Earlier this year I presented some of my findings at the 2016 Australasian Society for the Study of Animal Behaviour Conference in Katoomba. These findinges were based on data collected over the last 2 years during which my field sites have been impacted by 2 cyclones. Below is an adaptation of the talk from the conference.
The Importance of Sociality
There are lots of examples in nature of animals that form social groups. These species gain advantages and incur disadvantages from their social behaviour. For example an advantage might be better predator detection while foraging (known as the “many eyes” hypothesis) while a disadvantage could include higher rates of disease transmission. Studies suggest that the evolution and maintenance of sociality is likely to be influenced by environmental factors. Changes in the environment, like those caused by extreme weather events, are therefore likely to impact upon the social organisation of social species.
For social species, the balance between the advantages and disadvantages of sociality are vitally important in determining reproductive output, competitive ability, foraging success and survival, factors which can ultimately impact on a species’ ability to recover from a major impact.
The Study System
My research focuses on the coral gobies at Lizard Island, Queensland. Coral gobies are small fish, approximately three to four centimeters in length and they spend their entire adult lives within the branches Acroporid corals (corals of the genus Acropora). They suffer high mortality outside of their corals, and as such rarely move between corals once they have established themselves. I have observed up to 16 species of coral goby at Lizard Island which range in social organisation from strictly pair-forming species (which I will refer to as ‘Asocial’ species) to highly social species which can be found in groups of 12 or more (the largest group I’ve found was over 20 individuals).
During my studies, two cyclones have impacted my sites at Lizard Island which has been quite disruptive to my research, but has also presented a rare opportunity to gain an insight into the rarely studied effects of cyclones on social organisation. There is no doubt (unless you’re a cyclone skeptic) that cyclones cause severe damage to the physical structure of the reef. This destruction obviously has impacts on the abundance, diversity and distribution of reef species following the event. For example, obligate reef-dwelling species (species which depend on the structure of coral reefs for protection and food) tend to decrease in abundance while algal grazers tend to increase in abundance. However we know relatively little about how these events affect social structures of reef inhabitants which is a potential driver of the diversity and abundance patterns we observe. As I previously mentioned, social organisation is important in determining factors such as reproduction, foraging success and survival, all of which are critical for a species’ recovery from a major disturbance.
Methods and Results
We have been surveying sites around Lizard Island since 2014. During these surveys we search each Acroporid coral on our transects for coral gobies. We identify the gobies to species and count the number of individuals living within each coral head (which constitutes a group). We also identify the coral to species and measure it along three axes to estimate an average diameter. I’ve used average diameter in my research so that my findings are directly comparable to previous work which has used this measurement.
The next few slides show some graphs and conceptual diagrams in which I’ve tried to use consistent symbols which I’ll briefly explain. The yellow fish represent ‘asocial’ species (they’re actually pictures of Gobiodon axillaris, a strictly pair-forming species). The green fish represent the social species (these are pictures of G. erythrospilus which is often founds in groups of 3 or 4). I’ve used a little cyclone symbol with an arrow on the graphs to indicate when a cyclone affected the field sites.
We found that social species decreased in group size following each cyclone while asocial species group size remained the same. This indicates that group size decreases observed in social species were unlikely due to direct mortality from the cyclones (otherwise we would have seen a corresponding drop in average group size in the asocial species as well). A year after the first cyclone, the social species had returned to their pre-cyclone group sizes (keep this point in mind as I’ll return to this in a minute). However, a year after the second cyclone the social species had not returned to pre-cyclone group sizes. This may indicate that multiple impacts have longer lasting effects on social organisation.
Unsurprisingly, we found that coral size had decreased significantly throughout the study. This was the case for for both social and asocial species. The last set of bars on this graph shows the corals that were uninhabited. I’ve included this to illustrate that corals that were uninhabited at the beginning of the study (darkest bar) were of a similar size to the corals that the gobies were inhabiting at the end of the study. This means that at the end of the study, gobies were cramming into small corals that they previously wouldn’t have inhabited.
Let’s return now to that point I made about the social species returning to pre-cyclone group sizes a year after the first cyclone. From the coral size graphs we can see that these larger groups were cramming into smaller corals than before the cyclone.
I might pause here for a second to explain the underlying mechanism of the hypotheses of social evolution which I have looked at in this study, the ‘ecological constraints’ and ‘benefits of philopatry’ hypotheses. For both of these hypotheses we need to consider the proposition that social groups arise because subordinate individuals make the decision to delay their dispersal (often at a considerable cost to their own reproductive opportunities). The question of why some individuals will delay or forgo their own reproductive opportunities in order to remain within a group is one of the fundamental questions of evolutionary ecology. There are of course other ideas about why social groups arise, but this idea of delayed dispersal is what I will focus on for this study. It is also important to note that these hypotheses are not mutually exclusive and often act together. So why separate them out? Well, because each hypothesis contains its own set of testable parameters. These parameters can be used to create a statistical model which we test against the real data and we can determine which hypotheses best describe the observed social structure.
This hypothesis looks at ecological factors which might constrain dispersal from a territory such as a lack of available habitat or high predation pressure. In relation to my work, one of the reasons that the social gobies might have re-formed their large social groups in smaller corals could be that they were constrained by a lack of available habitat. i.e. Gobies displaced by the cyclone might have had no choice, but to move into a coral which already had a small group of fish living in it. In this case, we would expect to see that most of the corals would be inhabited because vacant corals would be quickly taken up by gobies dispersing from crowded corals.
Benefits of Philopatry
This hypothesis looks at the idea that animals gain some benefit of remaining on a site that outweighs the benefits of dispersing. For example, the site might be of a high quality which improves the animal’s fitness to survive and reproduce. Dispersing from this site risks, losing this benefit, unless it can find a site which confers the same or better benefits. In my project, it is likely that there was a lot of variation in coral quality following the cyclone. While fish might have quickly moved into whatever shelter they could find, they might have realised later on that their coral was not very good (indicated by the green, algae covered coral in the diagram), and decided that it was more beneficial to vacate their low quality coral and move into a high quality coral (white coral in the diagram) with an existing group of fish. In this scenario, we would expect to find a lower proportion of inhabited corals than we would under the ecological constraints scenario as fish would have vacated low quality corals in favour of high quality corals.
What we found was that after the cyclone, there was indeed a substantial drop in the proportion of inhabited corals. While this doesn’t definitively prove that benefits of philopatry are causing the observed social patterns, it does lend some support to the idea. There was also a drop in the proportion of inhabited corals for the asocial species, but it was not as substantial as that observed for social species. This likely due to a methodological ‘artefact’ which I won’t get into, but suffice to say, for social species, there is some support for benefits of philopatry playing a role in the observed social pattern following the first cyclone. Stay tuned for a more in-depth analysis of this data.
So, in summary, the major findings of this study were that after a cyclone, social species reduced in group size but asocial species did not. A year later social species had returned to their pre-cyclone group sizes, but in smaller corals. There is some evidence that benefits of philopatry are contributing to this pattern. The fact that asocial species did not alter their social organisation could indicate that the asocial strategy is either more robust to such an impact or that it is less flexible. Unfortunately, my surveys were not designed to examine patterns in abundance and I can’t really say whether either strategy is better or worse for recovery following a cyclone. This would be an interesting avenue for further research. Following a second cyclone, social species again decreased in group size, but did not return to pre-cyclone levels another year down the track. This might be because multiple impacts have longer lasting effects on social structure or because corals had reduced to such a small size that they were not capable of supporting larger groups.
Social organisation in social species is influential in determining survival. The effects of cyclones on social structures has received little attention thus far in the scientific literature. While my research raises many questions, I hope that it can provide a foundation to build upon and move toward a better understanding of how severe weather events might impact upon social organisation.
I would like to thank my supervisors and field assistants who have contributed to this work. Also a shout out to the Hermon Slade Foundation for funding this research and the Lizard Island Research Station for accommodating us.