BIOLOGICAL INDICATORS FOR THE OTAY RIVER WATERSHED
INTRODUCTION
Riparian areas may be the most important natural habitat in the western United States. Although
comprising less than 1 percent of land area, riparian habitats support the most diverse and abundant
wildlife communities. Yet they are disappearing at an alarming rate. In California, an estimated 95
percent of riparian habitat has disappeared during the last hundred years (Grupel and Elliott, 2001). In
most cases the loss of riparian habitat was preceded by a degradation in habitat quality that could have
been measured by a change in species abundance and habitat composition. These changes in the
ecological community have the potential to function as indicators of changes in the riparian and wetland
ecosystem as a whole. If appropriately selected, these bioindicators can be uses as an effective means to
monitor the effectiveness of watershed management activities. The objective of this discussion is to
identify bioindicators that will be useful for evaluating the effectiveness of wetland and riparian habitat
conservation and management measures implemented in the Otay River watershed.
Simply stated, a bioindicator is an ecological unit (such as an individual species or a group of species like
a vegetation community or taxonomic assemblage) that can be monitored over time and that has a
correlation of its condition with the condition of other ecosystem elements. Bioindicators that are chosen
to monitor the wetland and riparian habitat in the Otay River watershed should have the following
characteristics:

Bioindicators should be taxonomically well known and easy to identify and distinguish from other species.

The ecology and/or general life history of the bioindicators should be well understood.

Bioindicators should be readily surveyed and manipulated such that a field monitor can find, observe, and mark
the bioindicators easily.

Bioindicators should be specially adapted to the conditions of the target riparian or wetland system type. The
more specialized the bioindicator, the more sensitive it is to changes such as pollution and habitat modification.

Patterns observed in the bioindicators should be correlated in other related and unrelated species or components
of the ecosystem.
RIPARIAN AND WETLAND HABITAT IN THE WATERSHED
In the Otay River Watershed Management Plan (ORWMP) study area, the riparian habitats include
riparian forest, riparian woodland, riparian scrub, and natural floodchannel/streambed vegetation. These
general vegetation types are associated with rivers, streams, drainages, and other watercourses. Riparian
vegetation communities in the watershed area generally are dominated by willows (Salix spp.), cottonwoods
(Populus spp.), mulefat (Baccharis salicifolia), and western sycamores (Platanus racemosa). Willows and
cottonwoods are often dominant along the active stream channel when permanent water is present, whereas
sycamores tend to be a minor component of riparian habitats associated with permanent water, but increase in
abundance on higher flood terraces or along intermittent and ephemeral drainages. In several reaches of the
lower Otay River as well as other localized areas throughout the watershed, the non-native, invasive giant
reed (Arundo donax) and tamarisk (Tamarix spp.) are the dominant species in the riparian zone.
Riparian plant species recruitment and survival are strongly associated with riverine hydrology and fluvial
processes (Scott et al., 1996 and 1997; Shafroth et al., 1998; Stromberg, 1993 and 1998). Woody riparian
plant species establish in positions along streams where there are suitable conditions for seed germination
and sufficient water for seedling survival, and where they can tolerate physical disturbance from floods
(Stromberg and Patten, 1992; Hupp and Osterkamp, 1996; Scott et al., 1996; Mahoney and Rood, 1998).
Thus, the structure of riparian vegetation communities is often a mosaic (at varying spatial scales) of
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species and age classes produced by spatial and temporal variations in stream discharge patterns (Auble
and Scott, 1998; Stromberg et al., 1997; Shafroth et al., 1998).
Many willow species are recognized as “pioneer species” that are among the first to colonize newly
exposed substrates along streams. In the Mediterranean climate zones of coastal California, riparian tree
species tend to follow a dominance gradient with willows occurring on lower, wetter sites, cottonwoods
on slightly higher first terraces, and sycamores on higher, dryer stream terraces (Walters et al., 1980). In
Southern California, sycamores and coast live oaks dominate intermittent and ephemeral streams, whereas
willows and cottonwoods dominate the banks of perennial streams (Faber et al., 1989).
Aquatic habitat quality is largely determined by substrate composition and water quality.
Macroinvertebrate diversity is generally highest in streams with coarse substrates (coarse sands, gravels,
and cobbles), moderate nutrient and high dissolved oxygen concentrations, and adequate tree canopy
cover to moderate water temperatures. Many species associated with aquatic habitats require undisturbed
adjacent upland areas to complete portions of their life cycle. Vegetation in adjacent upland areas also
provides carbon and nutrients to aquatic habitats in the form of leaf litter, woody debris, and terrestrial
insects and serves to moderate sediment input.
Other non-riparian wetland habitats in the watershed include the southern coastal salt marsh, alkali marsh,
freshwater marsh, open water, estuarine, and saltpan/mudflats vegetation communities. The species
composition in these communities is variable and dependent on elevations relative to tidal fluctuations
and soil and pore water salinities. Characteristic species in areas of higher salinity may include cordgrass
(Spartina foliosa), pickleweed (Salicornia spp.), alkali-heath (Frankenia salina), shoregrass
(Monanthochloe littoralis), and saltgrass (Distichilis spicata var. spicata). In areas with reduced salinities,
cattails (Typha spp.), bulrushes (Scirpus spp.), and rushes (Juncus spp.) typically dominate.
THREATS TO RIPARIAN AND WETLAND HABITAT INTEGRITY
Storm water runoff from developed areas can carry significant loads of urban pollutants (Paul and Meyer,
2001). Runoff from impermeable surfaces such as buildings, streets, and landscaped areas transports a
number of water quality constituents, such as silt, metals, fertilizers, herbicides, and pesticides, to
downstream water bodies. These constituents have been shown to cause toxicity to aquatic organisms and
cause eutrophication of receiving waters. Eutrophication generally depresses dissolved oxygen
concentrations, particularly in pools and slow-moving waters. Sewage effluent can contain contaminants.
The effect of high levels of estrogens in sewage effluent on biological communities is unclear.
Less studied, but potentially as significant, is the influence of altered stream hydrology on riparian
biological communities. Alteration of hydrology and sediment supply affect riparian habitats by altering
the amount and timing of flows. Many species have evolved under specific hydrologic regimes and can be
sensitive to changes in the magnitude, frequency, and duration of flows. There is increasing evidence that
modifications of riverine hydrologic characteristics by urban development and irrigated agriculture can
greatly affect the composition of the riparian and aquatic communities. In many instances, altered
hydrologic characteristics favor non-native species at the expense of native species. For example, recent
research by the USGS (Fisher, unpubl. data) shows that historically intermittent drainages that now have
permanent base flow from irrigated landscaping or agriculture no longer support arroyo toads. This
pattern has been attributed to the successful establishment of non-native aquatic species (e.g., bullfrogs,
bass, and sunfish) that prey on or compete with larval toads. Permanent summer flow can also encourage
the establishment of non-native plant species, such as giant reed.
A number of factors can reduce breeding success of riparian bird species. Excessive noise and lights can
adversely affect mating behaviors of songbirds. Nest parasitism by brown-headed cowbirds (Molothrus
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ater) has the potential to significantly reduce reproductive success, and cowbirds can be particularly
abundant in agricultural areas with livestock. Non-native predators, such as house cats, can also prey on
riparian birds.
Development and human uses facilitate the invasion of non-native plant species into adjacent natural
habitats. Residential developments in close proximity to natural open space areas generally result in
increased disturbances from foot, bicycle, and motorized vehicular traffic as well as an increase in trash.
Illegal migrant worker encampments also contribute to trash and disturbance in riparian areas.
Establishment of unauthorized trails is a large management issue in most open space areas in San Diego
County, resulting in the loss of vegetation and compaction and erosion of underlying soils. These trails
are also routes for the invasion of non-native species. In some instances, these disturbances can produce
severe, virtually permanent habitat degradation. Buildup of trash or litter in and adjacent to the preserve
can attract house rats and promote the abundance of mesopredators, such as raccoons and skunks. An
unnaturally high abundance of mesopredators can affect nesting success of native birds.
SUGGESTED BIOINDICATORS FOR THE OTAY RIVER WATERSHED
The following list identifies the general ecological groups identified as having potential bioindicators.
Individual species and monitoring approaches are outlined below.

Vegetation Communities

Freshwater macroinvertebrates

Amphibian species

Riparian and wetland birds

Exotic species.
Monitoring of bioindicators in the Otay River watershed as a component of the Otay River WMP and
SAMP implementation should, to the maximum extent practical, be coordinated with the species and
habitat monitoring of the MSCP (subregional and individual subarea plans). A number of the species
covered by the MSCP are dependent on wetland and riparian habitats, several of which are suitable
bioindicators (Table 1). Several of these species have been selected as potential bioindicators for the Otay
River watershed, and are described below in detail.
Table 1 Wetland and Riparian Species Covered by the San Diego MSCP
that may Make Good Wetland/Riparian Bioindicators
Species
Riparian Species
Southwest willow flycatcher
Least bell’s vireo
Cooper’s hawk
Arroyo southwestern toad
California red-legged frog*
Southwestern pond turtle
Freshwater Marsh Species
Tricolored blackbird
White-faced ibis
Saltmarsh Species
Salt marsh bird’s beak
Salt Marsh Skipper
Reddish Egret
Light-footed clapper rail
Long-billed curlew
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Habitat Type(s)
Riparian Woodland
Riparian Woodland, Riparian Forest
Oak Woodland (breeding), Riparian Woodland
Breeds near water
Aquatic/riparian
Aquatic/riparian
Freshwater Marsh
Freshwater Marsh, estuaries
Saltmarsh
Saltmarsh
Saltmarsh
Saltmarsh
Saltmarsh
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Species
Belding’s savannah sparrow
Large-billed savannah sparrow
Northern harrier
River Mouth/Bay Species
Western snowy plover
Habitat Type(s)
Saltmarsh
Saltmarsh, Grassland, Freshwater Marsh
Saltmarsh, Grassland, Agricultural fields
Ocean/bay shoreline, river mouths
*Extirpated, but would be a good indicator if the species were reintroduced into the watershed.
MONITORING OF VEGETATION COMMUNITIES
Riparian and wetland vegetation communities should be used as a bioindicator to provide information for
a variety of different purposes, including identifying and prioritizing management actions, tracking the
response of communities to management actions, assessing systematic vegetation community patterns
that may be an expression of human-induced stresses, and evaluating vegetation patterns that may help
explain observed distributions and abundance of wildlife species.
Identifying the structural diversity is important in managing riparian biodiversity and integrity for the
following reasons:

Young communities support a different fauna than mature communities.

Healthy riparian ecosystems have mature, intermediate, and young components.

If a mature plant community is not being replaced by younger individuals, the vegetation type and associated
species may eventually be lost over time.
Vegetation community monitoring data should be used to evaluate the following:

The distribution of vegetation communities, seral phases, levels of disturbance, and change over time.
Disturbance factors include relative abundance of exotic species, vehicular traffic, trampling, erosion, urban
runoff, trash, habitat loss as a result of development activities, etc.

Changes in vegetation communities related to changes in sensitive species distributions.

Changes in vegetation communities that may require management actions.
A baseline of riparian and wetland vegetation should be established against which future monitoring
efforts will be compared. The existing vegetation mapping should be updated in coordination with MSCP
mapping updates.
Baseline surveys should be conducted to accurately delineate riparian and wetland vegetation
communities, and to describe relevant attributes of vegetation stands (e.g., level of disturbance, relative
abundance of exotics, successional stage, etc.). Information from these surveys will be mapped into a GIS
database with appropriate attributes.
Vegetation community classification and mapping will be conducted on all riparian and wetland habitats
in the watershed, using both remote sensing information and field verification. The community
classification system and specific attributes to be used must be consistent with those used in the MSCP.
At this time, the modified Holland classification is being used; however, the wildlife agencies may
recommend a different classification system in the future. In addition to mapping vegetation community
polygons, the relevant attributes for each polygon should be described, such as the dominant species for
each area, the health or condition of the patch, and the general level of disturbance (e.g., percent
composition of invasive species, percent of bare ground caused by trails or off-road vehicles, evidence of
grazing or tilling, etc.). The minimum mapping unit for riparian and wetland vegetation communities
should be 0.5 acre.
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At 5-year intervals, the vegetation community maps should be updated and analyzed to identify changes
in the boundaries or attributes of vegetation community polygons (e.g., changes in the spatial distribution
of vegetation communities or attributes such as level of disturbance). It may be desirable to refine maps
more frequently if vegetation community changes occur more frequently (e.g., by fire, flood disturbance,
adjacent development, or frequent recreational activities).
Using riparian vegetation as a bioindicator has the following objectives:

Document changes in the distribution or characteristics of habitats (e.g., level of exotic species, type change as a
result of urban runoff) that may trigger management actions.

Document changes in habitats that may correlate with factors such as adjacent land uses, fire, floods, etc.
MONITORING OF FRESHWATER MACROINVERTEBRATES
Freshwater macroinvertebrates are small invertebrate fauna, large enough to be seen with the naked eye,
and that inhabit the bottoms of streams, lakes, and wetlands. They include snails, worms, and the
multitude of insects and their larvae (such as midges, stoneflies, caddisflies, and some beetles). Using the
macroinvertebrate assemblages that characterize these freshwater environments as bioindicators provides
several advantages to monitoring riparian and wetland condition:

They are sensitive to the cumulative impacts of a wide range of disturbances.

They are differentially sensitive to various pollutants; the pollution responses of many common species are
known; and they can detect and respond to intermittent pollution.

They react quickly and are capable of a graded response to a broad spectrum of stresses.

They are ubiquitous, abundant, and relatively easy to collect.

They generally move only small distances, and therefore, their distribution may reflect the various impacts on
river health over time at the sampling site.

They live long enough to provide a record of environmental quality.

Qualitative sampling and analysis are relatively simple.
Because many species of aquatic macroinvertebrates require undisturbed adjacent upland areas to
complete portions of their life cycle, they can also be good indicators of upland conditions.
A thorough discussion of the utility of aquatic macroinvertebrates as bioindicators in this watershed is
included in the accompanying Baseline Water Quality Indicators for the Otay River Watershed, and
therefore, is not discussed in detail here.
MONITORING OF AMPHIBIAN SPECIES
Amphibians are well known for their sensitivity to habitat degradation caused by pollutants and changes
in hydrologic conditions. They must spend at least part of their life cycle in the water, and because they
tend to have highly permeable skin, they are highly vulnerable to toxins, which are readily absorbed. Eggs
are particularly susceptible to pollutants, and exposure often results in abnormal development. In addition
to poor water quality, amphibians are sensitive to environmental alterations that result in changes in
sedimentation rate and water flow. Amphibian species often have very particular micro-habitat
requirements for each life stage in order to fully develop, and any changes in these conditions can have
devastating effects that are manifested very quickly. However, long-term monitoring of amphibian
species is essential. Populations may fluctuate dramatically due to natural conditions, such as drought,
and therefore, a drop in numbers could be due to hibernation or dormancy rather than to an actual decline
in numbers. The introduction of non-native predatory species, such as bullfrogs, game fish, and bait fish
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has become a significant problem for amphibians. Therefore, any monitoring of one or more amphibians
as a bioindicator should also include parallel monitoring of non-native species. Amphibian species in San
Diego County include four species of salamanders, three species of toads, and two species of treefrog.
Arroyo Toad
The arroyo toad has the potential to be a good bioindicator for aquatic, riparian, and adjacent upland
habitat integrity. The arroyo toad breeding habitat is affected by water quality, stream
hydrogeomorphology, and sediment substrate conditions. The non-breeding adjacent upland habitat must
be available for toads to persist in a given area. The arroyo toad has been well-studied in recent years, and
therefore, has the characteristics of a good bioindicator as outlined in the introduction to this section.
Furthermore, it is an MSCP-covered species that must be monitored and managed as a part of the MSCP
implementation.
The primary populations of arroyo toad in the watershed are on Dulzura Creek; however, potentially
suitable habitat occurs in many areas of the watershed (County of San Diego, 2004). Therefore,
monitoring for this bioindicator should occur in all suitable habitat using the following protocol:
First, survey for potential arroyo toad habitat. If potential habitat occurs in the area, conduct night-time
surveys for toads, tadpoles, and/or egg masses. In areas of potential breeding habitat, conduct surveys
once every 3 years. Conduct at least three site visits between late March and late May. The survey should
be conducted by a permitted biologist familiar with the male arroyo toad's breeding call and identification
of toad eggs, tadpoles, and adults. Conduct surveys between 1 hour after dusk and midnight on nights
lacking a full moon and nights when air temperatures are >55ºF. Avoid surveying during rain, high winds,
or flood flows. Surveyors must be silent during surveys so as not to disturb calling toads. Use strong
flashlights to visually identify adult toads; otherwise, lighting should be kept to a minimum. Surveyors
must not enter the water near mating pairs and should not handle any toads.
Survey along the bank of the watercourse 10 ft back from the water's edge. If possible, survey up one
bank and back along the other, concentrating on open habitats adjacent to suitable breeding habitats. Stop,
listen for calls, then proceed to the next listening point until all suitable habitat has been covered. Shine a
bright light ahead to detect eye-shine, and also survey for toads at close range. When crossing the stream,
cross at the downstream end of potential breeding areas or on stable substrate to avoid trampling eggs or
larvae and to avoid clouding the water with silt, which can smother eggs and young.
Each sighting of a toad, egg mass, or group of tadpoles should be entered as a separate line on a standard
field form, and a GPS reading should be recorded for the location. Condition and degree of disturbance to
the habitat should be recorded, and management actions to control or reduce habitat disturbance should be
monitored for effectiveness.
MONITORING OF RIPARIAN AND WETLAND BIRDS
The relative abundance of bird species in riparian and wetland habitats is a useful bioindicator. Riparian
and wetland habitats with a diverse array of bird species including a number of habitat specialists (e.g.,
least Bell's vireos, southwestern willow flycatchers, yellow-breasted chats) is generally considered of
higher ecological integrity than habitats with lower diversity and only generalist species but few or no
specialists. The objectives of riparian monitoring are to: (1) increase our knowledge of habitat use by
breeding riparian birds, in general; (2) identify variables that influence the dynamics of populations of
least Bell's vireos, southwestern willow flycatchers, yellow-breasted chats, and other obligate riparian
bird species that are particularly sensitive to habitat degradation, nest parasitism, nest predation, and other
adverse edge effects; and (3) assess the effectiveness of watershed management actions. General riparian
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bird monitoring data should be used to determine the distribution and abundance of riparian and wetland
bird species populations in the watershed.
Southwestern Willow Flycatcher, Least Bell’s Vireo, Cooper’s Hawk, Yellow-Breasted Chat
Focal species monitoring for the southwestern willow flycatcher, least Bell's vireo, and Cooper's hawk,
which are MSCP-covered species, and the yellow-breasted chat as bioindicators for riparian habitats
should address the following issues:

Number of pairs of these bird species estimated to be present in the watershed, and what factors influence their
occupancy over time.

Factors that are positively or negatively affecting these species (e.g., are recreational users negatively impacting
nesting success).

Management actions that are effective in maintaining or enhancing the population.
Monitoring for these bioindicators should use the following protocol:

Surveyors should establish systematic survey routes through patches of suitable habitat, such that the suitable
habitat is completely covered. Survey routes should be varied relative to time of day between visits. The
surveyors will visit these patches three times during April through June, with at least a 7-day interval between
site visits. Taped vocalizations will be used, as needed. The number of pairs of each covered species will be
recorded, and notes will be taken on the condition of the habitat (e.g., level of vehicular disturbance, trampling
of habitat, relative abundance of exotic species, trash, erosion, drainage conditions, etc.).

The observer should be skilled in identification, including knowledge of the songs and calls of birds. Surveys
should begin within 1 hour after sunrise and end by noon. Surveys should not be conducted under extreme
conditions, i.e., during heavy rain or when the temperature is >95°F or <40°F or with winds >10 mph.
Condition and degree of disturbance to the habitat will be recorded, and management actions to control or
reduce habitat disturbance will be monitored for effectiveness.
Western Snowy Plover
The western snowy plover, a federally listed species covered by the MSCP, is a potential bioindicator for
relatively undisturbed shoreline habitat at the mouth of the Otay River and nearby vicinities in San Diego
Bay. Monitoring of the snowy plover as a bioindicator should address the following issues:

Identify and monitor areas this species uses and how it changes over time.

The status and trends in the number of breeding pairs.

The number of breeding pairs relative to habitat availability and distribution and to the activity of mammalian
and avian predators.
Monitoring for this bioindicator should use the following protocol:

Survey all potential western snowy plover breeding habitat annually in April. Map locations supporting this
species, and record the number of breeding pairs. Record condition and degree of disturbance to the habitat, and
monitor management actions for effectiveness in controlling or reducing habitat disturbance.
Belding’s Savannah Sparrow
The Belding’s Savannah sparrow is State listed as endangered and is covered by the MSCP. It is a
potential bioindicator for high quality intact saltmarsh habitat. Monitoring of this species as a bioindicator
should address the following issues:

Identify and monitor areas this species uses and how it changes over time.
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
The number of breeding pairs relative to habitat availability and distribution and to the activity of mammalian
and avian predators.

Response to efforts at reducing predation or disturbance.
Monitoring for this bioindicator should use the following protocol:

Annually count the total number of breeding Belding’s Savannah sparrow pairs in March. The surveys will
consist of circuitously walking through salt marsh habitat and mapping locations of territorial birds, using
existing CDFG and USGWS protocols. Note and record the condition and the degree of disturbance to the
habitat, and monitor management actions for effectiveness in reducing habitat disturbance. An alternative
protocol, which may be used by CDFG, requires conducting annual censuses only in areas of potential human
disturbance or where a restoration project is proposed. Under this protocol, a lagoon-wide census of all
potentially occupied habitat, as described above, should be conducted every 3 years.
Light-Footed Clapper Rail
The light-footed clapper rail is federally and State listed as endangered and is covered by the MSCP. It is
a also potential bioindicator for high quality intact saltmarsh habitat. Monitoring of this species as a
bioindicator should address the following issues:

Identify and monitor areas this species uses and how it changes over time.

The number of breeding pairs relative to habitat availability and distribution and to the activity of mammalian
and avian predators.

Response to efforts at reducing predation or disturbance.
Monitoring for this bioindicator should use the following protocol:

Annually conduct spring call counts in appropriate habitat. Conduct call counts between March and early May,
in early morning (until two hours after sunrise) or late afternoon (two hours before sunset). In locations where
rails are relatively common, all spontaneous rail calls should be mapped. In marshes with few rails, or in long
narrow channels or narrow strips of habitat, use taped "clappering" calls sparingly. No surveys should be
conducted under rainy or windy conditions. "Duets" and "clappering" should be treated as a rail territory
(Zembal, pers. comm.). Note and record the condition and the degree of disturbance to the habitat, and monitor
management actions for effectiveness in reducing habitat disturbance. Collect data to test the effects of factors
hypothesized to influence the distribution or habitat use by the birds. High tide counts may also be appropriate
to survey for rails. Observers should be stationed around the perimeter of a flooded marsh to observe all clapper
rails (Zembal, pers. comm.).
MONITORING OF EXOTIC SPECIES
Invasive, exotic species may be the greatest threats to sensitive species and the ecological integrity of the
watershed. Careful monitoring and management will be necessary to identify invasions or expansions of
these exotic pests and hopefully to control them or minimize their impacts on native resources. Therefore,
exotic species presence and abundance can be a bioindicator of an area that has been disturbed from a
more natural state. In some cases, exotic species can be completely removed from a system, and in most
cases, the exotic species abundance can be reduced to a level that has minimal impact to the natural
system. Identifying an exotic species early before it gains a foothold and causes serious damage also
provides the best opportunity to remove it from the system with the least cost.
Invasive plant species pose one of the greatest threats to the characteristics of ecosystems. These species
can dominate and cause permanent damage to vegetation communities by altering natural processes and
reducing biodiversity. Invasive weeds can destroy wildlife habitat; displace many threatened, endangered,
or sensitive species; and result in reduced plant and animal diversity where they form monocultures.
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Direct competition between native and exotic plant species is well documented (Alberts et al., 1993).
Furthermore, the successful invasion of exotic species may alter habitats and lead to displacement or
extinction of native species over time. For example, exotic invasions have been shown to alter
hydrological and biochemical cycles and disrupt natural fire regimes (MacDonald et al., 1988; Usher,
1988; Vitousek, 1990; D’Antonio and Vitousek, 1992; Alberts et al., 1993).
Invasive or potentially invasive weed species occurring in or near the watershed that may pose threats to
native species include but is not limited to tamarisk (Tamarix spp.), Pampas grass (Cortaderia selloana),
eucalyptus (Eucalyptus spp.), giant reed (Arundo donax), mustard (Brassica spp.), African fountaingrass
(Pennisetum setaceum), tocalote (Centaurea melitensis), purple falsebrome (Brachypodium distachyon),
artichoke thistle (Cynara cardunculus), castor bean (Ricinus communis), fennel (Foeniculum vulgare),
and ice plant (Mesembryanthemum chilensis).
Exotic animal species also can have a significant effect on biological resources, which has been well
documented (e.g., Gates and Gysel, 1978; Brittingham and Temple, 1983; Wilcove, 1985; Andren and
Angelstam, 1988; Langen et al., 1991; Donovan et al., 1997); most of this literature pertains to effects on
wildlife species. For example, both domestic dogs and cats are known to adversely impact native wildlife,
with effects ranging from harassment to disturbance of breeding activities to predation (Kelly and
Rotenberry, 1993).
Disturbed habitats are often considered vulnerable to Argentine ant invasions. There is evidence that this
exotic species rapidly invades disturbed areas within stands of native habitat (Erickson, 1971; Ward,1987;
DeKock and Giliomee, 1989; Knight and Rust, 1990; Suarez et al., 1998). Suarez et al. (1998) found
Argentine ants are most abundant along the edge of urban/wildlands interface, with densities of ants in the
natural areas decreasing with distance from the edge. They found that ant activity was highest within
about 325 feet of the nearest urban edge, whereas areas sampled beyond 650 feet contained few or no
Argentine ants. However, Argentine ants have also been found at distances of approximately 1,300 feet
and 3,280 feet from the edge, respectively, in other urban reserves in southern California (Suarez et al.,
1998).
Argentine ants appear to be confined to low elevation areas with permanent soil moisture (Erickson,
1971; Ward, 1987; Knight and Rust, 1990). Tremper (1976) reported that Argentine ants desiccate more
easily and are less tolerant of high temperatures than native ants. Suarez et al. (1998) indicated that the
presence of the Argentine ants in urban reserves might be dependent on water runoff from developed
areas. Holway (1998) found that the rate of Argentine ant invasion is primarily dependent on abiotic
conditions (e.g., soil moisture), rather than on disturbance. He suggested that disturbed areas are often a
point of introduction, but encourage invasions only if they increase the availability of a limiting resource
such as water. Blachly and Forschler (1996) found Argentine ants thriving in areas disturbed by human
activity, but indicated that their presence is also related to added ground cover, permanent water supplies,
and a simplified native ant fauna. Monitoring of the presence and density of Argentine ants in natural
areas is a potential bioindicator of the edged effects from nearby urbanization. However, because
Argentine ants are primarily limited by areas with sufficient soil moisture, once they invade from urban
fringes into riparian and wetland habitats, it is possible that they may persist in these moist environments
independent of any continued edge effect.
Invasive faunal species (e.g., Argentine ants, parasites) have the potential to negatively impact pollinator
populations. Loss or limitation of pollinators may adversely affect the long-term survivability of rare
plant species by reducing seed output (e.g., reproductive failure) if there is no selfing (Jennersten, 1988;
Bawa, 1990). The Argentine ant is known to displace native ant species (Erickson, 1971; Ward, 1987;
Holway, 1995; Human and Gordon, 1996; Suarez et al., 1998). Ants may also function as primary or
secondary dispersers of seeds (Roberts and Heithaus, 1986; Louda, 1989). They have been reported to
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contribute to the spatial heterogeneity of seed distribution (Reichman, 1984, 1979), and they decrease
seed abundance of some numerically dominant ruderal species in relation to less dominant native annual
species (Inouye et al., 1980). Displacement of native ant species by the Argentine ant could negatively
affect persistence of rare native plant species by reducing seed number and distribution.
Other non-native animals that may be a threat to sensitive species include red fox, opossum, cats, dogs,
black rats, cowbirds, bullfrogs, African clawed frogs, non-native turtles, and nonnative fish. The presence
and relative abundance of these species in natural areas throughout the watershed should be observed and
recorded during monitoring of other bioindicators.
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