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1. What We Don't Know About Diet-Breadth Evolution in Herbivorous Insects

   https://doi.org/10.1146/annurev-ecolsys-011720-023322  

   Hardy, Nate B. ; Kaczvinsky, Chloe ; Bird, Gwendolyn ; Normark, Benjamin B. ( November 2020 , Annual Review of Ecology, Evolution, and Systematics)

   null (Ed.)

   Half a million species of herbivorous insects have been described. Most of them are diet specialists, using only a few plant species as hosts. Biologists suspect that their specificity is key to their diversity. But why do herbivorous insects tend to be diet specialists? In this review, we catalog a broad range of explanations. We review the evidence for each and suggest lines of research to obtain the evidence we lack. We then draw attention to a second major question, namely how changes in diet breadth affect the rest of a species’ biology. In particular, we know little about how changes in diet breadth feed back on genetic architecture, the population genetic environment, and other aspects of a species’ ecology. Knowing more about how generalists and specialists differ should go a long way toward sorting out potential explanations of specificity, and yield a deeper understanding of herbivorous insect diversity. 

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2. Competition shapes individual foraging and survival in a desert rodent ensemble

   https://doi.org/10.1111/1365-2656.13583  

   Manlick, Philip J. ; Maldonado, Karin ; Newsome, Seth D. ( September 2021 , Journal of Animal Ecology)

   Intraspecific variation, including individual diet variation, can structure populations and communities, but the causes and consequences of individual foraging strategies are often unclear.

   Interactions between competition and resources are thought to dictate foraging strategies (e.g. specialization vs. generalization), but classical paradigms such as optimal foraging and niche theory offer contrasting predictions for individual consumers. Furthermore, both paradigms assume that individual foraging strategies maximize fitness, yet this prediction is rarely tested.

   We used repeated stable isotope measurements (δ¹³C, δ¹⁵N;N = 3,509) and 6 years of capture–mark–recapture data to quantify the relationship between environmental variation, individual foraging and consumer fitness among four species of desert rodents. We tested the relative effects of intraspecific competition, interspecific competition, resource abundance and resource diversity on the foraging strategies of 349 individual animals, and then quantified apparent survival as function of individual foraging strategies.

   Consistent with niche theory, individuals contracted their trophic niches and increased foraging specialization in response to both intraspecific and interspecific competition, but this effect was offset by resource availability and individuals generalized when plant biomass was high. Nevertheless, individual specialists obtained no apparent fitness benefit from trophic niche contractions as the most specialized individuals exhibited a 10% reduction in monthly survival compared to the most generalized individuals. Ultimately, this resulted in annual survival probabilities nearly 4× higher for generalists compared to specialists.

   These results indicate that competition is the proximate driver of individual foraging strategies, and that diet‐mediated fitness variation regulates population and community dynamics in stochastic resource environments. Furthermore, our findings show dietary generalism is a fitness maximizing strategy, suggesting that plastic foraging strategies may play a key role in species' ability to cope with environmental change.

    

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3. High resource overlap and small dietary differences are widespread in food‐limited warbler (Parulidae) communities

   https://doi.org/10.1111/ibi.13006  

   Kent, Cody M. ; Huh, Kyu Min ; Hunter, Sarah Chieko ; Judson, Kathryn ; Powell, Luke L. ; Sherry, Thomas W. ( August 2021 , Ibis)

   Although both interspecific competition and coexistence mechanisms are central to ecological and evolutionary theory, past empirical studies have generally focused on simple (two‐species) communities over short time periods. Experimental tests of these species interactions are challenging in complex study systems. Moreover, several studies of ‘imperfect generalists’, consistent with Liem's Paradox, raise questions about the ability of evolved species differences to partition niche space effectively when resources vary considerably across the annual cycle. Here we used a recently developed theoretical framework to combine past research on population‐level processes with observational data on resource use to test for ongoing interspecific competition and understand the nature of resource overlap. We compared species diet overlaps and differences in several distinctive communities centred on a focal species, the American RedstartSetophaga ruticillareplicated both spatially and seasonally, in combination with documentation of population regulation to assess the ability of similar species to partition dietary niche space and limit interspecific competition. Our results document high dietary overlap in most of the communities studied, with only subtle differentiation consistent with known species differences in foraging behaviour and morphology. These findings are largely consistent with species foraging as imperfect generalists. However, in contrast to past studies, the high diet overlaps observed here during times of inferred resource scarcity were driven by low‐value prey taxa (e.g. small ants) and did not involve truly ‘private’ resources. All of these factors increase the potential negative impacts of interspecific competition, and limit the ability of these birds to avoid competition if food availability deteriorates further than observed in our study, either seasonally or at longer intervals.

    

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4. Adaptive foraging in the Anthropocene: can individual diet specialization compensate for biotic homogenization?

   https://doi.org/10.1002/fee.2380  

   Manlick, Philip J. ; Newsome, Seth D. ( July 2021 , Frontiers in Ecology and the Environment)

   Biotic homogenization is a ubiquitous consequence of human disturbance. Through a mix of local extinctions and invasions, diverse communities of specialists are often replaced by or inundated with generalist species, resulting in uncertain consequences for ecological functions. While concern about biotic homogenization is growing, intraspecific variation and individual diet specialization (IS) have also emerged as key drivers of ecological functions. The niche variation hypothesis predicts that when a population is released from interspecific competition (ie “ecological release”), intraspecific competition will promote resource niche expansion and IS. It then follows that if biotic homogenization reduces taxonomic diversity andinterspecificcompetition,intraspecificcompetition within the remaining taxa should expand population resource niches and increase the prevalence of IS. Here, we integrate the rapidly growing fields of biotic homogenization and intraspecific variation and assess the potential for generalist species to compensate for multiple forms of biotic homogenization via adaptive foraging, resource niche expansion, and IS.

    

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5. Underrepresentation of dietary‐specialist larval Lepidoptera in small forest fragments: Testing alternative mechanisms

   https://doi.org/10.1111/1365-2656.70020  

   Mickley, James_G ; Anderson, Riley_M ; Wagner, David_L ; Brown, Leone_M ; Connors, Christian_J ; Davis, Miranda_L ; Elphick, Chris_S ; Kilpatrick, Howard ; LaScaleia, Michael_C ; Marren, Courtney ; et al ( February 2025 , Journal of Animal Ecology)

   Growing evidence suggests that organisms with narrow niche requirements are particularly disadvantaged in small habitat patches, typical of fragmented landscapes. However, the mechanisms behind this relationship remain unclear. Dietary specialists may be particularly constrained by the availability of their food resources as habitat area shrinks. For herbivorous insects, host plants may be filtered out of small habitat fragments by neutral sampling processes and deterministic plant community shifts due to altered microclimates, edge effects and browsing by ungulates.

   We examined the relationship between forest fragment area and the abundance of dietary‐specialist and dietary‐generalist larval Lepidoptera (caterpillars) and their host plants in the northeastern USA. We surveyed caterpillars and their host plants over 3 years in equal‐sized plots within 32 forest fragments varying in area between 3 and 1014 ha. We tested whether the abundances and species richness of dietary specialists increased more than those of dietary generalists with increasing fragment area and, if so, whether the difference could be explained by reduced host plant availability or increased browsing by white‐tailed deer (Odocoileus virginianus).

   The overall abundance of dietary specialists was positively related to fragment area; the relationship was substantially weaker for dietary generalists. There was notable variation among species within diet breadth groups, however. There was no effect of fragment area on the diversity of dietary‐specialist or dietary‐generalist caterpillars. Deer activity was not related to the abundances of either dietary‐generalist or dietary‐specialist caterpillars.

   Plant community composition was strongly associated with fragment area. Larger fragments were more likely to include host plants for both dietary‐specialist and dietary‐generalist caterpillars. Deer activity was correlated with decreased host plant availability for both groups, with a slightly stronger impact on host plants of dietary specialists. Although dietary specialists were more likely to lack host plants in fragments, the relationship between fragment area and host availability did not depend on caterpillar diet breadth.

   This study provides further evidence that decreasing patch area disproportionately impacts specialist consumers. Because this relationship was derived from equal‐sized plots, it is robust to some criticisms levelled at fragmentation research. The mechanisms for specialist consumer declines, however, remain elusive.

    

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