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I've often read and been told that with foraging less is often more. Whilst you may need X kilograms of a berry or flowers (like elderflowers) you should never pick a bush clean.
How can you estimate when to move on to the next bush, or not pick at all, would it be like leave at least half?
What's the reason to leave an amount on the bush?
(TL:DR -- The "safe" amount to harvest varies enormously with plant species and context. When in doubt, take 5% or less. 1/3 is probably safe for common, prolific species.)
This rule is given out because if you harvest every fruit on a plant, you've stopped it from reproducing this year. You don't necessarily even have to harvest all the fruit, if the plant has a generally low seed-germination rate. It gets a little more complicated if you're harvesting things other than fruit, too: taking a wild carrot kills it outright, meaning that unless its seeds have already ripened and been dispersed, that's 100% of its reproductive capacity gone. Same, and even worse, for annual species harvested that way.
In scientific collections, the general rule I was taught is to leave at least 95% of a population -- that is, to kill no more than 1 in 20 plants, or to strip no more than 5% of the population's reproductive capacity. This is a good solid rule, especially when you're dealing with relatively uncommon wild species or species that are slow to recover from harvest. Frankly, though, foragers shouldn't take any of those unless it's literally a matter of starvation.
Elderberry* is neither rare nor slow to recover; it flowers and fruits prolifically, year after year. You could likely harvest a third of its fruit, or even more, without damaging the population's long-term reproductive capacity. Be careful to leave enough for the animals that depend on the fruits, though! Personally, I definitely won't take more than 1/3 of a native species population.
All that said, there are some edible plants whose extirpation would be a public service. Look up your local noxious weeds, and harvest those to your heart's content. Take more than you need! Take all you can carry! Kill them off, please!
Did you have any specific plants in mind, other than elderberry? I could do some research and get you firmer numbers for them.
* Assuming you mean one of the more common Sambucus species -- S. nigra or S. racemosa or something -- in North America or Europe. I'm not well-up on the whole genus.
I think this is a question of sustainability. If you strip a bush of its flowers and/or fruit, then it loses the opportunity to reproduce this year, or ever in the case of annuals and/or patches of wild veg in which you harvest the whole plant.
Basically you want to leave some behind to continue the species where you found it.
In desert cultures this is even taken one step further. If you see a lone flower in the desert, carefully dig it up and relocate it to a place with others of its kind, this allows that lone flower a greater chance to reproduce.
