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Two gerbils in the Negev Desert of Israel: The two gerbils differ mostly in size: Gerbillus pyramidum (ca. 40 g) Gerbillus allenbyi (ca. 26 g) • 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. .. .. .. .. .. .. .. .. .. .. .. .. G. pyramidum on semi-stabilized dunes .. .. .. .. G. allenbyi on stabilized dunes .. .. .. .. .. .. .. .. Does the habitat distribution of the two gerbil species change, when their densities change? Stabilized dune Semi-stabilized dune Two 2-ha exclosures, foraging activity measured as the total length of tracks left in the sand What habitats do gerbils use when the competitor is gone? No G. allenbyi Activity of G. pyramidum No G. pyramidum Activity of G. allenbyi Both gerbils prefer the semi-stabilized dune habitat, but become less picky (more opportunistic), as their densities increase. What habitat do gerbils use when the competitor is there? G. pyramidum at intermediate density Activity of G. allenbyi G. allenbyi at intermediate density Activity of G. pyramidum Both species become more selective. But G. pyramidum increases its use of semi-stabilized dunes, while G. allenbyi increases its use of stabilized dunes. The shared preference model of habitat selection: N2 (G. allenbyi) 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 Species select different habitats N2 uses both habitats. N1 uses both habitats. N1 uses the preferred habitat, N2 the less preferred habitat. N1 (G. pyramidum) N1 isocline N1 in habitat B, N2 in habitat A. N2 (G. allenbyi) N2 isocline N1 in habitat A, N2 in habitat A and B. N1 & N2 in A. N1 in habitat A and B, N2 in habitat B. N1 (G. pyramidum) The distinct preference model: Species prefer different habitats. But at high density, each species spills over into the less preferred habitat. N1 in habitat B, N2 in habitat A. N1 in habitat A, N2 in habitat A and B. N2 N1 isocline N2 isocline N1 in habitat A and B, N2 in habitat B. N1 Fundamental niche: the set of environmental and biotic conditions where a species can survive in the absence of interspecific competition. + Realized niche: = fundamental niche for .. the set of environmental and biotic conditions that a species actually utilizes in the presence of interspecific competitors. = realized niche for .. Evolutionary past of two hypothetical rodent species: .. .. .. .. .. The surviving species evolves to become a better habitat generalist. .. .. .. .. .. .. Natural selection makes each species better adapted to the habitat used the most (distinct preference). .. Slight size differences won the bigger species the better habitat (shared preference). .. .. .. .. One species goes locally extinct. .. .. .. .. .. .. Species’ ranges shifted so that two very similar species found themselves in the same place, wanting the same resources. Two species evolved independently from a distant common ancestor. GHOST OF COMPETITION PAST (Connell) The evolutionary avoidance of competition which leads to niche differentiation (e.g. habitat specialists). .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. Character Displacement: The co-evolutionary divergence of species with similar resource/habitat requirements (niches). .. Reduced or no competition. .. Coexistence. .. .. Initially: strong competition. Possibility of competitive exclusion or character displacement. 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. Rodent sizes in three North American deserts: Great Basin 7 species of granivorous rodents: Mojave Smallest: pocket mouse Sonoran Largest: Kangaroo rat Body mass (log scale) Similarly sized rodent species do not overlap much in range: Small pocket mice (<11g) Large Kangaroo Rats (>100g) 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) Growth rate in open water (mg/day) The two species are habitat specialists. When there is only one species: • A generalist feeder that eats both benthic invertebrates and floating zooplankton Benthic (large) Solitary generalist (intermediate) Proposed evolutionary history: larger Schluter 2000 Testing hypotheses of the second invasion: 1) Do ancestral forms compete more than contemporary forms? Growth(marine species with solitary generalist) < Comp(marine species with benthic species) 2) Does natural selection favor morphological divergence in competing populations In competition with the small, limnetic species: fitness (smaller fish)< fitness (larger fish) 1) Do ancestral forms compete more than contemporary forms? Plastic divider +1000 marine stickleback +1000 marine stickleback +2000 solitary generalist +2000 benthic stickleback Experimental pond Marine stickleback growth 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. 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 Step 2: let the hybrid population compete against a marine stickleback competitor: Plastic divider +hybridized stickleback +hybridized stickleback +limnetic planktiviore Experimental pond After 3 months, fish were killed by poison, collected, and measured for length - 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.
