Cities are expanding and causing a loss of habitat for pollinators, but urban areas can hold surprising areas of diversity. Kathleen Prudic and colleagues in Arizona examined botanical gardens in the southwest USA to see what butterfly species were present. Their paper, published in Insects, shows that biodiversity-friendly botanical gardens can act as a refuge for pollinators.
The findings are the results of a study of two citizen science projects. Prudic and colleagues analysed observations made on the iNaturalist and eButterfly projects logged by users of these projects in five cities. Volunteers recorded over ten thousand observations in the period 2000 to 2022. The scientists then looked to see which observations were made in the botanical gardens in these areas: Desert Botanical Garden in Phoenix, AZ, Tohono Chul and Tucson Botanical Garden in Tucson, AZ, Living Desert in Palm Desert, CA, ABQ BioPark Botanic Garden in Albuquerque, NM, and Chihuahuan Desert Gardens in El Paso, TX. Comparing the observations within and outwith the gardens, the scientists could then see what effect the gardens had. The difference was dramatic, write Prudic and colleagues.
“The observed botanical gardens had disproportionately higher butterfly richness and diversity for their size. Species richness of gardens ranged in the 86–100 percentiles across permutation replicates…. We expected botanical gardens to be around the 50th percentile if they were capturing the same richness as the rest of the city. Instead, the species richness scores were high enough at all botanical gardens to reach our hotspot criteria, or >75th percentile. Two gardens, Tohono Chul of Tucson, AZ, and the Chihuahuan Desert Garden of El Paso, TX, had species richness higher than all permutation replicates of the corresponding city.”
“Species diversity, as measured by Shannon’s Index, were also disproportionately higher in botanical gardens than in the corresponding city. Here, too, we expected botanical gardens to be around the 50th percentile if they were capturing the same diversity as the rest of the city. Botanical garden species diversity percentile ranged from 76–98 across permutation replicates in 5 of the 6 botanical gardens meeting the hotspot criteria. One botanical garden, the Living Desert Botanical Garden of Palm Desert, CA was in the 53rd percentile of permutation replicates and did not meet hotspot criteria…”
The authors note that the low score for the Living Desert Botanical Garden is almost certainly due to how much green space there is in Palm Desert and not a reflection of the garden. “The incorporated area of Palm Springs includes large greenspaces starting at the base of the San Jacinto Mountains and extends across a substantial elevational gradient (~950 m in elevation gain over undeveloped land). This led to a higher estimate of species richness and diversity at the city level due to the inclusion of a significant number of non-urban locations.”
As far as the evidence of butterflies goes, it would seem that botanic gardens are performing an important role in supporting biodiversity. Prudic and colleagues say the same study with other species, such as bees or flies, could be done if people reported them with iNaturalist. They also say iNaturalist could be valuable in recording another vital feature of the botanic gardens. “Additional efforts to record plant affiliations through iNaturalist would also be helpful and could be accomplished by submitting a photograph twice: once for the animal, once for the plant.”
The team conclude with a suggestion to take the study beyond botanic gardens into other green spaces.
“Given current climate projections, we anticipate botanical gardens and other green refugia will be critical habitat for urban wildlife as water becomes less available and redistributed across the landscape. Encouraging and coordinating botanical volunteers to monitor the pollinators in other urban greenspaces such as parks, schools, cemeteries, and community gardens would be helpful to understand and predict urban ecosystem function, climate mitigation, and restoration efforts.”
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Prudic, K.L., Cruz, T.M.P., Winzer, J.I.B., Oliver, J.C., Melkonoff, N.A., Verbais, H. and Hogan, A. (2022) “Botanical gardens are local hotspots for urban butterflies in arid environments,” Insects, 13(10), p. 865. https://doi.org/10.3390/insects13100865