Homework SourseEcosystem ecologists may identify top down effects or bottom
Solution
Coral reefs contain a high diversity and abundance of piscivorous fishes, which primarily consume smaller fishes. Food web organization is the balance of top-down and bottom-up effects and fish are an important component in this context as they are higher in trophic levels and for their role in top-down control on food web dynamics (i.e. direct control on grazers and indirect control on fleshy macroalgae.
The concepts of “top-down” and “bottom-up” have been used to understand what occurs actually on coral reefs . Indeed the abundance of macroalgae on coral reefs is controlled by complex interactions of the bottom-up control such as nutrient levels and top-down control such as grazing by herbivorous organisms. Understanding the processes of bottom-up and top-down conjointly is critical to the elucidation of mechanisms that regulate the algal-herbivore balance.
A top-down cascade is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer. On the other hand, a bottom-up cascade occurs when a primary producer, or primary consumer is removed, and there is a reduction of population size through the community.
An example of such trophic cascade occurred in the Baltic Sea at the end of 1980s. After a decline in Atlantic cod (Gadus morhua), the abundance of its main prey, the sprat (Sprattus sprattus), increased and the Baltic Sea ecosystem shifted from being dominated by cod into being dominated by sprat. The next level of trophic cascade was a decrease in the abundance of Pseudocalanus acuspes, a copepod which the sprat prey on. - On Caribbean coral reefs, several species of angelfishes and parrotfishes eat species of sponges that lack chemical defenses. Removal of these sponge-eating fish species from reefs by fish-trapping and netting has resulted in a shift in the sponge community toward fast-growing sponge species that lack chemical defenses.These fast-growing sponge species are superior competitors for space, and overgrow and smother reef-building corals to a greater extent on overfished reefs.
Sea urchins can have both positive and negative effects on coral reefs. In some ecosystems, they are key herbivores and play a critical role in maintaining the balance between coral and algae. In some situations, where urchin populations reach outbreak densities, they can lead to unsustainable bio-erosion. Some measures have been explored to increase sea urchin populations, such as the building of artificial reefs which provide niches for urchins to hide from predators, and restrictions on fishi ng of urchin predators (e.g., triggerfish and larger wrasses) which could support the recovery of urchin populations. Some are thinking to increase urchin populations to support reef recovery especially areas where populations of other herbivores (e.g., parrotfishes and rabbitfishes) have been depleted. Hawaii, managers have raised and released collector urchins (Tripneustes gratilla) to graze the invasive algae and promote reef restoration. The area has been overgrown by invasive algae, including Gracilaria salicornia (also known as gorilla ogo) and the gristly yellow-green Eucheuma denticulatum, which smother the reef and destroy habitat for fish. In a healthy system, a native herbivorous fish and urchin community grazes the invasive algae. To restore health to the reef overgrown by algae, hatchery-raised juvenile collector urchins (Tripneustes gratilla) have been released onto the reef to graze the invasive algae and promote recovery of the reef.
