Environmental drivers of endemism in the Canary Islands: linking traits to island theory

Islands have been very influential in the fields of ecology and evolution. They are frequently described as dynamic because species richness and evolution are especially sensitive to island geological processes, leading to high biodiversity and endemism, which fluctuate through space and time. Islands thus serve as natural laboratories with which to test theories about processes that alter biodiversity. Traditional approaches in island biogeography used species richness to assess island ecological and evolutionary patterns but there is now a movement towards the use of functional traits as they can provide more mechanistic insight. Functional trait ecology has greatly increased our understanding of biodiversity patterns on the mainland and more recent advances are using multiple traits to position species in multi-dimensional space, thus indicating their role in a community. The adoption of multi-trait approaches on islands is part of the growing field of functional island biogeography. The overarching aim of this thesis was to understand how the abiotic environment influences both the functional and geographical composition of plants endemic to islands (island endemics). This objective was investigated through a series of scientific papers, which are either published or under review, focussing on the flora of the Canary archipelago as a model system. Firstly, I investigated patterns of endemic richness along topographic gradients. Endemic species typically occur in higher proportions at high elevations and it is theorised that increasing isolation with elevation leads to higher speciation rates (per species). Chapter 4 (which follows Introduction, Literature Review and Methods chapters) investigates the links between topographic isolation and endemism. Here, I extend the theory of isolation by elevation to make predictions at a finer scale, focusing on the isolating influence of small-scale fluctuations in topography in the form of deep barrancos (ravines). I assessed how the depth of barrancos influenced the proportions of species in different endemism categories. Depth did not influence endemism, but elevation did: endemic species were present in higher proportions at lower elevations within the barrancos. This is opposite to what was expected but suggests that, although barranco depth as a measure of isolation did not influence endemism, the presence of barrancos in general plays a role in shaping patterns of endemism. Trait-based approaches in island biogeography are still in their very early stages because, as island endemics are typically rare on a global scale, trait data are often severely lacking. Chapter 5 addresses this issue by evaluating the quality of existing plant trait data that are available for Canary Island species in an open access database, as well as comparing recently collected plant trait data from field expeditions with trait data that have been digitised from the literature. The outcome of this revealed a severe lack of trait data for Canary Island native species and lack of overlap between trait data collated from open access databases and traits expressed by island endemics. However, on a more promising note, trait data digitised from the literature can accurately represent what is measured in the field. This means that not all future research may require intensive field expeditions, which can be costly, time-consuming and destructive. Classic approaches in island biogeography have a strong focus on species richness and the historical lack of functional trait data means that trait variation within and across islands is largely unknown. In Chapter 6, I explore the functional composition of island species and how this relates to island age. I use plant functional trait data to investigate the link between species traits and their environment through the lens of rarity. The rarity of species traits is investigated using the index ‘functional distinctiveness’, defined as the average distance in trait space from the focal species to all other species. Functional distinctiveness of endemic and non-endemic species is analysed with respect to their occupancy in climatically rare habitats. The main result of this analysis is that endemics have an affinity for rare habitats but they are not more distinct in their traits than non-endemic species. Furthermore, as islands increased in age, species occupied fewer rare climates but no change in functional distinctiveness occurred. In other words, I found no link between trait distinctiveness and occupancy of rare climates. There has been a recent call for the inclusion of functional traits into island research. This thesis endeavours to answer this call by incorporating functional traits into island theory. To summarise the main findings, topography and climate influence the distribution of endemic species on islands, but no clear signal in their traits could be found. Trait data for Canary Island species are still lacking but the viability of some other sources of trait data, besides field data, should be encouraging to future researchers

Environmental drivers of endemism in the Canary Islands: linking traits to island theory

Islands have been very influential in the fields of ecology and evolution. They are frequently described as dynamic because species richness and evolution are especially sensitive to island geological processes, leading to high biodiversity and endemism, which fluctuate through space and time. Islands thus serve as natural laboratories with which to test theories about processes that alter biodiversity. Traditional approaches in island biogeography used species richness to assess island ecological and evolutionary patterns but there is now a movement towards the use of functional traits as they can provide more mechanistic insight. Functional trait ecology has greatly increased our understanding of biodiversity patterns on the mainland and more recent advances are using multiple traits to position species in multi-dimensional space, thus indicating their role in a community. The adoption of multi-trait approaches on islands is part of the growing field of functional island biogeography. The overarching aim of this thesis was to understand how the abiotic environment influences both the functional and geographical composition of plants endemic to islands (island endemics). This objective was investigated through a series of scientific papers, which are either published or under review, focussing on the flora of the Canary archipelago as a model system. Firstly, I investigated patterns of endemic richness along topographic gradients. Endemic species typically occur in higher proportions at high elevations and it is theorised that increasing isolation with elevation leads to higher speciation rates (per species). Chapter 4 (which follows Introduction, Literature Review and Methods chapters) investigates the links between topographic isolation and endemism. Here, I extend the theory of isolation by elevation to make predictions at a finer scale, focusing on the isolating influence of small-scale fluctuations in topography in the form of deep barrancos (ravines). I assessed how the depth of barrancos influenced the proportions of species in different endemism categories. Depth did not influence endemism, but elevation did: endemic species were present in higher proportions at lower elevations within the barrancos. This is opposite to what was expected but suggests that, although barranco depth as a measure of isolation did not influence endemism, the presence of barrancos in general plays a role in shaping patterns of endemism. Trait-based approaches in island biogeography are still in their very early stages because, as island endemics are typically rare on a global scale, trait data are often severely lacking. Chapter 5 addresses this issue by evaluating the quality of existing plant trait data that are available for Canary Island species in an open access database, as well as comparing recently collected plant trait data from field expeditions with trait data that have been digitised from the literature. The outcome of this revealed a severe lack of trait data for Canary Island native species and lack of overlap between trait data collated from open access databases and traits expressed by island endemics. However, on a more promising note, trait data digitised from the literature can accurately represent what is measured in the field. This means that not all future research may require intensive field expeditions, which can be costly, time-consuming and destructive. Classic approaches in island biogeography have a strong focus on species richness and the historical lack of functional trait data means that trait variation within and across islands is largely unknown. In Chapter 6, I explore the functional composition of island species and how this relates to island age. I use plant functional trait data to investigate the link between species traits and their environment through the lens of rarity. The rarity of species traits is investigated using the index ‘functional distinctiveness’, defined as the average distance in trait space from the focal species to all other species. Functional distinctiveness of endemic and non-endemic species is analysed with respect to their occupancy in climatically rare habitats. The main result of this analysis is that endemics have an affinity for rare habitats but they are not more distinct in their traits than non-endemic species. Furthermore, as islands increased in age, species occupied fewer rare climates but no change in functional distinctiveness occurred. In other words, I found no link between trait distinctiveness and occupancy of rare climates. There has been a recent call for the inclusion of functional traits into island research. This thesis endeavours to answer this call by incorporating functional traits into island theory. To summarise the main findings, topography and climate influence the distribution of endemic species on islands, but no clear signal in their traits could be found. Trait data for Canary Island species are still lacking but the viability of some other sources of trait data, besides field data, should be encouraging to future researchers

Thermal sensitivity of feeding and burrowing activity of an invasive crayfish in UK waters

Climate change and invasive species are among the biggest threats to global biodiversity and ecosystem function. Although the individual impacts of climate change and invasive species are commonly assessed, we know far less about how a changing climate may impact invading species. Increases in water temperature due to climate change are likely to alter the thermal regime of UK rivers, and this in turn may influence the performance of invasive species such as signal crayfish (Pacifastacus leniusculus), which are known to have deleterious impacts on native ecosystems. We evaluate the relationship between water temperature and two key performance traits in signal crayfish—feeding and burrowing rate—using thermal experiments on wild‐caught individuals in a laboratory environment. Although water temperature was found to have no significant influence on burrowing rate, it did have a strong effect on feeding rate. Using the thermal performance curve for feeding rate, we evaluate how the thermal suitability of three UK rivers for signal crayfish may change as a result of future warming. We find that warming rivers may increase the amount of time that signal crayfish can achieve high feeding rate levels. These results suggest that elevated river water temperatures as a result of climate change may promote higher signal crayfish performance in the future, further exacerbating the ecological impact of this invasive species

Climatic and biogeographical drivers of functional diversity in the flora of the Canary Islands

Aim Functional traits can help us to elucidate biogeographical and ecological processes driving assemblage structure. We analysed the functional diversity of plant species of different evolutionary origins across an island archipelago, along environmental gradients and across geological age, to assess functional aspects of island biogeographical theory. Location Canary Islands, Spain. Major taxa studied Spermatophytes. Time period Present day. Methods We collected data for four traits (plant height, leaf length, flower length and fruit length) associated with resource acquisition, competitive ability, reproduction and dispersal ability of 893 endemic, non-endemic native and alien plant species (c. 43% of the Canary Island flora) from the literature. Linking these traits to species occurrences and composition across a 500 m × 500 m grid, we calculated functional diversity for endemic, non-endemic native and alien assemblages using multidimensional functional hypervolumes and related the resulting patterns to climatic (humidity) and island biogeographical (geographical isolation, topographic complexity and geological age) gradients. Results Trait space of endemic and non-endemic native species overlapped considerably, and alien species added novel trait combinations, expanding the overall functional space of the Canary Islands. We found that functional diversity of endemic plant assemblages was highest in geographically isolated and humid grid cells. Functional diversity of non-endemic native assemblages was highest in less isolated and humid grid cells. In contrast, functional diversity of alien assemblages was highest in arid ecosystems. Topographic complexity and geological age had only a subordinate effect on functional diversity across floristic groups. Main conclusions We found that endemic and non-endemic native island species possess similar traits, whereas alien species tend to expand functional space in ecosystems where they have been introduced. The spatial distribution of the functional diversity of floristic groups is very distinct across environmental gradients, indicating that species assemblages of different evolutionary origins thrive functionally in dissimilar habitats.publishedVersio

The effect of small-scale topography on patterns of endemism within islands

Topography influences evolutionary and ecological processes by isolating populations and by enhancing habitat diversity. While the effects of large-scale topography on patterns of species richness and endemism are increasingly well documented, the direct effect of local topography on endemism is less understood. This study compares different aspects of topographic isolation, namely the isolating effect of deep barrancos (ravines) and the effect of increasing isolation with elevation in influencing patterns of plant endemism within a topographically diverse oceanic island (La Palma, Canary Islands, Spain). We collected plant presence–absence data from 75 plots in 8 barrancos on the northern coast of La Palma, spanning an elevation gradient from 95 to 674m a.s.l. Using mixed-effects models, we assessed the effect of barranco depth and elevation on the percentage of single-island endemics, multi-island endemics and archipelago endemics. We found that percent endemism was not significantly correlated with barranco depth, and correlated negatively with elevation within barrancos (rather than the expected positive relationship). The topographic barriers associated with the deep island barrancos thus appear insufficient to drive speciation through isolation in oceanic island plants. The decrease in endemism with elevation contradicts findings by previous broader-scale studies and it may reflect local influences, such as high habitat heterogeneity at low elevations

Diversification in evolutionary arenas : Assessment and synthesis

Understanding how and why rates of evolutionary diversification vary is a central issue in evolutionary biology and ecology. The concept of adaptive radiation has attracted much interest, but is metaphorical and verbal in nature, making it difficult to quantitatively compare different evolutionary lineages or geographic regions. In addition, the causes of evolutionary stasis are relatively neglected. Here we review the central concepts in the evolutionary diversification literature and bring these together by proposing a general framework for estimating rates of diversification and quantifying their underlying dynamics, which can be applied across clades and regions and across spatial and temporal scales. Our framework describes the diversification rate (d) as a function of the abiotic environment (a), the biotic environment (b) and clade-specific phenotypes or traits (c); thus d~a,b,c. We refer to the four components (a-d) and their interactions collectively as the 'Evolutionary Arena'. We outline analytical approaches to this conceptual model that open up new avenues for research, and present a case study on conifers, for which we parameterise the general model. We also discuss three conceptual examples based on existing literature: the Lupinus radiation in the Andes in the context of emerging ecological opportunity and fluctuating fragmentation due to climatic oscillation; oceanic island radiations in the context of archipelago isolation and island formation and erosion; and biotically driven radiations of the Mediterranean orchid genus Ophrys. The results of the conifer case study are consistent with the long-standing scenario that large niches, lack of competition, and high-rates of niche evolution differentially promote diversification, but these results go further by quantifying the statistical interactions between variables representing these three drivers. The conceptual examples illustrate how using the synthetic Evolutionary Arena framework results in highlighting gaps in current knowledge, and thus help to identify future directions for research on evolutionary radiations. In this way, the Evolutionary Arena framework promotes a more general understanding of variation in evolutionary rates by making quantitative results comparable between case studies, thereby allowing new syntheses of evolutionary and ecological processes to emerge

Effects of climate change on the distribution of plant species and plant functional strategies on the Canary Islands

Aim Oceanic islands possess unique floras with high proportions of endemic species. Island floras are expected to be severely affected by changing climatic conditions as species on islands have limited distribution ranges and small population sizes and face the constraints of insularity to track their climatic niches. We aimed to assess how ongoing climate change affects the range sizes of oceanic island plants, identifying species of particular conservation concern. Location Canary Islands, Spain. Methods We combined species occurrence data from single-island endemic, archipelago endemic and nonendemic native plant species of the Canary Islands with data on current and future climatic conditions. Bayesian Additive Regression Trees were used to assess the effect of climate change on species distributions; 71% (n = 502 species) of the native Canary Island species had models deemed good enough. To further assess how climate change affects plant functional strategies, we collected data on woodiness and succulence. Results Single-island endemic species were projected to lose a greater proportion of their climatically suitable area (x ̃ = −0.36) than archipelago endemics (x ̃ = −0.28) or nonendemic native species (x ̃ = −0.26), especially on Lanzarote and Fuerteventura, which are expected to experience less annual precipitation in the future. Moreover, herbaceous single-island endemics were projected to gain less and lose more climatically suitable area than insular woody single-island endemics. By contrast, we found that succulent single-island endemics and nonendemic natives gain more and lose less climatically suitable area. Main Conclusions While all native species are of conservation importance, we emphasise single-island endemic species not characterised by functional strategies associated with water use efficiency. Our results are particularly critical for other oceanic island floras that are not constituted by such a vast diversity of insular woody species as the Canary Islands

Scientific floras can be reliable sources for some trait data in a system with poor coverage in global trait databases

Aim: Trait‐based approaches are increasingly important in ecology and biogeography, but progress is often hampered by the availability of high‐quality quantitative trait data collected in the field. Alternative sources of trait information include scientific floras and taxonomic monographs. Here we test the reliability and usefulness of trait data acquired from scientific floras against trait values measured in the field, and those in TRY, the most comprehensive global plant trait database. Location: Tenerife and La Palma, Canary Islands, Spain. Methods: We measured leaf area and specific leaf area (SLA) in the field for 451 native vascular plant species and compared them with equivalent trait data digitised from the most recent and comprehensive guide of the Canarian flora, and data sourced from TRY. We regressed the field‐measured traits against their equivalents estimated from the literature and used the regression models from one island to predict the trait values on the other island. Results: For leaf area, linear models showed good agreement between values from the scientific flora and those measured in the field (r2 = 0.86). These models were spatially transferable across islands. In contrast, for SLA we found a weak relationship between field‐measured values and the best estimates from the scientific flora (r2 = 0.11). Insufficient data were available in the TRY database for our study area to calculate trait correlations with other data sources. Conclusions: Scientific floras can act as useful data sources for quantitative plant trait data for some traits but not others, whilst the TRY database contains many traits, but is incomplete in species coverage for our study region, and oceanic islands in general

Climatic and biogeographical drivers of functional diversity in the flora of the Canary Islands

Aim: Functional traits can help us to elucidate biogeographical and ecological processes driving assemblage structure. We analysed the functional diversity of plant species of different evolutionary origins across an island archipelago, along environmental gradients and across geological age, to assess functional aspects of island biogeographical theory. Location: Canary Islands, Spain. Major taxa studied: Spermatophytes. Time period: Present day. Methods: We collected data for four traits (plant height, leaf length, flower length and fruit length) associated with resource acquisition, competitive ability, reproduction and dispersal ability of 893 endemic, non-endemic native and alien plant species (c.43% of the Canary Island flora) from the literature. Linking these traits to species occurrences and composition across a 500m×500m grid, we calculated functional diversity for endemic, non-endemic native and alien assemblages using multidimensional functional hypervolumes and related the resulting patterns to climatic (humidity) and island biogeographical (geographical isolation, topographic complexity and geological age) gradients. Results: Trait space of endemic and non-endemic native species overlapped considerably, and alien species added novel trait combinations, expanding the overall functional space of the Canary Islands. We found that functional diversity of endemic plant assemblages was highest in geographically isolated and humid grid cells. Functional diversity of non-endemic native assemblages was highest in less isolated and humid grid cells. In contrast, functional diversity of alien assemblages was highest in arid ecosystems. Topographic complexity and geological age had only a subordinate effect on functional diversity across floristic groups. Main conclusions: We found that endemic and non-endemic native island species possess similar traits, whereas alien species tend to expand functional space in ecosystems where they have been introduced. The spatial distribution of the functional diversity of floristic groups is very distinct across environmental gradients, indicating that species assemblages of different evolutionary origins thrive functionally in dissimilar habitats

Effects of climate change on the distribution of plant species and plant functional strategies on the Canary Islands

peer reviewedAim Oceanic islands possess unique floras with high proportions of endemic species. Island floras are expected to be severely affected by changing climatic conditions as species on islands have limited distribution ranges and small population sizes and face the constraints of insularity to track their climatic niches. We aimed to assess how ongoing climate change affects the range sizes of oceanic island plants, identifying species of particular conservation concern. Location Canary Islands, Spain. Methods We combined species occurrence data from single-island endemic, archipelago endemic and nonendemic native plant species of the Canary Islands with data on current and future climatic conditions. Bayesian Additive Regression Trees were used to assess the effect of climate change on species distributions; 71% (n = 502 species) of the native Canary Island species had models deemed good enough. To further assess how climate change affects plant functional strategies, we collected data on woodiness and succulence. Results Single-island endemic species were projected to lose a greater proportion of their climatically suitable area (x ̃ = −0.36) than archipelago endemics (x ̃ = −0.28) or nonendemic native species (x ̃ = −0.26), especially on Lanzarote and Fuerteventura, which are expected to experience less annual precipitation in the future. Moreover, herbaceous single-island endemics were projected to gain less and lose more climatically suitable area than insular woody single-island endemics. By contrast, we found that succulent single-island endemics and nonendemic natives gain more and lose less climatically suitable area. Main Conclusions While all native species are of conservation importance, we emphasise single-island endemic species not characterised by functional strategies associated with water use efficiency. Our results are particularly critical for other oceanic island floras that are not constituted by such a vast diversity of insular woody species as the Canary Islands