Thursday 2 August 2018

Mosses on your roof? Here's what you should know...

As an ecologist and bryologist, I've spent a long time studying the little green things we call mosses. If you've never taken a minute to appreciate the beauty and intricacy of these tiny plants, I suggest you do so. The first time I looked at a fresh clump of moss under a micrcoscope I was hooked. Unseen to most, mosses form entire forests on a microscopic level, all within a centimeter or two of the soil or organic material upon which they're growing. These micro-forests contain complete food chains, with primary consumers (herbivores), several different tiers of secondary consumers (carnivores), and of course, the decomposers (mostly fungi). It's a fascinating world!



One of the most common questions people ask me, however, when they discover that I study mosses, is "how do I keep them from growing on my roof?". If you even Google this the phrase "mosses on roof shingles", the entire first page is packed with recommendations from various home-renovation and landscaping contractors on how to get rid of them - there's even a Wikipedia entry for it!

There is an assumption underlying this question though, that the mosses are somehow bad for the roof shingles, when in truth we simply do not know whether this is the case in the general sense - simply put, nobody has really studied the question.

To be sure, mosses may regularly be found growing in places where the shingles are somewhat degraded. But correlation does not equal causation. In other words, the mosses could be responding to the conditions that caused the shingle degradation, rather than causing the degradation themselves. And in some cases, mosses grow in places where there is little to no degradation whatsoever. My working hypothesis is that, while there are likely cases where the mosses contribute (slightly) to the degradation of shingles, most of the impact of mosses will be neutral to slightly beneficial, because (1) degradation is caused by other things, and the mosses are simply responding to those same things, and (2) mosses have several traits that might actually slow such degradation. Allow me to explain further.

Mosses are what biologists refer to as "poikilohydric"; this means they absorb all their water passively from the atmosphere through their leaves and stem. They do not have roots like the large herbaceous plants we're more familiar with. Instead, they have tiny little hair-like appendages called "rhizoids", which help them to stick on their chosen surface. These rhizoids are highly unlikely to cause damage to your shingles, because they don't usually penetrate into the ground. And even if they do find a nice opening, they're physically incapable of growing more than a couple of milimeters into it, because these rhizoids are the size of the fuzz on a piece of synthetic fleece or felt fabric. Simply put, mosses are pretty much physically incapable of doing physical damage to your rooftop in the way that a herbaceous plant does.

Before I go further with discussion of mosses, let's take a step back and address the primary causes of shingle degradation: erosion and fungal digestion.

The primary cause of shingle degradation is almost certainly water and wind-driven erosion for most rooftops. Think about it, where do you first see shingles with the corners and edges turning up? In places where water is channeled (e.g., joints between angled roofs), and where the shingles are more exposed to winds from multiple directions (the apex of the roof). Both of these actions will knock those little sandy particles loose, and these particles will then bounce down over other shingles, knocking other particles loose as they go. So let's start by acknowledging that erosion is the primary source of physical shingle degradation.

Next, we must acknowledge the primary chemical means by which shingles are degraded. Most asphalt shingles are made of 3 main things: recycled cellulose fiber (i.e., old newspapers), tar, and coarse sand. Cellulose fiber is undigestible for most animals (humans included), but fungi LOVE the stuff. There are thousands of species of fungi dedicated to digesting wood fiber. Some prefer certain tree species, others prefer particular parts of the tree. Some are more generalist in their choices, and will go for just about anything that contains cellulose (like paper or shingles). These fungi actually grow vast microscopic webs through the cellulose fibres, slowly breaking down the cell walls and incorporating the carbon from the cellulose into their own bodies. Because cellulose fibres are essentially hollow tubes that have been sealed off, breaking down this cellulose means that something formerly more like rigid PVC pipe is now more like empty paper-towel tubes; the more digested it becomes, the more the mat of cellulose fibre begins to work like a sponge, capturing and holding water tightly. If you've ever had to deal with mould in your house, you know that the continual presence of water can exacerbate the problem, because moisture brings on a whole-other set of fungal colonizers.

Recap: shingles are physically broken down by erosion, and chemically broken down by fungi. 


Now, let's bring the mosses back in - I'll make three points that relate to what mosses may be doing in this rooftop ecosystem: two that suggest the mosses are responding to conditions, and one that suggests they are contributing to the conditions.

The first point in favour of mosses responding to rooftop conditions, but not causing them, is that some mosses produce antimicrobial compounds. These compounds can inhibit the growth of certain bacteria, fungi, and parasites. In fact, certain species (e.g., Sphagnum fuscum) are so good at this that they were extensively used as absorbent wadding for bandages during the world wars. Ecologists have also shown that fallen trees decay more slowly when covered by certain moss species, particularly in peat bogs. To my knowledge, nobody has studied whether the moss species that commonly grow on rooftops will inhibit the growth of shingle-decay fungi, but let's keep our minds open to the possibility at this point. If they do, they would actually be reducing the rate of shingle decay, not contributing to it.

The second point in favour of mosses responding to rooftop conditions, but not causing them, is that by growing over your shingles, these mosses are likely shielding the shingles from erosion. Several scientists are actively studying their role as a critical component of something called a biological soil crust (BSC), which reduces soil erosion in sandy ecosystems (like pine barrens and deserts). By covering up loose soil (or shingles), the mosses block the actions of water and wind. Their presence also leads to the creation of a thin layer of organic compounds, which can act as a type of glue to hold particles in place. So even though mosses may not colonize until shingles are a bit eroded, their presence probably inhibits further erosion.

Now a point in favour of mosses contributing to shingle degradation. (Note that I'm saying "contributing to" instead of "causing", because the mosses wouldn't be there if the roof wasn't already regularly experiencing high moisture levels & erosion, combined with a bit of fungal degradation.)  Since they absorb their water passively from the atmosphere, mosses like it to be wet. They tend to colonize wet spots on your roof before dry spots because of this. But they also enhance their own habitat as they grow. The tendency to form dense, spongey mats, allows them to hold onto rainwater a little bit longer than if they were isolated shoots. So where you have mosses, your shingles may be wet for a little longer than other places on your roof. If an antimicrobial-compound-tolerant species of fungus can colonize your shingles, it will have a habitat that is slightly more wet than other parts of your roof, and may therefore grow a little more quickly through those cellulose fibres than it otherwise would.

So between the first two points, mosses may be reducing shingle degradation on both the physical and chemical front. From the last, they might be slightly enhancing the growing environment for decay-fungi, which would contribute to shingle decay.

Now to answer the question that I'm so frequently (not) asked: Should you attempt to get rid of the mosses on your roof by the use of moss-killing strips or sprays?

Probably not.

Given the arguments I've made above, getting rid of the mosses will be unlikely to increase the lifespan of your shingles, and may actually reduce it instead. Moreover, if your roof shingles are really degraded (e.g., leading to a leaky roof), the problem is not the mosses, it's the degradation of the shingles by cellulose-digesting fungi.

So if you're concerned about mosses degrading your rooftop, but the roof itself seems to be okay (not leaking or obviously coming apart), you can probably rest easy and enjoy the nice green carpet you got for free. You may even enjoy some additional benefits from having mosses on your roof, like increased insulative value and moderated stormwater runoff. This is actually an active area of research for me, so stay tuned to hear more about the benefits of having a mossy-green roof.

If your roof is obviously no longer doing its job (e.g., leaking), you don't have a moss problem, you have a shingle-rot problem: killing or removing the mosses will not solve this, and may even make it worse. In this case, it's probably time to re-shingle and re-seal the roof.

Friday 18 May 2018

The impacts of anthropocentrism on our thinking about evolution

The impacts of anthropocentrism on thinking about evolution

In evolutionary biology, one of the big myths we are often tasked with dispelling is that that the traits which seem like obvious indicators of success to us are adaptive in the evolutionary sense. I call this an anthropocentric interpretation of evolutionary theory. For example, people have asked me, if evolution leads to survival of the fittest, why aren't humans getting smarter as a species? While the cynic in me wants to respond in the immortal words of Harvey Danger "...that only stupid people are breeding...", and there is an element of truth to this, the question calls for a much more detailed examination of the misconceptions behind the question.

There are actually several assumptions built-in to this question that are either universally false, or at least wrong most of the time. They are: (1) that the human (anthropocentric) definition of success = the evolutionary definition of fitness; (2) that the evolution of humans (and presumably other large vertebrates) is easily measurable over time-scales that are relevant to modern science; and (3) that past selective forces = current selective forces.

Before I explain further, let's re-examine the central tenets of evolution by natural selection:

- More individuals are produced each generation than can survive
- Within populations, individuals vary in their phenotypic traits, and this variation is heritable
- Individuals with beneficial traits are more likely to survive than those with detrimental traits

The first assumption is wrong because for a trait to be considered evolutionarily adaptive (beneficial), it must demonstrably be selected for, or be selected against less than alternative traits. In the context of human evolution, this means that for intelligence to be an adaptive trait, more intelligent people should not only be more capable of gathering resources (e.g., land, wealth, etc.), but also have more mates and produce more children than less intelligent people. Both parts are pretty soundly refuted by scientific research. Once you factor out education level, inheritance, and other influences, most smart people are no better at accumulating wealth than most dumb people.1 In addition, smart people (more specifically, those with high IQ scores) have been shown to produce fewer offspring than dumb ones2. So not only is being smart not an evolutionary advantage, being "dumb" may confer a slight advantage, if differential reproductive output is the primary measure of success.

However, such research also assumes that (1) people with high and low IQ's are on equal footing in other ways, and (2) that our data on IQ and reproductive rates in humans have been of consistently high quality over enough generations to allow for a reliable test, and (3) that the selective advantages have remained consistent over time. So for the time being, we'll have to let this rest with the statement that being smart is probably not an advantage in the evolutionary sense, although it may be in others.

The second assumption, that human evolution occurs over easily measurable time-scales, is in the category of "probably false". From examining fossil skulls of early homonids, anthropologists know that brain volume has increased at very coarse scales (i.e., over millions of years) from our earliest ancestors up until the last 10,000 years or so. Since then, our ancestors brain volume remained static or even declined slightly, although there is some evidence that this is linked to population growth and the impacts of the development of agriculture on human survival, with a slight increase occurring after the industrial revolution3. However, there is much more to human intelligence than brain size. Newer studies have shown that, at best, brain volume is only weakly correlated to intelligence; a much more effective measure is the number of synapses in a brain4, but this is pretty much impossible to measure without modern MRI techniques, so we're mostly outta luck on testing this question. In addition, recent research shows that for large anatomical changes to become widespread in a population, it can take up to a million years!5 Presumably, smaller changes can become widespread in a smaller amount of time, but to expect them to be evident in as short a time as we have had MRI scans for is clearly ridiculous. We actually don't (and probably can't) know whether the intelligence (in the modern sense meaning that which is measured by IQ or similar tests) has changed since our species was separated from our ancestors.

The third and final assumption, that current selective forces are the same as past ones, is almost certainly wrong. As mentioned above, brain volume changed a bit between hunter-gatherer humans and agricultural humans, but that's not the only thing that changed. Hunter-gatherer humans had far more robust skeletons overall. Although recent research6 has shown that this change is mainly just in the phenotype, rather than the genotype, of humans, it does illustrate a point: the environment in which early humans lived was very different than the one in which we now live. Different selective forces were at play. Consequently, the farthest back we should really be looking is the last 10,000 years, when agriculture was becoming more widely adopted. Post-agricultural humans are more anatomically similar to modern humans than pre-agricultural ones were, and therefore probably had more comparable selective forces at play. And I would argue that, realistically, we should be comparing no further back than the industrial revolution, when humans once again went through an abrupt transition towards increasingly sedentary lives.

So in summary, the answer to the question "why aren't humans evolving to be smarter", is that (1) intelligence does not currently seem to confer greater fitness (i.e., survival AND reproductive success), (2) actually testing this question would require brain scans or IQ tests of humans going back MUCH farther than we currently have, because evolution takes a long time, and (3) due to rapid technological changes, we are not operating under the same selective pressures as our human ancestors were, so even if we did have good information dating back a million years, it's unlikely the trend would continue to modern days uninterrupted.

In case this whole thing just went over your head, or the heads of the people you're attempting to explain it to, don't worry. I have a simpler way to explain it.

Remember when you were a teenager, and there were popular kids and jocks and nerds and Jesus freaks and goths and other easily identifiable subcultures in your school? Which ones do you think had the most sex over their lifetimes?  Probably jocks and preps, right? Which ones do you think used the least birth control?  Probably the Jesus freaks, right?

(More sex) + (less birth control) = more babies
 
and, when it comes to evolution,

more babies = greater fitness

This is assuming, of course, that the babies survive, as they probably would in today's world.

Let me leave the last words to Harvey Danger: https://www.youtube.com/watch?v=ZVOD5D4IArw