letters to nature NATURE | VOL 399 | 10 JUNE 1999 | www.nature.com 579 between 270 and 4,000 ms after target onset) and to ignore changes in the distractor. Failure to respond within a reaction-time window, responding to a change in the distractor or deviating the gaze (monitored with a scleral search coil) by more than 1Њ from the fixation point caused the trial to be aborted without reward. The change in the target and distractors was selected so as to be challenging for the animal. In experiments 1 and 2 the animal correctly completed, on average, 79% of the trials, broke fixation in 11%, might have responded to the distractor stimulus in 6% and responded too early or not at all in 5% of the trials. In Experiment 3 the corresponding values are 78, 13%, 8% and 2%. In none of the three experiments was there a difference between the performances for the two possible targets. Differences between average eye positions during trials where one or the other stimulus was the target were very small, with only an average shift of 0.02Њ in the direction of the shift of position between the stimuli. Only correctly completed trials were considered. Firing rates were determined by computing the average neuronal response across trials for 1,000 ms starting 200 ms after the beginning of the target stimulus movement. Tuning curves. Tuning curves were derived by fitting the responses to the 12 directions presented with gaussian functions: r null þ dirGain ϫ exp ð Ϫ 0:5‫ء‬ðdir Ϫ prefdirÞ 2 =width 2 Þ . The four parameters of a gaussian curve capture the four features of a direction-selective cell: preferred direction ( prefdir), response to the anti-preferred direction (r null ), the directional gain (dirGain; the maximal response modulation) and the selectivity or tuning width (width; the range of directions the neuron responds to).

Habitat degradation and climate change are thought to be altering the distributions and abundances of animals and plants throughout the world, but their combined impacts have not been assessed for any species assemblage. Here we evaluated changes in the distribution sizes and abundances of 46 species of butterflies that approach their northern climatic range margins in Britain-where changes in climate and habitat are opposing forces. These insects might be expected to have responded positively to climate warming over the past 30 years, yet three-quarters of them declined: negative responses to habitat loss have outweighed positive responses to climate warming. Half of the species that were mobile and habitat generalists increased their distribution sites over this period (consistent with a climate explanation), whereas the other generalists and 89% of the habitat specialists declined in distribution size (consistent with habitat limitation). Changes in population abundances closely matched changes in distributions. The dual forces of habitat modification and climate change are likely to cause specialists to decline, leaving biological communities with reduced numbers of species and dominated by mobile and widespread habitat generalists.

There is growing concern about increased population, regional, and global extinctions of species. A key question is whether extinction rates for one group of organisms are representative of other taxa. We present a comparison at the national scale of population and regional extinctions of birds, butterflies, and vascular plants from Britain in recent decades. Butterflies experienced the greatest net losses, disappearing on average from 13% of their previously occupied 10-kilometer squares. If insects elsewhere in the world are similarly sensitive, the known global extinction rates of vertebrate and plant species have an unrecorded parallel among the invertebrates, strengthening the hypothesis that the natural world is experiencing the sixth major extinction event in its history.