Parrot Grazing Effects on Coral Health
Parrotfish, as well as many other marine species, plays a significant role in the growth and development of coral reefs. There are fourteen various species of reef parrotfish, known in the Caribbean Sea (Rotjan & Lewis, 2005). These fish are considered as the most abundant and the most dominate biomass of herbivorous fishes on Caribbean reefs. They consume macro algae that would otherwise outcompete hermatypic corals for space. Nevertheless, they have a negative effect on coral reefs’ health by regularly consuming live corals. This work discusses and evaluates the positive and negative effects of parrotfish on coral health.
Unfortunately, nowadays the coral cover is in decline on many Caribbean reefs. Thus, any process that can damage or destroy corals is a potential concern. Many factors, such as pollution, temperature change, and coral diseases cause an irreversible damage to the coral colonies. The results of various researches show that frequent and intense disturbance has increased in several past decades. Such disturbance has a negative effect on the main demographic processes of coral communities, such as population size and composition. Consequently, the repetitive decline in the great amount of long-lived structure building corals has resulted in a prevalence of short-lived “weedy” coral species (Roff, Ledlie, Ortiz, & Mumby, 2011). These drastic changes in ecosystem structure and function of corals have aroused interest in processes that have an impact on demographic rates in corals. Different scientists have investigated the impact of parrotfish on the coral colonies in the Caribbean Sea.
According to Mumby (2009), parrotfish grazing benefits coral recruitment in three different ways. First of all, it promotes encrusting coralline algae and reducing macro algae, which contributes to increase of the settlement space for corals. It also helps to prevent the growing thick algal turfs that might trap sediment and produce post-settlemen destruction in coral recruits (Mumby, 2009). In a similar manner, reducing macroalgae may decrease the repetitiveness and endurance of competitive interactions between coral recruits and macro algae (Mumby, 2009). According to the researcher, such interactions may drastically decrease recruit growth rates, and their survivorship. Despite the fact that parrotfish benefits corals by consuming macro algae, the number of recent researches made in the different areas of the Caribbean Sea demonstrate the damage caused by these species.
In their research, McCauley et al. (2014) used the bumphead parrotfish (Bolbometopon muricatum) to evaluate the supposed connections between the functional importance of the species and effects on ecosystem structure and dynamics. The research was performed by scientists in two biogeographically similar central Pacific atolls in the Northern Line Islands Archipelago. The researchers spent 1745 min of focal follow data from five bump head parrotfish individuals (McCauley et al., 2014). The results of their research demonstrate that the bite approximate frequency is 3.3 bites/min (McCauley et al., 2014). Scientists note, that about 59.3% of bites were taken from the dead coral substrate, and 40.1% were taken from living scleratinian corals (McCauley et al., 2014). However, only 0.6% of bites were taken from macroalgae (McCauley et al., 2014). Thus, according to the data provided by McCauley et al. (2014), a single adult parrotfish individual consumes approximately 4480 kg of material from the reef every year, which includes living and dead coral, as well as macroalgae. According to this data, B. muricatum gave their preference to pocilloporid corals with mass-based selectivity index 4.9. furthermore, Mumby (2009) suggests that interaction between Caribbean macroalgae and the coral M. annularis during the period of gametogenesis might contribute to reducing egg size, a number of eggs per gonad, and the quantity of gonads per polyp (Mumby, 2009). Hence, the reduction of a number of macro algae by grazing increases corals’ fecundity.
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The study that was conducted by Rotjan and Lewis (2005) in at Carrie Bow Cay on the Belizean barrier reef examined six major habitats with a great abundance of parrotfish species. The researchers also studied the incidence and extent of coral grazing on the different levels of coral reef, backreef (1–2 m depth), lagoon (2–4 m), upper spur and groove (3–8 m), lower spur and groove (8–20 m), inner reef slope (20–30 m), and the crest of the outer ridge (15–25 m) (Rotjan & Lewis, 2005). They established that the highest density of adult parrotfish was noticed at inner reef slope level. Rotjan and Lewis (2005) indicated that the main targets of parrotfish grazing were such corrals as Agaricia agaricites, Diploria strigosa, Montastraea annularis, M. cavernosa, M. faveolata, M. franksi, Porites porites, P. astreoides, and Siderastrea siderea. As a result, the researchers established that different species dominate different habitats. For example, M. annularis were usually more common in the lagoon and upper spur and groove, while M. franksi dominated in the outer ridge, the quantity of Sc. vetula was low across all levels, Sp. viride and Sp. aurofrenatum showed high abundance across many reef habitats (Rotjan & Lewis, 2005).
The highest parrotfish grazing was noticed on M. annularis corals. Randi Rotjan and Sara Lewis (2005) discovered that three live Montastraea species were also preferentially grazed among all reef habitats, but M. cavernosa had permanently low level of election. The scientists state that parrotfish are responsible for partial and total destruction of P. astreoides colony in the backreef habitat in Belize (Rotjan & Lewis, 2005). They explain such preferences among parrotfish species by such qualities of corals, as low defensive characteristics and high nutritional quality.
A similar study was made by Roff et al. (2011) in Bahamas archipelago. The results of this study show that despite the cover of corals throughout the Bahamas is mainly low, parrotfish demonstrated selectiveness, as much as in other areas on the Caribbean Sea. However, in these areas, parrotfish showed preferential corallivory of Porites porites among all four islands (Roff et al., 2011). The intensity of corallivory was the highest on such corals taxa as Montastraea annularis, Montastraea faveolata, and Porites astreoides (Roff et al., 2011). As well as the results of studies provided by Rotjan and Lewis, the study results made by Roff et al. (2011) demonstrate that colony density and taxa have a positive effect on the number of bites per square meter. The researchers established that morphology can play a significant role in defining the grazing behavior of parrotfish (Roff et al., 2011). Parrotfish were mostly noticed biting parts of massive corals, adult S. viride showed preference of consuming convex surfaces over flat (Roff et al., 2011).
In the further researches, Randi Rotjan, together with James Dimond (2010), have made several experiments that determine the reasons of parrotfish coral selectiveness. As a result of their experiments, they discovered that decreased nutritional quality of corals, which includes increased C:N ratio and reduced overall percentage of carbon and nitrogen, is directly connected to parrotfish grazing (Rotjan & Dimond, 2010). This discovery was unexpected because parrotfish were known to prefer algal, which has higher nitrogen value. Rotjan and Dimond (2010) suggest that permanent corallivory reduces nitrogen percentage in corals, however, parrotfish re-graze these colonies (Rotjan & Dimond, 2010). The researchers suppose that the parrotfish preference of corals with highest ratios of C:N can be explained by the compensation of lower nutritional quality by higher consumption. Moreover, parrotfish may be attracted by lipid-rich coral gonads (Rotjan & Dimond, 2010). Having examined the connection between the coral skeletal hardness of the live Montastraea species and parrotfish grazing, Rotjan and Dimond (2010) came to the conclusion that parrotfish removed a larger amount of live coral on species with softer skeletons.
Parrotfish produce by biting distinctive scars on the coral skeleton and tissue (Rotjan & Lewis, 2005). Nevertheless, it still remains unclear whether parrotfish corallivory has a significant negative impact on physiology of the coral holobiont (Mumby, 2009). The researchers point out that recovery rates of recently bleached Montastraea species were lower in colonies that had grazing scars than in colonies that were not damaged by parrotfish (Mumby, 2009). Thus, corallivory might contribute to stress in Montastraea.
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Parrotfish are important reef inhabitants. However, they have a dual effect on the maintenance of coral health. From one perspective, they help corals to stay healthy through herbivory and bioerosion. However, from the other perspective, they have a negative impact on the corals by consuming live species. Thus, the role of parrotfishes on Caribbean reefs is diverse. Despite all the researches that were made around this subject, it is still not completely clear if parrotfish grazing can substantively contribute to long-term coral decline (Mumby, 2009). Moreover, Rotjan and Lewis (2006) show in their study that corallivorous parrotfish abundance has been increasing on some Caribbean reefs, while live coral cover has progressively declined. Thus, it is not unreasonable to suggest that the ecological role of parrotfish corallivory is both negative and positive. Understanding the mechanism and causes of selective parrotfish grazing is essential in order to predict the connections between corallivores on reefs and percentage of live cover.