Competitor density and food concentration: an empirical approach to elucidate the mechanism of seasonal succession of two coexisting Bosmina
- DOI : 10.5141/ecoenv.2013.267
- Author: Mano Hiroyuki, Sakamoto Masaki
- Organization: Mano Hiroyuki; Sakamoto Masaki
- Publish: Journal of Ecology and Environment Volume 36, Issue4, p267~271, 27 Dec 2013
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ABSTRACT
To examine the density effect and food concentration in the competitive output of two
Bosmina species, the population growths ofBosmina fatalis were investigated by manipulating the density ofB. longirostris and the concentration of algae. TheB. fatalis density did not increase in conditions with abundantB. longirostris regardless of the food concentrations. TheB. fatalis increased only at low densities ofB. longirostris with high food concentrations. Based on the current results, a possible mechanism underlying the seasonal shift fromB. longirostris toB. fatalis in Japanese eutrophic lakes will be explored below.
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KEYWORD
Bosmina , Cladocera , competition , priority effect , seasonal succession
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Natural communities in freshwater ecosystems show seasonal successions, which are caused by changes in environmental conditions and biological interactions (Connell and Slatyer 1977, Drake 1991, Murdock et al. 2010). Seasonal succession of two small coexisting bosminid cladocerans is commonly observed in Japanese eutrophic lakes, where
Bosmina longirostris dominate the zooplankton community in spring andB. fatalis dominate in summer (Fig. 1).There are two possible mechanisms explaining this succession pattern. The first is selective predation by
Leptodora kindtii , which occurs in early summer (Chang and Hanazato 2003). The direct effect of temperature could not explain the seasonal succession of twoBosmina species (Hanazato and Yasuno 1985a), and thus Chang and Hanazato (2003) concluded that selective predation byL. kindtii onB. longirostris influenced the competition between two populations ofBosmina . The second is food density (Hanazato and Yasuno 1987). When the two species were reared together with the same initial numbers of individuals with a large food supply,B. fatalis overcameB. longirostris . The opposite occurred with a small food supply. These results indicate that the threshold food concentration (TFC), the ambient concentration of food that allows population losses to be compensated for by the production of offspring (Lampert 1977, Kreutzer and Lampert 1999), forB. longirostris is lower than that forB. fatalis . Moreover, a high population density ofB. longirostris inhibits the subsequent increase ofB. fatalis , indicating a density effect ofB. longirostris on the population growth ofB. fatalis . However, the importance of density effects in the seasonal succession of cladocerans has notbeen experimentally examined.
Here, the question of whether a high population density of
B. longirostris inhibits the growth of aB. fatalis population will be addressed. In the experiment, the summer decline ofB. longirostris was simulated by preparing high and low density groups representing their seasonal populations. Food resources were also manipulated to examine whether the effect ofB. longirostris on the population growth ofB. fatalis is affected by food concentrations.The stock culture of each species (single clone) was established from an individual collected in Lake Suwa (36°2′ N, 138°5′ E) a year before the experiment. The animals were maintained in a 20-L cylindrical polyethylene tank (30 cm diameter, 31 cm height) with 20 L of dechlorinated tap water for two weeks before the experiment. The tanks (one tank for each
Bosmina species) were kept at a constant temperature of 22 ± 1 ˚C and a regime of 16 h light and 8 h darkness. The green alga,Chlorella vulgaris (Chlorella Industry Co. Ltd., Fukuoka, Japan; 2 × 109 cells), was introduced into tanks as food forBosmina every second day. The culture medium in each tank was exchanged 24 h before the experiment, in which aggregated individuals near the light source (Bosmina shows positive phototaxis) were transferred to a new tank by pipetting. After the exchange of medium, no food was added into the tank until the start of the experiment.The experiments were carried out in sixteen 2-L polymethylpentene beakers (Sanplatec Co. Ltd., Osaka, Japan) as experimental aquariums. To create conditions of high and low
B. longirostris densities, 40 ml of theB. longirostris stock culture was added to 8 experimental beakers and 200 mL to the other beakers before the inoculation ofB. fatalis , respectively. The numbers of individuals in three samples of the 40-mL and 200-mL stock culture media were counted, and the initial densities (mean ± SE) ofB. longirostris (NBl-0) were 22.0 ± 1.0 individuals/L and 126.0 ± 8.4 individuals/L in low- and high-density conditions, respectively. The balance of each beaker was filled with dechlorinated tap water so that each contained 2 L. Experimental aquariums were assigned to low and high food concentrations of 0.5 × 105 and 5 × 105 cells/mL, respectively, every second day. All combinations ofB. longirostris initial density and food concentration were run in four replicates. Experimental aquariums were maintained for eight days to increase populations ofB. longirostris .Eight days after the start of the experiments, the contents of the beakers were gently mixed, and 500 mL of water from each was sampled to estimate the density of
B. longirostris (NBl-8). Each of the collected media was filtered through a 40 µm mesh net and preserved in a 4% sugar-formalin solution. After the sampling,B. fatalis was introduced into each beaker by adding 40 mL of gently mixedstock culture medium. The density of
B. fatalis (NBf-8) was estimated by counting the individual numbers in 40 mL of medium obtained from the other three samplings. Fifteen days after the start of experiments, the water remaining in each beaker (1.54 L) was filtered, and the samples were preserved in a 4% sugar-formalin solution. To estimate the densities ofB. fatalis andB. longirostris at 15 days (NBf-15 and NBl-15), the number of each species in each sample was counted under a dissecting microscope and the total was recalculated into number per liter. The experiment was conducted under the same laboratory conditions as used for the stock cultures.Population densities of
B. longirostris andB. fatalis were analyzed to test the hypothesis. To confirm theB. longirostris density treatment and to examine the temporal change in its density for each condition, the population densities ofB. longirostris at 8 and 15 days (NBl-8 and NBl-15) were analyzed with three-way repeated-measures ANOVA with time, initial density, and food concentration. The effects ofB. longirostris population density and food concentration on the population density ofB. fatalis (NBf-15) were tested with two-way ANOVA. To test increases in the population density ofB. fatalis for each condition, a multiple comparison between the density ofB. fatalis (NBf-8) and theB. fatalis density of each condition at 15 days (NBf-15) was performed using Dunnett’s test. Before statistical analyses, the densities ofB. longirostris andB. fatalis were log-transformed to achieve variance homogeneity. Statistical analyses were conducted with R ver. 2.11.1 (R Development Core Team 2010).Temporal changes in densities of
B. longirostris andB. fatalis cultured with high and low food concentrations were shown in Fig. 2. Although density treatment and time significantly affected the density ofB. longirostris , there were no effects of food concentration or interaction terms (Table 1). These results confirmed that the density ofB. longirostris increased with time irrespective of food concentrations.The NBf-8 value was 43.3 ± 4.6 individuals/L (Fig. 2). The values of NBf-15 were influenced by
B. longirostris density, food concentration, and the interaction term between them (Table 2). Only the NBf-15 in the condition with low initial density ofB. longirostris and high food concentration was significantly higher than all NBf-8 (Dunnett’s test,t = 3.718,P = 0.007). These results support the prediction that the growth ofB. fatalis population was inhibited by the abundantB. longirostris . ThatB. fatalis increased only with a lowB. longirostris density at a high food concentration suggests the presence of a density-dependent effect.Results suggest that food competition can be a possible mechanism underlying the observed density-dependent effect of
B. longirostris . In previously conducted competitionexperiments,
B. fatalis was depressed byB. longirostris at low food concentrations (Hanazato and Yasuno 1987, Chang and Hanazato 2004). Therefore,B. fatalis may have a higher TFC than doesB. longirostris . In the present experiment, the algal concentration in the low-food conditions might be comparable with the TFC value forB. fatalis and higher than that forB. longirostris . In the condition with a high food concentration and a high density ofB. longirostris , consumption by abundantB. longirostris individuals might decrease the algal concentration down to the TFC ofB. fatalis . On the other hand, in the condition with a high food concentration and a low density ofB. longirostris ,B. fatalis was able to feed enough to reproduce. Previous studies showed that when they had the same initial density, the larger of two species was a better competitor than the smaller species under high food conditions because it could feed more (Romanovsky and Feniova 1985). AsB. fatalis is larger thanB. longirostris (Hanazato and Yasuno 1987), the feeding rate ofB. fatalis may be higher than that ofB. longirostris and thus be dominant under conditions with a high food concentration and a low density ofB. longirostris .The population shift from
B. longirostris toB. fatalis is commonly observed during the early summer season in Japanese eutrophic lakes, as shown in Fig. 1 (Hanazato and Yasuno 1985b, Chang and Hanazato 2003). The present results demonstrate that the seasonal succession unconditionally requires very low population densities ofB. longirostris .L. kindtii is an effective predator, which can reverse the dominance relationship between the two species through selective predation ofB. longirostris (Chang and Hanazato 2003, 2004). In this study, laboratory experiments were conducted by usingB. longirostris andB. fatalis collected from one site, Lake Suwa. BecauseB. longirostris is a cosmopolitan species andB. fatalis inhabits in East and Southeast Asia, they can present intraspecific variations in traits related to food competition. To test the effect of the intraspecific variations on the density effect ofB. longirostris on the population growth ofB. fatalis , further experiments by using the two species from other lakes are required. Investigating the density effect of species that are already present in a community on other species that appear in the community at some later time may help us to explain seasonal successions in zooplankton communities in natural lakes and ponds.-
[Fig. 1.] Seasonal and reciprocal succession of two Bosmina species in Lake Suwa from 1997 to 1999 (redrawn from Chang and Hanazato 2003). (a) Seasonal changes in densities of B. longirostris, B. fatalis, and L. kindtii, and (b) relative abundance of B. fatalis (density of B. fatalis divided by total Bosmina density).
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[Fig. 2.] Changes in density (mean ± SE) of Bosmina longirostris and B. fatalis cultured with high (a, c) or low (b, d) food concentrations, which is 5 × 105 or 0.5 × 105 cells/mL, respectively. Upper (a, b) and lower (c, d) figures show results of high and low B. longirostris initial density treatments, respectively. High- and low initial density of B. longirostris, are 22.0 ± 1.0 individuals/L and 126.0 ± 8.4 individuals/L, respectively. Open and filled circles indicate B. longirostris and B. fatalis, respectively.
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[Table 1.] The result of three-way repeated-measures ANOVA with time, initial density, and food concentration for population densities of B. longirostris at 8 and 15 days
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[Table 2.] The result of two-way ANOVA with initial density of B. longirostris, and food concentration for the density of B. fatalis at 15 days (NBf-15)