Biological invasion

“ These enemies next to wars and fungi are the most destructive agencies on earth. Could they but be eliminated man’s struggle against opposing forces would be in large measure overcome. The results of recent work in economic zoology, both in regard to the destruction of enemies and protection of useful mammals, birds and fishes, furnish a bright outlook for the future.
Protozoology.—Partly as an experimental field for the solution of general biological problems and partly because of its practical applications the study of protozoa has now developed into a special science. ”

“ Man and animals affect invasion by the destruction of germules. Both in bare areas and in seral stages the action of rodents and birds is often decisive to the extent of altering the whole course of development. Man and animals operate as marked barriers to ecesis wherever they alter conditions unfavorably to invaders or where they turn the scale in competition by cultivating, grazing, camping, parasitism, etc. ”

“ And here I must add, that endless questions of interest will arise to those who will study, not merely the invasion of that truly European flora, but the invasion of reptiles, insects, and birds, especially birds of passage, which must have followed it as soon as the land was sufficiently covered with vegetation to support life. ”

“ But even when the war is open and declared, as between a raptorial species and its victims, the former is manifestly driven by necessity. And in this case the species preyed on are endowed with peculiar sagacity to escape its persecutions; so that the war is not one of extermination, but, as in a border war, the invader is satisfied with carrying off the weak and unwary stragglers. ”

“ But many of the means of animal defence are in themselves almost inscrutable; we see the weapon used, but cannot account for its present existence. Natural selection may explain the improvement and survival of such useful organs, but their origin is still obscure. ”

“ In France, the deficient crops are avowedly owing, to no small extent, to the unchecked ravages of insect plagues; and in England, the gamekeepers are shown how they are ruining their neighbours’ crops and fences without saving their eggs, young birds, and leverets, which fall a prey to rats more than they would to weasels and hawks, and owls, if these latter were allowed to make war on the rats and mice. ”

“ Indeed we might compare the cases of fungi which attack the root and collar, and so strike at and cut off the water supply, to a compact army which at once cuts off the enemy from his narrow base; whereas the innumerable units which bring about an epidemic attack on the leaves, and so surround the enemy and cut off his food supplies all round, is rather like a much larger army which cannot get in [116] beyond the natural barriers of the tissues, and so puts a cordon all round the territory and seizes the multitudes of food-stuffs at the frontiers. ”

“ Before the days of the steam engine, animal plagues advanced slowly and uncertainly, excepting in the presence of a great European war, which drew animals from all available sources, congregating them in large mutually infecting droves in commisariat parks, and marching them in the wake of the army for its daily supply. ”

“ From the latter statement, it follows that invasion occurs only when a species migrates to a new place, in which it germinates, matures, and reproduces. Maintenance by annual invasion simply, in which the plants of each year disappear completely, can not then be regarded as invasion proper. On the other hand, though such instances are rare, it is not necessary that the invaders produce fruit, provided they are able to maintain themselves, or to increase by propagation. ”

“ The invaders, if they gain a foothold at all, soon secure an advantage by reason of their terrific rate of increase. In some instances they carry on the war by producing poisonous substances, in others they rob the animal cells of food and oxygen. If the organism can withstand these assaults, can keep up its nutrition during the long siege, can ultimately destroy its assailants, it wins the battle. Fortunately for us, victory for once means victory forever, at least in many cases. Most contagious diseases attack an individual but once in his lifetime. ”

“ If disturbed during the period of incubation, they will attack large birds and even human beings, directing their long, needle-like bills at the offender's eyes. Our informant told us the particulars of a man who, under such circumstances, came very near losing both of these organs. ”

“ Invasion takes place by the penetration of single individuals or groups of individuals. This will depend in the first place upon the character of the disseminule. It is evident that, no matter how numerous the achenes may be, the invasion of those anemochorous species with comate or winged seeds or one-seeded fruits will be of the first type, while all species in which the disseminule is a several or many-seeded fruit or plant, as in hooked fruits, tumbleweeds, etc., will tend to produce a group of invaders. ”

“ She always seeks to preserve her balances. If a certain plant, or bird, insect, beast, man or nation, rises by intrinsic force and predation to dangerous increase, a destroying parasite is invariably fostered within its shadow. ”

“ When invasion is scattered, factor instruments can not be used to advantage, but where the invading line is well marked, or where extra-formational areas occur, a knowledge of the physical factors is a great aid.
An invasion that has been completed can not be studied in the manner indicated. A method of comparison must be used, in order to determine the original home of the invaders. For this an exact knowledge of the contiguous formations and of the abundance of the species common to all is a prerequisite. ”

“ In like manner, the army worm may be defeated by burning up her eggs with the grass-stalks in which they are deposited. Or we may enlist in our service the natural enemies of the various insect-pests, such as birds, toads, snakes. But their greatest foes are "those of their own household," predaceous or cannibal and parasitic insects. The study of the habits of these insect allies and insect enemies of the husbandman is the occupation of the practical entomologist. ”

“ Again, we now know that the most deadly foes of the higher forms of life are various lower forms of life, such as insects, or microscopic creatures conveyed into the blood by insects. There are districts in South America where many large animals, wild and domestic, cannot live because of the presence either of certain ticks or of certain baleful flies. In Africa there is a terrible genus of poison fly, each species acting as the host of microscopic creatures which are deadly to certain of the higher vertebrates. ”

“ He is tied to a tree which is in the path of this bashikouay army. What a terrible death it must be!
THE ATTACK. Every animal that lives on the line of march where they have spread is pursued, and, though instinct seems to indicate the forthcoming danger, many are caught. In an incredibly short space of time the mouse, the insect, and many small animals are overwhelmed, killed, eaten, and their bare skeletons only remain. If they ever get into a fowl-house, it is all over with the fowls. The insects seem to be the greatest sufferers. ”

“ We want to keep biological weapons out of the hands of people who are intent on destruction. At the same time we can be prepared--if these germs and agents fall in the hands of a potential terrorist--by preparing an effective response plan. ”

“ Not only that, but when the enemy actually gains entrance either by storming a wound or sneaking in through some unguarded opening, then the white cells rally to the attack, surrounding and destroying the invading microbes. ”

“ We have no hunting of wild beasts; they have all been exterminated long ago. This extermination has extended to vermin and insect plagues, and even to some kinds of animalculæ. There is nothing that can bite, sting, or injure us in any way. ”

“ In other words, the great fertility of a race only suffices to make up the wastage from enemy attacks. But the figures serve to show what might happen if the natural control by Weasels, Stoats, Hawks, and Owls were suspended for a short time. But Rats are disseminators of bubonic plague with the aid of their special species of flea. ”

“ Even upon the most careful man, the growth of parasites in the clothing or upon the person was a certainty. Within twenty-four hours the carcass of a horse, left on the field of battle, seemed to move with new and multitudinous life suddenly generated. The stench of the great battle-fields was unspeakable, and the sudden creation of incalculable hosts of insects to do nature's scavenger work was a phenomenon necessary, but to human nerves horrible. ”

“ Rapacious egg-collecting, legislation against birds wrongly thought to be harmful to grains and fruit, and the selfish wholesale killing of birds by professional and amateur hunters, help in the work of destruction. Apart from the brutality of such slaughter, and the extermination of the most beautiful and enjoyable of our animal companions, this destruction [18] works strongly against our material interests. Birds are the natural enemies of insect pests, and the destroying of the birds means the rapid increase and spread, and the enhanced destructive power of the pests. ”

“ Invasion at a level different from that of the facies is inevitably followed by modification. If the invasion takes place below the facies, the invaders will be exterminated gradually, or slowly assimilated. In either case, there is little structural change in the formation, and its stability is affected slightly or not at all. ”

“ Farmers and gardeners fight these enemies all the time. They spray plants and trees with poisons. They plow land in the fall, to turn up buried cocoons to the frost. They plant trap strips to catch the larva, and burn the strips. In gardens they pick off grubs and caterpillars by hand. They cutdown trees and limbs and burn them. All the year around these enemies, too small, often, to be seen, too high to be reached, too hidden in the earth, in the fruits, the bark, the hearts and roots of plants, to be found, are fought. But all that human beings can do, is to keep them in check—sometimes. ”

“ Thirty years ago the fear of tropical diseases would have made these regions seem impossible for white men, even as foremen or overseers. To-day the discovery that insects are the chief poison carriers of disease has reduced our fears. ”

“ That some epidemic prevailed amongst the insects there can be no doubt; and it is curious that it should have attacked so many different species and classes. I am not sure that it was confined to the insects, for there was also a great mortality amongst the fowls, many dying from inflammation of the crop, and two large parrots fell victims to the same disease. This disease amongst the birds may not, however, have been connected in any way with that amongst the insects. ”

“ Again we find that the introduction of domestic creatures or others kept as pets has brought diseases which may prove fatal to the indigenous fauna. Another means by which man causes immense destruction, is by destroying the natural habitat of various species. ”

“ But, observe again, fighting the soil demons and conquering them is only half the battle. There is not a tree, plant, shrub, vegetable, fruit, nor flower, in any latitude or zone, but that must contend with pests, parasites, and insects of all kinds. The herbivorous enemies are not limited to insects and creeping things, but actual diseases. ”

“ IT is universally admitted that economic entomology, like such other branches of applied biology as medicine and sanitary science, is to a very considerable extent the strategics of our warfare with a host of parasites, which are forever endeavoring to destroy our bodies, our domestic animals, our food supply, our clothing and the very materials with which we construct our dwellings and on which we write or print our interpretations of the wonderful world in which we live. ”

“ Now, dropping the entomologist for the moment, let us consider the insects themselves, and here we find their influence extending in every direction; sometimes to our benefit, more often to our injury. Those that affect us injuriously we are able to subdivide into those that attack us directly either as parasites or merely as a source of food supply, and those that prey upon our crops, supplies or farm stock. ”

“ Blow-flies deposit their eggs on fresh or decaying flesh, and this is one of the great sources of trouble to the officers of the Army Service Corps. But they are not content with killed flesh. They will lay their eggs on any living flesh which is exposed, or in sores or tumours, and here their larvae will thrive. Dr. Graham-Smith tells us he once found the exposed muscles of the broken leg of a living rabbit seething with a mass of small blow-fly larvae, which were nourishing themselves upon the living tissues. ”

“ In almost all cases they obtain their prey by the exercise of their own energies, either striking it down upon the wing, or pouncing upon it on the ground: all the vertebrated animals that they can overcome and kill are their victims, though some species are more restricted in their choice than others, and a few even feed upon large insects. ”

“ Youth has the greatest receptivity, the older animals having probably acquired immunity through an earlier attack. The animals that live in herds infect each other by contact, fighting, licking, etc., others are affected by eating the vegetation or drinking the water soiled by the diseased, wild boars by eating the carcasses, and all animals by the attacks of biting or blood-sucking insects which have just come from the diseased. It is claimed that the infection is carried by men and animals, and by the sale in villages of the flesh of infected animals. ”