The Fremont provide an important case study to examine the resilience of ancient farmers to climatic downturns, because they lived at the far northern margin of intensive maize agriculture in the American West, where the constraints on maize production are made abundantly clear. Using a tree-ring and simulation-based reconstruction of average annual precipitation and maize growing degree days, along with cost-distance to perennial streams, we model spatial variability in Fremont site density in the eastern Great Basin. The results of our analysis have implications for defining the ecological envelope in which farming is a viable strategy across this arid region and can be used to predict where and why maize farming strategies might evolve and eventually collapse as climate changes over time.

We recovered larval trombiculid mites (i.e. chiggers), vectors of scrub typhus, from small mammal hosts in Taiwan, and compared the relative degree of engorgement (RDE) of the dominant chigger (Leptotrombidium imphalum) from different hosts. Naturally occurring chiggers recovered from Rattus losea and Bandicota indica were 1·4x and 1·3x as engorged as those from Apodemus agrarius. Within each host species, RDE was negatively related to chigger loads, but was mostly unrelated to gender or to body or reproductive condition of hosts. We documented significant variation in chigger engorgement both within and among host species; to the extent that RDE is a proxy for fitness, this contradicts predictions of the Ideal Free Distribution (IFD) that the per capita fitness of vectors should be similar among hosts. Failure to meet predictions of the IFD may reflect the limited mobility of chiggers, which consequently must be less selective in the hosts on which they feed. Further disease control efforts should consider vector feeding success in addition to vector abundance and may be able to capitalize on the unsuitability of certain hosts in supporting disease vectors.

Existing models of the basic case reproduction number (R0) for vector-borne diseases assume (i) that the distribution of vectors over the susceptible host species is homogenous and (ii) that the biting preference for the susceptible host species rather than other potential hosts is a constant. Empirical evidence contradicts both assumptions, with important consequences for disease transmission. In this paper we develop an Ideal Free Distribution (IFD) model of host choice by blood-sucking insects, predicated on the argument that vectors must have evolved to choose the least defensive hosts in order to maximize their feeding success. From a re-analysis of existing data, we demonstrate that the interference constant, m, of the IFD can vary between host species. As a result, the predicted distribution of insects over hosts has 2 desirable and intuitively plausible behaviours: that it is heterogeneous both within and between host species; and that the intensity of heterogeneity varies with host and vector density. When the IFD model is incorporated into R0, the relationship with the vector: host ratio becomes non-linear. If correct, the IFD could add considerable realism to models which seek to predict the effect of these ecological parameters on disease transmission as they vary naturally (e.g. through seasonality in vector density or host population movement) or as a consequence of artificial manipulation (e.g. zooprophylaxis, vector control). It raises the possibility of targeting transmission hot spots with greater accuracy and concomitant reduction in control effort. The robustness of the model to simplifying assumptions is discussed.