There are many sources of stress on forests, including climatic, human, and biotic. Naturally occurring and introduced insect pests and diseases can kill large numbers of trees in contiguous areas of forest until natural limits curtail the insects or diseases. Introduced (weeds) or naturally-occurring plants can stress trees, for example, mistletoe is a parasitic plant that can create structural and nutritional pressure on individual trees. Over-browsing or grazing by animals can reduce succession or seedling recruitment rates in forests (e.g., cattle consumption of seedling oak trees).