Competitive exclusion of ostracod species in a temporary environment
Creators: name: McLay, Colin Lindsay
Publisher: name: University of British Columbia
Year: 2011

description: Two species of ostracods (Cyprinotus carolinensis Ferguson, 1958 and Herpetocypris reptans (Baird, 1835) living in a temporary puddle were studied to find out whether one species could cause the extinction of the other. The time during which the animals can reproduce is limited because the puddles are temporary. While the puddle is wet each species may be present at any time as both animals and eggs. The puddle in which the animals lived was a long (31.1 m), harrow (34.0 cm) and shallow (2.5 cm) rut on a gently sloping field. It was supplied with water from run-off and seepage. The puddle filled in the fall and dried up in the spring; during this interval the eggs present in the fall had to become more eggs by the spring for the species to survive. The presence or absence of a species was assessed after drying-up in the spring. The effect of one species upon another is expressed by a single quantity - the number of eggs of the species that are present at this time. One species of ostracod can affect another by altering the number of animals which hatch in any unit of time, the number and length of time required for them to reach maturity and the length of time which they spend laying eggs. A computer model was built which was capable of simulating the populations of eggs and animals of both species when alone and together. The model was built up from observations and experiments on the ostracods. Where possible the models of processes which were developed were tested upon the natural population. After the addition of water to the puddle there is a delay of from 6-7 days before the eggs of each species begin to hatch. The eggs are capable of hatching at any time after the addition of water. A simple model, in which the mortality in an interval of time related only to the numbers present, was found to be satisfactory. The mortality of Herpetocypris was less than that of Cyprinotus when both were alone. Cyprinotus affected the mortality of Herpetocypris in the same way as did the members of Herpetocypris, but there was an additional effect of Herpetocypris upon Cyprinotus. The number of day-degrees required for the animals to reach maturity was constant, regardless of the time at which the animals hatched. The number for Cyprinotus was half that required for Herpetocypris, and it increased linearly with the total initial input of animals. The egg-laying rate of both species is almost the same and was found to decrease in an exponential manner as the temperature decreased and/or the density increased. There is a delay between the laying and hatching of the eggs, which increases as the temperature-decreases. The delay for Herpetocypris eggs is much shorter than it is for Cyprinotus eggs. The final number of eggs left in the mud when the puddle dries is given by the sum of the old eggs that remain and the new eggs which have been added. The number of new eggs is the difference between the total number which have been laid and the total number of these which have hatched. Given an input of eggs, the times that the puddle filled and dried up and the temperature during the wet-period, the output of eggs in the spring could be predicted. The ability of the model to imitate the real populations was tested. Given the same input of eggs the output of eggs by the model was compared with the natural output in the following spring. The expected numbers agreed closely with the observed. During filling and drying-up not all parts of the puddle covered or dried at the same time; hence the mean daily puddle-length fluctuates. A simple model of the fluctuations in length was constructed. It used only the record of rainfall during each period. Using the mean times of filling and drying and mean temperatures for the years 1961-69, for selected points in the puddle, the effect of each species upon the other was investigated on the computer. The outcome depended upon the initial numbers of eggs. Herpetocypris could cause the extinction of Cyprinotus over most of the range of inputs. Cyprinotus could only become the dominant species when its initial numbers were 7-8 times larger than those of the other species. The number of 'seasons' required to arrive at these relative positions depended upon the initial numbers. The model was used to simulate the populations of both species for the years 1962-69. The conditions fluctuated from year to year. Initial concentrations of from 0.1 - 1.0 eggs/cm² for each species were tried and the outcome was found to be extraordinarily sensitive to the initial density. A narrow range of inputs led to the observed coexistence in 1969. Below this range only Cyprinotus remained and above it only Herpetocyprisc It is therefore concluded that competitive exclusion is indeed possible for these animals, even when the environment fluctuates from year to year.