Biogeographic Patterns in Density, Recruitment, Body Size and Zonation of Rocky Intertidal Predators Suggest Increased Population Vulnerability Near Southern Range Limits

Abstract

Aim Surveying the demography of populations near species range edges may indicate their vulnerability to range contractions or local extinction as the climate changes. In the rocky intertidal, not only are latitudinal ranges constricted by thermal stress, but tides also create zonation or a ‘vertical range’ driven by sharp environmental gradients. By investigating demographics along the latitudinal and vertical ranges simultaneously, we can investigate whether populations may be vulnerable to a changing climate. Location Rocky intertidal habitats along west coast of the United States. Taxa Ochre sea star Pisaster ochraceus , six‐armed sea star Leptasterias spp., emarginate whelks (Nucella ostrina and N. emarginata ) and channeled whelk N. canaliculata . Methods In 2018, we surveyed the demographics of the taxa above at 33 sites spanning > 11° latitude from central Oregon to southern California, near the southern range limits of each taxon. We counted and sized individuals from the high to low intertidal zone. To understand how environmental stress changed with latitude, we evaluated intertidal temperatures in situ , as well as tidal extremes, tidal amplitude and wave exposure using offshore buoys. Results For all taxa, population density, the relative proportion of smaller individuals (except for emarginate whelks) and the upper vertical limits on the shore declined from north to south as temperatures increased and high tide height, tidal amplitude and wave heights decreased. In addition, smaller individual Leptasterias spp. generally inhabited lower shore levels while smaller individual emarginate whelks inhabited higher shore levels coastwide. For N. canaliculata , smaller animals were higher on shore northward, but lower on shore southward. Main Conclusions While this study is a snapshot in time and cannot assess impacts of climate change, our surveys suggest environmentally‐related demographic limitation toward southern range limits and demographically vulnerable southern populations. Therefore, a warming climate may cause local extinctions or range contractions near southern limits.

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