For those followers of the Compass Blog you may remember my past ramble into dealing with the current lack of permanence and ties and therefore care of place. This piece takes us further into the micro aspect of that, how species become affected, become rare or even extinct. This is often the result of habitat degradation through changed land use such as building development or agricultural intensification, the latter often going hand in hand with use of pesticides. These general issues will often affect a whole suite of species relying on each other in some respect or other. So if one goes, others inevitably follow.
A few years after I had arrived on my patch I found a beetle. Now please you political geographers don’t switch off, I know the following will not influence the world situation and its current horrors. Just take a break though and come into the world of bio-geography.
Finding a beetle here is not unusual and it illustrates one aspect of rarity. Some things are rare simply because they are not found, maybe too small to see or occupying places few people ever go. This was different, it was large and it was in my back yard. Other species Have become rare because their habitat has been abused, changed mostly by humans so that they can no longer survive in great enough numbers to maintain viability. The other main reason for rarity is that the conditions have never been right for the organism and somehow little pockets have survived where there is no chance of the population expanding, extinction is the future.
My beetle, and I will now give it a name, Meloe rugosus or the Rugged Oil Beetle, suffers from a combination of the above, and also seemingly one of its own making. Habitat change has been one of its problems but it also has an insane lifestyle. It is common knowledge among entomologists that most species have interrelationships with others, either by benefiting from their presence in some way or by acting as a parasite on a neighbour.
There are five species of oil beetle in the UK all are listed as rare even by old writers working when ‘times were good’. Evolution as you will know works on the principle of changing form being either beneficial or not. If the former the species thrives and possibly out competes its predecessor. If on the other hand the organism has problems with either form or lifestyle it will fade away or become rare. This is where poor Meloe sits.
First it cannot fly as many beetles do but that should not present a problem as there are large numbers of non -flighted beetles which are common. It is though, a very slow-moving awkward walker. The term oil beetle refers to a defence mechanism it has developed in order to protect its vulnerability. If disturbed or handled it exudes a yellowy oily substance from between the joints in its exoskeleton. This stains, is smelly and also to some degree toxic to its predators causing rashes or blistering. The adults need to maintain this toxicity for their life (overwinter from October to April) so need to be located immediately adjacent to a plant which provides this. In the case of my colony it is Ranunculus repens, creeping buttercup. This provides Ranunculin (protoanemonin) and here I quote it is the “lactone of 4-hydroxy-2, 4-pentadienoic acid”.
I move now to another element in the rarity stakes – finding them. The fact that the beetles are nocturnal doesn’t help in being easily found, the fact that they occur over winter as adults makes it harder. Although when fully grown they can be 17mm long when first emerged are quite small and easily missed in the dark
I think I should now introduce you to a fully grown female.
It can be seen that the abdomen is bloated and the elytra (wing cases) are minimal, there are no wings beneath them. The bloated abdomen contains thousands of eggs. It has been found that each adult can lay between 20,000 and 30,000 eggs during its short life.My study indicates that I may have up to 200 adults.Now, if this is a success story why is the earth not knee deep in oil beetles.
So we move to its next problem.
In nature if an animal produces high numbers of offspring it is done to counter a very high mortality rate, and here is why. Meloe rugosus relies on a host species of solitary mining bee to provide sustinence for its larval stages. Habitat degradation has played havock with many wild bee species and you cannot have escaped the current media buzz concerning pollinators. Solitary mining bees are called so as they dig a nest each to lay their eggs and provision them with pollen. They can though form colonies with a number of nests in close proximity.
The larvae of the oil beetle uses the host, the mining bee, not directly as an adult (the adult beetles are vegetarian) but via its larva, which feeds on the bee larvae and the food store with which it has provisioned its nest. But how does the beetle larva get into the bees nest? Read on!
The oil beetle lays its eggs in a cavity dug in the ground. In spring these eggs hatch into the first stage larva called a triangulin. The name due to the fact that it has an extra claw on its front legs forming a triangle with the normal two tarsi. These climb into nearby flower heads and wait for a host species to visit, grab on to with the extra claw and get carried back to its nest. Problem. They are non specific with what they catch on to, anything furry will do, and even with bees it has to be the right species. It is at this stage then that many thousands of larvae get carried off to die attached to all sorts of insects. The few that make it to the correct host (one of the many species of the genus Andrena) then stand a chance of parasitising the bee’s nest initially by eating the bee larva and then the pollen store, to then change into the second stage true larva.
The host bee tend only to dig nests where the vegetation is fairly short and the well warmed soil can be easily accessed. So, light, well drained soil on an exposed south facing slope.
When all these factors are added together you can easily understand why my oil beetles are listed in the red data book as being very vulnerable to extinction. It is thought that all existing populations are relict, dating back to a period before the loss of the land bridge to the European mainland.
With the loss of continuous habitat there is little chance of such colonies spreading from these small island sites. This is just one of the rarities for which I find myself responsible in my island of ecological continuity.
Ian Rabjohns
DISCLAIMER: THE VIEWS EXPRESSED IN THIS ARTICLE ARE THOSE OF THE AUTHOR ONLY AND DO NOT REPRESENT THE VIEWS OR OPINIONS OF COMPASS MAGAZINE AS A WHOLE OR THE UNIVERSITY OF CAMBRIDGE DEPARTMENT OF GEOGRAPHY.