pyqgis - How can I get the path for an input layer in a QGIS 3 script?

Writing scripts in previous versions of QGIS, I would get the layer path to pass to algorithms using processing.getObject(layer). In QGIS 3, the scripting system has been rewritten, and I now define a 'source' for input layers like this:

SPECIES_POINTS = 'SPECIES_POINTS'      # INPUT = 'INPUT'

[...]

speciesPointSource = self.parameterAsSource(

            parameters,
            self.SPECIES_POINTS,
            context
        )

I then try to pass it to an algorithm:

pointsReprojected = processing.run("native:reprojectlayer", {
            'INPUT': speciesPointSource,
            'TARGET_CRS': CRS,
            'OUTPUT' : speciesExtent_id

        }, context=context, feedback=feedback)['OUTPUT']

but when I try to run this, I get an error:

Unable to execute algorithm
Could not load source layer for INPUT: invalid value
Traceback (most recent call last):
File "", line 191, in processAlgorithm
File "C:/OSGEO4~1/apps/qgis/./python/plugins\processing\tools\general.py", line 96, in run
return Processing.runAlgorithm(algOrName, parameters, onFinish, feedback, context)
File "C:/OSGEO4~1/apps/qgis/./python/plugins\processing\core\Processing.py", line 142, in runAlgorithm

raise QgsProcessingException(msg)
_core.QgsProcessingException: Unable to execute algorithm
Could not load source layer for INPUT: invalid value

I can see that speciesPointSource has type QgsFeatureSource, so I'm not surprised this doesn't work, but I can't see any attributes that would give me the file path for the layer. I have also tried self.SPECIES_POINTS but then I get the error Could not load source layer for INPUT: SPECIES_POINTS not found. What should I be passing as 'INPUT'?