Staring at Rocks, an Update:
When I was employed by Dale Fort Field Centre, I was fortunate to be managed by people who understood the value of monitoring the marine environment and I was allowed to devote time to intertidal surveillance (staring at rocks), (see blogs passim). Despite this, more time would have been welcome. Looking over old notes I find many notes to myself saying things like:
“When you have time, plot out the relative abundance of limpets at site 4, Noddy, 1999 to 2004. Is there a link to fucoid % cover, grazing and the meaning of the environmental thingy?”.
Mostly, I didn’t find the time. Now I’m retired I have the leisure to act on some of the suggestions I made to myself. This blog resulted from some of those suggestions.
In the last few years, I’d noticed that a small black lichen that usually lives near the top of exposed shores seemed to be doing rather well on one of my monitoring sites. Last week, I finally got around to plotting out my estimates for its % cover in a single 0.25m2 sample area towards the top of Noddy’s Transect on Castle Beach. Regular readers (both of you) will recall that this is known as Noddy’s Transect after a rock outcrop that resembled Noddy’s hat (now sadly eroded but still remembered).
The species concerned is Lichina confinis (designer stubble lichen), (common name: ©me).
Lichina confinis is what’s known as a fruticose lichen. This means it’s bushy, like a shrub, in this case a tiny shrub about a millimetre or so high. It’s black in colour and resembles speckled, bristly blots of paint on the rock. In short; it looks like designer stubble. It’s common high up on south facing sunny seashores.
Noddy’s Transect:
The graph below shows the % cover of L. confinis over a 25 year period at one site (site 17) on my transect at Castle Beach.
The % cover values do not seem to vary very much, (particularly after the early years), however, there does seem to be some sort of pattern in the minima and maxima.
If we take 1997 as the minimum early years value, cover values increase until 2002 and then drop to a minimum in 2003. Post 2003 cover values increase until 2008 and then drop to a minimum in 2009. Post 2009 cover values rise again and stay like that until beginning to drop and reaching another minimum in 2017.
This means that from 1997 it took 6 years to reach a peak and fall to a minimum in 2003.
From 2003 it took 6 years to reach a peak and fall to a minimum in 2009 and from 2009 it took 8 years to reach a peak and fall to a minimum in 2017.
Post 2017, cover values have increased by 2020 to the highest seen over the period of observation.
Given the roughly 6 year oscillation seen over the past 25 years might we expect a drop off post 2022? If so will it be dramatic or will any drop off be more gradual?
Is this anything to do with the climate? The weather? Light? Sunspots? Competition? Grazing? Let us discuss these factors:
Climate.
If the climate is changing, it’s probably a directional change. Our pattern, if real, seems to be a more or less regular oscillation suggesting that the climate is not a determining factor.
Weather.
Might there be a regular pattern in the weather? If there is, given that 25 years is just a tiny fragment of time, could it just be a coincidence?
Presumably L. confinis as a photosynthetic lichen responds to light energy. Is there a six year cycle of dull to sunny weather? Luckily, Natural Resources Wales (NRW) have mounted a light and temperature sensor very close to Noddy’s Transect (about 1m away from site 8). This means we have data on the light and water temperature regimes taken every twenty minutes since 2007. Site 17 is high up on the shore (about 7.5m) and is immersed only on the highest tides. In 2022 this amounts to about seven times. Therefore, it seems unlikely that water temperature will be a significant factor. Happily, NRW have an air temperature sensor at Martin’s Haven and have kindly let me have access to the data. Martin’s Haven is only about 5 miles away from the transect and faces the same direction, so it should provide a reasonable indication of what air temperatures were like on Noddy’s Transect.
Light:
The graph below show the % cover of L. confinis at site 17 between 1996 and 2021. The graph beneath it shows the mean annual light intensity at Castle Beach between 2007 and 2019.
As might be expected, there does not appear to be any obvious relationship between mean annual light intensity and % cover of L. confinis. Given the relaxed pace of lichen life, any responses are not likely to be immediate enough to be detected over a period of only a year or so.
Sadly, the mean light values for some years are affected by missing readings due to damage to the sensor. However we can say that 2017 was the brightest year and 2018 was the dullest year because there were full compliments of values for both years. The % cover values for 2017 and 2018 are the same. The values for 2020 and 2021 however reached 30%, the highest recorded over the 25 years. Could this be a delayed response to the two brightest and dullest years recorded over the time period? I don’t know.
Air Temperature:
Error bars show 2 standard deviations on either side of the mean.
As can be seen above, the mean annual air temperature varied by about 2OC over the time monitored. All of the error bars (which tell us how confident we can be about the data) overlap massively. Without going into statistical la la land, what this means is that we can’t make any sort of link between our lichen cover values and mean annual temperature. There is no significant rise or fall in mean air temperature between 1996 and 2021.
Rainfall:
L. confinis lives high up on the seashore. It is rarely immersed in seawater. In summer it may be baked by the sun for weeks on end and never wetted (well, maybe in about one summer out of ten). In winter it may be rained on for weeks on end and be continuously damp (always, without fail). It seems unlikely that it would be affected by minor changes in rainfall. The graph below shows that rainfall does indeed vary from year to year but it’s very unlikely to have a significant effect on our lichen cover.
Sunspots.
Sunspots are known to be a cyclic phenomenon, could they have an effect? Solar output varies with sunspot activity and photosynthetic L. confinis has been around for millions of years (not certain how many millions, but land plants began about 470 million years ago and it’s probably longer than that) and like all of us, evolved under the influence of the sun and its spots.
The graph below (now familiar?) shows % cover of L. confinis between 1996 and 2021. The arrows below the graph show when peaks and troughs of sunspot activity occurred over the same time period.
The arrows indicating years where sunspots peaked and troughed were taken from data produced by The Belgian Royal Observatory. Our L. confinis peaks and troughs correspond quite well with them. The obvious exception to this being the L. confinis trough of 2017 to 2019 which is less of a trough than the previous two and longer lasting. Percentage cover shows every sign of rising to a peak again roughly in line with predicted sunspot activity. Sunspots can have dramatic effects on radio and electronic equipment. Organisms are sensitive to electronic phenomena. Is there a causal effect? Am I going mad?
Grazing.
There are very few potential grazers of L. confinis at site 17. The only feasible candidate that might have a grazing effect is a small snail called Melarhaphe neritoides (the small winkle).
To investigate a potential link between M. neritoides grazing and % cover of M. neritoides it seemed sensible to plot the abundance of one against the other. If the snails increase with increasing lichen cover, then that implies that there might be some sort of relationship between the two. Similarly, if one variable goes down as the other increases then that might imply an inverse relationship. The graph below shows numbers of the snail plotted against % cover of the lichen.
There appears to be be a slight, roughly linear trend whereby numbers of M. neritoides increase with % cover of L. confinis. The trend line on the graph (see above) is almost flat and so is not very convincing.
For those that like these things to be demonstrated statistically (maybe you should get out more?); Spearman’s Rank Correlation Coefficient calculated for these data sets comes out at 0.185 (p = 0.366). Which is not statistically significant. (For non statisticians, the nearer the value comes to +1 the better the correlation and we would want a p value of 0.05 or smaller).
In 2016 there were exceptional numbers of M. neritoides (710) I thought that this might be messing up any correlation that could be present in the data.
If we remove the 2016 data we get what seems to be a clearer linear relationship between the two species:
The more of the lichen there was the more of the grazers there were.
Spearman’s Rank Correlation Coefficient is now 0.319 (p = 0.119), which is a more convincing correlation than we got with the data that included the 2016 anomaly. However it’s still not statistically significant. Why there were so many winkles in 2016 is and probably will remain a mystery.
Overall, there’s not sufficient evidence to conclude much about any effects that grazing by M. neritoides might have on L. confinis. I’ve also been unable to find many specific references as to what M. neritoides’s precise diet is. Most sources refer to black lichens and detritus.
The numbers of M. neritoides are often large and vary a lot. The % cover of L.confinis is small and does not vary very much. In order to compare the two year on year, the graph below shows the relative abundance of each species. The values shown are percentages of the maximum that occurred for each species. Thus the graphs for each species are scaled similarly so that (hopefully) more meaningful comparisons might be made.
There was wide disparity between the two at the beginning (in 1996). This disparity narrowed in
1997. The two species then underwent a period when their relative abundances followed each other closely. Apart from a blip in 2003, this continued until 2005, a period of eight years. There followed a dip in snail relative numbers in 2006, followed by the years 2007 to 2010 when relative numbers followed each other quite well again. From 2010, L. confinis entered a stable period until 2015 when a dip occurred that ended in 2018 when relative cover rose to a whole time maximum by 2020, which was maintained in 2021. From 2018 the snail relative numbers followed the lichen relative % cover closely.
What might all this mean? These number and % cover values have been obtained from a very small (0.25m2) sample area, by the same person over the 25 years. This ought to have the effect of making the values reasonably reliable, or at least comparable. However I’ve done this for long enough to know that despite my best efforts, factors like poor weather and personal health (colds, man-flu, cold, wind, rain, snow, fatigue, boredom etc.) might well have an effect. This might explain some of the variation seen. Obviously, a large study area would be preferable, but getting anyone to spend the time and effort required for that would not be an easy task. Getting someone to pay for it would prove even more challenging.
Another factor might also be related to the small size of the area being studied. Melarhaphe neritiodes can, for instance, serve as prey for birds like rock pipits. I have no actual observations of this but it’s easy to imagine a population of tiny snails occupying a small area of shore being devastated rapidly and possibly completely by a ravenous pipit. This might explain some of the variation seen in the snail population.
Mostly, it seems that the snail population has followed the lichen coverage reasonably closely, with occasional departures. This might be because the snails are eating the lichen. If the lichen does well the snails get more food and therefore reproduce more successfully. If the lichen does not do so well, then the snails have less food and less energy to put into reproduction and so reproduce less successfully.
It might also be that the lichen is enhancing snail survival as it expands by providing cover as well as food and thus reducing predation. Any of these factors (or something else altogether) could explain the wild variation in Melarhaphe neritoides relative numbers between 2011 and 2017 (2016 was so deliriously unrestrained that I removed it from the data).
A further factor might be related to the fact that M. neritoides has a pelagic larval stage. Settling out from the plankton is likely to be a perilous and chancy affair, especially high up on the shore where M. neritoides lives. This might help explain why 2016 had a massive 710 snails, far more than any other year. Maybe it was just a lucky year.
Having said all of that, I ought to add that I have been unable to find a direct reference to M. neritoides eating L. confinis. I only pursued this line of thought because there seemed to be very little else for the snails to eat at this site. Most sources say that they eat detritus and black lichens (by implication Hydropunctaria maura or black tar lichen). The only specific reference I found was from Laurand and Riera (2006) who used radioactive isotopes to prove that the M. neritoides they studied ate mostly H. maura and Caloplaca marina (an orange lichen).
The fact that trying to understand the variation in abundance of these two creatures* (see footnote) of no economic significance and completely unknown to most humans proves so challenging is testament to the immense complexity of ecosystems. The huge number of interactions between what many perceive as simple organisms and their environment and other organisms makes this sort of study both attractive and frustrating. The fact is, there’s no such thing as a simple organism. Everything alive now has survived since life itself began (admittedly probably not in its present form). We have all been shaped by billions of years of interactions with the physical environment and with other organisms. All this makes it very difficult to find complete explanations for all the variation you find.
That is not to say that we should give up trying. If we don’t look at the natural world around us, how can we know what is normal/usual/within acceptable parameters?
If we don’t know what’s normal (or even if there is a normal) how can we hope to decide what to do if it all goes horribly wrong?
Imagine for instance, if some powerful godlike being took a whole planet and doubled the concentration of a rare gas in its atmosphere?**(see footnote).
What might happen?***(see footnote).
Without some baseline information, how would we know if anything had changed? Even if we noticed changes, without some quantitative information, would we perceive changes in time to do anything about them if we wanted to?
One day, might we even convince those who make important decisions about cake and can’t see the point of any of this, that it’s well worth pursuing?****(see footnote).
As I mentioned in Blog Number 91, field centres are in a great position to do this sort of work. That is, if they have staff that stay there for a long enough period to get to know their part of the world and develop the necessary expertise. Field centres with high staff turnover are unlikely to achieve this.
Monitoring doesn’t cost much more than time, practice and persistence. I believe that field centre staff should be encouraged to stay in their jobs long-term and be allowed enough work time to learn the names and habits of the creatures that live in their part of the world and to monitor them quantitatively. They will then be in a position to know if things are changing or going astray. They might even be able to say why. Then, they will be of even more use to their students and they will contribute massively towards bringing environmental understanding to all.
Footnotes.
*Or three or four or more creatures if you count the lichen as several symbionts. I’d count it as a single entity that has resulted from endosymbiotic speciation.
**This happened to a planet called Earth over a period of about 200 years (no godlike beings involved of course).
*** Nobody knows, but it’s unlikely to be good.
**** Unlikely, they’re probably off their unmasked faces at a strictly work-based garden party event.
References.
Burt,TP, Howden, NJK and Osborn, TJ (2020)
Analysis of rainfall records from Dale Fort. Field Studies (http://fsj.field-studies-council.org/)
Laurand, S and Riera, P (2006)
Trophic ecology of the supralittoral rocky shore (Roscoff, France): A dual stable isotope (δ13C, δ15N) and experimental approach. Journal of Sea Research Volume 56 (1) pp 27 – 36)
Apologies for the strange formatting in places. I have tried very hard to make it consistent but for some inexplicably bizarre reason, WordPress won’t let me
Look out for Blog Number 94, which may not be quite as long as this one…
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