Socorro Island - Ecology

Ecology

The lowlands of Socorro - except on the northern, more humid side - are covered with thick shrubland, consisting mainly of endemic Croton masonii and a cactus, probably Engelmann's Prickly Pear (Opuntia engelmannii). Above 650 metres (2,130 ft) and on the northern side, a richer vegetation occurs. This includes small trees such as Ficus cotinifolia, Black Cherry (Prunus serotina), and the endemic Guettarda insularis, which bear epiphytic orchids (Epidendrum nitens, E. rigidum and the endemic Pleurothallis unguicallosa).

The native land fauna is depauperate, with birds predominating and mammals absent. There is one endemic species of iguanid lizard (Urosaurus auriculatus) and the land crab Gecarcinus planatus which occurs on islands throughout the region.

Sheep, cats and rodents were introduced to the island by human activity; more recently, the locust Schistocerca piceifrons has also established itself on the island. Unlike the mammals on Guadalupe Island or Clarión, their impact on the local flora was minor, but cat predation had a drastic effect since the mid-1970s due to the fauna's island tameness and the locusts swarm twice a year and seriously damage vegetation during that time. There have been no recorded extinctions of plants on Socorro; several birds have been drastically affected by cat predation however and one taxon appears to have gone extinct.

Socorro is an important breeding location for several seabirds, many of which have here one of their most north(east)ernmost breeding colonies. The present status of these birds is not well known, and they presumably have suffered from cat predation. In 1953, the following taxa were present:

• Wedge-tailed Shearwater, Puffinus pacificus (or Ardenna pacifica)
• Western Red-billed Tropicbird, Phaethon aethereus mesonauta - breeding suspected but not verified
• Nazca Booby, Sula granti - breeding suspected but not verified
• Northeast Pacific Brown Booby, Sula leucogaster brewsteri - breeding suspected but not verified
• East Pacific Great Frigatebird, Fregata minor ridgwayi - breeding suspected but not verified; a doubtfully distinct subspecies
• East Pacific Sooty Tern, Onychoprion fuscatus crissalis - a doubtfully distinct subspecies
• East Pacific Brown Noddy, Anous stolidus ridgwayi

Non-endemic landbirds and shorebirds occur mostly as vagrants or use the island as a stopover during migration; the Northern Mockingbird has become established in the late 20th century. Among those that are recorded not infrequently are Great Blue Heron, Hudsonian Curlew, Spotted Sandpiper and Wandering Tattler. Opposed to the situation on smaller and more isolated Clarión, wind-blown or vagrant birds seem to consititute the bulk of the records, including species such as Brown Pelican, Osprey, Peregrine Falcon, Semipalmated Plover, Willet, Sanderling, Belted Kingfisher and Buff-bellied Pipit. It may be that this puzzling observation is due to the presence of the Red-tailed Hawks and cats, which has at least made the local Urosaurus more wary than its relative on Clarión, and might deter passing birds from stopping on Socorro.