Fossil chironomid from the Andes
CHIRONOMIDAE AS A PALAEO-ECOLOGICAL TOOL
Chironomidae is a family of two-winged flies more commonly referred to as non biting midges. This diverse group of insects have been known for a long time to be sensitive environmental indicators. Early research in the field showed that the trophic status of lakes could be classified according to the characteristic chironomid assemblages found within them (Thienemann, 1922). Furthermore the head capsules of the larvae are well preserved within the sedimentary record. As a result palaeolimnological researchers became increasingly interested in the potential for using Chironomids to track the trophic development of a lake through time by examining the changing assemblages within the accumulated sediments. With geographically close lakes displaying significantly different midge faunas the potential for the insects being used as climatic indicators was dismissed and the following hypothesis became established: Chironomid assemblage composition reflects in-lake variables, e.g. lake depth, pH, dissolved oxygen, trophic status and substrate. However work by Walker and Matthews (1989) demonstrated that temperature was by far the most significant variable in controlling the broad scale distribution and abundance of midge fauna.
Walker and Matthews realised the potential for the non biting midge to be used as a palaeoclimatic indicator from two initial observations. Firstly within the fossil records, as climate began warming following the deglaciation of the northern hemisphere, the relative abundance of taxa associated with cold oligotrophic lakes (Heterotrissocladius) abruptly declined. Secondly they noticed the best analogues for late glacial assemblages were found in modern day arctic and alpine settings. Overall Walker and Matthews concluded that the northern limit of temperate taxon was controlled by cold summer air and/or water temperatures. The southern limit of Arctic species was instead driven by cold oxygenated refugia in the profundal zone of deep, temperate lakes. These temperatures were significant with respect to the insect’s life cycles as many species require critical temperature thresholds to complete pupation and emergence stages.
Since the pioneering work of Walker and Matthews (1989) and others the debate linking Chironomids to temperature has raged. Debate has centred upon what controls chironomid distribution and how suitable, if at all, the insects are in the context of palaeoecological studies. Recently Velle et al. (2010) discussed some key factors which must be considered when working on chironomid based temperature resonstructions.
Below I present some of the debate around the midge-environment-temperature debate; focusing on both midge distribution and identification and the potential of this proxy as a indicator of past environmental and climatic change.
CHIRONOMID BASE PAST ENVIRONMENTAL AND CLIMATE RECONSTRUCTIONS
Quantitative studies using Chironomids rely heavily on the training set/transfer function method (Brooks, 2005). This approach applies our modern understanding of species distribution to assemblage changes we see through time. The training set represents the modern species; it combines fauna and environmental data from lakes along the environmental gradient of interest. Once developed the past changes in the desired variable can be reconstructed. The transfer function applied to the down core section is dependant on the assumption that species tolerances do not vary through time. Therefore a fossil assemblage which is visible in the modern training set is responding to the same environmental variables present today and can be used to infer past conditions.
Such reconstructions have produced fantastic records which correlate with independent proxies; for example comparison with the stable isotope record from Greenland show that the method has successfully reconstructed fluctuations during the Lateglacial and follow broad scale temperature changes throughout the Quaternary (Heiri et al. 2007). Despite the strong independent correlation Velle et al. note several factors which, if overlooked, may affect the validity of a midge based temperature reconstruction. These are:
- spatial heterogeneity of organisms,
- the use of too coarse taxonomic resolution,
- inaccurate chronology,
- the choice of numerical models, and
- the complexity of ecological interactions between biological communities.
DISTRIBUTION CONTROLS
Velle et al. argue that the success of chironomid-temperature reconstructions is a consequence of a biased training dataset, i.e. because the temperature gradient is maximised it has a false influence over the reconstruction. However, Brooks et al. (2011) point out in their critique, chironomid-temperature performance statistics far outcompete those designed to infer other environmental variables (e.g. salinity, pH).
The influence of other environmental variables no doubt influences Chironomid distribution and will consequently affect the accuracy of any temperature reconstruction (Brooks et al. 2007). However, it can be adequately argued that they are perhaps over emphasised and certainly do not surpass temperature in being the driving variable. Trophic status has the potential to disrupt fauna during the Holocene: a time of significant deglaciation changing the surrounding landscape. Evidence however suggests the contrary; few lakes studied show unrealistically high Holocene temperatures which would be attributed to productivity. Crucially as Brooks et al. (2011) highlight many of the published reconstructions adequately reproduce known climate oscillations at 11.3, 10.3, 9.3 and 8.2 ka BP (thousand years before present).
IDENTIFICATION
Chironomid taxa are notoriously difficult to indentify to species level; commonly individuals can only be resolved to generic level. The degree of taxonomic resolution will ultimately influence the environmental conclusions made. Velle et al. argues that taxonomic inconsistencies compromise chironomid reconstructions to the point were low taxonomic resolution is actually recommended. Brooks et al. (2011) argues the contrary; coarser taxonomy reduces the specificity of environmental information taken from an assemblage. The weight of literature regarding this issue highlights a clear correlation between greater performance statistics and high taxonomic resolution.
CONCLUSIONS
The work produced by Velle et al. (2010) highlights the importance of a good understanding of chironomid ecology and many of the points highlighted are widely acknowledged and accepted by the research community. However, to a non specialist the paper could suggest that midges are not actually suitable tools for palaeoclimatic work at all and instead have been moulded to fit the wishes of the palaeoclimatologist. The weight of evidence suggests the contrary; chironomids do have the same problems associated with other proxies but also show a clear response to temperature which can be exploited effectively. As research continues issues such as taxonomy, model validity and driving variables will all slowly become more and more understood. Rather than presenting midges as flawed tool in palaeo studies it is much fairer to emphasize their true potential. Very few aquatic habitats do not support a Chironomid population; they have a global distribution and have done for millennia. The midge is potentially a palaeolimnologists strongest tool for reconstructing temperature change.
Over the course of my PhD I will deveop a Chironomid training data set for the Andes and hopefully apply this to the fossil record to test palaeo-envrionmental and climatic reconstructions from other proxies.
Brooks S.J, Axford Y, Heiri O, Langdon PG, Larocque-Tobler I. 2011 Chironomids can be reliable proxies for Holocene temperatures. A comment on Velle et al., 2010. in press, The Holocene
Thienemann, A., 1922. Die beiden Chironomusarten der Tiefenfauna der norddeutschen Seen. Ein hydrobiologisches Problem. Archiv fur Hydrobiologie 13, 609-646.
Ecology of the past 