Rainforest species rise in trophic level after forest logging, study says

Image

Red-naped Trogon by CK Leong

There are many known fatalities of habitat destruction in rain forests; however, not much focus is given to the trophic system, or how the ecosystem interacts as a whole. Faculty from institutions across the globe came together to understand how logging affects trophic positions and ranges of local species.  

In the understory of Bornean rain forests, researchers conducted a study on 73 species of birds to support the theory that logging raises trophic position.

By analyzing nitrogen isotopes, scientists have found that there is an increase in trophic level among bird species in degraded areas. 

 The scientists sampled nail clippings of 52 bird species from a protected forest and 57 from a logged forest. They analyzed the clippings to determine the concentration of nitrogen isotopes as compared to that of the surrounding forest. 

The ratio of Nitrogen-15 to Nitrogen-14 atoms increases at higher trophic levels, with a higher number of Nitrogen-15 isotopes indicating a rise in trophic level.  

Accounting for families of birds within the same species, the scientists charted the isotope data to map the trophic positions of the birds, determining whether there was a change in their place in the food chain. 

The birds were flexibly grouped based on primary diet previous to logging, leading to such categories as fruit, nectar, and insect eaters. 

So why does rainforest destruction raise the average trophic level of an ecosystem?

The rise in trophic level indicates that the bird species, or potentially their insect prey, become more predacious in the forests that experienced logging. The loss of their main food source, fruit from the trees, forces the local populations to seek alternative forms of energy, often at higher trophic levels. 

Any species that consumes fruit would face a food shortage due to the lack of fruiting trees in logged forests. The alternative sources of energy might come from insects if the birds had not previously been a carnivore, or the prey insects might have began consuming other insects. 

This behavior in turn increased the overall trophic position of the logged forest by half of a trophic level. Logically, the species originally occupying the lowest trophic level experienced the largest rise in trophic position.

In addition, the trophic range of the species shrank dramatically in the degraded forest due to lower variation in prey (Edwards et al., 2013). With fewer resources at the lower trophic levels, species are inclined to eat more restricted diets at higher trophic levels, which led to an upward shift.  

But what happens if a species fails to change trophic levels? Those bird species with more rigid trophic niches are more likely to face extinction following habitat loss than their more flexible counterparts.  

Changing the trophic structure changes the ecosystem as a whole, potentially causing a loss of species in more rigid trophic structures.

While 80% of species persisted in logged forests, those with more restricted diets faced local extinction due to habitat disruption (Edwards et al. 2011). Because the data show that the range of trophic levels within a species shrunk significantly, species that had smaller ranges prior to logging were more vulnerable.  

The fact that smaller ranges lead to species extinction has significant consequences for logged forests. The species that survived in logged forests at higher trophic levels also had narrower ranges; therefore, the ecosystem as a whole in logged forests is particularly fragile.

The forests of Borneo and Southeastern Asia as a whole are subject to deforestation and logging. While most species appear to stabilize after logging and the ecosystem is higher energy overall, the species in these forests are in grave danger. 

Conservation efforts should focus on areas that have been subjected to logging to prevent the further degradation of these fragile ecosystems and prevent a tragic loss of one of the richest sources for biodiversity in the world. 

Sources: 

Article:

EDWARDS, DAVID P.,  WOODCOCK , PAUL, NEWTON, ROB J., EDWARDS, FELICITY A.,  ANDREWS, DAVID J. R.,  DOCHERTY, TEEGAN D. S., MITCHELL, SIMON L.,  OTA, TAKAHIRO, BENEDICK, SUZAN, BOTTRELL, SIMON H., HAMER, KEITH C. (2013), Trophic Flexibility and the Persistence of Understory Birds in Intensively Logged Rainforest. Conservation Biology, 27: 1079–1086. doi: 10.1111/cobi.12059

Image:

By CK Leong (Red-naped Trogon) (http://borneobirds.com/red-naped-trogon/

http://borneobirds.com/red-naped-trogon/) via Google Images

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