And here is a different leaf of the same species – but this one is red! Why?

To answer this question, you need to know one more fact: the red leaf is actually a young, not an old leaf. In temperate climates, red leaves tend to be autumnal and reflect leaf senescence. In the tropic, red leaves tend to be associated with new growth. So what’s going on?

There are several hypotheses for the adaptive significance of red coloration. One idea is that the red pigmentation (containing anthocyanins) protects leaves from damage from solar radiation before they develop a full complement of photosynthetic pigments like chlorophyll. Because chlorophyll is expensive to produce, it makes more sense to invest first in a less expensive pigment, then later deploy chlorophyll if the leaf survives to maturity. A second idea is that the anthocyanins inhibit fungal growth on the leaf. A third idea is that anthocyanins are unpalatable to insects. A fourth idea is that the red coloration camouflages the leaves from colorblind herbivores. A fifth idea is that the red coloration serves as a warning to non-colorblind herbivores. Some of these ideas are discussed in the scientific literature (e.g. Dominy et al. 2002) but there is not yet a strong consensus – any or all of these hypotheses could be true, depending on the species. Ultimately, red coloration may be a complex trait with multiple evolutionary drivers. It makes for beautiful walks in the forest.

Many other things in the forest are also interesting colors – insects and fungi too. Sometimes this coloration has a warning function, but not always. Here are two examplees for you to consider – what do you think the coloration is doing?

First, plants often secrete a latex when their stems or leaves are cut. This latex is a milky fluid comprised of many different types of chemicals that can be sticky, poisonous, or both. Latex can come in many different colors, and may appear to either ooze slowly out of a stem or spurt out immediately – thus, scientists can use latex as an important character for identifying an unknown plant. Here is the latex from Manilkara bidentata, a common tree that grows here. It is thick and very sticky – even after several hand-washes it will remain attached to your skin.

Some plants also exude resins, to seal themselves after existing damage and to prevent future infection. Pines commonly produce resins. Here is an example of the solid white resin of Dacryodes excelsa, one of the most common species here. This resin can be collected and used for sealing wood or making candles!

Many plant defenses are also dangerous to people. The examples I showed here are safe to touch, but many are not. For example, the latex of Euphorbia cooperi is used as a fish poison and can easily cause permanent blindess in humans. It is dangerous to cut open the stem of an unknown plant in some parts of the world!

In the photo below, Amanda is sitting on a liana with a diameter of nearly nine centimeters. Despite this large size, the stem is very flexible because nearly all its carbon is invested in water-transporting tissues rather than dense wood. Lianas are parasites that use other plants for structural support (some plants may appear to begin growth as trees but will quickly reach a prone position and then twine up the nearest real tree). Here this liana forms a catenary curve, draped between high points in two parts of the canopy.

This is the same liana, looking up into the branches of a nearby tree. Lianas can be very difficult to identify, because they typically have leaves only at the very top of the canopy where sunlight is available. Also, lianas often branch repeatedly, forming long arcs and loops that dive up and down through the forest, sometimes passing underground, only to reappear elsewhere. Tracing one of these plants is like working one’s way through a maze.

Hemiepiphytes are also quite common. Here is a Philodendron that rises sixty feet into the sky and that has nearly killed its host tree. The long structures you see me holding are aerial roots that appear from different points on the plant’s stems. Even if the host tree were to die and fall, this Philodendron would not die, because its growth extends through a large patch of swampy forest and up several nearby trees too. It is a plant which appears sinister, but which also demands respect.

These climbing and twining plants are increasing in abundance in forests worldwide. No one is quite sure why yet, through several ideas exist (Schnitzer et al. 2011). Certainly these large-scale changes will have dramatic effects on forest diversity and performance in the coming decades.

Puerto Rico is an example of an island where several niches appear to be unfilled. One of the most striking examples is the lack of leaf herbivory – most forests are filled with leaves that have been eaten away by a wide range of insects. Here, nearly all the leaves we collect are intact, with no signs of damage at all.

There is also a dense population of land snails – more than thirty species in this forest. Here is one, Caracolus caracolla. It is six feet above the ground, hanging from a sierra palm leaflet –  a testament to the ‘slow and steady’ approach to life. Nothing seems to eat these snails, despite their high population density and thin shells.

And finally, there are many species of small lizards, but nothing (aside from a few birds) that eat them. Here is an anole that caught a ride on a headband after a day of fieldwork.

What’s happening here? Why are there apparently missing links in food webs, and unexploited resources? Islands have two unique properties – first, their distance from mainlands means that all species must have immigrated to the island at some point. Perhaps the missing herbivores and predators couldn’t reach it. But insects can disperse over very long distances, and this island is not very far from other parts of the West Indies and North America. Another possibility is human impacts – perhaps our activities have caused the local extinction of all the relevant species. This idea is plausible, because there is fossil evidence for several mammals (several rodents and shrews) that were on-island before human colonization but that disappeared shortly after the arrival of Europeans (Turvey et al. 2007).

We don’t have the answer, but ponder it each day we walk through this strangely empty island!

First, Urera baccifera – a plant in the nettle family (Urticaceae). Its leaves and stems are covered in stinging hairs that cause a painful sensation when touched. The pain lasts for about fifteen minutes and it is somewhat itchy. Fortunately, this plant is not very common!

Second, an unknown liana, with very odd fruit deployment. I had a look through Vines and Climbing Plants of Puerto Rico and the Virgin Islands and don’t yet have a good identification for this one. The thorns are very large, and all over – not fun when the forest is tangled with this on the ground and in the air.

Third, termites – here, Nasutitermes costalis, probably. The termites make small covered galleries in the bark of nearby trees out of saliva and feces. Unfortunately these galleries are easily crushed by passing scientists, causing large numbers of termites to swarm out and bite. The pain is not bad but it is multiplied many times, and termites have a way of finding their way underneath clothing too…

But still – no poisonous snakes, large mammals, or bee swarms. We are very lucky!

There are many fireflies which light up the night – they seem to prefer the edges of open areas, where they position themselves on leaves. I’ve also seen some on the ground. They rarely fly. The light they emit is a pure green, with a variable intensity. The closer the camera lens comes, the dimmer they get! There is a second species of luminescent insect that also lives here, the cucubano (Pyrophorus luminosus). It is a larger click beetle which glows from both its rear and middle segments. I think I’ve seen them flying around, but they are very skittish around cameras and don’t hold still for very long – the photograph below was a fifteen second exposure!

There are also several Lepidoptera species active at night. The identification of this butterfly (?) is ambiguous, so please comment if you have any ideas!

And finally, spiders. The trees here are mostly linked by a tangle of large webs. There are several kinds, but they all seem to enjoy web-making at face height. Every morning we crash through several on the way to work. I think I’ve accidentally eaten one of these – mouth open at the wrong time while walking!

Culebra is a very dry island, with a very different flora than our rainforest. It is so dry that there are no running streams or fresh bodies of water anywhere. This fact provides strong constraints on the types of organisms that can live here. The plants are short, a tangle of branches with small or thick leaves – adapted to salty, sunny, and hot conditions.

The size and climate of the island also constrains the activities of human settlers. Everything has to be imported from somewhere else. Plants and animals must immigrate, and all human supplies must be shipped in. Towns like the one below could not exist without this ongoing stream of imports. Oftentimes it is possible to learn something about the history of a place based on the things one finds there – for example, a lack of large mammals may indicate their inability to cross large bodies of water, or insufficient resources on an island, while very different species on an island compared to a mainland may indicate that speciation is occurring (think of Darwin and his finches).

Of course, human imports are also visible – here are the remains of a tank, brought here for amphibious landing exercises decades ago. The history of these islands is deeply intertwined with military usage and abuse.

For me, seeing this very small island highlights the amount of resources our lifestyle requires – everything we are accustomed to having, and to using, comes from somewhere. Only occasionally do we get to see that dependency so starkly.

We crossed the ocean by ferry at sunset. Back into the rain and clouds for more work and more thinking tomorrow.

The first step is to cut these leaves out of the canopy with pruning poles – the petioles are more than an inch thick, and the leaves drop quickly with a creaking cracking sound. To give you an idea of how big the leaves are, here is an image of part of one leaf – probably one eighth fraction of the total leaf. We cut them up into pieces in order to drag them out of the forest back to our field station.

Normally we can measure area with a digital flatbed scanner – but clearly these leaves are far too big for this. First, we remove all the leaflets, to make the leaf more manageable.

Next, we photograph the leaf pieces on a neutral background, simulating the action of a flatbed scanner. This poses a challenge at a field station – where to find somewhere with even illumination, a good background, and a large enough space to accommodate our samples? We had to improvise – here is our solution.

A balcony on an apartment for distance, old bedsheets for background, a meter tape for scale, another bedsheet for a lightbox, held up by a palm petiole. We then image all the leaf sections, and create silhouettes on the computer, and scale each image based on the conversion between pixels and meters. Our final answer – leaves of this species have surface areas that can be greater than four square meters. This is huge – more than an equivalent square with six foot sides. The diversity in leaf size across the world is immense – but for our part, as a field crew, we we prefer small leaves!

First, a plant in the Melastomataceae family, with four-petaled flowers.

And second, a species (probably) in the Begoniaceae.

This latter flower is interesting. What you might think are petals are actually sepals, which normally are protective tissues that protect the petals – but in this family, there are no petals at all – evolution has selected for showy sepals that converge on the same function as true petals! Begonias tend to have either four or five sepals per flower. Also, this flower has only stamens but no ovaries – it is a male flower, but female flowers can be found on the same individual plant. This is very different how humans reproduce!

Notice the large holes in these leaves – insects are especially fond of them. One of the tradeoffs that comes with rapid growth, large leaves, and height, is a reduced ability to defend against herbivores. Defense chemicals have a high carbon cost, and little is left over after so much is invested in growth. Below is a very small caterpillar eating its way through the underside of a Cecropia leaf.

Some herbivores are themselves defended against predation – this caterpillar is covered in a formidable array of spines. These spines inject a chemical into whatever is unlucky enough to brush against them, causing a painful reaction and discouraging further investigation. I found this out the hard way. It is a sharp but numbing pain, that persists for several minutes – impressive for a creature not more than five millimeters long…

But not all insects are covered in spines – here is a hoverfly (I think) that greeted us on return from the forest today!