8.1. Insect biogeography

Viewers of television nature documentaries, biologically alert visitors to zoos or botanic gardens, and global travelers will be aware that different plants and animals live in different parts of the world. This is more than a matter of differing climate and ecology. Thus, Australia has suitable trees but no woodpeckers, tropical rainforests but no monkeys, and prairie grasslands without native ungulates. American deserts have cacti, but arid regions elsewhere have a range of ecological analogs including succulent euphorbs, but no native cacti. The study of the distributions and the past historical and current ecological explanations for these distributions is the discipline of biogeography. Insects, no less than plants and vertebrates, show patterns of restriction to one geographic area (endemism) and entomologists have been, and remain, amongst the most prominent biogeographers. Our ideas on the biological relationships between the size of an area, the number of species that the area can support, and changes in species (turnover) in ecological time — called island biogeography — have come from the study of island insects (see section 8.7). Researchers note that islands can be not only oceanic but also habitats isolated in metaphorical “oceans” of unsuitable habitat — such as mountain tops in lowlands, or isolated forest remnants in agrolandscapes.

Entomologists have been prominent amongst those who have studied dispersal between areas, across land bridges, and along corridors, with ground beetle specialists being especially prominent. Since the 1950s the paradigm of a static-continent Earth has shifted to one of dynamic movement powered by plate tectonics. Much of the evidence for faunas drifting along with their continents came from entomologists studying the distribution and evolutionary relationships of taxa shared exclusively between the modern disparate remnants of the once-united southern continental land mass (Gondwana). Amongst this cohort, those studying aquatic insects were especially prominent, perhaps because the adult stages are ephemeral and the immature stages so tied to freshwater habitats, that long-distance trans-oceanic dispersal seemed an unlikely explanation for the many observed disjunct distributions. Stoneflies, mayflies, dragonflies, and aquatic flies including midges (Diptera: Chironomidae) show southern hemisphere disjunct associations, even at low taxonomic levels (species groups, genera). Current distributions imply that their direct ancestors must have been around and subjected to Earth history events in the Upper Jurassic and Cretaceous. Such findings imply that a great many groups must have been around for at least 130 million years. Such time-scales appear to be confirmed by increasing amounts of fossil material, and by some estimations of the purported clock-like acquisition of mutations in molecules.

On the finer scale, insect studies have played a major role in understanding the role of geography in processes of species formation and maintenance of local differentiation. Naturally, the genus Drosophila figures prominently with its Hawai’ian radiation having provided valuable data. Studies of parapatric speciation — divergence of spatially separated populations that share a boundary — have involved detailed understanding of orthopteran, especially grasshopper, genetics and micro-distributions. Experimental evidence for sympatric speciation has been derived from research on tephritid fruit flies. The range modeling analyses outlined in section 6.11.1 exemplify some potential applications of ecological biogeographic rationales to relatively recent historical, environmental, and climatic events that influence distributions. Entomologists using these tools to interpret recent fossil material from lake sediments have played a vital role in recognizing how insect distributions have tracked past environmental change, and allowed estimation of past climate fluctuations.

Strong biogeographic patterns in the modern fauna are becoming more difficult to recognize and interpret since humans have been responsible for the expansion of ranges of certain species and the loss of much endemism, such that many of our most familiar insects are cosmopolitan (that is, virtually worldwide) in distribution. There are at least five explanations for this expansion of so many insects of previously restricted distribution.

  1. Human-loving (anthropophilic) insects such as many cockroaches, silverfish, and house flies accompany humans virtually everywhere.
  2. Humans create disturbed habitats wherever they live and some synanthropic (human-associated) insects act rather like weedy plants and are able to take advantage of disturbed conditions better than native species can. Synanthropy is a weaker association with humans than anthropophily.
  3. Insect (and other arthropod) external parasites (ectoparasites) and internal parasites (endoparasites) of humans and domesticated animals are often cosmopolitan.
  4. Humans rely on agriculture and horticulture, with a few food crops cultivated very widely. Plant-feeding (phytophagous) insects associated with plant species that were once localized but now disseminated by humans can follow the introduced plants and may cause damage wherever the host plants grow. Many insects have been distributed in this way.
  5. Insects have expanded their ranges by deliberate anthropogenic (aided by humans) introduction of selected species as biological control agents to control pest plants and animals, including other insects.

Attempts are made to restrict the shipment of agricultural, horticultural, forestry, and veterinary pests through quarantine regulations, but much of the mixing of insect faunas took place before effective measures were implemented. Thus, pest insects tend to be identical throughout climatically similar parts of the world meaning that applied entomologists must take a world- wide perspective in their studies.

Chapter 8