The lifespan of a human is perhaps 80-90 years and increasing with every generation. That's a lot of living for an organism. It's easy to forget that for most species life is much shorter and much harder. There is a rough rule of thumb that the larger the organism the longer the lifespan the graphic below illustrates this quite neatly.
There are complex ecological reasons for this involving life history strategies and energy flow but in essence, species generally exhibit one of two life strategies. Complex large animals have longer lives than smaller less complex ones. These strategies reflect two extreme modes of reproduction and survivability and can explain why and how different species behave the way that they do.
The two strategies are r and K. r/K selection was developed in the 1960s as part of MacArthur and Wilson's famous theories on Island Biogeography. The theory dictates that species have either an r or a K reproductive strategy. r-strategists are related to the rate of production of a species and so these species produce large numbers of offspring, have short lifespans, and have a low probability of survival.
K-strategists are dictated by the carrying capacity of an ecosystem, these species have fewer young and live longer. Offspring develop slower and they tend to have higher greater parental care and therefore greater chance of survival.
All this information is pertinent at this time of year as the country erupts in a reproductive fever. Everywhere organisms are mating and having young and it is in this month that one of the organisms that typifies r-selection is abundant.
May is the time in which Mayflies emerge from our rivers in great clouds of dancing pixies. On some rivers, this display of flying mayflies is awe-inspiring but on my patch, it is somewhat subdued. It is still fascinating and this year has been particularly good. There are in fact 51 species of mayfly in the UK and they play an important role in the ecology of a river. They provide food for fish of many species as well as birds given the huge mouthfuls I saw the local Great Spotted Woodpecker taking into its nest.
On my stretch, I managed to identify two species, the Common, or Green Drake Mayfly (Ephemera danica) pictured below and Drake Mackerel Mayfly (Ephemera vulgata). These mayflies have hindwings and three tails. These species may be simple but they are an ancient species, mayflies were around before the dinosaurs, and they are a very successful species.
Most mayflies live but a few hours some as much as a day. I say mayflies, what I actually mean are the adult forms. They emerge from the water in mid-may as dull-winged insects, they then fly into undergrowth and moult again into a much brighter mating form.
The males dance above the water with females darting into the cloud to select a mate where they mate on the wing before the female descends to the water below. She will lay her eggs in the water and then exhausted collapse.
We tend to pity the mayfly's short lifespan but that is only the adult form. The eggs hatch into larvae which will spend up to two years developing on the river bed feeding on algae and vegetation. So feel a little less sorry for the mayfly, yes their appearance to us is fleeting but they in fact have a reasonable life out of sight. The paternal sacrifice of the adults feeds the bellies of fish and birds in a complexly balanced ecosystem.
And to be philosophical for a moment a two-year lifespan may seem like a blink of an eye to us but this species has lived on earth for perhaps 300 million years whilst Homo sapiens has existed a mere 300,000 years, now who lives in a blink of an eye.
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