The majority of organisms/taxonomic groups follow the Latitudinal Diversity Gradient (LDG), e.g. the increase in species richness from poles to the tropics. There are some rare exceptions, mainly discovered & published for animals (e.g. penguins or shorebirds) but rarely for plants (e.g. grasses). Recently we have demonstrated the same atypical gradient in gymnosperms. Who knows other plant groups/families which do not follow the LDG?
There are a number of plant families that have higher diversity in the mid-latitudes (temperate) so it depends how far towards the poles you go where climate becomes limiting. Many of these groups extend into the tropics but only at high altitudes so that they are not really seeing a tropical climates as they move up the hill. I suspect that this pattern is more pronounced in the southern hemisphere than the north. A good website to check out that shows family distributions and gives generic/species diversity is the Angiosperm Phylogeny website maintained by Peter Stephens at Missouri Botanic Garden http://www.mobot.org/MOBOT/research/APweb/welcome.html
Thanks a lot for all the interesting hints, links and publications.
Best regards,
Greg
Pine trees and Eucalyptus are more diverse on temperate zones than in the tropics
Macroecological and biogeographic patterns heavily rely on the scale you were talking about, so these examples are not so rare. At local and regional scales there are many cases that do not follow such latitudinal pattern. Globally, a good example is the world diversity of Gymnosperms as was mapped by Barthlott and colleagues (Erdkunde, 61: 305–316)
There is a whole range of plant families that are far more speciose in the temperate and subtropical zone than in the real tropics (Fagaceae, Betulaceae, Caryophyllaceae, Brassicaceae, Rosaceae, etc.), although most of them become rare again towards the poles (where higher plant hardly occur anyway). When you go down to genus level there will be whole genera not reaching the tropics, but I don't think that is a very useful analysis. When going down enough at the taxonomic level one can always find exceptions to the LDG rule. Only when th rule does no apply at high taxonomic level there is something going on that needs to be explained.
Theoretically the most diverse places on earth should be mountains in the tropics, where all climatic zones are present; a similar mountain on the pole will only have an arctic layer at the bottom and ice for the rest. But on that mountain in the tropics all those species are not living together, they are merely present within the same 10 x 10 km grid cell. That situation should be kept out of the comparison when testing/applying this LDG rule.
I would like to commend Sean Haughian on his answer referring to bryophytes. Several recent studies could not find conclusive evidence for a latitudinal diversity gradient in moss (Bryophyta) species richness - see Global moss diversity by Geffert et al. (2013) J. Bryol. 35(1): 1–11. DOI: http://dx.doi.org/10.1179/1743282012Y.0000000038
For your information: here is the paper on gymnosperms and their atypical LDG mentioned in my first message (now available online).
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