The net effect of interacting ecological processes is not straightforward. A good example is the interaction of competition and herbivory between the weedy exotic Japanese honeysuckle (Lonicera japonica) and the native trumpet honeysuckle (L. sempervirens) (Schierenbeck et al., 1994). In the absence of herbivory, Japanese honeysuckle will be outcompeted by trumpet honeysuckle because it has a higher growth rate and accumulates more biomass (Fig. 9.4). However, being a native, trumpet honeysuckle is more vulnerable to herbivores and the resultant damage makes it less competitive than Japanese honeysuckle. Why does trumpet honeysuckle not adapt by developing strong defences against herbivores so it can be both impervious and a good com petitor? The main reason is because there is a trade-off of limited resources allocated to competitive traits like growth, versus those allocated to defences against herbivores (Herms and Mattson, 1992).

A more complicated example of interactions between ecological processes is that of weedy tree-of-heaven (Ailanthus altissi-ma) (Facelli, 1994). White oak (Quercus alba) leaf litter: (i) provides habitat for herbivores that attack tree-of-heaven saplings and adults, and (ii) delays the seedling emergence of tree-of-heaven. Separate experiments also show that competition from species like giant foxtail (Setaria faberii) reduces seedling biomass of the tree-of-heav-en. We might expect that a combination of white oaks and species like giant foxtail would harm tree-of-heaven even further. However, when white oak litter and competition from species like giant foxtail occur simultaneously, the leaf litter has a greater effect on giant foxtail than it did on tree-of-heaven. Contrary to expectations from examining leaf litter and competition separately, a combination of both actually increases the ability of tree-of-heaven to survive.

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