Disturbance

Although biotic interactions (e.g., competition and her-bivory) are emphasized as having important consequences for coral reef structure, abiotic disturbances such as hurricanes, temperature fluctuations, sedimentation stress, and sea-level change also produce long-lasting effects on reefs. Coral reefs are one of the hallmark ecosystems strongly influenced by disturbance as the frequency and intensity of hurricanes or disturbance events determines how many species of corals coexist on reefs. If disturbance is very frequent or very intense, then only species that can recolonize disturbed areas quickly or that can withstand intense disturbances will persist. If disturbance is infrequent and mild, then the most competitive species eliminate the less competitive species and come to dominate. However, if disturbance is of an intermediate frequency and intensity, then species with different life-history characteristics (i.e., good colonizers vs. good competitors) can coexist because the disturbance-intolerant species are not displaced frequently and the poor competitors are not outcompeted.

Reefs often recover from acute disturbances such as storms but infrequently recover from chronic disturbances. The coupling of acute natural disturbances with chronic anthropogenic disturbances often leads to precipitous declines in coral reef health. One of the best examples of compounded disturbances driving coral reef decline is from the reefs of Jamaica. Chronic overfishing of herbivorous fishes compounded with two hurricanes and the mass mortality of the herbivorous sea urchin Diadema antillarum acted synergistically to force these once coral-dominated reefs into an alternate state of seaweed dominance (Figure 5). In more than two decades, these reefs have shown few signs of recovery. In fact, the episodic effects of natural physical disturbances, coral disease, and coral bleaching along with the constant anthropogenic disturbances of overfishing and pollution have combined to decrease coral cover an average of 80% on reefs throughout the Caribbean over the past few decades. Although disturbance is a natural and integral part of coral reef ecosystems, the compounding of many disturbances over short timescales is often more than reefs can withstand.

Positive Interactions

Ecologists now realize that positive interactions between species can have strong, cascading effects on natural communities and are no less important than negative interactions (i.e., predation or disturbance) in affecting community structure. On reefs, the most obvious positive interaction is the mutualism between corals and their symbiotic algae. Another is the positive feedback between herbivores and corals that maintains a coral-dominated ecosystem. Other crucial positive interactions come from species that are normally thought of as competitors but can mutually benefit each other under the right conditions. Sponges, for example, compete with each other, but can also interact positively. It is more common to find morphologically similar species of sponge growing intermingled in multispecies groups than it is to find a sponge colony growing alone. When growing in these groups, the growth rates of the different species of sponge are often greater than what they would be if these sponges were growing by themselves.

o7 m

o7 m

100 r

1975

1980

1985 Year

1990

Figure 5 Degradation of Jamaican coral reefs over two decades. Changes in (a) coral cover and (b) seaweed cover at four depths in Discovery Bay, Jamaica. This decline in corals and increase in seaweeds was the result of synergistic interactions of natural and anthropogenic disturbances including overfishing, hurricanes, disease, and eutrophication. A similar decrease in coral cover occurred throughout Jamaica with coral cover nationwide declining from about 60% to about 4%. From Hughes TP (1994) Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef. Science 265: 1547-1551.

1975

1980

1985 Year

1990

1995

Figure 5 Degradation of Jamaican coral reefs over two decades. Changes in (a) coral cover and (b) seaweed cover at four depths in Discovery Bay, Jamaica. This decline in corals and increase in seaweeds was the result of synergistic interactions of natural and anthropogenic disturbances including overfishing, hurricanes, disease, and eutrophication. A similar decrease in coral cover occurred throughout Jamaica with coral cover nationwide declining from about 60% to about 4%. From Hughes TP (1994) Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef. Science 265: 1547-1551.

This enhancement of growth rates may stem from differences among the species in their susceptibility to predation, pathogens, and physical disturbance. The summed traits of the sponge consortia may enable participants to survive environmental challenges that would be insurmountable for any of them growing alone. Further, sponges are important to the stability and integrity of the reef itself. Sponges actually act as a type of cement that binds the reef together and holds corals in place. When sponges are removed from reefs, storms displace and kill more corals from these reefs than from reefs that have an abundance of sponges.

Net positive interactions may also occur even between consumers and their prey. Herbivorous damselfishes often

form mutualisms with some seaweeds on tropical reefs. Through aggressive defense of the algal mats on which they feed, damselfish create patches of species-rich algae on reefs where these algae would normally be grazed to near local extinction by large herbivorous fishes. Although the rapidly growing filamentous algae in the damselfish's territory are its prey, they are also dependent on the territorial behavior of the fish for their persistence at high density. If the territorial fish is removed, its algal lawn is consumed within hours. However, the positive interactions between damselfishes and their algal gardens can be overridden by cooperation among other species of herbivorous fishes that forage in large schools. While schooling would appear to increase the competition for resources among herbivores, parrotfishes and surgeon-fishes often form feeding schools to overrun territories of pugnacious damselfishes. For these large herbivores, increasing school size allows for more bites per individual fish when foraging in and around damselfish territories. The benefits of acting mutually to overwhelm damselfish and gain access to resource-rich habitats must outweigh the potential for competitive interactions between the fishes using these schools. Similarly, piscivorous fishes such as grouper and moray eels often hunt in mixed-species or cooperative foraging groups. For these fishes that forage cooperatively, many mouths may be better than one in terms of overall prey yield to each predator when summed over a lifetime of hunts.

Finally, small wrasses (Labridae) and gobies (Gobiidae) act as cleaner fishes on tropical reefs and remove parasites, mucus, and dead or infected tissue from larger fishes. Reef-based cleaner fish are found at specific cleaning stations, usually situated on prominent portions of the reef. These fish can clean up to 2300 individuals of 132 different species in a day, and some client fish visit cleaners over 100 times a day. If these cleaner fish are removed from reefs, diversity of reef fishes declines, especially for large, transient fishes that may visit reefs specifically to be cleaned. Cleaner fishes can, thus, have a strong effect on parasite loads of their client fishes and on fish-usage patterns across patchy reef environments.

Oplan Termites

Oplan Termites

You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.

Get My Free Ebook


Post a comment