Abstract

The north-american crayfish species Pacifastacus leniusculus (Dana) was once brought to Europe as a substitute for native crayfish species Astacus astacus (L.), which was driven close to extinction by the fungus Aphanomyces astaci (Shikora). This fungus was also introduced from North-America before. However, some populations of A. astacus survived, only to find themselves confronted with a new threat: P. leniusculus, a seemingly superior competitor for their habitat.
Our aim is to get a better understanding of population dynamics in the two competing crayfish species A. astacus and P. leniusculus by means of a spatial implicit, individual based simulation model.

There is no doubt that these two species compete with each other, if they occur sympatric. But there are questions which might not be that easy to answer:

• What do these crayfish compete for?
• What mechanisms work in this competition?
• How exactly do these mechanisms act?
• Are there any interactions between these mechanisms and, if there are any, how do they act?
• How does all this affect the population dynamics?

In 2002 Westmann et al. published their long time observations on competing populations of A. astacus and P. leniusculus in an enclosed lake (Replacement of the native crayfish Astacus astacus by the introduced species Pacifastacus leniusculus in a small, enclosed Finnish lake: a 30-year study; Ecography 25-1, p. 53-73). Strongly simplified they report the following:
At first A. astacus clearly dominated over P. leniusculus by number. There seemed to be a kind of stable state. But then, within just a few years, this state turned into the opposite. There might be even a trend leading to the total extinction of A. astacus in this lake. We use these observations as the base for our model. Thus our questions could be subsumed under one question:
How can the observed population dynamics be explained?
There are two main mechanisms considered to play a role in the process of displacement: Competitive exclusion by antagonistic behaviour and reproductive interferences. To approach this question we will test the following hypotheses with our model:

1.) Antagonistic behaviour leads to a reversal in the sizes of the populations.

2.) Below a critical size of population, reproductive interferences leads to a additional decline in the size of population.