Wednesday, 1 December 2010

A relevant grand opening..

At risk of appearing like a beetle maniac, I decided to choose a beetle-related post to start with. As the British geneticist and evolutionary biologist J.B.S. Haldane said: "If the Creator exists, he had an inordinate fondness for beetles". This refers to the fact that the Coleoptera order contains more described species than any other order in the animal kingdom. Beetles constitute 25% of all known species, with an estimated 400,000 species in total.

Last year I went on a field trip to Uganda, spending part of my stay studying in Kibale forest. I carried out a study, focussing on the Dung Beetle (Scarabaeidae) preference for cow dung age and forest location. The dung beetle family display huge variation with 27,800 species worldwide, over 30 of which were identified in Kibale Forest by Nummelin & Hanski (1989). 

Dung beetles are characterised by modified forelimbs for digging, lamellate or clubbed antennae and a large head and pronotum. Strong competition over the high quality dung resource has led to specialization of the Scarabaeidae family. There are three classes of dung beetle, which use the dung in different ways: rollers; which make balls of dung and relocate them to hide in the soil, tunnellers; which build tunnels and nest under the dung, pushing it into chambers, and dwellers; which live and nest within the dung. They exhibit coprophagy; the ingestion of faeces. 

Dung beetles are of high ecological importance, as they decompose dung and other decaying organic matter and augment the processes of seed dispersal and nutrient recycling within ecosystems. They facilitate ecosystem growth by cleaning away dung, consequently reducing vector and disease. Despite the decomposition of dung occurring naturally; intensive farming methods lead to a large deposition of dung. This can cause environmental problems such as nutrient leaching and reduced pasture production because of increased forage fouling. Livestock will not graze in their excrement, thus reducing productivity. Dung beetles are consequently vital in agriculture, and enhance grazing quality by removing manure quickly and efficiently. 

Dung beetles can also be used as Biological control agents, and there have been several instances of introductions which have successfully increased productivity. They are regarded as a keystone species, exerting a large and stabilising influence throughout ecological communities and having a close relationship with the mammalian fauna. 

It is known that dung beetles are differentially adapted to utilize various types of dung, and where there are two or more large herbivorous species present; a niche dimension is observed. Studies into dung beetle use of different dung ages, however, are lacking, with little knowledge of how this differs in different environments. One would presume that new dung contains a higher concentration of nutrients and that the composition of dung breaks down more quickly in exposed secondary forest as opposed to the moist conditions within primary forest. It is clear that if dung is not recycled, nutrients are not put back into the ecosystem, thus decreasing the productivity. What needs to be made clear, however, is the potential effect on the ecosystem if the nutrients are not recycled by dung beetles. Concerns such as logging cause forest fragmentation, producing more secondary forest. If dung beetles are less active in secondary forest, does this mean there is a lower productivity? Would a buildup of dung affect plants? Perhaps it would prevent light access? This topic raises several questions which should be answered before decisions to continue to interfere with forests across the globe are made.

In light of this lack of knowledge, a project was designed to look at dung beetle activity across areas of primary and secondary forest within Kibale Forest. Kibale Forest is a moist evergreen forest in western Uganda (0° 27' N, 30° 26' E). There are two wet seasons; the first occurring between late August and early December and the second occurring from between early March and early May.  The annual rainfall is approximately 1500mm (Mahaney et al, 1997). It resides at a medium altitude of 1500m. The forest area is approximately 550km2, isolated from other areas of forest by approximately 50km.

A hypothesis was formed based around the question: Does the age and location of cow dung have an effect on the abundance and species diversity of dung beetles? We predicted that in the primary forest; the abundance and diversity would be higher due to the moist climate being “locked in” by the forest canopy. In addition to this, previous studies have shown dung beetles to have a preference for dark conditions. We also predicted the abundance and diversity to be higher for the new dung treatment as it would have a higher nutrient content, potency and preferred consistency. 

We found that Scarabaeidae abundance was significantly greater in areas of primary forest compared with felled secondary forest and on dung which has aged for shorter lengths of time (Figure 1). Probable reasons for this include the higher nutrient content, moistness and softer composition of younger dung. New dung is more useable and beneficial to dung beetles. This has implications in ecosystems where dung is more exposed to dry, hot and exposed conditions, allowing for the dung composition to change and “age” much more quickly. As well as this, the moist climate and dense canopy of the primary forest allow for the dung composition to change at a slower rate, retaining nutrients and moistness.  There is a clear preference for established primary forest, enabling nutrient recycling to occur at a faster rate. This suggests that in primary forest ecosystems that are reliant on dung beetles as principle nutrient recyclers, deforestation and fragmentation would have a detrimental effect on sustainability.

Figure 1. Effect of location and treatment on mean abundance (per pot) 
of dung beetles (Location: df=1, 21, F=6.77, P=0.018, Treatment: df=2, 21, F=16.92, P<0.001). Green represents primary forest, and black represents secondary forest.

Scarabaeidae species diversity, however, did not significantly differ across different ages of dung or across areas of primary and secondary forest (Figure 2). This suggests that both the age of dung and the ecosystem type has a larger effect on the abundance of beetles processing dung than on the diversity. It is, however, possible that due to the small sample size, the results were not entirely accurate, as a higher diversity of beetles was recorded on new dung and in primary forest, but the difference was not statistically significant. As most of the mammals in Kibale forest, use both primary forest and secondary felled forest, resources are largely available for dung beetles in both forest types. This could explain why a larger difference in species diversity wasn’t observed. The fact that the diversity data follows the same pattern to that of the abundance calls for further studies into this area to allow for a more concrete analysis.

 Figure 2. Effect of location and treatment on mean diversity index (per pot) 
of dung beetles (Location: df=1, 19, F=0.47, P=0.503, Treatment: df=1, 19, F=1.77, P=0.201). Green represents primary forest, and black represents secondary forest.

To conclude, this study further confirms the importance of dung beetles within ecosystems. There are potential implications of logging, which produces more secondary forest, on the abundance of dung beetles. This should be considered by organizations implementing forestry control, as long-term effects on productivity could be detrimental.

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