Populations 85 



(Erodiiim, Brassica, and Avena) have extirpated native plants in 

 some areas of California. Shipborne rats have virtually destroyed the 

 fauna of small vertebrates on numerous islands. 



Changes in the availability of places to live also constantly occur. 

 Silting and slowing down of streams make them untenable for 

 larval and pupal black flies and other organisms depending on 

 swift-running, oxygen-rich water. Planting of trees across the Great 

 Plains has permitted range extensions by tree-nesting woodland 

 birds. The slow accumulation of humus and disintegration of rocks 

 into soil make homes for orbatid mites, centipedes, fungi, and other 

 lovers of dank, dark places. Often the organisms living in areas sub- 

 ject to frequent catastrophic change have specialized genetic sys- 

 tems affecting their genetic behavior (see Chap. 9). 



All these examples are of relatively spectacular variation in the 

 environment. Important also are the smaller, more frequent changes: 

 the day-to-day temperature variation that affects the plankton popu- 

 lation of a shallow pond or the yearly precipitation changes that 

 determine the condition of a butterfly's food plant. None of these 

 changes can be viewed as an isolated event. Increased moisture may 

 improve the condition of the larval food plant of a butterfly, and a 

 large adult population may result. Among other things, this may 

 mean more food for nestling song birds and, in the long run, more 

 food for hawks. Large numbers of butterflies may mean more cater- 

 pillars next year, a year when little moisture means a poor crop of 

 food plant. Thus few adults survive, almost no food plants survive, 

 and topsoil is lost through erosion. Large numbers of song birds 

 from the previous season may find no suitable substitute food, with 

 resultant starvation, emigration, and unsuccessful reproduction. The 

 hawks go hungry. 



This somewhat overdrawn example illustrates only a few of the 

 many permutations of effects which might be hypothesized as result- 

 ing from a simple change in precipitation. Actually such gross effects 

 are relatively infrequent, for most ecological systems are made up 

 of a great many elements and have a historical dimension. Complexity 

 leads to less one-to-one dependence. Drought which reduces butter- 

 fly populations may lead to an increased supply of grasshoppers, and 

 the song birds readily shift their diet. Long-term associations have 

 presumably experienced most of the usual variations in climate, and 

 their members presumably can respond to it. Thus the butterfly food 

 plant will probably survive a drought (perhaps, if it is an annual, 

 as ungerminated seeds ) and be ready to return to abundance when 

 moisture reappears. If erosion has not proceeded too far, it may be 

 stopped. Doubtless, however, many organisms would be permanently 

 affected. 



