Banca de DEFESA: NATÁLIA FERREIRA DOS SANTOS

DIVERSITY OF AQUATIC ORGANISMS: A GLOBAL REVIEW ON DISTRIBUTION PATTERNS

Biodiversity, Zooplankton, Altitudinal gradients, Climate, Host-parasite interactions

Identifying the factors that accelerate changes in biodiversity has been a central theme in ecological
debates since the 18th and 19th centuries and continues to be a fundamental issue to this day. Changes
in biodiversity refer to any modification on species richness, abundance, and identity, which
determine changes on community structure over time. Although these biodiversity changes can be
accelerated by human activities or occur due to extreme temperature and precipitation events, the
underlying mechanisms determining these changes remain ambiguous. Freshwater ecosystems are
particularly sensitive to climate change due to their irregular features surrounded by terrestrial
landscapes, and because they are generally located in environments modified by human activities. In
these systems, climatic and anthropogenic factors have strongly altered the biodiversity of aquatic
communities, affecting them directly or indirectly. However, biotic groups that are phylogenetically
distinct or even have antagonistic relationships may respond in specific ways to these environmental
impacts. This thesis investigates the dynamics of diversity and distribution of organisms in aquatic
ecosystems, elucidating how different ecological and geographical factors influence spatial and
temporal patterns. We explore these issues by integrating empirical and meta-analytic methods, in
three chapters that cover distinct ecological structures: Chapter I – We conducted a systematic
review and meta-analysis to understand how elevation, at the global scale, influences the structure of
biotic communities, assessing the species richness of zooplankton and aquatic macrophytes along
altitudinal gradients, and identifying possible ecogeographic drivers underlying these relationships.
We compiled data from 84 studies covering 48 countries and extending to 4,200 meters above sea
level in six distinct freshwater ecosystems. We identified context-dependent patterns varying among
different taxonomic groups and habitat categories. In our study, elevation generally, but not
consistently, inversely correlated with species richness. Zooplankton richness, particularly among
Cladocera, showed a general negative effect with increasing elevation, while macrophytes show such
a trend mainly in flowing waters. Additionally, for both groups, the negative elevational influences
are more pronounced at higher latitudes, especially in the Northern Hemisphere. Chapter II – We
investigate the effects of seasonality on the zooplankton community in five temporary rock pools,
thus at the local scale. The main question of this chapter was to explore how local seasonal conditions
of rain and drought, drive abundance, richness, and evenness in the three different groups, Cladocera,
Copepoda, and Rotifera. We highlight seasonality as the main driver of the changes observed, with
differentiated effects among the taxonomic groups. Chapter III – We explored how climate and
habitat size shape the diversity of natural populations of Daphnia, their parasites, and disease risks at
the regional scale. We did this using a seven-year study database from 15 Michigan lakes. We found 

interdependencies between host and parasite, revealing that host richness and evenness fluctuate with
habitat size, but not consistently with parasites. We found no correlation between host and parasite
diversity, but host evenness is a key factor in modulating parasite transmission, highlighting a
negative relationship between host diversity and disease prevalence. This study reinforces the
understanding of how temporal, climatic, and altitudinal variables impact the biodiversity of aquatic
species. In all chapters, there is a common thread of extension of existing ecological theories,
highlighting the importance of taxonomic specificity, habitat complexity and the roles of abiotic and
biotic factors in biodiversity.