The Californian Urban Butterfly Fauna Is Dependent on Alien Plants
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DDI_120.fm Page 31 Thursday, December 27, 2001 10:06 AM Diversity and Distributions (2002) 8, 31–40 BIODIVERSITY VIEWPOINTS TheBlackwell Science Ltd Californian urban butterfly fauna is dependent on alien plants ARTHUR M. SHAPIRO Center for Population Biology, University of California, Davis, California, 95616, U.S.A., E-mail: [email protected] Abstract. Using the unusually well-documented abundance greatly reduced, the urban-suburban butterfly fauna of Davis, Yolo County, Califor- butterfly fauna would disappear. This might be nia, it is shown that the mainly native species regarded as an unfortunate, and perhaps intoler- commonly observed in gardens breed mostly or able, side-effect of such programs. entirely on alien plants, especially naturalized weeds. Over 40% of the fauna has no known Key words. Weeds, urban ecology, host plant native hosts in the urban–suburban environment. switching, butterfly gardening, invasive plants, Were certain alien weeds to be eradicated or their California Central Valley. x for over 30 years, I am regularly consulted by INTRODUCTION garden clubs, park and planning commissions, Concern about the impacts of alien organisms civic organizations and members of the public on native biota and environments has risen dra- as to methods of environmental enhancement matically in the past decade, accompanied by a for butterflies. The recent popularity of ‘butterfly spate of alarmist rhetoric and often dramatic gardening’ has increased the frequency of such metaphor. In a special section headed ‘Biological contacts, but I have also been asked to advise on Invaders Sweep In’, the journal Science claimed butterfly aspects in the restoration of degraded that ‘... exotic species can devour or out-compete sites, augmentation of host plants and nectar species that have called an ecosystem home for sources in parks, and the like. I know of no sys- tens of thousands of years. Biological invasions tematic surveys of public attitudes on butterflies are the second biggest cause of biodiversity loss in the United States, but my own experience has in the United States, after habitat destruction convinced me that many people have positive ... they could soon become the first (Enserink, feelings about butterflies and enthusiastically 1999).’ In California, a state particularly heavily want them as part of the urban and suburban affected by naturalized aliens (Bossard et al., environment. Most adults who discuss the sub- 2000), agencies managing parks and wildlands ject with me eventually claim that there are fewer have devoted increasing attention and resources butterflies today than in their childhood, and to the management of introduced species. Such regret that ‘fact.’ efforts sometimes appear driven more by ideo- My interactions with garden clubs and private logy than ecology. Often the managers are un- citizens demonstrate, however, that a clear gap aware of benefits conferred by aliens and of exists between perception and reality in identify- potentially undesirable side-effects from their ing resources for butterflies. The vast majority of removal. Whitson (1996) reminded readers that my interlocutors believe, with no evidence, that ‘the term “weed” does not always indicate that a butterfly gardening is grounded in the use of plant is totally undesirable, or that it cannot be native plants. Most of them prefer to use natives beneficial under certain situations.’ As an ecolo- on more or less ideological grounds, and it seems gist working on butterflies in northern California ‘natural’ to them that since native plants are © 2002 Blackwell Science Ltd. http://www.blackwell-science.com/ddi 31 DDI_120.fm Page 32 Thursday, December 27, 2001 10:06 AM 32 A. M. Shapiro ecologically ‘good’, they must be better than aliens along the south boundary of the city (Putah as resources for butterflies. I have been disabus- Creek). Most of the native plants in Davis occur ing them of this previously harmless conceit here. Within the city there are two artificially these many years. More recently, however, an constructed ‘urban wetlands’ (storm water actual threat to the well-being of urban and ponds) which are planted with mostly native suburban butterfly faunas has emerged in the riparian plants and managed as wildlife habitat. form of eradication or control programs directed The urban vegetation is dominated by a relat- against essential butterfly resources which happen ively short list of (nearly all alien) woody species to be naturalized aliens. I now find myself widely recommended and planted in inland Cal- arguing before government agencies to take into ifornian Mediterranean climates. The city has a account the value of such species as butterfly street tree program which is also strongly domi- resources. Some managers react with disbelief to nated by aliens, though some species belong to the claim that ‘native’ butterflies could depend genera with native members. Increasing numbers on ‘weeds.’ of individual gardeners are planting natives both This paper documents the degree of depend- woody and herbaceous; this has led to a conspicu- ence of one urban–suburban butterfly fauna on ous increase in the diversity of the urban flora alien plants and attempts to define the extent to since 1970. However, hardly any of these species which the results can be generalized. are of any significance to butterflies, and the few that are remain too rare to affect urban butterfly numbers. There are several areas of community THE FAUNA AND ITS SETTING gardens or allotments, most of which are weedy, Davis, Yolo County, California sits in the Sacra- scattered in different parts of the city. Very few mento Valley, the northern half of the California vacant lots remain within established neighbour- Great Central Valley. Except for man-made hoods, and nearly all the drainage/irrigation structures it is essentially flat, with an official ditches within the city have been culverted. There elevation of 15.85 m a.s.l. The population as of are significant corridors of ruderal vegetation March 2001 was 60 308 and the area, including bisecting the city north–south on the shoulders the campus of the University of California, was of Interstate Highway 80 and east–west on State approximately 6100 ha. The climate is Mediterra- Highway 113 and along the right-of-way of the nean, with cloudy, wet but mild winters (with California Northern Railroad. The highway frost, but rarely hard frosts), and sunny, dry and embankments have been landscaped in places hot summers, somewhat tempered by intermittent (with natives, in some cases) and are mowed in shallow intrusions of marine air through the late spring for fire control. There is no compre- nearby Sacramento-San Joaquin Delta. Davis is hensive flora of Davis, and the list of cultivated surrounded by agricultural land, producing a species is so labile that one would be very diffi- mixture of row and tree crops. Between Davis cult to maintain. I estimate — probably very and Sacramento is the Yolo Bypass, a managed conservatively — that at any given time aliens floodplain farmed in summer and used to divert outnumber natives by at least 5 : 1. The vast excessive flows from the Sacramento River to the majority of aliens are of no butterfly significance. Delta in wet years to prevent urban flooding. The The butterfly fauna of Davis has been intensely city is a patchwork of neighbourhoods of mostly monitored since winter 1971/72 and may be the single-family homes developed at different times best-documented such fauna in the United between the late 19th Century and the present, States. Given the domination of the urban flora now ringed by large peripheral subdivisions built and vegetation by aliens, the current fauna of 32 on former agricultural land. Older neighbour- breeding butterfly species in the city appears rich. hoods have relatively small single-family homes In addition, five species (Satyrium californica, on small lots with mature landscaping, forming a Erynnis persius, Epargyreus clarus, Ochlodes syl- nearly continuous canopy. Newer neighbour- vanoides and Poanes melane) have become extinct hoods have larger houses on larger lots and land- in Davis within the past 30 years. Of these, scaping is correspondingly younger. There is a E. clarus fed only on aliens and P. melane (prob- narrow fringe of disturbed riparian woodland ably) in part on aliens. © 2002 Blackwell Science Ltd, Diversity and Distributions, 8, 31–40 DDI_120.fm Page 33 Thursday, December 27, 2001 10:06 AM Urban butterfly fauna 33 The Appendix itemizes the extant fauna and elsewhere in the butterfly’s range and were its known host plants in the city. Two species present in California in the Tertiary. Others are which occur only in the ‘urban wetlands’ or in phylogenetically and/or chemically closely related native riparian vegetation on Putah Creek (Satyr- to native hosts (alien Fraxinus, Platanus and ium sylvinus and Limenitis lorquini) and are not Prunus all have native congeners which are hosts otherwise seen in the city are excluded. Approxi- of Papilio rutulus elsewhere; Ligustrum and mately 15 other species have been seen at least once Syringa, like Fraxinus, are in the Oleaceae). But- in the city; these include strays from the nearby terflies that breed frequently on cultivated herb- Coast Range or Sierra Nevada foothills as well as aceous aliens in gardens include Vanessa cardui, long-distance migrants (e.g. from the deserts of Pieris rapae, Strymon melinus and Papilio zeli- south-eastern California). One species which has caon. All of these also have weedy alien hosts, bred and been common in Davis (Leptotes marina) which are more common and more frequently is excluded because it is a sporadic long-distance used. The common garden hollyhock (Alcea rosea) migrant apparently incapable of persisting. All is the only cultivated alien used by butterflies in the butterflies except Pieris rapae are presumed three families, but its use is only occasional as native, though there is some suggestion of range compared to the ubiquitous weedy Malva.