Habitat Selection: An Experimental Field Test with Two Gerbil Species

Abstract

We provide experimental evidence for the isoleg theory of habitat selection in a pair of psammophilic gerbil species. Gerbillus allenbyi (mean mass: 26 g) and G. pyramidum (mean mass: 40 g) coexist in Israel's Negev desert in areas that may contain three distinct sandy habitats: stabilized sand fields, semistabilized dunes, and drifting dunes. When all three habitat types are available, coexistence between the two species has been explained by a centrifugal model of community organization that has been untested until now. To begin testing it, we conducted a field experiment in six 1 ha enclosures, each containing similar proportions of two of the sandy—habitat types (stabilized sand and semi—stabilized dune). This experiment tested the following hypotheses concerning the coexistence of the two species: (1) both species prefer the same primary habitat type; (2) G. allenbyi and G. pyramidum exhibit intraspecific density—dependent habitat selection; (3) habitat preference of both G. allenbyi and G. pyramidum is affected by the interspecific density of the other species; and (4) in the presence of the two habits of our experiment, the theory predicts that the habitat preferences of the two gerbil species should collapse from a centrifugal to a shared—preference model of habitat selection. We tested these hypotheses during two summers by measuring activity of the two species after introducing predetermined and different densities of gerbils into the enclosures during eight 3—4 wk long temporal replicates. During each temporal replicate, we further manipulated the density of G. allenbyi. Activity and habitat utilization of the rodents was measured by tracks left in the sand. Results of the experiments supported all four of our hypotheses and allowed the construction of their isoleg graph.

Inter-specific competitors reduce inter-gender competition in Negev Desert gerbils

“…We conducted this study in two 1-ha enclosures, completely surrounded by rodent-proof fencing, located in a sandy area within the Holot Mashabim Nature Reserve (31°01¢N, 34°45¢E), 35 km south of Beer Sheva, Israel [for a more detailed description of the enclosures see Abramsky et al (1990)]. The sandy areas in the study site can be classified into two clear habitat types, semistabilized and stabilized sand dunes, based on mobility of the sand and the dominant perennial plant species (Danin 1978).…”

“…The sandy areas in the study site can be classified into two clear habitat types, semistabilized and stabilized sand dunes, based on mobility of the sand and the dominant perennial plant species (Danin 1978). Each enclosure contained similar representations of each habitat type (Abramsky et al 1990). …”

Experimental test of predation's effect on divergent selection during character displacement in sticklebacks

“…Whereas there are now many examples from nature, the underlying mechanisms have rarely been examined (5,6). Resource competition has been demonstrated between species undergoing character displacement in a few cases (7)(8)(9)(10)(11)(12)(13)(14), but resource competition is not the only interaction that can lead to divergence. Populations may also diverge through shared enemies (e.g., predators, parasites, and pathogens; refs.…”

Bacteria and game theory: the rise and fall of cooperation in spatially heterogeneous environments

“…To better understand and model these population distribution dynamics, the concept of the IFD [27][28][29] is particularly useful. This framework, first proposed by Fretwell and Lucas and used to describe the male-female sex ratio of bird populations in the Mississippi River Valley [26], states that individuals will populate habitats according to the amount of available resource to maximize their fitness while reducing intra-species resource competition.…”