Coastal ecosystems and the services they provide are adversely affected by a wide variety of human activities. In particular, seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50 km of them. Seagrass meadows provide important ecosystem services, includ- ing an estimated $1.9 trillion per year in the form of nutrient cycling; an order of magnitude enhancement of coral reef fish productivity; a habitat for thousands of fish, bird, and invertebrate species; and a major food source for endangered dugong, mana- tee, and green turtle. Although individual impacts from coastal development, degraded water quality, and climate change have been documented, there has been no quantitative global assess- ment of seagrass loss until now. Our comprehensive global assess- ment of 215 studies found that seagrasses have been disappearing at a rate of 110 km2 yr 1 since 1980 and that 29% of the known areal extent has disappeared since seagrass areas were initially recorded in 1879. Furthermore, rates of decline have accelerated from a median of 0.9% yr 1 before 1940 to 7% yr 1 since 1990. Seagrass loss rates are comparable to those reported for man- groves, coral reefs, and tropical rainforests and place seagrass meadows among the most threatened ecosystems on earth.
Relationships among members of the seagrass genus Halophila (Hydrocharitaceae)
were investigated using phylogenetic analysis of the internal transcribed spacer (ITS) region of the nuclear ribosomal DNA. The final aligned ITS sequence data set of 705 base pairs from 36 samples in 11 currently recognised species included 18.7% parsimony informative characters. Phylogenetic analysis yielded two most parsimonious trees with strong support for six groups within the genus. Evolutionary trends in Halophila appear to be toward a more reduced simple phyllotaxy. In addition, this study indicates that long distance ‘jump’ dispersal between major ocean systems may have occurred at least in the globally distributed H. decipiens. Results of ITS analyses also indicate that the wide- spread pacific species H. ovalis is paraphyletic and may contain cryptic species. Like- wise, the geographically restricted species H. hawaiiana and H. johnsonii could not be distinguished from H. ovalis with these data and warrant further investigation.