438 



THE COMMUNITY 



serve as a link between their insect inhabit- 

 ants and the forest. Within certain hmits 

 the association is fairly close, certain kinds 

 of insects, inhabiting certain kinds of fungi, 

 breeding on or near the latter, passing their 

 life cycles within the fungus tissues, and 

 feeding thereon. In the cyclic development 

 of this habitat a point is reached at which 

 the fungus is no longer a suitable environ- 

 ment for its animal inhabitants. Its density 

 of population is rising wliile the potential 

 food supply is falHng, and this is accom- 

 panied by relative dehydration through 

 perforation and loss of woody tissues, with 

 resulting lowering of the relative humidity 

 of the interior. The occupants leave the 

 fungus to feed and oviposit on another 

 similar fungus substratum. The fungus is 

 the home or habitat niche of these myceto- 

 coles, providing them with food and shel- 

 ter. At the same time, the fungi grow upon 

 dead or dying trees, and are unable to exist 

 without such a food supply; the tree, there- 

 fore, is the habitat niche of the fungi. 



At either end of this example, other ad- 

 justments are made. The mycetophagous 

 insects are themselves fed upon by carniv- 

 orous animals that are facultative or oblig- 

 atory inhabitants of the surrounding forest. 

 The trees are primary constituents, since 

 they indirectly support this chain of activi- 

 ties, as well as numerous other sequences 

 of food and feeder. Nevertheless the trees 

 are restricted to a given area, the restric- 

 tion being a function of bacteriological, 

 edaphic, and chmatic influences. They pro- 

 duce a forest by more or less successful 

 competition for hght, soil moisture, and 

 soil salts and by inter-specific and intra- 

 specific cooperation; for example, their in- 

 creasing bulk serves as a windbreak and 

 insures increasing annual increments of 

 leaves remaining on the forest floor each 

 autumn for future incorporation into the 

 growing mold. Such a forest community is 

 self-sustaining. 



From this viewpoint, the forest is a 

 major community as previously defined, 

 whereas the fungus alone is not. Unfor- 

 tunately, the problem of community bound- 

 aries is not so simple as it would appear 

 from the foregoing. In a limited sense, each 

 habitat is a microcosm containing a bio- 

 coenose. Not only are the associated organ- 

 isms limited by their immediate environ- 

 ment, but they change the environment 

 through their own multifold activities, or 



indirectly through the products of their 

 metabolism. Again, as many cells of the 

 organism are continually being replaced by 

 other cells, so the elements of the forest 

 community are continually in the process 

 of replacement. This replacement is at 

 different rates and different levels of im- 

 portance. Thus the community arises, 

 matures, eventually becomes senescent, 

 and its location is occupied by another type 

 of community. Destructive influences may 

 be of such violence, as in prolonged flood- 

 ing due to a change in water table, or soil 

 impairment and stand injury by fire or hu- 

 man influence, that community wound 

 repair is not possible and the forest ceases 

 to have an effect upon the eventual oc- 

 cupiers of the area. Obviously, between 

 progressive evolution and eradication there 

 are diverse intermediate conditions that 

 do not result in loss of forest personality 

 since their impact can be absorbed. 



The initial example of the bracket fungus 

 and its inhabitants, in relation to the whole 

 forest, presents no novelties. The chain of 

 events could have been illustrated by other 

 habitats within the same community, such 

 as the nest of the forest deer mouse {Per- 

 omyscus leucopus noveboracensis) and its 

 associated organisms. Pertinent illustrations 

 could be taken from any other community 

 —for example, the burrows of prairie ro- 

 dents, such as Citellus and Cynomys 

 (Bailey, 1905; Gregory, 1936; Howell, 

 1938); the burrows of the gopher tortoises 

 (Gopherus) in dry sandy soils (Hubbard, 

 1893); the burrows of the crayfish (Cam- 

 barus diogenes) on the floor of temporary 

 ponds (Greaser, 1931); the gastropod 

 shells appropriated by hermit crabs (Eupa- 

 guridae) of the marine littoral; the brome- 

 liad epiphytes of the neotropical rain forest 

 (Picado, 1911, 1913); the ant-plant 

 (Tachigalia) of the British Guiana rain 

 forest (Wheeler, 1923). The fist could be 

 greatly expanded. These instances of sub- 

 ordinate habitats, drawn from a wide range 

 of communities, involve the dependence of 

 organisms upon the habitat for food and 

 shelter, or both, and of the relation of the 

 habitat to the more permanent, self-sus- 

 taining community. The habitat may be 

 created by the original occupant, as in the 

 case of Citellus, partially created as in the 

 perforation of the petioles of Tachigalia by 

 silvanid beetles, or simply occupied (Eupa- 

 gurids). The habitat may be part of the 



