Lecture 28. Human Effects on Ecosystems -let’s move now from theoretical understanding of how communities and ecosystems work -to examine effects of hum an activities on those systems -I’m going to approach this from a slightly different perspective: -we all know that human activities are profoundly influencing the function of the entire planet -we see this in daily news articles about climate change, pollution, fishery collapses, endangered species, loss of rainforest etc. etc. etc. -instead of compiling a comprehensive list of human effects on ecosystems, -let’s see if we can classify human activities from an ecosystem perspective -earlier I argued that humanity is the sixth state factor -humanity is now a fundamental driver that determines the state of all ecosystems -along with climate, parent material, topography, biota and time ** if this is true, then there should be a predictable suite of effects from human intervention -human beings en masse should behave consistently, even if individuals vary -here is my top-ten list the principal effects of human beings on the ecosystems of the earth -more or less in order of importance -Note: students in the past have correctly pointed out that this list conflates activities (what humans do) and effects (results of human activities) instead of sticking with one or the other. However, I haven’t had a chance to rework the list, and it is intended mostly as a starting point for further thought. Here, I have annotated each item. -as you read through the list, think about items you would add, or delete, or re-order. What would your list look like? 1. Climate change [Effect] -in first place because this affects the whole planet -caused by an increase in greenhouse gases by combustion of fossil fuels
Transcript
Lecture 28. Human Effects on Ecosystems
-let’s move now from theoretical understanding of how communities and ecosystems work
-to examine effects of hum an activities on those systems
-I’m going to approach this from a slightly different perspective:
-we all know that human activities are profoundly influencing the function of the entire planet
-we see this in daily news articles about climate change, pollution, fishery collapses, endangered
species, loss of rainforest etc. etc. etc.
-instead of compiling a comprehensive list of human effects on ecosystems,
-let’s see if we can classify human activities from an ecosystem perspective
-earlier I argued that humanity is the sixth state factor
-humanity is now a fundamental driver that determines the state of all ecosystems
-along with climate, parent material, topography, biota and time
** if this is true, then there should be a predictable suite of effects from human intervention
-human beings en masse should behave consistently, even if individuals vary
-here is my top-ten list the principal effects of human beings on the ecosystems of the earth
-more or less in order of importance
-Note: students in the past have correctly pointed out that this list conflates activities (what
humans do) and effects (results of human activities) instead of sticking with one or the other.
However, I haven’t had a chance to rework the list, and it is intended mostly as a starting point
for further thought. Here, I have annotated each item.
-as you read through the list, think about items you would add, or delete, or re-order.
What would your list look like?
1. Climate change [Effect]
-in first place because this affects the whole planet
-caused by an increase in greenhouse gases by combustion of fossil fuels
-not burning of oil and coal that is the problem,
-rather, the rate of that combustion
-from so many people demanding so much energy,
2-CO and other gases are produced faster than the planet can assimilate them
-global warming will not only increase mean temperatures,
-it will also drive a redistribution of species and ecosystem types across the planet
-what happens when the range of a species cannot migrate as fast as climate is changing?
-a great potential for extinctions, invasions and even ecosystem collapse
2. Mass extinction [Effect]
-much debate about the rate of species loss
-many species may be disappearing before we even find them
-recall that most species are small and inconspicuous,
-mostly insects, fungi, arthropods
-we do not even have a solid estimate of how many species there are on the planet
-estimates of total number range from 2 million to >10 million
-gross estimate of species loss in the 20 century is 300 000 species, th
-representing 3-10% of total
** we are losing species at a rate unparalleled since the dinosaurs
-some scientists have called it “The Sixth Extinction”
-because there have been five other times in the history of life when most species disappeared
-difference here is twofold:
** first, this is the first extinction that is biologically driven — by humans
-second, this extinction is much faster than any that happened before
-because we are changing the environment faster than any geological event
-all right, except meteorites
-what is driving the extinction?
** by far greatest cause of species decline is habitat loss
-three main causes of habitat loss:
(1) conversion of wildlands to agriculture
-including row crops, cattle grazing, and rotating culture (tropics)
(2) harvesting and clearing of forests
(3) conversion of land to human habitation, especially spreading urban areas
-in addition, species are lost through over-exploitation (hunting, fishing, trapping, collecting),
pollution, competition with introduced species, and accidentally
A clear-cut in a
(supposed) provincial
wildlife sanctuary,
Alberta. Destruction
of forests is one of
the main drivers of
species extinctions
Implications of species extinctions
-along with species we lose genetic diversity
-also the intrinsic value in the diversity of life on the planet
-in addition, important practical considerations
**along with the lost species, we may lose functional diversity
-degree of overlap and functional redundancy in communities declines with species loss
-therefore, capacity of ecosystems to resist disturbance is compromised
-when species loss arrives at a critical point, whole community becomes fragile
-we don’t know enough about most communities to know where that point is,
-or to know what all those species do
** we also lose so-called ecosystem services when species are lost
-wild species are important reservoirs of information for science, about evolution and biology
**also sources of new products, medicines, foods, materials
-the contain information on biochemical pathways and biological processes that we may use
**more than 1/4 of all medicines in use today were discovered in tropical rainforest
-best example: taxol (paclitaxel), a natural chemical in bark of Western yew trees
-effective and widely used in treatment of breast cancer
** loss of species across the planet is not even or random
-losses are concentrated in particular taxonomic or geographical groups
-rate of species loss is greatest in the tropics
-because that is where greatest number of species resides
-and where disruptive human activities from an expanding population is greatest
-greater losses among: large organisms (require large territories),
rare species (less habitat to maintain populations),
specialists (narrow range of suitable conditions)
and those that are harvested by humans for food and other products
** flora and fauna on islands also particularly vulnerable to human disturbance
-before a species is truly extinct, it may become functionally extinct
-where populations are too low for the species to fufill its ecological function
Example: most large sharks are now too rare to influence fish populations as they used to
3. Homogenization of Flora and Fauna [Effect]
-this effect has not been considered major until recently
-humans act as a powerful biogeographic force shaping global distributions of species
we do this through accidental and deliberate species introductions
** most common weeds in garden and field in North America came from Europe
-a meadow in Italy now looks like a meadow in Ontario
-we have deliberately introduced rainbow trout, honey bees, apples, pheasants, all over the world
-but accidental introductions abound, especially weeds and pest species
-starlings, house cats, rats, have disproportionate effects because of their size or predation
-a bird called a rose-ringed parakeet is becoming a problem in southern England
-about 30% of the flora of Nova Scotia is introduced, mostly from Europe
-most common tree in Toronto is the tree of heaven, native to China
A rose-ringed parakeet. Not a species one would
expect to become invasive
-we also remove barriers
-principal factor keeping species apart in different regions
-rivers, mountain ranges and especially oceans
-global shipping, airplanes, long-distance trucking, tourism, canals, allow species to spread
**simultaneously, loss of regional, specialist and rare species
-that cannot adapt to habitat modifications created by humans
-relatively small number of common, widespread, adaptable species thrive
Examples: rats, coyotes, house mice, European sparrows, starlings, cockroaches, roadside weeds
**result is loss of diversity at both regional and global scales
-ironically, local diversity may increase, at least in the north, because of multiple introductions
4. Simplification of Ecosystems [Effect]
-closely related to Number 3, above
-along with species removal and homogenization of communities, we also simplify the landscape
-by creating large, uniform tracts of one habitat type that used to be many small patches
-recall importance of habitat complexity created by patches of many sizes and kinds
-largest cause is extensification of agriculture
-modern agriculture requires large tracts of land
-that can be efficiently managed with machines
-these crops are pure monocultures
-virtually no habitat diversity or even non-crop species
-“improvement” of rangeland for cattle grazing is essentially a simplification exercise
-same thing happens in forestry
-patch mosaic of old and young forests, many ages and time from disturbance
-replaced by a uniform blanket of young forests regrowing from clearcuts
-old growth forests are virtually eliminated,
-range of stand age and species complement is greatly narrowed
** <2% of forest in Nova Scotia is more than 100 years old
-essentially all the remaining old-growth forest is in parks
-most forest is harvested on a strict scheduled after a few decades of regrowth
-in many places, natural forests are systematically replaced with plantations
-composed of one or a few desirable species
-plantations are common in U.S.A., Chile, England, Fennoscandia, Canada, New Zealand
-net result of all this is whole-scale loss of habitat complexity,
-destruction of the natural patch mosaic
**results in:
(1) species loss, (2) weakening of ecosystem functions and (3) reduced resistance to disturbance
Example: monocultures much more susceptible to diseases and pests
Example: much greater soil erosion from farmland compared with native ecosystems
-not to mention that the world becomes a less interesting place
Massive soil erosion on disturbed rangeland in southern Alberta.
I would like to say that this picture was taken during the Dust Bowl
years in the 1930s. Unfortunately, I took this picture myself in 1986.
5. Acceleration of Biogeochemical Cycles [Effect]
L faster cycling through short-term pools
L transfers from long-term to short-term pools
-we discussed acceleration of cycling earlier
-especially with respect to the N cycle
**same applies to all biologically active elements, on a global scale
6. Modification of Hydrology and the Water Cycle [Effect]
-as with biogeochemical cycles, big effect on water cycle is acceleration
** biggest modification of hydrology is by irrigation
-until you have seen irrigation works, the scale of these operations is hard to grasp
Examples : all rivers crossing the Canadian Prairies suffer heavy withdrawals for irrigation
-Bow River, which flows through Calgary, ~100 m wide, discharge is >100 m /s3
-water is diverted shortly downstream from Calgary at the Bowden Dam
** irrigation system has the capacity to take all the water in the river
-and the farmers would if it were permitted
An irrigation canal in
southern Alberta. This
canal, about 2× size of
Brierly Brook, carries water
from reservoirs to farm
districts.
An old, on-farm irrigation
canal, the final capillary in
the water distribution
system. Note the
dysfunctional control
structure in foreground.
An irrigation reservoir in the
midst of the Canadian
prairie. Small impound-
ments like this are scattered
across the plains throughout
the world.
-yet ours are relatively small irrigation works by international standards
-an irrigation canal in Kazakhstan is 1500 km long
-largest irrigation system in the world is in Libya (seriously)
-most applied water does not come back (lost to evapotranspiration)
-movement of solutes and sediments also modified by the extra water
-soil water in semi-arid regions tends to move upward from evaporation
-rather than downward from gravity, as in wetter regions
-therefore irrigation water tends to move up, carrying salts that accumulate near surface
-resulting salinization can render soil useless for agriculture
-vast areas of land in Murray River basin, Australia, have been ruined by salinization
-only cure is to apply even more water to flush the salts out
-irrigation greatly modifies flows and habitat in the affected rivers
-requires massive dams, dykes, canals to transport water
** human beings build dams as automatically as do beavers
-anywhere that has human settlement has dams,
-with attendent habitat modifications
-all the great civilization at the dawn of history were based on dams and irrigation
-we also tend to accelerate drainage,
-so that homes and farmland are not flooded
-leads to higher river flows after rain, lower river flows in dry periods
** removal of wetlands also contributes to faster water drainage
-a movement afoot to remove some older dams, allow rivers to run free
-other big effect on water cycle comes from climate change
** cutting down forests can strongly change local climate and drainage
Examples: in arid SW United States, forest harvesting is used to augment flows in rivers
-cities such as Tucson, Arizona, depend on periodic clearing of pine forests in the Sierra Nevadas
-so that water is not lost to evapotranspiration instead of filling the meagre rivers
-text says: On average, plantation forests produce 38% less runoff than non-forest vegetation
-conversion of Australian heathland to irrigated cropland:
