Tropical rainforest fragmentation - Fragment size, species diversity and composition
Anthropogenic activity such as road construction, urbanization and agriculture has led to the division of continuous forest area into smaller blocks, a process known as “fragmentation” (Echeverría et al. 2011). This sectioning of space alters forest structure by increasing the ratio of edge habitat to interior forest (Rusak 2002). Fragmentation thus results in changes in community composition and ecological processes, including reproduction, predator-prey interactions and nutrient recycling (Drinnan 2005). The size of the fragment determines how and to what extent fragmentation will impact the ecological profile of a forest area.
Fragmentation of forests increases the edge habitat area while sacrificing interior habitat space. A range of physical changes occurs at these edges – such as variations in temperature, moisture and sunlight – creating a different ecological environment to that of interior forest (Echeverría et al. 2011). This impacts species composition because some species are sensitive to edge habitat, while others thrive in it. A study done by Echeverria et al. (2011) looked at the composition of tree species in patches of different sizes located in southern Chile. These tropical forests are characteristic of how recent man-directed modifications in land use can put strains on different endemic species. They found that fragment size did not have a direct impact on overall diversity of the forests (in some cases, fragmentation even increased overall diversity), but as patches got smaller, shade-intolerant species that are restricted to edge habitat – such as Embothrium coccineum and Aristotelia chilensis – become more abundant. In contrast, the abundance of species that normally thrive in internal forest habitat – such as Nothofagus nitida and Amomyrtus meli – decreased as patch size got smaller. Thus, fragmentation resulted in a microenvironment that gave shade-intolerant species a competitive advantage in that space, including the alien species Rimapenaeus constrictus, which extended considerably into the smaller fragment habitats. Thus, by dividing forest space in this way, man alters the physical landscape and its trajectory of succession.
Forests establish microclimates that are partially protected from changes in the external environment. For example, the forest canopy provides physical cover from the external atmosphere, buffering against temperature extremes. Internal forest typically has warmer minimum temperatures and cooler maximum temperatures compared to the macroclimate (Ewers & Banks-Leite 2013). This buffering not only applies to temperature, but also protects against disturbances like droughts or storms. Effectively, buffering bolsters the diversity of many tropical plant and animal species because extreme changes in the environment could act as extinction events. The larger the size of the forest fragment, the more able it is to buffer against extreme macroclimate conditions. In a study by Ewers and Banks-Leite, the researchers looked at temperature buffering in the Atlantic forests of Brazil. They compared temperatures internal and immediately external to forest habitat, finding that the protective effect of forests decreased maximum external temperatures by one third at ground level. The researchers found that this buffering effect was reduced at forest edges, with the edge effect actually reaching as far as 20 meters into the forest from its perimeter. An increased number of smaller fragments would thus result in a serious loss of buffering capacity, rendering the plot more susceptible to disturbances. However, one must note that the forest itself cannot protect against all disturbances. Forest fires, for example, though arguably may be beneficial in the long term, prove more destructive when forest patches are larger (Cochrane et al. 1999).
Fragmentation of forest habitats can affect many inter-species biotic interactions, including pollination, migration, herbivory, seed predation and reproduction (Hill et. al 2011). As a fragment becomes smaller, it is more likely that species within the plot will need to migrate outwards in search of resources and space. But this migration can be difficult, and when populations become isolated, species are more prone to decline in abundance because of inbreeding. The species is also expected to have “swinging” population levels because of the over-exploitation of the habitat (Rusak 2002). Even if a species is not driven to extinction, these ecological processes can impact its development. A study carried out by Quevedo and colleagues found that there were differences in patterns of genetic structure between individuals of Clusia sphaerocarpa at the edge and in the interior of the forest (Quevedo et. al 2013). These changes were attributed to changes in plant-animal mutualisms at new forest edges, as well as different pollination and dispersal functions. Fragmentation thus can set pressures on the genetic development of species in unpredictable ways.
Fragmentation alters the ecological profile of internal forest space, putting the species that require this special microclimate at a competitive disadvantage, and establishing pressures on the species that can survive to develop in different ways. This is problematic from a conservation standpoint. Although overall diversity might remain the same following fragmentation, species composition becomes radically different. Furthermore, edge habitat is easier to construct anthropogenically (by planting fast-growing, shade-intolerants) as opposed to internal forest space, which relies on structures that have taken decades to develop and is therefore more valuable. By not preserving the uniqueness of internal forest habitat, deforestation could lead to the elimination of species that might have proven medicinally or economically valuable had they been recovered and studied. It is thus significant to be mindful of the sizes of plots we create by fragmentation.
The function of damaged forests can be improved through restoration activities. For example, the “Committee of the Friends of the Cedar Forest” in Lebanon started a reforestation initiative in 1985 by clearing the forests of detritus, treating the sick plants and refertilizing the soil. Most importantly, the remaining fragments are now rigorously protected to ensure they can regrow (Saab 2013). The effects of this reforestation program will only be appreciable in a few decades, due to the slow rate of growth of the forest. But careful planning can be used to preserve the continuity of these natural ecosystems. For example, this can be done by leaving natural connections or corridors between fragments, or integrating woodland into urbanized space. Actively enriching deforested space with equilibrium species can expedite the succession of these forest spaces too (Cayela et al. 2006).
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Cochrane, M.A., Alencar, A., Schulze, M.D., Souza, C.M., Nepstad, D.C., Lefebvre, P. & Davidson, E.A. (1999). Positive feedbacks in the fire dynamics of closed canopy tropical forests. Science 284:1832–1835.
Drinnan, I.N. (2005). The search for fragmentation thresholds in a southern Sydney suburb. Biological Conservation 124:339– 349.
Echeverría1, C., Newton, A.C., Lara, A., Benayas, J.M. & Coomes, D.A. (2007). Impacts of forest fragmentation on species composition and forest structure in the temperate landscape of southern Chile. Global Ecology and Biogeography. 16:426-439.
Ewers, R.M., Banks-Leite, C. (2013). Fragmentation Impairs the Microclimate Buffering Effect of Tropical Forests. PLoS ONE 8(3):e58093.
Hill, J.K., Gray, M.A., Khen, C.V., Benedick, S., Tawatao, N. & Hamer, K.C. (2011). Ecological impacts of tropical forest fragmentation: how consistent are patterns in species richness and nestedness? Philosophical Transactions of the Royal Society B. 366:3265–3276.
Quevedo, A.A., Schleuning, M., Hensen, I., Saavedra, F. & Durka, W. (2013). Forest fragmentation and edge effects on the genetic structure of Clusia sphaerocarpa and C. lechleri (Clusiaceae) in tropical montane forests. Journal of Tropical Ecology 29:321-329.
Rusak, H. (2002). Woodlands at Risk. In: Ontario Nature. <http://www.ontarionature.org/discover/resources/PDFs/factsheets/fragmentation.pdf> Downloaded on March 15 2014.
Saab, N. (2013). Ecological Footprint of Arab Countries. In: Lebanese Environment Forum. <http://www.lbeforum.org/arab-environment5-survival-options> Downloaded on March 13 2014.