Lqx

52 some instances it must be assumed that these radiations directly paralleled the evolution of angiosperms, although, it must be emphasized, the fossil record of angiosperm flowers is sparse. Further, as noted above, there is a suggestion that much insect radiation preceded the angiosperm diversification by as much as 100 million years (Labandeira and Sepkoski, 1993). Nevertheless, some extremely close interrelationships have evolved between plants and their insect pollinators (see Chapter 23, Section 2.3). For other orders, the radiation was only indirectly related to plants; for example, a large variety of parasitic Hymenoptera appeared, correlated with the large increase in numbers of insect hosts. By the middle Cretaceous, 84% of the insect fauna belonged to families that exist today.

The Cretaceous period was also rather active, geologically speaking, for a good deal of mountain making, lowland flooding (by the sea), and formation and breakage of land bridges took place. All of these processes would assist in the isolation and diversification of the insect fauna.

The decline of reptiles, which became accelerated during the late Cretaceous period, was followed by the increase in numbers of mammals and birds. These groups became very widespread and diverse during the Tertiary period. Paralleling this diversification was the evolution of their insect parasites. Throughout the Tertiary period the climate seems to have alternated between warm and cold. In the Paleocene epoch the climate was cooler than that of the Cretaceous. Thus, cold-adapted groups became widely distributed. A warming trend followed in the Eocene so that cold-adapted organisms became restricted to high altitudes, while the warm-adapted types spread. It appears that by the end of the Eocene period (approximately 36 million years ago) most modern tribes or genera of insects had evolved (Ross, 1965). In the Oligocene the climate became cooler and remained so during the Miocene and Pliocene epochs. However, in these two epochs new mountain ranges were formed, and some already existing ones were pushed even higher. The Tertiary period ended about 1 million years ago and was followed by the Pleistocene epoch of the Quaternary period. Temperatures in the Pleistocene (Ice Age) were generally much lower than in the Tertiary, and four distinct periods of glaciation occurred, at which time most of the North American and European continents had a thick covering of ice. Between these periods warming trends caused the ice to recede northward. Accompanying these ice movements were parallel movements of the fauna and flora. However, the overall significance of the Ice Age in terms of insect evolution remains uncertain.

To conclude this section on the importance of environmental changes, the effects of humans on insect populations and biodiversity must be mentioned. Initially, the hunter-gatherer habits of humans would have had little effect. However, starting about 60,000 years ago, hunting by the growing human population began to drive to extinction many species of large mammals (and presumably their insect parasites). The trend began in Africa, was then evident in Europe, and has since spread (at an increasing rate) to North and South America, Australasia, and many oceanic islands. But by far the greatest human impact on ecosystems, hence on insect diversity and numbers, was the evolution of agriculture. Conversion of native grassland, deforestation, drainage of wetlands, and use of herbicides and insecticides have occurred at an ever-increasing rate, leading to massive declines in ecosystem diversity. While many species are in terminal decline or are already extinct as a result of these practices, others (both plant and animal) have thrived as a result of monoculture, to become major agricultural pests. Many organisms were introduced accidentally into new areas as a result of commerce. In the absence of natural enemies these, too, have become pests (see also Chapter 24). Some species were deliberately released under the misguided idea that they would enhance their new environment, with no thought given to their possible long-term impacts, for example, displacement of native species.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment