Regulation and Environmental Variability in Experimental Populations of Protozoa

Abstract

Regulation of population size is described for 2 protozoan species in environments that become progressively more variable in time. the ciliate, Colpidium campylum, rapidly adjusts population levels to fixed equilibria set by bacteria food supply. As the equilibrium is varied more frequently, tracking populations of this species destabilize and eventually become extinct. A much larger species, Paramecium primaurelia, regulates slowly to fixed equilibrium but assumes more even densities in variable environments. Effects of changing environments are determined here by the frequency and not the magnitude of changing equilibria.

Environmental colour affects aspects of single–species population dynamics

“…The results from this experiment about how single species track a variable environment and how populations vary through time show encouraging agreement with simple models (May 1973;Pimm 1991) and a previous empirical investigation (Luckinbill & Fenton 1978) of how the intrinsic properties of populations and environmental variability determine population dynamics. The results agree with many theoretical studies which have indicated important di¡erences between the e¡ects of white and reddened environments on population dynamics (Mode & Jacobson 1987;Lawton 1988;Foley 1994;Halley 1996;Ripa & Lundberg 1996;Johst & Brandl 1997;Johst & Wissel 1997;Kaitala et al 1997b;Petchey et al 1997;Cuddington & Yodzis 1999;Morales 1999;Halley & Kunin 1999).…”

“…My results show that population variability is determined by an interaction between the intrinsic rate of increase (which equates to the resilience of a singlespecies population) and the extent (and colour) of environmental variability (the same result as Luckinbill & Fenton (1978)). Because of the interaction between the intrinsic rate of increase and environmental variability we cannot make general inferences about population variability from resilience, or resilience from population variability, without understanding the e¡ect of environmental variability (Pimm 1991;Horwood 1993).…”

“…I conducted this experiment and also tested fundamental hypotheses (e.g. Roughgarden 1975;May 1976;Luckinbill & Fenton 1978;Pimm 1991) about how the intrinsic properties of populations and extrinsic factors control population dynamics.…”

A review of extinction in experimental populations

“…However, the influence of environmental fluctuation frequency can depend on population growth rates, which may in turn depend on the size of the organism. Luckinbill & Fenton (1978) showed that a small protozoan (Colpidium campylum) adjusted its population size quickly in response to food levels, increasing in population variability with increasing frequency of food fluctuation, until populations eventually became extinct. In contrast, population sizes of a larger protozoan (Paramecium primaurelia) tracked food abundance much more slowly and thus was little influenced by the frequency of environmental fluctuation.…”

“…**While there is some support for this simple hypothesis, the relationship between environmental autocorrelation and population dynamics is likely complex (Heino et al 2000), depending on the time scale of autocorrelation (Orland & Lawler 2004) and its synchrony with population generation time (Luckinbill & Fenton 1978).…”