Competition between salamanders

The first example concerns two species of terrestrial salamanders, Plethodon glutinosus apd P. jordani, which live in the southern Appalachian Mountains of the USA. Generally, P. jolani lives at higher altitudes than P. glutinosus, but in certain areas their altitudinal distributions overlap. Hairston (1980) carried out an experi­ment at two sites, one in the Great Smoky Mountains and the other in the Balsam Mountains. These sites both had populations of the two salamander species, they had similar salamander faunas overall, they were at the same elevation and they faced the same direction. At both sites, seven experimental plots were established in I974: two from which P. jordani was removed, two from which P. glutinosus was removed and three as controls. Then, six times in each of the next 5 years, the numbers of both species and their ages were estimated in all plots.

In the control plots, and naturally, P. jordani was by far the more abundant of the two species; and in the plots from which it was removed, there was a statistically significant increase in the abundance of P. glutinosus at both locations. In the plots from which P.glutinosus was removed, there was no significant reciprocal increase in the abundance of P. jordani. However, there was, at both sites, a statistically significant increase in the proportion of p. iordani in the 1- and 2-year-old age classes. This was presumably a result of increased fecundity and/or increased survival of young, both of which are crucial components of the basic reproductive rate.

The important point is that individuals of both species must_' originally_' have been adversely affected by individuals of the other species_' since when one species was removed, the remaining species showed a significant increase in abundance and/or fecundity and/or survivorship. It appears, therefore, that in the control plots and in the other zones of overlap generally, these species competed with one another but still coexisted.

Competition between bedstraws (Galium spp.)

 

A.G. Tansley, one of the greatest of the 'founding fathers' of plant ecology, studied competition between two species of bedstraw. Galium hercynicum is a species which grows in Great Britain at acidic sites, whilst G. p umilurn is confined to more calcareous soils (in Tansley's time they were known as G. saxatile and G. sylvestre). Tansley found that as long as he grew them alone, both species would thrive on both the acidic soil from a G. hercynicum site and the calcareous soil from a G. pumilum site. Yet, if the species were grown together, only G. hercynicum grew | successfully in the acidic soil and only G. pumilum grew successfully in the calcareous soil. It seems, therefore, that when they grow together the species compete, and that one species wins, whilst the other loses so badly that it is competitively excluded from the site. The outcome depends on the habitat in which the competition occurs.

 

Competition between barnacles |

The third study concerns two species of barnacle in Scotland: Chthamalus stellatus and Balanus balanoides.   These species are frequently found together on the same Atlantic rocky shores of north-west Europe. However, adult Chthamalus generally occur in an intertidal zone which is higher up the shore  than that of adult Balanus, even though young Chthamalus settle in considerable numbers in the Balanus zone. In an attempt to understand this zonation, Connell monitored the survival of young Chtharnalus in the Balan us zone. He took successive censuses of mapped individuals over the period of 1 year and, most important, he ensured at some sites that young Ch thamlus that settled in the Balanus zone were kept free from contact with Balanus. In contrast with the normal pattern, such individuals survived well, irrespective of the intertidal level. Thus, it seemed that the usual cause of mortality in young Chthamalus was not the increased submergence, times of the lower zones, but competition from Balanus in those zones. Direct observation confirmed that Balanus smothered, undercut or crushed Chthamalus, and the greatest Chthamalus mortality occurred during the seasons of most rapid Balanus growth. Moreover, the few Chthamalus individuals that survived I year of Balanus crowding were much smaller than uncrowded ones, showing, since smaller barnacles produce fewer offspring, that interspecific competition was also reducing | fecundity.

Thus, Balanus and Chthamalus compete. They coexist on the same shore, but on a finer scale their distributions overlap very little. Balanus outcompetes and excludes   Chtharnalus from the lower zones; but Chthamalus can survive in the upper zones where Balanus, because of its comparative sensitivity to desiccation, cannot.