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2,2 Procedures for the coUection of data

Chapter 2: mutehaîs and meihods

2.2iv Foraging rates and flower visitation rates

Foraging rate data were based on counts of the number of individual flowers of faba beans or Phacelia visited by nectar gathering bumblebees of different species over a certain period of time. These measurements were used to obtain estimates of flower visitation rates (in numbers of flowers per minute). Foraging rates on faba beans were studied separately for bees making positive visits to flowers and those engaged in nectar robbing.

For the purposes of this study, the flower visitation rate of a bee is defined as the mean number of flowers visited in one minute. Flower visitation rate was estimated from the mean time taken by one individual bee to visit a known number of individual flowers, and includes the flight component as well as the probing component of foraging time (Harder 1983). Timings began at points convenient to the observer during the bee's foraging trip. On some occasions it was possible to time the bee and count its visits throughout an entire foraging trip within the plot; most data, however, represent just a segment of a foraging trip. One flower visit for field beans was recorded for each occasion on which the bee landed on the standard petal, pressed it down with its weight (or forced the lower petals apart with its front tarsi) and attempted to push its head into the corolla opening. For Phacelia, one flower visit was recorded for each time the bee landed on an inflorescence and probed one or more of the florets. Sufficient data were obtained to make estimates of flower visitation rate for positive and negative flower visits to field bean flowers, and for different bee species on Phacelia.

For Phacelia, the flowers of which have an entirely different architecture to field beans (see Plates 1 & 2), additional records were kept of the number of inflorescences visited during a certain time period. Separate flower heads were only counted as inflorescences when the bee was forced to make a short flight to reach them (a criterion used by Prys-Jones, 1986, to differentiate between floral architectural types). Some flower heads were so close together that the bee simply clambered or walked from one to the next, in which case they were counted as a "single" inflorescence.

2.2v Species composition and abundance

Data reflecting the species composition of the foraging community took the form of counts at regular intervals. Following the methods of previous studies of community structure (e.g. Fussell & Corbet 1992), bees were recorded during timed walks. For the faba bean plot, timed walks lasted exactly 20 minutes at a constant slow pace, and were carried out at hourly intervals. For each observation of a foraging bee a record was made of its species, foraging behaviour (e.g. positive or negative flower visits or visiting extrafloral nectaries) and the cultivar on which it was observed. Each walk involved on average 2 circuits of the plot including diversions into the lateral and longitudinal paths.

Timed walks in the Phacelia field also lasted 20 minutes and followed the western margin of the field so as not to cause damage to the crop. Because of the large numbers of bees active on Phacelia at any one time, thorough counts and species records were made at regular intervals of three paces. Timed walks on Phacelia were carried out at intervals of 1-2 hours. For both field beans and Phacelia, only bees that were actively foraging on the crop were counted as 'observations'.

2.3 Analytical Methods

Numeric data was analysed using Minitab (Release 8.2). Standard statistical methods were applied in an attempt to highlight and to define the most apparent trends and relationships within the data. Statistical analysis focused on points which were felt to be of potential relevance during the field observations. Much of the reasoning conceived prior to field observations was discarded afterwards, as the observations themselves had brought to light more pertinent questions.

2.3i Statistical procedures

Simple statistical methods such as analysis of variance and regression were used wherever possible. This was partly due to the volume of the data, and partly to the conviction that biological relationships within ecosystems can be described without the need for complicated diagnostic tools. Before applying any statistical methods.

Chapter 2: materials and methods

individual datasets were first displayed graphically or as a table and studied by eye in order to obtain an impression of possible patterns and interactions. For most datasets, for example those concerning nectar volumes and concentrations, foraging rates, floral dimensions and floral densities, a normal distribution function was assumed. For the situations in which a normal population could not be assumed, equivalent nonparametric techniques were applied.

2.3ii Combining datasets

Several different types of data were collected over a number of days, for example nectar data and foraging rate data. In some cases it was necessary to pool individual datasets from different days in order to obtain a larger picture of events, for example the species composition data for the bee community in the field bean plot or on the Phacelia crop as a whole, or to search for differences in nectar production between two cultivars which were studied on different days. Since the data were collected over a period of several weeks, during which weather conditions ranged from hot and very dry to cool, overcast and humid, the climatic data for each day were used as a benchmark to compare and combine different datasets. Ambient temperature and ambient relative humidity were strongly and negatively associated (p = 0.000, R2 = 61.6%) across all days of the study (see Fig. 2.2),

2.3iii Problems encountered during the analvsis

The main obstacle to the analysis arose from the number of experimental cultivars studied in the plot, and from the fact that for any one factor, multiple options existed. For example, any one of five bumblebee species observed to be foraging on field beans could have been visiting any one of twelve cultivars, and might have displayed one of at least three different types of foraging strategy while visiting flowers for nectar. Not only did ambient temperatures and relative humidities, light intensity and air movement differ from day to day, but there were concurrent changes in flowering stage for the different cultivars on the plot and changes in the community of foragers reflecting seasonal changes in the colony cycle of each species. For some such situations, interpretations have been offered based on a graphical presentation of the data. Another significant problem arose from the non-independence of datapoints. This made it difficult to isolate effects; for example, when studying the effect of

ambient temperature and nectar concentration on numbers of bees. In situations where two or more variables of high potential predictive value were known to be highly correlated, simple graphical methods were used to display the data in relation to each predictor variable.

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Ambient temperature (degrees C) R.H. = -3.303(Ta) + 122.826

Fig. 2.2 The relationship between ambient temperature (''C) and relative

humidity (%) 30/6-28/7/95.