Tourism and Coral Reefs: What's the Damage?

Tropical coral reefs cover less than 1% of the ocean, yet rival only rainforests as the most diverse ecosystems on Earth, providing vital services such as home and nursery to a quarter of all marine fish species. They are of utmost importance to humans and our economy, with 500 million people dependent on them for livelihoods. Healthy coral reefs are especially important for local tourism, through providing appeal for diving tours and fishing trips; furthermore, businesses based near reef systems such as hotels provide millions of jobs and income globally (Figure 1). Yet the latest global coral reef assessment states an estimated 19% of coral reefs are now dead and lost, and the tourism industry is far from guilt-free.

Figure 1. The breakdown of component values that contribute to the global annual economic value of coral ecosystems. Source.

Over these last three decades (e.g. 1990s, 2000s, and 2010s much of the research on tourism’s impact on coral reefs has centred on recreational activities, particularly snorkelling and scuba diving.

For example, a recent paper (Gil et al., 2015) based in Akumal Bay in Mexico, found that increasing snorkeller numbers generally have a significantly negative impact on coral cover. Coral reefs' structure and functioning centres on the balance between corals and algae, with which corals have to compete for benthic substrate.

A survey of the bay’s benthic community in 2013 compared sites in a snorkeller-intense zone with those at a proximate control zone featuring 13 times lower tourist numbers (Figure 2). The results show lower coral cover across all coral morphologies in the high-tourism zone, (although this effect was only significant for plating coral cover). Furthermore, tourism-intense sites featured significantly increased dead coral cover (a 50.5% increase from control to tourism sites; p < 0.05), and a greater abundance and number of taxa of benthic algae, although this was not significant.

Figure 2. May 2013 Mean (+/- 95% confidence intervals) percentage cover of corals and numbers of algal taxa/fishes, from the control zones (light bars) and the tourism-intense zones (darker bars). Source.

The study focuses on the increasing number of snorkellers as the causative variable for reductions in coral cover in the high-tourism zones. I agree that the presence of snorkellers and SCUBA-based reef tourism can drive coral reef degradation. Indeed, similar results are found in a number of different studies throughout the years. You can imagine why – to describe just a couple of mechanisms: direct, physical contact of snorkellers with corals (trampling, touching) can easily degrade coral skeleton and tissue, and snorkellers can cause localised water column sediment re-suspension, reducing recruitment and survival of corals through choking them. *

However, Gil et al.’s (2015) use of variables such as ‘no. of tourists in water’ and ‘number of snorkellers’ effectively lumps together all activities associated with snorkelling – from the individual snorkellers to the dive boats and anchors and their associated impacts. Because of this, I was sceptical as to whether the reductions in coral cover in the high-tourism zone (Figure 2) were really down to individual snorkellers, or whether there could be other tourism-related activities associated with the increasing numbers of snorkellers driving that difference, which was encapsulated within their metric of ‘number of snorkellers’, but not quite specified.

A bit of digging later, I found an excellent (albeit slightly dated!) study quantifying the degree to which separate tourism activities individually impact on coral reefs. Two of these impacts - dropping anchors and their chains on reefs and physical diver contact – both could have been encapsulated in Gil et al.,’s (2015) variables ‘no. of tourists in water’ and ‘number of snorkellers’ and thus driven the reductions in coral in the high tourism zone (as an increasing number of snorkellers could also entail an increasing number of dive boats dropping anchor). The results of this study took me by surprise; given the volume of research on the impacts of snorkelling and SCUBA diving on coral reefs, I assumed the greatest impact on coral cover would be from physical diver contact. However, anchors and their chains (from dive boats) had a whopping mean coral cover damage of 7.11% in contrast to a very moderate diver contact mean of 0.67%. Dropped anchors can break live portions of a coral colony into fragments, gouging out large chunks of coral skeleton and tissue – which often die and turn into rubble clouds, choking the surrounding corals. In contrast, the model results indicate that SCUBA divers and snorkelers tend to damage coral reefs on a smaller, individual scale. This would suggest that perhaps ‘number of snorkelers’ is a slightly simplistic metric when advising management, as it largely overlooks the difference in impact between separate activities associated with rising snorkeler numbers. A manager reading Gil et al.’s (2015) paper may focus on educating snorkelers on coral damage – when in fact they should be focusing on initiatives such as designated mooring lanes for dive boats to reduce coral damage!

Furthermore, the focus on snorkelling and SCUBA diving in the wider field of tourism-related coral disturbance research fails to appreciate the more indirect impacts of coastal tourism development on reefs. Such development increases land-based runoff events that drive nutrient enrichment (a global problem which can occur for example where hotel sewage is allowed to enter the coastal system) and sedimentation, promoting benthic algal domination over corals. Indeed, the National Oceanic and Atmospheric Administrations (NOAA)’s ranking of major threats to coral reefs ecosystems by region shows that coastal development and runoff poses a high priority threat in 10/14 regions of the world, as opposed to only 1/14 for tourism and recreation (Figure 3).

Figure 3. Ranking of major threats to coral reef ecosystems by region. Source.

A recent experimental study based on coral reefs in the Gulf of Panama showed that coral loss and algal growth induced by overfishing was magnified and accelerated when sediment addition or nutrient enrichment was combined with overfishing (Figure 4). It has been argued that overfishing is the fundamental factor driving community shifts to algal-dominated tropical coral reefs. This indicates that coastal tourism development can play a major role in coral reef degradation through exacerbating the pressures already being placed on coral reefs, such as overfishing.

Figure 4. Change in benthic composition over the experimental period. Changes in percent cover from initial values displayed in separate panels for different community members. Lines, styles and colours indicate the effects of different combinations of stressors: overfishing = dashed lines; sediment addition = yellow lines; nutrient enrichment = blue lines; both nutrient enrichment and sediment addition = green lines. Source.

An overall point of this post is that the localised impacts of tourism on coral reefs often co-occur, and damage cannot be attributed to divers alone. A final take-home point is that these local impacts are always experienced in the presence of global anthropogenic stressors, such as increased sea surface temperatures and ocean acidification, and regional-scale natural disturbances to coral reefs such as the El Nino-Southern Oscillation or hurricanes, which could also see shifts in their regimes in the near future due to climate change. In fact, this year’s El Nino event may even outdo the peak event of 1997-98, meaning that coral die-off in 2016 could be particularly severe as the bleaching event causes them to lose their algal coating. The tourism industry itself is increasingly contributing to these global pressures, for example through releases of greenhouse gases associated with increasing air travel (see my previous post!). This is important because the background effects of climate change are expected to generally reduce ecosystem resilience, increasing the impact of tourism’s local stressors.

This online photo gallery dramatically illustrates some of the horrific effects of human impacts on coral reefs discussed in this blog post.

*For more info on the direct impacts of snorkelers and SCUBA divers on coral reefs – check out this blog!