Feature Image: Photo credit: https://pixabay.com/photos/stump-wood-tree-trunk-bokeh-933702/
Source Article: Hannah, L., Le Roux, D.S., Milner, R.N.C., Gibbons, P., 2019. Erecting dead trees and utility poles to offset the loss of mature trees. Biological Conservation 236: 340-346. https://doi.org/10.1016/j.biocon.2019.06.001
Mature trees are important habitats
For many birds, trees are like fine wine: they get better with age. Mature trees, according to Hanan et al. 2019, are trees that act as high perches, have hollow cavities, peeling bark, and dead limbs that birds use for a variety of reasons. For example, robins use hollow cavities, which are only present in mature trees, to make and hide their nests. Birds also use trees to rest, avoid predators, or find food sources. Peeling bark and plants that grow on mature trees can provide important sources of food to a variety of bird species as well.
Despite the importance of mature trees, the growing human population has endangered these resources through tree removal to expand agricultural fields or build new homes, roads, or other infrastructure. Mature trees are particularly vulnerable because they are also used for industrial purposes (e.g. wood products). This loss of forests, known as deforestation, can have devastating consequences for the organisms that depend on trees for habitat, safety, or food. According to the International Union for Conservation of Nature (IUCN), over 80% of the world’s species live in forests and deforestation is the biggest threat to forests worldwide. In light of this threat, conservationists and resource managers alike are exploring strategies to protect forests ecosystems and to offset the impacts of forest loss in urbanized areas.
Can artificial structures provide similar services as mature trees?
In many areas of the world forests and mature trees are being replaced by homes, schools, libraries, and other urban infrastructure. When mature trees are removed, the species that rely on the trees for all or part of their lives also disappear. In order to offset the loss of species from mature tree loss in urbanized areas, conservationists, scientists, and citizens alike have turned to artificial structures. Birdhouses, for example, are artificial structures that mimic hollow cavities that mature trees provide for birds to create their nests.
Utility poles are another common artificial structure in urbanized areas. Is it possible that these poles could also support birds in areas where tree loss has occurred? A recent study published by L. Hannan and colleagues tried to answer this question. In particular, the researchers were interested in understanding how utility poles and erected dead trees (dead trees that are propped back up so they are still standing) impacted the number of bird species in areas where mature trees were lost.
Hannan and colleagues conducted bird surveys in four different areas: 1) areas with no mature trees, 2) areas with mature trees, 3) areas with utility poles, and 4) areas with erected dead trees. At each site, surveyors recorded each species of bird that flew within a 60 foot radius over the course of 20 minutes. The researchers conducted two surveys before the dead trees and utility poles were placed on the landscape and two afterwards.
In total, they identified 651 birds representing 45 different species during the study. Before the installation of utility poles and dead trees, the researchers recorded less than 1 bird per survey in each of these areas. After the installation, more than 6 birds were observed in the area surrounding utility poles and more than 8 birds were observed in the area surrounding dead trees. While the artificial structures supported more birds than the barren landscape, they did not support as many as mature trees. Areas with mature trees had the highest number of birds both before and after the installation of the artificial structures, with more than 17 birds recorded near mature trees prior to the installation and over 9 birds observed after the installation.
These results suggest that artificial structures can offset the loss of birds in deforested areas, but they cannot fully replace mature trees.
Conservation strategies should be diverse
When comparing conservation strategies, scientists and resource managers must consider the ecological value of a habitat and the cost of conservation activities. According to analyses by Hannan and colleagues, protecting mature trees is the costliest conservation strategy followed by installing artificial structures and planting seedlings. While planting seedlings may be cheap in the short-term, Hannan and colleagues determined that it would take 85 years for a tree to replace the function of a mature tree! That might seem like a long time, but consider that the giant sequoias that make up Sequoia National Park in the U.S. have been aged to over 3,200 years! Since artificial structures do not support as many birds as mature trees, conservation approaches should be diverse and combine efforts to plant seedlings, preserve mature trees, and install artificial structures to minimize the loss of species and ecological function. These conservation approaches are not only being undertaken to address deforestation. For more information, check out this envirobites story written by Lushani Nanayakkara about how artificial wetlands can offset the loss of birds due to loss of natural wetland areas.