Insects The Ultimate Survivors

Insects have been around for more than 400 million years. Dinosaurs (non-avian) only lasted 180 million. What determines how long families, genera, and species survive? When biological and physical events impact a species so that the death rate continuously exceeds the birth rate, that life form has begun a downward spiral towards extinction. Ultimately a low population threshold is reached where recovery is impossible and the fate of the species is sealed. That loss has a ripple effect throughout the ecosystem, the severity of which depends on the importance of the species. Demise of a keystone species invokes the greatest aftermath; the cessation of one with global distribution has worldwide consequences, producing a cascade effect throughout the species range. Depending on the organism's importance, the effect can be short or long term, but nothing disappears from the earth without producing some change.

To see how the cascade effect works, let's assume that a species of large tree in a tropical rain forest vanishes in a short period of time, say 100 years, due to a fungal infection. It was a keystone species, a dominant canopy tree that supported a wide range of flora and fauna. The first to follow are those specialists dependent on that tree's roots, foliage, flowers, fruits, seeds, and so on. These would probably be small life forms such as insects and fungi. Some trees host 1,200 or more invertebrates, and one-tenth of them could disappear. That is an immediate effect. But there are more gradual effects. The loss of the species to the ecosystem causes competition and changes in the community composition. Other plant species may die, taking their associated flora and fauna with them. Animals that fed on the invertebrates might move on to other prey, upsetting the balance and forcing more changes to occur. Event A leads to events B, C, D, and these in turn lead to E, F, G, H, I, and so forth. The overall magnitude of the events can never de determined. In the normal course of life, species composition in communities oscillates, a factor that leads to evolution and diversity. Sometimes, the disappearance of a species is not a terminal event but a pseudoextinction, which occurs when a plant or animal undergoes a gradual phyletic transformation into a new one. The organism never really disappears, but slowly becomes a daughter species. This gradual transformation provides its associates time to adapt and has minimal consequences to the ecosystem.

Extinctions are generally classified as background or mass. Background extinctions vary in intensity over time and are sto-chastic—the result of normal, random population changes. The rate of species loss from background extinctions has been estimated to vary from a hundred to thousands per year. Mass extinctions occur when a large percentage of the global population of plant and animal species (usually greater than 50%) die off abruptly. These tend to be more deterministic in nature, a fate that is inescapable though adaptation. All species of plants and animals today are the result of a gradual change of ancestral forms into modern descendants. Species found as fossils in the Cretaceous have all disappeared as the result of background, mass, and pseudoextinctions.

Natural selection infers that some determinants exist in species that makes them susceptible or resistant to background or mass extinction when confronted with a particular combination of biotic and/or abiotic circumstances. Some are listed in appendix B. Examples of biotic (biological) factors are the ability or inability to adapt, ward off infections, disperse, or feed on a wide range of foods. Abiotic (physical) factors are usually environmental, such as a rise or drop in temperature or loss of habitat. An animal or plant may be viewed as vulnerable to extinction when faced with more negative factors than positive ones. The question becomes, how many adversities can a species take before it succumbs?

Insects have been remarkable survivors over geological time, and their success can be attributed to a cumulative evolutionary selection of characters that shield them from extinction. To what do they owe their good fortune? They have been able to prosper since their origin in the Paleozoic because they had the advantage of adapting certain survival strategies, such as small size and vast diversity. Compare this with non-avian dinosaurs that elected for large size and limited diversity. The combination of just these two factors can affect the overall survival of genera and families. Because of their extreme diversity, insects can afford to have many more specialists. And small size makes it easier to find adequate food and shelter.

Another feature we should stress, besides their immense biomass, is their amazing range of reproductive strategies, including the production of large numbers of eggs and short generation times. Insects can complete one (univoltine), two (bivoltine), or multiple (multivoltine) generations every year. In general, smaller insects have more generations than larger ones. The most common form of reproduction is oviparity, where fertilized eggs are deposited in the environment. However, there is also ovoviparity (eggs hatch within the female and active larvae are deposited), viviparity (living young are produced), paedogene-sis (reproduction by larval forms), polyembryony (one egg divides to produce multiple eggs), and finally hermaphroditism and parthenogenesis in the absence of males.

Under ideal conditions some flies can develop and reproduce rapidly, with 25 or more generations per year. It has been estimated that a pair of Drosophila flies could produce 1.19 x 1041 offspring in a single year.292 A housefly pair could generate 1.91 x 1020 progeny in a single summer, and honeybee queens are thought to lay 1,500 to 2,000 eggs in a single day.293 Although the number of eggs varies from a few to thousands, the majority of insects deposit between fifty to several hundred at a time.

These reproductive strategies differ markedly from those sug gested for dinosaurs. The majority, if not all, dinosaurs laid eggs. Unfortunately there is no evidence to suggest how frequently these events were. Some are thought to have been viviparous.294 Inference from birds and reptiles suggests they had only one brood per season, and fossils indicate that the number of eggs varied from only 2 or 3 in many species to 20 in some hadrosaurs and even 26 in the case of one tyrannosaur. The number produced over a lifetime is unknown and probably varied with life span. The period from egg to reproductive adult is also a mystery, although estimates of 8-10 years in hadrosaurs and up to 62 years in sauropods have been suggested.295 The shorter the generation time and the more offspring produced each lifetime, the better able a species is to adapt to unfavorable environmental conditions through mutation and natural selection.

The ratio between egg or hatching weight and adult weight, an indication of how much organisms have to grow to reach reproductive age, can be an important factor in survival. This ratio varies among insects because the eggs of exopterygotes (those with external wing development) are larger that those of en-dopterygotes (insects with internal wing development).

Dinosaur egg weights were a small percentage of adult weights, especially with large dinosaurs. Eggs of hadrosaurs have been estimated to weigh about 2 pounds, and while some adults reached 4,400 to 8,800 pounds, the largest weighed in at 17.5 tons. That means an adult could have been 2,200 to 17,000 times bigger than its hatchling. Putative monumental dinosaur eggs (probably from the sauropod Hypselosaurus) have been described as 12 inches long and 10 inches wide.114 This is similar to those of an extinct ostrich (Aepyornithidae), which had eggs 3 feet in circumference, or nearly a foot in diameter, with an estimated liquid content of two gallons.335 Even with eggs of such immense size, the adult would still be more massive than the hatchling by quite a bit.

Insects frequently lay their eggs in sheltered areas and/or enclose them in a protective material provided by the mother, such as cases or nests of body hairs. Some are laid in cracks and crevices in the ground or inside plant or animal hosts. Many are attached to a food source. Most hatch as precocial young, which are on their own after birth. With rare exceptions, insects do not provide any care to their young.

There is evidence that at least partial parental care existed with some dinosaurs. Fossil eggs have been found in nests that were buried and others in depressions made in soil and vegetation. This and other data suggests that nesting activities may have ranged from actual brooding to protection and feeding of the young in colonies. However, the debate over whether the youngsters were precocial and hatched to fend for themselves or altri-cial and required parental care is still ongoing, and a range of behaviors is likely. If dinosaur hatchlings suffered attrition rates seen in many extant ground-nesting animals (90% or more the first year), and had to attain an adult size some five hundred to several thousand times their birth weight with little or no parental care before reproducing, then the survivors represented quite an achievement.

One cause suggested as contributing to dinosaur extinction is a temperature- sensitive sex determination factor such as occurs in crocodiles and alligators.296 When crocodile eggs are incubated at temperatures below 31.7°C, they produce females; from 31.7°C to 34.5°C the result is males, and above 34.5°C only females appear. There is no evidence that such a mechanism existed in any dinosaur, but if so, it may have become a significant aspect in endangered populations. Most insects, on the other hand, are not known to have temperature-dependent sex-determinate factors.

Insects have many additional survival strategies that have contributed to their overall success. One of the most important is their ability to enter diapause, a prolonged dormancy that is hor-monally regulated. Diapause can either be obligatory or facultative. The first type occurs irrespective of environmental stimuli, while the second is tied to them. In the tropics, these stimuli may be drought, high temperatures, or food shortages, while in more southern or northern latitudes a shorter photoperiod triggers the onset of diapause. No mechanism similar to diapause or even aestivation or hibernation has been suggested for dinosaurs.

Diapause affords insects the ability to survive environmentally stressful times. The condition is associated with a decrease in oxygen consumption and a lowered metabolism. Oxygen is normally carried to insect tissues by diffusion via a system of cuticle-lined tubes and branches called tracheas and tracheoles. Smaller insects may lack trachea and simply depend on diffusion across the cuticle. While this type of breathing bypasses the use of blood hemeproteins, it does limit the insect's size. The capacity to obtain oxygen by diffusion and regulate oxygen consumption contributes to their survival.

Breathing mechanisms for large dinosaurs are still a matter of speculation because no fossilized lung tissue has been found. Debate on whether their lungs were bird-like or crocodile-like continues. Whatever the mechanism for getting air to the tissues, the organs must have been immense to accommodate the oxygen requirements of a 50- or 100-ton sauropod. On the other hand, the needs of a 400-pound dinosaur would be considerable lower. Also, ectothermic animals that take on the temperature of the surroundings and endothermic animals that maintain their own body temperature would have different oxygen requirements. Since the issue regarding thermoregulation in dinosaurs is still being contested, lung size and function cannot be resolved.

A significant feature of insects that makes up for small size, aids in their dispersal, and gives them the ability to exploit many niches is the power of flight. If you have ever tried to catch a fly, you know about their extreme maneuverability and speed, as well as their ability to move sideways and backward in flight. About 90% of insects fly in the adult stage.

What were the aeronautical abilities of dinosaurs? Those who feel birds evolved from theropods could use the few Cretaceous birds as examples of a dinosaur line exploiting the air with insects. One theory for the origin of bird flight suggests it was the result of ground-dwelling pre-avian dinosaurs leaping into the air in pursuit of flying insects. Today about 80% of warm blooded vertebrates fly (900 species of birds and 1,000 of bats).297 However, the great majority of Cretaceous dinosaurs probably did not have the power of flight.

An overall comparison between dinosaurs and insects illustrates differences between what are called r and K strategists. The r-species are small organisms with rapid growth, high reproductive rates, and short lives. The K-species are those with slow development, large body size, low reproductive rates, and long lives. Mortality of the former is usually independent of population density, while in the latter, it is frequently density related. Also, the former characteristically occur in variable or unpredictable environments, while the latter thrive under uniform or predictable conditions. Looking at the animal world today, its obvious that r-selected populations are more diverse and successful than K-selected populations.66 Large dinosaurs represent the ultimate K-strategists while insects are the epitome of r-strategists (appendix B).

The list of adaptive features in insects that contribute to their success is extensive. In fact, there are few things about them that don't spell success. But one of the most amazing accomplishments of some is their ability to feed above their trophic level. In the normal scheme of things, animals only feed on others smaller than themselves unless they hunt in packs. Thus wolves can kill a 500-pound caribou. Insects, however, have jumped the scale and feed fearlessly on animals thousands of time their size—herbivores, carnivores, it doesn't make any difference to them, and this makes them the top predator in any ecosystem! They don't usually kill: there are only a few records of animals dying from exsanguinations by mosquitoes and blackflies. However, they can indirectly cause their host's death from what they carry, disease-causing pathogens.

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