1 INTRODUCTION

It is well known that coping with increasing aridity is a physiological challenge for all organisms (Chown et al., 2011). Aridity influences biodiversity at multiple levels, shaping species distributions at fine and large geographical scales (Craine et al., 2013; Rajpurohit et al., 2013; Watling & Braga, 2015), driving lineages diversification (Catullo & Keogh, 2014; Dorn et al., 2014; Pinceel et al., 2013; Razeng et al., 2017) and community turnover (Vander Vorste et al., 2021), and regulating ecosystem structure and function (Berdugo et al., 2020).

Aridity is a critical stressor in many freshwater ecosystems around the world (Bond et al., 2008; Woodward et al., 2010). Indeed, desiccation resistance traits have been associated with the distributions of freshwater invertebrate species along gradients of drought intensity and water availability (e.g., Céréghino et al., 2020; Perez-Quintero, 2012). For aquatic organisms with aerial dispersal, such as true water beetles (i.e., species with all life stages aquatic, Jäch & Balke, 2008), the physiological desiccation resistance capacity of the adult (dispersive) stage may limit dispersal capacity and thus could be a key determinant of distribution patterns, as demonstrated for other animal taxa prone to desiccation (e.g., amphibians—Watling & Braga, 2015—or terrestrial arthropods—Dias et al., 2013; Kellermann et al., 2012; Kellermann et al., 2020; Rajpurohit et al., 2013). In arid and semi-arid areas, where temporary inland aquatic systems are common, spatial connectivity is disrupted during droughts and many small and shallow water bodies can remain completely dry for long periods (Datry et al., 2016; Davis et al., 2018; Morán-Ordoñez et al., 2015; Murphy et al., 2015). Some aquatic invertebrates possess traits to resist desiccation in situ, but those that lack such traits are forced to disperse (resistance vs. resilience strategies, see Chester & Robson, 2011; Chester et al., 2015 or Strachan et al., 2015 for reviews on this topic). This is the case for water beetles, whose main mechanism to cope with desiccation consists of minimising cuticular transpiration by improving the waterproofing capacity of the cuticle (Botella-Cruz et al., 2021). Adult beetles disperse among wetted reaches recolonising dry sites when flow returns (Bilton et al., 2001; Cañedo-Argüelles et al., 2015; Velasco & Millán, 1998), experiencing dehydration during such aerial exposure (Bogan et al., 2017; Strachan et al., 2015). Consequently, their capacity to control cuticular water loss during such events could be an important physiological constraint for the occupation of aquatic habitats in arid and semi-arid regions, as well as a determinant of meta population dynamics (Cañedo-Argüelles et al., 2020; Chester et al., 2015; Razeng et al., 2016).

The study of the factors that shape species distributions has been typically addressed by correlative approaches relying on distributional data (e.g., species distribution modelling; SDM) and has often focused on realised rather than fundamental niches (Hutchinson, 1957; Jiménez & Soberón, 2022). Biogeographical studies of water beetles relating their thermal tolerance (Calosi et al., 2008; Hidalgo-Galiana et al., 2014), dispersal ability (Arribas et al., 2012), or lithology (Abellán et al., 2012) to distribution patterns have also shown that physiologically determined variables are important constraints on the global distribution of some water beetle clades. However, the role of aridity and desiccation resistance in shaping the distribution of aquatic insects has been less explored (but see Carey et al., 2021; Céréghino et al., 2020; Davis et al., 2013; Perez-Quintero, 2012).

The degree of congruence between fundamental and realised niches is a central issue in biogeography and evolutionary ecology (Araújo & Pearson, 2005; Soberón & Arroyo-Peña, 2017). Exploring the relationship between climate, physiological constraints, and species distributions requires studying aspects of both the fundamental and realised niches (Kearney, 2006; Soberón, 2007). While physiological data can be used to estimate the fundamental niche, the area currently occupied by a species can only provide partial environmental information on the full spectrum of abiotic conditions under which a species can potentially survive and reproduce (Colwell & Rangel, 2009; Sánchez-Fernández et al., 2011). The degree of congruence between such aspects is linked to the extent to which species are at equilibrium with current climate and to the relative role of physiology versus other contingent factors (e.g., historical factors, biotic interactions or dispersal limitations) in shaping distributions (Bozinovic et al., 2011; Helaouët & Beaugrand, 2009; Sánchez-Fernández et al., 2012). In water beetles, estimates of thermal limits derived from laboratory experiments (proxies of the fundamental thermal niche) and from species’ occurrences (realised thermal niche) have shown limited agreement (Sánchez-Fernández et al., 2012). Such a relationship has not been explored in terms of aridity, at least in freshwater ecosystems, so the question remains as to what extent the aridity conditions at which the species are currently exposed (realised aridity niche) are associated with the degree of desiccation that they can physiologically tolerate (i.e., the fundamental aridity niche). Understanding the influence of physiological constraints on species distributions is fundamental for predicting the impact of climate change and increasing aridification on biodiversity (Craine et al., 2013).

One limitation for exploring aspects of the fundamental niche is the unavailability of physiological data for most species. However, detailed examinations of a few carefully studied cases can be of great value in dissecting the nature of the underlying processes by which environmental conditions constrain species distributions (Kearney et al., 2018). Physiological desiccation resistance has been well studied for a few lineages of water beetles, such as the subgenus Lumetus within the genus Enochrus (fam. Hydrophilidae) (Botella-Cruz et al., 2017, 2019; Pallarés, Arribas, et al., 2017; Pallarés et al., 2016). This information, together with distribution and climatic data and a well-resolved molecular phylogeny of a set of representative Palearctic species of this clade, distributed across a large aridity gradient, provides an excellent study case to explore the relationship between the fundamental and realised aridity niche within an evolutionary framework. In this clade of beetles, reconstruction of the evolution of desiccation and salinity tolerance suggested that improved desiccation resistance provided a physiological basis to develop salinity tolerance and colonise highly saline waters, which are naturally stressful habitats located in arid and semiarid areas (Arribas et al., 2014; Pallarés, Arribas, et al., 2017). Accordingly, it could be hypothesised that physiological desiccation resistance might have been pivotal for the occupation of extreme environments in general, not only in terms of salinity, but also within an aridity gradient.

Here, we explore the association between experimentally measured desiccation resistance (as a proxy of the fundamental aridity niche) and the realised niche (i.e., climatic conditions in which the species are present, focusing on aridity-related climatic variables) in Lumetus species, accounting for the phylogenetic relationships among the species. Additionally, we use an SDM technique to explore the extent to which aridity-related variables are able to explain the geographical distribution of these species. If species distributions along aridity gradients are mainly shaped by physiological constraints (desiccation resistance), species with lower desiccation resistance should be restricted to wetter climates, showing narrow realised niches, whilst species with higher desiccation resistance would be potentially able to occupy the full spectrum of the aridity gradient, having wider realised niches (Figure 1a). Under this scenario, aridity-related environmental variables should have a higher contribution in explaining the distribution of those species with lower desiccation resistance. Alternatively, if factors other than physiological constraints (e.g., biotic interactions or dispersal limitations) are also important in shaping species distributions, a lack of relationship between desiccation resistance and realised niches would be expected. For example, desiccation-resistant species could be outcompeted in mesic areas—where their competitors may show higher fitness as a lower investment in specific adaptations to cope with desiccation stress is needed—and therefore they would be restricted to arid areas, showing narrow realised niches as well (Figure 1b). In this case, the contribution of aridity-related environmental variables in explaining species distributions should be relatively high for all species independently of their physiologically determined desiccation resistance. Finally, if desiccation resistance is not an important physiological constraint for the occupation of arid areas, realised niche breadth, the species niche position along the aridity gradient and the contribution of aridity variables would be unrelated to their desiccation resistance (Figure 1c).

中文翻译：

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水甲虫谱系中基本和已实现的干旱生态位之间缺乏一致性

1 简介

众所周知，应对日益增加的干旱是所有生物体面临的生理挑战（Chown 等人，  2011 年）。干旱在多个层面影响生物多样性，在精细和大地理范围内塑造物种分布（Craine 等人，  2013 年；Rajpurohit 等人，  2013 年；Watling 和 Braga，  2015 年），推动谱系多样化（Catullo 和 Keogh，  2014 年；Dorn 等人） al.,  2014 ; Pinceel et al.,  2013 ; Razeng et al.,  2017 ) 和社区更替 (Vander Vorste et al.,  2021 )，以及调节生态系统结构和功能 (Berdugo et al.,  2020 )。

干旱是世界各地许多淡水生态系统的关键压力源（Bond 等人，  2008 年；伍德沃德等人，  2010 年）。事实上，干旱抗性性状与淡水无脊椎动物物种沿干旱强度和水可用性梯度的分布有关（例如，Céréghino 等人，  2020 年；Perez-Quintero，  2012 年）。对于具有空中传播的水生生物，例如真正的水甲虫（即具有所有生命阶段的水生物种，Jäch & Balke,  2008），成年（分散）阶段的生理抗干燥能力可能会限制分散能力，因此可能是分布模式的关键决定因素，正如其他容易干燥的动物分类群所证明的那样（例如，两栖动物-Watling＆Braga，  2015-或陆生节肢动物 ——Dias 等人， 2013 年；Kellermann 等人，  2012 年；Kellermann 等人，  2020 年；Rajpurohit 等人，  2013 年）。在干旱和半干旱地区，临时内陆水生系统很常见，在干旱期间空间连通性被破坏，许多小型和浅水体可以长时间保持完全干燥（Datry 等人，  2016 年；戴维斯等人，  2018 年）; Morán-Ordoñez 等人，  2015 年；墨菲等人，  2015 年）。一些水生无脊椎动物具有抵抗原地干燥的特性，但那些缺乏这些特性的无脊椎动物被迫分散（抵抗力与恢复力策略，见 Chester & Robson,  2011 ; Chester et al.,  2015或 Strachan et al.,  2015的评论关于这个话题）。水甲虫就是这种情况，其应对干燥的主要机制包括通过提高角质层的防水能力来减少角质层蒸腾（Botella-Cruz 等人，  2021 年）。当水流返回时，成年甲虫分散在湿润的河段重新定居干燥地点（Bilton et al.,  2001; Cañedo-Argüelles 等人，  2015 年；Velasco & Millán，1998 年），在这种空中暴露期间经历脱水（Bogan 等人，2017 年；Strachan 等人，  2015 年）。因此，它们在此类事件中控制表皮水分流失的能力可能是占用干旱和半干旱地区水生栖息地的重要生理限制因素，也是元种群动态的决定因素（Cañedo-Argüelles 等，  2020；Chester 等人，  2015 年；Razeng 等人，  2016 年）。

对影响物种分布的因素的研究通常通过依赖于分布数据的相关方法（例如，物种分布建模；SDM）来解决，并且通常侧重于已实现的而不是基本的生态位（Hutchinson，  1957 年；Jiménez & Soberón，2022 年） . 水甲虫的生物地理研究与其耐热性（Calosi 等人，  2008 年；Hidalgo-Galiana 等人，  2014 年）、分散能力（Arribas 等人，  2012 年）或岩性（Abellán 等人，  2012 年）有关）分布模式也表明，生理决定的变量是一些水甲虫进化枝全球分布的重要限制因素。然而，对干旱和抗干燥性在塑造水生昆虫分布中的作用的探索较少（但参见 Carey 等人，  2021 年；Céréghino 等人，  2020 年；Davis 等人，  2013 年；Perez-Quintero，  2012 年） .

基本生态位和已实现生态位之间的一致性程度是生物地理学和进化生态学的核心问题（Araújo 和 Pearson，  2005 年；Soberón 和 Arroyo-Peña，2017 年）。探索气候、生理限制和物种分布之间的关系需要研究基本生态位和已实现生态位的各个方面（Kearney，  2006 年；Soberón，2007 年）。虽然生理数据可用于估计基本生态位，但一个物种目前占据的区域只能提供有关物种可能生存和繁殖的全部非生物条件的部分环境信息（Colwell & Rangel,  2009 ; Sánchez-费尔南德斯等人，  2011）。这些方面之间的一致性程度与物种与当前气候平衡的程度以及生理与其他偶然因素（例如，历史因素、生物相互作用或扩散限制）在塑造分布中的相对作用有关（Bozinovic 等等人，  2011 年；Helaouët 和 Beaugrand，  2009 年；Sánchez-Fernández 等人，  2012 年）。在水甲虫中，来自实验室实验（基本热生态位的代理）和物种发生（实际热生态位）的热限制估计显示出有限的一致性（Sánchez-Fernández et al.,  2012）。至少在淡水生态系统中，这种关系尚未在干旱方面进行过探索，因此问题仍然是该物种目前暴露的干旱条件（已实现的干旱生态位）与它们的干燥程度相关联。可以在生理上耐受（即，基本的干旱生态位）。了解生理限制对物种分布的影响对于预测气候变化和干旱化对生物多样性的影响至关重要（Craine 等人，  2013 年）。

探索基本生态位方面的一个限制是大多数物种的生理数据不可用。然而，对一些经过仔细研究的案例进行详细检查对于剖析环境条件限制物种分布的潜在过程的性质具有重要价值（Kearney 等人，  2018 年）。一些水甲虫谱系的生理干燥抗性已经得到很好的研究，例如Enochrus (fam. Hydrophilidae)属中的Lumetus亚属(Botella-Cruz et al.,  2017 , 2019 ; Pallarés, Arribas, et al.,  2017 ; Pallarés 等人，  2016 年）。这些信息，连同分布和气候数据，以及该进化枝的一组代表性古北物种的分子系统发育，分布在一个大的干旱梯度上，为探索基本干旱生态位和已实现的干旱生态位之间的关系提供了一个极好的研究案例在进化框架内。在这个甲虫进化枝中，对干燥和耐盐性进化的重建表明，提高耐干燥性为发展耐盐性和定殖高盐度水域提供了生理基础，这些水域是位于干旱和半干旱地区的自然压力栖息地（Arribas et al. ,  2014 年；Pallarés、Arribas 等人，  2017 年）。因此，可以假设生理干燥抗性可能对于一般极端环境的占领至关重要，不仅在盐度方面，而且在干旱梯度内。

在这里，我们探讨了 Lumetus 中实验测量的抗干燥性（作为基本干旱生态位的代表）与已实现的生态位（即物种存在的气候条件，侧重于与干旱相关的气候变量）之间的关联种，说明种间的系统发育关系。此外，我们使用 SDM 技术来探索干旱相关变量能够解释这些物种地理分布的程度。如果沿干旱梯度的物种分布主要受生理限制（抗干燥性）的影响，那么具有较低抗干燥性的物种应该被限制在较潮湿的气候中，显示出狭窄的实际生态位，而具有较高抗干燥性的物种将有可能占据全谱干旱梯度，具有更广泛的已实现生态位（图 1a）。在这种情况下，与干旱相关的环境变量应该对解释那些具有较低抗干燥性的物种的分布有更高的贡献。或者，如果生理限制以外的因素（例如，生物相互作用或扩散限制）在塑造物种分布方面也很重要，那么预计干旱抗性与已实现的生态位之间缺乏关系。例如，抗干燥物种可能在中干地区被击败——在这些地区，它们的竞争对手可能表现出更高的适应度，因为需要对应对干燥压力的特定适应进行较低的投资——因此它们将被限制在干旱地区，显示出狭窄的已实现生态位以及（图 1b）。在这种情况下，与干旱相关的环境变量对解释物种分布的贡献对于所有物种来说都应该相对较高，而与其生理决定的抗干燥性无关。最后，