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The community ecology of ants (Formicidae) in Indonesian grasslands with special focus on the tropical fire ant, Solenopsis geminata.



The community ecology of ants (Formicidae) in Indonesian grasslands with special focus on the tropical fire ant, Solenopsis geminata.


Rebecca Sandidge

Doctor of Philosophy in Environmental Science Policy and Management, Berkeley

Professor Neil Tsutsui, Chair

Invasive species and habitat destruction are considered to be the leading causes of biodiversity decline, signaling declining ecosystem health on a global scale. Ants (Formicidae) include some on the most widespread and impactful invasive species capable of establishing in high numbers in new habitats. The tropical grasslands of Indonesia are home to several invasive species of ants. Invasive ants are transported in shipped goods, causing many species to be of global concern. My dissertation explores ant communities in the grasslands of southeastern Indonesia. Communities are described for the first time with a special focus on the Tropical Fire Ant, Solenopsis geminata, which consumes grass seeds and can have negative ecological impacts in invaded areas. The first chapter describes grassland ant communities in both disturbed and undisturbed grasslands. The second chapter narrows in focus to describe the utilization of grass seeds as a food resource for S. geminata and the potential for spread of this invasive ant given an unlimited food supply. The third, and final, chapter describes competition between ant species at food resources. Solenopsis geminata competes with several native and introduced ant species. These interactions are analyzed and I report on the ability of invasive ants to dominate invaded habitats.

Chapter 1 includes a large-scale diversity survey across eight islands in and around Komodo National Park, Nusa Tenggara Timur, Indonesia. Invasive ants are able to disrupt native habitats and the communities of ants found in them. Land use plays a role in invasion as human-mediated disturbances facilitate the introduction of non-native species. Biogeography, land use, climate, and habitat structure help to shape local communities. We surveyed eight islands using pitfall trap transects. Transects were placed in undisturbed and disturbed grasslands. Data on ant species counts and identification was used to test four hypotheses related to species distribution and abundance. 1. Introduced species will have broader distributions than native species across space, habitat, and seasons. 2. Habitat structure (grass and trees) influences ground-dwelling ant species abundance, diversity, and composition. 3. Grassland disturbance is associated with an increase in introduced species richness and abundance. 4. Development (human-made landscape elements) and distance from a port (measures of connectivity) have a greater influence on diversity than island size. We found that introduced species are generally more numerous than native species, but that some introduced species present in the region were fairly restricted both spatially and numerically. Season and vegetation structure were related and influenced by land use. Differences are linked to differences in ant community compositions. In addition to differences related to habitat structure, species richness and diversity was influenced by the distance between the site and the port of Labuan Bajo.

The second chapter of my dissertation focuses on fire ant diets. The Tropical Fire Ant consumes large quantities of grass seeds. We described the diet of S. geminata in populations that inhabit grasslands and villages with comparisons to other ant species. Omnivorous ants are able to utilize a wide range of food resources, and this can enable invasion and even dominance in novel landscapes. Dietary specialization has evolved numerous times and can confer competitive advantages, optimal nutrition, and reduced cognitive needs in foraging. The Tropical Fire Ant, Solenopsis geminata, is a globally distributed invasive species. Solenopsis geminata has evolved a major worker caste for seed milling and granivory while the species is largely a dietary generalist. Here we used stable isotope analyses and behavioral assays to describe dietary shifts in S. geminata during range expansion from villages into disturbed grassland. These methods were used to question the benefits and draw-backs of dietary specialization in range expansion across habitat types with seemingly unlimited seed resources. We analyzed δ13C and δ15N in four ant species in adjacent village, disturbed grassland, and undisturbed grassland habitats on one island in Nusa Tenggara Timur, Indonesia. Isotope analysis was coupled with behavioral assays that delineated preferences for some grasses over others. Seed-size was manipulated to assess the influence of a worker’s ability to move a seed relative to its attractiveness. Solenopsis geminata shifts from a diet of animal-based foods and C3 plants in villages to a largely C4 grass seed-based diet in grasslands. Co-occurring ant species are within the same trophic level but show lower utilization of C4 plants in savannas. Solenopsis geminata collects seeds from exotic grasses common in disturbed Indonesian grasslands at higher rates than larger seeds of native grasses. When the larger native seeds are cut to smaller sizes, they are collected at a higher rate. Foraging assays show that large seed size in native grasses may pose a barrier to seed consumption and expansion into undisturbed grasslands with otherwise similar structure and environmental conditions.

In chapter 3 I look at competition between ant species at food resources. Competition for food resources within a community of native and invasive ant species was investigated in disturbed and undisturbed grassland ecosystems on islands in Komodo National Park. Several species of invasive ants have established populations across the region, threatening endangered ground-nesting wildlife. Invasive ants are generally very abundant and may have negative impacts on local flora and fauna. Competition with dominant native ant species may limit access to resources for introduced ants offering some level of biotic resistance to the impacts of invasion. We examine intraspecific competition between ants at food resources in disturbed and undisturbed grasslands. Our approach examines forager abundance, temporal resource partitioning, and dominance-discovery tradeoffs, three concepts related to foraging behavior and access to resources that sustain ant colonies. We test the following hypotheses to describe competitive interactions. 1. Introduced species will be more abundant and widespread than native species. 2. Common and widespread species partition resources by foraging at different times of day. 3. Dominance-discovery tradeoffs exist between dominant species and weaker competitors.

Tuna baits were observed at five sites on three islands in Nusa Tenggara Timur, Indonesia.

Observations were made during morning, mid-day, and evening hours of the wet and dry

seasons. A total of 81,188 ants were observed. Ants were collected at the end of each observation

period for identification. Two invasive ant species, Solenopsis geminata and Trichomyrmex

destructor, dominated baits in all disturbed habitat types. In undisturbed grassland, native ants

were as common as invasive ants, though they did not exclude invasive ants from baits. We did

not find evidence of temporal partitioning of the food resource within a 24-hour period, but

results suggest that there may be seasonal partitioning between the top competitors. Solenopsis

geminata was able to outcompete T. destructor but could be resisted by a native species,

Iridomyrmex sp 2. No evidence of a discovery-dominance tradeoff was found between top

competitors though Nylanderia vaga and, to a lesser extent, Paratrechina longicornis may

benefit from earlier arrival and departure when dominant species arrive. Interspecific

competition between a network of dominant species may prevent any one species from becoming

overly abundant and excluding less successful competitors. Complex interactions between

dominant species and asymmetric foraging behaviors most likely create opportunities for weak

competitors to access resources and may further limit resource access for invaders.

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