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Increased Urbanisation

Increased Urbanisation: Old Growth & Secondary Growth Forest

Deforestation in southern Mexico for agriculture

  • Similar w/ time
  • Except Specialists
  • Dispersal Important

Secondary growth forests (woody forest that re-grows after complete forest clearance due to human activity) is thought to increase during the next century as increased urbanisation and industrialisation cause agricultural land to be abandoned7.

Old growth forests are of main interest to tourists10 because they offer a glimpse of primeval forests before the rise of mankind. However, they are also of interest to logging companies and for commercial exploitation of products2. Old growth forests are most desirable for exploitation as secondary forests often lack large valuable trees associated with old growth2. So what else is different about the diversity of primary forests and how do they compare with second growth forest?

The Difference Between Old Growth and Secondary Forest

We can safely say that secondary forest species composition is different from old-growth forest2,3,11 but this doesn’t write them off completely.

A look at secondary forests from 62 published articles (conducted by Dent and Wright5), where old-growth forest was completely removed then abandoned allowing forest recovery, revealed interesting results.

Surprisingly, they found that many tropical forest species tolerate secondary forest and that secondary forests were found to grow rapidly more similar to old growth forest with time. Secondary forests obtain many aspects of old-growth forests, such as structure, environment, diversity, and species composition at the seedling layer, but that plant species composition in the canopy could take between 80 – 500 years to recover5.

Research shows that secondary forests approach species richness levels identical to old growth forests after around 80 years and in some cases exceed old growth forest in diversity4. However, animals dependent on specific plant species exhibit a stronger preference for old-growth forest6

In one study, a patch of secondary forest around 12 years in succession was studied adjacent to old growth forest to look at primate populations. They found the majority of primates were using secondary forest, but primary forest was concluded to be more important, containing a higher diversity and a greater number of individuals6.

Bird species richness in Amazonian secondary forest was found to consistently show similarity to old growth forest after only 10 years1.

A study of moth diversity in the neo-tropics revealed that secondary forest is not significantly different from primary forest, but that forest types exhibited a distinct community structure. Around 50% of the primary community was replaced in secondary forest by edge or disturbance specialists8.

The Importance of Dispersal

Importantly, these studies were conducted where dispersal is possible i.e. the recovering forest is located close to old growth forest, so seeds and animals can easily move across. Dispersal is highly important for regeneration and secondary forests often contain only a fraction of the original plant species due to absence of dispersal vectors. Because of this, commercially useful species are often added as part of enrichment plantings9. In areas where there is no nearby old-growth forest, regeneration will take many, many more years, unless the forest is assisted with the help of restoration ecology.

Conclusion

When we read about biodiversity differences between primary and secondary forests, it is important to think about the individual species that comprise that diversity and factor in if they are rare, common, endemic, or edge species. Secondary forests should be protected enabling recovery of the forest to a pre-human state but should not reduce the significance of preserving old-growth forest.

References

1. Andrade, G.I., and H. Rubio-Torgler. 1994. Sustainable Use of the Tropical Rain Forest: Evidence from the Avifauna in a Shifting-Cultivation Habitat Mosaic in the Colombian Amazon. Conservation Biology. 8: 545-554

2. Bawa, S.K., R. Seidler. 1998. Natural forest management and conservation of biodiversity in a tropical forest. Conservation Biology. 12: 46-55.

3. Bihn, J.H., M. Verhaagh, and M. Brandle. 2008. Do secondary forests act as refuges for old growth forest animals? Recovery of ant diversity in the Atlantic forest of Brazil. Biological Conservation. 141: 733-743.

4. Brown, S., and A.E. Lugo. 1990. Tropical secondary forests. Journal of Tropical Ecology. 6, 1–32.

5. Dent, D.H., and J. Wright .2009. The future of tropical species in secondary forests: A quantitative review. Biological Conservation. 142: 2833–2843.

6. Fimbel, C. 1994. The relative use of abandoned farm clearings and old forest habitats by primates and a forest antelope at Tiwai, Sierra-Leone, West-Africa. Biological Conservation. 70, 277–286.

7. Guariguata, M.R., and Ostertag. 2001. Neotropical secondary forest succession: changes in structural and functional characteristics. Forest ecology and management. 148: 1-3.

8. Hawes, J., C.D. Motta, and W.L. Overal. 2009. Diversity and composition of Amazonian moths in primary, secondary and plantation forests. Journal of Tropical Ecology. 25: 281-300.

9. Lamb, D., P.D. Erskine, and J.A. Parrotta. 2005. Restoration of degraded tropical forest landscapes. Science. 310: 1628-1632.

10. Naidoo, R., and W.L. Adamowicz. 2005. Biodiversity and nature-based tourism at forest reserves in Uganda. Environment and Development Economics. 10:159–178.

11. Zimmerman, J.K., J.B. Pascare, and A.T. Mitchell. 2000. Barriers to forest regeneration in an abandoned pasture in Puerto Rico. Restoration Ecology. 8: 350-360.



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