DOROTA L. PORAZINSKA, RICHARD D. BARDGETT, MARIA B. BLAAUW, H. WILLIAM HUNT, ANDREW N. PARSONS, TIMOTHY R. SEASTEDT and DIANA H. WALL. In Review, Ecology. RELATIONSHIPS AT THE ABOVEGROUND-BELOWGROUND INTERFACE: PLANTS, SOIL MICROFLORA AND MICROFAUNA, AND SOIL PROCESSES
Nicole M. DeCrappeo. Master Degree Program in Ecology. INFLUENCE OF BIOTIC AND ABIOTIC FACTORS ON THE DISTRIBUTION OF ENTOMOPATHOGENIC NEMATODES IN TALLGRASS PRAIRIE
Abstract:
Interactions at the aboveground-belowground interface provide important feedbacks that regulate
ecosystem processes. Organisms within soil food webs are involved in processes of decomposition and nutrient
mineralization and their abundance and activity have been linked to plant ecophysiological traits such as
species identity and the quality and quantity of plant tissue. We tested aboveground-belowground diversity
relationships in a naturally developed plant community of native tallgrass prairie by taking soil samples
from beneath naturally established grass tillers of chosen characteristics (e.g. homogeneous vs. heterogeneous
plant combinations or C4 vs. C3 photosynthetic pathway) without imposing any disturbances to existing
plant-soil relationships. The goal of this study was to elucidate the consequences, for soil microbiota
(microflora, protozoa, and nematodes) and for C and N mineralization, of plant community properties such as
species richness, resource quality, resource heterogeneity, species identity, and presence of exotics.
None of the biotic or abiotic soil variables were related to plant resource heterogeneity. Protozoa were not
responsive to any of the plant community traits. Some bacterial and nematode groups were affected by plant
characteristics specific to a particular plant species but no uniform pattern emerged. Invasive and native
plants generally were similar with respect to soil variables tested in this study. The lack of clear
responses of soil variables to plant community traits indicates that idiosyncratic effects dominate both at
the plant and soil biotic level, and that generalized plant and soil diversity effects are hard to predict.
Keywords:
bacteria, C mineralization, C3 plants, C4 plants, diversity, grassland, nematodes,
N-mineralization, plant richness, plant species, prairie, resource heterogeneity, resource quality, PLFA, protozoa, soil biota,
soil ecosystem
Abstract:
Entomopathogenic nematodes (EPN) in the families Steinernematidae and Heterorhabditidae are obligate insect-parasites
that are widely distributed in soils throughout the world. EPN are unique among entomogenous nematodes due to their
symbiotic relationship with bacteria in the genera Xenorhabdus or Photorhabdus. The bacteria reside in the intestine
of the nematodes and, when released into the haemocoel of an insect host, cause septicemia and produce suitable conditions
for EPN growth and reproduction. Because EPN-bacterial complexes are able to cause host mortality in a wide range of
insect orders, are not harmful to vertebrates or plants and can be mass produced, much research has been focused
on optimizing their potential as biological control agents in agricultural ecosystems. However, very little is known about
EPN population dynamics and ecology in natural systems, even though this information is vital to the successful application of
nematodes in biocontrol programs. Further, EPN may play an important role in regulating insect populations at different spatial
and temporal scales, which in turn could have ecosystem-level impacts by altering plant species richness, composition and productivity.
In order to better understand EPN ecology in a non-agricultural setting, I examined the abiotic and biotic factors that affect
the distribution, persistence and infectivity of entomopathogenic nematodes in the tallgrass prairie.
I investigated three hypotheses: 1) EPN occur more frequently in areas with high plant species richness,
2) EPN occur more frequently in areas with high belowground insect species richness and abundance, and
3) EPN distribution and activity will be affected by soil temperature and moisture. To test these hypotheses,
surveys were conducted at the Konza Prairie Long Term Ecological Research (LTER) site in northeastern Kansas in May and June 2000.
Historically, natural disturbances such as fire and bison grazing maintained the tallgrass prairie in the United States;
these disturbance regimes are now experimentally imposed at the Konza Prairie LTER site. I sampled for EPN along elevational
transects in watersheds subjected to burning frequencies of 1-, 4- and 20-yr intervals. The burn frequency creates a plant diversity
gradient such that annually burned areas have lower species richness and are dominated by the C4 grass Andropogon gerardii
(big bluestem), while areas burned every 4 or 20 years have higher species richness and more C3 grasses, forbs and woody vegetation.
A combination of field and laboratory methods was used to assess EPN presence and activity in the prairie.
Entomopathogenic nematode traps baited with Galleria mellonella (wax moth) larvae were used to exploit the nematodes'
attraction to live insects and assess what conditions are necessary for EPN infectivity in the field. Soil cores and assays,
in which cores were taken from each sampling transect and then baited with G. mellonella larvae in the laboratory, were used to
measure presence or absence of EPN and to optimize the conditions necessary for host infection in a controlled environment.
To examine the biotic factors influencing EPN distribution, I conducted vegetation surveys and collected belowground arthropods
from each sampling transect. The biotic parameters measured were plant species richness, vegetation cover, average vegetation height
and belowground arthropod species richness, abundance and biomass. Abiotic factors measured were soil temperature, moisture, texture and pH.
Entomopathogenic nematodes were recovered from 33.3% of transects sampled across all burn treatments. Entomopathogenic nematode occurrence
was highest in 20-yr burn watersheds (50% of total transects), intermediate in 4-yr burn watersheds (33.3%) and lowest in annually
burned watersheds (16.7%). Nematode recovery rates using the soil core and assay method were 38.9%, 27.8% and 11.1% from 20-,
4- and 1-yr sites, respectively. The trap method proved to be effective in isolating EPN only when there was a significant amount
of rainfall while the traps were buried in the soil; no trap insects were infected by EPN during weeks when there was less than 1.5 mm of rain.
After a rainfall event, nematodes were recovered from traps in 66.7% of transects in 4-yr burn areas and 50% of transects in 20-yr burn areas.
The annually burned watersheds had the lowest EPN occurrence with only 16.7% of transects positive for nematodes.
Several biotic factors differed significantly between burn treatments. Plant species richness, average vegetation height,
insect species richness and abundance were all highest on either 4- or 20-yr burn sites, while percent vegetation basal cover was
highest on annually burned sites. Soil temperature was the only abiotic variable that differed between burn treatments and
the only variable that was significantly different in all 3 burn treatments (F = 26.3, P = 0.005). Annually burned watersheds
had the highest soil temperatures, followed by 4-yr and 20-yr sites.
Simple logistic regression analyses revealed that the probability of finding EPN was positively correlated with belowground
insect species richness (χ² = 2.89, P = 0.089) and abundance (χ² = 4.62, P = 0.032) and
negatively correlated with % vegetation basal cover (χ² = 4.67, P = 0.031) and soil temperature
(χ² = 7.36, P = 0.007). Based on multiple logistic regression using a backward elimination model selection
procedure (α = 0.1), the probability of finding EPN had a positive relationship to belowground
insect abundance (χ² = 3.12, P = 0.0773) and a negative relationship to soil temperature
(χ² = 7.35, P = 0.0067).
Entomopathogenic nematodes were recovered more frequently from areas with high plant species richness,
belowground insect species richness and abundance and low soil temperature, however, the probability of finding EPN was
statistically correlated only with insect abundance and soil temperature. It is possible that high insect abundance and
diversity reduces the likelihood of local extinctions of the nematodes by providing a constant supply of insects with
asynchronous lifecycles, making susceptible hosts available over a longer period of time. Lower soil temperatures may
further contribute to nematode survival and persistence by delaying or prolonging the EPN lifecycle or by reducing the activity
of EPN predators and pathogens in the soil. It is also possible that the nematodes are avoiding the higher surface soil temperatures
associated with annually burned watersheds by migrating downward in the soil profile. Sampling at lower soil depths would be useful
in assessing whether EPN are present in these areas or if their survival decreases with high temperatures, as appears to be the case
in the present study.