The relationship between biodiversity and latitude is controversial. Research that includes immigration and not just origin or extinction support the “out of the tropics” model. This model indicates that taxa prefer to originate from the tropics. Then, they migrate to the poles while remaining in the tropics. Thus, the tropics are a cradle and a museum.
The latitudinal diversity gradient (LDG) is the phenomenon that as one moves from the poles to the equator (tropics), there is an increase in the number of species. LDG occurs in all different biomes. However, many researchers have disagreed over why and what led to the differences in variation at different latitudes.
Cradles and Museums
Biodiversity of an area depends mainly on three factors. These factors are origination rates (O), extinction rates (E), and immigration rates (I). Thus, diversity in the tropics (DT) can be defined as OT - ET + IT (origination and immigration minus extinction). The diversity is analogous to the number of species. The diversity of the tropics is greater than the diversity of the extratropics (DE). The extratropics are areas that are outside of the tropics.
The most basic model for LDG is when there is no immigration (IT = IE = 0). Thus, a greater diversity in the tropics is due to a higher rate of new species appearing or a lower extinction rate in the tropics compared to the extratropics. Or, the origination rate can be very high in the tropics to compensate for a higher extinction rate in the tropics.
These models for biodiversity in the tropics are referred to as a cradle or a museum. In a cradle, the biodiversity is due to the high origination of new species in the tropics compared to the extratropics. The extinction rates and the immigration rates are the same. In a museum, the extinction rate of the tropics is less than the extinction of the extratropics, contributing to tropical biodiversity. The origination rates and the immigration rates are the same. To distinguish the two, one must look at the origination and extinction rates separately (Figure 1). Typically, areas that have a high rate of diversification are cradles.
Rate Differences and Range Shifts
Besides the simple models of a cradle or museum, the researchers considered that that taxa may change geographic locations throughout time because of climate change. Another factor is that many taxa occupy both tropical regions and extratropical regions. Thus, it is necessary to consider the whole spatial distribution of taxa across time to understand LDG.
Out of the Tropics: A Dynamic Model
The researchers suggest the “out of the tropics” (OTT) model, which indicates that the tropics are both a cradle and a museum. This means that there is a higher origination rate in the tropics and lower extinction rate in the tropics. But, there is an increased immigration rate to the extratropics (Figure 1).
Testing the OTT Model
The researchers use the marine Bivalvia to study the OTT model. Bivalves clearly show LDG, and they currently live in all latitudes. Furthermore, there is a very rich fossil record of bivalves, which provide evidence of when and where bivalves appear.
To test that OT > OE, the researchers looked at the fossil record to find where and when a particular species first originated. The researchers took into account the proportion of living taxa within a bivalve family to overcome sampling bias. They find that the origination rate of species in the tropics is much greater than the extratropics origination rate of species.
Testing that ET is less than or equal to EE, the researchers find that there is more bivalve extinction at higher latitudes. But, more data is needed to confirm this.
To test that IT < IE, the researchers compared where species originated and where they are now. They find that over 75%% of species that originated in the tropics are now in extratropical areas. This indicates a very high immigration rate to extratropical regions.
Insights from Modern Biogeography
Species are not as unique to a particular geographic location as the latitude decreases. Furthermore, species are older at higher latitudes, and the tropics contain both old and new species. Both are consistent with the OTT model, although more research and modeling needs to be done.
The tropics are both a cradle and a museum. Species tend to originate in the tropics and then immigrate to areas towards the poles. Since the tropics are such a large source of new species globally, species at higher latitudes will be tremendously impacted if the tropics are ever in a crisis.