Species interactions have received less attention in global change biology than individual species’ responses. In large part, this is because long-term data on species interactions spanning the period of intense anthropogenic environmental change are rare. For example, first flower dates of Japanese cherry blossoms have been recorded in diaries since the ninth century, but we have no equivalent long-term records of cherry tree pollination, leaf microbial communities, or disease incidence. Data describing species interactions are laborious to collect and, in many cases, require technology, such as electron microscopy or DNA sequencing, that was not available until recent years. The lack of long-term data inhibits assessment of how species interactions are impacted by global change and limits our ability to determine how these effects mediate individual species’ distributions, abundances, and ecologies. Variation in species responses to global change has generated concern that interactions which were tightly coupled historically might become decoupled owing to phenological asynchronies. Phenological asynchrony arises when interacting species respond differently to global change—for example, if earlier flowering as a consequence of global warming is not matched by earlier pollinator emergence. Recent meta-analyses have suggested that phenological sensitivity to climate change differs among trophic levels, with lower trophic levels advancing more than higher trophic levels. In one well-documented example, great tit reproduction did not advance in sync with peak food availability for young, leading to potential fitness costs. Similarchanges in interactions between trophic levels may happen as a consequence of other differential responses to global change, such as spatial mismatches between species whose ranges expand poleward or upward in elevation to different extents.
While predictions for phenological and spatial asynchronies are clear, empirical data are sparse, square plant pots and there is still no consensus on whether asynchrony is common or rare, or which traits regulate when asynchronies arise. In the absence of long-term observational data, global change biologists increasingly mine museum collections to investigate how species interactions have shifted over time. Diverse types of data are available only in natural history collections, and they could therefore have wide applicability in global change biology. While natural history specimens are not collected systematically—and their use in ecological and evolutionary research can present challenges—they represent time-series data across much of the globe, span the tree of life, and may be able to fill gaps in species interactions data. Importantly, a large proportion of specimens were collected prior to the intensification of anthropogenic change and therefore may serve as baselines for studying consequences of, for example, invasive species spread, pollution and habitat alteration. Here, we explore the potential for museum specimens to provide insights into interactions between insect herbivores and their host plants. Insects have been eating plants for nearly 400 million years, and these interactions have given rise to much of macroscopic diversity. Herbivores co-evolved with plants, tracking plant speciation or defensive profiles and are frequently specialized. Over the past 12 000 years or so, humans have altered these relationships by domesticating plants and moving them beyond their natural ranges, spraying pesticides, building cities, and changing the global climate. Effects of these global changes on herbivores and their host plants is of critical importance to ecosystem functioning and the provision of ecosystem services. Insect damage—‘herbivory’—drives ecosystem processes, including decomposition and primary productivity. Herbivory also influences ecosystem properties that are of direct importance to people, including food production and tree cover, which are linked to human physical and mental health. In general, warming is expected to increase insect herbivore abundance where insects are living below their thermal optima.
While most insects in temperate and boreal climates probably occupy niches well below their thermal optima—and thus may benefit from warming—warming may cause temperatures to exceed insect thermal optima in areas that are already relatively warm, including the tropics. Insect fitness is not, however, a simple function of mean annual temperature, especially at the local scale. For example, many species have thermoregulatory behaviours that decouple body and air temperatures. In some cases, warming may also have negative effects on insect fitness by reducing snowpack or disrupting diapause. Furthermore, and contrary to the prediction that warming should increase temperate insect herbivore abundance, recent evidence suggests that insect biomass has declined precipitously in Germany since the 1980s. The extent of these declines across space and the mechanism driving them remain unknown, as does whether these declines have occurred in other continents, where comparable long-term insect data were not recorded. While shifts in the global climate will undoubtedly shape species interactions, the local drivers of global change may have complex and nonlinear impacts. Urbanization is a more localized form of global change that is accelerating, with important consequences for plant –insect interactions, and has thus attracted much recent attention in ecology and evolution research. Most people now live in urban areas for the first time in history, and the proportion of the world human population living in urban areas continues to grow. Impacts of urbanization on plant –herbivore interactions could thus have increasing consequence for society via its effects on urban greenery and agriculture. However, the key mechanisms driving insect and plant responses to urbanization remain debated because: urban development, like climate change, has multiple, concurrent effects that are difficult to tease apart experimentally, and we lack long term observational data to determine effects of urbanization over time. Despite these data challenges, studies in the past few decades consistently show that urban development can have profound impacts on plants and insect herbivores. The suite of competing predictions of how insects might respond to global change, and the multitude of mechanisms linking insect herbivore abundance and fitness to herbivory, makes predicting changes in insect damage to plants difficult in the absence of long-term data on historical responses.
Further, herbivory data for assessing competing hypotheses are currently sparse and thus ill-suited for making projections about herbivory change into the future. According to a recent compilation of studies, most short-term herbivory studies include just one year of data , focus on forest plants , are biased toward the northern hemisphere , and record only chewing damage, which represents a fraction of the damage types made by insects. Though short-term studies over space have highlighted that disturbance can have profound effects on plants and herbivores, only few studies focus on disturbed habitats, including cities and farms, areas where plant –herbivore relationships are likely to affect human well being. Here, we propose that biological collections could contribute data that would allow us to test major hypotheses on how diverse plant –herbivore relationships respond to global change. In contrast to observational or experimental studies of insect herbivory, data from museum collections span decades or sometimes centuries, include diverse growth forms and taxa from across the globe, and capture multiple types of herbivore damage. We focus on hypotheses that may be particularly well served by the 380 million vascular pressed plant specimens—herbarium specimens—and the more than 500 million insect herbivore specimens held in museums worldwide that are increasingly available online in the form of images and metadata. In general, we focus on above ground, not below ground herbivory, because intact roots required for assessing below ground herbivory are rarely available on herbarium specimens. Digitization of insect specimens has lagged behind plants owing to the difficulty of capturing three-dimensional specimens and the information from their labels. However, enormous efforts to digitize both plant and insect specimens are underway. Digital collections are also increasingly aggregated in online databases, e.g. the Chinese Virtual Herbarium , LepNet , Symbiota , GBIF , and iDigBio .These databases facilitate ‘big’ data analysis, but are equally as important in helping focus data collection efforts when physical specimens need to be examined. As we discuss below, these specimens can provide a wealth of ecological data that is difficult or impossible to collect using more traditional approaches. In the following sections, we demonstrate how natural history collections may provide unique insights into changing plant –insect herbivore interactions. We focus on species shifts in time and space as a response to recent anthropogenic climate change, and impacts of urbanization, representing one facet of habitat transformation, which is a major driver of current global change. In subsequent sections, we discuss how collections can also reveal species’ rapid adaptive responses to recent global changes, an application that may be particularly consequential for agriculture. Finally, plastic potting pots we review the challenges of natural history collections as sources of long-term data and suggest approaches to some of these challenges, with the goal of removing barriers that have prevented collections from becoming a standard source of data for twenty-first century ecology.Phenology—the seasonal timing of life-history events such as flowering and leaf-out in plants—is both a response to and an indicator of global change. Phenological models, such as the Spring Indices, allow us to map with increasing accuracy the transition from winter to spring across the northern hemisphere. These models integrate daily climate variables from meteorological records to predict day of year of first leaf and first bloom. However, the Spring Indices are calibrated using an extensive network of phenological observations on a single cloned lilac cultivar and two honeysuckle cultivars across the temperate United States .
While meta analyses reveal a consistent fingerprint of climate change on plant phenology, they also reveal large interspecific variation in plant responses. Thus, responses to climate change remain poorly characterized for the majority of plant species. The vast wealth of vouchers within herbaria greatly expand the spatial, taxonomic, and temporal extent of phenological observations and, as a consequence, the inference we can draw across climate space, even for species for which phenology has been documented in long-term observations. Animals are also shifting their phenology with climate change; many species are migrating sooner, advancing seasonal breeding times, and insects are emerging earlier. Natural history collections have been valuable in demonstrating how animal species respond to climate warming. Thanks to the biases of early Victorian naturalists and their attractiveness to contemporary collectors, the Lepidoptera— butterflies and moths, which in their immature stages are herbivorous—have been collected more comprehensively than many other groups and are thus the subject of a large proportion of collections research on animals. Butterfly collections document occurrences of species in time and space and, importantly, the seasonal timing of butterfly flight. Using data from approximately 48 000 collection records of Canadian butterflies, Kharouba et al. were able to show that timing of flight season predictably responded to temperature, and that species with early flight seasons and low dispersal ability appear most sensitive. However, in one recent study, Brooks et al. collected data from 83 500 specimens of British butterflies spanning100 years of climate change which suggested that early flying species might be approaching the limits of their phenological advancement. If advances in butterfly phenology are slowing but their host plants continue to leaf-out and flower earlier, we might observe phenological asynchrony between them. In collections research the potential for phenological synchrony between plants and their pollinators has tended to attract most attention, and collections data have been less frequently used to explore plant–herbivore interactions. However, the few studies that have considered phenological asynchronies between plants and insect herbivores demonstrate the potential for collections to inform such analyses. For example, Kharouba et al. showed that flowering time was more sensitive to temperature than the timing of nectar-feeding butterfly flight, suggesting that caterpillars or adult butterflies of these species might become phenologically mismatched with their host plants if warming continues. While collections data can be extensive for particular taxonomic groups, it is nonetheless rare for collections to capture temporally and spatially matching data on interacting species, such as on both butterflies and their host plants. However, herbarium collections can offer data on phenological sensitivity of plant species and, indirectly, data on their herbivores as captured by the amount of leaf area removed by herbivory. Herbaria may thus offer a unique opportunity to explore how shifts in plant phenology have affected herbivory since the onset of climate change. In figure 2, we outline some possible scenarios describing how warming could affect plant and insect phenology, and how these responses might translate to changes in herbivore damage to plants. For simplicity, we focus on spring phenology and specialized herbivores, though for plants that are commonly eaten primarily by generalists, we could derive an additional set of predictions. Additional factors, such as herbivore developmental plasticity, host plant nutritional quality , relationships with natural enemies, and differential responses among herbivores of a single host plant, might add complexity to the predictions described in figure 2, but could be placed within this general framework.