Abstract
The positive relationship between habitat heterogeneity and species richness is a cornerstone of ecology. Recently, it was suggested that this relationship should be unimodal rather than linear due to a tradeoff between environmental heterogeneity and population sizes. Increased environmental heterogeneity will decrease effective habitat sizes, which in turn will increase the rate of local species extinctions. The occurrence of the unimodal richness–heterogeneity relationship at the habitat scale was confirmed in both empirical and theoretical studies. However, it is unclear whether it can occur at broader spatial scales, for meta-communities in diverse and patchy landscapes. Here, I used a spatially explicit meta-community model to quantify the roles of two species-level characteristics, niche width and immigration rates, on the type of the richness–heterogeneity relationship at the landscape scale. I found that both positive and unimodal richness–heterogeneity relationships can occur in meta-communities in patchy landscapes. The type of the relationship was affected by the interactions between inter-patch immigration rates and species’ niche widths. Unimodal relationships were prominent in meta-communities comprising species with wide niches but low inter-patch immigration rates. In contrast, meta-communities consisting of species with narrow niches and high immigration rates exhibited positive relationships. Meta-communities comprising generalist species are therefore likely to exhibit unimodal richness-heterogeneity relationships as long as low immigration rates prevent rescue effects and patches are small. The richness-heterogeneity relationship at the landscape scale is dictated by species’ niche widths and inter-patch immigration rates. These immigration rates, in turn, depend on the interaction between species dispersal capabilities and habitat connectivity, highlighting the roles of both species traits and landscape structure in generating the richness–heterogeneity relationship at the landscape scale.
Highlights
One of the fundamental ecological concepts is that heterogeneous habitats can support more species, there is a positive relationship between species richness and habitat heterogeneity (MacArthur & MacArthur, 1961; Cody, 1981)
The area-heterogeneity tradeoff, has three main predictions: (1) there is a negative relationship between population sizes and habitat heterogeneity; (2) there is a positive relationship between habitat heterogeneity and local extinction rates; and (3) there is a unimodal relationship between species richness and habitat heterogeneity
The type of the richness-heterogeneity relationship at the landscape scale was affected by complex interactions between niche width and inter-patch immigration rates
Summary
One of the fundamental ecological concepts is that heterogeneous habitats can support more species, there is a positive relationship between species richness and habitat heterogeneity (MacArthur & MacArthur, 1961; Cody, 1981). A recent study has challenged the ubiquity of the positive richness–heterogeneity relationship, and suggested that the relationship should be unimodal (Allouche et al, 2012). The reasoning behind this theory, termed the area–heterogeneity tradeoff, is that as habitats become increasingly heterogeneous in finite geographical space, the area comprising a given set of environmental conditions becomes smaller. In different systems it is possible to find positive, unimodal, or even negative relationships between richness and heterogeneity, depending on the characteristics of the species in those systems, such as their niche widths (Allouche et al, 2012), their fecundity (Kadmon & Allouche, 2007), and the rate of immigration into the local community from the regional species pool (Kadmon & Allouche, 2007); as well as the hierarchical scale of the analysis (Bar-Massada & Wood, 2014), the size of the local habitat (Kadmon & Allouche, 2007), and the environmental variable whose heterogeneity is measured (Bar-Massada & Wood, 2014)
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