On a recent trip to an Ecuadorian cloud forest I was fascinated by the large numbers of diseases and deformities that riddle the plants. Many of these take the form of plants galls.
Galls are abnormal tissue growth on the surface of plants caused by parasites, such as fungi, nematodes, insects, mites or bacteria, the galls are tailored as the perfect place for the parasitic organism to live in. In the past I’ve looked for and found plant galls in England, but it seemed that they were more numerous and varied in Ecuador. I believe there are three reasons for this, all related to Latitudinal Diversity, which is the phenomenon whereby animal and plant species diversity increases the closer you get to the equator, it applies most notably to rainforests.
What makes galls particularly bizarre, is that these growths are not attachments to the plant, but the plant itself, made to alter its normal growing behaviour in order to benefit its parasite host. Although it is not clear how insects cause this change, bacteria is known to insert its own DNA into the plant cells to alter behaviour. (example here) Insect larvae have been found actually inside the cells of the plant, which might suggest similar interference. It is thought that one wasp (Cynipinae) works in conjunction with a virus (viruses reproduce by inserting their own DNA into the DNA of a host) that lives in the wasps saliva and gets into the plant as the wasp eats it. This is an example of mutualism, since the insect benefits from the virus by getting to make the gall and the virus benefits by getting to reproduce.
These galls are formed by the insect or mite either feeding or laying eggs. When adults lay their larvae on a leaf, excreta or saliva from the insect affects the cambium and causes it to grow differently (more detail above). The larva then grows inside the gall, feeding on the gall itself, eventually eating its way out and escaping. Sometimes the insect control over the plant tissue extends beyond the gall and starches and sugars are drawn in from elsewhere in the plant to increase the food store for the insect.
Sometimes I found insects that had taken over leaves, or even entire plants to make a home in. These were not galls, because while leaves were often distorted, the cells were not expanded or changed, but they were still quite bizarre to see.
Note: for other insect photos I took in Ecuador, see here
Although some galls I was clearly able to determine as being caused by insects or mites because I could find the animal or see its exit point, others I am just not sure about. The following are those less easy to decipher galls that may be caused by fungi or bacteria.
The plant in the next photo is a puzzle, the fluffy looking outgrowths at the base of the leaves (and in between) may be a normal part of the plant, perhaps even be the flowers, but they also look similar to the growths in the above picture, which are definitely galls.
This final gall, I believe, is caused by insects because I think it is possible to see them, the black mass at the heart of the distortion.
Some interesting and useful websites on plant galls:
Some books I used for reference:
I took a recent trip to a cloud forest in the mountains of Ecuador, working at a research centre called Los Cedros. While there I was able to take many hikes out into the forest, taking photos and trying to understand how the forest worked as a system.
A cloud forest is a type of rainforest, but at a higher altitude and therefore cooler and with a frequent covering of cloud. During the day, the cloud could be seen moving through the forest, like mist, and up and down the mountain.
The plants in a cloud forest and a rain forest are similar, with the same high species diversity, the same density of plants and the same complex interaction between plants, animals and fungi.
The majority of the trees were very tall, very thin, with no branching until reaching the top of the canopy, this is typical of the rainforest. The forest was always dark because the canopy was so dense and so leaves were concentrated as high up as possible where they could reach the light (what looks like white sky behind the trees is actually misty cloud between them). Lianas and aerial roots hung down between the trees.
Among the trees were tree ferns, palms, strangler figs and walking trees.
Trees were covered in plants, some were climbers, such as Philodendron, others were epiphytes that grew around the trunks of trees, using moss as an anchor, these were mostly orchids, bromeliads and ferns. Epiphytes grow high in order to use the increased light in the canopy layer, they have a number of methods to gain nutrients and water, normally provided by the soil. For example, bromeliads have stiff leaves that form a cup at the centre, water collects in this cup and insects defecate and drown in it, leading to a release of nutrients.
Mosses were abundant, covering leaves and trunks, they were virulent and colourful. Some more detail on mosses is here.
Mostly the forest floor was covered in leaves, thick plasticky leaves, a little like cherry laurel. The soil in rainforest is thin and low in nutrients, this is because there are so many organisms with cunning ways of exploiting death, snatching plant and animals corpses before they reach the soil. There is also very little light on the forest floor, perhaps as low as 2%, however, there were some plants that managed to grow and thrive.
Stellaria media (chickweed) and Plantago major (greater plantain) are both familiar weeds in England that have been introduced to the area, presumably by accident, and I found them growing wherever the forest had been cut back.
Warm, humid conditions are ideal for many diseases, add to that the large number of insects and parasitic plants and fungi, meant that most plants were damaged extensively. Non native trees, such as citrus, were the most affected, so presumably the native plants have built up some resistance, but the forest was still filled with diseases and decay.