The outcome of many high-order processes in ecology depends on the way in which the abundance and distribution of plants affect the eating rate of mammalian herbivores. However, simple, mechanistic models describing the operation of the functional response of these animals have failed to emerge. We offer new models describing the effects of spatial and morphological characteristics of plants on the intake rate of plant tissue by mammalian herbivores feeding within plant patches. We structure our models to respond to three patterns of plant availability: (1) spatially dispersed, apparent plants; (2) spatially dispersed, nonapparent plants; and (3) spatially concentrated plants. We depart from the traditional representations of predator functional response in assuming that searching for food and processing it can overlap in time. Our models illustrate that several distinct mechanisms can account for Type II functional responses frequently seen in herbivores. We show how differences among these mechanisms can explain anomalies in the empirical literature on regulation of intake rate of mammalian herbivores including divergence in functional responses between grazers and browsers, linear functional response curves, and curves showing zero slope throughout the domain of food availability.

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Type II functional responses are frequently observed in herbivores feeding in patches where plants are concentrated in space. We tested a mechanistic model of regulation of intake rate of herbivores foraging in food-concentrated patches (Laca and Demment 1992, Spalinger and Hobbs 1992) that accounts for asymptotic, Type II responses. The model is based on the hypothesis that competition between cropping and chewing regulates instantaneous intake rate in response to changes in the size of bites obtained by the forager. We tested this hypothesis and examined the ability of our model to account for observations of intake rate of 12 species of mammalian herbivores ranging in body mass over 4 orders of magnitude.We measured short-term intake rates of mammalian herbivores feeding in hand-assembled patches of plants. We varied bite size by changing plant height and density in patches offered to herbivores, and observed dry matter intake rates in response to this variation. Averaged across species, our model accounted for 77% of the variance in food intake rate (P < .001 for all species). Predictions of maximum intake rate closely resembled observations of processing capacity, demonstrating that processing rather than cropping sets an upper limit on short-term intake. Tests of model mechanisms provided strong support for the hypothesis that competition between cropping and chewing is responsible for the Type II functional response seen in herbivores feeding in food-concentrated patches. The model was able to consistently predict intake rates observed in 16 previous studies. These results indicate that plant characteristics regulating bite size (e.g., leaf size and geometry, spinescence) frequently control instantaneous rates offood intake by mammalian herbivores.

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Because of their agricultural value, there is a great body of research dedicated to understanding the microorganisms responsible for rumen carbon degradation. However, we lack a holistic view of the microbial food web responsible for carbon processing in this ecosystem. Here, we sampled rumen-fistulated moose, allowing access to rumen microbial communities actively degrading woody plant biomass in real time. We resolved 1,193 viral contigs and 77 unique, near-complete microbial metagenome-assembled genomes, many of which lacked previous metabolic insights. Plant-derived metabolites were measured with NMR and carbohydrate microarrays to quantify the carbon nutrient landscape. Network analyses directly linked measured metabolites to expressed proteins from these unique metagenome-assembled genomes, revealing a genome-resolved three-tiered carbohydrate-fuelled trophic system. This provided a glimpse into microbial specialization into functional guilds defined by specific metabolites. To validate our proteomic inferences, the catalytic activity of a polysaccharide utilization locus from a highly connected metabolic hub genome was confirmed using heterologous gene expression. Viral detected proteins and linkages to microbial hosts demonstrated that phage are active controllers of rumen ecosystem function. Our findings elucidate the microbial and viral members, as well as their metabolic interdependencies, that support in situ carbon degradation in the rumen ecosystem.