The most powerful explanation for hump-shaped species richness distribution on the gradient was mid-domain effect (Colwell and Hurtt 1994). This pattern occurs in different regions and different taxa. During the past decade, ecologists tried to challenge this view of point, either demonstrating the hump-shaped pattern in some situations does not exist or explaining this pattern by other ecological variables. In Taiwan, the peak of woody species richness occurs at the elevation range with the most heterogeneous landscapes. Our question is whether heterogeneity or null model of mid-domain effect has better explanation to the hump-shaped diversity distribution pattern.
We selected 696 woody species (within 6 597 sampling plots) from National Vegetation Database of Taiwan for producing both the curves of the species richness distribution and null model on the elevation gradient. Temperature layers were calculated from satellite data of MODIS and precipitation layers were downloaded from the website of WORLDCLIM. The sum of standard deviations of temperature, aspect, slope and precipitation along elevation represented the pattern of heterogeneity. Generalized linear model (GLM) was used to estimate how much this diversity pattern could be explained by heterogeneity and null model.
Results/Conclusions
The species richness distribution, null model and heterogeneity all have the hump-shaped patterns along the elevation gradient. The peak of species diversity curve is at 900 m to 1 200 m, the peak of heterogeneity is at 1 000 m to 1 400 m and the peak of the null model occurs between 1 700 m to 2 200 m. The result of GLM reveals that the heterogeneity has three times higher explanatory ability (65.1 %) than the null model (22.9 %). Although the species curve is hump-shaped, the number of woody genera along the elevation decreases. The number of species per genera on the gradient is also hump-shaped. This indicates that high species number in middle elevation could be a result of higher speciation rate in more heterogeneous landscape.
Here we conclude that the peak of the hump-shaped curve of species richness coincides with the range of the elevation where the most heterogeneous combination of other gradients occurs.