The anthropogenic allee effect is an ecological phenomenon which very few people understand. But it’s actually born from an economic concept that many more people can grasp: Scarcity.
The idea is that as an item becomes scarcer, people will be willing to pay more money to acquire it.
This concept unfortunately also holds true for the animals in our world’s oceans who are over-exploited for their meat. As species like sharks and tunas become scarcer, people become willing to pay more money to consume the last of these dying species – This is the Anthropogenic Allee Affect.
But What’s An Allee Effect?
Most of us are familiar with the idea of a species’ carrying capacity – the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment.
The idea of carrying capacity is based off something known in ecology as the Population Growth Curve of a species (see image).
The population of a species grows very slowly at first when there are few individuals (because it is hard to find a mate), but then experiences exponential growth as resources and mates become easiest to find. Eventually, the population will level off at the Carrying Capacity when there aren’t enough resources for the population to grow any further.
An Allee Affect is essentially the opposite of a carrying capacity. It’s what makes it difficult for a small population to grow. For example: difficulty in finding a mate.
So an Anthropogenic Allee Affect, as described earlier is a driving force that pushes rare animals to extinction.
Human value for these species makes the difficulty in finding a mate the least of their worries.
Three Species Feeling The Anthropogenic Allee Effect
Great hammerhead shark – found in warm, tropical, coastal waters around the globe. The great hammerhead is hunted for its fins, mainly its very large dorsal fin, however it is often caught in commercial fishing nets as bycatch. When caught, its fins are cut off and it is thrown back into the sea, still alive, where it bleeds to death and drowns. Due to the long gestation period of around 11 months, it can take time for the population to bounce back if at all. As shark fin soup is a delicacy in many Asian countries the demand for shark fins is always there. However as the numbers fall the value rises, therefore the price of shark fin soup also rises, leading to a greater commercial incentive to find and fin these sharks.
Bluefin Tuna – Found in the Atlantic, Pacific and Southern oceans as 3 different (but closely related) species, they can grow to be very large fish, some exceeding 500kg and 3m in length. For this reason, as well as its meat being considered delicious, it is widely hunted. The Atlantic Bluefin Tuna is highly prized in Japan to be used for sushi and sashimi so the fishermen try to catch the largest individuals they can, which can lead to a decline in the average size of the species, its meat can go for as much as $3603 per lb.
All bluefin tuna species are considered threatened, with the southern bluefin tuna classified as critically endangered. It is estimated that in the last 40 years the Atlantic and Southern Bluefin Tuna populations have declined by 80% or more and the pacific bluefin tuna has only 2.6% of its total largest population remaining. The fishing of these fish is simply not sustainable and if this rate continues we won’t see these fish for very much longer!
The hawksbill turtle – found in the tropical regions of the Indian, Pacific and Atlantic Oceans, this turtle is hunted for its meat and shell. The hawksbill is widely regarded as the most beautiful of the seven species of sea turtle for its colourful shell and because of this it is estimated that 90% of the population has been wiped out over the last 100 years.
Due to the excessive hunting of these turtles, the sponges that make up the majority of its diet are taking over the coral reefs, degrading corals and other species that live in these reefs. As the hunting continues the population decreases further causing a sharp decline in offspring being reproduced. Furthermore the age of sexual maturity is estimated to be 20 years so many hawksbills won’t even have the chance to reproduce before they are caught.
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