Lec 11-Relationship Betw Distribution and Abundance
Transcript of Lec 11-Relationship Betw Distribution and Abundance
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Relationship Between
Distribution & Abundance
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How do we define a speciesgeographic range (Fig. 8.1)?
worldwide, continental, regional,physiographic area, cluster, locality,colony or clump?
Spatial scale of GeographicRanges
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Spatial scalesand correlations
Fig 9.2 showsthat as thespatial scale
increases thecorrelation gets
Survey plots, 35m radius
Survey plots,24km diameter
for ChristmasBird CountsCBC)
5o latitude andlongitude blocks
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Variations in Geographic RangeSize
Most species in a group have small
geographic rangesbutfew havelarge geographic ranges Fig 8.2, Fig.9.3 birds and vascular plants showsthe hollow curve
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85% species in a group havesmallgeographic ranges
(1 millionsquare km)
5%, 3%, 2% and 1%
have large geographicrangesup (9 millionsquare km)
55% species in a grouphave smallgeographic ranges(1 million square km)
10%, 5%, 3%, 2% and 1%have large geographicranges (up to 10million square km)
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Rapoports Rule (Stevens, 1989)
Within the Mammals, geographic range
size decreases from polar to equator range is smaller in tropics
Fig. 8.3 for trees, fishes, some
birds, reptiles (Gaston et al., 1998)
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Canadian (60N)mammalspecies inhabit ranges thatare an average 25x largerthan in Mexican (23N)mammals (Pagel et al., 1998)
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Why do some follow Rapoportsrule while others dont? whatare the ecological mechanisms ofsuch a pattern?
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1. Climatic variability is greater at higherlatitudes animals need to havewide/broad tolerance range (broadtolerance broad ranges)
2 predictions
i. for terrestrial plants and animals tolerance forclimate should increase from tropics (lowerlatitude) to polar (higher latitude)
- higher latitude have greaterclimate variability
true for amphibiansii. for marine shallow water fish, temp tolerance is
minimal in both tropics and polar but wider at
Ecological Mechanism of DistributionPattern
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Temperate zone
Tropicszone
Polarzone
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2. Glaciation hypothesis as glaciersmelted (North Hemisphere) only specieswith high dispersal capacity were able torepopulate and these species have widegeographic ranges. This hypothesis is acontributing factor but not major cause
3. Lack of competition in polar communities no support for this mechanism as yet it is untested!
Ecological Mechanism of DistributionPattern
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A species geographic distribution is acontour map
Density falls off at edges (Fig. 8.5)
Gradual decline from centre of density toedges.
There may also be unexpectedboundaries due to physiological and
biological factors
Contour Map
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Correlation bet. Range Size(Distribution) & Abundance
If a species is widespread is it alwaysabundant?
If a species is rare or threatened is its
geographic range small? Data shows that more widespreadspecies generally tend to be moreabundant
(Fig 8.6) showing relationship betweendistribution and abundance of 263 mothsp.
Similar patterns in birds, plants and other
groups
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more widespread
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more widespreadspecies (430 plotsoccupied) generallytend to be moreabundant (80%)
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Hanskis (1982) Rule 3 Reasonswhy Distribution & Abundance are
Correlated
I. Sampling model
observed relationship is an artifact
does not require biologicalexplanation
the above applies as in Fig 8.6
but positive correlation has beenshown among birds, butterflies andmammals
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Hanskis (1982) Rule 3 Reasonswhy Distribution & Abundance are
Correlated
II. Ecological Specialization Model
Browns Model
Species that can exploit a widerange of resources becomewidespread and common
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Hanskis (1982) Rule 3 Reasonswhy Distribution & Abundance are
Correlated
III. Local Population Model
Species differ in their capacity todisperse, some occupy morepatches
Based on dispersal ability in localpatches of populations
Species that disperse more, occupymore patches and become morecommon, when compared to less
migratory species.
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If a species is declining in numbersdoes its geographic range alsodecreases? This is important for
conservation of declining species
Are species increasing in theirnumbersextend their geographic
range? Fig 8.7 shows data on birds
meadow larks reduced their
geographic range but grackles
Distribution (Range) &Abundance
A th i
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As theirgeographic rangedecreased(proportion ofroutes occupied)
their abundancedecreased
As theirgeographic rangedecreased(proportion ofroutes occupied)
their abundanceincreased