Barriers to Animal Dispersal

SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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Barriers to Animal Dispersal
Successful long-distance dispersal usually requires that organisms survive significant
periods in environments very different from their usual habitats. These unusual
environments constitute physical and biological barriers that successful colonists must
cross (although most animals cannot do so). The effectiveness of such barriers in
preventing dispersal not only depends on the nature of the environment but also on
the characteristics of the organisms themselves. These characteristics vary from one
taxonomic group to another, so that particular barriers may not affect all residents of
a habitat equally. Thus barriers are species-specific phenomenon.
A barrier is any physical, chemical, or climatic obstruction that the
animals cannot ordinarily cross. It is any factor in the environment
that restricts the distribution of animals.
Organisms that inhabit temporary or highly fluctuating environments are much more
tolerant of extreme or unusual physical and biotic conditions than are species that are
confined to permanent or stable habitats. Plants and animals from fluctuating
environments are also more likely to have resistant life-history stages, which not only
allow them to survive periods of unfavorable conditions, but can also serve as effective
propagules. Hence the “weedy” species that inhabit temporary or fluctuating
environments are likely to be better dispersers and less limited by any kind of barrier
than are species from more permanent or constant habitats.
General Classification of Barriers to Animal Dispersal
Because the effectiveness of a particular kind of barrier depends on both the physical
and biotic challenges it poses and the biological characteristics of the organisms
attempting to cross it, it is difficult to make sweeping generalizations about the nature
of barriers. But since there are so many different types of barriers in nature, they can
be tentatively classified into the following four general categories:

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1. Physiological Barriers
Probably the most severe barriers are presented by physical environments so far
outside the range an organism normally encounters that it cannot survive long enough
to disperse through them. Some of the factors that test the physiological limitations of
animals are as follows:
Physical Obstructions/Topographic Barriers: Animals cannot cross physical or
topographical obstructions like mountains, lowlands, deserts, rivers, canals, oceans,
etc. The Himalayan Mountains are an effective barrier for animals of the Palearctic
and Oriental regions, whereas the Sahara Desert acts as a natural boundary between
the Ethiopian and Palearctic regions. Lowlands serve as a barrier for mountain-
dwelling animals. Rivers also tend to act as barriers for most land animals that cannot
swim and do not have access to any route around them. The oceans separating
various continents are also an effective barrier as they prohibit cross migrations of land
animals.
Inhospitable Climate/Climatic Barriers: Usually, extremely high or low levels of climatic
factors like temperature, humidity, light, rainfall, etc. are not suitable for most
organisms. Organisms adapted to a particular climatic condition will not be able to
tolerate a great deal of deviation from that particular level. These conditions are
different from place to place and hence so are the accompanying flora and fauna.
Salinity: Aquatic organisms face another problem – salt concentration. Most freshwater
organisms cannot resist high salinity levels and the opposite is also true for marine
organisms, although there are certain obvious exceptions to both cases.
Pressure: Pressure becomes a major problem for marine organisms, it increases with
depth, and hence the deeper an organism can live in the ocean the more it is tolerant
to high pressure. Atmospheric pressure can also become a problem on mountain tops.

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2. Ecological Barriers
Many ecological factors restrict animal distribution such as food availability,
incompatibility with an ecosystem, predators, competitors, etc. They are discussed as
follows:
Food Availability: The type and amount of vegetation fluctuate from area to area, which
is especially problematic for monophagic animals such as pandas and koalas which
feed on only one type of food – bamboo leaves and eucalyptus leaves respectively. In
the case of carnivores, the availability of prey species can pose similar limitations.
Incompatible Ecosystems: Animals adapted to one type of ecosystem will not be able to
survive in other neighboring ecosystems, hence it would be extremely hard for them
to disperse through them. For example, extensive forests serve as barriers to the
dispersal of grassland animals, while grasslands serve as barriers to forest animals.
Forest animals do not usually cross the grassland, while grassland animals do not
cross large forests.
Predators & Competitors: Animal movements to other areas are also checked by
predators and competitors already present in those areas (this is made even more
effective by the fact that those animals cannot have possibly adapted any evasive
strategies to evade these predators or any competitive strategies to out-compete these
competitors).
3. Behavioral (Psychological) Barriers
Most organisms have mechanisms for habitat selection, the ability to recognize and
respond appropriately to favorable environments. In some animals, these traits are so
well developed that they strongly inhibit active dispersal. Many bird species are
perfectly capable of flying long distances but are unwilling to cross certain kinds of
barriers. Species adapted to successional habitats must continually disperse to new
environments as their habitat patches undergo succession and become unsuitable.
Such species are therefore much more likely to strike out across barriers and disperse

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successfully over long distances than are species that are restricted to very stable
environments.
4. Artificial Barriers
These include both manmade structures and anthropogenic activities (pollution, global
warming, etc.) that restrict or even reduce animal distribution. In fact, in many severe
cases, species have been pushed into the abyss of extinction while many more are
desperately clinging to the rope of existence (as in being critically endangered).
Barriers in Marine Environments
The barriers that restrict animal dispersal in marine environments are as follows:
1. Land Masses
Landmasses that project into the sea make an effective barrier even if they are very
small. Sometimes, the barrier may be small (i.e. Isthmus of Panama) and very fast-
moving marine animals may be able to cross it. Even in such cases, however, many
animals living on one side are different from their counterparts on the other side.
Figure: The Isthmus of Panama serves as a barrier for marine animals – species of
shrimp on either side of the Isthmus are different from one another.
2. Temperature
Those animals which can tolerate a change in temperature are known as Eurythermal
animals. On the other hand, the animals which cannot tolerate a change in

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temperature, are known as stenothermal animals. Temperature is an effective barrier
for stenothermal animals while eurythermal animals can migrate relatively easily.
Stenothermal animals are usually present only in shallow waters (littoral zone).
The Polar Regions are characterized by low temperatures and the Tropical Regions
are characterized by high temperatures. Fish and invertebrates present in the Indian
and Pacific oceans are similar due to the continuity of the two oceanic waters and the
similarity of temperature. Mollusks are widely distributed in all oceans as they are
eurythermal animals. The molluscan fauna of the Atlantic and Pacific Ocean is
however different.
Figure: A world map showing modern annual mean surface sea temperature (SST)
fluctuations in the world's oceans.
Some animals have a bipolar distribution, i.e. they are confined to the temperate
latitudes of either one or both of the poles (North and South). For instance, the
southern whales and grey dolphins are only found in the seas of southern regions,
whereas some animals like the basking sharks are found in the seas of both northern
and southern regions but not in the tropical regions.

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The occurrence of bipolar distribution is caused by the conditions that prevailed in the
ice age, when the temperature in the tropics dropped and northern organisms were
able to spread to the tropics and reach the southern hemisphere. Later when the
temperature rose, these organisms died out in the tropics, thereby giving rise to an
interrupted range. On the other hand, many animals like whale sharks (the largest
shark species and the largest known fish) are only found in the seas of the tropical
regions as they cannot tolerate the cold temperatures of the northern and southern
waters.
3. Salt Concentration
Salinity does not appear to change much in the marine environment and hence is not
an effective barrier. It is an important barrier in only those regions where large rivers
enter the sea. Animals that can tolerate change in salinity are called euryhaline
animals, i.e. Neries sp. (an annelid). Those animals which cannot tolerate change in
salinity are called stenohaline animals, i.e. haddock (marine fish). Stenohaline animals
cannot migrate from the seas to rivers because of the difference in salt concentration
(which would result in immediate death).
Figure: Average sea surface salinity (this does not fluctuate too much).
Most of the marine fish are stenohaline and they live and migrate wholly in the sea
(they are referred to as oceanodromous). Euryhaline fish species on the other hand

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can move to and fro between marine and freshwater bodies. Some fish migrate from
the sea up into freshwater to spawn, such as salmon, striped bass, and the sea
lamprey (they are referred to as anadromous). There are other fish such as the
aggressive bull shark which migrate from freshwater to the seas, or vice versa, but not
for breeding (they are referred to as amphidromous).
4. Pressure
The animals which can tolerate a change in the pressure are called eurybathic
animals. On the other hand, the animals which cannot tolerate change in pressure are
known as stenobathic animals. Pressure, therefore, acts as a limiting factor for the
distribution of marine animals. Pressure increases with the depth of the sea. Animals
that can tolerate change in pressure are widely distributed.
Figure: As the depth of the ocean increases so does the pressure. Only those
organisms can live in the lower depths, which can tolerate high pressure.

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Pressure is an effective barrier for the distribution of shallow water-dwelling animals,
they cannot reach deep waters because of the increased pressure which is beyond
their tolerance range. One of the deepest diving mammals, the Sperm Whale can go
as deep as 2,250 meters (most other marine animals cannot dive so deep). At a depth
of around 4,000 meters, in the Abyssal zone, frightening creatures like the fangtooth,
flashlight fish, and anglerfish are found.
Deeper than the Abbysal zone lies the Hadal zone, and at a depth of around 6,000
meters, the animal life is sparse and includes: fish, sea cucumbers, bristle worms,
bivalves, isopods, sea anemones, amphipods, and gastropods. The benthic zone (the
lowest level of a body of water, such as oceans, including the sediment surface and
some sub-surface layers) hosts larger invertebrates, such as crustaceans and
polychaetes (which live in close relationship with the substrate and many are
permanently attached to the bottom).
Barriers in Freshwater Environments
The major barriers for animals in the freshwater environment are as follows:
1. Landmasses
Landmasses are the major barrier for the dispersal of freshwater animals from one
freshwater body to another freshwater body, i.e. in between lakes. This is because
most of the aquatic organisms cannot tolerate dryness (and with only a handful of
exceptions – most fish cannot breathe air), hence there is no chance for them to move
across a landmass to invade another water body. Amphibians and certain "walking
fish" – such as mudskippers can migrate over land but in most cases, it is only possible
if the distance is not very much and the land is not completely dry, otherwise even they
cannot migrate from one water body to another.
2. Salt Concentration
Although all the rivers open into the sea, but the migration of stenohaline animals from
freshwater bodies to the sea is not possible because of the difference in salinity of
these two regions (the difference in salinity would kill the animal). Hence most of the

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freshwater fish species migrate wholly within freshwater (they are referred to as
potamodromous). On the other hand, there are some euryhaline freshwater fish that
migrate from freshwater down into the sea to spawn, such as eels (these are referred
to as catadromous).
3. Infrastructures
Manmade structures like dams and barrages create an artificial barrier in rivers which
interrupt the migration of fish species. This has also pushed many species such as
sturgeons to the verge of extinction, as their migratory routes (for breeding) are
blocked.
Barriers in Terrestrial Environments
The major barriers to the dispersal of animals in the terrestrial environment are as
follows:
1. Mountains
A mountain is a large landform that rises above the surrounding land in a limited area,
usually in the form of a peak. A mountain is generally steeper than a hill. Mountains
are the main barriers on land which prevent the distribution of a large number of
lowland animals.
Figure: Major mountain ranges of the world.

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The extremely cold temperature, high atmospheric pressure, altitude, and sparse
vegetation of mountains are the factors that prevent lowland animals from dispersing
through them. The highest mountains of the world – the Himalayan Mountains are an
effective barrier between the Palearctic and Oriental regions, animals on either side of
them are different from one another as they cannot cross them. The opposite is true
for the animals dwelling in mountains, they cannot survive in lowland conditions.
2. Deserts
A desert is a barren area of landscape where little precipitation occurs and,
consequently, living conditions are hostile for plants and animals. The lack of
vegetation exposes the unprotected surface of the ground to the processes of erosion.
About one-third of the land surface of the world is arid or semi-arid. This includes much
of the Polar Regions where little precipitation occurs and which are sometimes called
polar deserts or “cold deserts”.
Figure: Major deserts of the world.
Deserts make sharply defined boundaries between two landmasses. Examples
include the Gobi (rain shadow desert – Himalayan Mountains block rainfall by keeping
clouds from the Indian Ocean from reaching it; it is generally a cold desert) and the
Sahara (the largest hot desert). Animals living in the north of the Sahara are different
from those living in its South. It is a natural boundary between the Palearctic and

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Ethiopian regions. Deserts do not allow the animals to cross them due to severe
environmental conditions: dryness, lack of vegetation, generally high temperatures,
etc.
3. Large River Channels
Large river systems or river channels act as major barriers for the distribution of land
animals, i.e. mammals, birds, butterflies, etc. An example is the Amazon River system
in South America. This because most land animals cannot swim, even those which
can swim may find it very hard to cross fast-flowing rivers. Another factor that makes
rivers effective as barriers is flooding, which may be regular or irregular (mostly
affected by rainfall, melting of glaciers, etc.) Although some animals like bears can
and do cross-river systems.
Figure: Amazon River System.

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4. Regions of Rainfall
Regions with very high annual rainfall have their specialized flora and fauna and do
not allow other animals to cross through, i.e. the Amazonian Tropical Rainforest. Some
phenomena associated with rainfall such as humidity, water availability, probability of
flooding, etc. also have a serious impact on animal distribution.
Figure: Rate of summer precipitation (rainfall) all over the world.
5. Ocean/Sea
Oceans act as a barrier for the distribution of land animals, continents separated from
each other via oceans have very different biodiversity. Most land animals are not very
good swimmers and the ocean covers a vast area, making it very hard for them to
migrate from one landmass to another one which is separated by water. For example,
since the Australian region is separated from the rest of the world and hence has a
very unique assemblage of animals such as egg-laying mammals – found nowhere
else in the whole world. Even smaller ocean masses such as the Mozambique
Channel which separates the landmass of Africa from Madagascar, can and do
account for much of the difference between the fauna of the two areas. The true lemurs
are only found in Madagascar and nowhere in the world because it is an island
(surrounded by water on all sides), similarly the tuatara is only found in New Zealand.

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6. Urbanization & Agriculture
Advancement of human civilization has paralleled with urbanization – the formation of
cities and agriculture – for food production. Both of these are responsible for the
fragmentation of animal habitats. Animals are unable to cross these barriers because
not only do they serve as obstructions but the animals might also come into
competition with humans (which would mean death for them). This results in habitat
fragmentation, one of the leading causes of species becoming threatened.

Barriers to Animal Dispersal

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