Recommendations for depletion modelling of granivorous birds
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Models exist for a range of common weeds, including Alopecurus myosuroides (Doyle et al. 1986), Anisantha sterilis (Lintell Smith et al. 1999), Avena sterilis (Perry & Gonzalez-Andujar 1993), Avena fatua (Cousens et al. 1986), Chenopodium album (Freckleton & Watkinson 1998a), Galium aparine (Lintell Smith 1995), and Poa annua (Law 1981; Munier-Jolain et al. 2002). Of these species two (C. album and P. annua) are regarded as useful sources of food for seed-eating farmland birds. We therefore adapted the models for these two species in constructing the full model for the weed community (Objective 12). We also adapted the model for Alopecurus myosuroides because, although this species is not a valuable food resource for birds, it is of economic significance, particularly in cereals and it was felt that at least one such species should be included in the model output. 4.3 Discussion and interpretation The information collated has yielded key baseline data on the dynamics of seed production of a range of weed species, and we have used this information to develop a model predicting weed seed production and dynamics over the winter period when farmland birds are feeding on weed seeds (Objective 12). Understanding the dynamics of weed seed production not only requires an understanding of the numbers of seeds produced by individual plants, but also of the factors determining plant density and the long-term survival of seeds in the soil. Based on previous experince (e.g. Freckleton & Watkinson 1998b; Lintell-Smith et al. 1999; Watkinson & Freckleton 2001), we were able to identify which parameters need to be estimated. The main results of the work are: (i) estimates of seed production for major weed species; (ii) estimates of seed mortality and germination for major weed species; (iii) estimates of the effects of management, in the form of cropping practice, on seed production. 4.3.1 Pattern of seed production Seed production by weeds is notoriously high (Salisbury 1942) and weeds of all kinds potentially produce thousands of seeds. This high seed production means that weed populations are potentially able to recover rapidly following depletion to low levels. Additionally, even single weed plants are capable of producing a large number of seeds that may be valuable as food for birds. However, seed production in the crops is critically dependent on the ability of plants to survive in the crop following management. Thus the dynamics of seed through the winter period during which birds are feeding cannot be separated from the impacts of management on weed plant survival. Winter stubbles are potentially very valuable in generating small numbers of seeds (compared with production in the crops) during the winter that could represent an important source of food for birds during the winter. A recent DEFRA project has explored this issue in considerable detail, although much work remains to be done in this area. Although winter stubbles currently represent only a small proportion of winter fields, it could be that increases in the proportion of winter stubbles could have important positive effects on seed abundance during winter. We address this issue with the simulation model (Objective 12).
Long-term declines in weed abundance are potentially reversible because the seed bank represents a potentially buffering reserve. However, the results on seedbank decline indicate that declines are likely to be reversible only in the short to mid term (less than 20 years). Over longer terms the extinction of weed species will occur if seed production does not occur or is reduced. Species producing small numbers of seeds per plant, or those which have seeds that are not persistent in the soil, will be particularly vulnerable to eradication. Again, a small rate of seed production by weeds in winter stubbles could play an important role in alleviating such declines. We also explore this issue in the modelling exercise (Objective 12).
Weed population models are not at the stage where exact densities of weeds can be predicted under particular conditions with any confidence, and probably never will be (Cousens 1995; Freckleton & Watkinson 1998b; 2002; Watkinson & Freckleton 2001). Successful applications of weed population modelling tend to be more strategic (e.g. see Watkinson, Freckleton & Dowling 2000 for a review of examples). The success of such modelling approaches depends on being able to reasonably accurately characterise broad responses of species to management, as well as to measure key demographic parameters in order to parameterise models. We apply this approach in the development of the simulation model (Objective 12). 4.4 References Barralis, G., Chadoeuf, R., & Gouet, J.P. (1986) Essai de détermination de la taille de l'échantillon pour l'étude du potential semencier d'un sol. Weed Research, 26, 291-297. Cousens, R., Doyle, C.J., Wilson, B.J., & Cussans, G.W. (1986) Modelling the economics of controlling Avena fatua in winter wheat. Pesticide Science, 17, 1-12. Cousens, R., Firbank, L.G., Mortimer, A.M., & Smith, R.G.R. (1988) Variability in the relationship between crop yield and weed density for winter wheat and Bromus sterilis. Journal of Applied Ecology, 25, 1033-1044. Cousens, R. & Mortimer, A.M. (1995) Dynamics of weed populations Cambridge University Press, Cambridge. Doyle, C.J., Cousens, R., & Moss, S.R. (1986) A model of the economics of controlling Alpecurus myosuroides Huds. in winter wheat. Crop Protection, 5, 143-150. Ervio, L. (1971) The effect of intra-specific competition on the development of Chenopodium album L. Weed Research, 11, 124-134. Firbank, L.G., Manlove, R.J., Mortimer, A.M., & Putwain, P.D. (1984) The management of grass weeds in cereal crops, a population biology approach. In 7th International Symposium on Weed Biology, Ecology and Systematics, pp. 375-384. Proceedings of the 7th International Symposium on Weed Biology, Ecology and Systematics. Firbank, L.G. & Watkinson, A.R. (1985) On the analysis of competition within two–species mixtures of plants. Journal of Applied Ecology, 22, 503-517. Firbank, L.G. & Watkinson, A.R. (1986) Modelling the pouplation dynamics of an arable weed and its effects upon crop yield. Journal of Applied Ecology, 23, 147-159. Freckleton, R.P. & Watkinson, A.R. (1998) How does temporal variability affect predictions of weed population numbers ? Journal of Applied Ecology, 35, 340-344. Freckleton, R.P. & Watkinson, A.R. (1998) Predicting the determinants of weed abundance: a model for the population dynamics of Chenopodium album in sugar beet. Journal of Applied Ecology, 35, 904-920. Freckleton, R.P. & Watkinson, A.R. (2002) Are weed population dynamics chaotic? Journal Applied Ecology. Froud-Williams, R.J. (1985) Dormancy and germination of arable grass weeds. Aspects of Appled Biology, 9, 1-19. Froud–Williams, R.J., Chancellor, R.J., & Drennan, D.S.H. (1983) Influence of cultivation regime upon buried weed seeds in arable cropping sysytems. Journal of Applied Ecology, 20, 199-208. Grime, J.P., Hodgson, J.G., & Hunt, R. (1988) Comparative Plant Ecology Unwin, London. Harris, G.R. & Lovell, P.H. (1980) Adventitous Root Formation in Veronica spp. Annals of Botany, 45, 447-458. Harrison, S.K. (1990) Interference and seed production by common lambsquarters (Chenopodium album L.) in soybeans (Glycine max). Weed Science, 38, 113-118. Hume, L., Martinez, J., & Best, K. (1983) The biology of Canadian weeds. 60. Polygonum convolvulus Canadian Journal of Plant Science, 63, 959-971. Jaggli, B. (1992) Ber Geobot Inst ETH, 58, 86-100. Krebs, J.R. Wilson J.D., Bradbury R.B., & Siriwardena G.M. (1999) The second silent spring? Nature 400, 611-612. Law, R. (1981) The dynamics of a colonizing population of Poa annua. Ecology, 62, 1267-1277. Lawson, H.M. & Wright, G.M. (1993) Seedbank persistence of five arable weed species in autumn-sown crops. In Brighton Crop Protection Conference - Weeds, pp. 305-310, Brighton. Lintell Smith, G., Freckleton, R.P., Firbank, L.G., & Watkinson, A.R. (1999) The population dynamics of Anisantha sterilis in winter wheat: comparative demography, and the role of management. Journal of Applied Ecology, 36, 455-471. Lintell-Smith, G. (1995) The population dynamics of weeds in winter wheat. Ph.D, University of East Anglia. Lonchamp, J.-P. (1988) Effets de l'enfouissement des semences d'Aethusa cynapium, Chenopodium album, Euphorbia exigua et Sinapis arvensis sur leur capacité germinative et leur levée au champ. Agronomie, 8, 591-601. Malik, N. & Vanden Born, H. (1988) The biology of Canadian weeds 86. Galium aparine L. and Galium spurium L. Canadian Journal of Plant Science, 68, 481-499. Mulligan, G.A. & Bailey, L.G. (1975) The biology of Canadian weeds, Sinapis arvensis Canadian Journal of Plant Science, 55, 171-183. Munier-Jolain, N.M., Chauvel, B., & Gasquez, J. (2002) Long-term modelling of weed control stategies: analysis of threshold-based options for weed species with contrasted competitive abilities. Weed Research, 42, 107-122. Perry, J.N. & Gonzalez-Andujar, J.N. (1993) Dispersal in a metapopulation neighbourhood model of an annual plant with a seedbank. Journal of Ecology, 81, 453-463. Rees, M. & Long, M.J. (1993) The analysis and interpretation of seedling recruitment curves. The American Naturalist, 141, 233-262. Roberts, H.A. (1958) Studies on the weeds of vegetable crops. I. Initial effects of cropping on the weed seeds in the soil. Journal of Ecology, 46, 759-768. Roberts, H.A. (1963) Studies on the weeds of vegetable crops. III. Effects of different primary cultivations on the weed seeds in the soil. Journal of Ecology, 51, 83-95. Roberts, H.A. (1964) Emergence and longevity in cultivated soil of seeds of some annual weeds. Weed Research, 4, 296-307. Roberts, H.A. (1968) The changing population of viable weed seeds in an arable soil. Weed Research, 8, 253-256. Roberts, H.A. (1970) Viable weed seeds in cultivated soils. Report on the National Vegetable Research Station, 1969, 23-28. Roberts, H.A. & Dawkins, P.A. (1967) Effect of cultivation on the numbers of viable weed seeds in soil. Weed Research, 7, 290-301. Roberts, H.A. & Feast, P.M. (1972) Fate of seeds of some annual weeds in different depths of cultivated and undisturbed soil. Weed Research, 12, 316-324. Roberts, H.A. & Feast, P.M. (1973) Changes in the numbers of viable weed seeds in soil under different regimes. Weed Research, 13, 298-303. Roberts, H.A. & Feast, P.M. (1973) Emergence and longevity of seeds of annual weeds in cultivated and undisturbed soil. Journal of Applied Ecology, 10, 133-143. Roberts, H.A. & Neilson, J.E. (1980) Seed survival and periodicity of seedling emergence in some species of AtriplexI, Chenopodium, Polygonum and Rumex. Annals of Applied Biology, 94, 111-120. Robinson, R.A. & Sutherland, W.J. (2001) Post war changes in arable farming and biodiversity in Great Britain. Journal Applied Ecology, 39, 157-176. Salisbury, E. (1964) Weeds and aliens Collins, London. Salisbury, E. (1976) Seed output and the efficacy of dispersal by wind. Proceedings of the Royal Society, Series B, 192, 323-329. Salisbury, E.J. (1942) The Reproductive Capacity of Plants Bell and Sons, London. Stevens, O.A. (1932) The weight and number of seeds produced by weeds. American Journal of Botany, 19, 784-794. Watkinson, A.R. (1981) Interference in pure and mixed populations of Agrostemma githago. Journal of Applied Ecology, 18, 967-976. Watkinson, A.R. & Freckleton, R.P. (2001) Climate change and weed populations. BCPC proceedings, in press. Watkinson, A. R., Freckleton, R. P. & Dowling, P. M. (2000) Weed invasions of Australian farming systems: from ecology to economics. In: "The economics of biological invasions", Perrings, C., Williamson, M. Dalmazzone, S. & Mooney, H.A. (eds) Edward Elgar Publishing, Cheltenham, UK. Wilson, J.D., Morris A.J., Arroyo B.E., Clark S.C., & Bradbury R.B. (1999) A review of the abundance and diversity of invertebrate and plant foods of granivorous birds in northern Europe in relation to agricultural change. Agriculture Ecosystems and Environment 75, 13-30 Wilson, B.J. & Lawson, H.M. (1992) Seedbank persistence and seedling emergence of seven weed species in autumn-sown crops following a single year's seeding. Annals of Applied Biology, 120, 105-116. APPENDIX 5 Propose groupings for different taxa in terms of thier value and availability as a food source for arable farmland birds. 5.1 Rank invertebrates according to the proportion in the diet for each bird species. The methods used were the same as for Task 3.1. An extensive literature review was carried out and quantitative dietary information was collected for 22 farmland bird species (see Table 3.1). Analysis of crop and stomach contents was the most commonly used sampling method for investigating the invertebrate component of the diet, and faecal analysis was also frequently used, particularly for chicks (Table 5.1). Table 5.1. Number of references containing information on the invertebrate taxa eaten by farmland birds, and the sampling methods used to estimate dietary composition.
The degree of taxonomic resolution varied considerably between studies making it difficult to create rankings for invertebrates. A few references listed taxa to species level, but the majority only identified invertebrates to the level of Order. The difficulty in identifying invertebrate fragments was probably a limiting factor, as was the entomological knowledge of the researchers. Because individual species were not identified in most studies, analysis was not possible at this level and importance rankings could only be determined for invertebrate Orders. Considerable variation exists between the size and consequently nutritional value of invertebrates at this level of taxonomic resolution therefore the results have limited value. In addition some biases may have occurred because of the way in which the diet was recorded (% of items, % biomass) because the methods are not directly comparable. Moreover the individual methods of dietary analyses have limitations in their accuracy. For example, soft bodied insects are under-estimated or absent from faecal samples. Rankings were calculated in two different ways: for all bird species combined (Section 5.1.1), and for each bird species individually (Section 5.1.2) (see Appendix 3, Section 3.1 for methods). 5.1.1 ranking invertebrate taxa which are important for all bird species Rankings were determined separately for adult diet in the breeding and non-breeding seasons and for chicks, and invertebrates are listed below from highest (rank 1) to lowest importance (Table 5.2). An overall ranking for each Invertebrate Order was also calculated by averaging the rankings from the three bird categories (adults – breeding, adults – non-breeding, and chicks). The most important invertebrate Orders overall in descending value were Coleoptera adults (average ranking of 1.3), Hemiptera adults (3.0), Arachnida (3.0+), Lepidoptera larvae/pupae (3.3), Diptera adults (4.0), Lepidoptera adults (5.5+), Diptera larvae/pupae (7.0), and Hymenoptera1 adults (7.5+). However, Lepidoptera adults, Hymenoptera1 adults, and Arachnida were not important food items during the non-breeding season, when a smaller number of invertebrate taxa were important as food (only 11 taxa, c.f. up to 28 taxa in the breeding season). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||