From population sources to sieves: The matrix alters host-parasitoid source-sink structure

Field experiments that examine the impact of immigration, emigration, or landscape structure (e.g., the composition of the matrix) on the source-sink dynamics of fragmented populations are scarce. Here, planthoppers (Prokelisia crocea) and egg parasitoids (Anagrus columbi) were released among host-plant patches that varied in structural (caged, isolated, or in a network of other patches) and functional (mudflat matrix that impedes dispersal vs. brome-grass matrix that facilitates dispersal) connectivity. Planthoppers and parasitoids on caged patches exhibited density-dependent growth rates, achieved high equilibrium densities, and rarely went extinct. Therefore, experimental cordgrass patches were classified as population sources. Because access to immigrants did not result in elevated population densities, source populations were not also pseudosinks, i.e., patches whose densities occur above carrying capacity due to high immigration. Planthoppers and parasitoids in open patches in mudflat had dynamics similar to those in caged patches, but went extinct in 4-5 generations in open patches in brome. Brome-embedded patches leaked emigrants at a rate that exceeded the gains from reproduction and immigration; populations of this sort are known as population sieves. For species whose suitable patches are becoming smaller and more isolated as a result of increased habitat fragmentation, emigration losses are likely to become paramount, a condition favoring the formation of population sieves. An increase in the proportion of patches that are sieves is predicted to destabilize regional population dynamics. © 2007 by the Ecological Society of America

Shared parasitoids in a metacommunity: Indirect interactions inhibit herbivore membership in local communities

The interaction between species, mediated by a shared natural enemy (i.e., apparent competition), has been the subject of much theoretical and empirical investigation. However, we lack field experiments that assess the importance of apparent competition to metacommunity structure. Here, I conducted a series of field experiments to test whether apparent competition, mediated by shared egg parasitoids (Anagrus nigriventris and A. columbi), occurs between two abundant planthopper species (Delphacodes scolochloa and Prokelisia crocea) of the North American Great Plains. The two planthoppers feed on different plant species within prairie potholes (wet depressions) and, thus, do not interact directly. At the scale of individual potholes, a five-fold pulse increase in D. scolochloa density (relative to control potholes) resulted in a steady decline in P. crocea density over two generations. As expected in cases of apparent competition, P. crocea eggs in these potholes suffered twice the level of parasitism as P. crocea eggs in control potholes. In contrast, a sixfold increase in P. crocea density had no effect on D. scolochloa density or parasitism in those potholes. The superiority of D. scolochloa over P. crocea likely can be attributed to a larger source population size, greater amount of host habitat, and/or the presence of a phenological refuge from parasitism for D. scolochloa. In another experiment, in which small populations of P. crocea were established either in close proximity to D. scolochloa or in isolation, I found that the likelihood of P. crocea persistence was 36% lower in the former than the latter populations. This difference was attributable to very high rates of parasitism of P. crocea when adjacent to D. scolochloa. These two experiments provide clear evidence that the two planthopper species engage in apparent competition and that the shared parasitoids may play a significant role in limiting membership in a local community. Based on these findings, I argue that metacommunity studies must be broadened to include higher trophic levels. © 2007 by the Ecological Society of America

Matrix heterogeneity and host-parasitoid interactions in space

In this study, I experimentally examined how the landscape matrix influenced the movement, oviposition behavior, and spatial distribution of Anagrus columbi, a common egg parasitoid of the planthopper Prokelisia crocea. Both species exist among discrete patches of prairie cordgrass (Spartina pectinata), the sole host plant of P. crocea. Based on out-planted cordgrass pots bearing host eggs (to assess parasitism) or sticky leaves (traps for adult A. columbi), I found that the distribution of adult female A. columbi and pattern of ovipositions within a cordgrass patch were strongly matrix dependent. Female densities were 59% lower on the edge than interior of patches embedded in a mudflat matrix, but were evenly distributed within patches embedded in a matrix consisting of either native grasses or the exotic grass smooth brome (Bromus inermis). In contrast, parasitism was higher in the interior than edge for patches in all three matrix types. The lack of correspondence between A. columbi density and parasitism was attributed to differences in oviposition behavior: A. columbi parasitized 71% more hosts per capita in the interior than edge for patches embedded in nonhost grasses, but equal numbers on the edge and interior of patches embedded in mudflat. Matrix-dependent differences in the within-patch distribution and oviposition behavior of A. columbi can influence the distribution of parasitism risk and host-parasitoid stability at the patch level. Matrix composition also affected the pattern of movement through the matrix and the colonization of nearby cordgrass patches. Anagrus columbi females emigrating from a mudflat-embedded patch were captured at very low, but constant, numbers with distance out into the matrix, suggesting that they were reluctant to enter or remain in the mudflat. In contrast, A. columbi females entering a nonhost grass matrix had numbers that were high near the patch border and then declined exponentially with distance. These patterns of movement were likely responsible for the very different colonization rates for experimental patches embedded in different matrix types and located 3 m from a source patch of A. columbi. Patches embedded in brome were colonized at a rate that was 3.0 and 5.7 times higher than for patches in native grass or mudflat, respectively. Finally, based on a census of cordgrass patches spanning five generations, A. columbi densities and proportion of patches occupied generally increased with increasing host density, patch isolation, and the proportion of the surrounding matrix that was mudflat. Patch size had no effect on the distribution of A. columbi. Overall, these data suggest that cordgrass patches in a nonhost grass matrix, particularly smooth brome, have high connectivity relative to patches in a mudflat matrix. Changes in connectivity due to changes in matrix composition can significantly influence host-parasitoid persistence at the metapopulation level

Spatial Ecology of the Palm-Leaf Skeletonizer, Homaledra sabelella (Lepidoptera: Coleophoridae)

Understanding the processes that determine the distribution of populations is a fundamental goal in ecology. In this study, I determined the relative contribution of space and the biotic and abiotic environment to the distribution of the palm-leaf skeletonizer Homaledra sabalella (PLS; Lepidoptera: Coleophoridae) among patchily distributed dwarf palmettos (Sabal minor; Arecaceae). Based on surveys conducted at two sites in the Sherburne Wildlife Management Area, Louisiana, I found that the distribution of the PLS was primarily related to local environmental conditions – number of PLS increased with palmetto height, was greater in dry versus wet habitats, and varied in an inconsistent way with the type of understory cover. Spatial structure of the forest and isolation of the host plant were of minor importance to the distribution of the PLS. Based on a series of experiments, the mechanisms underlying the effects of these environmental variables on PLS abundance were elucidated. Tall palmettos have a greater abundance of PLS because they are 2.5 times more likely to be colonized than small palmettos. Tall palmettos do not represent better hosts (in terms of PLS survival to pupation, pupal length, or risk of parasitism). Similarly, an open understory increased colonization by two-fold, relative to a shrub understory, but understory type had no effect on host quality. Wet soils greatly reduced palmetto quality as a host (survival and pupal length), but only for the smallest palmettos (<0.75 m height). Finally, corroborating the survey data, my dispersal experiment revealed that the PLS is a strong flier and that local PLS populations (i.e., infested palmettos) are likely well connected by dispersal. I conclude by discussing how landscape-level changes at Sherburne Wildlife Management Area, owing to recent hurricane activity, could affect the risk of palmetto infestation by the PLS

Response of a gall-forming guild (Hymenoptera: Cynipidae) to stressed and vigorous prairie roses

Two general hypotheses that describe the relationship between plant quality and host-plant preference of insect herbivores are the plant-stress and plant-vigor hypotheses. We examined the response of a gall-forming guild of insect herbivores associated with prairie rose, Rosa arkansana Porter (Rosaceae), to experimental manipulations of plant stress (addition of NaCl) and vigor (addition of nitrogen; NH4NO3). The most common members of the gall-forming guild on roses are Diplolepis ignota Osten Sacken, D. nodulosa Beutenmüller, and D. rosaefolii Cockerell (Hymenoptera: Cynipidae). The repeated application of nitrogen throughout the growing season to prairie plots resulted in significantly higher plant nitrogen levels and plant growth rates. Both low and high NaCl additions caused leaves to turn yellow and wilt, but reductions in rose growth rates or xylem water potentials with NaCl additions were not statistically significant. All three members of the cynipid guild responded similarly to nitrogen and NaCl additions to rose plots. Incidence of occurrence and density within a plot decreased with increasing nitrogen or NaCl, but the decline associated with increasing NaCl was not significant for any of the cynipids. Neither the plant-vigor hypothesis (higher abundance on fast-growing, vigorous plants) nor the plant-stress hypothesis (higher abundance on physiologically stressed plants) was supported by this study. For cynipids, there is growing evidence that larvae perform best on plant tissues low in nitrogen (less vigorous plants). Agricultural runoff, of which nitrogen is an important constituent, may be significantly altering cynipid distributions and their interactions with other members of the tall-grass prairie ecosystem

Description, life history, and parasitism of a new species of Delphacid Planthopper (Hemiptera: Fulgoroidea)

Delphacodes scolochloa Cronin & Wilson (Hemiptera: Fulgoroidea) is a newly discovered delphacid planthopper that feeds and oviposits exclusively on the stems of sprangletop, Scolochloa festucacea (Willd.) Link (Poaceae: Pooidae), in the prairie pothole region of northeastern North Dakota. D. scolochloa is bivoltine. It is also wing dimorphic, but populations are composed predominantly of flightless brachypters. Macropters incur a substantial cost in terms of reduced longevity and fecundity relative to brachypters. The parasitoid complex attacking D. scolochloa consists of two egg parasitoids, Anagrus nigriventris Girault and Anagrus columbi Perkins (Hymenoptera: Mymaridae), and one undescribed dryinid. The proportion of eggs parasitized averages 21%, and egg parasitization is density independent. © 2007 Entomological Society of America

Genetics of oviposition success of a thelytokous fairyfly parasitoid, Anagrus delicatus

The foraging behaviour of the salt-marsh parasitoid, Anagrus delicatus (Hymenoptera: Mymaridae), is distinguished by few eggs laid per patch of hosts and frequent dispersal among patches. We investigated the within-population genetic variability in six quantitative traits associated with this unusual behaviour: fecundity (lifetime number of eggs), time on a patch, number of ovipositions per patch, oviposition rate, ovipositor length and tibia length (a measure of body size). Forty-one wasp isolines were initiated from single parthenogenetic females from three isolated salt marshes, and were maintained for up to eight generations in the greenhouse. We estimated the genetic variance and broad-sense heritability (h2) of these traits and tested trait means for differences among isolines (genetic variation) and sites (geographical variation). We found significant genetic variability among isolines for all traits except oviposition rate. The behavioural and morphological traits had similar levels of genetic variance, indicating that the evolvability (ability to respond to selection) of the traits is similarly high. However, the behavioural traits had higher residual variances, resulting in lower heritabilities. Only two traits had significant heritabilities. Fecundity, which is probably a good proxy for fitness, ceteris paribus, varied from on average 26 to 40 eggs per isoline and had the highest h2, 0.47±0.16 (mean±SE). Ovipositor length had an h2 of 0.36±0.17. These results suggest that the traits comprising the foraging strategy of A. delicatus should be amenable to selection (e.g. isolines could be selected that lay more eggs per host patch and consequently visit fewer patches). Genetic correlations among traits were numerous and positive. One important prediction from these data is that selection for larger wasps will result in large offspring with greater egg loads and higher oviposition rates. Wasps with this combination of attributes are likely to be more efficient natural enemies for use in biological control. In addition, there was no significant divergence (genetic or otherwise) in wasp morphology or behaviour among the three sites, even though they were separated by 8 km or more. © 1996 The Genetical Society of Great Britain

Matrix composition affects the spatial ecology of a prairie planthopper

To date, there is a lack of well-controlled field experiments that disentangle the effects of the intervening matrix from other landscape variables (e.g., patch geography or quality) that might influence animal dispersal among patches. We performed a field experiment to investigate how the movement of a delphacid planthopper (Prokelisia crocea) among discrete patches of prairie cordgrass (Spartina pectinata) is affected by the composition of the matrix (mudflat, native nonhost grasses, and the introduced grass smooth brome [Bromus inermis]). Within each matrix type, marked planthoppers were released onto experimental cordgrass patches that were made identical in size, isolation, and host plant quality. We found that the emigration rate (planthoppers lost per patch per day) was 1.3 times higher for patches embedded in the two nonhost grass matrix types than for patches in mudflat. The rate of immigration (immigrants per patch per day) into patches isolated by 3 m was 5.4 times higher in the brome than in the mudflat matrix. Patches in the native grass matrix had intermediate immigration rates. In addition, a survey of planthopper distributions in nature revealed that both the within- and among-patch distributions of the planthopper were related to the composition of the matrix. Within patches, individuals accumulated against mudflat edges (relative to patch interiors) but not against nonhost grass edges. Among patches, incidence and density increased with the proportion of the matrix composed of open mud. The matrix was equal to that of patch geography (size and isolation) in its ability to explain the distribution of the planthopper. We suggest that the low permeability of the mudflat relative to a nonhost grass edge may explain these planthopper distributional patterns. Also, because natural cordgrass patches in mudflat were richer in nutrients than those in nonhost grasses, planthoppers may have been more likely to remain and build up densities on the former patches. We predict that the displacement of native matrix types by invasive brome will result in increased connectivity and greater spatial synchrony in densities of planthoppers among cordgrass patches

Hurricane activity and the large-scale pattern of spread of an invasive plant species

Disturbances are a primary facilitator of the growth and spread of invasive species. However, the effects of large-scale disturbances, such as hurricanes and tropical storms, on the broad geographic patterns of invasive species growth and spread have not been investigated. We used historical aerial imagery to determine the growth rate of invasive Phragmites australis patches in wetlands along the Atlantic and Gulf Coasts of the United States. These were relatively undisturbed wetlands where P. australis had room for unrestricted growth. Over the past several decades, invasive P. australis stands expanded in size by 6-35% per year. Based on tropical storm and hurricane activity over that same time period, we found that the frequency of hurricane-force winds explained 81% of the variation in P. australis growth over this broad geographic range. The expansion of P. australis stands was strongly and positively correlated with hurricane frequency. In light of the many climatic models that predict an increase in the frequency and intensity of hurricanes over the next century, these results suggest a strong link between climate change and species invasion and a challenging future ahead for the management of invasive species. © 2014 Bhattarai, Cronin