Language and Ecology — The Latitudinal Gradient

In ecology, there is a widely-recognised global pattern in distribution of species. Studying terrestrial vertebrates, ecologists found that species richness increases from poles to tropics. This pattern is also seen in marine organisms as well. Birds, on the other hand, have the highest species richness in regions corresponding to the tropics and mountain ranges. The pattern on species richness increasing towards the equator is often referred to as the latitudinal diversity gradient. It reflects how species are unevenly distributed across the globe, drawing great academic interest in explaining this pattern.

A curious thing is, such a gradient has also been identified in human languages. Indeed, the world’s 7000 or so languages are unevenly distributed throughout the world, with ‘hotspots’ like Papua New Guinea having well over 10% of the world’s languages despite covering a small proportion of the global land area, and relatively linguistically sparse countries like Russia, where 1.5% of the world’s languages is spoken in a country occupying 11% of the world’s land area. There is still a general trend; language diversity increases towards the equator. This is consistent with the latitudinal diversity gradient recognised in ecological studies. Additionally, within the tropics, languages tend to be restricted to smaller areas compared to languages in higher latitudes. In other words, the tropics tend to be linguistically denser than higher latitudes. Similar to the ecology scene, this gradient has also attracted academic attention to propose factors and hypotheses surrounding language diversity. Some have suggested similar underlying mechanisms between linguistics and biological diversification, while others may argue on a more anthropological school of thought. In this post, we will be discussing the possible presence of ecological drivers influencing global language diversity, drawing arguments from a paper by Hua et al. (2019). Their main finding was, the climate played a larger role than geographical isolation in possibly influencing language diversity.

Two general ecological mechanisms have been proposed to explain the latitudinal gradient in language diversity, namely isolation and ecological risk, or more simply put, climate.

Isolation refers to geographical barriers that pose as challenges to human movement, driving a reduction in interaction between people groups, and slowing the spread of language and its variants to other populations separated by these barriers. Over time, changes in linguistic features of respective languages accumulate, making them distinguishable compared to each language spoken in neighbouring populations. This takes landscape features into account, considering the roles of rivers, mountains and roughness in the landscape. Previous studies have shown a positive correlation between these landscape features and language diversity, which demonstrated how isolation has a role in driving language diversity.

On a global scale, however, the authors argued otherwise, that the landscape played an inconsistent role in language diversity. They suggested that previous studies reflecting association between landscape and language diversity were driven by autocorrelation and non-independence. For instance, river density, the number of rivers in a certain defined region like grid cells, is associated with smaller minimum speaker population size, while not associated with the average speaker population size. What this means is, rivers allow smaller speaker populations to persist, but not dividing them into further smaller speaker populations. Rivers, in this case, do not function as a means of isolation between populations, but instead, may be an ecological resource. This led them to suggest that ecological risk could play a bigger role than isolation in language diversity, at least from an ecological perspective.

Climatic variables are proposed to have a role in language diversity. These include seasonal temperature variation and annual precipitation. These factors can directly impact the ability of a given area to produce food. The less food that could be grown in an area, the higher the ecological risk. One of their propositions was, smaller social groups are more likely to be stable in regions of abundant and stable food supplies or resources, often requiring predictable rainfall and low seasonal variation. In regions with sporadic rainfall and high seasonal variation, social bonds have to be formed across bigger regions to achieve a similar level of sustainability, especially when resources are scarce and unpredictable. Therefore, within a given unit area, one would expect to observe more language diversity in a region more capable of reliable food production, by agricultural means or hunting and gathering. With this prediction, they assessed six climatic variables, including mean annual temperature and precipitation, seasonal variation in temperature and precipitation, net primary productive and mean annual growing season, i.e. the number of days in a year suitable for growing crops. They found that these climatic variables could explain the latitudinal gradient in language diversity well, but not fully, noting that on larger scales, seasonal variation in rainfall has the strongest association with language diversity, while on smaller scales, seasonal variation in temperature has the strongest association. While seasonal variations in temperature and precipitation were found to have large roles in shaping language diversity patterns, mean annual growing season also seemed to play a role as well, supporting the ecological risk hypothesis. A longer growing season is linked to higher language density in a given area, and these languages have a smaller geographical range. In comparison with the isolation hypothesis, the ecological risk hypothesis plays a more significant role in shaping language diversity. We must note that there is no mechanistic approach involved, meaning that we cannot draw causative relationships between climate and language diversity. Nevertheless, this study revealed much about the roles the variable play in language diversity on different scales.

We must note that these ecological drivers do not account for all the variation in language diversity. There are still regions with abnormally high or low number of languages even when the data are corrected for these ecological drivers. The authors noted that New Guinea, Mesoamerica, East Himalayas and West Africa contained high diversity which was unexplained by ecological factors, while the Amazon was revealed to be relatively lower. While a convincing proposition was brought up to account for the Amazon’s relative paucity, suggesting that the Amazon is undersampled, the same could not be said about the unusually language-rich regions. Evolution and diversification of languages could have occurred faster there, although no quantitative studies have been made. Unlike biological species, there is no agreement on the phylogenetic relationships between language families, and how long ago they diverged from one another. Languages predominantly exist in speech, with an increasing number now being written and digitised. But when a language goes extinct prior to documentation, no one would know that language has ever existed. The first languages spoken by humans remain a mystery for this reason; they were never recorded, and we could only look at what records of languages we have today, and at most, back to when writing was invented. In this regard, can ecological methods be truly applied to the field of linguistics?

Like species, languages can undergo migration events, diversify or go extinct. This paper made note that this study only represented a snapshot in language diversity, taking into account current climate data as well. The changing cultural diversity is often confounded with various historical events like conflict, migration and politics, factors which their analyses could not account for given the main scope. They added that, even under these human influences, the detection of environmental influences on language diversity adds another facet to the debate on the drivers of language diversity patterns. Perhaps, this may direct studies in the evolutionary aspect, where language diversification rates or accumulation could be modeled, shedding light on probable historical trends of language diversity despite the many challenges that follow.

Afterword

I chanced upon this Nature communications article about using ecology to explain global patterns in language diversity, drawing parallels to biodiversity. I remember writing about languages and biodiversity as part of my university applications, which lead me to reflect on how this paper has changed my view on languages and biodiversity. Being more academically informed about these topics definitely helped shape my opinion on this article, which I have written in a rather journal-club style. Anyway, I hope you have enjoyed reading this post, and I will see you in the next one.

Further Reading

Hua, X., Greenhill, S. J., Cardillo, M., Schneemann, H. & Bromham, L. (2019) The ecological drivers of variation in global language diversity. Nature Communications, 10, 2047. Available from: doi.org/10.1038/s41467-019-09842-2.

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