Urban ponds – oases of biodiversity

“These ponds serve a purpose. Besides preserving biodiversity, one of their functions is to purify water. They also provide numerous other ecosystem services. For example, they have educational value. Instead of hiring a bus and heading out of town, schools can take their students, or pupils, with nets and buckets and wander a few hundred metres to a pond. You can learn about biology in them, quite simply. Ecosystem services are a free service we humans get from the pond,” says Frank Johansson, Professor of Conservation Biology at the Department of Ecology and Genetics.

He leads the research project “Biodiversity conservation in urban ponds”, which has made an inventory of small fauna such as insects, snails and worms in 80 ponds in Stockholm. These are small ponds with a surface area of approximately 100 by 100 metres or less. Some ponds are natural while others, such as stormwater ponds, are man-made.

“Stormwater ponds are designed to collect all the water from streets and hard surfaces when it rains. Sediment is allowed to settle there before releasing the water into natural waters. So it’s a purification process. The ponds act as a filter. Of course this is also true of natural ponds,” Johansson explains.

The optimal pond

The main goal of his research project has been to find out what the optimal pond should look like to benefit both people and biodiversity.

“We have inventoried the ponds and measured a host of variables around them. We have measured how big they are, how much vegetation they have in them, how much vegetation is around them, how much built-up area there is around them, how much forest, how much meadow, their nutrient content. We have studied about twenty of these environmental variables and then been able to correlate them with biodiversity,” says Johansson.

Declining species may thrive

On average, they found 40–50 different species in each pond, though there were also ponds that contained no more than seven or eight different species. However, low species diversity is not always necessarily negative, as rarer species were more often found in these environments. Species whose populations have declined also showed up in the ponds. Two examples of species in decline that seem to thrive in urban ponds are the great crested newt and the large white-faced darter.

“We have shown that in an optimal pond, about 50 per cent of the water surface should be covered by vegetation, and that you need to have open blue water as well. What often happens is that the ponds become overgrown and then many species that want more open water disappear,” says Johansson.

Blasted rock not a good choice

So what should anyone who wants to build a congenial water oasis for diving beetles, dragonflies, water striders and other creatures think about? According to Johansson, a good start is to create a soft bottom, avoiding the use of blasted rock. This is appreciated by most pond dwellers, especially species living in bottom sediments.

Another important thing, he points out, is not to put fish in the pond. It may look nice but it is bad for biodiversity because fish are voracious eaters of the larger insects that are often the top predators of the water body when fish are not present.

From pond to pond

One thing that he singles out as particularly conducive to high species diversity in an urban environment is having several types of ponds that can meet the habitat requirements of different organisms.

“It’s really important that the ponds are not too far apart, because of course sometimes populations and species die out and then you need a possibility of spreading to the next one. A kilometre or so between the ponds is about right, then there’ll be no problem,” says Johansson.

It is not just the distance between the ponds that matters. Equally important, the ponds should be connected by green spaces so that creatures can move between them. This does not necessarily mean large parks – a ditch, a small stream, a grove of trees or a few bushes can suffice.

But does the location of the pond make any difference? Is there more wildlife in ponds in wealthy municipalities than in deprived suburban neighbourhoods? The researchers investigated this too.

“We proceeded 500 metres from ponds and drew a circle around them and then we looked at socio-economic factors: how much people earn, what education they had, what ethnic background and so on. We wanted to see if these factors had any influence. This has been shown in the United States, for example. Areas where there were many rich and well-educated people have higher biodiversity. But this was not the case in our study. Nothing significant emerged. The ponds in deprived areas of Stockholm did not have lower or higher biodiversity than the ponds in any of the rich municipalities,” Johansson says.

Åsa Malmberg