Recommendations for depletion modelling of granivorous birds
|
|  Yüklə 6.33 Mb.
|
|
Fig. 4.4 summarises the mean densities of weeds in the crops used in the IFS experiment. It is clear that densities of all species are generally very low. Only Poa annua is generally common in several crops, otherwise most species tend to be restricted to one or two crops. In terms of predicting seed densities, the patterns in Fig. 4.4 have to be viewed with caution for two reasons. Firstly, weed species vary hugely in seed production (Table 4.1). when seed production is calculated (Fig. 4.5) a very different pattern emerges and it appears that a number of crops are potentially capable of yielding high densities of seeds. Secondly, the mean density of plants may be a poor indicator of dynamics, since there may exist considerable variability in numbers. As shown in Table 4.6, the maximum density of weeds may be several orders of magnitude larger than the mean seed density. This may be of enormous significance to farmland birds in winter, which may rely disproportionately on such high density patches, for instance because they are easier to locate.  F      ig. 4.4 Densities of weed seedlings in the IFS experiment. These densities are used to infer the intensity of management on weed plant density, and hence weed winter seed production in the modelling exercise (Objective 12). (a) winter wheat; (b) spring barley; (c) winter oilseed rape; (d) potatoes; (e) spring peas. The filled bars represent integrated management, the open bars represent conventional systems. Fig. 4.5 Estimated maximum seed rain at the start of winter in the IFS experiment. These densities were estimated using the seed production data (Table 4.1), and assuming all weeds present in the spring survive to autumn. In practice mortality will occur and hence realised densities may be lower. (a) winter wheat; (b) spring barley; (c) winter oilseed rape; (d) potatoes; (e) spring peas. The filled bars represent integrated management, the open bars represent conventional systems.       Table 4.6 Mean and maximum densities of weeds in the IFS experiment (averaged across farming systems).
|