two gerbils in the negev desert of israel:. gerbillus pyramidum (ca. 40 g) gerbillus allenbyi (ca....

Two gerbils in the Negev Desert of Israel:

Gerbillus pyramidum (ca. 40 g) Gerbillus allenbyi (ca. 26 g)

The two gerbils differ mostly in size:

• solitary burrow dwellers• forage at night for seeds• hide and store seeds in their burrows• aggressively defend food sources.

The gerbils use mainly two habitat types:

Semi-stabilized sand dunes with sandy patches and scarce vegetation.

Stabilized sand dunes with lichen cover and dense vegetation.

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G. pyramidumon semi-stabilized dunes

G. allenbyion stabilized dunes

Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand

Semi-stabilized duneStabilized dune

Does the habitat distribution of the two gerbil species change, when their densities change?

What habitats do gerbils use when the competitor is gone?

Both gerbils prefer the semi-stabilized dune habitat, but become less picky (more opportunistic), as their densities increase.

Activity of G. pyramidum Activity of G. allenbyi

No G. pyramidumNo G. allenbyi

What habitat do gerbils use when the competitor is there?

Both species become more selective. But G. pyramidum increases its use of semi-stabilized dunes, while G. allenbyi increases its use

of stabilized dunes.

Activity of G. pyramidumActivity of G. allenbyi

G. pyramidum at intermediate density

G. allenbyi at intermediate density

The shared preference model of habitat selection:

Two species prefer the same habitat and share it at low density. But as both densities increase, the more aggressive or stronger species wins

and defends the preferred habitat.

Both species in preferred habitat

N2 uses both habitats.

N1 uses both habitats.

N1 (G. pyramidum)

N2 (G

. alle

nbyi

)

N1 uses the preferred habitat, N2 the less preferred habitat.

Species select different habitats

N1 (G. pyramidum)

N2 (G

. alle

nbyi

)

N1 in habitat A and B,N2 in habitat B.

N1 in habitat B,N2 in habitat A.

N1 in habitat A,N2 in habitat A and B.

N1 & N2 in A.

N2 isocline

N1 isocline

The distinct preference model:

Species prefer different habitats. But at high density, each species spills over into the less preferred habitat.

N1

N2

N1 in habitat A and B,N2 in habitat B.

N1 in habitat B,N2 in habitat A.

N1 in habitat A,N2 in habitat A and B.

N2 isocline

N1 isocline

Fundamental niche: the set of environmental and biotic conditions where a species can survive in the absence of interspecific competition.

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+ = fundamental niche for

Realized niche: the set of environmental and biotic conditions that a species actually utilizes in the presence of interspecific competitors.

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= realized niche for

Evolutionary past of two hypothetical rodent species:

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Species’ ranges shifted so that two very similar species found themselves in the same place, wanting the same resources.

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.... Slight size differences won the

bigger species the better habitat (shared preference).

Natural selection makes each species better adapted to the habitat used the most (distinct preference).

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Two species evolved independently from a distant common ancestor.

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One species goes locally extinct.

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The surviving species evolves to become a better

habitat generalist.

GHOST OF COMPETITION PAST(Connell)

The evolutionary avoidance of competition which leads to niche differentiation (e.g. habitat specialists).

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Character Displacement:

The co-evolutionary divergence of species with similar resource/habitat requirements (niches).

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Initially: strong competition. Possibility of competitive exclusion or character displacement.

Reduced or no competition. Coexistence.

Prediction: If competitors evolve in this way,

1)Members of a guild (functional group) that share the same geographic location should be more dissimilar on average than members of a guild sampled across geographic regions.

2)Highly similar members of a guild should not coexist in the same geographic location.

7 species of granivorous rodents:

Rodent sizes in three North American deserts:

Great Basin

Mojave

Sonoran

Body mass (log scale)Largest: Kangaroo rat

Smallest: pocket mouse

Small pocket mice (<11g) Large Kangaroo Rats (>100g)

Similarly sized rodent species do not overlap much in range:

The story of the threespine stickleback in the lakes of British Columbia

Threespine stickleback (Gasterosteus aculeatus)

The lakes were populated by a marine ancestor at the end of the last ice age.

• The marine ancestor is a small zooplankton eater.

Lakes have either one or two species.When there are two species:

• Benthic: feeds on invertebrates in the lake sediment of on vegetation

• Limnetic: feeds on floating zooplankton

Benthic (large)

Limnetic (small)

The two species are habitat specialists.

Growth rate in open water (mg/day)

When there is only one species:

• A generalist feeder that eats both benthic invertebrates and floating zooplankton

Benthic (large)

Solitary generalist(intermediate)

Schluter 2000

Proposed evolutionary history:

larger

1) Do ancestral forms compete more than contemporary forms?

2) Does natural selection favor morphological divergence in competing populations

In competition with the small, limnetic species:fitness (smaller fish)< fitness (larger fish)

Growth(marine species with solitary generalist) <

Comp(marine species with benthic species)

Testing hypotheses of the second invasion:

1) Do ancestral forms compete more than contemporary forms?

Experimental pond

Plastic divider

+1000 marine stickleback

+1000 marine stickleback

+2000 solitary generalist

+2000 benthic stickleback

Under otherwise identical conditions, marine stickleback grew faster in the presence of the more diverged benthic species than in the presence of the less

diverged intermediate species.

Mar

ine

stic

kleb

ack

grow

th

2) Does natural selection favor morphological divergence between competing populations?

Step 1: create a population with increased character variation by hybridization:

int X lim

int X ben

int X int

Experimental pond

Plastic divider

+hybridizedstickleback

+limnetic planktiviore

After 3 months, fish were killed by poison, collected, and measured for length

Step 2: let the hybrid population compete against a marine stickleback competitor:

+hybridizedstickleback

+ limnetic competitor

- limnetic competitor

More benthic(bigger)

More limnetic(smaller)

Fish more dissimilar to the competitor grew faster.

Summary:

The coexistence of competitors requires divergent resource/habitat requirements (niche differentiation).

If competitors do not exclude one another, they co-evolve by affecting each other’s resource availability.

The co-evolution of competitors has a tendency to make competitors more different over time, thus reducing competition between them, and stabilizing their coexistence even further (niche evolution).

Biogeographic consequences:

• Character displacement: members of a community are more dissimilar than would be expected if communities were assembled by chance from the greater species pool.

• Spatial exclusion of species with very similar niches.