When and how do species achieve niche stability over long time scales?

Niche conservation is not ubiquitous among all species or all environmental changes. Identifying the underlying basis for variable species responses is important for conservation efforts and macroevolutionary theory and requires comparing multiple clades and types of environmental change. To examine this issue, niche stability was analyzed for a phylogenetically diverse suite of invertebrates that inhabited the epicontinental seas of eastern North America during the Late Ordovician Period (approximate to 450 Ma). These taxa experienced fluctuating sea level and a regional biotic invasion event, the Richmondian invasion, during nine time slices spanning approximately three million years. Niche models were generated with Maxent and niche stability was assessed via geographic overlap, niche similarity, and environmental parameters. These taxa exhibited high fidelity habitat tracking and stable ecological niches during the pre-invasion interval characterized by gradual sea-level changes, whereas taxa exhibited statistically lower niche stability and more niche evolution during and after the Richmondian invasion. Conversely, environmental parameter similarity between niche models of adjacent time slices was lowest during the pre-invasion interval and increased following the Richmondian invasion. Declining niche similarity and geographic stability coincident with increasing environmental parameter similarity indicates ecological niche change resulted from contracting species' realized niches to a subset of the original fundamental niche dimensions rather than shifting to new ecospace. Niche contraction is interpreted as a response to increased competition from invasive species, which may have arisen from adaptation, sorting (differential birth and death processes) and/or ecological fitting. These results indicate that consideration of multiple hierarchical levels, temporal scales and ecological contexts will yield new insights into the processes that generate both macroevolutionary and ecological patterns. They also suggest the widely employed assumption of niche conservatism through time and space may be more frequently valid for intervals of gradual abiotic change than for biotic changes caused by invasive species.