Creatures of the marine world exist in a balance. Changes in the population of one species affects
the population of another, through the many interactions that take place from predation
and competition to symbiosis. But at the heart of these diverse and vibrant communities are
certain species that hold the system together. These are the keystone species, whose presence
keeps the balance in the ecosystem by determining the numbers of other species in the community.
Without keystone species, ecosystems would look very different, and if they disappeared,
the oceans would be more vulnerable to environmental change and would experience a rapid decline
in biodiversity. The three different kinds of keystone species, the predators, ecosystem
engineers, and mutualists, have very different roles to play in order to maintain their ecosystems.
Let’s take a closer look at these, and examine the keystones of the deep.
Predators in the oceans are often thought of as detrimental due to the threat they pose
to us, but they are in fact the most important group of organisms in terms of ocean health.
Sharks are apex predators, meaning they feed on a variety of prey animals, but have very
few natural predators themselves. Often, they prey upon old or sick fish, leaving healthier
animals to flourish. By controlling the abundance of their own prey in this way, sharks in turn
preserve the quantity of plants and animals further along the food web. If sharks were
to disappear, their prey, like tuna, would see a population boom. But that boom would
then cause rapid decline of the tuna’s prey. With their prey now wiped out, the tuna and
competing predators would also decline, and the same process would repeat throughout the
food web, reducing species diversity. This domino effect is known as a trophic cascade.
Even just the intimidating presence of predators can help to preserve vital habitats. By frightening
sea turtles, sharks inadvertently protect seagrass meadows as they cause turtles to
move around more, feeding at different meadows rather than continuously grazing from the
same one. This minimises the depletion of any one particular habitat. In areas where
sharks are less abundant, sea turtles, manatees and other grazers stay in one place and decimate
the seagrass, impacting countless other species that also rely on the habitat for food or
shelter.
To put it simply, predators ensure species diversity by stopping animals below them in
the food web from upsetting the vital balance. This is why many of them can be considered
keystone species in all parts of the oceans, with deep sea predators like squid and jellyfish
mirroring the role of sharks near the surface.
While it may be true that most organisms can impact their environment in some way, the
term ‘ecosystem engineer’ helps us distinguish organisms with the strongest impact on their
respective habitats. These are the animals that contribute to the diversity and richness
of an ecosystem, for they maintain balance by creating, changing, or even destroying
a habitat. There are two different forms of ecosystem engineers, depending on how they
alter their environment. Those that alter habitats through their own biology are known
as autogenic engineers. The second type, allogenic engineers, alter habitats by physically changing
the abiotic and biotic factors around them.
To better highlight the differences, let’s take a look at an example of an autogenic
engineer. Cold-water corals in the deep sea are a living part of their environment. Just
like trees on land, as corals grow, they provide food and shelter to other organisms. And when
they die, they leave behind their exoskeletons which continue to shape the ecosystem.
The rocky skeletons create new solid surfaces, therefore providing support for new living
corals to grow from. As the process continues over an extended period of time, the growth
and death or corals helps build one of the most biodiverse habitats on Earth. Coral reefs.
For this reason, coral is known also as a foundation species, a classification that
encompasses organisms with a strong role in structuring a community.
In contrast to the ways in which corals use their own biology to engineer habitats, a
number of benthic burrowing creatures instead create change by altering the existing environment
around them. These are allogenic engineers, and out on the abyssal plain their presence
is vital in supporting the diversity of life that dwells here. Rays and skates often forage
in the soft mud for food, kicking up clouds of sediment and thus contributing to an important
ecosystem function known as bioturbation. This makes buried organic matter accessible
to deposit feeders like eels, sea cucumbers and crabs that roam the sea floor. The organic
matter is a vital food source, and without bioturbation making it available to these
animals, the structure of the abyssal plain ecosystem would be very different. Deposit
feeders would not be able to survive. Instead of hosting macrofauna, the sea floor would
support only microfauna and bacteria, and the entire marine food web would be impacted.
The extent of the changes that would arise from the absence of an ecosystem engineer
demonstrate just how interconnected the oceans are. Communities are isolated geographically,
but the processes and interactions that link them together means that a disruption to any
of the ocean’s scattered communities would have a significant impact on the wider ecosystem.
For example, the great whales can be considered as the ocean’s most important ecosystem
engineers. They prey on many species of fish and invertebrates, therefore keeping their
populations in check. Whales are also prey themselves for other predators like killer
whales. But perhaps most importantly, they distribute nutrients via a process known as
the whale-pump. In this process, whales feed at depth and then defecate near the surface,
creating nutrient-rich plumes that support the growth of plankton. Whales therefore indirectly
support the entire marine ecosystem, for plankton form the base of nearly all aquatic food webs.
The large-scale migrations that whales undertake means that they help to engineer nearly all
of the ocean’s communities, either directly or indirectly. Even those that spend most
of their lives near the water’s surface have a huge impact on the deep sea when they
die, for their carcasses sink to the sea floor and bring an abundant supply of nutrients
to hundreds of species that can only survive on this carrion. Some species have become
so specialised to living here, like the bone-eating osedax worm, that they can no longer survive
away from the carcasses. And incredibly, these whale-falls can support life for over 50 years.
Both whale-pumps and whale falls mean that whales can be defined as nutrient vectors
- a term to describe important species that transfer nutrients from one habitat to another,
allowing otherwise barren locations to host life.
Some animals involved in symbiosis can be considered keystone species, particularly
when two species interact for one another’s benefit. In such cases, the two symbionts
are called mutualists. While the importance of mutualism on land is best demonstrated
by the interactions of bees pollinating flowers, the deep ocean exhibits its own examples of
mutually beneficial interactions. In particular, a mutualism between microbes and a wide range
of animals is responsible for the ability of life to survive in the dark where the lack
of sunlight makes photosynthesis impossible.
In its place, chemosynthesis occurs at hydrothermal vents or cold seeps. Microbes use the dissolved
chemicals to produce nutrients and release energy, but they are helped by animals like
tube-worms and mussels. The bacteria often live inside these animals, providing nutrients
in return for protection from grazers. By being protected, they are able to exist in
higher numbers, producing more nutrients and allowing more complex communities to thrive
around these hotspots. Nutrient vectors like wandering predators then transfer this energy
to other locations in the deep, by consuming the residents of cold seeps, and releasing
nutrients via excretion as they venture further out into the abyssal plain.
Not all mutualists are keystone species. The specific importance of animals at cold seeps
that interact with microbes is because they help the process of production. Without production
at vents and seeps, nutrients in the deep sea would be sparse. Animals throughout the
depths would have to depend only on the scraps of marine snow that fall from above. In other
words, symbiotic animals can be considered keystones when their relationship helps to
maintain balance and diversity in their ecosystem.
Overall, deep sea keystone species help to define the many communities of the marine
ecosystem. Without these organisms, marine processes would be radically different, and
diversity would suffer. The importance of understanding keystones is highlighted by
the extent of damage that can be caused by removing just one of these species. Overfishing
and pollution threaten a great number of keystones. Sharks populations have plummeted around 70%
in the last 50 years. When we consider how this can lead to trophic cascades and a loss
of diversity, we can see how damage to one species causes damage to all. No organism
is idle in its community. All have vital roles to play, and any minor offset in a population
could be catastrophic. Now more than ever is it vital we understand these processes,
and realise that no change in the oceans is without consequence.