What is the edge of a forest (EAFE)

Source: Meeting of the EAFE (European Association for Forensic Entomology),

Lausanne, Switzerland, 27.-29. April 2005):21--22
Meeting of the EAFE (European Association for Forensic Entomology)
Lausanne, Switzerland, 27.-29. April 2005
Halbach M.'; Sinclair B.'; Benecke M.' '

' Zoologisches Forschungsinstitut und Museum Alexander Koenig, Adenauerallee 160, 53113 Bonn, Germany
' International Forensic Research & Consulting, Postfach 250411, 50520 Cologne, Germany, E-mail forensic@benecke.com

We tried to experimentally address a "simple" question that came up during a homicide trial in Aachen, Germany. Some dead adult flies of the genus Bradysia (Sciaridae) had been recovered from a corpse that was found on grassland close to a forest. Bradysia is known to live on the borders of forests. Relating to this expert witness statement, defense and prosecution asked: "Alright, but what is an edge of a forest?"

To determine if this question could be answered ecologically, we monitored all Diptera during decomposition of two pigs -- inside of the forest, and on the border of the forest. Over a period of six weeks in Autumn 2003, adult insects resting on the corpses were caught by hand/with a net. With the help of recent determination keys and the extensive collection of Museum Koenig in Bonn, Germany, we determined the following fly families by sex, and to the lowest taxonomic level possible (often to species level): Anthomyiidae, Calliphoridae, Dryomyzidae, Fanniidae, Muscidae, Heleomyzidae, Phoridae, Piophilidae, Scatophagidae, and Sepsidae.

On the inorganic side, the most obvious difference between both habitats was simply the outside temperature (mean T = 0.9°C, p<0.001). This is of course interconnected with outside humidity, and inside temperature of the two corpses (mean T = 2,2°C, max. T 44.7°C vs. 35.7°C on 7th/8th day of decomposition).

Successional waves were represented not only by presence or absence but also by proportion of insect groups. These differences were of course dependent on the location of the pigs. For example, numerous Heleomyzidae were found during post-decay stage on both pigs. Piophilidae were found mostly after bloated decay inside the forest, but during three stages (bloated stage, decay stage, and post decay) on the pig on the border of the forest.

A much better definition of the classical successional waves was calculated out of the relative number of adult individuals on the corpses. Here, the relative decrease of Calliphorid individuals compared to the increase of Heleomyzids, Piophilids and Dryomyzids was one of the most prominent markers.
Whilst α-diversity (Shannon-Wiener index: 3.6 vs. 4; Evenness 0.7 vs. 0.7) and β-diversity (Renkonen Percentage Similarity Index: 70.2%) showed that both habitats were similar, i.e., no dominant insect species were present in general, this was not true for the fauna of the pigs. The decomposing pig on the border of the forest maintained a significantly higher number of different families and species of Diptera compared to the pig inside of the forest. This was not only due to an excess of a Neoleria (Heleomyzidae) species inside of the forest, but was a constant tendency that became more obvious the further decomposition progressed.

Several Diptera species had a clear preference for the border of the forest, e.g., Parapiophila vulgaris during post decay. Some 35.7% of the flies were found exclusively on the corpse of the pig at the border of the forest, whilst 10.7% were exclusively found on the corpse inside of the forest.
Differences in sexes of insects were very clear, too, but not as expected: Calliphoridae (23 ♂ / 301 ♀), Muscidae (4 ♂ / 89 ♀), and Fanniidae (1 ♂ / 87 ♀), yet Piophilidae: 116 ♂ / 41 ♀. This shows that male Piophilids -- in contrast to all other monitored flies -- try to establish a small, personal territory on the corpse whilst female Piophilids prefer to sit elsewhere, preferably in the sun. They only approach the corpse to lay their eggs, and probably to mate.

Unfortunately, the sex ratio of Heleomyzids (238 ♂ / 18 ♀) cannot be explained as of yet, because of a lack of ecological knowledge of this group.
An interesting observation was that Lucilia ampullacea arrived very early in and on the border of the forest, whilst L. caesar arrived 1-2 days later.
To answer our initial question from the trial: Yes, we did find marked ecological differences between both habitats. However, these differences can only be monitored by traps, or, like in our case, by catching adult flies by hand.

In this particular case, it took an ecologist, a taxonomist, a forensic entomologist, and a critical experimental investigation into one defined area over more than 600 working hours to determine these "simple" differences. However, some species do clearly prefer to live on the border between forest and grassland, and can therefore be used as a marker hinting towards this habitat.