Habitat Change

​By Bruce Hyde

While I have always known that certain dragonfly species like certain habitats, I did not realise what a difference even small changes can make. Sometimes, usually I suppose,  these changes can be gradual, but sometimes they can be dramatic.
 
Recently I visited two small areas by the river Tech, which I hadn't been to for about a year. I was very surprised to find that in one place there were no dragonflies at all, and in the other there were very few.  In each location I had seen around 15-20 different species in the past. One would not necessarily expect to see them all at the same time, but even so to see none at all was quite a surprise.

Large Pincertail (Onychogomphus uncatus) Chevaine Catalan - one of the rarer species previously found here.

This set me thinking - why should there have been such a change in a relatively short period of time? After all, there had been no major weather events in the last year.
 
What I believe had happened was that the riverside vegetation had grown rapidly and the change had caused the dragonfly and damselfly populations to move to more acceptable locations. The ones I had previously seen here were all species that liked fairly slow flowing or still water, with vegetation on the bank that was quite low, offering plenty of places for them to perch in the sunlight.
  
One damselfly that seems to have moved on is the attractive Common Bluetail.

Common Bluetail female (Ischnura elegans rufescens) L'Agrion élégant

 
Another very common damselfly now absent from that particular spot is the White Featherleg.
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White Featherleg male (Platycnemis latipes) L'Agrion blanchâtre

​As an example of slow changes, these two photographs taken from roughly the same place, and under two years apart, show the extent to which vegetation has grown in a shortish period of time. Even though the seasons are quite different you can see how much growth has taken place.

​Near Moulin de Breuil January 2015 and September 2016

​When a major weather event arrives, such as a big storm or a prolonged period of heavy rain, it is not surprising that the habitat, and the species in it, change.
 
As an example of this, during the winter of 2010/11 there was a period of prolonged rainfall resulting in enormous amounts of water coming down the river Tech from the slopes of Canigou. These floods caused the level of water at Le Boulou to rise 6 metres above its normal level. This meant that the weight of water rushing through the river basin caused even large trees to be swept away, which in turn devastated the lesser vegetation on the river banks. Even local landmarks such as large trees and bushes disappeared.

​The size of the tree trunks and the height of the log jam gives some idea of how high the water was.
 
The following photos were taken from the identical place before and after the big inundation. From lush growth to absolute desolation - note Canigou peeping up in the background.

What is really surprising is that anything living in the water can survive the massive flooding and pounding of stones, rocks and detritus. Dragonflies usually spend the winter underwater as larvae, eating other aquatic insects and, sometimes, other dragonfly larvae. They emerge in the spring and early summer and develop into the beautiful flying insects with which we are all so familiar. I don't know how they survive such turmoil. Nevertheless, even fish survive such conditions.

Common Carp (Cyprinus carpio) Carpe Commune

Catalan Chubb (Squlius laietanus) Chevaine Catalan

Of course, when habitats change, local species that can fly will move elsewhere, so all one has to do is track them down again! Fish just have to adapt to their new environment. One is reminded that the ever-changing landscape is all part of nature, and ecology has been evolving since the beginning of time; and it is also part of the Darwinian process of survival of the fittest.