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Los efectos de la elevacin y el uso de la tierra en la actividad y diversidad de especies de hormigas
The effects of elevation and land use on ant activity and ant species diversity
In todays world, it is becoming increasingly important to understand the effects that humans have on Earths ecosystems. Approximately 39 to 50% of the Earths land has been transformed or degraded by human activity (Vitousek et al. 1997). Ten to fifteen percent of that land has been altered for row-crop agriculture or for industrial development and six to eight percent has been converted to pastureland. These transformations represent the leading causes of decreases in biodiversity. As a result, they alter the global biogeochemical cycles and have considerable effects on climate change. Humans contribute more and
more everyday to climate change and it is known that many species are moving up in elevation in response to these changes. The purpose of this study was to examine the effects of elevation and land use on ant
activity and ant species diversity. I recorded the time it took for ants to arrive at a bait trap on an elevational gradient (1000 masl, 1375 masl, 1550 masl, and 1800 masl) in forest sites versus pasture sites in the Monteverde area. Then, I separated the collected ants into morphospecies in order to calculate species diversity. I found that as elevation increases, ant activity and ant species diversity decreases in the pasture sites. Also, ant activity and species diversity were higher in the pasture sites than in the forests sites. These results indicate that elevation and land use affect ant activity and ant species diversity. Provided that ants are good bioindicators, elevation and land use also impact invertebrate activity and diversity.
El propsito de este estudio fue examinar el efecto de la altitud y el uso de las tierras en la actividad y diversidad de especies de hormigas.
Text in English.
Diversidad de especies
Tropical Ecology 2009
Ecologa Tropical 2009
t Monteverde Institute : Tropical Ecology
Heil 1 The Effects of Elevation and Land Use on Ant Activity and Ant Species Diversity Laura Heil Department of Biology, St. Michael's College ABSTRACT In today's world, it is becoming increasingly important to understand the effects that humans have on Eart h's ecosystems. Approximately 39 to 50% of the Earth's land has been transformed or degraded by human activity (Vitousek et al. 1997). Ten to fifteen percent of that land has been altered for row crop agriculture or for industrial development and six to eight percent has been converted to pastureland. These transformations represent the leading causes of decreases in biodiversity. As a result, they alter the global biogeochemical cycles and have considerable effects on climate change. Humans contribute more and more everyday to climate change and it is known that many species are moving up in elevation in response to these changes. The purpose of this study was to examine the effects of elevation and land use on ant activity and ant species diversity. I recorded the time it took for ants to arrive at a bait trap on an elevational gradient (1000 masl, 1375 masl, 1550 masl, and 1800 masl) in forest sites versus pasture sites in the Monteverde area. Then, I separated the collected ants into morphospecies in order to calculate species diversity. I found that as elevation increases, ant activity and ant species diversity decreases in the pasture sites. Also, ant activity and species diversity were higher in the pasture sites than in the forests sites. Th ese results indicate that elevation and land use affect ant activity and ant species diversity. Provided that ants are good bioindicators, elevation and land use also impact invertebrate activity and diversity. RESUMEN En el mundo actual se est volvien do sumamente importante entender el efecto que los humanos ejercen sobre los ecosistemas en la Tierra. Aproximadamente entre un 39 y un 50% de la tierra en el planeta ha sido transformada o degradada por la actividad humana. Diez a quince por ciento de l a tierra ha sido alterada con fines agrcolas o para desarrollo industrial, y entre un seis y ocho por ciento ha sido convertido en pastizales. Esta transformacin representa una gran causa en la disminucin de la biodiversidad. Como resultado, se han al terado los ciclos biogeoqumicos globales teniendo un impacto importante en el cambio climtico. El propsito de este estudio fue examinar el efecto de la altitud y el uso de las tierras en la actividad y diversidad de especies. Tom el tiempo que les tom aba a las hormigas para llegar al cebo en un gradiente elvacional de 1000 a 1800 msnm en bosque y pastizales en el rea de Monteverde. Luego separe las hormigas colectadas en morfoespecies para calcular la diversidad de especies. Encontr que conforme au menta la altitud la actividad y diversidad de especies disminuye en los pastizales. Adems, la actividad y diversidad de hormigas es mayor en pastizales que en bosques. Estos resultados demuestran que la elevacin y el uso de las tierras afectan la activ idad y diversidad de especies. Demostrando que las hormigas son buenos bioindicadores, la elevacin y el uso de la tierra tambin impactan la actividad y diveersidad INTRODUCTION Effect of humans on ecosystems It is necessary that we perform an environm ental impact assessment to better understand the intensity of humans' effects on Earth's ecosystems and bioindicators can be used to
Heil 2 aid us in this process. Invertebrates are becoming an increasingly common taxon that are used as bioindicators because the y are ubiquitous, abundant, and easily sampled (Rosenberg et al. 1986; Anderson et al. 2002). They fill important roles in various ecological functions such as soil decomposition, soil nutrient cycling (Hancock 1994), and they provide food for many fish a nd wildlife species. Similarly, land use alterations may alter invertebrate species richness and abundance, indicating that invertebrates are useful bioindicators for assessment of the impact of land use (Rosenberg et al. 1986). An example of how differen t land uses can affect invertebrates is with dung beetles in tropical dry forest habitats (Andresen 2008). Anderson found that dung beetle community diversity decreased with a decrease in forest cover. A specific invertebrate taxon that is commonly used a s a bioindicator in studying the effects of land use is ants. Like many other invertebrates, ants occupy a variety of niches in nature and thus, fill many important roles in ecological processes (Bestelmeyer and Wiens 1996; Post and Jeanne 1982). This al lows ants to represent the effects of land transformation and elevation for the entire ecosystem (Anderson and Majer 2004). In addition to the various effects of land use on ecosystems, elevation also impacts ecological interactions. In recent years these elevation effects have been especially important to understand because many species are changing their elevational ranges in response to climate change (Hodkinson 2005). In 1994, Olson conducted a study that examined how leaf litter invertebrates were di stributed along a Neotropical altitudinal gradient. He found that as elevation increases, species richness decreases and suggests that these trends correlate with ecotones. When the cloud layer was reached, he observed a much sharper decline in invertebr ate species richness than in other altitudinal transitions. He proposes that some invertebrates species could not occur at these elevations because of the low temperatures, reduced solar radiation, and high mist levels. These conditions affect metabolic processes and limit optimum foraging conditions. I collected data on the time it took for ants to arrive at a bait trap, the proportion of baits visited and the ant species diversity all along an elevational gradient in forest sites versus pasture sites. I predicted that as elevation increased, ant activity and ant species diversity would decrease. In addition, I predicted that ants in the forests would respond more quickly to the bait traps than ants in the pastures. MATERIALS AND METHODS I conducted m y study across a gradient that consisted of two habitats, a forest and a pasture, at four elevations in the Monteverde Area. The first elevation was located at 1000 masl and was conducted at The University of Georgia at Costa Rica (UGA). The second eleva tion was located 1350 masl for the forest site in Bajo del Tigre and 1400 masl for the pasture site at Frank Joyce's pasture in Bajo del Tigre. Since the elevations for these locations were different for the forest and the pasture, I averaged them togethe r to have a common number of 1375 masl for figures and tables. The third elevation was located at 1550 masl and was conducted at the Monteverde Biological Station. The fourth elevation was located at 1800 masl and was conducted at the continental divide for the forest site and the television towers for the pasture site. For each forest site, I walked in approximately 20 m to avoid edge effects.
Heil 3 At each location I set up three transects with three baits per transect. The transects and the baits were all five meters apart. At each of the nine locations I placed a marking flag, and a Petri dish with a nickel sized portion of tuna. I collected data for four hours from 1200 to 1600. checking the baits for ants every half hour. As I checked the baits, I rec orded the time it took for the first ant to arrive, and collected that ant in a separate vial. I then collected any other additional ants from the baits. I placed the collected ants in separate vials for each bait for later identification. Ants were pres erved in 95% alcohol. Ants were identified to subfamily and separated into morphospecies. The diversity of ant species at different elevations and between different land uses were estimated using Shannon Weiner's Diversity Index and tested for differences in diversity with a t test. RESULTS Effects of Elevation As elevation increases, the time for the first ant to arrive increases in pastures beginning at 1375 masl (Fig. 1). The proportion of baits visited decreases as elevation increases in all pasture s ites (Fig. 2). At the 1000 masl pasture, 89% of the traps were visited and at the 1800 masl pasture, 11% of the traps were visited (Fig. 2). As elevation increases, the diversity of ant species in the pasture sites decreases (R 2 = 0.964, p value = 0.018 N = 2605 individuals). The pasture site at 1000 masl is the most diverse with a Shannon Weiner's Diversity Index value of 1.10. The forest site at 1550 masl is the most diverse with a Shannon Weiner's Diversity Index value of 0.57 (Fig. 3). Effects o f Land Use The proportion of baits visited is higher in the pasture sites than in the forest sites at all elevations (Fig. 2). Overall, the ant species collected in pasture sites are more diverse than those of the forest sites (Fig. 3).
Heil 4 FIGURE 1. The a verage time (hrs.) it took for the first ant to arrive (N = 9 baits per site) at each forest and pasture site among four elevations in the Monteverde area, Costa Rica. FIGURE 2. The proportion of baits visited (N = 9 baits per site) at each forest and pasture site among four elevations in the Monteverde area, Costa Rica.
Heil 5 FIGURE 3. The Shannon's Diversity Index Values (N = 2605 individuals) at forest and pasture sites for four elevations in the Monteverde area, Costa Rica. DISCUSSION I expected that elevation and land use would both affect ant activity and ant species diversity. I found that as elevation increases, ant activity decreases in pastures. This is observed in the relationship between elevation and the time it takes for the first ant to arrive and also between elevation and the proportion of baits visited. This trend is found in many previous studies and is often attributed to the impaired efficiency of invertebrates at lower temperatures and misty conditions (Hodkinson 2005). As ele vation increases, the cloud layer is eventually reached and at these conditions, invertebrates decrease their activity because they are poikilothermic. This means that their body temperature tends to be very close to that of the ambient temperature (Romos er and Stoffolano 1998). This could also explain why I found that the species diversity in the pasture sites decreases as elevation increases. An explanation as to why this relationship was observed in the pasture sites and not in the forest sites could be because the ground layer, the location of the bait traps, in pasture sites are more immediately affected by the temperature and misty conditions than in the forest sites. There was more activity and more species diversity in the pasture sites than in th e forest sites. Despite this result being the opposite of many other studies (Silva et al. 2007) and of my prediction, it could be due to the increased amount of obstructions such as leaves, twigs and saplings for the ants in the forest sites. As a resul t, these obstructions could cause a change in ant foraging strategies (Jeanne 1979). In the pastures, the bait sites were very open and easily accessible for the ants. However, in the forests, the bait sites were surrounded by leaf litter, twigs and sapl ings. Jeanne (1979) found that ant predation rates were higher in low second growth vegetation than in forests. He attributed this result to changes in foraging strategies between the two sites. Some other explanations as to why I received these results are my very small sample size and my flawed experimental design. I placed the tuna baits on Petri dishes
Heil 6 and this may have had an effect on the ant activity. Also, I think that if I had used more tuna per bait, more ants would have arrived. To conclude, it is clear that an increase in elevation causes a decrease in ant activity and species diversity. This indicates that ants are less active and less diverse at higher elevations. With global annual temperatures increasing at alarming rates and recent cha nges in species ranges, it would be interesting to repeat this study in the future to investigate whether ant activity and ant species diversity is increasing at higher elevations and whether this change is significantly correlated with changing temperatur es. I also found there to be higher ant activity and ant species diversity in pasture sites than forest sites. It is evident that land use and elevation have major impacts on ecological processes of ants and of other species as well assuming that ants ar e good bioindicators. ACKNOWLEGEMENTS I would like to first and foremost thank Anjali Kumar for her endless assistance and enthusiasm. I would also like to thank my parents, William and Philinda Heil, for their encouragement and for giving me the opport unity to study abroad and St. Michael's College for supporting all of my academic endeavors. Also, thank you to Yi men Araya for his vast knowledge of biological statistics and to Jos Carlos Cadern Ullmoa for providing me with all of my supplies. A fin al thank you to the Monteverde Biological Station, The University of Georgia at Costa Rica and Bajo del Tigre for allowing me to collect data on their property. LITERATURE CITED Anderson, A.N., B.D. Hoffmann, W.J. Muller, and A.D. Griffiths. 2002. Using ants as bioindicators in land management: Simplifying assessment in ant community responses. Journal of Applied Ecology 39: 8 17. Anderson, A.N., and J. D. Majer. 2004. Ants show the way down under: Invertebrates as bioindicators in land management. Front Ecol. Environ. 2:291 298. Andresen, E. 2008. Dung beetle assemblages in primary forest and disturbed habitats in a tropical dry forest landscape in western Mexico. Jour. Insect Conserv. 12: 639 650. Bestelmeyer, B.T., and J.A. Wiens. 1996. The effects of land use on the structure of ground foraging ant communities in the Argentine Chaco. Ecological Applications 6: 1225 1240. Hancock, P.W. 1994. Land use, root mass and diversity of soil macroinvertebrates. In: CIEE: Tropical Biology and Conservation 100 11 6. Hodkinson, I.D. 2005. Tropical insects along elevation gradients: species and community responses to altitude. Biol. Rev. 80: 489 513. Hunter, M.L. 1999. Maintaining Biodiversity in Forest Ecosystems Cambridge University Press, Cambridge, U.K., p. 436. Jeanne, R.L. 1979. A latitudinal gradient in rates of ant predation. Ecology 60:1211 1224. Olson, D.M. 1994. The distribution of leaf litter invertebrates along a Neotropical altitudinal gradient. Journal of Tropical Ecology 10: 129 150.
Heil 7 Post, D.C., and R. L. Jeanne. 1982. Rate of Exploitation of arboreal baits by ants in an old field habitat in Wisconsin. American Midland Naturalist 108: 88 95. Romoser, W.S. and J.G. Stoffolano. 1998. The Science of Entomology, sixth ed. The McGraw Hill Companies, Inc., MA, pp. 268 269. Rosenberg, D.M., Danks, H.V., Lehmkuhl, D.M. 1986. Importance of insects in environmental impact assessment. Environ. Manage 10: 773 783. Silva, R.A., Machado Feitosa, R.S., Eberhardt, F. 2007. Reduced ant diversity along a habitat regenerat ion gradient in the Southern Brazilian Atlantic Forest. Forest Ecology and Management 240: 61 69. Vitousek, P.M., H.A. Mooney, J. Lubchenco, and J.M. Melillo. 1997. Human domination of earth's ecosystems. Science 277: 494 499.