Rodents can be a real pain if left to feed undisturbed in your polytunnel. If you see evidence of mice, voles or even rats in your polytunnel then intervention will be needed in order to save your crops from their nibbling. If you notice that you have a problem with rodents then swift action is needed, since a mouse can chomp through a lot in even a single night.
As usual in an organic garden, keeping pest populations in check is the first and most important step to take. If you have huge numbers of rodents then whatever measures you take are likely to be insufficient to deal effectively with the problem. First of all then, it is a good idea to turn your attentions not to the mice themselves but to the creatures that can help you to keep their numbers down. Owls are amongst the predators that you should work to encourage in your garden. Build owl nest boxes and create a habitat suited to their habits. Keeping chickens can also help. Foxes will also eat mice – though will not be welcome visitors for those who want to keep chickens. Generally keeping a wildlife friendly garden will help with pest problems.
If mice have already found food in your polytunnel then keeping the ecosystem in balance will not be enough. You will have to create physical barriers between the rodents and your seeds and seedlings. To make sure your seeds come to fruition, it can be a good idea to sow seeds indoors before transplanting seedlings to the polytunnel later. You can also cover tender new seedlings with cloches and/or netting to stop them from being eaten. (Plastic drinks bottles cut in half work fine and will reduce the cost of protecting the plants in your polytunnel.
Planting wildlife friendly plants and creating wilder areas in a different part of your garden will help to draw mice away from your crops. They will be less likely to eat your food if they have a source of wild food such as berries to keep them fed. Good hygiene in the garden and polytunnel and keeping the place clear of debris can also help to reduce the incidence of problems with mice and other rodents.