Chapter 10: Invertebrates and Plants
Wendell R. Haag, Robert J. Distefano, Siobhan Fennessy, and Brett D. Marshall
Invertebrates and plants are among the most ubiquitous and abundant macroscopic organisms in aquatic ecosystems; they dominate most habitats in both diversity and biomass and play central roles in aquatic food webs. Plants regulate and create habitats for a wide array of organisms (Cooke et al. 2005). Snail grazing and bivalve filtering profoundly alter habitats and communities (Harvey and Hill 1991; Vaughn and Hakenkamp 2001). Aquatic habitats in North America support extremely diverse floras and invertebrate faunas; groups such as crayfishes and freshwater mollusks reach their highest worldwide diversity here. Crayfishes are important economically for human food, fishing bait, and the aquarium pet trade industry (Nielsen and Orth 1988; Huner 1997). Freshwater mussels have been exploited heavily in North America since at least the mid-1800s for freshwater pearls, button production, and currently for cultured-pearl bead nuclei (Anthony and Downing 2001).
Macroinvertebrates and aquatic plants are the foci of many fisheries studies because of their importance in aquatic ecosystems, the imperiled status of many species, and the increasing presence of harmful invasive species. A disproportionate number of aquatic plants and invertebrates are imperiled relative to terrestrial species. For example, more than 50% of the 579 plant species considered of special concern in Pennsylvania are aquatic (Cronk and Fennessy 2001). Freshwater mussels and snails are among the most imperiled animals in North America (Strayer et al. 2004); about 35 mussel species and 40 snail species have become extinct in the last 50 years (Neves et al. 1997). Although only four species of crayfishes are federally listed as endangered in the USA, about half of the 363 species in the USA and Canada are considered imperiled (Taylor et al. 2007), illustrating the frequent disparity between formal conservation status and actual conservation risk. Conservation status of aquatic insects is poorly known; however, the local extinction rate of aquatic insects in Illinois exceeds that of fishes and mussels (DeWalt et al. 2005), and stoneflies may be comparable to mussels and fishes in their degree of imperilment (Master et al. 2000). Moreover, invasive aquatic plants and invertebrates have jeopardized the integrity of many aquatic ecosystems (Pimentel et al. 2000; Holeck et al. 2004).