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Chapter 6: Habitat degradation and loss


Chapter 6: Habitat degradation and loss ... Chapter 7: Habitat Fragmentation Habitat fragmentation: Reduce area of habitat and break into isolated patches. – PowerPoint PPT presentation

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Title: Chapter 6: Habitat degradation and loss

Chapter 6 Habitat degradation and loss
Biggest threat to biodiversity over 80 of
land on Earth is affected by humans to some
extent at least 60 is used unsustainably and/or
is damaged. Aquatic systems too e.g., over 1/5
of coral reefs gone, another 1/5 degraded.
Degradation vs. loss Many human factors, direct
or indirect (e.g., some forms of pollution
global warming). Degradation Some species
affected, not all not necessarily
permanent. Loss All or most species affected
recovery time (if even possible) usually long.
Habitat conversion (transformation) Usually
some variation on degradation or loss change
land from one type to another -- e.g, forest to
grassland, grassland to agriculture. Degradation
often escalates to loss.
Fig. 6.1 For many biomes, much of loss was
pre-1950 since then, biggest effects have been
in tropics. Projection (at current rates)
degradation/loss of approx. 50 of most by 2050.
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Terrestrial Can track history of vegetation
using pollen cores satellite imagery now allows
tracking in real time. Evidence of forest to
agriculture started at least 7,000 years ago in
parts of the Old World some permanent by 2,000
years ago.
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Degradation and loss increases exponentially
with human population growth. Only about 40 of
natural vegetation left in U.S. some habitat
types almost gone similar patterns in Europe,
now SE Asia, and other areas, especially with
rapid urbanization and agriculture.
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In last 300 years, 50 of worldwide forests
cleared most gone in Europe by 1700 now over
70 of forest loss per year is in the
tropics. Forests critical for biodiversity also
filter and protect drinking water, carbon
sequestration, climate (e.g., evapotranspiration
and rainfall patterns, temperature), control of
erosion/prevention of flooding, contribute to
soil fertility...
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Grasslands/shrublands over 40 of Earths
terrestrial area easily transformed for
agriculture heavy grazing by livestock changes
vegetation and can increase erosion and
Freshwater Affected by dams, rerouting,
changes in flow rates, pollution. Over 90 of
U.S. threatened or endangered fishes due to water
development, use for agriculture and urban areas,
Consider Aral Sea in Kazakhstan combined effects
of water use Rivers diverted for agricultural
use (especially cotton) lose fishes (including
ones used for food) salt concentrates in water
dry areas that accumulated pesticides etc. create
toxic dust. Quality of life declines (and cotton
industry will probably decline too).
Wetlands Major effect to decrease loss over
50 lost in US, and 70 in Europe.
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Marine About 20 of coastal habitats extensively
modified drained, dredged, used for aquaculture,
polluted by runoff etc. diversion of rivers can
decrease nutrient input and increase erosion due
to loss of sediments. Runoff can cause
eutrophication, algal blooms, loss of
Tropical coastal mangrove forests severely
damaged important for breeding fishes and other
organisms many cleared or used for aquaculture
(especially shrimp in SE Asia). Unsustainable
Massive pollution, nutrient input, destruction...
huge areas of mangrove forests disappearing.
Other coastal ecosystems seagrass beds
(important e.g., for larval fishes even if
species richness of plants is low) coral reefs
-- inherent species richness.
6.13 The Exxon Valdez oil spill contaminated
over 1900 km of shoreline in Alaska
Recall that some of the major types of habitat
destruction are due to agriculture extraction,
including coal, mining exploitation -- fishing,
timber etc.
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Agriculture Growing rapidly uses water for
irrigation often heavy use of pesticides,
fertilizers. Worst are monocultures (large areas
of one species e.g., corn, soybeans). Better
polyculture use of agricultural matrices,
creation/maintenance of habitat corridors.
Huge areas used to produce grain for livestock
-- better to use pasture land (and of course,
reduce demand for meat and dairy products).
Distribution of cultivated systems worldwide
6.19 Distribution of the 305 crisis ecoregions
that are at-risk of elimination
6.17 Biodiversity hotspots (dark gray) and major
tropical wilderness areas (light gray)
Case 6.2 Table A
Extraction mining, oil, gas, coal -- often very
damaging. Logging is also extraction in a
sense some improvements, e.g., selective logging
in tropics. Intensive fishing (especially
trawling of sea floor) is also a form of
extraction often causes medium- to long-term
damage (usually less so than logging).
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6.21 Economic value from managing ecosystem in
sustainable ways vs. unsustainable uses
6.20 How habitat degradation caused by land use
change is based on economic and social drivers
Development Building cities etc. usually
completely destroys habitat. Roughly 3 of land
on Earth is urbanized this is occupied by at
least 50 of world population. But -- can
consider this in terms of actual surface area
covered, or by the ecological footprint.
Ecological footprint how far do the effects of
the city extend? Estimate for Vancouver B.C.
Footprint is about 174 times the city area. UN
estimate for the Earth (2007) human ecological
footprint 1.5 Planet Earths (i.e., people are
using resources 50 faster than they can be
renewed). Per person (UK) about 5.5 global
hectares (worldwide range 1-10).
Case study (from Chapter 18) Approaches to
pollution, sustainability, etc. how can cities
reduce their impact? Many answers.
Cities historically have been built near sources
of good water and agricultural land. Influenced
by the industrial revolution, starting in the
1700s exponential growth and spread to
developing countries. More and more people moved
to cities trend continues in 3rd world.
Urban sprawl not just cities suburbs, more
cars, pollution, inefficient land use, demands on
natural resources. All of this increases the
ecological footprint. How to change? W. Europe
especially active change patterns of use of
food, goods, and energy.
Changes in agriculture organic, or at least low
pesticide/herbicide. How much food is wasted? In
developed countries, up to 1/3 of food
purchased. Food miles How far does food
travel, on average, from producer to consumer?
U.S. about 1300 miles. So -- think
Case 18.2 Table A
Many cities are getting better at getting people
to separate garbage recycling, organic matter
for composting (and make compost available for
local vegetable growing). Vienna and other
European cities urban gardens/allotment gardens
local fresh produce, low cost. And --
recycling and urban agriculture can boost local
Recycling can be very complex, but overall,
especially useful if materials dont have to be
transported long distances. Make it economically
viable for local people to collect recyclables
and profit (aluminum cans are a prime
example). Especially effective in some 3rd World
Compacted living Urban sprawl (suburbs etc.)
bigger ecological footprint, more cars -- build
up, not out. Concentrate more people in smaller
areas, and increase efficiency of transportation,
energy use etc. Urbanization may even lead to
restoration of natural areas (at least locally).
Case 6.1 (B) The landscapes of New England land
use and population density (black line)
Ecoprocurement Evaluate use of goods used by
local governments, ecologically AND economically
often saves money and encourages local production.
Poverty People who cant afford expensive goods
and services may have a negative impact
disproportionate to their income. Poor
sanitation may lead to pollution. Clearing local
areas for (often inefficient) agriculture. Produc
tion/use of charcoal (e.g., Madagascar).
What if the land could be used for sustainable
purposes? Example Kenya Use natural forest
areas to raise butterflies. Big market in rich
countries also encourages ecotourism. Sustainabl
e, and has doubled average income.
Case 18.2 Table B
Water A growing issue as population expands,
need to get water from further and further
away. Developed countries much of water used
for nonessentials (lawns etc.). Often huge
losses (leaking pipes etc.) -- and, often most
extreme in poor areas. Repair leaks, recapture
water, monitor flow also reduces runoff
Pollution isnt just chemical. Consider light
Can change behavior of organisms, even
flowering/seed set. More light some organisms
more active at night, some less. Florida Reduce
night light hatchling sea turtles move to ocean
(moonlight) instead of toward artificial light
Air pollution Industry, cars, acid rain from
sulfur oxides released by burning fossil
fuels. 1990 Clean Air Act has helped a lot, plus
growing awareness, and economic incentives to
companies and consumers.
Solid waste Aside from landfill, some less
obvious effects. Plastics In fresh and marine
waters, animals get entangled, try to eat them,
etc. And -- most plastics require
photodegradation -- even then, break into smaller
and smaller pieces (and small pieces are used
too e.g., body scrubs).
Plastics can soak up and concentrate toxins like
PCBs consumed by small aquatic organisms, and
build up through the food chain. All sorts of
other chemical pollutants some less obvious ones
include hormones and medicines that can affect
organisms (e.g., reproduction) in water and soils.
Not all pollutants are inherently
toxins. Consider nitrogen often a limiting
nutrient in many systems (as is
phosphorus). Amounts of N (and P) have increased
tremendously in many areas. For N, mainly the
result of fertilizer use.
Under natural conditions, N can be fixed, and
also released artificially introduce large
amounts (anthropogenic fixation) major
consequences. Can promote rampant growth of
certain plants and other organisms (e.g., algal
blooms) changes not only species composition of
photosynthesizers, but also herbivores and others.
Artificial addition of N to native
grasslands total number of insects increased,
but insect and plant species richness decreased.
6.15 Plant species richness declines along a
nitrogen gradient in a prairie ecosystem
Aquatic systems adding N can speed up
decomposition and use up oxygen. Eutrophication
initial burst of organismal abundance, then death
and rapid decomposition. Often creates a
feedback effect result of fertilizer use in
coastal areas (or sewage) -- this is often
controllable, through sewage treatment, and
reduction/changed timing of fertilizer use (may
even save ).
6.14 The amount of nitrogen deposited in the
worlds major watersheds has increased
6.16 World distribution of hypoxic or dead
So -- the recurring question -- what to
prioritize? Really, another kind of cost-benefit
analysis. Conservation International Identified
34 hotspots of biodiversity with combination of
high endemism and species richness that are
losing habitat fast these represent about 16 of
Earths surface, 77 of vertebrate species, plus
at least 300,000 species of true plants.
Has been very successful at drawing attention
and support from other countries, plus the
countries involved. e.g., West Africa Liberian
6.17 Biodiversity hotspots (dark gray) and major
tropical wilderness areas (light gray)
World Wildlife Fund Global 200 ecosystems
more than half are terrestrial consider
combination of endemism, species richness, rare
or phylogenetically divergent taxa, ecology,
evolutionary potential. Note Species richness
is often a function of recent evolutionary
radiation is this a good way to preserve
evolutionary potential?
World Conservation Society Protect the last
10 of ecoregion or biome least affected by
humans. Identify wilderness areas that have
potential for preservation and at least represent
a broad range of biodiversity.
World Wildlife Fund/Nature Conservancy
Conservation Risk Index (CRI) Percentage of
habitat converted by humans relative to
percentage protected, by biome or ecoregion.
  • Three categories (crisis levels) modeled on IUCN
    species threat levels.
  • Critically Endangeredgt 50 conversion CRI gt 25
    (64 ecoregions).
  • 2) Endangered gt 40 conversion CRI gt 10 (80
  • 3) Vulnerable gt 20 conversion CRI
  • gt 2 (161 ecoregions).

Many of the regions under greatest threat are
temperate grasslands, savannahs, and scrublands,
and Mediterranean forests, scrub, and woodlands.
6.18 Protected areas provide better coverage of
some biomes than of others (Part 1)
6.18 Protected areas provide better coverage of
some biomes than of others (Part 2)
6.19 Distribution of the 305 crisis ecoregions
that are at-risk of elimination
How to implement? People are the key one
approach is debt-for nature swap foreign debt
for commitment to preserve certain areas. (Debt
itself may contribute to habitat
destruction). Generates credits that can be
used as tax writeoff, PR, or purchased by
conservation agencies.
Usually accompanied by efforts to develop
sustainable resource base, create incentives to
maintain the system long-term, and more evenly
distribute costs and benefits between rich and
poor countries.
6.21 Economic value from managing ecosystem in
sustainable ways vs. unsustainable uses
6.20 How habitat degradation caused by land use
change is based on economic and social drivers
Chapter 7 Habitat Fragmentation
Habitat fragmentation Reduce area of habitat and
break into isolated patches. Isolation lack of
gene flow among individuals that occupy that type
of habitat (and perhaps key species interactions
prevented etc.). Many kinds of fragmentation
some are worse than others in general (and may
have different effects on different species).
7.1 Changes in a wooded area of Wisconsin during
the period of European settlement
7.4 A fragmentation sequence
Absolute fragmentation vs. habitat shredding
(for example). Shredding maintains strips of
habitat, often with some degree of
connection. This may maintain metapopulation
structure (again, very species-specific).
Metapopulation Complex set of natural
populations (subpopulations) that have varying
degrees of dispersal/gene flow, spatially and
temporally. Population densities and locations
vary according to changing conditions.
Often determined by patchiness/heterogeneity of
environment -- so can we develop or manage areas
such as to maintain metapopulation structure?
7.2 Topographic distribution of vegetation on an
idealized west-facing slope on the Great Smoky
Mountains National Park
Environmental grain scale of patchiness
function of sizes and connectedness of patches.
Usually the result of disturbance (and this is
often natural).
7.3 (A) Fire mortality patches from 1800 to
1900 in the Cook-Quentin study area, Oregon
7.3 (B) Stand development phases in a 1 km wide
section of virgin forest in Yugoslavia
7.5 This short-tussock grassland in New
Zealand illustrates fine-scale fragmentation
Some key points 1) Fragmentation affects
extent and connectedness of habitat patches
different species respond differently.
2) Often there are differences between naturally
patchy landscapes and artificially
(human-induced) patchiness. Structure of
naturally patchy habitat is usually more complex
may be more conducive to gene flow/movement. Arti
ficial fragmentation often just creates separate
areas that arent able to sustain exchange of
3) Degree of contrast in habitat types is often
greater in artificially fragmented
situations. Fuzzy edges vs. sharp boundaries
may determine whether a species can persist, and
whether populations can exchange genes. 4) Some
species respond more strongly than others a bird
may cross a road, a beetle might not.
  • Consequences of fragmentation
  • Initial exclusion Species with very limited
    ranges/requirements may simply disappear --
    nowhere to go.

  • 2) Crowding Reduce habitat area individuals
    more tightly packed.
  • Population may crash due to lack of resources.
  • Or -- may be more subtle -- e.g., reduced
    reproductive success of individuals (e.g., birds
    in NE N. America).

  • This may turn a metapopulation source (area of
    high production of the species) into a sink (a
    dead end where mortality exceeds natality).

7.6 Probability of migrant birds nesting in
United States mid-Atlantic forests of various
sizes (1)
7.6 Probability of migrant birds nesting in
United States mid-Atlantic forests of various
sizes (2)
  • 3) Island effects reduced area may lead to
    reduced number of individuals of a species, or
    the species altogether, along with reduced
    richness overall.
  • Plus, some species, including keystone
    predators etc. may need large individual ranges,
    high degree of spacing.
  • And, some species wont breed in small areas
    (e.g., birds in previous slides).

  • Land-bridge hypothesis For oceanic islands, have
    greater species richness for islands periodically
    connected to one another and/or mainland.
  • Fragmented habitats are similar e.g., large
    mammals in some state and national parks rate of
    loss of some mammals (especially large) exceeds
    rate of colonization if connections stop.

7.7 Predicted species richness over time for
land-bridge islands (A) and oceanic islands (B)
  • Relaxation of richness start with high richness
    then get fragmentation and isolation
  • Number of species drops due to crowding, resource
    limitations, catastrophes, etc.
  • Or -- richness may even increase, but due to
    invasion of weedy, often non-native species.

  • 4) Isolation Saving many small areas may be
    worse than one or a few large areas (especially
    if unconnected).
  • Partly a problem of genetics (drift, inbreeding
    causes loss of genetic variation and unmasking
    of deleterious recessive alleles extinction
    vortex) -- discuss this more later.

  • 5) Edge effects Reduce area -- ratio of edge to
    interior increases (especially well studied for
  • Boundary between original habitat and new becomes
  • Many abiotic effects Edges dry out erosion,
    greater light exposure warming (can affect
    insects, decomposers, as well as more conspicuous

  • Forest species may decline a long way in, while
    edge and open area species increase.
  • Ecological trap High concentrations of natural
    edge species attracts parasites, promotes
  • And (birds best studied) predators and nest
    parasites like cowbirds move in, decimate species
    formerly less densely spread in wide edges (or
    have been forced near edges).

7.11 Percent of experimental nests preyed upon
as a function of distance from forest edge
  • Also increases vulnerability to humans, either by
    direct hunting or other disturbance.
  • Overall, extent and proportion of edge can be
    critical for many species.

  • 6) Habitat matrices May have natural patches
    that differ from surroundings, and many in close
    proximity may support individuals or populations
    whereas isolated ones cant.
  • And -- some species may need access to multiple
    resources found in closely spaced patches.

7.8 A constellation of separate habitat patches
may be critical to the survival of individuals
or populations (A supports more individuals than

7.9 Many animals require a suite of different
habitats or resources to meet life history needs
Even roads can have strong effects besides road
kill, some species (including many invertebrates,
small mammals etc.) wont cross them. Barriers
dont necessarily have to be very big to disrupt
an ecosystem and affect gene flow.
7.13 Roads can be significant barriers to the
movement of small vertebrates and invertebrates
(here, beetles).
Roads also provide corridors for invasive
species, disease, parasites, etc. Seemingly
minor changes can cause major fragmentation and
changes in species composition.
7) Allee effect Below some threshold population
size/density, plants may no longer be attractive
to pollinators (or, more generally, as population
size decreases, average fitness
decreases). Especially problematic for
specialized pollination systems (e.g., only one
species acts as pollinator for a plant
species). And -- can be amplified by catastrophes
such as floods in already small patches.
7.15 Extinction rates were highest in small,
isolated patches of the plant Clarkia concinna
(Part 1)
Light gray low/no reproduction dark gray
catastrophe (flood etc.).
7.15 Extinction rates were highest in small,
isolated patches of the plant Clarkia concinna
(Part 2)
7.15 Extinction rates were highest in small,
isolated patches of the plant Clarkia concinna
(Part 3)
Recall importance of fire in some ecosystems
with fragmentation, may not start in the first
place (e.g., less chance of lightning
strike). If it does, may not be able to
spread. Even a road could be the barrier.
  • How do species survive fragmentation (or not)?
  • Some do well in disturbed habitat -- but often
    weedy and non-native.
  • 2) Some are simply adapted to fragmented habitat
    (as long as some remains).
  • 3) Some are good dispersers and can make use of
    remaining fragments.

But many dont make it, unless theres a
continuous source population nearby -- and
eventually the fragments just become
sinks. Climate change may increase the effects
of fragmentation -- e.g., block northward
movement of species that might otherwise be able
to shift ranges.
Chapter 8 Overexploitation
Whats sustainable? A growing issue. Case in
point The bush meat trade in tropical African
forests rate of exploitation for many groups
(e.g., primates, carnivores) exceeds level
thought to be sustainable, especially for
long-lived species. Solid line removal
production Dotted removal 20 of production
(sustainable for long-lived species?)
8.2 Hunting rates are unsustainably high across
large tracts of tropical forests
Economics of sustainability -- and incentives --
are often complex and can fluctuate not just with
immediate needs, but with the economy. e.g.,
Bolivian mahogany trees most valuable at large
size, but growth takes a long time if interest
rates increase faster, creates incentive to cut
down young trees.
8.1 Productivity of Bolivian mahogany trees as a
function of size
Brazil nuts Often considered sustainable (a
non-timber forest product), but high levels of
harvest from mature trees prevent growth of new
8.3 Relationships between historical levels of
Brazil nut collection and mean tree size
Fisheries At least 75 are at maximum or
declining for some, aquaculture works, but in
other cases, aquaculture can be worse (e.g.,
shrimp and salmon farming). Atlantic cod 99.9
decline since 1960s Canadian fishery now shut
down doubtful if stocks will recover.
8.4 Trends in global fisheries gray capture,
black aquaculture
But awareness is growing, and now guides are
available for sustainable seafood e.g.,
ch.aspx And, chefs and others are becoming much
more selective and eco-conscious.
Overexploitation can be especially problematic
when populations are fragmented. Abalone
bivalve in high demand harvested from Pacific
west coast. Overcollecting creates small,
fragmented populations and isolated individuals,
many limited to deep water.
Male sperm can only travel about 1 m creates
Allee effect, where fertilization success
declines, remaining individuals are even more in
demand downward spiral. But -- collecting is
much more regulated now, and low-impact
farming/reintroduction is becoming more
And of course, removal of keystone species,
ecological engineers, and other essential species
can have huge effects sea otters, beavers, large
How to assess sustainability? One approach try
to work within framework of density
dependence. Basic idea Remove just enough
individuals to reduce competition and make room
for others to replace them. But can be tricky,
and often doesnt take into account changing
conditions etc.
How to assess sustainability? One approach try
to work within framework of density
dependence. Basic idea Remove just enough
individuals to reduce competition and make room
for others to replace them. But can be tricky,
and often doesnt take into account changing
conditions etc.
8.8 Density dependence stems from relationships
between population density and per capita
rates of birth and death
r intrinsic rate of natural increase
8.9 (A) Logistic population growth of a
population up to a maximum population size, Nmax
(determined by K, carrying capacity of
8.9 (B) Sustainable yield, Y against population
size for the logistic case shown in (A)
So Aim for the harvest that is at the point of
maximum rate of population growth -- maximum
sustainable yield, MSY. Often leads to constant
quota approach set level of harvest year after
year. BUT Populations fluctuate naturally,
conditions vary etc. -- can lead to overharvest.
8.10 Equilibria and population stability under
constant quota exploitation
  • Possibilities
  • High quota Dangerous if too many taken (yield
    too high), then population will crash even if
    large to begin with.

  • 2) MSY quota Number that could be taken under
    ideal conditions that exactly balances removal
    and population growth rate.
  • If population is larger than NMSY, then will push
    back to NMSY equilibrium if at all smaller, then
    the MSY quota will reduce numbers.

  • 3) Low quota Cautious usually based on limited
    data though.
  • If population is low when implemented (N1), can
    still cause crash if just a bit higher, will
    benefit from removal of some larger population
  • Ideal Rate of removal that stabilizes population
    at N2.

8.10 Equilibria and population stability under
constant quota exploitation
8.11 Equilibria and population stability under
proportional exploitation
Better Proportional quota (constant
effort) Base level of harvest on population
size establish fraction that can be taken for a
given population size. Try to keep yield
(harvest) below intrinsic rate of natural
increase r. Ideally should allow establishment
of different sustainable equilibria.
Martens in Canada (weasel relative) trapped for
fur. Population sizes fluctuate depending on
food etc. Constant quota If high, high
probability of extinction. But tracking yearly
numbers and using proportional quota keeps
population size stable.
8.12 Probability of overexploitation in relation
to mean yield of marten in commercial trapping
Quota approaches are essentially cost-benefit
approaches. Tragedy of the Commons If one
individual or entity controls a resource, then in
their interest to maintain it. But -- if many
exploit the resource, some exploit the caution of
others (e.g., international fisheries). Then --
take what you can while it lasts.
Consider too that even if it might be beneficial
to maintain a resource long-term, economically
might be just as well to get what you can and
invest the money. So, often need other arguments
to justify maintenance of a species or ecosystem.
And -- models are helpful, but usually
simplistic, and dont consider all possible
parameters or fluctuations. So really, should
couple with sensitivity analyses in which
different variables are perturbed -- how much
does changing one change the outcome?
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