Transcript BCB341_Chapter2_threats_to_biodiversity_habitat_loss_fragmen

FACULTY OF NATURAL SCIENCES
THREATS TO BIOLOGICAL DIVERSITY
1: HABITAT LOSS AND
FRAGMENTATION
Lecturer: Richard Knight
Knight.rich[at]gmail.com
Assistance provided by Sam Hopkins
WHAT IS HABITAT LOSS AND
FRAGMENTATION ?

Habitat loss is when a habitat is changed from
one type to another

Habitat fragmentation is where habitats are
divided into smaller parts
WHY IS IT SO IMPORTANT ?

Habitat loss reduces the amount of habitat available

Thought to be the most important threat to
biodiversity at the moment

Fragmentation results in the pieces of habitat
increasing in insularity with larger edges as well as a
loss of total habitat
WHAT CAUSES HABITAT LOSS AND
FRAGMENTATION

Habitat loss and fragmentation are caused primarily by humans

For example

Building

Draining wetlands

Nutrient enrichment
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Grazing

Cultivation

Dredging

Stopping fire

Clearing
EDGE EFFECTS 1

As the habitat becomes fragmented the edge
of the forest increases

The edges are different to the interior of the
fragment in climate, soil environment and
species composition

Each fragment of habitat is surrounded by a
“matrix” which is the term used for the
modified habitat
EDGE EFFECTS 1

The species at the edge of the habitat tend to differ from the ones
in the centre of the fragment

This is seen in the vegetation present in forest fragments where

Stem density increases at the edge
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Species richness increases

Shade intolerant species grow at the edge

Shade tolerant species stay in the centre

Tree mortality at patch edge
EDGE EFFECTS –
HERPATOFAUNA OF
MADAGASCAR 2

On Madagascar 90% of the forests have been lost

The remaining forests persist as small isolated
fragments

This increases the numbers of edges

It is suggested that these edges will affect the
herpatofauna as they have quite specific
physiological needs
EDGE EFFECTS –
HERPATOFAUNA OF
MADAGASCAR 2

In one study the edge effects were found to influence
the distribution of many species

The study was conducted in patches of rainforest with
anthropogenic scrub in between

Some species only inhabited the interior of the forest

Some species preferred the edge of the forest

It was found that the season affected certain species
affinity for the edge or interior
EDGE EFFECTS –
HERPATOFAUNA OF MADAGASCAR
2

Edge avoiders


Interior avoiding

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Mantidactylus bicalcaratus
Omnipresent


Mabuya elegans
Seasonal

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Mantidactylus boulengeri
Hemidactylus mercatorius
Edge avoiding reptiles and amphibians are more prone to extinction
ISLAND BIOGEOGRAPHY 3

As each fragment of habitat is surrounded by an
altered habitat it is possible to look at these fragments
as islands

A lot of the theory on island communities is relevant
to habitat fragments

The risk of local extinction in single fragments
becomes higher as the fragment is often too small to
support a viable population

Re-colonisation from other fragments is possible
however this depends upon the distance of the
nearest fragment
ISLAND BIOGEOGRAPHY 3

A number of studies have looked at the island effect on
mammals, birds and lizards in Western Australia
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There are 23 preserves varying in size from 34 – 5119 hectares

The birds did not seem to be affected by this fragmentation,
possibly due to their ability to re-colonise areas using shrubby
growth and tree lined roads

The lizards and mammals were affected by the isolation as if
their were on an oceanic island

Some of the lizards and bird had already become extinct in some
of the preserves
HABITAT DESTRUCTION CAUSES SPECIES
TO DECLINE –
ANURAN SPECIES IN ONTARIO CANADA 5
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70% of wetlands in southern Ontario have been
destroyed

Much of this land is now used for agriculture

In a study on the anurans in the area, anuran diversity
as well as density decreased in agricultural areas as
well as land down stream from the agriculture

This was seen for 7 species of frog
FRAGMENTATION CAUSES SPECIES
TO DECLINE –
THE MICRO FROG

The micro frog
(Microbatrachella capensis)
now on the IUCN red data
list as critically endangered 6
is

This has been caused by
habitat destruction

The habitat that the frog relies on is sand plain fynbos, it is
unique in its composition of acidophilic plants such as
ericas and proteas 7
FRAGMENTATION CAUSES SPECIES TO
DECLINE –
THE MICRO FROG

Over half of the sand plains
fynbos on which the micro frog
relies is under human settlement
and only 0.013% of the fynbos
type remains 7

One of the few remaining sites
is in centre of Kenilworth race
course

It has only lasted here because of the protection
that the racecourse has provided

As you can see from the photo it is surrounded by human
settlement

The area is only 56 hectares
SOME SPECIES BENEFIT


Where habitat is destroyed or fragmented
animals benefit
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Grey squirrels
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Rats
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Foxes
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Canadian Geese
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Pigeons
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European sparrows
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House mice
These species are often thought of as pests
some
SO WHAT CAN WE DO ?

A controversial concept is to have
corridors between fragments to allow
the movement of species

The idea behind a corridor is that a strip
of natural habitat is allowed to remain in
the unnatural matrix

It should link two fragments of
habitat that are being conserved

This is meant to allow for local
extinctions and re-colonisation

In practice it is not so simple
CONSERVATION CORRIDORS

There are arguments and studies supporting and
refuting the use of corridors

For a look at a conservation corridor plan in
Minnesota go to www.de-chant.com/tim/nhc/index
FOR CONSERVATION
CORRIDORS

Fahrig and Merriam (1985) 8 tested a
model that compared the age
structure and population size of
patches of habitat that were linked
or not
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They then applied the model to the
white footed mouse (Peromyscus leucopus)

The white footed mouse lives in patchy forests and doesn’t like
open areas

They applied the model to populations of mice that were isolated
or linked to another population by a corridor

They found that isolated populations had a lower growth rate
FOR CONSERVATION
CORRIDORS

A study by Aars and Ims (1999) 9 focused on voles

They looked at population sizes and the genetics of the
population

The voles had the same population size with or without the
corridor

The corridor enhanced the movement of female voles from one
population to the other

The corridor did not affect the movement of males from one
population to the other

This small increase in movement by the female voles could
enhance genetic diversity within the fragmented populations
AGAINST CONSERVATION
CORRIDORS

An overview by Mann and Plummer (1993) 10 discussed the use of
habitat corridors

The report was written after a conference where only 5 out of the
36 papers presented empirical data and three of them showed
that animals rarely move along them

The problems with corridors are:

Fire
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Disease
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Introduced species
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Costs
ACROSS BORDER PARKS
11

The largest example of an
across border (transfrontier)
park is in Southern Africa

It is an agreement with
Kruger in South Africa,
Limpopo in Mozambique
and Gonarezhou in Zimbabwe

The area includes 4 main habitat types and boasts species
numbers of at least 147 mammals, 116 reptiles, 49 fish, 34 frogs,
500 birds and 2000 plants

It is about 40,000 square km

Aims to allow more natural migration patterns by taking down
fences
REFERENCES
1.
Freidenburg, L. (1997) Physical effects of habitat fragmentation in Fielder, P. and Kareiva, P. Conservation Biology for the
coming decade pp 66-79
2.
Lehtinen, R. M. et al. (2003). Edge effects and extinction proneness in a herpatofauna from Madagascar. Biodiversity and
conservation 12: 1357-1370
3.
Cox, G. W. (1997) Conservation biology. Pp 123- 131
4.
Debinski and Holt (2000) A survey and overview of habitat fragmentation experiments. Conservation Biology 14: 342-355
5.
Bishop, C. et al. (1999) Anuran development, density and diversity in relation to agricultural activity in the Holland river
watershed, Ontario, Canada (1990- 1992). Environmental monitoring and assessment 57: 21-43
6.
De Villiers, A. L. (2004). Microbatrachella capensis (Boulenger, 1910). In Atlas and red data book of the frogs of South
Africa, Lesotho and Swaziland. Eds Minter, L.R., Burger, M., Harrison, J. A., Braack, H. H., Bishop, P. J. and Kloepfer, D.
pp 241-244
7.
McDowell, C. (1989). Conservation and horse racing: the unseen connection. Veld and Flora 75: 36-39
8.
Fahrig and Merriam (1985) Habitat patch connectivity and population survival. Ecology 66: 1762- 1768
9.
Aars and Ims (1999) The effect of habitat corridors on rates of transfer and interbreeding between vole demes. Ecology
80: 1648- 1655
10.
Mann and Plummer (1993) The high cost of biodiversity. Science 260: 1868- 1871
11.
www.environment.gov.za/projprog/tfcas/gltp