The Feral Pig in a Low Impacted Ecosystem: Analysis of Diet Composition and Its Utility

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Abstract

The feral pig is an exotic species in the Sierra La Laguna Biosphere Reserve, Baja California Sur, Mexico, and is considered harmful to the woodlands and tropical deciduous forest because of its foraging habits. To determine the taxonomic, nutrient, and energy content of the feral pig diet in this area, stomach contents of forty feral pigs were analyzed during the dry and rainy seasons in the woodlands and the tropical deciduous forest. Results indicate that feral pigs are frugivore-herbivore and stenophagous, feeding on a high percentage of fruits and seeds (77–87%) that provide high metabolizable energy (8293–11,585 kJ kg−1 dry matter) and moderate crude protein (94.1–151.6 g kg−1 dry matter) year round in the woodland and during the dry season in the tropical deciduous forest, or high percentage of leaves, stems, and tubers (44%) that provide low metabolizable energy (4740 kJ kg−1 dry matter) and moderate crude protein (107.5 g kg−1 dry matter) in the rainy season in the tropical deciduous forest. Therefore, diets based on fruits and seeds favor a lower quantity of food to meet the energy for maintenance of feral pigs than those based on other plant tissues: pine nuts < acorns ≈ wild figs < leaves, stems, and tubers. The observed diet based on acorns, along with estimates of acorn production results in a carrying capacity estimate of 0.031 adult feral pig ha−1 yr−1 during years of low acorn production. Thus, feral pigs in the area use fruits and seeds in both woodlands and tropical deciduous forest because of their quality as food resources. The availability of the food resources preferred by pigs (pine nuts, acorns, fan-palm fruits, and wild figs) during the year can be used to inform potentially sustainable feral pig carrying capacities and establish hunting programs.

Introduction

Feral pigs (Sus scrofa) are an exotic, invasive, and successful species in several ecosystems around the world (Thomson and Challies, 1988; Taylor and Hellgren, 1997; Campbell and Long, 2009). Their success is attributed to their generalist and opportunistic foraging habits, commonly feeding on plants, vertebrates, and invertebrates (Wood and Roark, 1980; Taylor and Hellgren, 1997). Their foraging behavior causes multiple undesirable changes in ecosystems, such as: depletion of the original biodiversity by increasing the number and biomass of exotic plants (Cushman et al., 2004), and reduction of plant cover and the density and biomass of native invertebrates (Howe et al., 1981). Therefore, feral pigs are a target for control or eradication where they have invaded (Campbell and Long, 2009).

The Sierra La Laguna Biosphere Reserve (the Reserve), Baja California Peninsula, Mexico, encompasses an isolated mountain range that supports mesic plant communities in stable states (León-de la Luz and Domínguez-Cadena, 2010), which host feral pigs. Since feral pig populations have been largely described as a pernicious agent in several ecosystems that they have invaded elsewhere in the world (Wood and Roark, 1980; Taylor and Hellgren, 1997; Campbell and Long, 2009), the Reserve managers have concerns about their effect on the plant communities and are also interested in the implementation of effective, low-cost, ethical, and socially acceptable management strategies for its control.

Knowledge about the composition of the feral pig diet could assist to assess the impact on the area and develop strategies for their management within the Reserve. Diet composition and quality are valuable indicators of the potential reproductive success and animal activity distribution on the communities (Singer et al., 1981; Baber and Coblentz, 1987; Bieber and Ruf, 2005; Santos et al., 2006). Although the composition of feral pig diets has been examined in different sites, such as in a forest in New Zealand (Thompson and Challies, 1998), and a forest, marsh, scrubland, and chaparral in the USA (Wood and Roark, 1980; Baber and Coblentz, 1987; Taylor and Hellgren, 1997), there is little information in the Reserve. Similarly, limited information is available about the nutritional composition of diet in feral pigs (Santa Catalina Island, USA; Baber and Coblentz, 1987) and even in wild pigs (Chile; Skewes et al., 2007). Therefore, the objectives of this research were: (a) to identify the components of the seasonal diet selected by feral pigs in different plant communities; (b) to determine the nutritional composition of the selected diet; (c) to estimate the amounts of food resources that feral pigs consume, and (d) to use the diet composition to make recommendations for feral pig control.

Section snippets

Study area

The Reserve occupies an area of 112,437 ha and is located between 23°42’N and 23°20’N, and between 109°46’W and 110°11’W. Topographically, it is a rough mountainous ecosystem composed by three plant communities along the altitudinal gradient: a) tropical deciduous forest between 400 and 1100 m asl; b) oak woodland between 1000 and 1500 m asl; and c) oak-pine woodland between 1500 and 2200 m asl (Fig. 1). Some other plant associations present are the riparian vegetation that follows the drainage

Diet composition

Plant matter prevailed in the stomach contents. The fruits and seeds category was the most consumed, followed by the categories leaves and stems, and underground plant matter. Meanwhile, the intake of the categories woody matter and litter, vertebrates, invertebrates, and minerals were low, ≤3.7% each one (Table 1).

Season, plant community, and their interaction had effects on the presence of the categories fruits and seeds, leaves and stems, and vertebrates in the stomach contents (Table 1).

Diet composition

Feral pigs are classified as generalist consumers (Baber and Coblentz, 1987; Taylor and Hellgren, 1997), which also describes their feeding habits in the Reserve. Feral pigs ate at least 59 identifiable items during a year and 17 to 32 items from different plant communities and seasons, and individually ate 7 items from different origin (plant, vertebrates, and invertebrates). These feral pigs can be also classified as stenophagous since 1 to 4 items with both a high frequency of occurrence

Implications

The biomass production of the main diet components and the food required to meet the energy maintenance costs of feral pigs can be used to estimate the carrying capacity of the Reserve. For instance, the oak Q. devia is a dominant species in 3514 ha of the woodlands and produces 38.6 ton acorns yr−1 (≈ 11 kg DM acorns ha−1 yr−1) during years with low production (2009–2011; Arnaud et al., 2014). Therefore, the area dominated by Q. devia could meet the requirements of metabolizable energy for

Declaration of Competing Interest

None.

Acknowledgements

Our sincere thanks go to the Centro de Investigaciones Biológicas del Noroeste SC and the Facultad de Medicina Veterinaria y Zootecnia of Universidad Nacional Autónoma de México for sample processing. Special thanks go to Abelino Cota-Castro, Carlos Palacios-Cardiel, Franco Cota-Castro, Miguel Domínguez-León, Reymundo Domínguez-Cadena, Joaquín Rivera-Rosas, and Catarino Rosas-Espinosa for their collaboration in this work. Thank you to the reviewers for their help to improve this manuscript.

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