Literature Review: Oaks as Islands

by Carl Strang

One paper in the journal Ecology this year caught my eye because of its relevance to my study of leaf miners in maples (Barber, Nicholas A., and Robert J. Marquis. 2011. Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community. Ecology 92:699-708). They studied white oaks in Missouri.

White oak leaves

Barber and Marquis looked at leaf chewing herbivores on accessible lower tree branches, as I have done in sugar/black maples, but with much greater elaboration. They grouped the species they found into guilds based on their feeding style: free-feeding (e.g., chewers of holes or gouges in leaves), shelter building (insects that roll leaves or tie them with silk to form little hiding places), leaf-mining (tiny insects that live between the upper and lower surfaces of the leaves), and arthropod predators. They recognized separate herbivore assemblages divided by season with peaks in May, early July, and late August-early September, and found little or no species overlap between these periods.

They assessed predation by birds, using exclosures to prevent avian access to some branches, and found that birds had a negligible impact on insect abundance or community structure.

They also measured the nutritional value of the individual trees, and found that most of the invertebrate guilds preferred plants with high nitrogen and low tannin levels. This was especially true late in the season. Higher quality plants had larger communities, and sometimes higher species counts. On the other hand, similarity between communities on different trees was not based on similarity of nutritional value, but rather on how far apart the trees were. They concluded that community species composition is driven more by stochastic processes (the likelihood that dispersing insects will find a tree) than by host plant quality. This tied their study to the venerable body of research on island biogeography. Their insects behave as though these white oaks are islands, with some trees more remote than others and thus less likely to be reached by the less effective dispersers.

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