Waterbird-mediated dispersal of aquatic organisms: an introduction

Abstract

Human activities are modifying many of the Earth’s ecosystems through the direct loss and fragmentation of habitats, the addition of pollutants, the alteration of functional processes and the resulting decrease of species richness. Aquatic ecosystems are among the most affected. Over recent decades, the total area of well-conserved aquatic habitats has been decreasing at an increasing rate. Approximately, two thirds of all European wetlands have been lost since the beginning of the 20th century (CEC, 1995) and more than half of the world’s wetlands may have been destroyed during the same period (Ramsar Convention Bureau, 1996). Despite recent conservation efforts, the total number of wetlands is still decreasing in most countries (Finlayson et al., 1992; Groombridge and Jenkins, 1998). As a direct consequence of wetland loss and deterioration, both habitat availability and the number of populations of wetland species decrease. In addition, indirect effects include severe reductions in local dispersal due to the increased distances and degree of isolation among those wetlands that still remain. Wetlands are fragmented ecosystems; therefore, the diversity of their biota depends on the equilibrium between species immigration and local extinction (Cox and Moore, 1993; Dodson, 1992). Thus, despite being geographically isolated, important ecological links between these wetlands may exist. As a result, the consequences of wetland loss for local and regional diversity may be larger than, and non-proportional to what would be expected from the habitat loss per se. They could include decreases in local and regional genetic variation and species richness, as well as the reduction of species distributions. The effect of reduced dispersal will be particularly critical in wetlands that already show decreased species richness, including those under anthropogenic stress or recovering from it, and those in extreme climates such as semiarid and arctic areas. One of the most important (natural) dispersal vehicles between river catchments and isolated waterbodies are waterbirds. Changes in waterbird migration and diet could thus bear consequences not only for the diversity of birds but also for the diversity of aquatic organisms that rely on bird dispersal. The loss of suitable habitats has already affected wintering and breeding grounds as well as stopover habitats, thus distorting present migration routes and causing increasing rates of population decline of migrants (Sutherland, 1996, 1998a). In addition, the impact of various human activities has also modified the diet of several migratory species, with the majority of individuals of some species now feeding on crops and grassland instead of on aquatic organisms (Madsen, 1991). The problem could be exacerbated by the effects of climate change, which may affect coastal wetlands through sea-level rise, reduce the Mediterranean wetlands through increased aridity, damage important breeding areas in the Arctic tundra and modify the seasonal dynamics of food organisms (WWF, 1997; Sutherland, 1998b). Adequate knowledge of the vectors and mechanisms regulating the dispersal of aquatic organisms among wetlands may thus be of key importance to evaluate the effects of human intervention on the diversity and functioning of wetland ecosystems. Because the widespread distribution of many aquatic organisms contrasts with the fragmented character of the aquatic habitats, their modes of dispersal received early notice by eminent scientists, such as Darwin (1859) and Ridley (1930). However, it has awakened surprisingly little interest since then (with the exception of work by V.W. Proctor and collaborators during the 1960s). In response to the recent awareness of the importance of dispersal for both terrestrial and aquatic species (Bilton et al., 2001; Bullock et al., 2002), the scientific community is developing new research programmes (Charalambides et al., 2000; Nathan et al., 2001). As a contribution to these recent developments, a workshop on the ‘Dispersal of aquatic organisms’ was held in April 2001 at the Netherlands Institute of Ecology. The objective of this symposium was to gain a better perspective on the research carried out in Europe within this framework, evaluate the latest findings * Corresponding author. E-mail address: santamaria@cl.nioo.knaw.nl (L. Santamaria). Acta Oecologica 23 (2002) 115–119