Everyone knows that the bisophere is a complex and interdependent web of interaction; disruption of a single element can have far-reaching consequences that bring ecosystems to their knees. Just how far-reaching those consequences can be though is often quite surprising, and is something that conservationists must constantly bear in mind. The following paper provides an excellent illustration of how two seemingly-unrelated ecological elements – rainforest logging in Gabon and sea turtle nesting activity on African beaches – can affect and be affected by each other.
William F. Laurance, J. Michael Fay, Richard J. Parnell, Guy-Philippe Sounguet, Angela Formia and Michelle E. Lee (2008). Oryx, 42, pp 246-251. doi:10.1017/S0030605308006625.
At first glance, it seems like these two things should be unrelated. Rainforest logging is a major concern in conservation of course, but its impacts on marine life are not normally high on the list of reasons why it is considered problematic. After all, logging obviously happens in forests, rather than in the ocean – so how could it be that these activities would be having an affect on a group of animals that spends almost all of its time in the sea? The pictures below, taken from the article, hint at what has been going on.
Laurance et al, 2008
You can probably already see where this is heading. Sea turtles do have to come out of the ocean to lay their eggs on beaches, and both they and their young can easily be thwarted by obstructions – obstructions such as stray logs which earlier broke free from rafts during river transport, and which wash up on nearby beaches in great numbers. At the time the research for this paper was conducted, the extent of this phenomenon was unknown. In a moment we will look at how the researchers went about assessing that extent, what they found in so doing, and what some of their recommendations are for helping to alleviate the problem. Let’s start, as always, with the abstract:
Industrial logging is expanding rapidly in Central African rainforests. We suggest that logging operations in this region pose an indirect threat to nesting marine turtles, especially the Critically Endangered leatherback turtle Dermochelys coriacea. This occurs because some logs are being lost or abandoned during downriver transport to coastal timber yards; the lost logs float out to sea and then often wash ashore, where they accumulate on beaches used by nesting turtles. We used a light aircraft to survey logs along the entire coastline of Gabon, and also studied the impacts of logs at Pongara Beach, one of the world’s most important turtle nesting areas, during the 2002-2003 and 2003-2004 breeding seasons. Nearly 11,000 lost logs were counted along Gabon’s beaches, with an estimated commercial value of $11.1 million. Logs were unevenly distributed along the coast, reaching a peak density of 247 logs/km. At Pongara, logs blocked 30.5% of the beach. These logs had a number of negative effects on marine turtles, causing 8-14% of all nesting attempts (n = 2,163) to be aborted or disrupted. Initiatives to remove lost logs and driftwood from critical nesting beaches may be the most effective means to reduce their deleterious impacts on threatened marine turtles.
It’s easy to see how a 30.5% blockage of a beach could have a significant impact on the success of turtles attempting to nest there. What is also striking is the fact that the abandoned timber has a significant monetary value! This suggests some obvious solutions, but before we get there let’s talk a little about how the authors got their results.
First, a little more background on the problem. Gabon (like much of central Africa) has been going through a major timber boom over the last couple of decades. Oftentimes, trees which are cut in the forest are transported out to the mills by binding them together into large rafts and floating them out of the jungle on rivers. When they reach the mill, the rafts are broken up and the logs are taken out of the river for processing.
At least, that’s how it’s meant to work in theory. Occasionally, logs get loose at some point in the process and float away – and as the authors mention, some less-than-ethical operators sometimes simply abandon logs which are defective for one reason or another. These rogue logs then travel down toward the mouth of the river, and if they make it to the sea they have a tendency to wash up a short time later on nearby beaches.
Gabon’s beaches are major nest sites for several species of marine turtle, and the presence of large numbers of abandoned logs on their beaches seem like they could easily have a serious negative impact. In the words of the authors:
Marine turtles, which must nest above the high tide line on beaches to be successful, and move laboriously on land, appear especially vulnerable to beached logs. Gabon is one of the world’s most important nesting areas for the leatherback turtle Dermochelys coriacea, as well as for olive ridley Lepidochelys olivacea, green Chelonia mydas, and hawksbill Eretmochelys imbricata turtles.
However, all this theorizing is no good without data. It would be great to discover, for instance, that turtles are capable of navigating around logs and making their nests despite obstruction from the abandoned timber. The first step in determining whether abandoned timber is having an impact on turtle nesting was to quantify the amount of timber present on the beaches in the first place. Here’s how they did it:
Aerial surveys of logs on Gabonese beaches were conducted during daylight hours on 11-12 January 2003, using a Cessna 182 light aircraft. … The entire coastline of Gabon was surveyed but our focus was on areas of beach (including coastal dunes) that provide potential nesting habitat for marine turtles. … We recorded logs on beaches using a video camera linked via a time stamp on the images to data from a Global Positioning System (GPS) with external antenna. … Data on log densities for each beach segment were imported into the geographical information system ArcView v. 3.2 (ESRI, Redlands, USA), which was used to produce maps of the distribution of beached logs along the coastline.
I left out some detail there, but I’m sure you get the idea. They also estimated the value of the timber that they saw by simply taking the value per cubic meter of okoumé roundlogs (which they state make up a significant majority of the timber exported by Gabon) and then estimating the total volume of timber seen in their surveys.
The researchers come up with a figure of 10,969 logs total with an average density of 29.6 logs/km, clumped in two major zones which together comprise about half the total coastline and include the two most important turtle nesting beaches, Pongara and Mayumba. At Pongara specifically, 30.5% of the beach was blocked by logs. They further estimate that the total value of the lost logs they surveyed was approximately $11.1 million – a significant amount of money!
Laurance et al, 2008
But was this blockage affecting sea turtles attempting to nest on these beaches? To find out, the researchers collected data on several kinds of nesting failures:
Along the same section of Pongara Beach we estimated the impacts of logs on female marine turtles during the 2002-2003 and 2003-2004 nesting seasons (November-March), as part of a larger study of this population. Data on the incidence of turtles entangled in logs (found dead and wedged into log piles) and of aborted nesting attempts (in which the female returned to the sea without nesting) were recorded during both nesting seasons. In the second nesting season, we also attempted to determine (a) whether logs were responsible for any aborted nesting attempts (whereby the female failed to circumvent the blockage and then returned to the sea without nesting), (b) the incidence of nests constructed too near the waterline to be viable, because of log blockage, and (c) the number of disoriented turtles that failed to return to the sea and were found in inland savannah during the daytime (i.e. apparently because logs blocked their view of the sea, which is brighter than the land at night).
This kind of observational data is necessarily a little subjective, which the authors acknowledge. It would’ve been ideal if the researchers had had an unaffected-but-otherwise-identical beach to compare to, but such is the life of the field biologist. Even if they had had the opportunity to compare nesting activity on Pongara itself both before and after logging began in Gabon, there would be numerous potential confounding factors which would render such an experiment difficult to interpret. Oftentimes all that can be done is to set careful sampling rules which strive to maximize objectivity while simultaneously acknowledging whatever ambiguity cannot be avoided.
In this case, one way that the authors tried to remove (or at least compartmentalize) the ambiguity of their data was by allowing for two classes of disruption: cases where interference from logs clearly had an effect, and cases in which an effect was merely deemed likely.
They ended up with quite a bit of information. The paper itself breaks it down into various types of disruption such as aborted attempts, improper nest site placement, and disorientation, but here’s the bottom line:
The results of our study suggest that, along one of the world’s most important marine turtle nesting beaches,8-14% of all nesting attempts are being disrupted by lost or abandoned logs that largely originate from inland timber operations. The most frequent impacts of log obstructions on beaches involve females either digging their nests below the high tide line, where their eggs would almost certainly be killed by seawater inundation, or aborting their nesting attempts altogether. In addition, c. 1.2% of all nesting individuals were trapped or disoriented by logs. This figure may appear insubstantial but for long-lived animals with delayed maturation such as marine turtles, even small increases in adult mortality could potentially have a significant impact on population viability.
Laurance et al, 2008
An 8% decline in reproductive success can indeed make a huge difference to a species that is already threatened, as most sea turtle species are, especially species with the kind of life history that turtles display. The study rightly points out that their figure represents a conservative lower bound for the effects of abandoned logs on marine turtles in Gabon. There are several potential disruptions which they do not consider. For instance, newly-hatched turtles may have to spend precious time and energy navigating around logs, tiring them out prematurely and exposing them to predators. Logs may also pose a crushing hazard to turtles while they are still being tossed around in the surf, a phenomenon which the authors point out has been witnessed in crocodiles. These are dangers which were not accounted for in this study, but which if included could easily increase the figures on degree of disruption.
The fact that the abandoned timber has significant monetary value would seem to provide a natural solution to the problem, but there is a hitch: in Gabon, abandoned timber is legally considered government property – but the Gabonese government does not seem interested (as of the time the paper was written) in going to the effort to recover it. The authors suggest that this situation be changed:
Revoking the current restriction on harvesting beached timber could potentially facilitate log removal but such operations may degrade beaches if tractors or other heavy equipment are not used carefully. One option would be to allow local communities or contractors to remove timber from critical nesting beaches but only under the supervision of NGOs working locally to promote turtle conservation. Such organizations could ensure that timber removal operations were conducted outside the turtle nesting season, and that all efforts were made to minimize physical damage to beaches.
Of course, they then go on to say that logging efforts in Gabon are likely to further intensify in the near future, due to international development pressure at the time that the article was written – a future which would clearly only increase the degree to which marine turtles were disrupted. The last ten years have borne out this prediction (the rate of logging in central Africa continues to increase) but I have so far heard nothing about efforts to reduce logging waste or to curtail the kind of indirect effects that this paper describes. If you or anyone you know is aware of such efforts, or has any detail or update to offer on the matter, I for one would love to hear about it. It would also be interesting to hear some ideas for creative solutions to this problem, as I feel like that is one area where the authors could have spent a little more time than they did. I hope that this paper has reinforced for you the reality of the interconnectedness of our environment, and the importance of remembering that interconnectedness when considering the impacts of our actions.