Tropical Zoology 20: 163-186, 2007
Patterns of interspecific and intraspecific microhabitat segregation of two rodents Praomys jacksoni (De Winton 1897) and Hylomyscus stella (Thomas 1911) (Rodentia) in an African rainforest subjected to various levels of anthropogenic disturbance A. Mortelliti and L. Boitani Dipartimento di Biologia Animale e dell’Uomo, Università di Roma “La Sapienza”, Viale dell’Università 32, 00185 Roma, Italy Received 28 April 2006, accepted 22 February 2007
The aim of this study was to evaluate whether anthropogenic disturbance in tropical rainforests, in the form of alteration of lower vegetation structure, affects the patterns of interspecific and intraspecific microhabitat segregation in small mammals. We chose two areas of a Kenyan tropical rainforest: one disturbed and the other relatively undisturbed. Rodents were live-trapped with Sherman traps and 30 microhabitat variables were measured around each trapping site. Due to small sample sizes we concentrated our analysis on the two most abundant species: Praomys jacksoni (De Winton 1897) and Hylomyscus stella (Thomas 1911). We structured trapping patterns and data analysis to examine patterns of microhabitat segregation on various spatio-temporal scales. First, all available data (696 captures — all sessions of all grids in both areas) were pooled. Step by step this large sample was divided into comparisons concerning single areas, single grids (merging all sessions) and finally single sessions of single grids. The species segregate in structurally different microhabitats and are vertically stratified. In the disturbed forest there is a higher overlap between microhabitats, and patterns of microhabitat segregation are different, suggesting a response to the reduction of microhabitat variability occurring in the disturbed forest. Intraspecific differential microhabitat use was also found for Praomys jacksoni. We discuss whether juveniles, subadults and males are segregated in poorer quality microhabitats, and we hypothesize that anthropogenic disturbance affects this dynamic. key words:
tropical rainforest, microhabitat quality, vertical stratification, niche breadth, Kakamega, Kenya.
Introduction . . . Study site . . . . Materials and methods Results . . . .
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A. Mortelliti and L. Boitani Macrohabitat structure comparison . . Trapping results and vertical stratification Interspecific microhabitat-use comparisons Intraspecific comparisons . . . . . Niche breadth . . . . . . . Discussion . . . . . . . . . Vertical stratification . . . . . . Intraspecific comparisons . . . . . Final considerations . . . . . . Acknowledgements . . . . . . . References . . . . . . . . . Appendix 1 . . . . . . . . . Appendix 2 . . . . . . . . .
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INTRODUCTION
As a result of interspecific competition, coexisting rodent species often segregate into structurally different microhabitats (Dueser & Shugart 1978), responding to structural components of their environment on a scale of resolution much finer than gross habitat differences (Seagle 1985). Great efforts have been made in the past to study this fine microhabitat partitioning, particularly for desert rodents (Wondolleck 1978, Thompson 1982) in which microhabitat segregation is strongly expressed (Jorgensen & Demarais 1999). Notwithstanding the important role of rodents as seed predators and dispersers in tropical rainforests (Struhsaker 1997, Chapman & Chapman 1999) few studies have investigated patterns of microhabitat segregation in these highly threatened ecosystems. Habitat use may vary by individual, sex-age group and social status, and yet most studies pool individuals and do not sample adequately (Garshelis 2000). At the microhabitat scale, few studies have focused on intraspecific differences: Bowers & Smith (1979) and Ostfeld et al. (1985) demonstrated that female mice (Peromyscus sp.) inhabited more favourable microhabitats; Van Horne (1982) found that adult high density habitat was more favourable than juvenile high density habitat and hypothesized the presence of sink-source habitat dynamics, with dominant individuals occupying more favourable habitats. Alteration of habitat features, due to natural or human causes, can modify the ecology of rodent species, affecting population abundance, population dynamics and species richness and diversity (Abramsky et al. 1979, Seagle 1985, Isabirye-Basuta & Kasenene 1987, Fox 1990, Kirkland 1990, Kelt et al. 1994, Stephenson 1995, Canova & Fasola 2000, Avenant 2003, Suzuki & Hayes 2003). From a tropical rainforest management perspective, changes in the ecological parameters of rodent communities can have repercussions on tree regeneration and floral composition (GenestVillard 1980, Isabirye-Basuta & Kasenene 1987). The microhabitat approach provides a measure of the structure of the environment that is either known or reasonably suspected to influence the distribution and local abundance of the species (Dueser & Shugart 1978). The study of how human disturbance affects the environmental parameters which species rely on to coexist (i.e. some species may suffer from the reduction of preferred microhabitats whereas others may be favoured) will provide a further basis for tropical rainforest management decision-making. Here we focused on the effects of altered forest structure, as we presumed it might have direct consequences on the microhabitat segregation of species. We
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chose the Kakamega Forest (western Kenya) because of the presence of two contiguous areas subjected to different levels of anthropogenic disturbance. Our objectives are to: (1) compare microhabitat variables of both forest types (subsequently referred to as macrohabitats) and characterize the structural changes caused by the anthropogenic disturbance; (2) assess whether the alteration of forest structure has consequences on the patterns of interspecific and intraspecific microhabitat segregation; (3) assess the effects of alteration of forest structure on the “niche breadth” or index of variety of microhabitats occupied (sensu Carnes & Slade 1982). We structured the data analysis in a hierarchical way that would allow us to compare patterns of microhabitat segregation in the two macrohabitats subjected to different levels of anthropogenic disturbance and to examine local (single trapping grid level) and general (macrohabitats and whole forest) patterns of microhabitat segregation.
STUDY SITE Research was carried out from November 2002 to April 2003, during the dry season. The study site was located in the Kakamega Forest, Kenya (latitude 00°10’N-00°21’N, longitude 34°47’E-34°58’E; 1500-1700 m a.s.l.). The forest is the only reasonably large patch (15,480 ha) of Central African type lowland rainforest in Kenya (KIFCON 1994). Mean annual precipitation is 2000 mm (Cords 1990): precipitation occurs throughout the year, with maxima in April-May and August-September. Temperatures range from 18-29 °C (day maximum) and 11-12 °C (night minimum). The commonest trees are Celtis africana (Burm f.), Prunus africana (Hook f.), Albizia gummifera (Gmelin) and Antiaris toxicaria (Leschenault) (Cords 1990), among a total of 350 recorded species (Kifcon 1994). The forest is managed by two authorities: the northern part (the Buyango forest area) is managed by the Kenya Wildlife Service and locals are not allowed into the forest; the southern part (the Isecheno forest area) is managed by the Forestry Department and locals are allowed to enter the forest to collect wood and other forest resources such as fruit and medicinal herbs (Rogo et al. 1999). They are also allowed to pass with their cattle (pers. obs).
MATERIALS AND METHODS Rodents were live-trapped using Sherman large aluminium folding traps (23 × 8 × 9 cm); diced fried coconut mixed with peanut butter was used as bait. Traps were checked once a day, in the morning, and animals were marked by toe-clipping. Standard data were taken from each animal: specimen number, trap location, sex, reproductive condition and body weight, conforming to guidelines for capture and handling from the Kenyan Ministry of Education, Science and Technology. In each part of the forest (i.e. macrohabitat) 3 different trapping grid layouts were used (for a total of 347 traps per macrohabitat). Grid settings were determined by logistic constraints. Each grid covered an area of 0.81 ha. In each macrohabitat we used: (1) One 7 × 7 grid with 15 m trap spacing, two Sherman traps at each station, one at ground-level and one above ground (1-3 m). These grids were trapped 4 days during each session. (2) One 7 × 7 grid with 15 m trap spacing, one ground-level Sherman trap at each station. These grids were trapped 4 days during each session. (3) One 10 × 10 grid with 10 m trap spacing, two Sherman traps at each station, one at ground-level and one above ground (1-3 m). These grids were trapped 3 days during each session. The distance between the two trapping areas was 12 km, the distance between grids ranged from 600 m to 1 km.
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Three trapping sessions (December, February, March) were performed in the undisturbed Buyango forest area while, due to unexpected logistic problems, only 2 trapping sessions (February, March) took place in the disturbed Isecheno forest area. The 10 × 10 grid was used only in the February and March sessions of each forest. A removal-grid with a variable number of traps was occasionally added in each forest to collect 10 skulls of each sex of each species for accurate species identification. We followed the classification and diagnostic characters of Delany (1975) and Lecompte et al. (2001, 2002) and confirmed the identification with reference collections of local museums. Thirty-one microhabitat variables were measured around each capture station, following Dueser & Shugart (1978, 1979), Loy & Boitani (1982), Isabirye-Basuta & Kasenene (1987), Canova & Fasola (1991), Canova (1993), Stephenson (1995) and Morrison et al. (1998). All variables and sampling methods are described in Appendix 1. Age classes were determined mainly on the basis of pelage and weights, with further confirmation from signs of sexual activity. Group separation was performed using SPSS (Version 11) Discriminant Analysis. Groups were species and age classes within each species. Discriminant analysis is commonly used in microhabitat segregation studies (Dueser & Shugart 1978, Carnes & Slade 1982, Seagle 1985, Stephenson 1995). Only variables significant at the t-test (2 group-comparisons) or Anova (more than 2 groups comparisons) (P < 0.05) were introduced simultaneously in the discriminant analysis. The significance of the discriminant function was tested with Wilks Lambda and with ANOVA or t-test conducted on discriminant scores. The degree of group overlap and the separation of group centroids was evaluated with the percentage of cases correctly classified and by the absolute value of Wilks Lambda. In all the analyses, only the first capture of an individual in each site during each trapping session was included because repeated captures of the same individuals at the same trapping stations may introduce a bias (Kelt et al. 1994). Data analysis was organized hierarchically (4 Levels). First (Level 1), data for all the grids and sessions within habitats were pooled. Step by step this large sample was divided into comparisons within single macrohabitats (Level 2), single grids (merging all sessions, Level 3) and, finally, single sessions of single grids (Level 4). Macrohabitat structure comparison was performed with a 6 group (corresponding to 6 grids) Discriminant Analysis. Single groups were represented by pooled microhabitat variables of individual trapping grids: the 3 grids of the disturbed forest and the 3 grids of the undisturbed forest. At the macrohabitat level, two approaches were followed: the first separated data from the two macrohabitats and performed independent analyses. The second approach pooled data from both macrohabitats but discriminated among four groups: Praomys jacksoni (De Winton 1897) of the disturbed forest, Hylomyscus stella (Thomas 1911) of the disturbed forest, Praomys jacksoni of the undisturbed forest, Hylomyscus stella of the undisturbed forest. The variance of discriminant scores was used as an index of the variety of microhabitats occupied (Carnes & Slade 1982). This measurement was called “niche breadth” by M’Closkey (1976), Van Horne (1982) and Seagle (1985). Carnes & Slade (1982) noted that, compared to other methods, it is less influenced by differences in sample size between species. In the macrohabitat comparison, variance of discriminant scores was considered an index of microhabitat variability (Seagle 1985). Comparisons were performed using an F-test.
RESULTS
Macrohabitat structure comparison The analysis produced five significant functions to separate the six grids used as groups; the first two functions are shown in Table 1. The first discriminant function explained 45% of the variance between groups. Two clusters corresponding to
Microhabitat segregation of rodents
167 Table 1.
Discriminant analysis comparing microhabitat variables among live-trap grids. Results are shown for Wilks Test and Anova conducted on discriminant scores of the first two functions; centroid values are also included. N = sample size.
Wilks lambda Significance of separation: Anova
Function 1
Function 2
0.143 P
