In human society, the terms “free-riders” or “free-loaders” are often used to shame those who take advantage of other people. The same phenomenon can be found in nature too. Some “free-riders” only take more than their share. Some, however, may threaten the lives of their hosts while extracting their benefits!
Harming others to benefit oneself
In ecology, there is a relationship between some species called “parasitism”. The “parasite” will take benefits from the “host”, causing a certain degree of harm at the same time. One kind of parasitism is the absorption of nutrients from the host by a parasite living inside the body or on the surface of the host. The female barnacle larva of the genus Sacculina is one example. These larvae will grow within a crab’s body by taking up the host’s nutrients, thus rendering the crab malnourished and even infertile. The barnacles form an “externa” – a bulbous reproductive organ, on the underside of their host and force the host to release mature barnacle eggs through manipulating its nervous system – all for the reproduction of the parasite.
Another type of parasitism – brood parasitism – is to rely on others to raise their offspring. The best-known example is some birds of the cuckoo family, which lay eggs in the nests of other birds and evade all the hard work and costs of rearing their own young. Poor host parents – they have no idea they are raising babies for others!
Benefiting oneself without harming others
Harm is not a must in some case. There is another kind of relationship called “commensalism”, in which one species provides for or protects another species without any impairment involved. For instance, Eastern Cattle Egret (Bubulcus coromandus) sometimes enjoys meals by standing on the back of the cattle or other large mammals which roam insect-rich grasslands. It causes no harm to its carriers at all!
Many plants such as Urena lobate have hooked seeds that stick to animals passing by, which help their propagation. The animals are unharmed, and often even unaware of they are “lending a helping hand”!
Sometimes, the ones being exploited do not mind even if they are aware of the situation. Examples are worker ants and worker bees. These are social insects that are highly organised with efficient production systems, and all members have their assigned duties. However, scientists have discovered that in these systems, many workers are free of work! Over half of the members may not be involved in the transport, rearing of the young or foraging for food, yet can still enjoy the fruits of work of other members in the community.
What is the reason behind the existence of such “redundant members”? Scientists hypothesise that the “redundant members” are important standbys in the community, which can replenish lost members when faced with environmental challenge. This allows the community to become resilient in operation and reproduction, and is an advantage from the perspective of evolution. Therefore, there is no complaint regarding the free-riding “redundant members” whatsoever!