predator cycling


cryptid spotted in australia: doggy longlegs

anonymous asked:

Hi! I have a biology question for you. Its believed that many animals use bioluminescence to attract prey. I believe that works for insects, deep sea animals and stuff. But why then didn't those animals eventually evolve the instinct to avoid lights instead of going after them?

Yes, bioluminescence has widespread use amongst invertebrates, particularly marine invertebrates, as well as fishes and sharks etc., especially those that live in a deep sea habitat. 

Before I talk about bioluminescence specifically, I’ll just quickly run through some predator prey dynamics. The relationship between a predator and it’s prey can be seen as an arms race, i.e. the two are constantly evolving and adapting to try and have an edge over the other. For example, natural selection will favour a predator that can run faster and catch more prey than other members of the species, and be more likely to survive and produce more fast offspring, and thus the adaptation for faster running will evolve. On the flip side, there will also be a selective pressure for the prey species to become faster, and thus they evolve to outrun predators, and so you end up back to square one. This is also known as the Red Queen Hypothesis, after the Red Queen in Alice and Wonderland:

There is usually a balance met, an equilibrium between the effectiveness of both sides. Obviously animals can’t become infinitely fast - there were be physiological limitations, and various costs and for each adaptation - for example longer legs for running may make the animal more fragile and prone to breaking legs, or may make it lose body heat faster etc. Trade offs, and cost/benefit analyses, almost identical to those in economics, are fundamental to evolutionary biology. What adaptations are the most cost effective will change over a temporal scale, not only because of counter-adaptations in the other species, but also due to environmental and ecological factors which are constantly in flux. 

Usually the equilibrium tends to be in favour of the prey species, i.e. they have the edge, and in this example, more likely to outrun the predator. This due to something known as the life-dinner principle  - a predator will be running to catch it’s dinner, whereas the prey will be running for it’s life, i.e there is a much stronger selection pressure on the prey as for them it is a life or death situation, whereas the predator just loses a meal. This is why predators often pick off weak or injured prey - healthy adults will probably outrun them/be too dangerous/too much effort to catch. 

Additionally, one side of the race can “win” - if a predator becomes so good at catching prey it may wipe out the prey population, and will have to move on to different prey or die. Thus it’s usually in a predator’s interest not to be TOO good. This is also why predator population numbers are usually much smaller than their prey (predator populations rise -> more prey is eaten -> less food for everyone -> predators die of starvation and population falls -> prey population rises due to low numbers of predators -> cycle begins again, the classic example of this is the showshoe hare/lynx cycle ) 

Equally the prey could become so good at avoiding predation that, the predator again must move on to another prey source or die. We don’t really see these situations in nature because by definition, one side is extinct or is in a relationship with another species BUT we can see this happen with invasive species who outcompete native species and drive them to extinction. 

SO what your question seems to be asking is: why are bioluminescent predators that use light as a lure winning the arms race, why are prey species not evolving a counter adaptation, i.e. learning that bioluminescence is a trap? 

Well this luring behaviour is most common in the deep sea, and because of this, we have to take a lot of factors into account. First of all, bioluminescence in the deep sea is widely used and has many functions.  In can be used as camouflage, whereby light emitting organs line the belly of animals such as squid and fish, breaking up their silhouette from below.

It can be used as a defensive mechanism - many small crustaceans, plankton,  squid, and other invertebrates release clouds of bioluminescence ink to coat potential predators, making the predator itself more vulnerable to predation. 

Much like toxic animals on land like ladybirds and tree frogs use bright colours to advertise their toxic nature (aposematism), some animals like jellyfish use bioluminescence to signal their toxicity. Some animals, like dragonfish even use bioluminescent organs beneath their eyes as headlights to find prey.

A huge application for bioluminescence is communication, particularly for mate attraction. The deep sea is an unimaginably vast and empty place, so finding a mate in the darkness is very challenging. For example, ostracods (small shrimp like crustaceans) flash brightly to attract mates.

Since animals like ostracods are low in the food web, they are prey species for many deep sea animals, and thus many animals seek out flashing bioluminescence in order to find them. This is where luring animals, such as anglerfish come in, by mimicking bioluminescent prey species, they attract predators right to their mouth. Maybe now you can begin to see why simply avoiding lights to avoid predation wouldn’t be an option, it all comes back to costs and benefits

If an animal begins to avoid flashing lights, the benefit of not getting eaten may be outweighed by the cost of not finding food or not finding a mate (in evolutionary biology, not breeding is just as costly as dying). So, you can avoid predation, but will then either starve, or not pass on the genes for avoiding predation. Furthermore, the deep sea is so vast and spaced out that the probability of encountering a luring predator is negligible compared to  the probability of encountering your much more numerous prey species or mate. Thus, the proportion of the population lost to lure predation will be close to the rate of death from stochastic (random) events, and there will not be a strong selection pressure to develop an aversion for lures. 

Furthermore, luring predators like anglerfish tend to be opportunistic generalists - they will eat whatever they can get, and thus do not exert a strong predation pressure on any one species. If luring predators become more effective, and their population grows, then something similar to the lynx-hare cycle may occur. So all together, due to the deep sea environment and the ecology and population dynamics of the predator and prey species, arrive at equilibrium we have today - it’s not worth avoiding bioluminescence a) because it may have a direct cost to the prey, and b) the chances of being killed by traps is very low relative to the size of the prey population. 

We can look at terrestrial examples too. The larvae of fungus gnats (arachnocampa luminosa) in New Zealand live in caves and lay sticky bioluminescent threads from the ceiling to attract and trap flying insects. Flying insects such as moths use light (specifically starlight) for navigation, so adaptations for avoiding light are costly, as you may not be able to find food/mates as effectively. Furthermore, the populations of flying insects are so incredibly massive, that deaths from getting trapped in these rare caves is negligible, and thus there is not a strong selection pressure. 

Sorry if that was long, I hope it helps!



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