Small mammal diversity along elevational gradients in the Philippines: an assessment of patterns and hypotheses

Abstract

1 It is widely accepted that tropical lowland rain forest holds the greatest diversity of organisms, and it is often implied that this general pattern is also true for virtually all individual higher‐level taxa. Standardized elevational transect surveys of non‐flying small mammals (Insectivora and Rodentia) on geologically old, species‐rich islands in the Philippines consistently show maximum diversity and relative abundance in upper montane/lower mossy forest at 1500–2200 m, often exceeding lowland species richness and relative abundance by a factor of three or more. 2 On mountains where maximum elevation exceeds 2000 m, there is a decline in species richness above about 1500–2000 m, yielding a curvilinear pattern of species richness along the elevational gradient. The peak in species richness occurs at the area of transition from montane to mossy forest, which is also the point at which rainfall probably peaks. In parallel with species richness, relative abundance of small mammals in the Philippines also increases from the lowlands to 1500–2200 m, increasing by a factor from two to 10. 3 Twelve hypotheses concerning patterns of diversity along elevational gradients, plus the null hypothesis, are evaluated. The null hypothesis of no variation and the hypotheses that maximum diversity is in the lowlands, that diversity is highest in areas of least perturbation, and that diversity increases with increasing area, are rejected. There is weak or ambiguous support for the hypotheses that diversity is greatest in areas of community interdigitation, that diversity is highest in the area of highest productivity, that diversity is correlated with habitat complexity, and that diversity is correlated with habitat diversity. There is strongest support for the hypotheses that diversity is correlated with annual rainfall, with total abundance of individuals in the community, with food resource diversity, with areas of reduced competition from other organisms, and with areas characterized by high rates of speciation. 4 Causality is difficult to evaluate because many hypotheses make non‐exclusive predictions, they probably represent non‐independent aspects of causal factors (in other words, there is much interaction among the processes highlighted by the various hypotheses), and they represent the range from proximate to ultimate and from descriptive to causal. All the hypotheses probably represent phenomena that exist in nature, but few (or none) represent phenomena found in all taxa. The primary challenge in the future will not be simply to accept or reject individual hypotheses, but rather to determine the circumstances under which the various causal factors are most important, how they interact, and how they can be combined into a more comprehensive and general multi‐factorial model.

Cross-taxon congruence in tree, bird and bat species distributions at a moderate spatial scale across four tropical forest types in the Philippines

“…The lower bird species richness in montane forest in the NSMNP compared to lowland dipterocarp forest reflects the general higher species richness of Philippine birds at lower elevations: 61% of resident species are restricted to lowlands, 15% to montane areas over 1,000 m and the remainder of 24% occurs al all elevations (Kennedy et al 2000). Bat diversity also decreases with elevation in other areas in the Philippines (Heaney 2001) although montane forest in our study was still relatively species rich for bats, likely as a result of the large elevation range of our study sites (500-1,750 m).…”

Pequenos mamíferos não-voadores da Reserva Florestal do Morro Grande: distribuição das espécies e da diversidade em uma área de Mata Atlântica

“…Porém, os sítios comparados diferiam também quanto ao grau de perturbação, estádios sucessionais e fragmentação. Os dados de 20 localidades (Apêndice 1) que abrangem diferentes regiões da Mata Atlântica, mas estão localizadas em grandes remanescentes de matas tardias ou maduras, indicam padrão inverso e estão de acordo com trabalhos recentes que têm demonstrado maior riqueza de pequenos mamíferos não-voadores em altitudes intermediárias (500 a 1500 m), tanto na Mata Atlântica (Vieira & Monteiro-Filho 2003, Geise et al 2004, como em outras florestas tropicais (Goodman et al 1999, Heaney 2001, Nor 2001, Sánchez-Cordero 2001, McCain 2004. Os 13 sítios localizados em áreas de baixada apresentam de 2 a 11 espécies de pequenos mamíferos não-voadores, enquanto os sete sítios localizados a altitudes maiores apresentam riqueza variando de 4 a 18 espécies (Apêndice 1).…”

Scale effects and human impact on the elevational species richness gradients

“…Species richness along altitudinal gradients was previously assumed to increase universally from cool highlands to warm lowlands, mirroring the latitudinal increase in species richness from cool to warm latitudes 1,4,5 . However, since the more recent general acceptance of altitudinal gradients as model templates for testing hypotheses behind large-scale patterns of diversity [5][6][7][8][9] , these gradients have been used in support of all the main diversity hypotheses, although little consensus has been achieved. Here we show that when resampling a data set comprising 400,000 records for 3,046 Pyrenean floristic species at different scales of analysis (achieved by varying grain size and the extent of the gradients sampled), the derived species richness pattern changed progressively from hump-shaped to a monotonic pattern as the scale of extent diminished.…”

“…Today, with more than 1,000 studies 9 , the altitudinal pattern is seen to be more complex. However, monotonic declines and hump-shaped patterns with peak richness at a wide range of altitudes are the most commonly reported patterns [5][6][7][8][9]11,13,16,17 . The perception of varying altitudinal patterns and the current lack of consensus on the mechanisms controlling altitudinal variation may be due largely to scale effects.…”