PEDRO M. ANASTACIO, Teacher, University of Évora (Portugal) – Department of Landscape, Environment and Planning
After more than 30 years of presence of the crayfish in European wetlands, do you consider that its population have reached to certain equilibrium within the invaded ecosystems?
I would say that, whether we like it or not, this species is now a component of most of our freshwater ecosystems and total eradication is currently impossible. Although it has been related to several ecological and economical problems, P. clarkii is especially important in temporary water systems such as small streams and wetlands. In these areas it is an “easy” food source for many predators such as otters, egrets or storks and it is possible that it contributed for the increase of their population densities. On the other hand, its negative effects on biodiversity have been described and it is known that it may predate and consume several freshwater organisms. In addition it acts as a carrier of a disease which decimates native European crayfish species. In view of this, it is hard to talk about equilibrium since in some regions of Europe the species is present for 30 years but in other regions, e.g. of Italy and France, the species is still expanding. My answer to this would be that the species has reached a dynamic equilibrium state in the areas where it established long ago but that this has not yet been reached on the invasion fronts which are still active.
Which is the current situation of red swamp crayfish population in Portugal?
The species is currently present in every river basin in Portugal, but the invasion front is still progressing in a few small rivers. Although it is locally consumed by people, it is not a much appreciated food item since in Portugal there is a much stronger tradition of consumption of saltwater crustaceans. In fact there is some commercial capture of the species, mostly to satisfy the demands from the average Spanishman. The Portuguese legislation allows for the capture of the species but since it is considered an invasive species, it has to be killed immediately. This would largely remove the commercial value of the species and therefore it is not a common procedure by local fisherman. I believe that we should promote the harvest of crayfish in areas where it is considered to have negative impact. This would allow for some reduction of the field densities. Any attempt of eradication would in my view be a waste of money since we share our river systems with Spain which has a different legislation and a different perspective regarding the species.
Which are the impacts of the red swamp crayfish on rice crops in Portugal?
Several studies were performed by our group regarding the impact of crayfish in Portuguese rice fields. We found that a density of one adult crayfish per m2 is sufficient to reduce rice production by 42% and at 3 individuals per m2 no rice plants survive. We demonstrated that crayfish activity generates changes in rice field water quality, but that these changes do not affect rice production. We found out that the damage to rice production is related to the direct consumption of rice seeds and seedlings. Crayfish effects are more intense at an early rice development time (approx. 6 days) but crayfish prefer 12 days old rice (approx.). The reason for this is that the same amount consumed will have a different relevance on a young or an old plant. In addition to the consumption of rice plants we also have to consider the effects of crayfish burrowing activities, which lead to water loss. Nowadays rice farmers have adapted their culture practices to the presence of crayfish, minimizing their damages. It is also common that farmers facilitate the access of crayfish fishermen to the irrigation channels in their farms.
Which are the advantages of using models to study exotic species?
Models are wonderful tools to test scenarios, therefore preparing strategies for the management under future conditions. Moreover, with models we can perform a pre-study of the properties of a system without really having to physically manipulate it. However, in my point of view, we should not fall into the trap of building purely theoretical models. Calibration and validation of the models using real data are important steps of the ecological modelling process. With P. clarkii in particular, we have been using both dynamic and spatially explicit static models. The dynamic models we built allowed us to study the population dynamics of the species in the rice fields therefore predicting the best approaches to the reduction of the damages caused by them. Our more recent works using static models allowed us to identify the most suitable areas for invasion by the species worldwide. In addition we were able to produce consensus suitability curves for several invaders based on modelling approaches. These are curves that show the environmental suitability of the species regarding gradients of environmental variables. Its values were not obtained by the use of a single model but from the results of a set of different modelling approaches.
Contact : http://home.uevora.pt/~anast/ [1]