Ecological impact of beavers Castor fiber and Castor canadensis and their ability to modify ecosystems

• 1 The genus Castor comprises two species: the Eurasian beaver Castor fiber, and the North American beaver Castor canadensis. Both species suffered from overexploitation, but have seen a revival since the 1920s due to increased protection and reintroduction programmes. Increases in the populations and distributions of species that are able to modify ecosystems have generated much scientific interest. Here we review the available literature concerning the possible ecological impact of beaver species in the Old and New World.
• 2 Beavers, being ecosystem engineers, are among the few species besides humans that can significantly change the geomorphology, and consequently the hydrological characteristics and biotic properties of the landscape. In so doing, beavers increase heterogeneity, and habitat and species diversity at the landscape scale. Beaver foraging also has a considerable impact on the course of ecological succession, species composition and structure of plant communities, making them a good example of ecologically dominant species (e.g. keystone species).
• 3 Nevertheless, the strength of beavers’ impact varies from site to site, depending on the geographical location, relief and the impounded habitat type. Consequently, they may not be significant controlling agents of the ecosystem in all parts of their distribution, but have strong interactions only under certain circumstances. We suggest that beavers can create important management opportunities in the Holarctic, and this review will help land managers determine the likely outcome of beaver activity.

     

Phosphorus and nitrogen relationships of Cladophora glomerata in two lake basins of different trophic status

• 1 Cladophora has increased in abundance and cover in the South Basin of Windermere, English Lake District, in recent years. During the growing seasons of 1992 and 1993, the maximum biomass of Cladophora in the South Basin, at nearly 200 g dry weight m^-2, was nearly six times greater and the percentage cover, at 95%, was nearly ten times greater than in the North Basin.
• 2 Nutrient analysis showed that Cladophora from the South Basin had significantly higher average contents of N than that from the North Basin. The suggested rate-limiting tissue N content, 1.3% of dry weight, was never reached in the South Basin but was reached in 20% of samples from the North Basin. In contrast, the average P content was not significantly different, at about 0.24% in both basins. The suggested rate-limiting P content, 0.16% of dry weight, was reached in 31% of the samples from the South Basin and 37% from the North Basin.
• 3 On average, the standing stock of P in the South Basin was 0.12 g m^-2, twelvefold that in the North Basin at 0.01 g m^-2 and the standing stock of N in the South Basin was 1.81 g m^-2, elevenfold that in the North Basin at 0.16 g m^-2. Maximum standing stocks of P were 0.38 g m^-2 in the South and 0.05 g m^-2 in the North Basin. Maximum standing stocks of N were 5.02 g m^-2 in the South and 0.60 g m^-2 in the North Basin.
• 4 The maximal rate of PO₄–P uptake ranged between 298 and 2949 μg P g^-1 DW h^-1 and the specific affinity varied between 2.9 and 24.3 μg P g^-1 DW h^-1 (mg PO₄–P m^-3)^-1. The higher values of both characteristics are within the range which is believed to indicate severe P limitation.
• 5 Using the nutrient uptake characteristics, rates of uptake at pelagic and littoral concentrations of PO₄–P were calculated to be between 0.1 and 1.7% of the maximal rates apart from one site where rates were up to 5.6% of V_maxi and 40% of V_maxi based on pelagic and littoral concentrations of PO₄–P, respectively. Despite these generally low rates of uptake, calculations suggest they were just sufficient to account for the measured rate of increase of P standing stock, assuming no biomass loss. Similar calculations suggested that pelagic concentrations of NO₃–N could support between 21 and 30% of maximal rates and these were between four and six times (North Basin) and twenty-five times (South Basin) the observed rates of increase in N standing stock. Littoral concentrations of NO₃–N were similar and so would have allowed similar rates.
• 6 Evidence from nutrient tissue contents, standing stocks and rates of uptake suggests that the growth of Cladophora, in both basins of Windermere, is controlled primarily by the availability of P.

     

Parasitism of grain legumes by Alectra species (Scrophulariaceae)

In glasshouse pot experiments in the United Kingdom, the host preference of nine seed samples of Alectra vogelii from Eastern, Western and Southern Africa and of two samples of A. picta from Cameroon and Ethiopia, to cultivars of cowpea, groundnut, bambara and mung bean, was assessed. A susceptible cowpea cultivar, Blackeye, and four cultivars of groundnut were attacked by all samples of both parasitic species regardless of whether the host of origin was cowpea, groundnut or bambara. Five “strains” of A. vogelii were distinguished using two criteria: their ability to parasitise bambara and/or mung bean and their ability to parasitise cowpea B301 and bambara TVU 870. The latter proved in an associated experiment to be resistant to collections of the parasite from some locations. Strain 1, including populations from Mali, Nigeria and Cameroon, attacked all groundnuts, cowpea cultivar Blackeye, but not cowpea line B301, mung bean or bambara. Strain 2, from Botswana, differed in attacking B301 and mung bean. Three other strains were identified which attacked susceptible lines of all four legume species. Strain 3 from Kenya failed to attack either cowpea B301 or bambara TVU 870, strain 4 from Malawi attacked cowpea B301, but not bambara TVU 870, while strain 5 from Northern Transvaal, South Africa, attacked bambara TVU 870, but not cowpea B301. Cowpea B359 was resistant to A. vogelii samples from all locations and also to A. picta, which has a similar host preference to strain 1 populations of A. vogelii from West Africa. Two out of 13 groundnut lines tested showed low susceptibility to A. vogelii from Cameroon suggesting there is scope for selecting resistance in this crop also.     

The effect of temperature upon intensity of colour in the coronas of cut blooms of narcissus var. Soleil d'Or

Cut blooms of narcissus Soleil d'Or were subjected to constant temperatures ranging from 1.2° C. to 18.3° C. both in darkness and under illumination. The rate of opening of successive florets, the rate and final degree of colour development of the corona and subsequent display life were observed. Opening was negligible at 1.2° C. and 38 days were taken to open to five florets at 4.5° C. Full opening occurred in 4 days at 15.6° C. The change of colour of the corona from yellow to orange was negligible at 15.6° C, but final intensity of orange colour increased with decreasing temperatures. Temperature affected colour development both of opened and unopened florets, and after treatment at low temperature colour continued to develop when the blooms were removed to 18.3° C. The period of low-temperature treatment giving maximum colour development was 8 days. Artificial illumination of intensity 200 ft.-candles had little effect at 18.3° C. but it increased orange colouring at the lower temperatures. Increased orange colour whether at intermediate or low temperature was shown to be associated with a slight increase in the concentration of the total carotenoids, but more especially with greatly increased content of β-carotene which constituted approximately 10–20% of the total carotenoids.