| The increase in species number from poles to the equator is one of the most fundamental patterns in ecology.Although several hypotheses have been proposed,there is a lack of consensus on the mechanisms underlying this pattern.While most hypotheses provide plausible explanation for high tropical diversity of tropical clades,it is unclear if similar mechanisms drive the diversity of extra-tropical clades.Medicago L.is a genus of flowering plants in the legume family.It belongs to tribe Trifolieae,subfamily Papilionoideae in the family Fabaceae or Leguminosae.It likely originated in the Caucasus region or north of the Mediterranean,and mostly includes annual and perennial herbs.The center of diversity for this genus is located in the arid regions of Eurasia.Medicago species are excellent forage crops and are therefore widely introduced around the world.Additionally,they hold high application value in industries like medicine,food,green manure,and industrial enzymes.As a genus of legumes of temperate dryland origin,the distribution of Medicago shows clear environmental preferences,such as most species are found in the Mediterranean,large number of narrowly defined endemic species in the Mediterranean mountains and the highlands of Central Asia,and successful establishment of some Medicago species in environments similar to those of their ancestors.However,there are still no specific studies on their global distribution patterns and the driving factors influencing this pattern.In this thesis,we constructed one of the most comprehensive distribution database of 90 species of Medicago and mapped its global distribution patterns in 100 km x 100 km grids.We then analyzed the role of contemporary and past environmental factors driving the diversity patterns of Medicago,and finally elucidated the potential mechanisms limiting its diversity pattern.The main research results and conclusions are summarized as follows:(1)The distribution of Medicago species diversity is highly uneven.At the global scale,its diversity center is located in the Mediterranean region,extending eastward from the Strait of Gibraltar to the western Transcaucasus,with slightly lower diversity in North Africa,Central Europe,Central Asia,Transcaucasus,and southeastern Australia,and the lowest diversity in tropical and high-latitude regions.Based on the bootstrap analysis,we found significant differences in the species richness of Medicago in different climates,biogeographic regions,and biomes.The richness is more than twice as high in temperate,European,and woodland/bush than in tropical,other continents,and other biomes,respectively.Also,richness is significantly higher in both continents and biomes dominated by temperate climates compared to continents and biomes dominated by tropical climates.The analysis of latitudinal diversity gradients shows that the peak of diversity occurs near the subtropical boundary both globally and in the continents,with a clear downward trend towards the tropics,and the analysis of species-area relationships shows that Asia and Europe have distinct species richness accumulation curves,with most Medicago species concentrated in these two continents.(2)The distribution pattern of Medicago diversity is characterised by significant climatic niche conservatism.We used generalized linear model to analyze the trends between Medicago species richness and Mediterranean climate deviation,and further compared whether there were differences in the relationships between richness,distribution proportions and Mediterranean climate deviation for the two life forms of Medicago.The results showed the richness of global Medicago species decrease significantly with increasing distance from the Mediterranean climatic niche.However,on assessing this trend for annual and perennial species separately,we found that the two life forms exhibit dissimilar patterns.For example,the slope of the annual species was much steeper than the slope of the perennial species and while the proportion of annual species decreased with the increasing distance from the Mediterranean climatic niche,the proportion of perennial species increased.(3)The distribution pattern of Medicago diversity is primarily driven by Quaternary climate change and environmental energy.We used generalized linear models to quantify the relative effects of five categories environmental factors on the richness patterns of Medicago.The results showed that past climate change and environmental energy factors were the most important explanatory variables for richness,and their effects were consistent at global,continental and biome scales.Among them,the past climate change factor was always negatively correlated with the richness.Interestingly,however,energy was positively correlated with species richness in temperate Asia,Europe and North America and temperate biomes(tundra,boreal forest,temperate seasonal forest and temperate grassland/desert)but negatively correlated with species richness in Africa,South America and Oceania and tropical and subtropical biomes(tropical seasonal forest/savanna and subtropical desert).(4)The Medicago species diversity in the tropics is strongly limited by environmental energy.We explored the variation in richness-environment relationship across latitude using geographically weighted regression model,and further compared whether there were differences in the relationships between richness,proportion and environmental energy for two life forms of Medicago.The results showed that the local coefficient of the regression model changed from higher negative in the tropical latitude to higher positive in the temperate latitude,and became zero at the junction of subtropical and temperate region in the Northern Hemisphere.The richness and proportion of annual species were positively correlated with the environmental energy,whereas the richness and proportion of perennial species were negatively correlated with the environmental energy.In summary,Medicago species prefer to be distributed in environments similar to their ancestral Mediterranean climate,which may be the strong driving force for their aggregation in temperate climate regions.Our results suggest that the spatial variation in Medicago diversity patterns may be strongly influenced by heat tolerance as opposed to the tropical clades that depend upon cold tolerance.This dependency on heat tolerance can be attributed to strong niche conservatism exhibited by Medicago species.Our findings provide new insights into the richness-energy hypothesis and suggest that high environmental energy may not necessarily yield high species diversity but may also lower species diversity,especially of extra-tropical clades towards the tropics. |
