Biogeography and habitat use

Abstract

The geographic range of distribution, abundance and body size of a species is the result of the interaction between species-specific features, ecological processes and evolutionary history (Brown et al. 1995, Lemmon and Lemmon 2008). Patterns of species geographical abundance may thus provide insights into several central questions in biogeography, macroecology, conservation biology and evolutionary biology (Sagarin et al. 2006, Gaines et al. 2009, Quintero 2015). For example, on regional scales, geographic patterns of abundance provide information on species‟ range limits, gene flow, population dynamics and responses to climate and human impacts (Sagarin and Gaines 2002a, Sagarin et al. 2006). The abundance of a species across its geographic range is classically assumed to be higher at the centre of its distributional range and to decline towards the edges, because individuals living at the centre of their ranges have better physical and biotic conditions than their conspecifics living at the edges (Brown 1984, Guo et al. 2005). This concept is known as the “Abundant Centre Hypothesis” (ACH hereinafter) [Hengeveld and Haeck 1982, Cotgreave 1993, Hochberg and Ives 1999]. In recent years, other features such as genetic structure, life-history traits, physiological traits and sex ratio have been found to influence both distributions and range boundaries (Sagarin et al. 2006, Lester et al. 2007, Rivadeneira et al.2010). This hypothesis was tested, for instance, in five porcelanid crab species from Chile, and results showed that females are more abundant at the centre, while males are more abundant at the edges of the species‟ distribution range (Rivadeneira et al. 2010). Neohelice granulata is one of the most abundant semiterrestrial brachyuran crabs (Brachyura: Varunidae) in South America. It has a wide distributional range extending over more than 20 latitudinal degrees along the southeast Atlantic coast (from central Brazil to central Argentina, see Fig. 15.1), where it inhabits the intertidal zone of estuaries, salt marshes and mangroves (Spivak 2010). This species is considered to be an ecosystem engineer (Gutierrez et al. 2006) because its burrowing impacts the habitat where it lives, with effects such as (i) entrapment of detritus, increasing the content of organic matter in the sediment, (ii) modification of carbon dynamics, by taking carbon into the sediment, (iii) effects on the direction and magnitude of nutrient flow, and (iv) increase in the redox potential and oxygen availability in the sediment, as well as nutrient content, favouring biomass production and primary productivity (see Chapter 10 Volume I). In this context, understanding the processes underlying the patterns of abundance and distribution may be key for an ecosystem engineer. Moreover, the environments where N. granulata lives (such as intertidal zone of estuaries, salt marshes and mangroves) are particularly important because they provide several ecosystem services such as protecting us from flooding and erosion, reducing eutrophication, and providing mitigation zones for climate change by sequestering and storing carbon (see Chapter 10 Volume I). Despite the vast literature on N. granulata reporting size-frequency distributions and abundances for different sites (see Spivak 2010), there have not been any attempts to seek general patterns in a biogeographical context regarding the relationship between abundance and body size distribution of crabs. This chapter presents an exhaustive review of the available information from existing studies on abundance and body size of N. granulata populations over their distributions. The main goal is to compare the contributions of local and biogeographic processes to the variation patterns in N. granulata abundance and body sizes. Specifically, we intend to address the following questions: (1) On regional scales, what environmental variables, such as temperature, granulometry, organic matter and tidal amplitude, might affect N. granulata abundance and body size (2) Is there a systematic variation in N. granulata abundance and body size on latitudinal scales? (3) On local scales, how do abundance,space use and body size change in relation to environmental heterogeneity.