In Western Sydney, there’s a critically endangered vegetation type called Cooks River Clay Plain Scrub Forest, of which there’s less than 1% of its original extent. All the remaining patches are heavily fragmented and surrounded by very urban areas. The biggest remaining patch (~11 ha, so pretty small) is a 2 minute drive from my house, so I’ve started to survey it and record everything I can find in there.
In 1979, a local resident called Tony Price did a similar thing, but for plant species only; he spent 3 years surveying the area (not just to get a species count, but also looking at flowering times, fire ecology, etc. --> I’m only doing the species count). Since then, there have been zero major surveys of the patch, with 1 or 2 surveys in the 41 years since merely confirming the top 10% common/dominant species were still there, and then just assuming everything else had stayed the same since 1979…
So I’m recording everything (all the non-plant stuff will be ‘baseline’ data given it hasn’t been recorded there before), but I’m specifically comparing my plant species count with his to see which species have persisted over time, which have disappeared, which new ones have appeared, etc.
To cut to the chase, at what point do I stop searching for new plant species and assume I’ve found ‘everything’ that is still there. In 1979, Price recorded 467 species; after ~42 hours so far, I’ve found 267:
The figure is slightly dodgy because a) I don’t spend the entire time each survey looking only for plants and b) I don’t walk the exact same route through the reserve each time, but it’s clear that relatively soon, the curve will start to plateau, and it will become harder and harder for me to find new species each trip.
Quite a number of the species Price found will no longer be there (e.g. rare orchids smothered by weeds, garden escapees that appeared once or twice but never established), so I’m assuming the ‘true’ number of current species there is less than 467 (although this is probably slightly counterbalanced as I’ve found a number of new invasive species that weren’t there in 1979). Given I’m at 267 right now, at what point do I pack up shop and say I’ve found essentially everything, or at the very least it would not be feasible to continue to search. The dilemma of course is that if you survey over a long time and don’t find something, there are two possibilities: 1) it genuinely isn’t there or 2) you just haven’t looked in the right spot yet/have overlooked it each time. As long as you continue to not find that species, it is impossible to tell which of these 2 scenarios applies.
This has become a very long and rambling post, so I guess to sum up:
At what point on a species accumulation curve, for a relatively small area, do you stop surveying and decide you have found ‘everything’ within reason?