Here are a few plant families with interesting distributions you could check out as well:

• Ericaceae, the heather family, is insanely high in the Cape of South Africa. Try turning on the log scale (top right) to see the variation in rest of the world better
• Polemoniaceae (phlox) is centered on California. Click on California to see all the other plant families that are unusually high here.
• The parasitic "vampire-cup" family Cytinaceae has a weird ditribution around Mexico, Madagascar and the mediterranean.
• Sarraceniaceae (new world pitcher plants) are a cool family with an interesting distribution, highest around Florida and Venezuela.
• The gentian family has an interesting climate breakdown, the majority of species are found in "subalpine or subarctic" climate, which is very unusual.

Since the boundaries used for aggregation are somewhat arbitrary, this visualization can't be taken too seriously as representing "true" centers of biodiversity or distribution. See Sabatini et al. 2022^[2] for a much computational approach, although showing similar patterns, although other studies have used these boundaries for statistical analysis, controlling for area and climatic diversity within the boundaries^[3].

This is a central and still unresolved question in biogeography, sometimes called the Latitudinal Diversity Gradient (LDG, my notes ). The general pattern is that species richness tends to be higher around the equator and lower around the poles. It seems to be a relatively recent phenomenon which began after the end of the Cretaceous period, 30 or 40 million years ago. Before this, Earth usually had a much wider tropical band, and species richness was probably not much lower around the poles than at the equator. A planned feature for this site is a chart to visualize this LDG next to the map for each family.

One paper (Tietje et al. 2022^[3:1]) used this same dataset (filtered to seed plants) to try and explore some different hypotheses on this question. They found that one of the most important factors contributing to an area's plant species richness are temperature and precipitation. However, they also found that the diversification rate was generally lower in these areas and higher in dry, northern areas that experienced more climate change in the past 30 million years. They conclude that the reason why tropical areas have more diverse plants is not because of higher rates of diversification, but that they have conserved more species from the Earth's deep tropical past. Something to consdier when exploring the map!

As for why different families have different distributions than each other - this is a very complex question, but I believe it is related to:

• The LDG - tropical areas tend to be more diverse as a rule
• Environmental diversity or geodiversity of an area
  • areas with lots of different microclimates, soil types or altitudes will have more diversity
• the evolutionary history of the family and its interactions with continental drift
  • some families evolved quite recently in isolated areas, or have gone extinct everywhere expect for these areas
  • New Caledonia is an example of this, containing a few families of "basal angiosperms" that have gone extinct everywhere else.

I previously assumed the reason why some families are very diverse in one area and present in small numbers in the rest of the world is because their center of diversity is the same as their point of evolutionary origin. However, this seems to not be true in every case. For example:

Onagraceae (the "evening primrose" or fireweed family), is present around the world but extremely diverse in California. However, the earliest fossils we have are from Paleocene Colombia (as per Paleobiology Database), and its evolutionary history is much more complex, with its most diverse lineages originating in California, but with other genera originating in South America, eastern North America, and Chamanerion appearing to originate in the northern temporal zone^[4].

I hope this helps you dive into lots of rabbit holes!