Image Credit:Jami Dwyer
Tropical forest constitutes the most diverse and complex biomes on Earth. Since Humboldt to the present day, the ecology of these biomes has been captivating and challenging scientists. The speciality of the ecology of these biomes reaches further than pure natural history; they are of key importance to the world: from timber, to medicine, to regulating the global climate. Now many people consider these unique biomes to be at a crossroads. With increased deforestation and fragmentation of tropical forests in the shadow of global warming, there has never been a more pressing time to understand the ecology of these vital biomes than the present day. This highlights what is particularly special about the ecology of tropical biomes: we know very little about it. To conserve these biomes effectively and sustainably we must understand their ecological functions and systems.
Diversity, and complexity
In the tropics there is high annual rainfall, and relatively stable average temperature, with only two seasons: wet and dry. The high annual rainfall results in leaching of the soils causing nutrient poor soils. Although the tropics share these similar climate and soil conditions, there are many different types of tropical forests: from dry to rain, and from mangrove to eucalyptus. They are divided along gradients of temperature, humidity, altitude, and flooding; as well as being divided by their evolutionary history, with different species playing similar roles in different forest types, for example new world monkeys compared to old. From nutrient poor soils has arisen the most productive, diverse, and complex terrestrial ecosystems on Earth.
As well as a large diversity of forest types, the biodiversity within the forests is the highest on Earth, harbouring around 50% of the world species. One of the most noticeably biodiverse groups in the forest are invertebrates, particularly insects. Nigel Stork fogged ten trees of just five different species with insecticide, which brought down around 2800 different species of arthropod, and, what was truly amazing, was that most of these species were represented by a single individual! Now, with Bornean forest having as many as 300 tree species per hectare, and then considering the biodiversity of arthropods in the leaf litter, or in epiphytes, one starts to comprehend how special these ecosystems are. In fact, a study in 2004 investigated how much of the canopy biodiversity was living among large epiphytes. From just five basket ferns they collected around 250,000 individual arthropods, and they concluded that epiphytes contain about as much animal life as does the canopy above it, doubling the estimate of invertebrate biomass in the rainforest canopy from just five plants. This is also just invertebrate biodiversity; the plant diversity is equally staggering, with orchids, ferns, bromeliads, lianas, mosses, liverworts, algae, and lichen covering the branches of trees, and on top of that epiphytes growing on epiphytes, creating a fractal image of plant life.
Tropical forests are not just a random combination of organisms, but a complex ecological web of interactions between species. As written in ‘On the Origin of Species’ (1859): ‘When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance. But how false a view this is!’. I think this complexity is best shown by the forest’s sensitivities. The Biological Dynamics of Forest Fragments Project shows that when the forests become fragmented, the entire ecological community changes. For example, in a one-hectare fragment after 2 years of isolation, the number of bird species declined by 60%. The high complexity means changes in the population sizes and community compositions can trigger a chain reaction, with synergistic effects, that ripples through the forest, impacting many other species.
Practising ecology in tropical forests
In my opinion, what is particularly special about the ecology in tropical forests, is that we are still in the very early stages of understanding this field: ‘The science of biodiversity is not much farther along than medicine was in the Middle Ages. We are still at the stage, as it were, of cutting open bodies to find out what organs are inside’ – The Unified Neutral Theory of Biodiversity and Biogeography (2001). Tropical forest ecology was for a long time focused on describing specific patterns and processes, but Hubbell’s Neutral Theory disrupted this into carefully questioning many of its most fundamental assumptions.
Along with this theoretical turning point in tropical forest ecology, practical ones have come with modern technology. One of the reasons why we know relatively little about tropical forest ecology is because tropical forests present many challenges to ecologists, particularly how to representatively survey the most biodiverse area on Earth, which happens to be up to 30m high in the canopy. This challenge has produced some creative solutions: from training macaques to collect specimens; to mini zeppelins; to construction cranes. Now with modern technology it is becoming safer and easier to survey these areas, though it is still by no means easy. As with deep sea and space exploration, much of tropical forest ecology is still in the dark, which makes it very exciting.
The changing ecology
Now our lack of understanding and knowledge is more important than ever, because, as we run into the Anthropocene, the ecology of tropical forests is changing. This reveals another speciality of tropical forest ecology: how to sustainably conserve the most exploited, complex, least understood, and important terrestrial environment on Earth, while allowing for development of communities who live in/rely on tropical forests.
Tropical forests are not just changing due to deforestation for timber and agricultural land, but also due to selective extractions of plants, poaching, biological invasion, fragmentation, and climate change. These changes are particularly hard to combat because several threats are likely to interact synergistically with one another, as well as precipitating indirect and direct effects through poorly understood interaction webs.
It is important to conserve tropical forests not just because they support around 50% of described species, and possibly an even larger number of undescribed species, but because they also play a disproportionate role in global carbon and energy cycles. In the early 2000s forests in 75 tropical counties studied contained 247 billion tons of carbon (NASA). This means that the deforestation of tropical forests is a significant contributor to carbon emissions. Another important reason for their conservation is that tropical forest provides a livelihood for millions of people.
There is some debate about the extent to which tropical forests are being negatively affected by climate change, but if anything, this just highlights the need for more knowledge on tropical forest ecology. It is crucial to understand the ecological effects of this anthropogenic change if we want to effectively conserve tropical forests.