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Riqueza de especies de plantas vasculares en las zonas de transicin ribereos de la Quebrada La Cruz y La Quebrada el Socorro en Monteverde, Costa Rica
Species richness of vascular plants in the riparian transition zones of the Quebrada La Cruz and the Quebrada Socorro in Monteverde, Costa Rica
While many studies have been performed to determine the effects of urbanization in the later stages of development on biodiversity, little is known about how biodiversity is affected by urbanizations in its early phases. This research evaluates how the biodiversity of herbaceous plants in the riparian ecotones of two rivers is affected in the transition from a rural to town setting. Morpho species diversity was sampled on two rivers of the Monteverde, Costa Rica area; the Quebrada Socorro, relatively unaffected by human influence, and the Quebrada La Cruz, which is greatly influenced by both the growth of town infrastructure and agriculture. Ten sites were sampled at equal elevations on each river. Shannon-Weiner diversity index values were compared from each river at each elevation. It was found that the Socorro had a higher diversity value (H = 3.40) than the La Cruz (H = 2.63) (Modified t-test, t = -8.55; df = 246.95; P < 0.05). In addition, there were significant differences in the H values at six out of the ten different elevations, and of those six elevations, five had higher H values on the Socorro. Other observations found that species plant composition was radically different between the two rivers. This research implies a change in plant species composition and loss of diversity in riparian ecotones with increased urbanization and human influence which has repercussions for stream ecosystem food webs that depend on a diverse riparian input
of leaf litter at the primary trophic level.
Mientras que muchos estudios se han realizado para determinar los efectos de la urbanizacin en el estado avanzado de desarrollo sobre la biodiversidad, poco se sabe sobre como la biodiversidad es afectada por urbanizaciones en sus fases tempranas. Esta investigacin evala como la biodiversidad de las plantas herbceas en los ecotonos ribereos de dos ros afecta la transicin de un area rural al ajuste de un pueblo. La diversidad de las especies de morfo fue muestreada en dos ros de Monteverde, Costa Rica; Quebrada Socorro, relativamente inafectada por la influencia humana, y Quebrada La Cruz, que esta influenciado grandemente por el crecimiento de la infraestructura del pueblo y la agricultura.
Text in English.
Riparian plants--Costa Rica--Puntarenas--Monteverde Zone
Plant diversity--Costa Rica--Puntarenas--Monteverde Zone
Plantas ribereas--Costa Rica--Puntarenas--Zona de Monteverde
Diversidad de plantas--Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology 2008
Riparian ecotones--Costa Rica--Puntarenas--Monteverde Zone
Ecologa Tropical 2008
Ecotonos ribereos--Costa Rica--Puntarenas--Zona de Monteverde
t Monteverde Institute : Tropical Ecology
1nrnnn nn nn nnnnn !n"nn!n#n$ % ABSTRACT While many studies have been performed to determine the effects of urbanization in the later stages of development on biodiversity, little is known about how biodiversity is affected b y urbanizations in its early phases. This research evaluates how the biodiversit y of herbaceous plants in the riparian ecotones of two rivers is affected in the transition from a rural t o town setting. Morpho species diversity was sample d on two rivers of the Monteverde, Costa Rica area; the Quebrada Socorro, relatively unaffected by human influence, and the Quebrada La Cruz, which is great ly influenced by both the growth of town infrastruc ture and agriculture. Ten sites were sampled at equal el evations on each river. Shannon-Weiner diversity in dex values were compared from each river at each elevat ion. It was found that the Socorro had a higher diversity value (HÂ’ = 3.40) than the La Cruz (HÂ’ = 2.63) (Modified t-test, t = -8.55; df = 246.95; P < 0.05). In addition, there were significant differences in the HÂ’ values at six out of the ten different eleva tions, and of those six elevations, five had higher HÂ’ values on the Socorro. Other observations found that speci es plant composition was radically different between t he two rivers. This research implies a change in pl ant species composition and loss of diversity in ripari an ecotones with increased urbanization and human influence which has repercussions for stream ecosys tem food webs that depend on a diverse riparian inp ut of leaf litter at the primary trophic level RESUMEN Mientras que muchos estudios se han realizado para determinar los efectos de la urbanizacin en el est ado avanzado de desarrollo sobre la biodiversidad, poco se sabe sobre cmo la biodiversidad es afectada po r urbanizaciones en sus fases tempranas. Esta investi gacin evala cmo la biodiversidad de plantas herbceas en los ecotonos riparios de dos ros se a fecta en la transicin de un rea rural al ajuste d e la ciudad. La diversidad de morfo especies fue muestre ada en dos ros de Monteverde, Costa Rica; Quebrada Socorro, relativamente inafectada por influencia hu mana, y Quebrada La Cruz, que es influenciado grandemente por el crecimiento de la infraestructur a de la ciudad y la agricultura. Diez sitios fueron muestreados a elevaciones iguales en cada ro. Los valores de ndice de la diversidad de Shannon-Weine r fueron comparados de cada ro en cada elevacin. Se encontr que quebrada Socorro tena un valor ms alto de la diversidad (HÂ’ = 3.40) que La Cruz (HÂ’ = 2.63) (t-prueba modificada, t = -8.55; df = 246.9 5; < de P; 0.05). Adems, hubo diferencias significativas e n los valores de H en seis de las diez diversas elevaciones, y de esas seis elevaciones, cinco ten an valores ms altos de H en el Socorro. Otras observaciones encontraron que la composicin de pl antas de la especie es rdicalmente diferente entre los dos ros. Esta investigacin implica un cambio en l a composicin de especies de plantas y la prdida de diversidad en ecotonos riparios con la urbanizacin creciente y la influencia humana que tiene repercusiones en las cadenas alimenticias de las qu ebradas que dependen de una entrada de hojarasca al ro como el primer nivel trfico.
2 INTRODUCTION Future population growth in the American tropics is projected to increase, while rural populations are predicted to remain the same or dec line (Wright and Muller-Landau 2006). This implies that urbanization is a trend we will see more often in the American tropics. Consequently, there is a need to anticipat e the possible environmental effects of an increased density of populations, and evaluate w hat this will mean for biological communities. Specifically, increased urbanization will have dras tic effects on stream communities. Studies in the United Sates have shown an overall degradation of stream ecologies resulting in losses of species richness a nd of sensitive species as urbanization increases (Lussier et al. 2008). Increased polluted storm water run-off and sedimentation due to erosion are characteristics of increased urb anization (Lussier et al 2008). At risk in most cases is the water quality of a large portion of the worldÂ’s freshwater sources. Riparian transition zones are the buffer zones that minimize these effects of urbanization on stream ecologies. These ecotones serve many purp oses such as preventing erosion, reducing run-off, maintaining water quality, and pr oviding habitat to many diverse species and taxa (Burton et al. 2005; Naiman and D camps 1997). In spite of the invaluable services riparian ecoton es provide for the environment, they are often the most sensitive to environmental changes and most likely to be disturbed by humans (Nilsson and Svedmark 2002). Pl ant diversity exemplifies this phenomenon because is especially important to maint aining a healthy stream ecosystem, and simultaneously is very susceptible to human dis turbances. Studies have been carried out to studying the effects of human influences and urbanization on the biodiversity of different taxa such as woody plants (Burton et al. 2005) and macro invertebrates, and have found that increased urbanization reduces dive rsity across most taxa. Another study showed that this general decrease in diversity was the result of the loss of the less tolerant species, and the flourishing of the more tolerant s pecies (Lussier et al. 2008). These findings have large implications for the riparian e cotone and the stream ecosystems they border. A study by Naiman et al. (2000) found a dis proportionally large abundance of plant species in riparian transition zones compared to their surrounding ecosystems. This species richness of riparian ecotones provides the primary energy input via leaf litter for the food web of the streams they protect (Lecerf et al. 2005). If urbanization is having an affect on riparian plant species diversity or compo sition, the effects will be felt throughout the detritus based food web. While there is research that determines the effects of urbanization in the later stages of development on biodiversity, little resea rch has been done to study this relationship in the transition from rural to town s ettings. During this early period of urban growth there is a unique combination of the populat ion that depends on the rural setting, and others that live in the more developed areas. T he Monteverde region of Costa Rica provides an opportunity to examine this scenario of how an increase in infrastructure associated with growth of towns in otherwise agricu ltural areas can affect riparian plant species diversity. The region, because of itÂ’ rapid population growth, provides a good model that can be generally applied to many other L atin American areas of the same nature.
3 Due to previous research, I hypothesized that ripar ian forests situated in habitat experiencing the effects of increasing human densit y and concentrated use will be impacted in terms of plant community diversity and composition. I predicted that the impact of increased infrastructure growth in the Mo nteverde region, will be to decrease herbaceous plant species diversity, while changing the composition of species that are more tolerant of stress and disturbance. METHODS AND MATERIALS Study Sites This study was conducted in the Monteverde region o f Costa Rica during late July through the beginning of August, known as the Â“vera nillo de San JuanÂ” or little summer portion of the rainy season (MayOctober). During this season there are light to moderate winds accompanied by mist precipitation and occasio nal convective precipitation characteristic of traditional rainy season (Clark e t al. 2000). Monteverde is a relatively small region that ranges from a town setting in the Santa Elena area, to rural farmlands in San Luis, i ntermingled with sections of undisturbed and secondary growth forest. The popula tion density ranges from more dense in the Santa Elena region to sparse in the San Luis region. Monteverde provides a chance to research how urban and agricultural pressures co mbine to affect plant diversity and species richness along streams. Data were collected from two streams in the Monteve rde area. The first was the Quebrada Socorro, a river that begins at 1400 m in elevation and flows down a very steep mountain slightly to the east of Monteverde through the San Luis region. San Luis is a rural farming community with sparse populations and minimal environmental impact. There was noticeably more vegetation along these ba nks and no apparent signs of human interference. The Quebrada Socorro provided a good example of a river that has had minimal human disturbance recently. The second river sampled was the Quebrada La Cruz. This river begins at 1400 m in elevation as well. The La Cruz is west of Santa Elena and runs towards Caitas. Santa Elena is the most urbanized part of the community o f Monteverde, and has a much higher population density than San Luis. In addition, this is the area where most of the tourist economy is concentrated. There are more cars and ta xis, restaurants, hotels, and supermarkets to meet the needs of the influx of tou rists that come through the Monteverde area. As a result, water run-off and ove rall air quality is degraded in comparison to San Luis. Outside of the Santa Elena region the river flows alongside the rode, winding around houses and some farms. There w as noticeably more human influence such as litter and larger pieces of dispo sed of construction materials, and less vegetation along the Quebrada La Cruz then the Quebrada Socorro. The Quebrada La Cruz served as an example of a river that is highly influenced by urbanization in progress. Measuring Plant Species Diversity Data were collected from the two rivers starting at 1090 m and sampled at ten meter intervals in elevation for a total of 20 sites, ten on each river (range from 1090 m to 1180
4 m). Terrestrial, herbaceous plants directly along t he bank of each river was sampled. Any non-woody vascular plant under one and three quarte rs meters was sampled. Each of the ten sites along each river was selected at random by starting at the elevation of 1080 m (ten meters below my intended s tarting point), and walking up the middle of the river looking at the wristwatch altim eter. As soon as the altimeter indicated the right elevation, I immediately stopped, turned to the bank with easiest access, and put an indication stake in the ground at the meeting of the river and the bank. I then measured out a one meterby one meter area to sample, using the stake as an indicator of the starting point from which to measure. Once the area was marked, I started at one corner of the area and observed and recorded plant species and abundance for each site along the river. Plants were assigned a number or letter for identification, and distinguished based on morphological differences. In addition, I took a sample of each species as well and later confirmed morphological differences of the pl ants. Velocity, temperature and pH of the water were also monitored along each river in order to make sure that there were not considera ble differences among these variables on each river. Velocity was measure by timing how l ong it took for a ping pong to travel one meter down the river. Temperature of the water was measured using a standard thermometer. Samples of water were collected in via ls for later pH analysis using pH Wide Range TesTabs. Analyzing Data After data concerning the species richness and abun dance were recorded at each of the ten sites along the two rivers, the information for each elevation was put into an Excel template spreadsheet that calculated the number of individuals at each site (n), species richness (S), evenness (E), and the Shannon-Weiner diversity index values (HÂ’). Then I used a modified t-test to determine if there was a difference between the HÂ’ results. Both the Shannon-Weiner diversity index and the modified t-test were performed comparing each elevation (ten different elevations) and for t he entire sets of data from both of the rivers. RESULTS Analysis showed varying Shannon-Weiner diversity in dex values (HÂ’) among the different elevations (Table 3 and 4). The total sam ple size of plants sampled among the 20 different sites was 462; 357 from the La Cruz an d 105 from the Socorro (Table 2). Ten modified t-tests were then performed on the HÂ’ valu es at each elevation (P < 0.05). Analysis showed significant differences in the HÂ’ v alues at six out of the ten different elevations. T-values were significant at 1090 m, 11 00 m, 1140 m, 1160 m, 1170 m, and 1180 m (Table 1). At five of these six elevations, HÂ’ was greater for the Socorro than the La Cruz (Figure 1). In addition, Shannon-Weiner diversity i ndex was calculated for each of the two rivers across the altitudinal gradient a nd found to be significantly higher on the Socorro (HÂ’ = 3.40) than on the La Cruz (HÂ’ = 2.63) (Modified t-test, t = -8.55; df = 246.95; P < 0.05) (Table 2). There were a total of 69 different morpho species o f herbaceous plants observed at the 20 sites. Fifty-eight percent of the species were found only on the Socorro, 39% were found only on the La Cruz, and 3% were found o n both rivers (Figure 2). Velocity, temperature, and pH were found to be relatively con stant between the rivers (Table 5).
5 DISCUSSION The diversity value was greater on the Quebrada Soc orro (HÂ’ = 3.40) than on Quebrada La Cruz (HÂ’ = 2.63) (Modified t-test, t = -8.55; df = 246.95; P < 0.05). The data show a significant difference in the values of HÂ’ for si x of the ten elevations measured. At five of these six elevations, HÂ’ was greater on the Soco rro (Figure 1). Despite the fact that one of these elevations (1180 m) showed a higher HÂ’ val ue for the La Cruz, overall there is a greater species diversity and richness on the Quebr ada Socorro. These findings support my predictions that plant diversity would be greate r on the Socorro, a river that has less human influence, than the La Cruz, which is more af fected by higher human density and use. My results are also congruent with past studies tha t looked to compared urban vs. rural influences on diversities of various taxa. Lu ssier et al. (2008) found a negative correlation between in-stream biodiversity with inc reasing residential land use (RLU). Similarly, research by Burton et al. (2005) found t hat diversity of riparian woody plants decreased as the percentage of riparian forest decr eased and proximity to urban areas increased. My research shows a similar trend with h erbaceous riparian plants. In addition, I found a very small overlap in types of morpho species found on each river. These findings are also consistent with past studies of biodiversity loss. Burton et al. (2008) found in their studies of wood y riparian plants that as regeneration occurred from both natural and anthropogenic causes of disturbances, the diversity decreased as the percent of non-native species in t he regeneration layer increased. Implications The findings of my research indicate that increased human influence associated with the process of urbanization has a significant affect on herbaceous plant biodiversity in riparian ecotones along streams. Both the diversity and the composition of species were drastically altered between the Quebrada Socorro th e Quebrada La Cruz. Previous research has found that the vegetative diversity an d composition serves many functions in both the riparian and stream ecosystems. A study by Lecerf et al. (2005) found that plant species richness may indirectly affect ecosystem op erations because of the important role that riparian plants play in producing the leaf lit ter that is the primary energy source for the entire detritus-based food chain. This means th at the stream diversity of other taxa could be affected by the lack of herbaceous plant b iodiversity on the La Cruz. Another point to consider when faced with riparian plant species loss is the invasion of by non-native plant species. My researc h shows only a three percent overlap of species found on the Socorro and the La Cruz. Be cause both were sampled from the same general region, during the same season, and at the same elevations, one would expect to find more then just three percent of the species on both rivers. Even though over three times as many individual plants were fou nd on the La Cruz (n = 357) as on the Socorro (n = 105), species richness was considerabl y lower on the La Cruz (S = 28) than on the Socorro (S = 41) (Table 2). The species on t he La Cruz show higher dominance, which is typical of highly disturbed habitats, as o pposed to the situation for the Socorro where there were more species of plants with relat ively low abundances. But the general trend of a few species dominating the environment w e see on the La Cruz is characteristic
6 of a river with loss of species diversity. Wright ( 2005) discusses this homogenizing effect, comparing the species composition of plants and animals in secondary vs. primary forests. Disturbed areas allow for the weedy, non-n ative species to dominate the resources and take over. In pristine environments, like undisturbed riparian environments, communities are not dominated by one species and su pport a wide-range of plants and animals, as seen on the Socorro (Wright 2005). These findings may be precursors of the coming loss of plant diversity that we will see in the Monteverde region and throughout La tin America. As more people move away from rural areas to the urban centers we will see more of these effects percolating down through the intricate and inter-related ecosys tems. A loss of herbaceous plant species in the riparian ecotones of streams in Mont everde could mean changes in how the entire detritus-detritus based food web operates. ACKNOWLEDGEMENTS I would like to take this opportunity to thank Tani a Chavarria for advising me throughout the whole of this project, helping me formulate my idea, change it wh en I needed to, and walking me through this process of research and critical scientific analysis. Thanks t o Karen Masters for being the change that I wish to see in the world, and for showing me that biological resea rch is a necessary, immensely important step toward s making those changes. Thanks to Alan Masters for te aching me that there are no simple solutions to complex problems, which was a theme used many times throughout the process of this research. Thanks to the Estacin Biolgia for providing the environment necessary to finish this research. Thanks to Signe Spencer for accompanying me on one of my data colle ction days and defying death with me. Thanks to Pablo Allen and Moncho Calderon for tolerating my i ncessant questions and the technological help. Than k you to Nathan Spathe for electronically powering th e conception and analysis of this data. Finally, th ank you to my host family, Gaud and Raul Ramrez for supporting me through the whole of my data coll ection process, helping me address my injuries, and provid ing such a warm and loving environment for me to conclude this summer. LITERATURE CITED Burton, M.L., L.J. Samuelson, and S. Pan. 2005. Rip arian woody plant diversity and forest structure along urban-rural gradients. Urban Ecosystems 8 : 93-106. Clark. K.L., R.D. Lawton, and P.R. Butler. 2000. The Physical Environemt. In: Monteverde: Ecology and conservation of a trop ical cloud forest. N. M. Nadkarni and N. T. Wheelwright, eds. Oxford Univers ity Press, New York, NY, pp. 1530. Lecerf, A., M. Dobson, M. K. Dang and E. Chauvet. 2 005. Riparian plant species loss alters trophic dynamics in detritus-based stream ec osystems. Oecologia 146 : 432442. Lussier, S.M., S. N. da Selva, M. Charpentier, J. F Heltshe, S. M. Cormier, D. J. Klemm, M. Chintala and S. Jayaraman. 2008. The influence o f suburban land use on habitat and biotic integrity of coastal Rhode Islan d streams. Environmental Monitoring Assessments 139 : 119-136.
7 Naiman, R.J. and Dcamps, H. 1997. The ecology of i nterfaces: Riparian zones. Annual Review of Ecology Systems 28 : 621-658. Naiman, R. J., R. E. Bilby and P. A. Bisson. 2000. Riparian ecology and management in the Pacific coastal rain forest. BioScience 50 : 996Â–1011. Nilsson, C. and M. Svedmark. 2002. Basic principles and ecological consequences of changing water regimes: Riparian plant communities. Ume University, Ume, Sweden. Wright, S.J. 2005. Tropical forests in a changing e nvironment. TRENDS in Ecology and Evolution. 20 : 553-560. Wright, S.J. and H.C. Muller-Landau. 2006. The futu re of tropical forest species. Biotropica. 38 : 287-301.
8 Table 1. Shannon-Weiner diversity index values (HÂ’) for eac h of the 20 sites sampled, modified t-test values and critical values (cv) for each elevation testing for significant differences in the HÂ’ values (P < 0.05) Elevation (m) HÂ’ of Socorro HÂ’ of La Cruz Modified t-test value Critical Value (cv) Significant difference in HÂ’? ** 1090 1.84 1.01 -4.07 2.07 Yes 1100 1.72 1.19 -2.90 2.04 Yes 1110 1.33 0.93 -1.83 2.06 No 1120 1.16 1.22 0.26 2.05 No 1130 1.15 1.17 0.07 2.23 No 1140 0.64 0.28 -2.79 2.18 Yes 1150 1.48 1.35 -0.65 2.08 No 1160 1.75 0.65 -8.75 2.57 Yes 1170 1.38 0.64 -4.98 2.06 Yes 1180 0.64 1.97 9.17 2.09 Yes Table 2. The Quebrada La Cruz had a greater number of total individuals observed (n = 357), while the Socorro had a greater total species richness (S = 41). The significant HÂ’ values showed that overall plant biodiversity was g reater on the Socorro. River La Cruz Socorro Number of individuals (n) 357 105 Number of Species (S) 28 41 Evenness (E) 0.79 0.92 HÂ’ value 2.63* 3.40* *HÂ’ values found to be significant (Modified t-test t = -8.55; df = 246.95; P < 0.05) Table 3. The Shannon-Weiner diversity index values for the Quebrada La Cruz. Elevations of La Cruz n S S marg HÂ’ E 1090 17 4 1.06 1.01 0.73 1100 26 4 0.92 1.19 0.85 1110 18 3 0.69 0.93 0.85 1120 38 5 1.10 1.22 0.76 1130 33 4 0.86 1.17 0.84 1140 50 2 0.26 0.28 0.40 1150 89 9 1.78 1.35 0.65 1160 24 3 0.63 0.65 0.59 1170 32 2 0.29 0.64 .0.93
9 1180 30 9 2.35 1.97 0.90 Total La Cruz 357 28 4.59 2.63 0.79 Table 4. The Shannon-Weiner diversity index values for the elevations of the Quebrada Socorro. Elevations of the Socorro n S S marg HÂ’ E 1090 15 7 9.83 1.84 0.96 1100 16 7 11.05 1.72 0.88 1110 14 5 13.97 1.33 0.83 1120 11 5 26.61 1.15 0.72 1130 7 4 20.98 1.15 0.83 1140 3 2 -2.21 0.64 0.92 1150 7 6 10.29 1.48 0.92 1160 7 6 8.94 1.75 0.98 1170 19 5 12.33 1.38 0.86 1180 6 2 -2.21 0.64 0.92 Total Socorro 105 41 32.69 3.40 0.92 Table 5. Water conditions did not vary greatly between the two rivers. River La Cruz Socorro Velocity .4 m/s .3 m/s Temperature 21C 19C pH 8 6
10 0 0.5 1 1.5 2 2.5 *1090*1100111011201130*11401150*1160*1170*1180Elevation (m)H' Value H'La Cruz H'Socorro* implies a significant difference in HÂ’ values (P < 0.05) Figure 1. Elevation vs. Shannon-Weiner diversity index value (HÂ’) for each river. HÂ’ values of the Quebrada Socorro were higher than the HÂ’ values of the Quebrada La Cruz at five of the six elevations that were found to ha ve a significant difference in HÂ’. Species only found in La Cruz 39% Species only found in the Socorro 58% Species found in both 3% Figure 2. Relative proportions of the species found only in t he Socorro (58%), only in La Cruz (39%), and found in both (3%). The three perce nt overlap of the types of species found implies that each river has characteristic sp ecies unique to that environment. Total number of species found = 69.