A riot of species in an environmental calm: the paradox of the species-rich deep-sea floor

Abstract

Deep-sea ecosystems are the most extensive and remote ecosystems on Earth. Perception of the deep-sea benthic environment has changed dramatically in the last century from one of an azoic, or at least species-poor habitat to one that is rich in species. The early misconception was created, in part, by evidence of vast, monotonous expanses of cold, dark sediment plains with little obvious spatial or temporal heterogeneity. Given that many species-rich ecosystems on Earth are obviously heterogeneous, it is surprising that some estimates of species numbers in the deep sea (e.g. ∼10 7 macrofaunal species) rival those for tropical rainforests. Although other estimates are more conservative (e.g. 5 x 10 5 macrofaunal species), it is clear that deep-sea benthic habitats contain many species. The paradox of high deep-sea diversity has generated a number of explanatory hypotheses, including some that are currently difficult to test and others that are the focus of ongoing study. Approaches include analyses of local, regional, and global patterns, and experimental manipulations within habitats. Mechanistic generalisations are difficult to make because experimentation and sampling coverage are spatially and temporally limited, but evidence to date suggests that small-scale habitat variability and patchy disturbance, as well as global and regional variability, may play roles in maintaining deep-sea diversity. The importance of small-scale habitat variability and patchy disturbance has been demonstrated for only a small subset of species, many of which are opportunists. Broad inferences from global and regional patterns of species diversity are debatable because many areas remain poorly sampled and causes of patterns are ambiguous. Nonetheless, our understanding of diversity patterns in the deep-sea benthos has increased dramatically in the last three decades. If the approaching decades hold even a portion of the surprises seen in the recent past, then science can expect very exciting discoveries from the deep ocean in the near future.