# How to compare vascular flora richness among areas of different size

The question is: are there mathematical tools to make a comparison in floristic richness among areas of different size that are ecologically plausible?

It is obvious that in a given region, if factors such as geography, soil and climate are more or less homogeneous, the number of vascular plant species does not grow linearly as the area increases. In this light, for me there is no ecological sense in comparing a lowland wetland of, for example, 15 hectares and the whole region it lies in or with a mountain range.

Did someone propose that species richness should grow linearly based on area?

Hi,
of course, comparisons of species richness of different sizes are limited due to the confounding effect of area. This has a vast literature in ecology. Various models have been proposed for describing how species richness changes as a function of area. If you know which model applies to your system, you can predict what species richness should be found if you know the size of the sample site, and then you can compare how much the observed richness deviates from the predicted richness. Alternatively, you can try estimating the richness-area function for your sites separately, and compare them based on the steepness of this function. The steeper function should indicate higher diversity but it is somewhat different from comparing pure richness.
Cheers,
Attila

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Was it the question of the thread? It does not seem so.

I donâ€™t know. You said it things were obvious, I donâ€™t think they are obvious at all

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I simply wanted to know some background about this. Edit i.e. did someone suggest that species richness should increase linearly all other variables constant?

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I didnâ€™t think I was lecturing (?) I wanted clarification and asked a question. Thatâ€™s not lecturing is it?
I cannot answer the threadâ€™s question without that. I think that diversity is a better measure to compare areas of different size than richness

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I agree with this but one, sometimes, could have available only species richness and not diversity.

In my area we have â€śregional ecosystemsâ€ť. These are based on surface geology, area (area as in a broad location, not size) and the dominant canopy species. Richness or diversity doesnâ€™t really come into into it (apart from the canopy layer). Most of this can be done using geology mapping and aerial photos. Sometimes, obviously, ground-truthing is required but not as often as youâ€™d think (for my area). Species richness or diversity is interesting but Iâ€™m not sure that itâ€™d be linear. For example, in RE 12.8.2 I can be pretty confident of the canopy species (well, certain in the majority of cases), and of the plant families present in the understorey (but species richness being linear? Not sure at all). Itâ€™s an interesting thing. Iâ€™m sorry this doesnâ€™t answer the question, but I think there is a correlation but I donâ€™t know if it can be extended to the rest of the world

About the shapes of the species-area relationship, see:
https://onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2699.2003.00877.x
https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2699.2008.02038.x
https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12428
https://www.pnas.org/doi/10.1073/pnas.1818190116
This is just a brief excerpt from the vast amount of literature on this topic.

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None of those are linear models/relationships though, are they? Maybe Iâ€™m reading too much into the question

No one claimed it does grow linearly. OP is saying it doesnâ€™t. (Might be best to move on from that line of inquiry)

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Ok, sorry. I thought that someone had claimed it was linear and the OP was saying that they disagree with that (and I agree). Sorry for the misinterpretation

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We have many endemics, so a larger area would and does bring more species.

My understanding of species-area curves is that they are most appropriately applied to different areas of similar habitat heterogeneity (or lack thereof). So I agree that

Except, maybe, in a relative sense, if you take a mean species-area curve for a larger region, you could compare smaller habitat patches within that region to the mean expectation, and be able to say that particular habitats have higher-than-expected or lower-than-expected floristic richness for that region.

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I have heard that in general when you increase an area by a factor of ten, the number of species roughly double. This is a rather rough rule, and the latitudinal gradient will skew this result.

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