COS 111-1 - Monarch butterfly host use across a patchy urban landscape

Abstract: Cities — like many landscapes — are patchy, causing fragments of suitable habitat to be separated by a matrix of unsuitable habitat. In patchy landscapes, movement behavior is paramount, as it determines how animals find and subsequently use these sparse resources. Monarch butterflies, an urban-tolerant species of conservation concern, must find two kinds of plant: milkweed hosts on which to lay their eggs, and various nectar plants on which to feed as adults. Scattered host plants may limit monarch population growth via search time limitation, a situation in which growth rates are limited by the time needed to locate a sparse resource. Nectar plants might attract monarchs to some areas and slow their movement once there. To test for effects of both host plants and nectar plant densities on egg-laying behavior, we placed 20 identical pots of milkweed in Greater Boston neighborhoods that varied in host plant and flower garden densities (a proxy for nectar density) and monitored them for monarch eggs. We predicted that milkweed use (eggs per stem) would vary across these resource contexts.

In our landscape, milkweed stem densities and garden densities were only weakly correlated (r = 0.19, p > 0.4). Monarchs used all pots, laying 877 eggs over the course of the season (range = 8-147 eggs/pot). Our results indicate that monarchs effectively locate host plant even in fragmented urban contexts: in fact, the average number of eggs/stem (0.23) was comparable to values reported for natural areas. Monarchs also varied egg-laying rates based on the neighborhood resource context. Sites with higher neighborhood milkweed densities received fewer eggs per stem (neg. binomial GLM, χ² = 9.9, df = 1, p = 0.002). Sites in neighborhoods with more flower gardens tended to receive more eggs per stem, but this effect was not significant (neg. binomial GLM, χ² = 2.9, df = 1, p = 0.09). Given this non-significant trend, garden-rich areas might attract more monarchs or cause them to stay longer in an area; alternatively, gardening rates may be too low to affect flower resources or monarchs might not modify movement in response to flower density. Conversely, more isolated milkweed plants received more eggs per stem; in spite of this, neighborhoods with more total milkweed stems still supported substantially more total eggs thanks to their greater host plant densities. Our results support the notion of search-time limitation and highlight the value of urban habitat for monarch butterflies.