Why sharks are so important.

SHARKPROJECT has been advocating for shark conservation for over 20 years. The threats to sharks—including overfishing, the fin trade, and climate change—are numerous. But what exactly is the role of sharks in the marine ecosystem, and what might be the consequences of their disappearance in certain regions?

Ecosystems are complex and often difficult or impossible to fully understand, so determining the significance of sharks is not straightforward. A team from the University of Florida has examined this issue in detail and summarized over 100 publications in a review. It is evident that sharks play various roles in the ecosystem, and many aspects remain unexplored. Consequently, it is challenging to predict the exact consequences of shark extinctions in specific ecosystems and how severe these consequences might be.

An ecosystem is officially defined as the array of organisms living in an area and interacting directly or indirectly with each other and their physical environment. Therefore, there is no single "marine ecosystem"; rather, individual systems overlap and interact. In practice, the scope of an ecosystem often depends on our perspective, the questions we ask, and the scale we use.

An ecosystem can be as large as an entire sea or as small as a droplet of water with its microorganisms. It is important to recognize that we frequently generalize, and many aspects are merely approximations of reality. The complexity of interrelationships in the real world is far greater than we can fully comprehend.

Generalization of sharks

A common generalization is the term "sharks." Most people envision a large, gray fish with many teeth. However, "shark" encompasses over 500 species, many of which differ significantly. Some inhabit the deepest parts of the ocean, while others roam reefs.

Some inhabit the deepest parts of the ocean, while others roam reefs. Some grow to enormous sizes, while others are less than half a meter long. The so-called "standard" shark would be just over a meter in length and not necessarily grey.

The diversity among shark species means they occupy various positions and roles in the ecosystem. Therefore, their significance varies. However, a few basic points can be summarized.

Visually, we often think of apex predators as being at the top of the food chain, influencing lower levels from "top down." We remember the classic pyramid from our 10th-grade biology books, with apex predators at the top and producers at the bottom. However, marine food chains do not operate in such a simple linear fashion. They are more like complex networks, known as food webs, with species assuming different roles and engaging in multiple interactions.

Regulating sharks

Larger species, in particular, often occupy the highest trophic levels and act as top predators. These include demersal sharks (Carcharhiniformes; e.g., bull sharks, hammerheads, and tiger sharks), mackerel sharks (Lamniformes; e.g., mako sharks and white sharks), and grey sharks (Hexanchiformes; e.g., sixgill and sevengill sharks).

In this role, they regulate and stabilize the entire system by controlling meso-predators (predators slightly below the top trophic level). This regulation helps maintain healthy herbivore populations (e.g., algae-eating fish). Large shark species particularly impact oceanic balance, but they are often among the most endangered.

Exceptions include filter feeders like whale sharks, basking sharks, and megamouth sharks. Although these giants (whale sharks exceed 10 meters) are not apex predators due to their feeding habits, they play crucial roles.

Many species also change roles throughout their lives; for example, juveniles often occupy a mid-trophic level.

Most shark species are meso-predators, neither at the top nor the bottom of the food web. They are integral to these complex networks, influencing prey-predator relationships and helping control the spread of diseases by targeting weak, old, and sick animals.

Sharks keep ecosystems in balance

Sharks are therefore key marine species that help maintain ecosystem balance and regulate populations. Their disappearance could lead to irreversible changes and, in the worst case, ecosystem collapse.

Sharks and the carbon cyrcle

Recent research, particularly in the last 10-15 years, has explored how sharks (and other large predators) affect carbon storage in the ocean. Sharks act as indirect carbon sinks. When they die, they sink to the sea floor, where their carbon is sequestered.

However, fishing interrupts this process, releasing carbon into the atmosphere as CO2. It is estimated that over the past 50 years, shark fishing has prevented the storage of more than 2 million tonnes of carbon, with about 94 % of this carbon likely released into the atmosphere as CO2 (Mariani et al. 2020).

Additionally, sharks influence carbon reservoirs such as seagrass meadows.

Their absence can lead to the spread of species that consume seagrass, transforming these reservoirs into carbon sources, which negatively impacts the global CO2 balance (Atwood et al. 2015).

These findings illustrate the complex interplay between sharks and marine ecosystems. It is essential not to prioritize one species group over others, as functional groups (e.g., predatory fish) often provide a more accurate representation in scientific studies.

Sharks and humans

The impact of shark declines on humans is illustrated by a case from the early 2000s in the USA. The overfishing of large shark species along the east coast led to the collapse of oyster and mussel fisheries, resulting in job losses and the downfall of an entire industry. This collapse was attributed to an increase in certain ray species, which were formerly preyed upon by the sharks (Myers et al. 2007).

Small changes have a big effect

The disappearance of top predators is often difficult for ecosystems to compensate for due to their limited numbers. While lower trophic levels may have more species with similar functions, a decline in higher-level predators can have significant impacts. The current biodiversity crisis is exacerbating this issue, reducing ecosystem resilience and making even minor changes potentially catastrophic.

As demonstrated, the role of sharks in ecosystems is complex and varied. Their importance cannot be overstated, and their potential disappearance poses severe risks.

We remain committed to fighting for healthy shark populations worldwide because a healthy ocean ultimately benefits us all.