Academic journal article Estonian Journal of Ecology

Morphological Alteration of Land Reclamation Canals by Beavers (Castor Fiber) in Lithuania/Kopra (Castor Fiber) Tegevuse Moju Kuivenduskraavide Struktuurile Leedus

Academic journal article Estonian Journal of Ecology

Morphological Alteration of Land Reclamation Canals by Beavers (Castor Fiber) in Lithuania/Kopra (Castor Fiber) Tegevuse Moju Kuivenduskraavide Struktuurile Leedus

Article excerpt

INTRODUCTION

Land reclamation canals comprise more than 80% of the hydrographical network of Lithuania (Gailiusis et al., 2001). Many of these canals were abandoned due to unsuitable soil or infrastructure conditions necessary for intensive agriculture and are no longer maintained. These canals are deteriorating and beaver activity is accelerating this process (Kvaraciejus, 2001). Land reclamation canals are attractive habitats for these semi-aquatic mammals (Balodis, 1990). In the last decades more than one third of the Lithuanian beaver population was estimated to inhabit these canals (Balodis et al., 1999; Ulevieius, 1999; Ulevieius, unpubl.). The intensive impacts of beavers on the drainage canals are well documented in regions of developed agriculture in central Lithuania (Lamsodis, 2000, 2001).

The newest estimates of beaver abundance in Lithuania range from 80 000 to 120 000. These numbers result in a mean population density of approximately 16 individuals/1000 ha (Ulevieius, unpubl.). Observations of habitat distribution and beaver abundance indicate a considerable influence of these animals on different characteristics of land reclamation canals in Lithuania. Beavers are considered as important geomorphologic agents in aquatic environments, due to their dam building activity and its consequences (flooding effects, sedimentation) (Butler, 1995, for review).

Beaver activities in drainage canals can be controversial from both economic and ecological points of view (Balodis, 1990). Re-naturalization of the ecologically low-valued drainage canals can be considered as a benefit for ecosystem functioning (Balodis, 1990). However, economically re-naturalization can be a significant loss due to the initial drainage system installation expenses and damage to agricultural land. In this paper we examine the aspect of ecosystem function and concentrate on how beavers alter anthropogenic structures increasing their ecological value.

This paper describes and quantitatively evaluates specific aspects of beaver impacts on land reclamation canals in a human-transformed landscape with a high-density beaver population.

STUDY AREA

Lithuania is approximately 65 000 [km.sup.2] in area with a hydrographical network of approximately 77 000 km (mean density 1.18 km/[km.sup.2]). Streams [greater than or equal to] 3 km make up 49% of the hydrographical system and streams < 3 km account for 51% of the total length. Streams with a regulated bed (human-managed) comprise 82.6% and streams with a natural bed 17.4% of the total length of the hydrographical network (Gailiusis et al., 2001).

Canals that are 30 years old or older prevail in the hydrographical network of Lithuania. This creates a relevant problem of canal maintenance, especially if canals have been abandoned for a long time. The banks of the abandoned canals are usually overgrown by woody vegetation (Lamsodis et al., 2006) and have become a suitable habitat for beavers.

A study area of 625 [km.sup.2] (25 x 25 km) situated in central Lithuania (Fig. 1) (centre point co-ordinates: 55[degrees]37' N, 24[degrees] 11' E) was examined to evaluate the density of the beaver population and the extent of beaver activity in the drainage canals. First-order and second-order tributaries prevail among drainage canals in the study area. The density of the hydrographical network in this area is ca 1.2 km/[km.sup.2]. The major part is comprised of forest and riparian sections of canals (80%), whereas natural streams are of relatively little importance (ca 16%). Only a small part (ca 4%) is made up of lakes, reservoirs, and other water bodies (oxbows, peatland excavations, small artificial ponds). The territory is intensively exploited for agriculture.

Damming, burrowing, and other geomorphological activities of beavers in drainage canals were investigated also in various other areas, scaterred throughout the country. …

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