Gamma diversity: derived from and a determinant of alpha diversity and beta diversity. An analysis of three tropical landscapes

Abstract

Using three taxonomic groups of beetles we examine how alpha and beta diversity influence the species richness of a landscape (gamma diversity), and vice versa. That is, how the species richness of a  landscape – which is a historical and biogeographical phenomenon –  contributes to the values of alpha diversity (1) at a given site, (2) in a community, (3) in terms of cumulative species richness by community, and also contributes to (4) the intensity of species exchange between  communities. To explore this question, we used two subfamilies of  Scarabaeoidea: Scarabaeinae and Geotrupinae, and the family  Silphidae. In all analyses these three taxonomic groups are considered  as a single indicator group: the copronecrophagous beetles.  Information is also included on the subfamily Aphodiinae  (Scarabaeoidea), coprophagous beetles not included in the indicator  group. Several types of vegetation located in three landscapes (tropical, transition and mountain) were studied, and these are located along an  altitudinal gradient in the central part of the state of Veracruz, Mexico.  We base this study on the following concepts. The alpha diversity of an  indicator group reflects the number of species that use a given  environment or resource in a given place or community. Spacial beta  diversityis related to the response of organisms to spatial  heterogeneity. Gamma diversity depends primarily on the historical and  geographic processes that operate on the mesoscale level and is  also affected by alpha and beta diversity. It is on this scale of landscape  that human actions, such as the modification and fragmentation of  vegetation, have their most important effects. These are, however, often  beyond the scope of ecological analyses carried out on a local  scale. In the three landscapes, sampling was carried out regularly at 67  sites, with complementary sampling at another 69 sites. Twenty-six  types of vegetation communities were studied. A total of 16,152  specimens representing 60 species were captured (52 species of  Scarabaeinae, 4 Geotrupinae and 4 Silphidae). In the tropical landscape  the community richest in species was low deciduous forest. In the  transition landscape, cloud forest was the richest. Each of these  communities is the most representative of their respective altitudinal  bands. In contrast, the greatest species richness in the mountain  landscape occurred in the mountain grasslands and pastures; types of  community favoured by or even created by human intervention. This is  explained by the expansion of heliophilous species from the Mexican  High Plateau into these areas. In the tropical landscape the species  richness of the pastures is similar to that of its forests, but with a  partially different composition which is characterized by the dominance  of heliophilous and coprophagous species; the latter, in addition to the  more ubiquitous species that are shared with the tropical forest. In the  transition landscape the cloud forest and the coffee plantations with  polyspecific shade are important in the context of conserving the fauna. This type of community offers arboreal cover and occupies the majority  of this landscape, allowing the groups of insects studied to move  between remnant fragments of cloud forest. On the landscape scale but  not locally, the fragmentation of natural communities does not  appear to have reduced the number of species for the beetles of the  indicator group. In each landscape disturbance by human activity  appears to have been overcome for distinct reasons. In the tropical  landscape we find the heliophilous beetle fauna characteristic of  pastures, and this has increased by two species of recent invaders. In  the transition landscape, the coffee plantations with polyspecific shade  create a communication matrix, while in the mountain landscape the  expansion of the mountain pastures has made conditions more  favourable for heliophilous species. These results are not necessarily  expected for other groups of organisms.