Climate change and habitat destruction have been linked to global declines in vertebrate biodiversity, including mammals, amphibians, birds, and fishes. However, invertebrates make up the vast majority of global species richness, and the combined effects of climate change and land use on invertebrates remain poorly understood. Here we present 35 years of data on 159 species of butterflies from 10 sites along an elevational gradient spanning 0-2,775 m in a biodiversity hotspot, the Sierra Nevada Mountains of Northern California. Species richness has declined at half of the sites, with the most severe reductions at the lowest elevations, where habitat destruction is greatest. At higher elevations, we observed clear upward shifts in the elevational ranges of species, consistent with the influence of global warming. Taken together, these long-term data reveal the interacting negative effects of human-induced changes on both the climate and habitat available to butterfly species in California. Furthermore, the decline of ruderal, disturbance-associated species indicates that the traditional focus of conservation efforts on more specialized and less dispersive species should be broadened to include entire faunas when estimating and predicting the effects of pervasive stressors.biodiversity | elevational gradient | global change | Lepidoptera | phenology

Insects have diversified through more than 450 million y of Earth’s changeable climate, yet rapidly shifting patterns of temperature and precipitation now pose novel challenges as they combine with decades of other anthropogenic stressors including the conversion and degradation of land. Here, we consider how insects are responding to recent climate change while summarizing the literature on long-term monitoring of insect populations in the context of climatic fluctuations. Results to date suggest that climate change impacts on insects have the potential to be considerable, even when compared with changes in land use. The importance of climate is illustrated with a case study from the butterflies of Northern California, where we find that population declines have been severe in high-elevation areas removed from the most immediate effects of habitat loss. These results shed light on the complexity of montane-adapted insects responding to changing abiotic conditions. We also consider methodological issues that would improve syntheses of results across long-term insect datasets and highlight directions for future empirical work.

24Insects have diversified through 400 million years of Earth's changeable climate, yet recent and 25 ongoing shifts in patterns of temperature and precipitation pose novel challenges as they 26 combine with decades of other anthropogenic stressors including the conversion and degradation 27 of land. Here we consider how insects are responding to recent climate change, while 28 summarizing the literature on long-term monitoring of insect populations in the context of 29 climatic fluctuations. Results to date suggest that climate change impacts on insects have the 30 potential to be considerable, even when compared to changes in land use. The importance of 31 climate is illustrated with a case study from the butterflies of Northern California, where we find 32 that population declines have been severe in high-elevation areas removed from the most 33 immediate effects of habitat loss. These results shed light on the complexity of montane-adapted 34 insects responding to changing abiotic conditions and raise questions about the utility of 35 temperate mountains as refugia during the Anthropocene. We consider methodological issues 36 that would improve syntheses of results across long-term insect datasets and highlight directions 37 for future empirical work. 38 39 Key words 40 Anthropocene, climate change, population decline, extinction, extreme weather 41 42 43 51 52 53 54 From invasive species to habitat loss, pesticides and pollution, the stressors of the Anthropocene 55 are many and multi-faceted, but none are as geographically pervasive or as likely to interact with 56 all other factors as climate change (1, 2). For these reasons, understanding the effects of 57 anthropogenic climate change on natural systems could be considered the defining challenge for 58 the ecological sciences in the 21 st century (3). It is of particular interest to ask how insects will 59 respond to recent and ongoing climate change, because they are the most diverse lineage of 60 multicellular organisms on the planet, and of fundamental importance to the functioning of 61 terrestrial ecosystems. The issue also has new urgency in light of recent and ongoing reports of 62 insect declines from around the globe (4). Insects and climate change have been discussed 63 elsewhere (5-8), and our goal here is not to cover all aspects of the problem. Instead, we focus 64 on recent discoveries and questions inspired by long-term records of insect populations, 65

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Background: Climate change is challenging plants and animals not only with increasing temperatures, but also with shortened intervals between extreme weather events. Relatively little is known about diverse assemblages of organisms responding to extreme weather, and even less is known about landscape and life history properties that might mitigate effects of extreme weather. Our aim was to address this knowledge gap using a multi-decadal dataset of 163 butterfly species that recently experienced a millennium-scale drought. To understand faunal dynamics in the context of the millennium drought, we investigated the behavior of phenology (including date of first flight), species richness and diversity indices through time at 10 study sites spanning an elevational gradient. Linear models were developed to understand the differential sensitivity of butterflies to climate at low and high elevations. Results: Dates of first flight advanced across the elevational gradient during the drought, leading to an overall expansion of the flight window at low elevations and a compression of the flight window in the mountains. The number of species observed per year increased at lower elevations but decreased at higher elevations, apparently as a consequence of extreme sensitivity to hot and dry conditions.