Exploitative Competition In Differently Sized Daphnia Species: A Mechanistic Explanation

INTRODUCTION

Cladocerans like Daphnia are key species in pelagic freshwater food webs. Because they are relatively nonselective filter feeders (Lampert 1987), the food spectra of different species overlap to a large degree (Kerfoot et al. 1985) and hence there is a considerable range of seston particles that can be considered a single shared resource. This makes it possible to use Tilman's (1982) concept of [R.sup.*] to predict competitive relationships among various species of Daphnia. In Tilman's mechanistic theory of resource competition, [R.sup.*] represents the amount of resource that a species requires in order to maintain a stable equilibrium population. The main advantage of Tilman's theory is that [R.sup.*] is a species-specific physiological characteristic that can be measured for individual species and can be applied to predict the outcome of competition between species. The species with the lowest [R.sup.*] will always be the superior competitor.

Until today, Tilman's models have been tested for plankton exclusively in chemostats, which cannot be used for crustaceans because these animals are strong swimmers and can avoid being washed out. Rotifers are the only metazoans small enough to be cultured in chemostats and, thus, they have been used successfully to test the resource competition theory (Rothhaupt 1988).

A possibility to estimate the competitive abilities of different Daphnia species without using chemostats (Rothhaupt 1990) is the "threshold food concentration" concept (Lampert 1977), a special case of Tilman's [R.sup.*] concept. The threshold food concentration ([C.sub.0]) is the ambient concentration of food that allows an individual to meet its metabolic demands but not grow. Both [R.sup.*] and [C.sub.0] indicate resource concentrations at a zero net change in the consumer's biomass, but basic differences between the two characteristics originate from the way they are measured. [R.sup.*] is calculated from the residual resource concentration in a chemostat at equilibrium. [C.sub.0] is determined by measuring individual growth rates in a series of constant low food concentrations. The intersection of a regression of growth rate vs. food concentration (growth rate = zero) defines [R.sup.*] and [C.sub.0]. Thus, the growth rate is kept constant experimentally for estimating [R.sup.*] and the residual food concentration is measured in response, whereas the food concentration is kept constant and the resulting growth rate is measured when [C.sub.0] is determined. As a consequence [R.sup.*] cannot be determined for zero mortality (chemostat flow rate zero) while [C.sub.0] applies always to zero mortality, i.e., it predicts the competitive ability in a predator free environment.

The [C.sub.0] concept has been used in various ways. Lampert (1977) studied the effect of environmental conditions and body size on [C.sub.0] of Daphnia pulex, but did not consider differences among species. He calculated [C.sub.0] as the difference between carbon assimilation and respiration rate. Stemberger and Gilbert (1987) used population growth rates to compare food thresholds of differently sized rotifers. A clear negative relationship between body size and the [C.sub.0] of various cladocerans was found by Gliwicz (1990), confirming the competition part of the Size Efficiency Hypothesis (Hall et al. 1976). Gliwicz and Lampert (1990) demonstrated the differing effects of algal filaments interfering with the filtering process on [C.sub.0] of differently sized Daphnia. They used these data to explain the dominance of small cladocerans under hypertrophic conditions. Finally Achenbach and Lampert (1997) showed that temperature does not affect the rank order of [C.sub.0] in various cladocerans, hence concluding that changing competitive ability cannot explain the dominance of small species in subtropical waters.

Although [R.sup.*] and [C.sub.0] are theoretically similar, they differ in the predictive power of the conclusions. [R.sup.*] is a direct measure of the minimum resource concentration that can result from the consumption by organisms. [C.sub.0] in contrast is an indirect estimate from nonequilibrium conditions, and it still needs to be shown that growing Daphnia are efficient enough to suppress environmental resource concentrations to levels equivalent to their [C.sub.0]. At this point, they would no longer grow and be in equilibrium with the growing algal resource.

The aim of this study was to establish equilibrium conditions between Daphnia and its food resource and to measure the resulting equilibrium food concentration directly. If a Daphnia population at a fixed resource renewal rate grows towards its carrying capacity the food concentration will decrease until no further Daphnia growth is possible. Daphnia biomass and food concentration will remain constant. Although mortality in this case is not predetermined by the chemostat flow rate (only natural mortality is involved), the equilibrium food concentration results from the resource uptake of the daphniids. Hence it is equivalent to Tilman's [R.sup.*]. In order to point out the analogy we will call the resulting equilibrium resource concentration [C.sup.*]. [C.sup.*] and [C.sub.0] should be numerically identical at zero mortality.