Effects of large herbivores on grassland arthropod diversity

Abstract

Both arthropods and large grazing herbivores are important components and drivers of biodiversity in grassland ecosystems, but a synthesis of how arthropod diversity is affected by large herbivores has been largely missing. To fill this gap, we conducted a literature search, which yielded 141 studies on this topic of which 24 simultaneously investigated plant and arthropod diversity. Using the data from these 24 studies, we compared the responses of plant and arthropod diversity to an increase in grazing intensity. This quantitative assessment showed no overall significant effect of increasing grazing intensity on plant diversity, while arthropod diversity was generally negatively affected. To understand these negative effects, we explored the mechanisms by which large herbivores affect arthropod communities: direct effects, changes in vegetation structure, changes in plant community composition, changes in soil conditions, and cascading effects within the arthropod interaction web. We identify three main factors determining the effects of large herbivores on arthropod diversity: (i) unintentional predation and increased disturbance, (ii) decreases in total resource abundance for arthropods (biomass) and (iii) changes in plant diversity, vegetation structure and abiotic conditions. In general, heterogeneity in vegetation structure and abiotic conditions increases at intermediate grazing intensity, but declines at both low and high grazing intensity. We conclude that large herbivores can only increase arthropod diversity if they cause an increase in (a)biotic heterogeneity, and then only if this increase is large enough to compensate for the loss of total resource abundance and the increased mortality rate. This is expected to occur only at low herbivore densities or with spatio-temporal variation in herbivore densities. As we demonstrate that arthropod diversity is often more negatively affected by grazing than plant diversity, we strongly recommend considering the specific requirements of arthropods when applying grazing management and to include arthropods in monitoring schemes. Conservation strategies aiming at maximizing heterogeneity, including regulation of herbivore densities (through human interventions or top-down control), maintenance of different types of management in close proximity and rotational grazing regimes, are the most promising options to conserve arthropod diversity.

Keywords: defoliation; grazing; insects; invertebrates; large grazers; management; plants; soil compaction; species richness; ungulates.

Fig 1

Research focus of 141 published studies assessing the effects of large herbivores on arthropod diversity, conducted in open landscapes (grass- or heathlands) with arthropods identified to species level. (A) Studied taxa, (B) taxonomic spread (number of investigated taxa), (C) duration of sampling, (D) geographic location, and (E) year of publication (until 2012). We documented the identity of the most commonly assessed taxonomic groups (usually to order level, but sometimes to family or class level). A complete list of the analysed studies and definitions of arthropod groups can be found in Tables S1 and S3. *Arachnids: spiders, harvestmen, pseudoscorpions; **other groups: Mantodea, Phasmatodea, Neuroptera, Dermaptera.

Fig 2

Comparison of the response ratios of plant and arthropod diversity to an increase in grazing intensity (median ± interquartile range, whiskers represent 1.5 × interquartile range, dots represent outliers) (A) and the relationship between these response ratios (B) Data were extracted from 24 studies published between 1940 and 2013 reporting on the effects of grazing on both plant and arthropod diversity, supplemented with data obtained from several authors (see online Table S1).

Fig 3

A conceptual framework of the mechanistic pathways by which large herbivores directly and indirectly affect arthropod diversity. Arrows represent mechanisms. The first row of boxes represents biotic and abiotic conditions that are modified by large herbivores; the second row of boxes represents the mechanisms operating on arthropod individuals, populations and communities. (1) Direct effects: trampling and unintentional predation (Section III.12009); (2) direct effects: dung, carcasses, blood, live tissue (Section III.12009); (3) increase or decrease in plant species richness and changes in functional groups, the direction of which depends on large herbivore density and ecosystem properties (Section III.32006); (4) changes in vegetation structure: lowering of vegetation height through defoliation and changes in horizontal heterogeneity resulting from herbivore selectivity (Section III.22004); (5) changes in soil conditions (pH, bulk density) (Section III.42013); (6) changes in soil conditions can affect vegetation characteristics (Section III.42013); (7) changes in plant species richness can affect species richness of associated insect herbivores (Section III.32006); (8) a reduction in vegetation height can increase predation risk by vertebrate predators (Section III.22004); (9) direct competition for resources between the base of the arthropod food web and large herbivores (Section III.22004); (10) a reduction in vegetation height increases surface temperatures, but decreases shelter from climatic extremes and essential structures for egg deposition or web construction (Section III.22004); (11) changing soil properties may affect insects that spend part of their lives below ground (Section III.42013); (12–14) the combined changes in abiotic conditions, resources and predation determine the effects on each arthropod species, thereby affecting species richness; (15) due to the interactions between arthropod species, changes in species' abundances may have cascading effects on other species, with ultimate effects on total arthropod species richness.