Full text of DOI:10.1111/j.1600-0587.2000.tb00313.x

ECOGRAPHY 23: 693–701. Copenhagen 2000
On the insularity of islands
Michael D. Rose (†) and Gary A. Polis (†)
Rose, M. D. and Polis, G. A. 2000. On the insularity of islands. – Ecography 23:
93–701.
6
We propose that islands are ‘‘less insular’’ than is generally perceived. This belief
results, in part, from the paucity of studies on vagrant species that exploit islands but
are not permanent residents with continual breeding populations. We show, via
anecdotal evidence extracted from the literature and data acquired on Gulf of
California islands, that visitors to insular systems are fairly common. We delineate
three types of events beyond the bounds of current biogeographical analysis that
make islands less insular: 1) migrants and ‘‘accidental’’ visitors, 2) individuals of a
species whose foraging areas encompass many islands or the mainland and islands,
and 3) species who ‘‘colonize’’ islands during opportune periods or years but become
extinct during difficult times (source-sink situations). Such events potentially signifi-
cantly affect the ecology and evolution of island inhabitants by such means as
increased predation and/or competition, transport of parasites and pathogens, disper-
sal of seeds and eggs, and genetic introgression and hybridization. Discussion of
other ‘‘insular’’ habitats such as freshwater lakes and wildlife refuges illustrate that
vagrancy events may be nearly ubiquitous. Studies addressing the frequency and
ecological and evolutionary significance of vagrants are required, especially in light of
recent and rapid extinctions on islands and the increasing fragmentation of habitats.
M. D. Rose and G. A. Polis, Dept of En6ironmental Science and Policy, Uni6. of
California, Da6is, CA USA.
Are islands as insular and isolated as generally per- islands when highly vagile species visit (e.g. birds, in-
ceived? With the exception of species turnover, we tend sects, and some mammals).
to view islands as rather self-contained units with segre-
We discuss two other situations that are outliers of
gated evolutionary and ecological trajectories, rarely classic island biogeography, but nonetheless signifi-
influenced by introgression, disperal, or visitors from cantly affect the insularity of islands: 1) individual
other locations. One understudied aspect of insularity is fauna whose foraging range consists of several islands
the effect of visitors and unsuccessful colonizers, i.e. or island-mainland regions and 2) species whose popu-
‘
‘vagrants,’’ on islands. The presence of vagrants makes lations are characterized by mainland-island or island-
islands ‘‘less insular’’ and can significantly alter the island source-sinks. These species may significantly
ecology and evolution of resident insular biota. Conse- influence island ecology and evolution via several path-
quently, we assert that one cannot fully understand ways akin to vagrancy effects. Moreover, in source-sink
island ecology and evolution without including non-res- situations, species turnover likely does not reach a
ident interactions and influences. In this paper, we dynamic equilibrium as factors other than isolation and
focus on cases where individuals of non-resident species island area determine the colonization rates and sus-
periodically exploit islands during forays from other tainability of these island residents.
areas. These vagrants do not play by the rules of classic
We first establish that many species move across
island theory, as they do not colonize or go extinct. water barriers. We then describe types of vagrants that
This situation is probably quite common on islands visit islands and discuss consequences and implications
close to mainlands, but also occurs on more isolated of island non-residents. Finally, we address the impor-
Accepted 1 February 2000
Copyright © ECOGRAPHY 2000
ISSN 0906-7590
Printed in Ireland – all rights reserved
ECOGRAPHY 23:6 (2000)
693

tance of understanding the possible effects vagrants
may have on other isolated (‘‘island’’) habitats and urge
studies of vagrants in the contexts of island biogeogra-
phy, ecology, evolution, and under other applicable
circumstances.
Types of non-resident visitors to islands
Vagrants and migrants
Some taxa consistently visit islands for short periods,
but depart and do not establish viable populations
(
Saunders and De Rebeira 1985). It is highly probable
that many varieties of animals access islands and then
return to the mainland (either by swimming, flight, or
wind and sea-surface transport), especially on
nearshore islands. Johnson (1980) noted several historic
records of elephants Loxodonta africana and Elephas
maximus from coastal regions swimming to exploit
island resources and potentially escape from mainland
predators when giving birth. He also recorded an inci-
dent of lions Panthera leo swimming to islands as well,
presumably to hunt or scavenge insular prey. In Baja
California, Mexico, a coyote Canis latrans was ob-
served swimming ꢀ2 km from the mainland to
On the vagility of island inhabitants
We know that movements across oceans to islands and
subsequent colonizations are common across many
time scales. For example, dispersal events over thou-
sands to millions of years have populated all isolated
archipelagoes, e.g. the Hawaiian and Gal a´ pagos Islands
(Gulick 1932, Darlington 1957, Carlquist 1965, Paulay
1
994, Peck 1994a, b). We also know that colonization
events can happen swiftly. Colonizations of newly cre-
ated or sterilized islands (e.g. Krakatau; Motmot, New
Guinea; San Benedicto, Mexico; Surtsey, Iceland; Long
and Ritter Islands, New Guinea) occurred soon after
volcanic explosions or surfacing events (Dammerman
2
Gemelitos West, a tiny island (0.02 km ) in Bah ´ı a de los
Angeles containing double-crested cormorant Phalacro-
corax auritus and yellow-legged gull Larus li6ens
colonies (ꢀ70 and 15 breeding pairs, respectively;
Vazquez pers. comm.).
1
1
948, Carlquist 1965, Diamond 1974, 1975, Fridriksson
975, Williamson 1981, Thornton 1996).
Terrestrial avian species are also common vagrants,
using small islands as roosting and feeding sites and
migratory stopover points. Numerous instances of
avian vagrancy have been recorded on nearshore and
distant islands (e.g. Simberloff and Wilson 1969, Dia-
mond 1975, DeSante 1983, McNab 1994, Scott 1994).
McNab (1994) reported that pigeons (Ducula pacifica)
Experimental manipulations support this view. Upon
removing all bird fauna on a small island 3 km from
New Guinea, Diamond (1975) found that recoloniza-
tion occurred extremely rapidly, initially at 1 avian
species per hour, until ‘‘equilibrium’’ was reached B2
d later. Moreover, Simberloff and Wilson (1969) found
that it took B36 weeks to re-establish previous levels
of species richness of arthropods on 5 of 6 experimental
mangrove islands, with immigration and extinction
2
on Anuta Island (0.4 km ), an outlier of the Solomon
chain, relocated from 137 to \300 km in their search
for fruit. Black-shouldered kites Elanus caeruleus uti-
lized periods of favorable wind direction to inhabit San
Clemente Island \80 km from the California coast,
whereupon their relatively large population (16–26 in-
dividuals for two months) fed on mice Mus musculus
and Peromyscus maniculatus but did not breed (Scott
−
1
rates ranging from 0.05–0.50 species d . These colo-
nizations indicate that dispersal is continuous and can
occur at a grand scale. In addition, these data are very
conservative and do not indicate two potentially fre-
quent types of colonization/dispersal events. First, the
‘
‘rescue effect’’ (Brown and Kodric-Brown 1977) is a
1994). Likewise, several raptors (Falco peregrinus, F.
dispersal event that generally goes unrecognized be-
cause colonists merge into extant populations. Second,
not all arrivals ultimately establish populations, either
because of improper sex ratios, small colonizing groups
eventually become extinct, or they arrive at a premature
period during the species assembly of island food webs
spar6erius, Circus cyaneus, Accipiter cooperi, and A.
striatus) visited Socorro Island 460 km from Cabo San
Lucas, Baja California Sur, Mexico (Walter 1990). De-
Sante (1983) gives an account of \215 species of
migrant birds that visited Southeast Farallon Island off
(
e.g. Allee 1931, Terborgh and Winter 1980, Pimm et Point Reyes, California; he credits long-range, large
al. 1988, Holt et al. 1999).
scale weather factors as determinants of atypical peaks
It is commonly accepted that colonization takes place in vagrant abundance.
with regularity on islands. In fact, much research has
Such records indicate that avian vagrants locate is-
been conducted on the turnover and colonizations of lands and then make their way back to the mainland.
plant (e.g. Herwitz et al. 1996), avian (e.g. Diamond Nevertheless, these visitors may alter the islands that
1
969, Thornton 1996), mammalian (e.g. Heaney 1984, they visit. For example, the first vertebrate visitors on
Laurance 1991), and invertebrate (e.g. Simberloff and Motmot, a volcanic island created within a large fresh-
Wilson 1969, Thornton 1996) species. It should not be water lake on Long Island, New Guinea, were seabirds
much of an extension, then, to appreciate that vagrants and vagrants (avian predators and black ducks Anas
and other ‘‘non-residents’’ also reach far and near superciliosa). These species facilitated the ecological
islands routinely.
development of this island through seed dispersal and
6
94
ECOGRAPHY 23:6 (2000)

allochthonous nutrient deposition (Ball and Glucksman Individuals with multi-island and mainland-island
1
975).
foraging ranges
There are undoubtedly numerous other examples of
There are some vagile species whose foraging ranges
extend across water boundaries and encompass one to
many islands. Approximately 10% of insect species
sighted on Simberloff and Wilson’s (1969) experimental
islands, including cicadas, odonates, bees, wasps, but-
terflies, and moths (and birds as well), were this type of
vagrant. Simberloff and Wilson (1969) did not analyze
how the presence of these predators and competitors
altered colonization and extinction rates; a study that
could have proved interesting. Whitakker (1998) re-
vagrant species either swimming or flying roundtrip to
small islands from the mainland (or from larger is-
lands). However, witnessed or documented events are
infrequent because interested observers are in place so
rarely, the dispersal may occur at night or be of short
duration, and visitors are often inconspicuous. Thus,
the frequency of such behavior is largely unreported
and unknown. To remedy this shortage, we recorded
resident and vagrant bird species on 21 islands over 700
estimated visits for 10 yr (1990–1999) in the Midriff ported island-mainland and island-island range overlap
area of the Gulf of California. Our data suggest that for fruit bats Pteropus 6ampyrus and fruit pigeons
short-term island visits are likely very common. For Ducula bicolor, respectively, that inhabited Krakatau.
example, in the Midriff area of the Gulf of California These species were prime contributors to the plant
alone, we recorded 32 species within 16 families of assembly and successional dynamics of this island
migrants and visitors that rested and foraged on islands through seed dispersal (Whitakker 1998). In Baja Cali-
fornia, individual turkey vultures Cathartes aura, An-
na’s and Costa’s hummingbirds Calypte anna and C.
costae, red-tailed hawks Buteo jamaicensis, and ravens
Cor6us corax have foraging areas that apparently regu-
larly encompass mainland and island locations. We
have likewise observed large spider wasps (tarantula
hawks, Pepsis spp.: Pompilidae) and dragonflies (Order:
Odonata) flying over open ocean in direct lines to, and
foraging among, islands.
for short periods but did not establish multi-year resi-
dency (Appendix). Utilization of these arid islands by
avian vagrants and migrants is significantly linked to
2
wet years associated with El Ni n˜ o events (R =0.45,
DF=1, 7, F=7.59, p=0.03). Case and Cody (1983);
Appendix 8.8) report 35 additional migrant and vagrant
species on Gulf of California islands.
Our data also show that other vagrant and migratory
groups visit islands. In March 1998, we recorded ꢀ100
monarch butterflies Danaus plexippus resting on two
Pachycormis discolor elephant trees on Isla Ventana.
Moreover, in the wet El Ni n˜ o years of 1992/1993, 1995, Source-sink inhabitants
and 1998, we commonly observed butterflies from sev-
Although not vagrants per se, a source-sink configura-
eral families (e.g. Danaidae, Lycaenidae, Nymphalidae,
Pieridae, Papilionidae) flying over the ocean between
the mainland and islands and among islands (see also
Askew 1988, Ferguson et al. 1991).
tion with rapid species turnover likens these individuals
to non-resident visitors. Habitat quality in local patches
varies temporally and may control the dynamics of
‘‘sink’’ populations (Pulliam 1988, Moilanen and Han-
ski 1998). Thomas and Hanski (1997) note that some
species breed and/or persist over larger areas during
optimal seasons or years and then withdraw to more
suitable habitats during bad periods.
This is the case in our study system. Over the last 10
yr, we have observed seven instances of colonization
(
from the mainland and larger islands), persisting for
months to two years, with reproduction and extinction
events by two spider species (Peucetia 6iridens and
Argiope argentata) on islands in the Midriff region of
the Gulf of California. On these same islands, we also
recorded multiple episodes of colonization and breeding
by Costa’s hummingbirds, black-throated sparrows
Amphispiza bilineata, virdens Auriparus fla6iceps, cactus
wrens Campylorhynchus brunneicapillus, and rock wrens
Fig. 1. Winter precipitation and mean % difference in the Salpinctes obsoletus (Appendix). These birds colonize
number of breeding songbird species (S+1) between years. (from the mainland and larger islands) in wet El Ni n˜ o
2
Data was collected on 7 small (B1.5 km ) Gulf of California
years, breed, sometimes repeatedly, and then ‘‘become
islands from 1990 to 1999. Significantly more species inhabited
extinct’’ (i.e. die or return to the mainland) in subse-
quent dry years (Fig. 1). For example, on Isla Piojo in
islands during wet (El Ni n˜ o) years (1992–1995) than dry
(non-El Ni n˜ o) periods (1990–1991, 1996–1997, 1999).
ECOGRAPHY 23:6 (2000)
695

1
995 (a wet year), black-throated sparrows colonized time. However, source-sink populations in our system
and produced three clutches that persisted into early counter this principle. Significantly more songbird spe-
2
1
996, a very dry year. By late 1996, no individuals cies colonize small islands (B1.5 km ) during wet (El
2
remained and we actually found carcasses on the island. Ni n˜ o) periods than dry (non-El Ni n˜ o) years (R =0.55,
Overall, we have observed 13 events on 7 islands of DF=1, 7, F=10.61, p=0.01; Fig. 1). Extinctions
songbird colonization and breeding followed by extinc- occur relatively quickly (within B1–2 yr) as dry years
tion. Lastly, one Scleroporus lizard species colonized soon follow El Ni n˜ o. Colonization rates are dependent
Isla Cerraja during a good year (1995), but has subse- on precipitation; high rates correspond with opportune
quently disappeared. Clearly these are examples of wet years, low rates with inopportune periods. Thus,
source-sink driven populations where extinction rates source-sink occurrences on Gulf of California islands
exceed colonization rates during bad (dry) seasons/ represent non-equilibrium turnover, with net losses (ex-
years.
tinctions) of species during dry years. This is likely true
Many other species colonize and become extinct with for many other mainland-island source sink situations,
regularity during opportune and inopportune periods due in part because other factors besides island area
(
MacArthur 1972, Schoener and Spiller 1987, Thomas and distance from the mainland determine colonization
and Hanski 1997). Diamond’s ‘‘supertramps’’ (1974, and extinction rates.
975) follow this scheme. Even individuals within an Vagrants may influence bigeography measures in
1
island population can be vagrants; Pimm et al. (1988) other ways. For example, failure to distinguish vagrants
pointed out that roughly half of the insular pairs of from residents may erroneously alter classic equilibrium
Hawaiian crows Cor6us hawaiiensis, hawks, and owls analyses of immigration and extinction rates. Over-esti-
died or dispersed to other islands before breeding. mated extinction rates for plants and butterflies on
Hence, range contractions and expansions within Krakatau and Rakata were the result of inclusion of
source-sink and metapopulation systems potentially several migratory and non-residents in initial species
create vagrancy-like situations for some species.
surveys (Whitakker 1998).
Consequences and implications of reduced
island insularity
Island ecology
Vagrants, even when uncommon and few in number,
may significantly affect the flow of energy, nutrients,
and resources, and population spatial, temporal, and
behavioral organization on islands. Transient species
can exert a variety of effects on island resources and the
food webs which they exploit temporarily (e.g. Spiller
and Schoener 1994, Polis et al. 1998). We delineate five
types of interactions. 1) Predation: brief but intensive
predation may be central to resident dynamics (see
Polis 1994). For example, during their annual migration
from Africa to Europe, thousands of white storks Cico-
nia alba remove up to 95% of adult tenebrionid beetles
in certain regions of the Negev desert, Israel (Ayal and
Merkl 1994). 2) Parasites and pathogens: vagrants po-
tentially significantly affect insular populations and is-
land dynamics in their role as vectors for the transport
of parasites or disease. For example, it is well known
that migrant birds can spread avian diseases and trans-
port Ixodes ticks which carry Lyme disease to island
residents (Smith et al. 1996). 3) Competition: vagrants
may compete with residents for resources. Patten and
Burger (1998) found that spruce budworm Choris-
toneura fumiferana outbreaks attracted vagrant warbler
species who subsequently decreased resident magnolia
warblers Dendroica magnolia. 4) Zoogeomorphology:
vagrants can also affect island ecology directly through
habitat alteration: i.e. geophagy, trampling, wallowing,
Our data illustrate that islands may be relatively more
open to non-resident species than previously thought.
The potential frequency of (successful and unsuccessful)
colonizations and short term visits by vagrant and
migratory species does not conform smoothly with the
notion that islands are immune to the influences of
non-insular species. We argue that many islands are
actually quite open to individuals of species that can
move from other islands and the mainland. This view
carries several consequences:
Island isolation
Islands are not as species-poor as species-area curves
would implicate. The arrival of vagrants and utilization
of islands by biota during opportune periods means
that apparently depauperate islands may experience
many more species than expected by species lists of
long term residents made at a particular time. This
decreased isolation carries ecological and evolutionary
implications.
Island biogeography
Classic island biogeography models equilibrium digging, burrowing, and transport/accumulation of ma-
turnover, i.e., no net gain or loss in species number over terial for nest-building (Butler 1995). 5) Cascading
6
96
ECOGRAPHY 23:6 (2000)

events: lastly, infrequent visitors on islands may initiate to large herbivores and predators. Moose Alces alces
cascading events that can affect most elements in an and (especially) caribou Rangifer tarandus inhabit is-
island food web. For example, transport of seeds (via lands in Lake Nipigon, Canada for the summer months
mud on feet, stuck in feathers, excreta, or on prey to escape timber wolf predation (Bergerud et al. 1990).
carried to islands) and eggs (of aquatic invertebrates) Vagrant Sorex shrews on islands in Lake Sysm a¨ , Fin-
can influence island assembly structure and succession land, exist for the summer months but disperse to
of species with effects from the base of the food web larger islands to breed or, more likely, annually ‘‘go
upward (Fridriksson 1975, Ball and Glucksman 1975, extinct’’ (Peltonen and Hanski 1991). Indeed, Brown
Porter 1983). Additionally, top-down effects, e.g. by and Kodric-Brown’s (1977) ‘‘rescue effect’’ is empiri-
vagrants, may influence species compositions, nutrient cally derived from populations of arthropods that ‘‘vis-
dynamics, NAPP, and the distribution of biomass via ited’’ thistle plants (habitat islands) but did not colonize
several pathways (Polis 1999) on islands.
or maintain breeding populations.
Evolution of island biota
Insularity of refuges and reserves
Island vagrants may influence the direction of evolution Habitat refuges/reserves and oceanic islands are similar
on islands in many ways. First, occasional genetic in that ecologically influential yet distinctly different
introgression via visitors could undoubtedly impede the environments surround each. Vagrants from non-re-
evolution of all traits particular to island life (see serve habitats undoubtedly visit refuges and reserves,
Slatkin 1987). Most notably, traits associated with ab- probably regularly. A common, yet presently controver-
sence of predators (flightlessness, changes in body size, sial, example of this involves avian predation (e.g. by
tameness) and competitors (niche expansion, adaptive blue jays Cyanocitta cristata, weasels Mustela erminea,
radiation) would evolve at a slower rate, or cease and raccoons Procyon lotor) and brood-parasitism by
altogether. Coincidentally, such ‘‘non-insular’’ traits cowbirds Molothrus ater along the edges of woodland
could be advantageous for island species that are now habitats and/or in forest fragments (e.g. Wilcove et al.
facing increased predation and competition by invasives 1986, Donovan et al. 1995, Evans and Gates 1997).
(Stone et al. 1994). Second, hybridization of island Similar to situations on marine islands, genetic ‘‘ho-
species with vagrants will alter their genetic (and evolu- mogenization’’ of locally adapted traits with character-
tionary) existence via introgression, hybrid sterility, or istics of rife agricultural vagrants from juxtaposed
outbreeding depression (Rhymer and Simberloff 1996). croplands may decrease resident fitness (Janzen 1986).
These phenomena may occur rather rapidly and with As continental ‘‘island’’ refuges become smaller, va-
few interbreeding (vagrant) individuals (Huxel 1999). In grants will have greater access to insular resources
fact, Rhymer and Simberloff (1996) suggest that hy- (Crooks and Soul e´ 1999), and thus a mounting role in
bridization may be more common on islands (especially determining refuge structure, species composition
small ones) where breeding populations are isolated and within reserves, and evolutionary pathways of residents.
diminished, and the ‘‘other’’ hybridizing population
here vagrants) is concurrently small. Conversely, hy- residents also act as vagrants, moving between refuges
In contrast, it is important to note that refuge/reserve
(
bridization with vagrants has the potential to promote and/or exploiting urban areas, fields, and domestic taxa
prompt evolutionary change (Lewontin and Birch 1966, (e.g. most canid species, raccoons, waterfowl, insects,
Barton and Hewitt 1989) and increase fitness through vampire bats; Janzen 1983, 1986). This exploitation
hybrid vigor, augmented genetic variation, and adjust- carries positive and negative consequences. For in-
ment to novel environments (Grant and Grant 1992, stance, coyotes indirectly increased bird diversity by
Arnold and Hodges 1995, Rhymer and Simberloff killing domestic cats within urban habitat islands
1
996, Huxel 1999).
(Crooks and Soul e´ 1999), a positive consequence. How-
ever, high predation by domestic predators and pesti-
cide-induced mortality of visitors utilizing urban areas
and croplands negatively impacted insular populations
On the insularity of habitat and other
‘islands’’
(
Churcher and Lawton 1987, Boutin et al. 1999).
‘
Janzen (1986) describes further how utilization of adja-
cent habitats by refuge populations can alter insular
systems. Resource subsidy (e.g. for peccary Tayassu
Non-oceanic islands
We have restricted our examples to marine islands. But, spp.) from foraging outside of a Costa Rican reserve
vagrancy occurs within other types of insular systems. influenced the park in three distinct ways. Subsidized
For example, islands in lakes experience visitors with peccaries, with higher densities than enabled by reserve
some frequency, from migratory birds (e.g. Ball and resources alone, foraged within the refuge and greatly
Glucksman 1975) to small mammals (e.g. Hanski 1986) decreased insular resources, trampled soils and vegeta-
ECOGRAPHY 23:6 (2000)
697

tion, and dispersed invasive species of seeds fed upon are ubiquitous, and likely affect islands worldwide,
during forays to juxtaposed areas (Janzen 1986). Such especially nearshore ones. We propose that vagrancy
direct and indirect impacts can significantly influence events are much more prevalent than past documenta-
refuges and reserves.
tion suggests. Moreover, if our view of islands is ex-
panded to encompass lake island refuges, habitat
preserves and parks, caves, mountaintops, and isolated
bodies of waters (ponds, lakes and streams), then va-
grancy is arguably a near ubiquitous and potentially
significant phenomenon. Janzen (1983, p. 403) noted a
self-induced naivet e´ of researchers, in part, about va-
grancy and its effects: ‘‘Every field biologist…is aware
to some degree of this problem, but none of us wish to
fully acknowledge its potential impact on biotic interac-
tions and our ecological/evolutionary interpretations of
them.’’ Consequently, models and studies of such topics
as species turnover, community assembly, island ecol-
ogy, species evolution, extinctions, effects of competi-
tion and predation, and other facets of island systems
underrate or neglect entirely the importance of vagrants
Studies on the ecological and evolutionary
effects of vagrants
‘
‘One of the features imposed by the equilibrium
paradigm has been the restriction of focus to species
that are residents on islands’’ (Whitakker 1998, p. 187).
Little has been done, empirically and otherwise, to
understand the effects of vagrants on ‘‘island’’ systems.
Yet understanding vagrancy events is important for
island conservation, ecology, and evolution. Although
difficult in practice, we nevertheless urge empirical ex-
amination of the frequency and importance of island
visitation by vagrants. Are vagrants more common in
some geographical regions or on islands with certain
features? If so, what geographical and ecological factors
predict vagrant visitation to islands – distance to main-
land, archipelago geometry, high food availability, low
competition and/or predation, a combination of these
factors, or other causes? We advocate studies that
examine how vagrants influence island residents via
competition, predation, parasitism, or transport of
seeds and nutrients. In conjunction, we stress the im-
(
Laurance 1991). In fact, many accounts of island
vagrancy in this paper were gleaned from statements of
exclusion from analyses by other authors (e.g. Sim-
berloff and Wilson 1969, Diamond 1975, Walter 1990,
Peltonen and Hanski 1991). In light of recent and rapid
extinctions on islands and the extent of habitat frag-
mentation on mainlands, the role vagrants play in
population, community and food web dynamics for all
‘‘islands’’ cannot remain an ignored phenomenon.
portance of identifying what species are likely to be Acknowledgements – We extend our gratitude to those who
contributed discussion, reviews, and advice that greatly im-
highly influenced by vagrants; this could carry signifi-
proved this paper: Susan Harrison, Caitlyn Harvey, Gary
cant conservation implications for endemics. Lastly,
Huxel, Antonio Resendiz, Susan Rose, Francisco Sanchez-
there is a need to analyze the effect of vagrants on Pi n˜ ero, Anna Sears, David Spiller, Paul Stapp, Candy Supnet,
species turnover, i.e. are island colonization and extinc- and Abraham Vazquez. This project was supported by the
National Science Foundation (grants DEB-9207855 and DEB-
tion events influenced by visits from non-residents?
9
527888 to GAP) and the Earthwatch Foundation. The Mexi-
To address impacts of vagrants on insular evolution,
can government provided research permits 603-1412 and
we encourage studies looking at the frequency of hy- DAN-02201.
bridization of residents and vagrants on islands. What
are the potential positive and negative effects of hy-
bridization and how does this alter the survival/evolu-
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Appendix. A list of migrant and visiting avifauna present on Midriff Islands in the Gulf of California from 1990
to 1999. Islands of occupancy are listed by number. Bold numbers indicate sited nesting events.
Apodiformes
Trochilidae
Anna’s hummingbirds
Costa’s hummingbirds
Apodidae
white-throated swifts
(
8, 15, 20, 21)
(Calypte anna)
(Calypte costae)
(
7, 13, 17, 19, 20, 21)
(
3, 4, 7, 10, 11, 13)
(Aeronautes saxatalis)
(Zenaida macroura)
(Ceryle alcyon)
Columbiformes
Columbidae
mourning doves
Coraciiformes
Alcedinidae
belted kingfishers
Falconiformes
(
9)
(3, 21)
Accipitridae
red-tailed hawks
Cathartidae
turkey vultures
Passeriformes
(
21)
(Buteo jamaicensis)
(Cathartes aura)
(
6, 20, 21)
Cor6idae
ravens
(
19, 20, 21)
(Cor6us corax)
Emberizidae
magnolia warblers
brown-headed cowbirds
American tree sparrows
(
13, 17)
(Dendroica magnolia)
(Molothrus ater)
(Spizella arborea)
(Amphispiza bilineata)
(Spizella breweri)
(Passerculus sandwichensis)
(Aeronautes saxatalis)
(
13)
(
13,20)
(
3, 6, 9, 10, 13, 17, 20, 21)
black-throated sparrows
(
13, 20)
Brewer’s sparrows
(20)
savannah sparrows
white-throated sparrows
(
21)
Hirundinidae
violet-green swallows
barn swallows
Laniidae
loggerhead shrikes
Mimidae
(
21)
(Tachycineta thalassina)
(Hirundo rustica)
(
3, 17)
(
9, 20, 21)
(Lanius ludo6icianus)
(Mimus polyglottos)
(Polioptila melanura)
(Auriparus fla6iceps)
(20)
northern mockingbirds
Muscicapidae
(9, 21)
black-tailed gnatcatchers
Paridae
(
8, 14)
verdins
Troglodytidae
Bewick’s wrens
cactus wrens
canyon wrens
(
3, 17)
(Thryomanes bewickii)
(Campylorhynchus brunneicapillus)
(Catherpes mexicanus)
(
8, 14, 20, 21)
(
3, 4)
7
00
ECOGRAPHY 23:6 (2000)

Appendix. (Continued)
(
2, 3, 5, 6, 7, 9, 10, 11, 12, 13, 15, 17, 18, 20, 21)
rock wrens
(Salpinctes obsoletus)
(Chamaea fasciata)
(
14)
wrentits
Tyrannidae
ash-throated flycatchers
Cassin’s kingbirds
thick-billed kingbirds
tropical kingbirds
Say’s phoebes
Strigiformes
(1, 5, 20, 21)
(Myiarchus cinerascens)
(Tyrannus 6ociferans)
(Tyrannus crassirostris)
(Tyrannus melancholicus)
(Sayornis saya)
(20)
(21)
(13)
(20, 21)
Tytonidae
elf owls
(21)
(Micrathene whitneyi)
1
. Almagres Chico 2. Bota 3. Cabeza de Caballo 4. Calavera 5. Cardonosa 6. Cerraja 7. Coronadito 8.
Danzante 9. Flecha 10. Gemelos West 11. Jorobado 12. Mejia 13. Mitlan 14. Pajaros 15. Partida Norte 16.
Pata 17. Piojo 18. San Luis 19. San Pedro Mart ´ı r 20. Smith 21. Ventana
ECOGRAPHY 23:6 (2000)
701