I enjoyed it a lot and still enjoying when I watch documentaries on lush green tropical jungles full of life forms within it.
And, I started birding in my childhood, and ever since always wondered why there are so many birds in tropical regions.
Then, I realized it’s not only applying to birds but also to many animals.
The diversity increases with the latitude decrease where the tropical belt holds the maximum diversity of life forms on earth. In ecology, this phenomenon is known as the Latitudinal Diversity Gradient (LDG).
The tropical region of the world is situated between the Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere. The name implies about warm, hot, and the moist climatic situation persists year-round.
This tropical belt is known to hold many biodiversity-rich ecosystems such as tropical rainforests, seasonal tropical forests, spiny forests, deserts, and many more microhabitats to harbor many animals.
The tropical belt of the world hosts the highest diversity of the birds and highest species richness in the world. If you rank countries based on the highest number of bird species within a country, 8 out of 10 are situated within the tropical belt.
These are Colombia, Peru, Brazil, Indonesia, Ecuador, Bolivia, Venezuela, and Congo. The other two countries in the top 10 are China and India which are situated outside of the tropical belt. But the most important segment contributing to Indian birdlife in terms of species richness came from the wet tropical mountain known as the Western Ghats.
So, surely the tropical belt is home to many birds. But Why? In this post, I discuss four major reasons why there are so many bird species in the tropics.
1 – Direct Solar Radiation
The primary energy source of all living things on earth is solar energy. With the positioning of the earth, tropical regions get direct solar radiation compared with the other areas of the planet earth. So, the tropical region gets the maximum energy flux.
And the number of plant species in the tropics is several-fold high when compares with other parts such as temperate zones and polar regions. This higher plant diversity demands a greater amount of energy which can only be satisfied in this higher energy-intensity tropical region.
High plant diversity and richness enable high animal diversity since animals depend mostly on plants for food and other biological requirements. Thus, Net Primary Production provides energy to a whole ecosystem.
Because of the wind patterns and other climatic conditions, the tropical belt tends to have an optimum temperature throughout the year (averaged between 25 to 28 degrees of Celsius/ 77 to 82 degrees of Fahrenheit) which is suitable for most birds.
Even the birds which live in temperate and other higher latitude areas, migrate to the tropical region during the winter season to avoid harsh and cold climates with lower food supplies.
Abundance of Food in Tropics Year-Round
Another important reason to have a higher number of bird species in the tropics is well-established rainfall patterns.
Even though it says that there is no seasonality in the tropics (i.e.: Summer, Autumn, Winter, and Spring), we can identify one or two seasons with the wet condition due to heavy rains. On average, tropical regions are known to have more than 60 mm of precipitation during wet months.
Most of the forested areas in the tropics usually get mild rain during the evenings. These warm and wet climatic situations facilitate most of the natural cycles such as the water cycle, Carbon cycle, Nitrogen cycle, etc.
And also, those environmental and climatic conditions speed up the decaying process which facilitates nutrient recycling fast and stores most of the nutrients in plants.
So, year-round fruiting is available for most of the plants including one or two peak fruiting seasons. You can find many bird species of Bulbuls, Pigeons, Toucans, and Hornbills in tropical regions which are mainly frugivores in their feeding guild.
In other words, the tropics hold well-established well-distributed food resources throughout the year to feed the birds. Especially, those peek fruiting times of plants and blooming time-synchronized with breeding seasons of many birds in the tropics.
2 – Phylogenetic Lineages Origination With Higher Kinetics
When we think about the evolutionary time history of most of the life forms on earth, ancestors of those animals mainly originated in the tropics.
Such originated species have mostly settled in the tropics without migrating to other parts of the world. This is mainly due to the optimum climate conditions of the region which facilitate the fulfillment of basic life needs.
Because high energy and higher reproductive rate resulted in higher mutation to occur which resulted in a higher speciation rate as well.
Thus, the origination of new species in the tropics is also at a higher level compared with other regions. Those findings are well supported by fossil records as well
3 – Niche Conservatism
We define a niche as the space which holds all the necessary resources need for a species to survive. It is a very broad subject in ecology and very challenging to understand even for a single species.
Niche conservatism is a degree to which animals tend to retain their niche and related ecological factor through space and time.
Basically, in the tropics, most of the animals including birds tend to retain their habitats, food resources, moving space unchanged with time (evolutionary time).
This is mainly due to quite a stable environmental condition in the tropical regions. And also due to the lack of stressful environments for most of the animals.
Therefore, ecologists have identified niche conservatism as another reason to have much higher diversity in the tropics. We can see most of the bird species with a similar ecological role (e.g.: Bulbuls) has higher species richness in the tropics but tend to share some resources with resource partitioning to a level where they do not need to compete with each other.
Further, in the tropics, the aggregation of bird species with more specialized habitats is higher compared to high latitudes. It is a known fact that tropical bird communities hold more specialized and rare species than other regions (subtropics, temperate, and polar-caps).
4 – Less Anthropogenic Interactions
In recent times, geologists named this era Anthropocene due to various activities done by humans to make a geological layer made of many artificial things such as plastics and polythene.
Human activities now threatening most of the natural habitats all over the globe. But when compared with the other regions, the urbanized tropical region is still less disturbed. If you examine the global urbanization map, you will see there is still a considerable amount of land available in tropical regions compared to other parts of the earth.
Thus, I would like to suggest fewer human activities are also a reason at least with an indirect effect on higher species richness in the tropics.
Because when human activities tend to rise, such as urbanization, we can see local-level species extinction over time. Inaccessible geographic formations and human population migrating from rural/ village areas to cities may affect positively tropical natural ecosystems to persist with less harm by the people.
To conclude, I have listed four major reasons which influence having higher bird species richness in the tropics when compared to other parts of the world. And most of these reasons are related to each other to some degree.
Some scientists are still not satisfied with the findings, especially related to phylogenetic lineage histories in the tropics but point out that much more needs to do before we conclude.
So, this is a subject area with a huge knowledge gap with huge room to do more scientific work. There could be more reasons which I’m not aware of. So, I would love to hear your thoughts on this as well. If you have any, please share them in the comment section for the benefit of me and other readers.
Literature sources: Brown, J. H. (2014). Why are there so many species in the tropics? Journal of Biogeography, 41(1), 8-22.
Map credits: Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F., Present and future Köppen-Geiger climate classification maps at 1-km resolution“. Nature Scientific Data.