Conceptos básicos

1.8.1 Biological invasions and ecosystem engineering

Biological invasions are regarded as natural processes but the current rates of species invasions around the globe are wholly unprecedented (Williamson 1996, Vitousek et al.,

1997). Alteration of ecosystems by the activities of exotic invaders can be dramatic (Crooks 2002), affecting resource availability for other species, altering the flow of energy or biomass, changing food webs, and even changing the physical structure of the ecosystem itself (Simberloff 1991, Crooks 2002). In Britain the water plants, New Zealand Pigmy Weed Crassula helmsii and Parrot’s Feather Myriophyllum aquaticum are well-known habitat modifiers. These plants create thick beds that limit water movement and light penetration, but they also offer habitat for invertebrates and predation refugia for fish (Schmitz et al., 1993, 1997, Crooks 2002).

Biological invasions are a major cause of biodiversity loss (Willby 2007). Their impacts on native biodiversity include displacement of indigenous species through competition or predation, structural damage to aquatic habitats, and a loss of genetic integrity. Invasive species are of critical concern to conservation bodies worldwide (Willis & Birks 2006) as they can often threaten native species with extinction (Gurevitch & Padilla 2004). Invasive alien species are assigned with an ‘Impact Status’ according to risk assessments by the UK Environment Agency (Environment Agency, Water Framework Directive Programme, 2004). Water bodies most vulnerable to biological invasions are often subject to multiple pressures for which the key drivers are agricultural intensification and urbanisation (Kercher & Zedler 2004, Ervin et al., 2006). The presence of invasive species can, therefore, impair the ecological status of such sites. In an attempt towards urgent and effective control of invasive aquatic plants, together with post-control recovery of native communities, the European Water Framework Directive requires the restoration of degraded water bodies to ‘good ecological status’ by 2015 (EU 2000/60/EC).

1.8.2 Invasive free-floating plants

The invasion of dense mats of free-floating plants is acknowledged as among the most important threats to the functioning and biodiversity of freshwater systems (Scheffer et

al., 2003). Duckweeds (Lemnaceae) are well known to cause physico-chemical changes

in the water beneath them (Pokornŷ & Rejmankova 1983, Goldsborough 1993, Portielje & Roijackers 1995) by interfering with light penetration, reducing photosynthetic active

radiation (PAR) by up to 99% with associated temperature fluctuations which can lead to diurnal temperature stratification (Dale & Gillespie 1976, Goldsborough 1993). Moreover, they reduce gaseous exchange causing the predominance of respiratory activity beneath the mats by reducing dissolved oxygen and increasing carbon dioxide levels (Janes 1998), causing a reduction in pH (McLay 1976, Janes 1998) and an increase in conductivity (Sayer et al., unpublished data). Furthermore, dense duckweed mats are detrimental to ecological structure, functioning and biodiversity due to the loss of submerged macrophytes that often arises when they are present (Janse & Van Puijenbroek 1998). The anoxic and cold conditions, together with an increase in carbon dioxide can also cause fish kills (Lewis & Bender 1961) and loss of macroinvertebrates (Janse & Van Puijenbroek 1998). These prolonged anoxic conditions could result in just a few invertebrate species surviving these harsh conditions in small ponds, examples include Cloeon dipterum (Ephemeroptera) larvae that have adapted to anoxic conditions in small ponds in regions with long and cold winters (Nagell 1977).

Clearly, further work is required to assess the impacts of introduced invasive species on specific water bodies, both to enable an accurate assessment of ecological status and to design appropriate response measures (Environment Agency, Water Framework Directive Programme 2004). This thesis intends to contribute to future assessments of pond ecological status by attempting to shed light on the ecological impacts and effects of

Lemna minuta dominated duckweed mats in small ponds.

1.8.3 Lemna minuta Kunth.

Lemna minuta (American duckweed) is a native of temperate regions of North and South

America (Preston & Croft 1997, Lucey 2003) and has now become naturalised in Europe. It was first recorded in the British Isles in Coe Fen, Cambridge in 1977 (Landolt 1979). It was first recorded in Eire in 1993 at Blarney Castle, Cork (Lucey 2003). It has spread rapidly across the British Isles since the 1980s (Bramley et al., 1995) and is listed as one of the species showing the most dramatic increase in range and abundance in Britain during the twentieth century (Walker 2007). It has a significantly broader tolerance to nitrate concentration than the native species L. minor (Lüönd 1980), and according to

Bramley et al., (1993) the abundance of L. minuta is not significantly controlled by water chemistry. Despite its relatively small size it occurs in considerable quantities, often excluding other free-floating aquatic plants, and there is evidence that it is more aggressive than other lemnids (Leslie & Walters 1983). According to Landolt (1980) this is a species that favours a more Mediterranean climate and there is a significant risk that climate change will allow a rapid northerly expansion of some invasive aquatic plants that are already established in the south of England (Willby 2007). This will likely favour the rapid establishment of L. minuta. It has been seen to out-compete indigenous Lemnaceae (Leslie & Walters 1983, Oliver 1991) becoming the most dominant aquatic plant species, covering the surface area in dense and thick mats, creating anoxic conditions leading to high fish mortalities, and declining aquatic invertebrate diversity. L.

minuta appears to be prone to devastating and alternating boom-bust cycles leading to the

marginalisation of indigenous Lemnaceae (Bramley et al., 1995, Dussart et al., 1993). Despite the above knowledge, L. minuta is classified as ‘Unknown Impact’ and for which a full risk assessment is required (Environment Agency, Water Framework Directive Programme, Technical Assessment Method, 2004).

L. minuta appears to be capable of withstanding British winter temperatures (Leslie &

Walters 1983) and even sub-zero temperatures where protection is afforded by growth in thick, dense mats. Consequently it tends to over-winter more successfully than monolayer growths of native L. minor (Janes 1998). The introduction, spread and the biological interactions of invasive plant species have provided fascinating ecological and evolutionary insights (Walker 2007) and according to Max Walters (1970): “Most of them are unplanned experiments [sic], but if we watch we can learn a great deal from them”. This thesis intends to take this insight on board by studying the recent ecological impacts of L .minuta in a small pond.