The species social structure and behavior (Geist 1971) 

 along with its reproductive potential and morphological, 

 physiological, and genetic characteristics determine its rate 

 of dispersion and dispersal. Thus, given favorable 

 environmental conditions, distribution of microtine rodents 

 might progress from A to C and beyond (Fig. 12.3) in a year or 

 less. For deer (Odocoileus spp.) and moose (Alces spp.), 4 to 

 5 years might be necessary for expansion of their distribution 

 from that represented by Figure 12. 3A to that in Figure 12. 3C. 

 An equivalent expansion in distribution might take decades for 

 species such as mountain sheep (Ovis spp.) (Geist 1971). 



Clearly then, the species rate of dispersion and 

 dispersal is another time factor important in population 

 dynamics, particularly in variable environments. Species with 

 rapid rates of dispersion have the potential to reach "habitat 

 fill" (Fig. 12. 3C and beyond) during periods when 

 environmental conditions remain favorable. These species are 

 more likely to at least occasionally reach "habitat fill" and 

 perhaps display "density-dependent" dynamics. Species with 

 relatively slower rates of dispersion may seldom display 

 density-dependent dynamics because environmental conditions 

 suddenly may become unfavorable before sufficient time has 

 passed for them to reach "habitat fill". 



The relative stability of different environments is also 

 an important consideration in determining population dynamics. 

 Thus, relatively stable environments are more likely to 

 provide adequate time for "habitat fill" to be achieved while 

 more variable environments provide less time under favorable 

 conditions for "habitat fill" to occur. This combination of 

 factors leads us to the hypothesis that species with rapid 

 rates of dispersion that live in stable environments are more 

 likely (though not guaranteed) to reach densities where a 

 significant degree of "density-dependent population dynamics 

 might be observed than species with slower rates of dispersal 

 that live in variable environments. 



A counter to the foregoing argument might be that species 

 with slower rates of dispersion and dispersal are more likely 

 to increase within place, placing increased pressure on local 

 resources. Thus, these slower dispersing species might be 

 most likely to exhibit density-dependent dynamics. This 

 argument certainly needs more consideration and investigation. 

 Tentatively, however, our observation is that slower 

 dispersing species are usually those which also have lower 

 reproductive output. Thus, environmental conditions usually 

 turn unfavorable before local densities have the opportunity 

 to reach extreme levels. Also, species populations that live 

 in variable environments tend to have rapid, not slow rates of 

 dispersal. Dispersal rate may also vary within a species, 

 depending on the environment in which they live. 



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