The Earth is home to a wide variety of species, and a whole gamut of hypotheses exist in order to explain the diversity in life. Most of these draw a link between species richness and physical phenomenon such as climate or seasonality or environmental heterogeneity. This is, for instance, well reflected in what is known as the ‘latitudinal diversity gradient,’ in that the number of species are highest in the tropics (around the equator) and decrease as one moves to the polar regions. Often, factors such as time and space available for the accumulation of species are invoked as well.
In order to better unravel this puzzle, Tietje et al. (2022) tried to examine if they can quantify the impact various environmental variables have on the rate of diversification. (The rate of diversification is defined as rate of speciation minus extinction.) Secondly, if a correlation between the environment and the diversification rate does exist, does it offer a causal explanation for determining species richness? In other words, is species richness determined by environment via diversification rate? The study brings under purview five hypotheses:
- H1, the metabolic theory of ecology: it argues that high temperatures lead to higher metabolic activity among individuals, which causes more mutations and, therefore, speciation. Additionally, it argues that the high net primary productivity of tropical biomes creates a climate warm-and-wet enough to harbour more species.
- H2, the hypothesis of climate stability: the longer the climate of a place is stable, the more it will encourage speciation. Abrupt climate change leads to extinction.
- H3, the hypothesis of niche differentiation: temporal variations in a place’s climate (e.g. hot v cold, wet v dry) make its resident species better adapted. A place where precipitation and temperature is similar all-year-round is likely to have species that will perish should the climate change.
- H4, the hypothesis of environmental heterogeneity: the more types of soils, vegetation types (essentially, microclimates) a place has, the more species it is going to contain.
- H5, the hypothesis of evolutionary arenas: some biomes are naturally predisposed to have a high speciation-low extinction rate, and, therefore, high species richness.
In order to test these hypotheses, Tietje et al. (2022) analysed a global data on richness and diversification for all seed-producing plants (angiosperms and gymnosperms), encompassing 310 botanical countries covering almost all of earth’s land. These botanical ‘countries’ largely follow political boundaries, and the data was sourced from the World Checklist for Vascular Plants (WCVP).
The study’s mathematical model ‘found no connection between the two variables’ of species richness and diversification rate. In fact, the study even observed that while species richness does increase when one moves from poles to the equator, rates of diversification show the opposite trajectory as it decreases towards the equator.
‘The results firmly reject any hypotheses involving diversification as a mechanistic link between the environment and species richness.’
Apropos of the fourth hypothesis, for instance, spatial environmental heterogeneity showed a positive effect on species richness, but – largely – a negative effect on diversification. Similarly, with respect to the second hypothesis, climate stability showed no impact on richness, even as it increased diversification. Seasonality (the third hypothesis) had little to no influence on either diversification rate or species richness. Therefore, the study argues, that none of the hypothesis that invoke diversification rate in order to explain the link between environment and species richness hold much ground – as far as this study on seed-producing plants is concerned.
What other theories or hypotheses could then explain these observations? The metabolic theory of ecology could give way to the tropical conservatism hypothesis. The latter maintains that since the lineages of most species originated in the tropics, the tropics naturally had more time to ‘accumulate’ high diversity throughout earth’s history. Of course, the model fits tropical areas – one of the biomes considered for the model – perfectly. It is quite clear, according to this study, that tropical rainforests are species rich because they are widespread and old, and not because they witness quick speciation or little extinction.
The reason climate stability was not shown to have a significant effect on diversification was probably because extinction events that happened in the past were highly localised and would not have really affected the diversification rates. The absence of any effect of climate on species richness could also be explained by the ‘notorious difficulty of separating past and present climate trajectories.’
Seasonality returns a weak link, because the climatic niches of plants are not necessarily narrower in areas that are not seasonal. In fact, in the event of climate change, plants that are used to surviving in a narrow niche might just go extinct. However, authors find that seasonality can lead to a somewhat higher species richness directly – for finer niche partitioning can just permit more species to coexist.
Authors further argue that the absence of the impact of environmental heterogeneity on either diversification rate or richness was simply due to the fact that plants have been evolving for far longer than the topographic mosaic we see today was created. ‘While some of the most topographically rugged areas are renowned for spectacularly rapid radiations, these are usually recent and may not be reflected in the average diversification rate,’ the study adds.
The key takeaway here is that diversification rates and species richness are not linearly correlated for plants. Future research could, the study suggests, look at time as a parameter, and dive deeper into the nuances between environment and diversification rates.
The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik