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Structural Habitat Preferences in Norops tropidolepis in the Presence and Absence of Norops humilis
Digitized by MVI
Interspecific and intraspecific competitions are effectively reduced in populations of Anoline lizards through
the use of niche separation. While much research has been done on this phenomenon, not much research
addresses what happens to niche structure in the absence of a species. This project studies the effects of the
absence of Norops humilis on the structural habitat preferences of N. tropidolepis in an area of cloud forest in
Monteverde, Costa Rica. Between the 25th of October and the 14th of November 2003, an area of cloud forest
above the Estacin Biolgica de Monteverde was searched thoroughly for Norops (Zones 3 and 4, above
1470m in elevation). When a lizard was found, perch height and diameter were measured. These results
were then compared against findings by J. Alan Pounds (1988) with N. humilis present. Adult N.
tropidolepis were found to significantly shift their perch height down in the absence of N. humilis.
Significant differences were also found in stem diameter preference. I conclude that N. tropidolepis and N.
humilis experience facultative niche separation on the basis of structural habitat preference.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--Monteverde
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
Tropical Ecology Fall 2003
Ecologa Tropical Otoo 2003
t Monteverde Institute : Tropical Ecology
Structural Habitat Preferences in Norops tropidolepis in the Presence and Absence of Norops humilis Greg Maximov Department of Geology, T he Colorado College ______________________________________________________________ ABSTRACT Interspecific and intraspecific competitions are effectively reduced in populations of Anoline lizards through the use of niche separation. While much research has been done on this phenomenon, not much research addresses what happens to niche structure in the absence of a species. This project studies the effects of the absence of Norops humilis on the structural habitat preferences of N. tropidolepis in an area of cloud forest in Monteverde, Costa Rica. Between the 25 th of October and the 14th of November 2003, an area of cloud forest above the Estacin Biol gica de Monteverde was searched thoroughly for Norops (Zones 3 and 4, above 1470m in elevation). When a lizard was found, perch height and diameter were measured. These results were then compared against findings by J. Alan Pounds (1988) with N. humilis pr esent. Adult N. tropidolepis were found to significantly shift their perch height down in the absence of N. humilis. Significant differences were also found in stem diameter preference. I conclude that N. tropidolepis and N. humilis experience facultative niche separation on the basis of structural habitat preference. RESUMEN La competencia interespecifica e intraspecifica se reducen efectivamente en poblaciones de lagartijas Anolis por medio del uso de la separacin de nichos. Aunque se ha hecho mucha inve stigacin sobre este fenmeno, no hay mucha investigacin que enfoca lo que sucede a la estructura del nicho cuando no existe un a especie. Este proyecto estudia los efectos de la ausencia de Norops humilis en las preferencias del hbitat e structural de N. tropidolepis en un rea del bosque nuboso en Monteverde, Costa Rica Entre el 25 de octubre y el 14 de noviembre del 2003, un rea de bos que nuboso arriba de la Estacin Biolgica de Monteverde fue recorrida minuciosamente para encontrar Norops (Zonas 3 y 4, arriba de 1470m en elevacin). Cuando se en contraba una lagartija, se midi la altura de la percha y el dimetro. Estos resultados se compararon luego con los resultados de J. Alan Pounds (1988) cuando estaba N. humilis presente. Se encontr que los adultos de N. tropidolepis cambian significativamente la altura de su percha aun poco ms abajo en ausencia de TV. humilis. Se encontr diferencias significativas en la preferencia del dimetro del tallo. Concluyo que N. tropidolepis y N. humilis presentan separacin facultativa del nicho con base en preferencia del Hbitat estructural. INTRODUCTION Niche partitioning is an effective method for reducing interspecific as well as intraspecific competition within communities. Several studies have highlighted perch height stratification as an important form of niche partitioning in Anoline lizards (Rand, 1964; Schoener, 1975; Moermond, 1979). In 1988, J. Alan Pounds conducted a study in the Reserva Biolgica de Monteverde, Costa Rica, on the structural habitat partitioning, locomotion, and morphology of Anoline lizards. In his research, Pounds showed that the
Cloud Forest Anole, Norops tropidolepis, stratified its niche by height in the presence of ground dwelling N. humilis and generally occupied higher perches. In this study, I ask whether N tropidolepis use lower perches in the absence of N. humilis. Furthermore, has N. tropidolepis changed the diameter of its perch substrates? Very little research has been conducted on niche movement in the absence of an Anoline species. An article by Rummel and Roughgarden (1985) covers this topic, but the experiment took place in the Netherlands and used enclosures. If N. tropidolepis has a significantly different niche in the absence of N. humilis, then niche separation between the two species has evolved quite recently and is more facultative in nature. If N. tropi dolepis has failed to move its niche, one of two explanations could come into play. First, the two species have significantly evolved traits or behaviors locking them into their respective niches. Second, not enough time may have passed since the removal o f N. humilis to witness a change in niche dimensions. I believe that the insects that inhabit leaf litter and lower tree trunks provide an unparalleled food source for Anole populations. Therefore, N. tropidolepis should shift its niche to include this val uable resource in the absence of a possibly more talented competitor. METHODS On 16 days between the 25 th of October and the 14 th of November 2003, 1 walked the syst em of trails between the Estaci n Biolgica de Monteverde and the ridgeline above. This elevational block consists of both Zone 3 and 4 (above 1470m, as explained by Hayes et al.). It has been casually observed that the area is currently void of N. humilis (Karen Masters, pers. comm. 2003). I walked the trails for roughly five hours per day, using different paths each time and constantly searching from the ground up to about two meters in height I also spent equal time off the trail, searching the forest. I recorded any Anole for which I could determine species and relative age (male or j uvenile). If I was able to catch the lizard, I recorded its sex and snout to vent length. For every Anole recorded, I measured both the distance from the ground and the diameter of its perch substrate if possible (fallen logs, roots, etc.) for the spot at which it was originally sighted. In this way, I took a random sampling of the perch heights of each individual present, including a range of activities throughout the day. T tests were conducted to find differences in perch height and stem diameter between adults and juveniles within N. tropidolepis and N. intermedius. Mann Whitney U tests were conducted to find differences in perch height, stem diameter, and size between males and females of each species due to reduced sample size. T tests were also used t o compare N. tropidolepis and N. intermedius on the basis of perch height, stem diameter, and size. These tests were conducted between adults, juveniles, and all members to find differences in niche parameters. Finally, chi squared tests were conducted on the basis of perch height and stem diameter between N. tropidolepis as I found them, and J. Alan Pound's 1988 findings in the presence of N. humilis. RESULTS During the course of my study I found 128 Anoline lizards overall: 47 N. intermedius, 78 N. tropidolepis, and 3 Norops woodi (Appendix 1). N. woodi was excluded from all data analysis due to extremely small data size. I found that adult N. tropidolepis tended
to perch higher than juveniles (t = 2.330, P = 0.0225) (Fig. 1). No significant diff erence was found between choice of stem diameter between adults and juveniles in this species (t = 1.432, P > 0.05), but it seems that adults use a larger range of stems as well as a larger range of perch heights. When the same tests were conducted for N. intermedius adults and juveniles, no significant difference was found for perch height preference (t = 1.619, P > 0.05) while results were inconclusive for stem diameter preference. Norops intermedius could possibly show the same trends as N. tropidolepis in these regards, but the sample size of juveniles is too small to draw any conclusions. Comparative tests between males and females in each species proved to be inconclusive as well, due to small sample size. Interspecific comparisons between N. tropidole pis and N. intermedius showed that adults did not significantly differ in perch height preference (t = 0.547, P > 0.05). Finally, I found that adult N. tropidolepis significantly shifted their perch heights down in the absence of N. humilis (x 2 = 13.846; df = 4; P < 0.05) (Fig. 2a). Perch height preference for N. tropidolepis juveniles was shown to be quite similar in both the presence and absence of N. humilis (Fig. 2b). Both adult and juvenile N. tropidolepis significantly shifted their stem diameter pre ference (x 2 = 38.501; df = 6; P < 0.05 for adults) (x 2 = 32.633; df = 6; P < 0.05 for juveniles) (Fig. 3a, 3b). DISCUSSION NICHE EXPANSION OF NOROPS INTERMEDIUS Quite possibly the most surprising result of this study was the discovery of N. intermedius in my test site. This species was found at all elevations within my test site, up to the continental divide above the Estacin Biol gica (Zones 3 and 4). Originally, N. intermedius was only recorded as inhabiting Zones 2 and 3 in the Monteverde area, up to 16 00m in elevation (Hayes, et al.). Former limitation in N Intermedius range was probably dictated by another form of niche separation: climatic habitat selection. Climatic habitat selection can most easily be explained in terms of thermobiological differen ces between species (Pounds, 2000). While all anoles are diurnal, some are heliothermic (N. intermedius, for example), and some are thermoconformers (N. tropidolepis, N. humilis, and N. woodi, for example) (Savage, 2002). Heliothermic species are gap dependent and bask to raise their body temperature, reaping the benefits of faster sprint speeds and expedited maturation. On the other hand, thermoconforming species inhabit the shaded understory and let their body temperatures track the ambient air temperature (Pounds, 2000). These species use a low energy strategy to exploit their cool environment (low gain, low expenditure) and experience trade offs such as slow growth and maturation rates as well as reduced running speeds (Van Berkum, 1986). At the same time, they are extremely heat sensitive and become desiccated easily in direct sunlight. Despite these major morphological differences, I still found N. intermedius in a site thought only suitable for shade tolerant species. Additionally no niche separation by perch height or perch diameter (structural habitat parameters) was found between N. tropidolepis and N. intermedius. Two explanations can be used to explain these phenomena. First, perhaps not enough time has passed since N. interm edius began to inhabit my study site. It might be that the new lizard community is still in the process of n iche separation. This line of reasoning assumes that food sources are currently
sufficient to support the Anole community at its present population level. Once the community grows past the point at which it can survive off of local food resources, interspecific competition will force facultative niche separation. A second line of reasoning posits that perch height and diameter might not play large rol es in the niche separation of N. tropidolepis and N. intermedius. Anoline species inhabiting the same structural habitat (i.e. similar perch height and diameter trends) have been shown to separate climatically (Rand, 1964). These species might still be s eparated by climatic habitat choice, just on a smaller scale. It could be that N. intermedius continues to occupy sunny areas such as tree fall gaps and trail edges while N. tropidolepis inhabits areas with more constant shade. This hypothesis could not be tested within the scope of this experiment. More research needs to be done to this end. Another interesting line of investigation concerns morphological similarities between Anoline species inhabiting similar structural niches. Several sources have tied s imilar morphological features to structural habitat preference (Rand, 1964; Moermond, 1979; Pounds, 1988). All manner of morphological characteristics have been used to lump Anoles into classes by movement type and finally, by structural habitat preference Using Anole body measurements, Pounds (1988) was able to classify N. tropidolepis and N. humilis as "jumpers," and N. intermedius as "runners." In this case, it is important to ask whether microhabitat structure affects locomotion tendencies, or if it might be the other way around and evolved morphological differences dictate microhabitat selection. Both answers might be true if past competition led to niche separation that, in turn, led to locomotor adaptation (Pounds, 1988). This locomotor adaptation could then act as a form of obligate niche separation in the future as a species spreads its home range. NICHE EXPANSION OF NOROPS TRO PIDOLEPIS The classification of both N. tropidolepis and N. humilis as "jumpers" provides good support to my findings pointing to facultative niche separation between the two species in the form of structural habitat selection. I found that adult N. tropid olepis significantly shifted their perch heights down in the absence of N. humilis (Fig. 2a). N. tropidolepis juveniles, already perching significantly lower than adults (Fig. 1 ) did not significantly move their niche downward (Fig. 2b). I believe that thi s is because they have already maxed out their use of the ground. It was also found that both adult and juvenile N. tropidolepis significantly changed their perch diameter selection in the absence of N. humilis (Fig. 3a, 3b). Most of this change can be accounted for by an increased presence on the ground, as opposed to any measurable substrate (sticks, roots, tree trunks). With "ground" excluded from analysis, N. tropidolepis juveniles appear to have shifted to u sing smaller diameter substrates, while this trend is not as obvious in adults. This trend is to be expected as support diameter was shown to be positively correlated with height above ground in similar forest (Pounds, 1988). N. tropidolepis and N. humilis are very similar species in terms of thermoregulation, shade tolerance, body shape, and locomotor tendencies. These characteristics place the two species in roughly the same climatic niche, leaving them vulnerable to habitat overlap. Within the same climatic habitat (cool, shaded forest understory), body size easily differentiates the two species. Norops humilis is generally a lot smaller than N. tropidolepis (Savage, 2002), giving it the advantage when hunting and escaping predators in the leaf litter. Since distance between supports and height above ground are shown to be positively correlated in similar forest (Pounds, 1988), the
larger body size of N. tropidolepis should be to its advantage when jumping longer distances between supports farthe r from the ground. These morphological differences surely played a role in the niche differentiation by height found by J. Alan Pounds (1988). In the absence of competition with N. humilis, possibilities of a more varied diet and perhaps more varied escape routes lured N. tropidolepis into shifting its niche down onto the ground. Even though the N. tropidolepis and N. humilis are quite different in size, the rapidity of niche adjustment by N. tropidolepis points to a more facultative form of niche separatio n between the two species. CONSERVATION IMPLICATIONS It would be unfair to say that the amphibian and reptile declines of recent years in the Monteverde area have been underreported. Synchronous population crashes in 1987 led to the disappearance of twenty species of frog and toad in the Monteverde area (Pounds et al. 1999). These local disappearances included the global extinction of the Golden Toad (Bufo periglenes), a species that would thereafter act as a poster child for the event (as reported in Crump et al,, 1992; Pounds and Crump, 1994 among other numerous sources). While the anuran decline occurred in short devastating bursts (actually three demographic events in 1987 1994 and 1998), the incident affected populations of Anoline lizards in a much slower, sustained fashion (Pounds et al. 1999). Although some have speculated that the declines are no more than normal fluctuations in herpetofauna populations (Pechmann and Wilbur, 1994), there has been much evidence presented to refut e these claims (Pounds et al. 1997). A widely debated hypothesis for the declines points to climate change as a major factor. Pounds et al. (1999) correlate increasing sea surface temperatures to peaks in anuran decline and longer term reptile decline. In their analysis, they show how raising levels of cloud formation could lead to a decrease in mist frequency in Monteverde cloud forest during the dry season. I believe that many of the findings in this study could be related to climate change. The expansion of N. intermedius into Zone 4 is enough to raise some concern by itself. Unusually warm temperatures coupled with a higher frequency of tree fall gaps due to a strengthening of trade winds could serve as possible explanations for climatic niche expansion in this species (Pounds, 2000). On the other hand, I believe that thermoconformers such as N. tropidolepis and N. humilis are very vulnerable to increasing temperatures and seasonality. In these species, climate changes might affect reproductive cycles (Po unds, 2000), reduce suitable habitat (shaded understory), or facilitate the arrival of new competitors (N. intermedius). As a result, N. tropidolepis has retreated to higher elevations and has disappeared from the drier, western parts of its distribution ( Zone 2 and part of Zone 3) (Pounds, 2000). I believe that the results of this project can be applied to the larger problem of reductions in biodiversity in the tropics. As species continue to shift their niches in response to disappearances of other specie s, gaps are sure to be left in the complex fabric of trophic interactions. More importantly, what happens when obligate niche separation has left other species incapable of filling in voids left by the disappearance of species within ecological time? Reduc tions in diversity in the tropics should lead to reduced efficiency in trophic interactions as well as reductions in both resistance and resilience of tropical communities.
ACKNOWLEDGEMENTS I would like to thank Karen Masters for a great project idea and f or her undying enthusiasm for answering my seemingly endless stream of questions. I would also like to thank Alan Masters for introducing me to Monteverde and for some great jams. I would like to thank my familia Tica for pretending to understand my horrib le Spanish and for providing me with wonderful times. I would also like to thank my parents for providing me with support from afar. I would like to thank the students and TA's of this program for bringing me a constant supply of Norops. I would like to th ank Carmen Rojas for taking the time to help me with my Spanish and for helping me translate my abstract I would especially like to thank Andre's Vaughan for showing me the wonders of herping. Finally, I would like to thank all the Anoles that I used for my experiment for their perseverance in working through some potentially stressful and confusing times. LITERATURE CITED CRUMP, M. L., F. R. HENSLEY, and K. L. CLARK. 1992. Apparent Decline of the Golden Toad: Underground or Extinct? Copeia, pp. 413 420. HAYES, M. P., J. A. POUNDS, and W. W. TIMMERMAN. An Annotated List and Guide to the Amphibians and Reptiles of Monteverde, Costa Rica. Society for the Study of Amphibians and Reptiles, Herpetological Circular, No. 17. MOERMOND, T. C. 1979. Habitat Constr aints on the Behavior, Morphology, and Community Structure of Anolis Lizards. Ecology, Vol. 60, No. 1, pp. 152 164. PECHMANN, J. H. K., and H. M. Wilbur. 1994. Putting Declining Amphibian Populations in Perspective: Natural Fluctuations and Human Impacts. Herpetologica, 50, pp. 65 84. POUNDS, J. A. 1988. Ecomorphology, Locomotion, and Microhabitat Structure: Patterns In a Tropical Community. Ecological Monographs, 58(4), pp. 299 320. 2000. Amphibians and Reptiles. In Monteverde: Ecology and Conservation of a Tropical Cloud Forest Edited by Nadkarni, N. M. and Wheelwright, N. T. Oxford University Press, Oxford, pp. 149 177. POUNDS, J. A., and M. L. CRUMP, 1994. Amphibian Declines and Climate Disturbance: The Case of the Golden Toad and the Harlequin Frog Conservation Biology, Volume 8, No. 1, pp. 72 85. POUNDS, J. A., M. P. FOGDEN, J. M. SAVAGE, and G. C. GORMAN. 1997. Tests of Null Models for Amphibian Declines on a Tropical Mountain. Conservation Biology, Vol. 11, No. 6, pp. 1307 1322. POUNDS, J. A., M. P. FOGDEN, and J. H. CAMPBELL. 1999. Biological Response to Climate Change on a Tropical Mountain. Nature, Vol. 398, April 15, pp. 611 614. RAND, A. S. 1964. Ecological Distribution of Anoline Lizards in Puerto Rico, Ecology, Vol. 45, No. 4, Pg. 745 752. RUMMEL, J.D., and J. ROUGHGARDEN. 1985. Effects of Reduced Perch Height Separation on Competition Between Two Anolis Lizards. Ecology, Vol. 66, No. 2, pp.430 444. SAVAGE, J.M. 2002. The Amphibians and Reptiles of Costa Rica: A Herpetofauna Between Two Continents Between Two Seas. The University of Chicago Press, Chicago, pp.444 480. SCHOENER, T.W. 1975. Presence and Absence of Habitat Shift in Some Widespread Lizard
Species. Ecological Monographs, Vol. 45, No. 3., pp. 233 258. VAN BERKUM, F.H. 1986. Evolutionary Patterns of the Thermal Sensitivity of Sprint Speed in Anolis Lizards. Evolution, Vol. 40, No. 3, pp. 594 604.
Appendix 1: Data collected between the 25 th of October and the 14 th of November, 2003 in Zones 3 and 4 above the Estacin Biolgica de Monteverde, Costa Rica. Species Hgt. (cm) Diam. (cm) Size (mm) Age Gender N. intermedius 0 47 Adult F N. intermedius 0 50 Adult F N. intermedius 0 51 Adult F N. intermedius 0 51 Adult F N. intermedius 0 51 Adult F N. intermedius 0 52 Adult F N. intermedius 0 52 Adult F N. intermedius 0 53 Adult F N. intermedius 0 53 Adult F N. intermedius 0 53 Adult F N. intermedius 0 54 Adult F N. intermedius 0 Adult F N. intermedius 23 9 48 Adult F N. intermedius 30 51 Adult F N. intermedius 38 48 51 Adult F N. intermedius 39 0.1 53 Adult F N. intermedius 65 2 51 Adult F N. intermedius 0 31 Adult M N. intermedius 0 33 Adult M N. intermedius 0 46 Adult M N. intermedius 0 47 Adult M N. intermedius 0 47 Adult M N. intermedius 0 49 Adult M N. intermedius 0 53 Adult M N. intermedius 0 54 Adult M N. intermedius 18 2 52 Adult M N. intermedius 37 3.51 49 Adult M N. intermedius 55 2.5 52 Adult M N. intermedius 58 7.5 49 Adult M N. intermedius 0 Adult
N. intermedius 0 Adult N. intermedius 0 Adult N. intermedius 0 Adult N. intermedius 67 3.4 Adult N. intermedius 135 19 Adult N. intermedius 163 0.5 Adult N. intermedius 0 20 Juvenile N. intermedius 0 21 Juvenile N. intermedius 0 21 Juvenile N. intermedius 0 21 Juvenile N. intermedius 0 21 Juvenile N. intermedius 0 21 Juvenile N. intermedius 0 21 Juvenile N. intermedius 0 22 Juvenile N. intermedius 0 22 Juvenile N. intermedius 0 Juvenile N. intermedius 15 17 21 Juvenile N. tropidolepis 0 31 Adult F N. tropidolepis 0 40 Adult F N. tropidolepis 0 45 Adult F N. tropidolepis 7 43 Adult F N. tropidolepis 20 1.5 41 Adult F N. tropidolepis 0 37 Adult M N. tropidolepis 0 38 Adult M N. tropidolepis 0 42 Adult M N. tropidolepis 0 44 Adult M N. tropidolepis 0 44 Adult M N. tropidolepis 0 45 Adult M N. tropidolepis 0 46 Adult M N. tropidolepis 0 47 Adult M N. tropidolepis 0 48 Adult M N. tropidolepis 0 48 Adult M N. tropidolepis 0 49 Adult M N. tropidolepis 0 50 Adult M N. tropidolepis 0 52 Adult M N. tropidolepis 0 53 Adult M N. tropidolepis 20 0.82 48 Adult M N. tropidolepis 23 0.1 29 Adult M N. tropidolepis 23 3.57 46 Adult M N. tropidolepis 23 1.15 48 Adult M N. tropidolepis 24 0.85 45 Adult M N. tropidolepis 25 5 51 Adult M N. tropidolepis 26 22 31 Adult M N. tropidolepis 28 1.2 46 Adult M N. tropidolepis 30 8.3 50 Adult M N. tropidolepis 35 1.94 45 Adult M N. tropidolepis 38 1.11 42 Adult M N. tropidolepis 40 75 53 Adult M N. tropidolepis 44 5 48 Adult M N. tropidolepis 54 11 37 Adult M N. tropidolepis 56 100 43 Adult M N. tropidolepis 60 1.47 48 Adult M N. tropidolepis 62 69 47 Adult M N. tropidolepis 86 10 53 Adult M
N. tropidolepis 0 Adult N. tropidolepis 0 Adult N. tropidolepis 0 Adult N. tropidolepis 0 Adult N. tropidolepis 0 Adult N. tropidolepis 26 1.4 Adult N. tropidolepis 27 1.74 Adult N. tropidolepis 29 1.17 Adult N. tropidolepis 36 3 Adult N. tropidolepis 54 33 Adult N. tropidolepis 62 2.5 Adult N. tropidolepis 65 Adult N. tropidolepis 82 39 Adult N. tropidolepis 126 0.68 Adult N. tropidolepis 0 20 Juvenile N. tropidolepis 0 21 Juvenile N. tropidolepis 0 21 Juvenile N. tropidolepis 0 21 Juvenile N. tropidolepis 0 22 Juvenile N. tropidolepis 0 22 Juvenile N. tropidolepis 0 22 Juvenile N. tropidolepis 0 23 Juvenile N. tropidolepis 0 25 Juvenile N. tropidolepis 0 26 Juvenile N. tropidolepis 0 27 Juvenile N. tropidolepis 0 27 Juvenile N. tropidolepis 0 27 Juvenile N. tropidolepis 0 27 Juvenile N. tropidolepis 0 27 Juvenile N. tropidolepis 0 28 Juvenile N. tropidolepis 0 28 Juvenile N. tropidolepis 0 Juvenile N. tropidolepis 0 Juvenile N. tropidolepis 8 0.6 20 Juvenile N. tropidolepis 18 0.64 27 Juvenile N. tropidolepis 25 0.88 25 Juvenile N. tropidolepis 25 1.34 28 Juvenile N. tropidolepis 27 1.14 23 Juvenile N. tropidolepis 39 5.53 Juvenile N. tropidolepis 45 1 26 Juvenile N. tropidolepis 80 0.4 21 Juvenile N. woodi 49 1.3 83 Adult M N. woodi 82 8.77 81 Adult M N. woodi 0 Adult