Research Articles
Does Cattle Grazing Affect Ant Abundance and Diversity in Temperate Grasslands?

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Abstract

Half of the world's land base is grazed by domesticated livestock. Because of the important functional role of ants in grasslands, it is important to understand the effect of livestock grazing on ant abundance and diversity. The objectives of this study were to examine the effect of cattle grazing and site productivity on the abundance, species richness, and species diversity of ants in Lac du Bois Grasslands Provincial Park, British Columbia, Canada. We hypothesized that the measured ant variables would be lowest in grazed areas and at low site productivity. Pitfall trapping was conducted at four sites: two at each low and high site productivity levels. At each site an ungrazed (fenced exclosure) and grazed transect was sampled during May, July, and August of 2008. Captured ants were preserved in ethanol and identified. Eight genuses of ants were collected: Tapinoma, Camponotus, Formica, Lasius, Aphaenogaster, Myrmica, Solinopsis, and Temnotharox. The mean number of ants per pitfall was higher at high site productivity sites that were grazed (15.10 ±  SE) compared to high productivity sites ungrazed (3.28 ±  SE); grazing at low productivity reduced numbers of ants from 5.07 (± 0.70 SE) to 2.20 (± 0.39 SE) (F  =  21.806; P [ 0.001). Tapinoma sessile and A. occidentalis had the greatest numbers in the pitfall traps. Species richness (F  =  23.330, P [ 0.001) and diversity (F  =  11.764, P  =  0.001) followed a similar trend. Because productivity and cattle grazing affect ant diversity and abundance, and ants impact ecosystem functioning, these factors should be considered in management of grasslands.

Resumen

La mitad de las tierras del mundo son pastoreadas por animales domésticos. Debido a la importancia práctica de las hormigas dentro de los pastizales es importante comprender el efecto del ganado en pastoreo en la diversidad y abundancia de las hormigas. Los objetivos de este estudio fueron examinar el efecto del pastoreo y la productividad del sitio en la abundancia, la riqueza, y la diversidad de especies de hormigas en los pastizales de Lac du Bois en el parque Provincial, de la Columbia Británica, en Canadá. Nuestra hipótesis fue que las variables medidas en las hormigas serían menores en áreas pastoreadas y en sitios de menor productividad. Se utilizaron trampas en cuatro sitios: dos por cada sitio de alta y baja productividad. En cada sitio, transectos en áreas sin pastoreo (excluidas) y con pastoreo fueron muestrados durante mayo, Julio y Agosto del 2008. Las hormigas capturadas fueron conservadas en etanol e identificadas. Se recolectaron 8 géneros de hormigas: Tapinoma, Camponotus, Formica, Lasius, Aphaenogaster, Myrmica, Solinopsis, and Temnotharox. El número medio de hormigas por trampa fue mayor en de alta productividad y que fueron pastoreados (15.10 ±  SE) comparados a los sitios de alta productividad sin pastoreo (3.28 ±  SE); mientras que en los sitios de baja productividad se redujo el número de hormigas de 5.07 (± 0.70 SE) a 2.20 (± 0.39 SE) (F  =  21.806; P [ 0.001). Las hormigas Tapinoma sessile y A. occidentalis tuvieron los mayores números en las trampas. La riqueza de las especies (F  =  23.330, P [0.001) y la diversidad (F  =  11.764, P  =  0.001) siguió una tendencia similar. Debido a que la productividad y el pastoreo del ganando afectan la diversidad, la abundancia, y el impacto de las hormigas en el funcionamiento del ecosistema, estos factores deben considerarse en el manejo de los pastizales.

Section snippets

INTRODUCTION

Ants (Hymenoptera: Formicidae) represent 10–15% of animal biomass in the majority of terrestrial systems and are among the most dominant land organisms on Earth (Hölldobler and Wilson 1990). Ants contribute to ecosystem function in their ecological roles as pollinators (Hölldobler and Wilson 1990), predators, and prey (Raine and Kansas 1990; Bull et al. 1995; Torgerson and Bull 1995; Noyce et al. 1997); and have important roles in grasslands as soil engineers (Whitford et al. 1986), seed

Site Description

Ant specimens were collected from the temperate grasslands in Lac du Bois Provincial Park and adjacent areas within the bunchgrass grasslands 2–10 km north of Kamloops in the southern interior of British Columbia, Canada. The region is semiarid, with annual precipitation of 279 mm, 75.5 mm of which is snowfall. Average annual daily temperature for the region is 8.9°C, the warmest month is July, 21.0°C, and the coldest is January, -4.2°C (Environment Canada 2009). We selected two different

Species Identified

Ant species from eight genera were identified from the pitfall traps (Table 1). Most species were captured at both elevations; however, Camponotus vicinus Mayr, Solenopsis molesta Say, Temnothorax rugatulus Emery, and Temnothorax nevadensis W. M. Wheeler were captured only in the lower grasslands; Myrmica fracticornis Forel and ants in the Formica sanguinea species group occurred only in the upper grasslands. Formica puberula Emery was a new record for British Columbia, Canada. Tapinoma sessile

DISCUSSION

In all four measured response variables (total ant abundance, T. sessile abundance, species richness, and Hill diversity) we found interacting effects between elevation and grazing. We found that the number of ants captured increased in grazed sites at upper elevation (i.e., high productivity). However, ant numbers was reduced by grazing in lower-elevation sites. Because of the interaction effects, our results only show partial support for our two hypotheses.

Although pitfall traps are the most

MANAGEMENT IMPLICATIONS

Ants perform many different ecosystem functions and therefore have extensive impacts on their environment. Although grassland management is typically concerned with vegetation and mammals, consideration of invertebrates such as ants is important for maintaining biodiversity and ecosystem function. The results of this study are useful for informing grazing and biodiversity management of grasslands. This study suggests that when designing grazing regimes ant diversity could be maximized to

ACKNOWLEDGMENTS

We thank Jessica Gossling and Anna-Maria Pellet for help collecting pitfalls in the field. Dr. Rob Higgins verified identification of ant taxa. We thank Agriculture and Agri-Foods Canada, Kamloops Range Research branch for allowing use of cattle exclosures and the British Columbia Ministry of Environment for allowing access to Lac du Bois Provincial Park. This work was funded by a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant to L.H.F.

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