BIO120H1 Chapter Part 5: Struggle for Existence - Part 3 (Distribution and Abundance as Things We Need to Explain)

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27 Nov 2018
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BIO120: Struggle for Existence, James D. Thomson (Part 3
Distribution and Abundance as Things We Need to Explain)
In association with Lecture 13 & 14 and Reading Quiz 6
Species’ Ranges as a Foundational Component of Ecology
- Definition of the science of ecology: study of the factors determining the distribution and
abundance of organisms
- Most basic and coarsest formulation of a species’ distribution is its geographic range
comprised of areas of planet where species can be found; usually depicted as range maps
- As general rule, ranges are though of as rather stable parts of a species’ biology bird books use
range as a guide to identification along with physical and morphological details
o Birds that undergo seasonal migrations have maps with different colours to distinguish
southern over wintering range and northern summer breeding range
o What keeps their ranges restricted and predictable?
o Why don’t they just spread themselves over the whole world and appear everywhere?
o There’s a number of range – limiting factors at different spatial scales; partially
hierarchal
Broadest and most important is climate and its interplay with physiology
organisms will grow and survive best in places with certain combinations of
temperature and precipitation
- Due to ubiquity of tradeoffs, its hard for organisms to be equally well adapted to different
environments an animal well adapted to life in hot parching deserts will be poorly adapted to
cold, wet muskeg habitats of subarctic
- Global variation in climate is likely to draw outermost lines containing a species’ range
Biomes
- Spatial variation in climate affects many organisms in parallel ways very different sets of
characteristic organisms (biomes) in places with different climates
- Different biomes develop different types of soil limits which plant species can thrive there;
species range limits correspond to boundaries between different biomes allows biomes to be
accurately defined by their possession of characteristic collections of resident species
- Understanding how climate varied and how it resulted in different biological communities was a
significant intellectual breakthrough
- Most general rules of biome level variation are simple:
o Places with more precipitation develop vegetation that is taller, and usually more species
rich and productive, than places that are drier; driest places are sparsely vegetated
deserts increment in water allows deserts to be replaced by densely vegetated short
grass grasslands; further increments allow short grass communities to be replaced by
tall grass prairies, or grassy savannahs with scattered trees; adding more rain and snow
will result in open savannahs to be replaced by true forests with tall trees forming closed
canopies of foliage
o Warmer places support bigger and more complex vegetation than colder places; as you
go up in latitude toward cold polar regions, or up in elevation toward cold mountain tops,
plants diminish in stature until you reach extreme of tundra biomes
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o Seasonality of temperature and precipitation is important two regions can have same
annual cycle of temperatures and same total annual rainfall, but if one area gets summer
rains and other gets winter rains, they’ll have different sets of organisms
- It should be learnt and remembered both the main biological characteristics of each biome, the
climatic characteristics (annual patterns of temperature and precipitation), and how latter
influences the former
Selected Aspects of Soil Ecology in Relation to Biomes
- Soil characteristics contribute strongly to differences among biomes
- Precipitation affects distributions of plants and animals
- Type of soil is critical to:
o Whether water is available to organisms
o What mineral nutrients it contains
- Soil influences vegetation, and vegetation influences soil
- Soil is formed by action of living organisms and geophysical processes on some mineral substrate
called “parent material” parent material is a geological subject rather than ecological; its often
bedrock, but might also be sand deposited in an area by wind or water
- Parent material becomes altered and added to by:
o Biological processes
o Chemical actions such as dissolution and precipitation
- Soil has mineral component and some comes from parent material, some is imported, and some is
altered also has an organic component including decomposition and waste products of plants,
animals, fungi, and microbes
- Main variation among soil types is analogous to primary axis in human societies: rich vs poor
o Rich well suited to supporting plant growth, also known as fertile; includes three
main aspects:
Fertile soils offer generous concentrations of dissolved ions of elements needed
for plan growth, N, P, and K needed in quantity, and most important for plants to
collect through roots; also some necessary trace elements
Good soils have low levels of harmful substances such as toxic metal ions, Al,
and PB
Good soils offer intermediate water availability; dependent heavily on texture of
soil particles very dry soils are xeric, water saturated ones are hydric, and
intermediate ones are mesic; growing in hydric or xeric requires special
adaptations: water logged roots suffer lack of oxygen, and water starved
plants can’t conduct photosynthesis or cool tissues
Soil Development: Gains and Losses
- Soils are complex mixtures of inorganic breakdown products of whatever rock derived from, plus
critical additions of organic matter from animal waste and decaying dead organisms
- Plant roots and leaves usually biggest sources of organic matter
- When organic matter becomes decomposed to the point of no longer recognizable, it becomes
humus
- Inorganic or mineral component ranges from large particles (sand) to tiny ones (clay) ; wind
blown dust (loess) or water deposited sediments (alluvium) are important too
o E.g. agriculture in ancient Egypt dependent on enrichment of desert soils by annual
flooding of Nile River)
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- Particles of soil components hold ions on surfaces and ions are acted upon by rainwater or
snowmelt percolating through soil
- Depending on temperature and pH of water, different ions either dissolved or precipitated out
- If rainfall is heavy, dissolved nutrients carried far down to deep aquifiers in soil, below reach of
plant roots leaching, it produces nutrient poor soils
- If rainfall isn’t heavy, water may penetrate soil to moderate depths, but then be pulled back up by
plant roots water is returned to atmosphere by evapotranspiration; ions stay in rooting zone of
plants, rather than being leached downward out of reach
- Amount of leaching dependent on amount of water moving through soil annually and on age of
soil
o E.g. millions of years since Australia has experienced kinds of geological activity that
cause new rock to be exposed Australian soils tend to be ancient and heavily leached
even though rainfall is sparse in most places
o E.g. Ontario has young soils; retreat of the last glaciers redistributed and exposed fresh
parent materials; tectonic uplift and volcanic activity are important sources of fresh
parent material
- Leaching also dependent on water retaining capacity of soil
o Sandy soils don’t hold enough water; spaces between coarse silica too large to exert
enough capillary force; soils are well drained
o Clay soils hold water too tenaciously; causes waterlogging and prevents plant roots from
getting enough oxygen
o Best soils are loams; contains some sand, some clay, and a lot of organic matter; organic
content aids in sustaining mesic soil and keeps nutrients available
- Oxidized organic components make soil dark, so colour serves as rough indicator of agriculture
quality Black Earth, Wisconsin had rich, dark, laomy soils
- Temperature and precipitation are key drivers soil development varies with climate; some
generalizations:
o Soils in cold northern regions, especially in conifer forests , tend to be tan, sandy, acid,
and nitrogen depleted (podzolized soils)
o Lowland tropical soils tend to be ancient, red, clay rich, heavy in Fe and Al, and
leached of good nutrients Lateritic soils can support impressive forests, but tend to
bake into brick after logging; very poor for sustained farming and often converted to
cattle grazing because it can’t support nutrient – demanding crop plants
o Podsolization and laterization both produce poor soils
o Brown soils in temperate forested regions of Europe, China and North America are
somewhat leached, but loamier and better suited for agriculture
o Best agricultural soils are deep, black, organic rich mesic loams developed in
grasslands of central North America and Ukraine; rainfall is enough to allow plant
growth, but not so heavy to leach nutrients out of soil
- Perennial grasses and herbs populated in prairies developed massive root systems that struck
deeply in search of ground water roots died and decomposed; they contributed rich humus to
soil; prairie soils contain almost twice as much organic matter as forest soils nearby
- Deep soils accumulated over thousands of years now experiencing net loss because of erosion
caused by tillage agriculture
The Temperate Deciduous Forest Biome
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