Dr. Jaymee Marty, The Nature Conservancy,
CAL-PAC SOCIETY FOR RANGE MANAGEMENT SYMPOSIUM —GRAZING FOR BIOLOGICAL CONSERVATION —JUNE 23, 2006
The purpose of this study was to examine the effects of grazing on vernal pool plant species
diversity and aquatic invertebrate and vertebrate species diversity. Vernal pools occur throughout
California in grassland areas underlain by a hardpan or claypan layer that restricts percolation of
water through the soil. In short they are poorly drained depressional features. They are a
conservation concern because less than twenty percent of the original habitat remains, and they
contain a very high degree of diversity with more than 100 species of endemic plants and more
than 30 species of endemic crustaceans. Vernal pools are defined by their seasonality. They are
dry in the summer and fall. Water ponds in the pools each year with the onset of winter rain,
creating aquatic habitat that supports the activity of aquatic vertebrates including such species as
the California Tiger Salamander and the vernal pool tadpole shrimp. These unique species
require rather long periods of inundation in order to complete their lifecycles. In spring as the
pools begin to dry down they are ringed with wildflowers, and have become known as islands of
native diversity in a sea of exotic annual grassland. Each pool displays a distinct moisture
gradient from the pool, to the edge, and finally the upland. There is a significant correlation
between the moisture gradient and the percent absolute cover of native and exotic species. The
pool bottoms and edges are dominated by native species while the uplands surrounding the pools
are dominated by non-native annual species. The study measured how disturbances such as
grazing impact species composition along this moisture gradient. The study also investigated
what level of grazing maintains the highest native diversity; how grazing impacts exotic species
in and around the pools; whether exotic plant species impact vernal pool hydrology; and finally
whether or not these effects differ by soil type.
The study was conducted on the 12,362-acre Howard Ranch property in Eastern Sacramento
County. The ranch had been seasonally grazed by cattle for the past several decades. The ranch
contains two distinct soil formations each providing the setting for unique vernal pools. The
Northern portion of the ranch is underlain by the Valley springs formation which is defined by
shallow soils underlain by bedrock and contains smaller and shallower vernal pools compared
with the deeper pools found on the Southern portion of the ranch. These pools are associated
with the Laguna Formation, defined by alluvial terraces overlying a hard pan and claypan at
approximately three meters below the soil surface.
Three replicates of four grazing treatments were implemented on each formation for a total of six
replicate treatment blocks. In each of these six blocks, the grazing treatments included: one left
completely ungrazed, one grazed only during the wet season (when water was still in the pools),
and one that allowed for grazing only during the dry season (when no water remained). The
control consisted of continuous grazing of vernal pools from October through June at an intensity
of one animal unit per six acres.
The vegetation sampling scheme employed transects laid through the longest axis of each pool
studied. Three locations were selected at random along the baseline transect to run perpendicular
transects. Three 35 X 70 cm quadrats were placed along these perpendicular transects in the
pool, on the edge, and in the upland. Within each quadrat researchers recorded species, cover
class values, bare ground, litter, and anything else of interest.
The results demonstrated that the relative cover of native plant species remained highest in
continuously grazed plots, while declining in those where grazing was removed. Grazing
removal did not affect the cover of native vegetation in the pool quadrats but did negatively
impact native cover in both the edge and upland zones. When the study began in 2001 the area
was a forb-dominated system. After three years of grazing removal, the treatment areas quickly
shifted to grass dominance. These treatment effects were most pronounced under the ungrazed
treatment regime and less significant with the wet or dry season grazed treatments. The effects of
the treatments were also more pronounced in the edge and upland zones compared to those in the
pools. The effect of residual dry matter (RDM) values was also investigated, and it was found
that there is a significant decline in species richness with higher RDM levels. The possibility of
upper limits for RDM values being developed was pondered, but would prove challenging due to
the significant variability in RDM values caused by changes in precipitation from year to year. It
was also found that the change in native richness per quadrat over the first three years of the
study was positive in grazed pools and negative in ungrazed pools. There was a decline in
diversity with the removal of grazing after only three years, and this effect was most significant
on the edge.
The study also looked at the effect of grazing on the hydrology and invertebrate/vertebrate
community of the pools. Water quality measurements pertaining to temperature and clarity
during invertebrate sampling were recorded. Water depth was recorded weekly, and the period
of inundation was also determined. Sampling for invertebrates was performed twice a season and
involved quite tedious separation and counting of each species in the lab. The change in
hydrology as a result of the complete removal of grazing was quite significant resulting in an
average maximum ponding period 50 days less in ungrazed pools compared with those left
continuously grazed. This reduction in pool inundation is biologically very significant in that
many species depend on extended periods of inundation to successfully complete their lifecycles.
It was also observed that continuously-grazed pools dried completely fewer times than any of the
grazing removal treatments. Soil compaction was also noted to be lowest in the ungrazed pools,
leading to the possibility that in the case of vernal pools higher compaction may actually be a
good thing. However the main cause of the observed hydrologic change was most likely a result
of increased densities of grass resulting in increased rates of evapotranspiration essentially
sucking the pools dry. As a result of shorter periods of inundation in the ungrazed plots
invertebrate taxa richness was lowest in the ungrazed plots presumably because fewer species are
allowed sufficient time to complete their lifecycles. Invertebrate taxa richness was observed to
remain constant in continuously grazed pools and declined in ungrazed pools throughout the
season in 2003.
This study has several implications for management and restoration of vernal pool grasslands.
First, if a site is grazed and demonstrates high diversity, then it should be left grazed unless there
is a compelling, scientifically-based reason to change the management regime. Furthermore, if
changes are made to the grazing regime the change should be monitored utilizing a control so
that changes can be effectively observed and attributed to the grazing treatment rather than
changes that may be simply due to climatic variability. Grazing too little should be considered as
a threat to these systems as well as overgrazing. Finally, the removal of grazing may negatively
impact vernal pool hydrology and species requiring longer periods of pool inundation.
Full results of this study can be found in the following article:
Marty, J. 2005. Effects of cattle grazing on diversity in ephemeral wetlands. Conservation
Biology 19:1626-1632