Tropical rainforests can be loosely defined as the permanently wet wooded areas in the equatorial region between tropic of Cancer and Capricorn. There is huge diversity – not only within tropical rainforests – but also between them; there is no set model forest, as each has its own biogeographical and biological structures, which vary hugely in different regions, due to separate evolutionary pathways. Despite this, several environmental characteristics are useful in comparing diverse regions, such as consistent high heat, humidity, and rainfall, as well as large, evergreen trees, and specific plant adaptations/lifestyles such as epiphytes, buttressing, climbing, stilted roots, and cauliflory. Additionally, on top of regional distinctions between rainforest types (neotropical rainforests from central-south America, African rainforests in the Congo Basin and surrounding areas, and the Eastern Tropics which cover parts of Asia and Australasia), there’s also differentiation in forest types depending on the elevation of the site; for instance, lowland, montane, subalpine, and cloud are additional definitions of tropical rainforests, and come with their own set of characteristics.
Despite these many distinctions and categories for organising our knowledge of tropical rainforests, a feature they all share is extremely high species diversity. The American biologist Edward Osborne Wilson claimed in his 1992 work on biodiversity, that despite only covering 6% of the world’s surface, tropical rainforests contain more than half of the species on earth. Evidence and estimates supporting this claim have arisen since then, with species counts of plants and animals showing a significant latitudinal gradient, with diversity increasing greatly, the closer you get to the equator. When comparing temperate zones with diversity hotspots like Ecuador, we can see that this area has more than 10x the plant species the UK has, and when looking at animals, the UK is home to 55 mammals and 219 bird species, whereas Ecuador boasts 382 and 1435 respectively. The findings from these direct comparisons of a temperate vs. tropical zone are seen across the board, and although lower diversity tropical areas do exist, they are still much higher than temperate zones comparatively. In addition to increased diversity in tropical areas, there is also a much higher level of endemic species, which are native/only found in one certain place, leading to tropical rainforests often being named ‘biological hotspots’, as they hold large numbers of different species, but also, a lot of these species do not exist anywhere else.
So, we have an established and empirically reinforced trend of increasing species diversity and endemism with proximity to the equator, biologists have hypothesised a variety of theories to explain these findings. These proposals can be split into two groupings: evolutionary ideas, which focus on answering how these regions originally became so diverse, and what biogeographical features shared by tropical rainforests enabled the speciation events; and ecological theories which examine the current biophysical factors which facilitate the coexistence of so many different species together. Consequently, the debate in the scientific community can be simplified into asking whether the tropical environments promote fast speciation, or old lineages just persist well in these areas instead of being regularly replaced by new ones over evolutionary time. This essay will critically evaluate the leading theories that explain the latitudinal diversity gradient, beginning with evolutionary hypotheses and followed by ecological arguments.
When examining the evolutionary theories, the most common theory referenced is the Museum hypothesis. This theory states that because tropical rainforests are relatively stable habitats, without the stress of seasonal changes, the environment where new species emerge does not transform significantly over time, meaning that the habitat acts to preserve the new species, rather than forcing them to adapt to changing conditions. Because the conditions in tropical rainforests are stable, they preserve newly evolved species, which are normally only driven extinct or forced to adapt due to stochastic environmental events. This leads to a slow accumulation of many new species, which differentiated due to factors aside from environmental conditions, and have maintained success over time, due to the stability of tropical regions. Therefore, this leads to very low extinction rates, where high numbers of species can persist over time.
However, critiques of this theory point to the fact that speciation doesn’t necessarily rely on excessively long periods of time, as if the correct selection pressures are active on a group of organisms, speciation can occur in as little as 2 million years, which has been proven in the neotropical kingfishers. Furthermore, it is true that during the quaternary geological period (Pleistocene-Holocene), the habitat around the equator has been relatively stable, maintaining an average temperature of ~27°C, humidity ~90%, and an annual rainfall of 0.1-2m, across the last 2million years. However, further back in geological time there have been several significant climate changes, such as the Silurian and Pennsylvanian-Permian ice ages, where extensive glaciations occurred, and during these periods, the tropics became significantly cooler and dryer.
Advocates for the Museum hypothesis combat these rebuttals by claiming that the tropics acted as a ‘Pleistocene refugia’. A refugium is defined in population biology as a safe area, where organisms can survive during harsh conditions, such as widespread glaciation. This theory states that as the rainforests were contracted or split into sub-areas during climatic instability, this facilitated allopatric speciation of pre-Pleistocene organisms. Then, after the cool period subsided, and the rainforests expanded again, the newly differentiated species were united, permitting the refuge-area populations to spread and extend their territories.
In contrast to the Museum hypothesis, which concludes that low extinction has caused the high diversity in the tropics, the Cradle hypothesis, is a second evolutionary theory which suggests that the tropics may act as a nursery for new species to be born, and claims that high speciation rate is the cause of the latitudinal gradient. Many factors feed into the cradle hypothesis of high speciation, for instance, the geological instability (proximity to tectonic plate edges and mountain formations) can act as a species pump, and has been seen in the areas surrounding the Andes, as genealogy reveals that numerous organism lineages emerged at the same time as the mountain formation. Furthermore, the tropics are highly fragmented, by complex, large river systems, creating further species pumps, as allopatric speciation is driven by separation and barriers.
Another explanation claims that the warmer climate promotes faster speciation, fundamentally because of the greater available energy in the system, leading to greater population sizes, increased metabolic scope, and thereby allowing more species to exploit specialized niches.
A final evolutionary method within the Cradle hypothesis states that the diversity of elevation within tropical habitats facilitates differentiation, because the heterogeneity promotes adaptive divergence. As previously stated, distinct rainforest types cover different altitudes, meaning that if a species travels up a mountain with their habitat, this promotes adaptive divergence through clines.
In addition to new species developing, they must be able to coexist and thrive, while not exhausting the resources provided by their shared habitat, in order for highly diverse regions to persist. The ecological theory of Competitive Exclusion (Gause’s law) has 7 criteria that must be fulfilled; but can be simplified into the principle that, two species cannot coexist long-term if they directly compete for a limited resource. The conditions that must be met to satisfy Gause’s law are:
1. Rare species not evolutionarily favoured
2. Species actively compete
3. Mechanism/competition is limited by a single resource
4. Environment remains constant over time
5. Environment has been stable for long enough for mechanism to play out
6. There is no immigration from other populations
7. Shared habitat is not significantly heterogeneous.
These factors enable high levels of species to coexist without constantly driving each other extinct or draining shared resources. Many case studies have shown that competition, rather than neutral mechanisms drive speciation: in 2012, Renner and Ricklefs concluded that biological community structure in rainforests cannot be assigned to stochastic factors, as they found it was common to see hundreds of different tree species growing next to each other in all 3 tropical continents. This study shows that when taxonomic tree families are arranged by ranking species richness, the orders are significantly similar in all study sites. However, critics of this competitive exclusion research would claim that as the trees/ their ecological niches are not sufficiently segregated, the 7 criteria of Gause’s law are not met, therefore the findings are void.
In conclusion, it is clear that there is a large variety of factors acting collectively to facilitate the high species diversity we see towards the equator. The latitudinal species richness gradient is not explained by a singular evolutionary or ecological hypothesis, and is likely driven by a combination of factors, which create the perfect scenario for high speciation to arise, low extinction rates to exist, and maintain an equilibrium where this large number of species can coexist and proliferate over evolutionary time.