Does Predator Go for Size Selection or Preferential Toxic-Nontoxic Species Under Limited Resource?

Abstract

Problem statement: Selectivity is common in predator-prey interaction but the selection mechanism is still unexplored and a debatable issue in modern theoretical and experimental ecology for numerous species across the globe. In present investigation we emphasized the hypothesis that the zooplankton is less inclined to opt the food based on size selectivity criteria than the preferential selectivity for the safe non-toxic food species. Approach: As a test bed we select one nontoxic phytoplankton (Chaetocerous gracilis), one toxic phytoplankton (Microcystis aeruginosa) and one zooplankton (Artemia salina). Initially the experiment is setup through the small batch cultures of Nontoxic (NTP) and toxic Phytoplankton (TPP). Both the strains of phytoplankton are collected from the deltaic region of river Subarnarekha (87°31”E and 21°37”N) and the isolation is done in the laboratory. Similarly batches of zooplankton (Brand: Red Top, USA) are also hatched and maintained at optimal conditions in the laboratory. We set off the experiments with the physical parameters viz. Photo period: 12: 12 L: D cycle, Temperature: 26-27°C, Salinity: 10 ppt and pH of the medium 7.5. To evaluate our hypothesis in restricted environment we have introduced the zooplankton in a 3 liter beaker with 75: 25 (TPP: NTP) food ratio. Biological activities (feeding) are monitored for each of the species with regular recorded biomass count on each experimental day till the predator population goes to extinct. Results: The mean biomass profile of zooplankton remains more or less constant at the initial stage but a sharp decline trend has been observed after the 4th day of the experiment. A similar trend has been observed for the mean biomass profile of NTP leading the population toward extinction after 6th experimental day. The entire mean biomass profile trend of TPP can be interpreted as a convolution of three growth pulses viz., initially positive, followed by a negative and terminating with a positive growth. To evaluate the bias in the result of experiment we have estimated the variance levels of sample biomasses for each of the experimental time points for each of the three species. Conclusion: The observed stable nature of the zooplankton biomass may be due to initial NTP uptakes but a sudden decline suggests that they are forced to feed on the TPP for survival. In absence of grazing pressure, TPP initially showed a mild positive growth but when the predator switch to TPP for food it shows a negative growth and finally due to rapid mortality of zooplankton and excretal nutrient input the growth rate again kicks up. In summary we conclude that the zooplankter (Artemia salina) can discriminate toxic and nontoxic food species and more inclined toward the non-toxic species if the resource available. But shortage of nontoxic species, force them to feed on toxic one, in spite of drastic adverse effect on its survival.