Agriculture Reference
In-Depth Information
studied in regrown North American forests were aggregated in 15 of 23 plots and were
randomly distributed in the remainder. This was considered to result from the aggregated
nature of suitable nesting sites in the forests that she studied and to environmental
heterogeneity. On the basis of these results, Herbers concluded that the ants in her study
area were limited more by a dearth of appropriate nesting sites than by food. Gordon
(1991) also reported non-regular distributions of the nests of competing colonies of
the Nearctic harvester
Pogonomyrmex barbatus.
She attributed this to the regular inter-
digitating shapes of the foraging areas and considered it likely that it is the latter rather
than the nests that are evenly partitioned. In both of these studies, nest dispersion
patterns altered over time.
Even established ant colonies may be considered relatively mobile entities.
Some species apparently move their populations seasonally or at shorter intervals while
others with large, climatically-buffered nests do so only when considerably provoked.
Approximately 10 % of the colonies of the Nearctic seed-harvesting ant
Pogonomyrmex
barbatus
moved annually in the area studied by Gordon (1992) and Gordon and Kulig
(1996) while those of the other
Pogonomyrmex
spp. present moved more frequently.
However, colonies of many mound-building species often remain at a single location
for a number of years. Such influences as mechanical disruption of the nest, flooding,
predation, competition and environmental changes may lead to the movement of whole
populations of affected colonies (Hölldobler and Wilson, 1990). Army ants move their
nest sites continuously during the nomadic phase of their activities.
4.3.3.7
Ant populations and biomasses
Ant colonies differ widely in their abundances depending on the subfamily they belong
to, colony ontogeny and their environment. Where colonies are small, up to thirteen per
square metre may occur (Baroni-Urbani and Pisarski, 1978). At the other extreme, Higashi
and Yamaguchi (1979) reported an extremely large colony of
Formica yessensis
that
contained an estimated 307 million individuals living in approximately 45,000 nests and
covering an area of 2.7 Nonetheless, most colony population densities fall nearer
to the lower end of this range and the median colony density for the more than 145 sites
(mostly from areas of temperate climate) recorded in Baroni-Urbani and Pisarski (1978)
was the equivalent of 1700 colonies per hectare (interquartile range 300-4800 colonies).
The population abundances and biomasses of ants are very poorly known on a unit
area basis. The maximum population density estimates recorded in the summaries
of Baroni-Urbani and Pisarski (1978) and Petersen and Luxton (1982) are
ca.
7300
workers and this corresponds to a standing crop biomass of 2.4 g (dry weight)
(most ants weigh between 0.1 and 1.0 mg dry weight although a few exceptional species
reach 3 mg). Most populations are substantially less abundant than the maximum cited
above. Ants attain their greatest importance in tropical savannas where they may
comprise more than 16 % of the total invertebrate standing crop biomass; in other
biomes, their biomasses attain maximum values of
ca.
2 % of invertebrate biomass
(Petersen and Luxton, 1982).
