To our knowledge, all other herbivore species included in this analysis were native to the study region. We modelled the presence of each herbivore species using generalized linear models fit using the best predictors of herbivore damage, as estimated in the models described above: mean temperature of the coldest quarter,extracted from WorldClim 2.0 data at 30‐s resolution , and human population density . Twenty‐five percent of the occurrence records were assigned for testing, and the remaining 75% were used to train the models. Pseudoabsences were generated by randomly extracting 1.5 × the number of observations from the background data , delimited as New England. We used ANOVAs to compare the regression coefficients from the GLMs for insect herbivores grouped by host plant. Regression coefficients were weighted by the model AUC score , a measure of reliability of the estimates from the climate occupancy models. Thus, more reliable models contributed more to parameter estimation. Post hoc comparisons among pairs of host plant species were made using Tukey’s Honest Significant Difference.Using herbarium specimens spanning 112 years of rapid global change in the northeastern USA, we found a significant increase in herbivory over the past century. This trend is consistent across four ecologically and phylogenetically distinct host plant species with very different herbivore communities.
We suggest our observation of increasing herbivory through time is most likely driven by insect herbivore responses to warmer winter and early spring temperatures. Two environmental drivers—temperature and urbanization— explain much of the trend in herbivory through time, square pot but they have opposing effects. Over space and time, higher temperatures were associated with greater herbivory and higher probabilities of occupancy for 66 of 69 known insect herbivore associates. In contrast, high human population density was associated with lower herbivory and reduced occupancy for most insect herbivores that showed significant responses. In this region of the US, warming is projected to surpass 2°C by 2040 and is expected to be greater in winter than in summer . Our results indicate that damage to plants by insect herbivores may continue to increase with climate change but that, locally, urbanization may counteract this more general trend.Our analyses indicate that warming winter temperatures may drive increasing herbivory over time. Herbivory preserved in herbarium specimens was positively associated with mean winter to early spring temperature and decreasing latitude. Almost half of the variation in herbivory through time and across latitudes could be explained by temperature. These findings support theory as well as some empirical evidence suggesting that herbivory is largely driven by winter temperature at mid latitudes . As wintersin the northeastern US are projected to warm more than other seasons , we suggest herbivory may continue to increase in the future. We found compelling circumstantial evidence that insect herbivores known to feed on our focal plants also prefer warmer winters.
The vast majority of herbivore species examined had higher occupancy probabilities where winter temperatures were warmer. Occupancies of the other three species were not significantly related to winter temperatures, but the trend was also positive. While the association between winter temperatures and herbivore occurrence was estimated using contemporary data across space, we suggest this relationship is likely to be reflected in patterns through time. First, herbivore ranges may have extended northward in response to milder winters . Second, resident herbivore species have become more abundant due to greater survival in milder winters , and are therefore more frequently observed. Our data are also consistent with the possibility that shifting phenology might contribute to growing herbivory pressure. There is strong evidence indicating that many butterfly species are flying earlier in the UK, US, Canada, and Spain . Butterfly species that emerge earlier may have more generations within a year now than they did several decades ago , allowing more rapid population growth. Warming early in the growing season could also restructure phenological interactions so that they are more synchronous than they were historically , which may increase herbivory if host plants avoid herbivore damage by timing leaf‐out to be asynchronous with herbivore emergence. Warmer early springs might also alter phenological matching between herbivores and their natural enemies, reducing natural biological control of herbivores, as lower trophic levels may be more sensitive to climatic warming than higher trophic levels .
Some evidence suggests that this is the case for Lycaenid butterflies in our study region, which have advanced their flight more than has been shown for birds, which are their potential predators . Independent from its effect on phenology, warming may affect herbivores indirectly through altering host plant nutritional quality. In some cases, warming can induce water stress that alters plant nutritional quality, which can increase herbivore egg production on water stressed relative to unstressed plants . However, if this were the case here, we would expect that higher summer temperatures would be most closely associated with greater herbivore damage, contrary to results from our model comparisons. While warming may have driven increasing herbivory over time, urbanization was associated with reduced herbivory. This negative relationship was consistent with our herbivore occupancy models. Herbivore species that responded significantly were more likely to show significant negative than positive associations with human population density. It is possible that herbivores that showed positive responses favour urban areas because of factors such as the urban heat island effect ,natural enemy release , or higher host plant quality on urban compared to rural plants . However, overall patterns in our data add to mounting evidence that urban development locally reduces diversity in Lepidoptera, which are major herbivores. We focused on Lepidoptera in this study because they are well‐represented in observations. Future studies could extend efforts to collect long‐term data for other herbivorous taxa, such as beetles and grasshoppers, to determine if there is a more general reduction in herbivore diversity and damage with urbanization. Trends in herbivory through time across the four focal plant species were remarkably consistent. The relative importance of the different abiotic variables, however, varied among plant species. Different plant–herbivore relationships are likely sensitive to different drivers. For example, Q. bicolor did not show significant trends towards increasing herbivory with increasing temperature or decreasing latitude, even though the vast majority of herbivore species associated with Q. bicolor are more likely to occur where temperatures are warmer. Factors other than temperature and herbivore occurrence might thus be stronger drivers of herbivory in this host species. Geographical and temporal variation in driver intensity across the host range might also contribute to explaining different responses among species. For example, the negative association between herbivore damage and human population density was strongest for V. angustifolium. This species has a larger range than Q. bicolor and C. ovata that captures a larger urbanization gradient , providing a greater opportunity to detect the effects of human population density.Herbarium specimens collected by botanists provide long‐term estimates of herbivory that span the time frame of anthropogenic environmental change, filling a major data gap. Observational studies tend to span much shorter time frames, with herbivory studies rarely spanning more than 1–2 years . Field warming experiments are also typically short‐term and often address effects on herbivores rather than herbivory . We suggest that data from herbarium specimens may provide opportunities to assess herbivory across unprecedented temporal, spatial, and phylogenetic scales .
In addition, square plastic plant pots equivalent data from herbarium specimens on plant–pollinator interactions and plant–pathogen interactions may be used to further tailor land management strategies to changing environmental conditions. However, data from herbaria present challenges that require careful consideration . The spatial resolution of older specimens is coarse and, in our data, limited to the county level within the US. In addition, plant collectors tend to avoid damaged specimens, and thus, absolute values of herbivore damage are likely underestimates. Nonetheless, we have shown that it is possible to detect meaningful variation in herbivory that can be contrasted between species and time periods. While collecting biases could in theory confound interpretations—for example, if more recent collectors are more likely to collect specimens with herbivore damage, leading to an apparent increase in herbivore damage through time—we find no evidence to support any such bias. And, importantly, we can see no reason why collection of damaged specimens should be correlated with temperature or urbanization. In addition, we note that we observe increasing herbivory with day of year . As herbivory is cumulative through the growing season, these data indicate that, even if collectors show bias towards selecting more intact specimens, we are still able to detect expected temporal trends in herbivory preserved within herbarium collections. One exciting prospect is that long‐term herbivory data from herbarium specimens may provide the opportunity to compare effects of contemporary temperature change to predictions from fossils. Fossils are one source of long‐term data that may help in generating predictions of how plant–herbivore interactions will respond to projected anthropogenic change. It is currently unclear how reliably responses to temperature across epochs should predict effects of the rapid, anthropogenic change we are experiencing today. Because herbarium specimens, like fossils, can be scored for the presence and absence of herbivory, it should be possible to answer this question and assess whether patterns across millennia can predict effects of recent global change. Comparing herbivory on herbarium specimens and fossils would require adjusting the methods we have developed here to derive comparable fossil and herbarium data on herbivory that could be placed on a common axis. Our results hint that herbivory responses to contemporary and paleontological climate change might be congruent.Domesticated livestock are an integral part of agriculture as many high-quality foods come from their production such as eggs, milk, and meat. Ruminants, such as cattle, goats, and sheep have a specialized stomach, breaking down into four different chambers with the largest being the rumen. The rumen is an anaerobic environment that can hold anywhere between 113 to 226 liters of feed materials and fluids dependent on the age and size of the animal and hosts a variety of microbes such as bacteria, fungi, archaea, and protozoa. Anaerobic fungi possess carbohydrate-active enzymes such as cellulase that can break down structural carbohydrates . Bacteria are the most abundant microbes in the rumen and can contain billions within 1 milliliter of rumen fluid . Each having their own niches, they also contain enzymes that can break down carbohydrates ranging from plant cell walls to simpler carbohydrates like starch. Through their metabolism, they produce byproducts that can be utilized by the host in the form of volatile fatty acids . Acetate, propionate, and butyrate are the three major VFA that are metabolized and used by the host as their primary energy source . The proportion of VFA is highly dependent on the feed composition with acetate associated with higher fiber degradation and propionate with starch degradation . Microbes themselves also become a nutrient for the host as they are the source of microbial crude protein when they are digested and absorbed in the lower digestive tract . Through fermentation and digestion, host animals can utilize the VFA and MCP to promote growth, reproduction, and create products such as milk and meat . This symbiotic relationship between ruminants and microbes allows them to access a variety of plant-based feed sources without competing with humans. Aside from VFA and MCP, fermentation also produces other byproducts such as CO2 and hydrogen . It is crucial that the H2 formed from fermentation does not go unregulated as too much will cause the rumen environment pH to decrease . Many microbes in the rumen cannot function and may even die off if the rumen becomes too acidic . This can lead to a decrease in fermentation and depression in growth and production, and in some cases detrimental to the health of the animal . However, there are microbes in the rumen that can utilize hydrogen as substrates for their own metabolism. The main H2 utilizers are hydrogenotrophic methanogens by reducing CO2 with H2 to form methane gas. Other microbes found in the rumen can also utilize hydrogen that can act as competitors to methanogens such as homoacetogens, nitrate-, sulfate-, and fumarate-reducing bacteria. Homoacetogens produce acetate using both H2 and CO2, however, methanogens are more efficient at utilizing H2 when resources are limited . Even though sulfate and nitrate are thermodynamically more efficient, substrates for bacteria to use are limited as the concentrations are low in a ruminant’s diet .