It's hard not to see the world through the lens of our own preconceptions and biases. We tend to be more interested in other large mammals. We're drawn to human-like qualities in pets. But even the most benign insect is disturbingly alien when seen up close. We also tend to use the familiar as a metaphor for understanding the unfamiliar. Sometimes this gives us additional insight. Other times, it leads us down the wrong path.
This is the first in a series of posts in which I plan to examine one of the fundamental concepts in ecology - the species-area relationship Species conservation has always been intimately linked with the idea of habitat conservation. While habitat quality determines the amount of habitat required to protect a viable population of a given species, it’s only a modifier - the determining factor is area. Habitat quality can determine whether you need more or less area, but area is still the critical factor. While protected areas can be set aside for specific species, more commonly protected areas seek to protect as many species as possible. More land is likely to protect more species, but there are other factors that influence conservation decisions like the cost of land acquisition and the competing interests such as agriculture, mining or housing development. One way to maximise the number of species in a protected area is to include as many habitat types as possible. If you include a forest, a meadow, a marsh and a lake in your protected area, you are likely to get a lot more species than you would if you only had forest habitat. The heterogeneity of the area increases the number of species. (After all, you don’t find a lot of fish in a pine forest, or field mice in a lake.) But again, this overlays a simple factor of area. A larger tract of forest will probably have more species than a smaller tract of forest. A larger section of a marsh will probably have more species than a smaller section of marsh. This fact, known as the species-area relationship is fundamental in both ecology and conservation biology. The existence of a relationship between species richness and area is obvious to anyone who has taken the time to think about it, but it is still interesting enough that it has attracted the attention of generations of ecologists.
Succession is one of the first things that students learn about in ecology. Each intervening stage modifies the environment in such a way that lays the groundwork for the next stage, while making the environment less hospitable to its own offspring. Only the final stage is self-perpetuating and stable. Frederic Clements, one of the pioneers of community ecology, saw ecological succession as an ontogenic process in which the community - a superorganism - developed into its final, mature form. The orderly progression from bare ground to mature forest is orderly, progressive…and very Victorian.
When people think about the destruction and degradation of tropical forests, they tend to focus on rainforests. Tropical dry forests tend to get overlooked. They aren’t as striking - no cathedral-like understorey, no mind-boggling biodiversity. But more importantly, they often just aren’t there. Over much of their potential range they have simply been erased from the landscape. They may have covered as much as 42% of the land area in the tropics1, but have been reduced to less than 27% of their former range in Mexico2, and as little as 2% in Central America3 and New Caledonia4. Despite this fact, tropical dry forests are often seen as being quite well-adapted to human disturbance.
Plant leaves are photosynthetic organs. Their main job is to harvest energy from sunlight, and use that energy to convert carbon dioxide and water into carbohydrates. In addition to capturing sunlight, leaves need to be good at doing two other things - taking up carbon dioxide and conserving water. These requirements conflict - anything that lets carbon dioxide in also lets water out. To deal with these conflicting requirements, plants produce a waterproof cuticle and regulate carbon dioxide uptake by opening and closing their stomata.
Over the last decade, genetically modified crops have become widespread in agriculture. One of the more successful of these are Bt crops - transgenic plants that express genes derived from Bacillus thuringensis. These genes allow the plants to produce toxins which specifically affect certain groups of insects. Since these plants do not need to be sprayed, and since the toxins are relatively specific, the environmental effects appear to be lower than conventional agriculture.
When European naturalists first visited the New World Tropics they saw vast forests that seemed untouched by humans. While indigenous people often lived in these forests, their populations were small. This led to a perception of tropical forests as primeval, “virgin” forests. In the last few decades, this perception has changed - large areas now covered by mature forests have a history of cultivation. In many cases, “primeval” forests are less than 500 years old.
In response to Wright and Muller-Landau’s paper on the future of tropical forests (which suggests that declining rural populations can allow forest recovery; see my previous post), Sloan pointed out reduced rural population often leads to increased deforestation. Really that’s not a huge surprise - peasant farmers tend to have limited labour to clean and plant land, and being capital-limited they tend not to be able to switch to mechanised agriculture.
In a fairly optimistic paper published in Biotropica in 2006 Joseph Wright and Helene Muller-Landau suggested that declining rates of population growth in tropical countries, coupled with increased urbanisation "give reason to hope that deforestation will slow, natural forest regeneration through secondary succession will accelerate, and the widely anticipated mass extinction of tropical forest species will be avoided." Their assertions have been controversial, and have attracted a fair bit of criticism.
Anyone who has conducted field research knows that the very process of collecting data alters the system that you are studying. As you walk across a field, forest or stream to collect data, your footfalls trample vegetation, they compact the soil, they scrape algae off the rocks.
Survey work usually involves a single visit to a site - as long as you avoid sampling from the areas you have trampled, it’s usually pretty safe to assume that your presence is unlikely to have affected the data that you have collected. Permanent plots are a different matter - because these plots are repeatedly sampled, there is cumulative damage.
In larger plots, permanent trails may be established within plots. Since we can’t avoid these effects, the real question is whether the effects are significant. Ecological systems are inherently heterogeneous. Does the effect of disturbance fall within the range of natural variability within the sample? That’s what really matters when it comes to data collection.
In a forthcoming paper in the journal Biotropica, Liza Comita and Gregory Goldsmith “sought to quantify the significance and spatial extent of research trail impacts on the structure and dynamics of the seedling layer in the 50-ha permanent forest dynamics plot on Barro Colorado Island (BCI), Panama”.
Anoop Sindhu and colleagues report on a gene that may have played a key role in the evolution of grasses. The gene, Hm1, provides resistance against Cochliobolus carbonum race 1 (CCR1), a fungus that is capable of attacking and killing corn at any stage of its development (images of CCR1 infection). While CCR1 is only known to affect corn, the gene Hm1 and its relatives are present throughout the grass family, but are absent from other lineages.
Tropical forests are immensely species-rich. The question of what causes this diversity is a perennial one in tropical biology. In the 1970s Daniel Janzen and Joseph Connell independently came up with the same explanation - if the seeds or seedlings of more common species have a higher probability of being killed by a pest or pathogen (what is known as density-dependent mortality), then less common species will be favoured. If the organisms that are responsible for most seed and seedling mortality are specialists - if they focus on just a few plant species - then the pathogens and seed predators that specialise on common tree species should be more abundant (since there’s more food for them).
Janzen was able to demonstrate this with a few species of beetle whose larvae fed on (and killed) seeds. When seeds of the Hog Plum (Spondias mombin) were abundant, female bruchid beetles laid their eggs on (and ended up killed) well over 90% of the seeds. When the seeds were scattered, mortality rates were reduced. Unfortunately, while there were several good anecdotes, there was little evidence of density-dependent mortality playing a role at the community level. In fact, there was evidence that trees were more likely to be clumped than scattered, a finding which was not in keeping with Janzen and Connell’s hypothesis.
