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The hypothesis are: three version of Energy Hypothesis ("Productivity Hypothesis", 'Ambient Energy Hypothesis", Freezing Tolerance" Hypothesis), Habitat Heterogeneity Hypothesis and Historical Contingency.

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This article focus on productivity hypothesis, in which plant diversity is limited by solar energy tempered by water availability while animal diversity is limited by production of food required.

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Ambient energy= PET (Potential Evapotranspiration) and annual mean temperature.

founded on physiological requirements of organisms. (sunlight and temperature)

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They concluded that contemporary levels of energy and water availability are strong predictors of LDG.  Energy input operate indirectly via influences of energy on plant productivity. Water also influences plant productivity. Productive areas can support higher species diversity and thus it acts a  strong predictor of global extent geographical variation in species richness of terrestrial birds.

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Habitat Heterogeneity Hypothesis states that an area with high habitat heterogeneity will contain a high species diversity. The adaptation to one habitat would result in competitive inferiority - limited resources . Landscapes where distinct habitats can support different species results in high diversification when specialist exhibit distinct habitat preference.

Altenative habitat types -> Support distinct population

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Organisms limited at higher latitudes due to their inability of them to withstand winter temperature.

REJECTED HYPOTHESIS

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1. operate via plant productivity

2. latitudinal shift from ambient energy in far North to water toward the tropics

3. high latitudinal area ( ambient energy)

4. low latitudinal area (water-related variables - AET & annual rainfall)