Population Biology

and do not harm nontarget organisms.) All the exposed insects are stunned and fall to the ground, where they are collected, identified, and counted. In this way, almost complete samples can be taken. Cockroaches (Fisk 1982), wax bugs (Wolda 1980), grasshoppers (Roberts 1973), and carabid beetles (Erwin 1983) have been analyzed.

Some comparisons of species richness of temperate rheophilic (Stout and Vander-meer 1975) forms to those in Costa Rican and Andean (Patrick 1966) streams have been made. Results generally show that midlatitude stream faunas are significantly more diverse than those in comparable, high-latitude streams.

Soil and litter types are studied by means of the Berlese (Tullgren) funnel (Beebe 1916). Samples may also be taken with a core auger and separation by flotation in liquid suspensions. Substrata may be experimentally manipulated, which allows the investigator to measure minute habitat characteristics with great accuracy and compare them to natural plots (Stanton 1979).

From these various population studies, it is generally conceded that in tropical forests, insect abundance is low but the diversity very high. It seems also that the dispersion of insect populations over a particular area is seldom even but usually very patchy. This may be caused by the irregular distribution of habitats (Wiens 1976), but it can be so even if environmental needs appear continuously and abundantly available. Moisture content of litter and soil may be very unevenly distributed, for example, and a major factor affecting the presence or absence of ants, springtails, mites, and other arthropods (Levings and Windsor 1984). However, in spite of the enormous numbers of leaves available on a single tree or in a forest, only a few are normally utilized by herbivores at one time (notwithstanding population explosions when defoliation may occur).


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