A comparison of extinction risk in shallow versus deep water reefs

Background Often called the rainforests of the sea, coral reefs are one of the most diverse ecosystems on Earth, but this diversity is in trouble. Recently, coral reefs are thought to be experiencing a worldwide decline from increases in pollution and global temperatures, and certain coral reef ecosystems are more threatened than others (Jackson 2010). Coral reefs are considered to be one of the most resilient ecosystems throughout the history of life on Earth, but currently, more than sixty percent of remaining coral reefs are threatened with extinction (Bongaerts et al. 2010; Bridge et al. 2013). In particular, shallow coral reefs are currently facing a higher risk of extinction than deep coral reefs—this is largely due to more human contact and increased environmental variation in shallow reefs (Bak et al. 2005). However, it is important to consider that because of increased accessibility of shallow reefs, data on shallow coral reefs are typically more abundant than on deep coral reefs. Due to this shortcoming, extinction risk for deep coral reefs are likely underestimated (Menza et al. 2007a). While shallow reef conservation is typically prioritized, conservation of both shallow coral reefs and deep coral reefs is important to preserve the biodiversity of coral reefs. Shallow coral reefs and deep coral reefs share some fundamental ecological qualities, but they are also characterized by distinct traits. Shallow reefs are typically characterized as existing in a 0-30 meters depth range with light penetration (Men-za et al. 2007a). Due to relatively consistent light penetration, shallow reefs have the highest percent cover of photosynthet-ic organisms in comparison to other reefs (Slattery and Lesser 2012). Shallow reefs are the type of ecosystem that we usually think of when we think about coral reefs because they occupy warm, shallow, tropical waters. We have more contact with shallow reefs making them a much more studied and identifiable ecosystem. Deep coral reefs, on the other hand, are much less accessible because of their depth, and the costs associated with their exploration. Deep reefs are characterized as existing below 30 meters in depth and typically have little or no light penetration (Bongaerts et al. 2010). They are commonly found in deep water canyons or on continental slopes or shelves and have been known to reach depths of up to 3,000 meters (Hourigan et al. 2007). The majority of deep water corals have separate sexes, which is a contrast from shallow water corals, …