Ecological Implications for Pond Organisms


        As shown above, the numbers and landscape position of ponds have changed markedly within the Brandywine watershed. Combining this information with what is currently known about the ecological functions of individual ponds can thus provide a useful view of the aggregate impacts of ponds on the local landscape. The changing impact of increasing pond densities on aquatic organisms is considered here.
        Humans are not the only species affected by changes in the abundance and “landscape position” of ponds in the Brandywine watershed. A wide variety of aquatic organisms, including fish, amphibians, algae, plants and invertebrates, depend on suitable freshwater habitat for their persistence. For organisms that
history_fig24.jpg

Fig. 24. Aerial photo of a portion of Plum Run, East Bradford Township, showing ponds of varying size, age and landscape position.

require ponds for all or part of their life cycles, changes in pond habitat quality (e.g., by sediment accumulation, or changes in surrounding land use) represent a challenge to survival. Populations of particular species can be expected to rise and fall as habitat changes, with frequent episodes of local extinction. Regional persistence is thus in many instances strongly determined by the ability to disperse to other habitats – in effect to move among ponds.
        A small portion of the Plum Run sub-basin of the Brandywine in East Bradford Township, shown in aerial view in 2005, provides a visual sense of the dispersal problem for many pond species. The 6 ponds present in the photograph differ in size, age, surrounding shoreline habitat, and in their stream connections. In most instances, the colonization of new ponds from downstream is prevented by outfalls that descend rapidly from outflow pipes, and many ponds do not have stream inflows (for example, ponds 4, 67 and 116 in the photograph).
        Different kinds of organisms have different modes of colonizing new ponds. For example, many aquatic plants “passively” disperse their seeds on wind currents (consider the fact that almost no-one intentionally plants cattails or the Giant Reed Phragmites, and yet these species are often among the first to arrive wherever there is newly available water). Many aquatic insects are “active dispersers” that fly among ponds in search of suitable habitat. For many amphibians, the problem of locating suitable habitat is especially challenging, as the larval stage is aquatic and the adult stage is terrestrial. As pond densities have increased, distances among ponds have decreased. By contrast, increasing urbanization of the land between ponds may make even short-distance dispersal difficult for some species (consider the challenge for amphibians of crossing roads as an example).
        Biodiversity conservation must thus take into account the fact that individual ponds change, sometimes fairly rapidly, over time, whereas the distances and connections among ponds are changing. The need to consider ponds at the “landscape scale” thus extends not only to biogeochemical processes such as sediment retention and nutrient transformation, but also to the ability of ponds to sustain aquatic organisms.