310 Relations within the Species 



ample send out runners that take root, producing new plants, and 

 these in turn send out more runners. In this way dense aggrega- 

 tions of the species are formed. Unfortunately for man, briars and 

 poison ivy also produce thick clumps of individuals by this very suc- 

 cessful vegetative reproduction. 



Similar attached aggregations are formed by certain sessile organ- 

 isms of the animal kingdom. Many sponges, bryozoans, tunicates, 

 hydroids and other invertebrates grow in colonies, and coral animals 

 produce 'extensive formations as a result of colonial development. 

 Animals displaying this growth habit are sometimes very numerous, 

 but they are not capable of free locomotion, and they have not at- 

 tained as high a degree of development as non-colonial animals. 



In other groups formed by the progeny of a breeding unit the in- 

 dividuals are unattached but remain together. The parents and their 

 immediate offspring constitute a family among animals, and the same 

 term is sometimes applied to the progeny of a single plant. A few 

 relatives may join the family unit, as has been observed in a den of 

 wolves (Murie, 1944). Larger aggregations result from a further 

 extension of family groups. An animal and "his sisters and his 

 cousins and his aunts" may remain together as a clan, and unrelated 

 members of the species may subsequently join the group to form a 

 larger herd, pack, or flock. Seals, sea Rons, wolves, monkeys, prairie 

 dogs, many ungulates, and various kinds of birds are examples of ani- 

 mals' that form units of this sort. More complex groups of related in- 

 dividuals are represented by colonies of social insects, such as ants, 

 bees, wasps, and termites. In the plant kingdom the progeny of 

 neighboring adults may join to form larger groups of the same 

 species. If such a group is invading a bare area, it is termed a plant 

 colony, as will be discussed more fully in Chapter 12 in relation to 

 ecological succession. 



In other instances groups resulting from reproductive activities are 

 not necessarily the descendants of one breeding unit. Aggregations 

 of young are frequently formed by the simultaneous release of eggs 

 or larvae within a restricted area. The coincident production of 

 young by neighboring adults is often set off by some common en- 

 vironmental stimulus, such as the occurrence of a critical illumination 

 or temperature. The nearly simultaneous spawning of oysters, trig- 

 gered by temperature conditions as described in Chapter 5, results in 

 the sudden appearance of swarms of oyster larvae in estuaries of the 

 Atlantic coast. 



