I have species present and relative abundances from one visit for the historical data to sites that we have been able to resample extensively. We do not know the sampling effort for the historical work - just species lists and numbers of individuals caught/site. There seems to be a great deal of disagreement (and reviewer harshness!) about the most appropriate methods when (historical) data are limited. Our questions are really about how species' ranges have shifted, expanded, contracted, or remained the same over time and across sites.
You can compare species diversity (using indices) and ranges across sites and years. You need to figure out whether your sites represent true replicates or if they are just subplots within one consistent area. Either is fine, so long as you specify whether you have true replicates or not. Generally, the more widely spaced the plots, the more likely they are to be true replicates :)
I agree with the answer above. You can also use the Michaelis-Menten product momentum equation to project historical spp. Diversity and abundance using limited data. We used similar methods comparing fish diversity and abundance along a submarine canyon wall. You can find our methods in the attached.
Article Vertical stratification in the distribution of demersal fish...
In my opinion, the fig.24 is good for grafic altitudional gradient. For perennial dinamic of number, the Furie spectral analis (short and long periods) is the best.
In the attached we used historical plant species presence data in a substantial and steep elevation gradient (forest to arctic-alpine vegetation). Some of the issues may be similar. A lot may depend on spatial (100 m to kilometers) and temporal (decades, centuries, millennia) resolution.
Have you considered comparing SDMs for historical versus current species data plus environmental conditions?
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Article Fine resolution distribution modelling of endemics in Majell...
what is relative abundance? are you thinking of hierarchical distance sampling. whichever method, the best approach would be to have sites as random effects while elevation and years are exploratory variables or fixed effects if your are comparing species richness between well defined elevation gradients or amongst years
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