Effect of Propagule Size and Landscape Structure on Morphological Differentiation and Asymmetry in Experimentally Introduced Roesel's Bush‐Crickets

Abstract

Conservation efforts involving introductions, reintroductions, and translocations of populationshave an inherent and inescapable problem of small initial populations. Small founding populations are likely to have a small proportion of the genetic variability carried by the original population. This may manifest phenotypically through changes in individual morphology, such as decreased body size, increased degree of fluctuating asymmetry, or changing susceptibility to environmental stressors. I investigated the effects of population and landscape variables on the morphology and fluctuating asymmetry in individual bush-crickets by examining 584 individual Roesel's bush-crickets (Metrioptera roeseli) from 29 established populations from different propagule sizes that were introduced in areas previously unoccupied by the species. The introduction sites were in landscapes similar to where the species occurs naturally, but sites differed in connectivity and amount of surrounding suitable habitat. Individuals were caught up to 9 years after the initial introduction, and five different morphological traits were measured. All introduced individuals originated from the same population and individuals from this source population were also collected for comparison in the analyses. Male body weight and female body length were positively affected by initial population size and degree of connectivity of the introduction patch. Isolation also affected fluctuating asymmetry in male tibias, with a higher degree of asymmetry in males that came from more isolated populations. In a relatively short time period, I was able to detect the effect of isolation and small population sizes on morphology and asymmetry. Small propagule sizes and habitat isolation are both likely to have resulted in decreased genetic diversity, the latter by reducing population sizes through decreased survival. These results show the importance of both large propagule sizes and good connectivity of habitats when introducing populations. The differences between the sexes in response to the variables examined also indicate that studies on morphology and fluctuating asymmetry need to consider males and females separately to avoid inaccurate generalizations of the state of the population.Efecto del Tamaño de los Propágulos y de la Estructura del Paisaje sobre la Diferenciación Morfológica y Asimetría en Grillos (Metrioptera roeseli) Introducidos Experimentalmente Resumen: Los esfuerzos de introducción que implican introducciones, reintroducciones y translocaciones de poblaciones tienen un problema inherente e inevitable de poblaciones iniciales pequeñas. Es probable que las poblaciones iniciales pequeñas tengan una pequeña proporción de la variabilidad genética de la población original. Esto puede manifestarse fenotípicamente por medio de cambios en la morfología individual, como una menor talla corporal, un aumento en la asimetría fluctuante o cambio en la susceptibilidad a factores ambientales estresantes. Examiné los efectos de variables poblacionales y del pais...

The Relationship Between Morphological Symmetry and Immune Response in Wild-Caught Adult Bush-Crickets

“…An estimate of measurement error is required when testing for the presence of FA to determine whether between-sides variation is significantly larger than measurement error; we tested for this in a mixed model two-way analysis of variance performed on repeated measures. The analysis of measurement error showed that FA was significant relative to measurement error in all three traits: maxillary palps (MS = 1.53, p < 0.05), tibias (MS = 0.80, p < 0.0001) and wings (MS = 32.81, p < 0.0001; for more details see [13]). An additional composite measure of FA was also calculated for each individual as the first component of a principal components analysis (PCA) from the three FA measures.…”

“…Since then, these populations have been regularly monitored and show that propagule size and habitat connectivity affect population persistence and growth [12,14], and that patch size and connectivity are correlated with immune response and FA [13,15]. Because these relationships may be, at least in part, related to genetic diversity in isolated populations [13,15], the landscape and population history factors which may confound relationships between immune response and FA need to be accounted for. Such information is usually not available in studies of wild populations; thus, our study populations present a rare opportunity to account for these factors when studying relationships between the ecology, FA and immune function in wild insect populations.…”

“…In this study we utilise these population histories to control for landscape variables known to influence immune response and FA [13,15] when comparing the variation in immune response relative to the degree of FA in wild-caught bush-crickets. Individuals that are more symmetrical (lower FA) are generally predicted to have a higher immune response because: (1) poor condition (whether from genetic or environmental factors) should result in larger FAs and higher exposure or susceptibility to parasites [8,16], or (2) parasites may exploit the limited resources of their hosts during development, which, in turn, increases FA [6].…”

“…These populations were introduced as a part of a large-scale introduction experiment in 1994 and 1995 (for more details see [12,13]). At the time of collection, the bush-crickets had been in their adult form for approximately 2 months.…”

“…Populations were introduced to 70 locations with varying propagule sizes and landscape variables to study the effects of these factors on population establishment and persistence [12,13]. Since then, these populations have been regularly monitored and show that propagule size and habitat connectivity affect population persistence and growth [12,14], and that patch size and connectivity are correlated with immune response and FA [13,15]. Because these relationships may be, at least in part, related to genetic diversity in isolated populations [13,15], the landscape and population history factors which may confound relationships between immune response and FA need to be accounted for.…”