How do deer affect the local ecosystem?

Deer are an important part of their natural ecosystems. They are prey for larger carnivores, such as coyotes and black bears. They also have a strong influence on plant communities. Through their grazing and browsing, they can have both positive and negative effects on plant diversity. In areas where there is high plant diversity (many species of plants are present), deer appear to help some plant species survive, enhancing biodiversity (Cook-Patton et al., 2014).

However, when deer become overabundant, they can have negative impacts on the local ecosystem (see ecological carrying capacity in the FAQ on overabundance here). An individual deer can consume 4 to 8 pounds of flowers, shrubs, and seedlings a day. A large herd of deer can dramatically impact the plants they feed on. However, deer do not feed on all plants equally. There are many native and non-native species that they tend to avoid. The result of this is that certain species - often native species that deer prefer - are greatly reduced, and plant communities become dominated by the few species that deer avoid eating (which are often invasive). In addition, they help to disperse seeds of invasive plants, which may stick to their fur or be consumed and later excreted. Furthermore, seedlings of native plants that are preferred food sources are quickly eaten as they begin to grow, preventing regeneration of natural plant communities. The overall effect of this is to produce plant communities that are greatly diminished in biodiversity. This, in turn, can impact other species, such as birds, reptiles, amphibians, and invertebrates, that rely on those plant communities for habitat (USDA-NRCS and New Jersey Audubon Society, 2012).

References:

Cook-Patton, S.C., M. LaForgia, and J.D. Parker. 2014. Positive interactions between herbivores and plant diversity shape forest regeneration. Proceedings of the Royal Society B 281: 20140261. https://royalsocietypublishing.org/doi/epdf/10.1098/rspb.2014.0261

USDA-NRCS and New Jersey Audubon Society, 2012. NJ Biology Technical Note: White-tailed Deer Impacts and Forest Management. Accessed at https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs141p2_017980.pdf