cenozoic Era. By looking at their present distributions, we can see
exactly how these distributions are related to factors such as space, climate,
barriers, plant cover and each other.
A biological subdivision of the earth's
surface can take account of either the terrestrial or the marine biosphere.
In a terrestrial subdivision, the oceans are treated as lifeless. Different
systems of biogeographical classifications have been proposed based on different
groups of organisms, and disagreement over the ranks to be assigned to the
units and the exact locations of boundaries are inevitable. A system
may use up to four different ranks: the realm, region, subregion and province,
but the region is the unit most often referred to.
The regions that you are most likely to see referred to in the literature
Nearctic - including North America, northern Mexico and Greenland
Palearctic - Europe, northernmost Africa and northern Asia
Neotropical - Central America, South America, tropical Mexico and the Caribbean
Ethiopian - the remainder of Africa, Madagascar and southern Arabia
Oriental - India, Indo-China, southern China, Malaya (tropical Asia and
closely associated islands)
Australasian - Australia, New Guinea, New Zealand and associated islands
Antarctic - Antarctica
Oceanic - South Pacific islands, no large land masses
The regions are sometimes grouped
into 3 realms that reflect their physical relationship to one another:
(1) Megagea (Arctogea) - the great part of the world;
(2) Neogea - the Neotropical region alone; and
(3) Notogea - The Australasian, Antarctic and Oceanic regions.
The regions can also be grouped in other ways that emphasise the type of
boundary separating them from other regions - for example, climatic boundaries,
or salt water boundaries.
The designation of these regions represents
an average pattern of the distributions of different major groups of terrestrial
animals - and mammals and birds have been used most often. The regions
show, in a broad way, how animal distribution is fitted to the world and how
climate and barriers affect them most deeply. They can be used as a
sort of meter stick by which the distributions of different animals can be
measured, described and compared. They also allow special features to be
determined and important things about the animals and their histories to
be revealed. Deviations from the standard patterns are expected and
informative. If, for example, a particular group of animals reaches
the Australian region, but shows less than standard differentiation there,
this suggests recent dispersal and the ability to cross bodies of water.
Faunal regions have other uses.
For example, they help zoologists in different parts of the world set natural
limits to regional studies. Also the names of the regions are useful
terms. For example, to say "Oriental Region" is simpler, more exact and therefore
more useful than "tropical Asia and certain closely associated continental
Regional faunas are not homogeneous
assemblages of animals uniformly distributed, but instead represent animals
(1) more or less concentrated in favourable places,
(2) vary in composition in different places, and
(3) enter into complex transitions with adjacent faunas.
These transition zones are scientifically very interesting because they
tend to have depauperate faunas with few faunal elements from either side.
For example, Wallacea is the zone between the Oriental and Australasian regions.
In this figure there are two major lines, Wallace’s line and Weber’s line
- Wallacea is the area in between. True to transition zone characteristics,
there are few mammals in this area.
The regions constitute just one of
the tools used in the study of biogeography. Next week we will consider
several others that contribute to the methods by which we can interpret the
distributions of organisms over the earth.
Lecture 5 [Animal Diversity and Evolution] will be
given on October 18th.
* * * *
Midterm Test Example Questions
Which one of these factors does not contribute to sponge diversity?
a. folding of the body wall
b. polyp polymorphism
c. spicule structure
What are considered to have been the two most important steps in increasing
biological diversity on the earth (aside from the origin of life itself)?
Discuss the concept of Biological Realms, emphasising their utility.