Climate warming is implicated in the increase of exotic species invasion and the outbreak of pests and pathogens. To elucidate the mechanisms driving this pattern, we must understand how temperature affects population dynamics and environmental filtering. However, this requires determining compatibility between a species’ environment and its fundamental niche, the latter being historically difficult to measure.
Here, we show how to characterize the fundamental thermal niche of an ectotherm by developing necessary and sufficient conditions for population viability through use of a stage-structured model that explicitly incorporates the temperature response of life history traits (fecundity, development, survival). We highlight regions within the thermal niche for which (i) abundance, (ii) the ability to increase when rare, and (iii) the ability to respond to perturbation, are greatest for bounded and unbounded population growth. We use these regions to generate testable predictions regarding asymmetries of invasion success (warmer-adapted exotics introduced into cooler climates versus cooler-adapted exotics introduced into warmer climates). We test these predictions in a Southern California Hemipteran insect system consisting of a newly invading sub-tropical species (Bagrada hilaris) and a native species adapted to the Mediterranean climate of the region (Murgantia histrionica).
We find that, in populations experiencing density-dependent growth, the temperature at which a species has the greatest ability to increase when rare is much higher than the temperature at which abundance is maximized. In exponentially growing populations, however, (e.g., crop pests, new invaders) these two temperatures coincide. The novel finding of our trait-based framework is that the temperature at which invasion is most likely (i.e., temperature at which a species’ ability to increase when rare is greatest) is significantly higher than the temperature for which invasion resistance is strongest (i.e., temperature at which a resident species' abundance is maximized). Importantly, this leads to an asymmetry in invasion success: warm-adapted species from lower latitudes can more successfully invade higher-latitude habitats than vice versa because the temperature at which its invasibility is the greatest is likely to be higher than the temperature at which competitive pressure from the native species is strongest. Our framework provides a way to quantify the fundamental thermal niche of an ectotherm species based solely on information about the temperature responses of the underlying life history traits, thus producing predictions of species' climate envelopes completely independently of observed distributions.