Designing ecological and biodiversity sampling strategies

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starting out with a limited objective. They would rather ‘keep an open mind’ and seewhat they can see. Of course important discoveries in ecology have been made bychance rather than through planned studies, and every scientist should permanently beopen to the possibility of unanticipated observations, and truly novel explanations.But there are at least four reasons for trying to design a study with specific objectives,including testable hypotheses.1. Without a clear hypothesis, it is impossible to say whether finding no pattern is theresult of none existing or of inadequate (insufficient or inefficient) sampling. There isno basis for evaluating the success of such a study.2. Those serendipitous discoveries that might be made usually have the nature ofhypothesis formulation — observations which suggest explanations. Carefullyplanned studies are needed to test the explanations.3. The proponents of the ‘no hypothesis’ approach actually do have some hypotheses,but these are implicit. For example, without some notion of environmental factors thatmight be controlling biodiversity, it is impossible to choose which of an almostinfinite number of such factors should be measured at sample locations. If the implicithypotheses are made explicit, study designs can be improved.4. If we have specific hypotheses, it is often possible to improve the study design,making the study more efficient.The last point is behind much of what follows in this paper. Suppose the hypothesis isthat an indicator of below ground biodiversity (BGBD) in agricultural plots isdetermined by the level of disturbance (D) and the level of soil organic matter (SOM).If we collect data by SRS or grid sampling, then it is likely that:a) Most sample locations will have values of D and SOM around the average,with relatively few points with very high or low values. But when seekingto understand the relationship between BGBD and SOM or D, it is themore extreme points that provide most of the information (Figure 1).Stratification — dividing the population or study area into sub-populationsand deliberately sampling each — can be used to increase the number ofpoints with more extreme SOM and improve the estimate of therelationship without increasing the number of samples.4
) SOM and D may well be correlated, for example with plots with high Dtypically having low SOM. In such a case, it is hard or impossible todisentangle the effects of the two variables. However, the study could bedesigned to deliberately include some samples with high D and high SOMas well as others with low D and low SOM. Then the effects of bothvariables, and their combined effect, can be estimated.In practice, it may not be possible or useful to produce a single index of BGBD or D,as plotted in Figure 1, and relationships may be more complex than straight lines butthe same principles of design apply.a. b.BGBDBGBDSOMSOMFigure 1. Designs for estimating the relationship between BGBD and SOM.(a) Simple random or grid sampling will probably give most SOM values near theaverage, and a poor estimate of the line. (b) Deliberately including samples moreextreme SOM values through stratification increases the precision of the estimate ofthe line at no extra cost.Another example of a hypothesis implicit in many studies is that of the spatial scale atwhich interesting patterns occur. By choosing the distance between sample locationsand the overall size of the study area, the scientist is making choices and assumptionsabout the important scales to study. If these are made explicitly, then they are open todebate, with a likely improvement in the study design.The overall objective of a project may be to test the hypothesis that increasing landuse intensity changes an indicator of below ground biodiversity, as in the CSM-BGBD project. This is a rather general statement, but can still be helpful in focusingdesign of sampling. Ideally we would investigate it with an experiment. The only5
Page 1: Designing ecological and biodiversi
Page 4 and 5: LIMITED CIRCULATIONCorrect citation
Page 6 and 7: AbstractEmpirical studies of patter
Page 8 and 9: Contents1. Introduction............
Page 11: 2. Study objectives and sampling ba
Page 15 and 16: Step 1: Define objectivesAs outline
Page 17 and 18: ased on scientific grounds. Selecti
Page 19 and 20: A different hypothesis is ‘BGBD i
Page 21 and 22: 5. Focus on objectives: stratificat
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Page 25 and 26: grid. Likewise, it will not be poss
Page 27 and 28: • Not including all land uses fou
Page 29 and 30: ReferencesCochran W.G. (1977). Samp
Page 31 and 32: 25. The role of livestock in integr

Designing ecological and biodiversity sampling strategies

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