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 Victorian science provides a picture of nature that is, in its very essence, Victorian, there’s good reason to re-think your models. And in the study of succession, people have done that. People working on succession tend to stress the role of chance, and see the system as cyclical - there is no stable “climax” community, there are no undisturbed forests. But old ideas die slowly. Textbooks still teach succession using a number of early studies which were based on the idea that you could substitute space for time, what ecologists call chronosequence studies.

In a paper published in the May issue of Ecology Letters, by Edward Johnson and Kiyoko Miyanishi¹ take a look at some of the classic succession studies, and came to the conclusion that “empirical evidence invalidates the chronosequence-based sequences inferred in these classic studies“. While this is not totally surprising, it's important to document.

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. Being less species-rich than wetter forests, they tend to support fewer rare species, and may be less extinction-prone. In addition, dry forests are dominated by trees that sprout after being cut. This means that if you cut down a patch of dry forest, most of the stumps will re-sprout. This type of recovery is much quicker than you would get if the trees had to germinate from seeds - not only does it take much longer for seedlings to grow large (stump sprouts can draw on resources stored in the roots of the tree), but there’s likely to be a time lag as seeds disperse into the area from surviving trees (tropical forests tend to lack long-lived seedbanks).

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.

Photosynthetic rates tend to be limited by whatever is in shortest supply - water, light, carbon dioxide, nitrogen to make enzymes... If you never water your plants, they will die regardless of how much carbon dioxide and fertiliser you give them. But no matter how much water, light and carbon dioxide they have, if nitrogen is in short supply they can't make enzymes to run photosynthesis.

For plants in wet environments, access to water isn’t their biggest problem. When plants experience precipitation every day, access to carbon dioxide is likely to be more important than access to water. Since carbon dioxide is less soluble in water than in air, the presence of water on the leaf surface may hinder the uptake of water by the plant. It has been suggested that in environments like these, selection should favour the ability to rapidly shed water from leaves.

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.

However, Bt toxins face the same problem that other pesticides face - the evolution of resistance in target insects. The selective pressure of a compound in your food that kills you (unless you have a resistance gene) is huge. The more widely a Bt crop is planted, the greater the selective pressure. And unlike pesticides, to which a pest is only exposed periodically, Bt toxins will (presumably) be produced by the plants on a continuous basis. Since lab studies had showed that pest species possessed genetic variation in their resistance to Bt toxins (including this 1998 EPA study), the question is more one of how quickly resistance would evolve, rather than if resistance would evolve.

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.

Thus, they are likely to leave a substantial portion of the landscape either forested - either ’still’ forested (because they haven’t cleared the land yet) or reforested (secondary forest grows rapidly on abandoned agricultural land so long as fire is excluded). Larger operations are less likely to be constrained by these problems.

Cattle ranching, one of the major land uses behind the frontier, uses less labour and more land. Cattle ranchers are usually better capitalised, and are often supported by state subsidies. Fire and grazing prevent forest regeneration, and does limitation in seed supply (the bigger the opening, the greater the distance the seed has to travel) and competition from grasses. The other alternative, mechanised agriculture (as practised in Mennonite colonies in Bolivia, for example) also maintains large areas free of forest, and since they repeatedly crop the same land and depend on chemical inputs, there is no opportunity for forest recovery.

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.

CCR1 is only known as a disease in Zea mays, but the Hm1 family of genes throughout the grass family. Sindhu and colleagues silenced the corresponding gene in barley. This resulted in barley that was susceptible to CCR1. The fungus is able to invade susceptible grasses through the production of Helminthosporium carbonum* (HC) toxin. The ability of Hm1 and related genes to resist CCR1 comes from an enzyme known as HC-toxin reductase (HCTR), which detoxifies HCTR.

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.