Habitat quality matters for the distribution of an endangered leaf beetle and its egg parasitoid in a fragmented landscape

Abstract

Abstract. Fragmentation, deterioration, and loss of habitat patches threaten the survival of many insect species. Depending on their trophic level, species may be differently affected by these factors. However, studies investigating more than one trophic level on a landscape scale are still rare. In the present study we analyzed the effects of habitat size, isolation, and quality for the occurrence and population density of the endangered leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) and its egg parasitoid, the hymenopteran wasp Foersterella reptans Nees (Hymenoptera: Tetracampidae). C. canaliculata is strictly monophagous on meadow sage (Salvia pratensis), while F. reptans can also parasitize other hosts. Both size and isolation of habitat patches strongly determined the occurrence of the beetle. However, population density increased to a much greater extent with increasing host plant density (= habitat quality) than with habitat size. The occurrence probability of the egg parasitoid increased with increasing population density of C. canaliculata. In conclusion, although maintaining large, well-connected patches with high host plant density is surely the major conservation goal for the specialized herbivore C. canaliculata, also small patches with high host plant densities can support viable populations and should thus be conserved. The less specialized parasitoid F. reptans is more likely to be found on patches with high beetle density, while patch size and isolation seem to be less important.

The role of habitat quality in fragmented landscapes: a conceptual overview and prospectus for future research

“…Measured resources may vary from hostplant density for butterflies (e.g., Heisswolf et al 2009) to den availability (e.g., Virgos 2001) and food abundance (e.g., Schooley and Branch 2007;Mortelliti and Boitani 2008;Verbeylen et al 2009). Provided that the experimental design and the subsequent analysis allow the researcher to (1) separate the independent contribution of resources from the role of the geometry of the patch and (2) subsequently test for the interaction between resources in the patch and the geometrical features, the measurement of resources as a proxy for patch quality may be a good compromise in many cases.…”

“…Within this group of demographic measurements, density and/or abundance are often used as indicators of patch quality, particularly in studies on invertebrates (e.g., Rabasa et al 2007;Heisswolf et al 2009;Hodgson et al 2009a, b). Van Horne's (1983) main conclusion was that density is a misleading indicator of habitat quality, as is the case of despotic distributions where a higher concentration of individuals occurs in low-quality habitats.…”

“…When it comes to evaluating the role of patch quality, rather than a gradient of habitat degradation or a gradient of patch quality, in most studies (e.g., Rabasa et al 2007;Virgos 2001;Heisswolf et al 2009) researchers allow the quality of the patch (or, more often, its surrogate measurements) to vary without any control. Consequently, even if patches of variable quality are included in the sample, it is only rarely that the range of suitability is ever confirmed to be actually meaningful.…”

“…While most studies have focused on patch occupancy (Dennis and Eales 1997; Thomas et al 2001) or abundance (Lloyd 2008;Heisswolf et al 2009), there is evidence of an important role of colonization and extinction parameters (Fleishman et al 2002;Franken and Hik 2004) while Rabasa et al (2007) found no effect on dispersal. Franken and Hik (2004) found that patches of higher quality (measured as average survival throughout 4 years and with structural characteristics) had higher chances of being recolonized and lower chances of becoming extinct, while Fleishman et al (2002) found that patch quality affected most variance in occupancy and turnover rates.…”

“…An important role of ''patch quality'' as determinant of occupancy patterns has been shown for plants (Adriaens et al 2009), butterflies (Thomas et al 2001;Fleishman et al 2002), moths (Summerville and Crist 2004), other insects (Wettstein and Schmid 1999;Heisswolf et al 2009), amphibians (Marsh and Trenham 2001) birds (Verboom et al 1991;Lloyd 2008), small mammals (Holland and Bennett 2007), primates (Arroyo-Rodriguez and Mandujano 2006), and carnivores (Virgos 2001;Mortelliti and Boitani 2008). Several authors have made a plea for the incorporation of habitat quality in metapopulation models (Thomas et al 2001;Armstrong 2005), and the importance of habitat quality has been recognized by theoretical studies that have investigated the usefulness of including patch quality in metapopulation models (Moilanen and Hanski 1998;DeWoody et al 2005) and in connectivity metrics (Visconti and Elkin 2009).…”