A pirate spider. © Martín Ramírez   The Vital Spider How’s that for an unsettling thought? To put it in perspective (just in case you thought tigers or orcas held the title), a mere 4% of all animal species on the planet are vertebrates, and mammals make up less than four tenths of one percent. Within terrestrial ecosystems, the key players in maintaining habitats and structuring communities are plants and arthropods. (Arthropods are members of the invertebrate group, which includes insects, crustaceans, and spiders.) And spiders are far and away the Top Arthropods. Here’s why: There are an awful lot of them. * Over 36,000 species have been described, but they represent only a third to a half of those that still await discovery.      1 of 10