Ecological succession is the process by which the structure of a biological community evolves over time.
In the soil, plant exudates attract fungi and bacteria, which provide food for protozoa and nematodes, which are then eaten by other fungi and micro-arthropods who become food for worms, beetles, ants and millipedes, which are preyed upon by birds, moles and badgers, who bring in nutrients in their scat that – once the fungi and bacteria have broken them down – are eventually absorbed by the plant.
This process is called ‘nutrient cycling’. As it goes on the plant communities get increasingly complex and varied, and succession is under way.
Sterile soil contains none of this complexity. But the first colonising plant, lichen or weed will start the ball rolling. Those sloughed off cells and exudates from the plant roots will entice bacteria to move in and the process of succession keeps adding complexity of species and interactions between those species. The end of the line, perhaps 500 years later (in Scotland at least), is usually oak- or Scots pine-dominated forest, which is the richest soil with the greatest ecological output potential.
Competition or Cooperation?
Much of my formal biological training centred around the pivotal idea of competition and self-serving individual action as being a primary driving force in biological theory. The more I’ve learned about different aspects of our natural world, the more competition seems like the small picture and cooperation the big picture.
Competition certainly exists and it helps to fine tune living beings to their environment, so they can function better in that environment, and so more creatures can inhabit more environments – and that means more life, in more varied forms.
The greater the range of habitat types, whether it’s windy tree tops or mossy rocks in a forest, deep slow pools or braided channels in a river, the more living things you can find that have adapted to life in that particular place.
Wild life forms are out there doing their thing and the result, inadvertently or not, is that more life forms can come along and make use of the new space or resources provided by the previous ones.
For this web of life to function properly, and in tune with the greater orchestra of nature, it has to be generated by the wild itself. Natural processes build things up gradually, like the accumulation of organic material in a soil – only once a critical level has been reached can that soil support a tree, for example, and if we leave nature to do its thing a tree will eventually appear on that soil, given enough time.
What has this got to do with our rivers?
Adding more and more juvenile hatchery salmon to a depleted river system is like trying to run a formula one race using mass-produced, mechanically inferior cars on a bumpy farm track.
It’s like trying to bake a cake over a campfire, or trying to tighten a screw with a maggot.
If a system doesn’t have the correct fundamental parts you can’t expect the desired result. There will be no increase in output unless the system can support it.
Stocking a river with hatchery salmon is like pouring more and ever more water into a leaky bucket, instead of fixing the holes in the bucket. What’s more, a larger proportion of the salmon that end up in the river are less likely to survive to spawn again than the wild salmon they have replaced.
A formula one car needs the right habitat for a formula one car – a smooth racetrack. Cake baking habitat is an oven, and a screwdriver is what’s required for the screw.
Salmon need natural processes to dominate not just in, but also critically around, a river in order to have the right habitat to maximise recruitment. This is the only way the river can support more life.
This can be done top down by introducing the requisite keystone species or bottom up by providing lots of terrestrial and aquatic invertebrate habitat and clean spawning gravel.
Over time, and with nature allowed to be the driving force, the river will recover and will be able to increase recruitment, but it needs to be given the space to do so.
Some will argue that we need to shoot the sawbill ducks and seals, control numbers of dolphins, otters, kingfishers, cormorants, dippers, ospreys… let’s also kill all the trout and grayling (they eat salmon eggs, right?), and the pike and perch.
Or maybe not.
Predation does not limit life in the river. It is simply a part of the essential nutrient cycling system. Having said that, non-native species, like mink, that don’t fit in (don’t benefit) the system should be removed. Those millennia of co-evolution that otters, sawbills and our other native salmonid predators have alongside salmon means they are part of the system, and an essential part at that – they cycle nutrients.
‘Over predation’ can only happen in a broken system. In other words it’s a particular problem when there’s a lack of hidey-holes for the prey and an incomplete nutrient cycle (some key species are missing).
So what are the existing factors that currently limit life in our rivers? Here are a few…
- Industrial and agricultural pollution
- man made dams
- lack of biological uplift
- limited spawning gravel
- limited fry and parr habitat
- limited shelter
- limited food
- high temperatures
- flashy tributaries
- blanket shade from non native trees
- poorly designed culverts
- hard revetment
- little or poor structure for shelter
- disconnected flood plains
- limited diversity in depths and speeds of flow
- single stem rivers with uniform flow profiles
- road run-off