Lec 11-Relationship Betw Distribution and Abundance

8/4/2019 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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Correlated

II. Ecological Specialization Model

Browns Model

Species that can exploit a widerange of resources becomewidespread and common


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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

Lec 11-Relationship Betw Distribution and Abundance

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