Fragmentation mediates thermal habitat choice in ciliate microcosms

Abstract

Habitat fragmentation is expected to reduce dispersal movements among patches as a result of increased inter-patch distances. Furthermore, since habitat fragmentation is expected to raise the costs of moving among patches in the landscape, it should hamper the ability or tendency of organisms to perform informed dispersal decisions. Here, we used microcosms of the ciliate Tetrahymena thermophila to test experimentally whether habitat fragmentation, manipulated through the length of corridors connecting patches differing in temperature, affects habitat choice. We showed that a twofold increase of inter-patch distance can as expected hamper the ability of organisms to choose their habitat at immigration. Interestingly, it also increased their habitat choice at emigration, suggesting that organisms become choosier in their decision to either stay or leave their patch when obtaining information about neighbouring patches gets harder. This study points out that habitat fragmentation might affect not only dispersal rate but also the level of non-randomness of dispersal, with emigration and immigration decisions differently affected. These consequences of fragmentation might considerably modify ecological and evolutionary dynamics of populations facing environmental changes.

Keywords: dispersal; fragmentation; informed decision; phenotypic plasticity; temperature; thermal niche.

Figure 1.

Experimental design used to quantify the effects of habitat fragmentation (manipulated through the length of corridors) on temperature-dependent habitat choice at emigration (i.e. stay or leave) and immigration (i.e. where to settle). Six replicates per genotype for each corridor length and central temperature were performed.

Figure 2.

Fragmentation affects habitat choice at emigration (a,b) and immigration (c,d). Estimates ± s.e. of habitat choice under long corridors (y-axis) versus standard corridors (x-axis) are shown. Each dot represents one genotype, and s.e. of estimated habitat choice values are illustrated by error bars (table 1). Habitat choice estimates (a & c) vary between −1 and 1: h = 0 for random dispersal (illustrated by grey dashed lines), h > 0 for a preference for optimal habitats and h < 0 for a preference for suboptimal habitats. The black dashed line illustrates the null hypothesis where fragmentation does not affect habitat choice. In addition to basic habitat choice estimates, absolute habitat choice values are provided to illustrate habitat choice ability irrespective of its adaptive value (b,d): dots below the line correspond to a decrease in habitat choice with fragmentation, while genotypes increasing habitat choice with fragmentation are above the line. Red dots are genotypes significantly affected by fragmentation (table 1). (Online version in colour.)