Multiple facets of biodiversity: Assembly processes, trait composition, and functionality along tropical elevation gradients
Biodiversity provides the basis for species interactions and ecosystem functioning. However, anthropogenic impacts – habitat degradation and climate change being the most severe – lead to dramatic declines in biodiversity and have even caused researchers to warn against the Earth’s sixth mass extinc...
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|Biodiversity provides the basis for species interactions and ecosystem functioning. However, anthropogenic impacts – habitat degradation and climate change being the most severe – lead to dramatic declines in biodiversity and have even caused researchers to warn against the Earth’s sixth mass extinction (Ceballos et al. 2015). Decreasing biodiversity comprises a loss of taxonomic, genetic, and functional richness which affects ecosystems and related ecosystem processes. Despite its importance for effective conservation management, we are still far from a comprehensive understanding of the patterns of biodiversity, the processes that determine these patterns, impacts of changing environmental conditions, and the relationships between biodiversity and ecosystem functioning. Elevational gradients are well suited to study responses of species assemblages and their different taxonomic, phylogenetic, and functional components with changing environmental conditions. In this thesis, I focused on these different components of biodiversity to i) investigate assembly processes of tropical trees, ii) study responses of functionally important morphological traits of orthopteran assemblages to changes in temperature and productivity, and iii) disentangle relationships between changes in temperature, seasonality, and habitat degradation and the incidence, species richness, and functional richness of ants and the subsequent effects on the ecosystem process predation of herbivorous arthropods, thereby testing the suitability of ants as functional indicators for effective ecosystem monitoring.
In the second chapter, I aimed to reveal assembly processes of tropical tree assemblages in the Andes. I studied patterns of species richness, phylogenetic diversity, and family age of tree assemblages to test predictions from the phylogenetic niche conservatism hypothesis (PNC) along an elevational gradient in the Ecuadorian Andes. Based on the latest available phylogenetic megatree of plants and age estimates for branch length calibration, I calculated two different phylogenetic diversity measures and the mean family age of tree assemblages. The two phylogenetic diversity measures focus on different evolutionary time scales of the phylogenetic structure of assemblages: The mean nearest taxon distance (MNTD) detects patterns close to the tips of the phylogenetic tree and therefore reflects the most recent evolutionary history. The mean pairwise phylogenetic distance (MPD) detects tree-wide patterns of the phylogenetic structure of co-occurring species. The mean family age resembles patterns from the deepest nodes within the phylogenetic tree.
My findings contrast predictions from the PNC, which would suggest decreasing species richness, phylogenetic diversity, and decreasing mean family ages with decreasing temperatures along the studied elevation gradient. Instead, MPD and family age of tree assemblages increased with elevation, whereas species richness and MNTD were not related to elevation. Furthermore, tree assemblages were generally phylogenetically clustered which suggests environmental filtering as the main driver of tree assembly along the studied gradient.
My results revealed that the occurrence of elements from old gymnosperm and angiosperm floras with extra-tropical origins at high elevations drive the phylogenetic and family age pattern of tree assemblages. This suggests that extra-tropical taxa with adaptations to temperate environments followed a corridor of temperate habitats that emerged in the course of the Andean uplift during the Neogene (Hoorn et al. 2010) to finally reach the high elevation habitats in the tropics (Segovia and Armesto 2015). The relatively young geographical history of the Andes, therefore, plays a major role in today’s composition of tree assemblages in the Andes (Qian 2014). These findings challenge the general applicability of the PNC for tropical elevation gradients. Furthermore, the findings underline that the recent tree assemblages in the Ecuadorian Andes were shaped by an interplay of ecological processes (environmental filtering), biogeographic events (the uplift of the Andes), as well as historical processes (immigration of flora elements with extra-tropical origins). The biogeographic history of mountains can, therefore, play important roles for the composition of recent tree assemblages along elevational gradients.
In the third chapter, I aimed to reveal relationships between changing environmental factors and interspecific changes of body size and other morphological traits. In particular, I studied how four different morphological traits of orthopteran assemblages respond to changes in temperature and productivity along an elevational gradient at Mt. Kilimanjaro (Tanzania). The four traits cover four ecologically important aspects of Orthoptera assemblages: Body size is related to fecundity, wing length influences dispersal ability, hind femur length relates to jumping ability, and eye size is positively related to predator detection. First, I calculated a multivariate measure of the overall body size of species of ten morphological traits. I corrected the morphological traits for covariation with body size and used community weighted means as a measure of interspecific trait values. Finally, I used Bayesian linear mixed-effect models to analyze the effects of elevation (as a proxy for temperature) and productivity on the trait values of orthopteran assemblages.
I found that body size decreased linearly with increasing elevation. Independently of the pattern of body size, the relative wing length, hind femur length, and eye size also decreased with increasing elevation. My findings emphasize the importance of changes in temperature not only for body size but also for other morphological traits of orthopteran assemblages. The effect of temperature on body size is suggested to derive from temperature-dependent biochemical processes that shorten larval growth periods, and negatively affect growth rates (van der Have and de Jong 1996, Chown and Gaston 2010). Low temperatures at high elevations also decrease the metabolic rates of insects which hampers flight ability (Dillon et al. 2006) and would make the maintenance of long wings and flight ability resource intensive. The decrease of hind femur length and eye size might rather suggest a potentially decreasing predator pressure with increasing elevation. Besides temperature, productivity negatively affected body size and eye size of orthopteran assemblages. Productivity is positively linked to the availability of plants that provide hiding abilities and resources for orthopteran species. Areas with low productivity may, therefore, be associated with an increased risk of predation and starvation (Denno et al. 2003). Habitats with low productivity might thus favor Orthoptera assemblages with larger body sizes that reduce starvation risk, as well as larger eyes that enhance predator detection. The results underline that morphological adaptations to changing environmental conditions go beyond changes in body size and involve independent adaptations of other morphological traits. The inclusion of other morphological traits in studies of size clines along environmental gradients can, therefore, help to better understand interactions between organisms and their environment.
In the fourth chapter, I aimed to test the suitability of different measures of ant assemblages as indicators of ecological responses to environmental changes, habitat degradation, and of the ecosystem process predation of herbivorous arthropods. To do so, I studied the relationships between changes in elevation (as a proxy for temperature), season, and habitat degradation on the incidence, species richness, and functional diversity of ants and their combined effects on the predation process along an elevational gradient in the Ecuadorian Andes. First, I used an easily applicable baiting approach to sample the epigaeic ant assemblages. Then I determined the incidence and species richness of the ant assemblages. I used the multidimensional trait space of four predation-related morphological traits of ants to obtain a measure of the functional richness of ant assemblages. Furthermore, I quantified the predation of herbivorous arthropods using artificial caterpillars made of plasticine. In the last step, I used a path analysis to disentangle the causal relationships between the environmental factors temperature, season, and habitat degradation and the incidence, species richness, and functional diversity of ants and their combined effect on predation rates.
Both ant incidence and species richness decreased with increasing elevation and were higher during the dry season. Ant incidence and richness positively affected the predation of artificial caterpillars. These findings suggest that the forecasted global warming would support more active and species-rich ant assemblages, which would mediate increased predation of herbivorous arthropods. Opposing the finding of ant in-cidence and richness, predation rates were lower during the dry season and higher during the wet season. This emphasizes that relationships between changes in temperature, precipitation, species assemblages, and ecosystem processes are complex and therefore difficult to predict (Colwell et al. 2008, Lavergne et al. 2010). Surprisingly, I did not find significant effects of habitat degradation on the incidence or species richness of ants, nor on the predation of herbivorous arthropods. This suggests that degraded forests in the study area provide a suitable habitat for epigaeic, ground-dwelling ant assemblages that resemble assemblages in natural forests in terms of their incidence, richness, and predation of herbivorous arthropods. However, it is important to consider that the sampling approach (bait traps) might not have attracted resource-specialized ant species that might be more sensitive to habitat degradation. The functional richness of ant assemblages decreased with elevation, however, a null-model comparison revealed that the functional richness measure was biologically not meaningful. The measured traits were thus mainly driven by ant incidence instead of elevation. Altogether, my results suggest that the incidence and species richness of ants can serve as effective indicators of responses to changes in temperature and precipitation and of the ecosystem process predation of herbivorous arthropods.