Response of Grassland Vegetation on Santa Cruz Island to Removal of Feral Sheep

2012·5.5 MB·English

by  Dirk H Van Vuren

Madrono, Vol. 59, No. 4, pp. 190-195, 2012
RESPONSE OF GRASSLAND VEGETATION ON SANTA CRUZ ISLAND TO
REMOVAL OF FERAL SHEEP
Dirk H. Van Vuren and Lizabeth Bowen^
Department of Wildlife, Fish, and Conservation Biology, University of California, Davis,
CA
95616
[email protected]
Abstract
We
measured biomass and frequency of herbaceous species before and 12 years after removal of
feral sheep from a grassland on Santa Cruz Island, California. Native grasses, especially Stipa spp.,
increased dramatically following sheep removal whereas all exotic grasses except Avena spp.
decreased, probably reflecting the competitive ability of native grasses as well as differential
vulnerability to grazing. Native forbs showed a mixed response, with some species increasing but
many decreasing, perhaps because some native forbs benefit from grazing. Almost all exotic forbs
decreased, likely because these species benefit from herbivores and their activities. Our results provide
encouraging evidence that an island grassland dominated by exotic species has the potential for
recovery from severe overgrazing.
Key Words: Annual grass, exotic species, feral sheep, grassland, grazing, herbivory, island restoration,
Stipa.
Feral herbivores, especially goats and sheep, grazing had a major effect on island vegetation;
have been introduced to numerous islands for example, one grassland had 40% of the soil
around the world, and resulting overgrazing surface denuded and had an altered community
has caused major impacts on island vegetation composition on the 60% that supported vegeta-
(Coblentz 1978; Scowcroft and Giffin 1983; tion (Van Vuren and Coblentz 1987). All sheep
Wardle et al. 2001). Because of these impacts, were removed, first on the western 90% of the
feral herbivores have been eradicated from many island during 1981-1989, and then on the eastern
islands (Bullock et al. 2002; Campbell and 10% of the island during 1997-2001, with a total
Donlan 2005; Carrion et al. 2006), which has of 47,000 sheep removed (Schuyler 1993; Faulk-
often resulted in a dramatic recovery of native ner and Kessler 2011). Recovery of woody
vegetation. Most studies of vegetation recovery vegetation has been dramatic (Wehtje 1994;
have focused on shrub or woodland communities Cohen et al. 2009), but response of grassland
(e.g., Wehtje 1994; Bullock et al. 2002; Reddy species has received limited study (Klinger et al.
et al. 2012). The relatively few studies that 2002). On Santa Cruz Island, as on mainland
quantified plant recovery in grasslands found a CaHfornia, grasslands once dominated by native
general pattern ofincreased cover or frequency of perennial grasses have become dominated by
native vegetation, often dramatically so, follow- exotic annual grasses (Junak et al. 1995), leading
ing the removal or exclusion of feral goats or to the hypothesis that exotic grasses are compet-
sheep (Baker and Reeser 1972; Meurk 1982; itively superior (Seabloom et al. 2003; Corbin
Klinger et al. 2002). However, several researchers and DAntonio 2004). Evaluation of the initial
have expressed concern about the proliferation of response (<5 years) by grasslands on Santa Cruz
exotic herbaceous species following release from Island to sheep removal suggested an increase in
grazing pressure and how that proliferation annual grasses at the expense of native species
might hinder the recovery of native species (Klinger et al. 2002). Consequently, the potential
(Scowcroft and Hobdy 1987; Kessler 2002; for dominance of exotic species after removal of
Khnger et al. 2002; Knapp et al. 2009). feral sheep from Santa Cruz Island may be of
Santa Cruz Island, the largest (249 km-) of the particular importance. Our objective was to
eight Channel Islands located offshore of south- evaluate grassland vegetation before and 12 years
ern CaHfornia, supported a large population of after removal of feral sheep, in order to assess the
domestic sheep {Ovis aries). Sheep were intro- potential for recovery of a community dominated
duced in the 1850's, reached numbers of 50,000 or by exotic species.
more by the late 1800's, and became feral by the
1920's (Van Vuren and Bakker 2009). Sheep
Methods
' Present address: U.S. Geological Survey, Davis Santa Cruz Island is characterized by rugged
Field Station, Davis, CA 95616. topography and an east-west system of interior
VAN VUREN AND BOWEN: GRASSLAND RESPONSE TO SHEEP REMOVAL
2012] 191
valleys, dominated by the large Central Valley, sites again using the same procedures, including
which are bordered by the Northern Ridge randomizing the location of quadrats within each
(elevation 750 m) and Southern Ridge (elevation site. Winter rainfall on Santa Cruz Island differed
465 m). Climate is an oceanic, Mediterranean between 1980 and 1996 (Boyle and Laughrin
type characterized by hot, dry summers and cool, 2000). Precipitation during November through
wet winters. Most of the 51-cm average annual March totaled 64.1 cm for 1980 (13.3 cm of this
precipitation falls from November through April falling in March), compared with 35.7 cm (4.7 cm
(Boyle and Laughrin 2000). The island is in March) for 1996.
botanically diverse, with 627 species of vascular We calculated mean biomass (g/m-, dry basis)
plants ofwhich 157 are exotic (Junak et al. 1995). by averaging among all 50 quadrats for each
Several plant communities have been described species, and we calculated frequency of occur-
(Junak et al. 1995), with grassland (46%) and rence among all 50 quadrats for each species. We
chaparral (31%) being the most common in 1980 grouped species according to whether they were
in terms of proportion of the island covered native or exotic (Junak et al. 1995). For most
(Minnich 1980). Our study was conducted in species this was straightforward, but some taxa
Ram Canyon, a 2-km long drainage that begins identified only to genus contained both native
at the crest of the Northern Ridge and drains and exotic species. Two species of Festuca were
southward to the Central Valley. Our study area exotic and common, and two were native and
m
was at an elevation of 300 350 near the scarce (Junak et al. 1995); hence, we classified
headwaters of the canyon (34°0r30"N, Festuca spp. as exotic. Of the nine species of
19 45'45"W), where tributaries join from the Bromus, three of five exotic species were com-
1
west, northwest, and northeast. Vegetation in the mon, but only one of four native species was
study area was classified in 1980 as mostly common; the remaining five species were rare,
grassland with scattered patches of chaparral whether native or exotic. Hence, we classed
dominated by Quercus pacifica Nixon & C. H. Bromus spp. as exotic, though we recognize that
&
Mull and Ceanothus megacarpus Nutt. var. B. carinatus Hook. Arn. var. carinatus, the
insularis (Eastw.) P. H. Raven (Minnich 1980). common native species, could have been in our
We
Density of feral sheep in this area was estimated quadrats but not identified. combined Pseu-
at about 2/ha, which is considered exceptionally dognathalium spp. and Gamochaeta spp. into one
high (Van Vuren and Coblentz 1989; Schuyler group considered to be native, because several of
1993). Feral pigs {Sus scrofa) were present on the seven native species in the two genera were
Santa Cruz Island throughout our study, but they common, whereas the one exotic species was
were uncommon in our study area and we found considered occasional and generally occurred in
We
no evidence of pig rooting in or near any of our sites more mesic than ours (Junak et al. 1995).
quadrats. compared mean biomass between 1980 and 1996
We sampled vegetation 2-13 April 1980, for plant groups (native grasses, exotic grasses,
during the peak of the growing season, in order native forbs, exotic forbs) using a t-test. Statisti-
to determine forage availability in a study of feral cal comparisons for individual species were either
sheep diets (Van Vuren and Coblentz 1987). We precluded or rendered of questionable value by
used a stratified-random design in our sampling. the large number of zeros in the data.
We
chose nine grassland sites that appeared
representative of the vegetation in the area; these Results
sites were distributed throughout an 8-ha area on
the middle and lower slopes of the canyon, hence Total biomass increased substantially after
slope exposure varied among sites. Within each removal of feral sheep, from 29.8 g/m- in 1980
site we randomly located 0.75 X 1 .50 m quadrats, to 51.9 g/m- in 1996. However, native species
six at each of the first eight sites and two at differed from exotic species in their responses to
We
the ninth site, for a total of 50 quadrats. sheep removal. Biomass of exotic grasses in-
identified all green herbaceous species in each creased by only 32%, from 20.3 g/m- to 26.8 g/m-,
quadrat to the lowest taxon feasible, clipped them a difference that fell short of statistical signifi-
at ground level, and weighed them to the nearest cance (t = 1.76, P = 0.082). In contrast, native
gram; nomenclature followed Baldwin et al. grasses increased from 0.2 g/m- to 18.1 g/m-, an
A
(2012). sample of each plant species was increase of nearly two orders of magnitude that
clipped, weighed, dried at 65°C for 24 hours, was significant (t = 2.80, P = 0.007). Nafive forbs
and then reweighed to obtain dry biomass. Feral increased 59%, from 3.7 g/m- to 5.9 g/m-, though
sheep were removed from the area during July- the difference was not significant (t = 1.21, P =
December 1984 (P. T. Schuyler, personal com- 0.1 10). Exotic forbs decreased 80%, from 5.6 g/m-
=
munication), and fencing prevented recoloniza- to 1.1 g/m-, a difference that was significant (t
tion by sheep remaining on other parts of the 3.28, P < 0.001).
We
island (Schuyler 1993). returned to our study Individual taxa varied considerably in their
area during 4-13 April 1996 and sampled all nine response to removal offeral sheep. The proliferation
0
MADRONO
192 [Vol. 59
Table 1. Dry Biomass and Frequency of Plants among 50 Quadrats in a Grassland on Santa Cruz
Island, California During April 1980 and April 1996.
1980 1996
Taxa g/m- Frequency g/m- Frequency
Native grasses
Aristida spp. 0.0 0.00 2.0 0.10
Stipa spp. 0.2 0.12 16.1 0.32
Exotic grasses
Avena spp. 1.0 0.60 18.7 0.94
Bromits spp. 11.4 1.00 6.9 0.72
Festuca spp. 5.4 0.86 1.0 0.32
Hordewn niurinuni L. 1.1 0.36 0.0 0.00
Lamarckia aurea Moench 1.4 0.82 0.2 0.24
Native forbs
/iciiiispou Sii igosiis yrswiv.) Drouniei yj.1 u.zz U.U u.uu
/iLilllSpOn n1tUi^i:ilCUillS ^rlaCil. Oc ^^./\. IVlcy.; J_>'.L^. oUJvUlOll nU.11 u.uo z9.7/ nU.J'^Z9
Acouvtici iiiicvocephcdci DC. u. / n n9 u.u u.uu
Apicistruui cingnstifoliwii Nutt. u.u u.uu 1 nu.nu9z
Astvcigcdus didyjiioccirpus Hook. & Arn. nu.0z n /in U.J U.jO
0 0 0 09
\^ICl\HflllCl pt.11UlILIIll VVllJU. 5iUU5>p. pclllHlLllLl yJ.1I 1
d'oton setigevus Hook. U.J U.JO 1 .u nU.11^/I
LJuncus piisuius iviicnx. nU.J^Z9 u.u Ou.OuOu
Diche/ostenniia capitatuni (Benth.) Alph. Wood subsp. ccipitatum <0.1 0.10 <0.1 0.12
Eucrypta chrysantheuiifolia (Benth.) Greene var. chrysantheiiiifolia 0.2 0.40 0.0 0.00
Galium spp. <0.1 0.04 0.0 0.00
GUia spp. 0.0 0.00 <0.1 0.02
Lasthenia californica Lindl. 0.1 0.24 0.0 0.00
Layia pkityglossa (Fisch. & C.A. Mey.) A. Gray <0.1 0.16 <0.1 0.02
&
Logf'iafUaginoides (Hook. Arn.) Morefield 0.9 0.66 0.7 0.18
Lupirms bicolor Lindl. <0.1 0.06 0.6 0.40
Marah spp. <0.1 0.02 0.0 0.00
Phacelia spp. 0.0 0.00 <0.1 0.02
Pseudognaphaliiuu spp., Gamochaeta spp. yj.l U.4Z fU\.1I nU.TI^D
&
Pterostegia dryiuarioides Fisch. C.A. Mey. <0.1 0.02 <0.1 0.02
Thysauocarpus curvipes Hook. 0.0 0.00 0.1 0.14
Trifoliuin spp. 0.5 0.70 0.3 0.26
Exotic forbs
Capsella bursa-pastoris L. <0.1 0.04 0.0 0.00
Cerastiuui glonievatum Thuill. 0.2 0.16 0.0 0.00
Erodiuiu spp. 1.4 0.90 0.2 0.36
Hypochaeris glabra L. 3.0 0.86 0.0 0.00
Medicago polymorpha L. 0.2 0.24 0.0 0.00
SHene gallica L. 0.6 0.60 0.0 0.00
Sonchus asper (L.) Hill subsp. asper 0.0 0.00 0.9 0.40
Stelkirki medki (L.) Vill. 0.1 0.12 <0.1 0.08
of native grasses resulted from a dramatic these species did not respond to sheep removal or
\
increase in both biomass and frequency of Stipa were at such a low frequency that our sampling
Two
spp., as well as the appearance of Aristida spp. protocol did not detect a response (Table 1 ).
\
(Table 1). For exotic grasses, Avena spp. in- species {Acmispon wrcmgelianus (Fisch. and C.A.
!
creased in both biomass and frequency, while Mey.) D.D. Sokoloff and Lupinus bicolor Lindl.)
the remaining species either decreased in both showed substantial increases in both biomass and |
measures {Broinus spp., Lamarckia aurea frequency. Many native forbs, however, de- j
Moench, and Festuca spp.) or were not detected creased in biomass, frequency, or both between |
at all {Hordeum imirinuni L.; Table 1). 1980 and 1996, including several native forbs that
Several native forbs showed a low frequency of were common (frequency >0.40) in 1980 (e.g.,
occurrence among quadrats in both 1980 and Daucus pusillus Michx., Eucrypta chrysanthemi-
1996 (e.g., Acourtia microcephala DC, Apiastrum folia (Benth.) Greene var. chrysanthemifolia, ,
&
augustifolium Nutt., Galium spp., Gilia spp., Logfia jilaginoides (Hook. Arn.) Morefield,
Marah spp., Phacelia spp., and Pterostegia Pseudognathalium spp., Gamochaeta spp, and
drymarioides Fisch. & C. A. Mey.), hence either Trifolium spp.). Croton setigerus Hook., one of
I
VAN VUREN AND BOWEN: GRASSLAND RESPONSE TO SHEEP REMOVAL
2012] 193
the most common native forbs in 1980, decreased good colonizer of bare soil (Bartolome and
in frequency by 1996 but increased in biomass. Gemmill 1981; Seabloom et al. 2003), which
One common native forb. Astragalus cUdyniocar- was common in our study area in 1980 due to
&
pus Hook. Arn., remained relative unchanged grazing and trampling by feral sheep (Van Vuren
in biomass and frequency between 1980 and 1996 and Coblentz 1987). Once established, native
(Table perennial grasses can be strongly competitive
1).
All but one exotic forb decreased in both with exotic annual grasses (Seabloom et al. 2003;
biomass and frequency between 1980 and 1996 Corbin and D'Antonio 2004), especially at
(Table 1); three taxa in particular, Erodiwu spp., coastal sites where the summer drought typical
Hypocliaevis glabra L., and Silenc gallica L., were of California's Mediterranean climate is moder-
among the most important forbs, native or ated by marine influences (Corbin and D'Anto-
exotic, in 1980 in terms of both biomass and nio 2004). Our study site was located only 3.3 km
frequency but were greatly reduced or absent in from the coast.
1996. One exotic forb, Soiicluis aspcr (L.) Hill Native forbs varied greatly in their response to
subsp. aspcr, was not identified in 1980 but was removal of feral sheep. Much of this variation
common in 1996 (Table 1). likely resulted from differential rainfall between
years and its effect on germination and growth of
Discussion annual species. Most of the native forbs in our
study are annuals, as are all of the exotic forbs
The increase in biomass of grassland vegeta- (Junak et al. 1995). In annual grasslands on the
tion shown in 1996, 12 years after removal of mainland, species composition can vary greatly
feral sheep, is not surprising given the high from year to year, especially in response to
density of sheep present in 1980; the area was rainfall (Talbot et al. 1939; Pitt and Heady
considered to be severely degraded due to long- 1978; Heady 1988). However, some of the
term overgrazing (Van Vuren and Coblentz variation in native forb response likely resulted
1987) Indeed, the response might have been from the cessation of grazing. Most native forbs
.
greater if not for relatively dry conditions during may not have been greatly impacted by sheep
1996. Rainfall in 1996 was 38% below the long- herbivory, since native forbs other than Croton
term average, whereas rainfall in 1980 was 26% setigerus were rarely eaten by feral sheep (Van
above the average (Boyle and Laughrin 2000). On Vuren and Coblentz 1987). Consequently, many
Santa Cruz Island, cover of herbaceous vegeta- native forbs may have benefited from grazing
tion is positively correlated with annual rainfall that reduced mulch and suppressed competitors
(Klinger et al. 2002). (Coleman and Levine 2007), potentially explain-
As a group, exotic grasses showed only a ing the decline in several species after sheep were
modest increase in biomass, and that increase was removed. Research on grasslands elsewhere in
attributable entirely to one genus, Aveiia, which is California showed that grazed sites have a higher
represented by two annual species {A. barbata richness and abundance of native forbs (Hayes
Link and A. fatua L.) that are common on the and Holl 2003).
Two
island (Junak et al. 1995). Arena is especially native forbs, however, increased in both
vulnerable to heavy grazing pressure (Heady biomass and frequency. Acniispon wrangcliauus
1988) which characterized conditions in 1980, may have benefited from removal of feral sheep;
,
and it also may be a good competitor after release in a mainland California grassland, this species
from grazing because of its tall stature (Menke increased in abundance when herbivores were
1992). All other exotic grasses declined in both excluded (Hobbs et al. 2007). The response of
biomass and frequency; some of these grasses Lupiims bicolor is somewhat surprising, since
might be poor competitors in the absence of Thomsen et al. (1993) found that grazing had a
sheep disturbance, which is the case for La- positive rather than a negative effect on density.
marckia aurea (Crampton 1974). Croton setigerus increased in biomass but de-
In contrast to exotic grasses, native grasses creased in frequency, perhaps because it was a
showed a dramatic increase in both biomass and highly preferred food of feral sheep (Van Vuren
frequency, especially Stipa, which is represented and Coblentz 1987) but is also associated with
by three perennial species {S. cermia (Stebbins disturbed areas (Junak et al. 1995). Hence, a lack
&
Love) Barkworth, S. lepida Hitchc, and S. of herbivory coupled with a lack of disturbance
pulchra Hitchc.) (Junak et al. 1995). This resuh is in 1996 may have resulted in fewer but larger
somewhat surprising, since the wholesale replace- plants.
ment of native perennial grasses by exotic annual Exotic forbs as a group showed a dramatic
grasses and forbs throughout much of California decrease following sheep removal, a finding
during the 19"' century has been hypothesized to consistent with that of Klinger et al. (2002) for
have resulted because native grasses are inferior other grasslands on Santa Cruz Island. Research
competitors (Seabloom et al. 2003; Corbin and in mainland California indicates that several of
D'Antonio 2004). However, Stipa pulchra is a these species likely benefit from the presence of
MADRONO
194 [Vol. 59
herbivores and their activities, especially soil Literature Cited
disturbance and the removal of mulch. Erodium Baker, J. K. and D. W. Reeser. 1972. Goat
spp. decreased markedly in cover when herbi- management problems in Hawaii Volcanoes Na-
vores were excluded (Talbot et al. 1939); Medi- tional Park. Natural Resources Report Number 2,
cago polymorplia L. was more frequent on U.S. National Park Service, Washington, DC.
disturbed sites than on relatively undisturbed Baldwin, B. G., D. H. Goldman, D. J. Keil, R.
sites (Schiffman 1994); and the presence of mulch Patterson, T. J. Rosatti, and D. H. Wilkin,
depressed seed production in Erodium spp. (Rice (eds.). 2012. The Jepson manual: vascular plants of
1985) and interfered with seed germination in California, 2nd ed. University of California Press,
Hypochaeris glabra (Heady 1956) and Capsella Berkeley, CA.
hiirsa-pastoris L. (Bergelson 1990). Similarly, Bartolome, J. W. AND B. Gemhill. 1981. The
ecological status of Stipa pulchra (Poaceae) in
Harrison et al. (2003) found that grazing California. Madrono 28:172-184.
enhanced the richness of exotic forbs. The
Bergelson, J. 1990. Life after death: site pre-emption
increase in Sonchus asper subsp. asper was by the remains of Poa annua. Ecology 71:2157-
unexpected, since the species disappeared from 2165.
a grassland in Britain 13 years after herbivores Boyle, T. and L. Laughrin. 2000. California's Santa
were excluded (Watt 1981) and the congeneric Cruz Island weather. Pp. 93-99 D. R. Browne,
///
S. oleraceus L. declined on nearby San Miguel K. L. Mitchell and H. W. Chancy (eds.). Pro-
Island between 6 and 25 years after feral livestock ceedings of the fifth California Islands symposium.
were removed (Corry and McEachern 2009). U.S. Department of the Interior, Minerals Man-
agement Service, Camarillo, CA.
Perhaps Sonchus asper subsp. asper will decline as
well in our study area, given enough time. Bullock, D. J., S. G. North, M. E. Dulloo, and
M. Thorsen. 2002. The impact of rabbit and goat
Our study suffered from several limitations. eradication on the ecology of Round Island,
The composition of California annual grass- Mauritius. Pp. 53-63 in C. R. Veitch and M. N.
lands can vary greatly between years and also Clout (eds.), Turning the tide: the eradication of
seasonally, whether grazed or not (Talbot et al. invasive species. lUCN SSC Invasive Species
1939; Heady 1988), hence our measures may Specialist Group, Gland, Switzerland.
not have been representative. Further, our Campbell, K. and C. J. Donlan. 2005. Feral goat
results reflect only two points in time, providing eradications on islands. Conservation Biology
a limited ability to ascertain the patterns of 19:1362 1374.
change during intervening years, especially if Carrion, V., C. J. Donlan, K. Campbell, C. Lavoie,
those patterns are not linear. Finally, we AND F. Cruz. 2006. Feral donkey (Eqims asinus)
eradications in the Galapagos. Biodiversity and
sampled only one location, which may not be
Conservation 16:437-445.
representative of other grasslands on Santa
Coblentz, B. E. 1978. The effects offeral goats {Capra
Cruz Island because of variation in topography liircus) on island ecosystems. Biological Conserva-
and grazing history (Klinger et al. 2002). tion 13:279-286.
Klinger et al. (2002) sampled several grasslands Cohen, B., C. Cory, J. Menke, and A. Hepburn.
on Santa Cruz Island 1-5 years after sheep 2009. A spatial database of Santa Cruz Island
removal and found that exotic species, especially vegetation. Pp. 229-244 in C. C. Damiani and D. K.
grasses, had increased in frequency, were Garcelon (eds.). Proceedings of the seventh Cali-
dominant, and potentially were displacing na- fornia Islands symposium. Institute for Wildlife
tive species. Our results, showing a proliferation Studies, Areata, CA.
of native grasses and a decline in most exotic Coleman, H. M. and J. M. Levine. 2007. Mecha-
nisms underlying the impacts of exotic annual
species, may reflect geographic variation in grasses in a California coastal meadow. Biological
grassland response, or it may reflect an addi- Invasions 9:65-71.
tional 7-11 years of time. Time frames longer CORBIN, J. D. AND C. M. D-Antonio. 2004. Compe-
than 5 or even 12 years may be necessary to tition between native perennial and exotic grasses:
understand grassland recovery from overgrazing implications for an historical invasion. Ecology
(Hobbs et al. 2007). Despite these limitations, 85:1273-1283.
our results provide encouraging evidence indi- CoRRY, P. M. AND K. McEachern. 2009. Patterns in
cating that 12 years after removal of feral sheep post-grazing vegetation changes among species and
environments, San Miguel and Santa Barbara
from an island grassland, most exotic species
had decreased and some native species, espe- Islands. Pp. 201-214 in C. C. Damiani and D. K.
Garcelon (eds.). Proceedings of the seventh Cali-
cially native grasses, had increased.
fornia Islands symposium. Institute for Wildlife
Studies, Areata, CA.
Acknowledgments
Crampton, B. 1974. Grasses in California. University
We thank The Nature Conservancy for financial of California Press, Berkeley, CA.
support; M. Carroll, S. Junak, R. Klinger, L. Laughrin, Faulkner, K. R. and C. C. Kessler. 2011. Live
R. Philbrick, and S. Timbrook for help in identifying capture and removal of feral sheep from eastern
plants; and L. Laughrin and the UC Natural Reserve Santa Cruz Island, California. Pp. 295-299 in C. R.
System for logistical assistance. Veitch, M. N. Clout and D. R. Towns (eds.), Island
VAN VUREN AND BOWEN: GRASSLAND RESPONSE TO SHEEP REMOVAL
2012] 195
invasives: eradication and management. lUCN, response of the mamane forest to feral herbivore
Gland, Switzerland. management on Mauna Kea, Hawaii. Pacific
Harrison, S., B. D. Inouye, and H. D. Safford. Conservation Biology 18:123 132.
2003. Ecological heterogeneity in the effects of Rice, K. J. 1985. Responses of Erodiwn to varying
grazing and fire on grassland diversity. Conserva- microsites: the role of germination cueing. Ecology
tion Biology 17:837-845. 66:1651-1657.
Hayes, G. E. and K. D. Holl. 2003. Cattle grazing Schiffman, p. M. 1994. Promotion of exotic weed
impacts on annual forbs and vegetation composi- establishment by endangered giant kangaroo rats
tion of mesic grasslands in California. Conserva- {Dipodomys ingens) in a California grassland.
tion Biology 17:1694-1702. Biodiversity and Conservation 3:524-537.
Heady, H. F. 1956. Changes in a California annual Schuyler, P. 1993. Control of feral sheep {Ovis aries)
plant community induced by manipulation of the on Santa Cruz Island, California. Pp. 443^52 in
natural mulch. Ecology 37:798-812. F. G. Hochberg (ed.). Third California Islands
. 1988. Valley grassland. Pp. 491-514 in M. G. symposium: recent advances in research in the
Barbour and J. Major (eds.). Terrestrial vegetation California Islands. Santa Barbara Museum of
of California. California Native Plant Society, Natural History, Santa Barbara, CA.
Sacramento, CA. Scowcroft, P. G. and J. G. Giffin. 1983. Feral
HOBBS, R. J., S. Yates, and H. A. Mooney. 2007. herbivores suppress mamane and other browse
Long-term data reveal complex dynamics in species on Mauna Kea, Hawaii. Journal of Range
grassland in relation to climate and disturbance. Management 36:638-645.
Ecological Monographs 77:545-568. AND R. HoBDY. 1987. Recovery of goat-
Junak, S., T. Ayers, R. Scott, D. Wilken, and D. damaged vegetation in an insular tropical montane
Young. 1995. A flora of Santa Cruz Island. Santa
forest. Biotropica 19:208-251.
Barbara Botanic Garden, Santa Barbara, CA.
Seabloom, E. W., W. S. Harpole, O. J. Reichman,
Kessler, C. C. 2002. Eradication of feral goats and AND D. Tilman. 2003. Invasion, competitive
pigs and consequences for other biota on Sarigan
dominance, and resource use by exotic and native
Island, Commonwealth of the Northern Mariana
California grassland species. Proceedings of the
Islands. Pp. 132-140 in C. R. Veitch and M. N. National Academy of Sciences 100:13384 13389.
Cinlvoaustiv(eedss.p).eciTeusr.nilngUCtNhe tSidSeC: tIhnevaesriavdiecaStpioencieosf Talbot, M. W., H. H. Biswell, and A. L. Hormay.
1939. Fluctuations in the annual vegetation of
Specialist Group, Gland, Switzerland.
Klinger, R. C, p. Schuyler, and J. D. Sterner. ThomCsaleinf,ornCi.a.DE.,coWlo.gya.20W:i3l9l4i^a0m2s., M. Vayssieres,
2002. The response of herbaceous vegetation and
F. L. Bell, and M. R. George. 1993. Controlled
endemic plant species to the removal of feral
grazing on annual grassland decreases yellow
sheep from Santa Cruz Island, California.
Pp. 141-154 in C. R. Veitch and M. N. Clout starthistle. California Agriculture 47:36-40.
(eds.). Turning the tide: the eradication of invasive Van Vuren, D. H. and V. J. Bakker. 2009. Rapid
species. lUCN SSC Invasive Species Specialist morphological change in an insular population of
Group, Gland, Switzerland. feral sheep. Journal of Zoology 277:221-231.
Knapp, J. J., C. Cory, R. Wolstenholme, K. AND B. E. CoBLENTZ. 1987. Some ecological
Walker, and B. Cohen. 2009. Santa Cruz Island effects of feral sheep on Santa Cruz Island,
invasive plant species map. Pp. 245 252 in C. C. California, USA. Biological Conservation 41:253-
Damiani and D. K. Garcelon (eds.). Proceedings of 268.
the seventh California Islands symposium. Institute AND . 1989. Population characteristics of
for Wildlife Studies, Areata, CA. feral sheep on Santa Cruz Island. Journal of
Menke, J. W. 1992. Grazing and fire management for Wildlife Management 53:306 313.
native perennial grass restoration in California Wardle, D. a., G. M. Barker, G. W. Yeates, K. I.
grasslands. Fremontia 20:22-25. Bonner, and A. Ghani. 2001. Introduced brows-
Meurk, C. D. 1982. Regeneration of subantarctic ing mammals in New Zealand natural forests:
plants on Campbell Island following exclusion of aboveground and belowground consequences. Eco-
sheep. New Zealand Journal of Ecology 5:51-58. logical Monographs 71:587 614.
Minnich, R. a. 1980. Vegetation of Santa Cruz and Watt, A. S. 1981. A comparison of grazed and
Santa Catalina Islands. Pp. 123 137 in D. M. ungrazed Grassland A in East Anglian Breckland.
Power (ed.). The California Islands: proceedings Journal of Ecology 69:499 508.
of a multidisciplinary symposium. Santa Barbara Wehtje, W. 1994. Response of a Bishop pine (Pinus
Museum of Natural History, Santa Barbara, CA. nmricata) population to removal of feral sheep on
Pitt, M. D. and H. F. Heady. 1978. Responses of Santa Cruz Island, California. Pp. 331-340 in W. L.
annual vegetation to temperature and rainfall Halvorson and G. J. Meander (eds.). The fourth
patterns in northern California. Ecology 59:336-350. California Islands symposium: update on the status
Reddy, E., D. H. Van Vuren, P. G. Scowcroft, J. B. of resources. Santa Barbara Museum of Natural
Kauffman, and L. Perry. 2012. Long-term History, Santa Barbara, CA.