Charles Darwin is most known for. But in his writings, another concept also emerges almost just as clearly: that is the potential of animals to change local flora. Indeed, in his book “On the origin of species” he wondered whether neighbourhood cats can affect the abundance of flowers in local fields close to his house in Kent, UK1.
When it comes to ecology, every single species has a role in the wonderful and complex web that is our ecosystem. But there are some species that have a particularly unique role since due to their properties they can regulate the abundance of other species and often the way their habitats look. These species come in all shapes and sizes.
For example, they can be as small as the snowshoe hares that are a key food source for endangered species such as lynxes and mountain lions or birds, as is the hummingbird that is key to pollinating from flower groups that have evolved in such a way that they depend on hummingbirds for their survival1.
The crucial role of keystone species in the ecosystem
The name keystone species, coined by American zoologist Robert T. Paine in 1969, comes from a very different practice: construction. To build the arch of a bridge or other similar construction, builders would use a wedge-shaped stone to support the structure; without this wedge-shaped stone the structure would not hold.
According to Paine, the same applies in ecology: just as other stones in the construction depend on the keystone for support, other species in an ecosystem or habitat depend on the presence of a keystone species to maintain the community’s structure2.
In some ways, the cascading effects of keystone species are different depending on the habitat. Generally speaking, in any habitat, the disappearance of a keystone species would start a domino effect where other species would either also disappear or be affected in some other way, leaving the entire habitat out of balance3.
But it is conceivable that a species in one habitat plays a unique role that has the potential to regulate the populations of other species or even the entire landscape but it does not function in the same way in another habitat. In other words, the keystone attributes of a species can also depend on the delicate balance and network of relationships between species in the different ecosystems.
Life depends on figs
One clear example of how a species can be in specific situations a keystone species is the case of the fig tree in some forest communities in tropical America. Along with a few other plants, figs and fig trees are considered a keystone food resource because they are key to supporting the ecosystem.
Figs, in those climates, can bear fruits throughout the year, so they become the only food sources for a large number of birds and mammals during the times of the year when other food resources are scarce. In other words, without figs, many species would disappear from the local environment2. It is unclear though if the fig tree plays a similar role in other regions.
The example of the fig tree is also very interesting from another point of view: in contrast to the usual examples of keystone species, it is a food resource and does not fall neatly in any of the established categories of keystone species, such as mutualists (which can be species that pollinate plants, such as the hummingbird), engineers (for example, beavers that build damns or elephants that shape their surrounding environment) or predators that regulate the population of other species (such as the jaguar)4.
And it is not just the fig tree that is considered a keystone food resource. Sugar maple trees also perform a similar role: not only do their roots have the ability to transfer water from moist areas of the soil towards dryer ones, benefiting soil conditions for other plants, but their bark is a food source for different insects5.
Acorn banksia and honeyeaters
Another keystone food resource is the acorn banksia, which can be either a shrub or a tree, and can be found in Western Australia. In this region and for a certain period each year, this acorn-like plant is the sole food resource for honeyeaters, a local species of bird.
The honeyeaters are very important themselves as they play an important role in pollinating a number of plant species. If the number of acorn banksia shrubs or trees were to collapse, this could lead to a drop in the population of honeyeaters which in turn would affect the entire ecosystem4.
Indescribable complexity of the food chain
There are a myriad more keystone food resources that our plants provide, whether these are fruits, seeds, flowers, floral nectar, or bark. Just like other types of keystone species, these plants serve a disproportionately large trophic importance but there is much less research invested in better understanding their function. Indeed, despite their significance for different ecosystems no studies have attempted to examine the ecological attributes that might help us recognize them and evaluate their importance in species-rich plant assemblages6.
It could be argued that one of the reasons accounting for this lack of research outputs is that it might be very difficult to identify the links between those keystone food resources and the dependent species – most of the time the dependent species cannot be quantified or remain very loosely defined.
For example, some work done in Cuyahoga Valley National Park in the US, identified that in that specific ecosystem, keystone food resources were various flowers including grasses!
The rationale was the following: these flowers and grasses there the only food source of beavers and insects. Other food resources would dwindle if these flowers and grasses disappeared which would have a knock-on effect on predators such as the bald early and the coyote7. While the impacts of the loss of flowers and grasses would be tremendous, it is difficult to trace it back to a single species as is the case of beavers, otters or elephants.
In a way, and perhaps quite ironically, the most important lesson keystone food resources can teach us that of the importance of all species in an ecosystem because of the intricate was one species is linked to another.