How to predict the future? On niches and potential distributions of amphibians and reptiles in a changing climate

Abstract

The present Ph.D. thesis investigates relationships between variations in large scale climate, niches of amphibians and reptiles and their corresponding geographic distribution patterns. As suggested by several authors, anthropogenic climate change may threaten a large part of the world’s biota. However, our knowledge on processes leading to those threats and our ability to make robust predictions of possible impacts is still limited but pivotal to develop successful management strategies. The aim of the thesis at hand is to narrow some of those knowledge gaps. The results are presented in four sections, each with several complementary chapters focussing on several aspects of the link between macro-climate, species’ environmental niches and their distribution patterns. Each chapter is distinct in the question elucidated and the material and methods used to answere these questions. <br /> Section 1<br /> The first section provides a general overview over the current knowledge concerning impacts of climate change on biota, niche concepts, availability of both climate and species occurrence data and the methods used herein. Additionally, potential ‘pitfalls’ when applying environmental niche models or climate envelope models are highlighted, illustrated and discussed using numerous examples. <br /> Section 2<br /> This section focuses on the structure of climate niches. Climatic variability within species ranges and habitat choice are analysed and dicussed in the context of natural history properties of the respective species. <br /> Chapter 2.1. Climate is suggested to be one major driver shaping species range patterns. Especially species with temperature-dependent sex determination may depend on certain climatic conditions, such as the Slider, Trachemys scripta, from North America. In this chapter, we hypothesise that climatic requirements allowing successful egg incubation and balanced sex ratios in T. scripta are the major driver for the species’ geographic distribution. We tested if the observed variation in monthly mean temperatures at 377 records throughout the native distribution of T. scripta can be used as a predictor for its geographic range. Our study showed that apparently climatic requirements during egg-incubation are the major driver for the species’ geographic distribution. Freezing events during winter may regionally limit the species’ distribution only. Adaptive strategies such as nest site choice by females, plasticity in nesting phenology or regional variation in embryonic temperature sensitivity exist. However, they may account only for partial compensation of negative effects caused by regional differences in temperature related parameters or a changing climate. <br /> Chapter 2.2. Recently, several authors observed a climatic mismatch between native and invasive ranges predicted by Climate Envelope Models (CEMs). In chapter 2.2, we address the issue of niche shift in alien invasive species versus variable choice by deriving CEMs based on multiple variable sets. The first selection of predictors aims at representing the physiological limits of a well studied alien invasive Slider. This model was compared to numerous other models based on various subsets of environmental variables or aiming at comprehensiveness. The CEMs aiming to represent the species physiology depicts its worldwide potential distribution better than any of the other approaches. The results indicate that a natural history driven understanding is crucial in developing statistical models of niches while ‘comprehensive’ or ‘standard’ sets of explanatory variables may be of limited use. <br /> Chapter 2.3. The use of Climate Envelope Models (CEMs) to predict potential distributions of species is steadily increasing. A necessary assumption is that climatic niches are rather conservative, but recent findings of niche shifts during biological invasion indicate that this assumption is not valid in every case. Selection of predictor variables may be one reason for observed shifts. In chaper 2.3, we assess differences in climatic niches in the native and invaded ranges of the Mediterranean Housegecko (Hemidactylus turcicus) in terms of commonly applied climate variables in CEMs. We analyzed which variables are more conserved versus relaxed (i.e. subject to niche shift). Furthermore, we study the predictive power of different sets of climate variables. We developed models for the Mediterranean region and the conterminous United States (US) using MaxEnt and various subsets of variables out of 19 environmental layers. Occurrence data from the native range in the Mediterranean region were used to predict the introduced range in the US and vice versa. Niche similarity and conservatism per predictor and per set of predictor as assessed using both Hellinger distances and Schoener’s index. Significance of results was tested using null models. My results indicate that the degree of niche similarity and conservatism varied greatly among predictors and variable sets applied. Shifts observed in some variables can be attributed to active habitat selection whereby others apparently reflect background effects. The study was based on comprehensive occurrence data from all regions where Hemidactylus turcicus is present in Europe and North and Central America providing a robust fundament. My results indicate that the degree of niche similarity and conservatism varied greatly among predictors and variable sets applied. These results have important implications for studies of biological invasion, impacts of climate change, and niche evolution. <br /> Section 3 <br /> This part of the thesis at hand focus on the relative importance of dispersal abilities, accessibility and biotic interactions shaping a species’ realized distribution. <br /> Chapter 3.1. Globalization has led to a heightened spread of alien invasive species, which can alter mutualistic relationships, community dynamics, ecosystem function, and resource distributions. They can cause extinctions affecting thereby local and global diversity. Among reptiles two gecko species, Hemidactylus frenatus and Hemidactylus mabouia, have considerably increased their range during the last century. Both have caused already local decimations and extinctions of native species. Records of invasive populations of H. frenatus are known from tropical Asia through Central America and Florida and invasive populations of H. mabouia can be found in Central and Southern Africa as well as in large parts of Central and South America. Only few sympatric populations are known. Herein, we aim to identify areas potentially suitable for the geckos using a climate envelope approach, predict their potential distribution under current conditions and a future climate change scenario, and try to assess why sympatric populations of both geckos are apparently rare. The results presented in chapter 3.1 suggest that climatic suitable areas for both species can be found in nearly all tropical regions. Future projections revealed that the amount of climatic suitable areas will increase for H. frenatus on a global scale, but decrease for H. mabouia. Greatest changes are suggested for South America where further spreading of H. frenatus will be enhanced due to better climatic conditions. In contrast, climatic conditions for H. mabouia will be aggravated here. We conclude that both competitive exclusion and a non equilibrium in the ranges of the species explain the virtual absence of sympatric populations, although the impact of climate on competition success is pending further testing in the field. <br /> Chapter 3.2. It was suggested that Climate Envelope Models may be only of limited use if the target species’ range is not predominately limited by unsuitable climate. In chapter 3.2, I test this assumption using the alien invasive anuran Eleutherodactylus coqui as model species. This species is presently distributed in many Caribbean islands and Hawaiian Islands where it causes major ecological and socioeconomic problems, especially evident in the later. I use a maximum entropy ecological niche modeling approach to model the native geographic distribution of this species and to project that model into other potentially threatened areas. The projection results under current climatic conditions suggested high probabilities of occurrence in tropical regions including the Caribbean, Florida, major parts of the Amazon basin and adjacent Andes, the Pantepui region, the Congo basin, and most Asian islands. Using only native records within Puerto Rico for model training my results indicate that the invasive range in the Hawaiian Islands can be predicted with high acurrancy. Projections of potential distributions under future anthropogenic global warming scenarios within the Hawaiian Islands suggest an overall stable potential distribution, but fine scale patterns suggest a possible range allocation towards higher elevations which may affect natural reserves. If the predictive maps are interpreted as depicting invasiveness potential of E. coqui, strategies to prevent further invasion should focus on biosafety measurements within the areas highlighted. <br /> Chapter 3.3. In this chapter, the invasive alien Cuban treefrog Osteopilus sepentrionalis native to Cuba, the Bahamas and some adjacent islands was used as an second example for the case that climate is not the predominantly driver of the range. It was accidentally introduced to Florida, Puerto Rico and some Hawaiian islands; it has become predator and competes with native wildlife. In chapter 3.3, we have used a maximum entropy ecological niche modelling approach to model its potential spread derived from present climate conditions as present in its native geographic distribution and we project that model into future climate change scenarios in order to detect new areas that are potentially threatened. Our model applying current climatic conditions suggested high probabilities of occurrence in countries around the Gulf of Mexico. As in chapter 3.2, the results indicate that the invasive range in Florida can be predicted with high acurrancy using only native records within Cuba and the Bahamas for model training if the predictor variables are carefully chosen. <br /> Chapter 3.4. Biotic interactions such as competitive exclusion or predation may limit the realized distribution of species in some areas although climatic conditions are well suitable. In chapter 3.4 we assess such a pattern as observed in the Brown tree snake (Boiga irregularis). The snake is native to South-East Asia and Australia and has been introduced to Guam. Here it causes major ecological and socioeconomic problems and is considered to belong to the 100 worldwide worst alien invasive species. We use a maximum entropy-based Climate Envelope Model to identify areas outside the species’ known range which worldwide are potentially suitable under current climate. Projections revealed that this invasive alien species potentially occurs in tropical and in part subtropical regions. In the larger vicinity of the snake’s known distribution, highest suitability was found for the Northern Mariana and Hawaiian Islands, Madagascar, New Caledonia and Fiji Islands. However, although most East Asian mainlands and islands are climatically suitable the invasive populations of this species do virtually not exist. The predicted potential distribution is highly coincident with the general distribution of the genus Boiga. Since B. irregularis does not coexist with other members of the genus or other potential competitors in its native range, competitive exclusion may be the best explaination for the observed pattern. <br /> Chapter 3.5. Anthropogenic habitat alteration has a strong impact on native biota and can significantly shape distribution patterns. Eleutherodactylus johnstonei, native to the northern lesser Antilles, has established numerous invasive populations at Caribbean islands and the adjacent Central and South American mainland. The species is a highly successful colonizer, but only able to invade anthropogenic disturbed habitats. Here, I use a maximum entropy climate envelope modelling approach to model the geographic distribution of this species and to project that model into other potentially threatened areas. Results obtained from the model are compared with a measure of anthropogenic habitat disturbance (Human Footprint). My results suggest a high probability of occurrence in large parts of southern Central America, at the northern and north-eastern coast of South America, and in the Andes of Colombia, Ecuador and Venezuela. The Andean region harbouring a diverse amphibian fauna, which is highly threatened due to anthropogenic habitat alteration, appears to be at highest risk for further spread. If the predictive maps are interpreted as depicting invasiveness potential of E. johnstonei, strategies to prevent further invasion should focus on biosafety measurements within the areas highlighted. <br /> Section 4<br /> The focus of the last section are the breadths of climate niches, their evolution and dynamics in space and time.<br /> Chapter 4.1. If climate changes a species’ range may shift as a respond. If disprersal limitations exist hampering range shifts, the species’ range may (1) shrink, (2) the species’ niche breadth may be large enough to buffer changing environmental conditions, or (3) the species may adapt to them. In chapter 4.1 we use a gecko endemic to a small island as a case study. Phelsuma parkeri is an endemic gecko species native to the relatively flat island of Pemba (elevational range 0 to <100 m a.s.l.). Information on its distribution on the island and its conservation status is currently lacking. In this chapter, we provide information on the spatial distribution of P. parkeri and its adaptability to habitat modification and changing climates. Our results imply that P. parkeri was hitherto highly adaptable to anthropogenic habitat modification. A comparison with paleoclimatic conditions in the Last Glacial Maximum on Pemba revealed that P. parkeri and other endemic species of the island survived climatic conditions in the past completely different from the current climatic conditions despite absence of possibilities to compensate these changes by altitudinal range shifts. We expect that P. parkeri is currently unlikely to be threatened by climate change although projection of its current realized climate niche would suggest a complete range loss. The main potential threat identified may be the introduction of invasive species such as Phelsuma dubia, which is already established on the nearest Island Zanzibar. Based on our findings, we propose that P. parkeri should be categorized as Near Threatened on the IUCN Red List. <br /> Chapter 4.2. The disturbance vicariance hypothesis (DV) has been proposed to explain speciation in Amazonia, especially its edge regions, e.g. in Guianan harlequin frogs (Atelopus) having derived from a cool-adapted Andean/western Amazonian ancestor. In this chapter, in concordance with DV predictions, we expected that: (i) these amphibians display a natural distribution gap in central Amazonia; (ii) east of this gap they constitute a monophyletic lineage which is nested within in a pre-Andean/western clade; (iii) climate envelopes of Atelopus west and east of the distribution gap show some macroclimatic divergence due to regional climate envelope shift; (iv) geographic distributions of climate envelopes of western and eastern Atelopus range into central Amazonia but with limited spatial overlap. We tested if presence and apparent absence data points of Atelopus were homogenously distributed applying Ripley’s K function. A molecular phylogeny (mitochondrial 16S rRNA gene), by application of Maximum Likelihood and Bayesian Inference, was reconstructed to study if Guianan Atelopus constitute a nested clade within a larger genus phylogeny. We focussed on climate envelope divergence and geographic distribution by computing climatic envelope models with MaxEnt based on macroscale bioclimatic parameters and testing them with using Schoener’s index and modified Hellinger distance. All four expectations were corroborated leading us to conclude that DV predictions are well applicable to Amazonian harlequin frogs. We here for the first time draw attention to species’ climate envelope change and assessment in the ongoing debate on diversification and distributions in the Amazon basin and adjacent areas. <br /> General conclusions The results of this Ph.D. thesis, while updating our status of knowledge on the link between climate change and corresponding responses of species in terms of changes in their phenology and / or their distribution patterns, hopefully will enhance our ability to understand and probably manage some of the problems arising due to anthropogenic climate change. However, although our qualitative understanding of processes and mechanims causing patterns of species phenology and distribution has been largely extended during the last decades, it is still far from being comprehensive and our ability to make robust quantitative predictions is still very limited.